fV I Grr United states Department of Commerce

1 ^J In 3 1 National Institute of Standards and Technology

NATL INST. OF STAND S TECH R.I.C.

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NIST

PUBLICATIONS

NIST Technical Note 1265

Guidelines for Realizing the

International Temperature Scale of

1990 (ITS-90)

B. W. Mangum and G. T. Furukawa

QC

100

.U5753

#1265

1990

C.2
NATIONAL INSTITUTE OF STANDARDS &

TECHNOLOGY

Research Informatkm Center

Gakhersburg, MD 20899
146753

HIST Technical Note 1265

Guidelines for Realizing the

International Temperature Scale of

1990 (ITS-90)

B. W. Mangum

G. T. Furukawa

Center for Chemical Technology

National Measurement Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

August 1990

SfATE5 O* *

U.S. Department of Commerce

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

John W. Lyons, Director
National Institute of Standards U.S. Government Printing Office For sale by the Superintendent

and Technology Washington: 1 990 of Documents

Technical Note 1265 U.S. Government Printing Office

Natl. Inst. Stand. Technol. Washington, DC 20402

Tech. Note 1265

190 pages (Aug. 1990)

CODEN: NTNOEF
ACKNOWLEDGMENTS

The authors gratefully acknowledge the assistance of Mr. Gregory F. Strouse in

providing some of the figures and tables and the reports of calibration for

SPRTs, and the assistance of Mr. George W. Burns, Mr. Earl R. Pfeiffer and Miss

Jacquelyn A. Wise in providing copies of reports of calibration for

thermocouples, RIRTs , and liquid- in- glass thermometers, respectively.

111
Table of Contents

page

List of Tables ix

List of Figures x Abstract xiii

Scope xiii

1

Introduction 1

2. Definition of the ITS- 90 3

2.1 Between 0.65 K and 5.0 K , 3 He and 4 He vapor-pressure

thermometry 6 2.2 Between 3.0 K and 24.5561 K (the triple point of Ne)

3He and A He constant volume gas thermometry 10

2.3 Between 13.8033 K (the triple point of equilibrium

hydrogen) and 1234.93 K (the freezing point of silver),

platinum resistance thermometry 11

2.3.1 General relation between resistance ratios

and Tgo 11

2.3.2 SPRT specifications 12

2.3.3 Range 13 . 8033 K to 273 . 16 K 14

2.3.3.1 Subrange 24.5561 K to 273.16 K 16

2.3.3.2 Subrange 54.3584 K to 273.16 K 16

2.3.3.3 Subrange 83.8058 K to 273.16 K 16

2.3.4 Range 273.15 K (0 °C) to 1234.93 K (961.78 °C) 17

2.3.4.1 Subrange 273.15 K (0 °C) to 933.473 K

(660.323 °C, freezing point of

aluminum) 18

2.3.4.2 Subrange 273.15 K (0 °C) to 692.677 K

(419.527 °C, freezing point of zinc) 18

2.3.4.3 Subrange 273.15 K (0 °C) to 505.078 K

(231.928 °C, freezing point of tin) 18

2.3.4.4 Subrange 273.15 K (0 °C) to 429.7485 K

(156.5985 °C, freezing point of

indium) 18

2.3.4.5 Subrange 273.15 K (0 °C) to 302.9146 K

(29.7646 °C, melting point of

gallium) 19

2.3.5 Subrange 234.3156 K (-38.8344 °C, triple point of

mercury) to 302.9146 K (29.7646 °C, the melting

point of gallium) 19

2.4 Above 1234.93 K (961.78 °C, freezing point of silver);

radiation thermometry based on Planck's Law of

Radiation 19

3 . Realization of the ITS - 90 20

3.1 Vapor pressure thermometry and gas thermometry 20

3.1.1 Realization of the ITS -90 below 273.16 K 20

3.1.2 Vapor pressure thermometry and the CVGT range 20

3.1.3 Realization of the vapor-pressure and CVGT scales

at temperatures below the neon triple point 22

3.1.3.1 3He vapor pressure measurements 23

3.1.3.2 A He vapor pressure measurements 24

iv
Table of Contents (continued)

page

3.1.3.3 e-H2 vapor pressure measurements 24

3.1.3.4 Constant volume gas thermometry 24

3.1.3.5 Pressure measurements 26

3.2 Realization of the fixed points of the ITS-90 28

3.2.1 Effect of impurities 28

3.2.2 Triple points of e-H2 , Ne , 2 , and Ar 29

3.2.2.1 General consideration of apparatus

design 29

3.2.2.2 Realizations of the triple points and

their application to calibration 33

3.2.3 Triple point of water, 273.16 K (0.01 °C) 36

3.2.3.1 Realization and application of the

triple point of water 37

3.2.4 Freezing, melting, or triple points of metals: Hg,

Ga, In, Sn, Zn, Al , Ag, Au, or Cu 39

3.2.4.1 Container material 42

3.2.4.2 Metal fixed point devices for

calibrating SPRTs 42

3.2.5 Control charts of check thermometers 59

3 .

Radiation thermometry 60

4. Calibration of thermometers on the ITS-90 at various levels of

uncertainty and some approximations of the scale 61

4.1 Rhodium- iron resistance thermometers 63

4.2 Germanium resistance thermometers 64

4.3 Standard platinum resistance thermometers 64

4.3.1 Capsule SPRTs [13.8033 K to 429.7485 K

or 505.078 K] 64

4.3.2 Long-stem type SPRTs (83.8058 K to 1234.93 K) 64

4.3.3 Conversion of the IPTS-68 constants and W(T6Q ) tables to approximate ITS-90 constants and

W(TQQ ) tables 65

4.3.4 Uncertainties of calibrations and their

propagation 69

4.3.5 Estimates of possible errors introduced by

extrapolations beyond the range of calibration 70

4.4 Thermocouples (77 K to 2400 K) 78

4.5 Liquid- in- glass thermometers 78

4.6 Industrial platinum resistance thermometers 78

4.7 Thermistor thermometers, digital thermometers, and other

types of thermometers 79

4.8 The logo of the National Conference of Standards

Laboratories for the ITS-90 80

5 References 82
Table of Contents (continued)

page

Appendices 90

6.1 Copies of, or excerpts from, some papers related

to the work of the international bodies concerned

with the ITS - 90 90

6.1.1 Reproduction of the article by H. Preston-Thomas,

entitled, "The International Temperature Scale of

1990," published in Metrologia 27, 3-10 (1990),

with corrections 90

6.1.2 Reproduction of the article by T. J. Quinn,

entitled "News from the BIPM" , published in

Metrologia 26, 69-74 (1989) 99

6.1.3 Recommendations adopted by the CCT at its

17th Session 106

6.1.3.1 Recommendation Tl (1989), The

International Temperature Scale of 1990 106

6.1.3.2 Recommendation T2 (1989), Reference tables

for thermocouples and industrial platinum

resistance thermometers 107

6.1.3.3 Recommendation T3 (1989), The uncertainty

inherent in the realization of the

International Temperature Scale of 1990 107

6.1.4 Reproduction of article by B. W. Mangum,

entitled "Special Report on the International

Temperature Scale of 1990; Report on the 17th

Session of the Consultative Committee on Thermometry," published in J. Res. Natl. Inst.

Stand. Technol. 95, 69-77 (1990) 108

6.2 Excerpts from proceedings of some meetings of the

CIPM and of the CGPM that are related to the designation

of the size of the kelvin and to the ITS -90 118

6.2.1 Recommendation of the CIPM that the numerical value

of 273.16 °K, exactly, be assigned to the triple point

of water on the thermodynamic scale, excerpted from

Proces-Verbaux des Seances, Deuxieme Series, Tome XXIV,

Session de 1954 du Comite International des Poids et

Mesures, published by Gauthier Villars , EditeurImprimeur-Libraire , Paris , 1955 118

6.2.2 Definition of the thermodynamic scale of temperature.

This entails the assignment of the value 273.16 °K,

exactly, to the triple point of water as a fundamental fixed point, excerpted from Comptes Rendus

des Seances, Dixieme Conference Generale des Poids et

Mesures, Reunie a Paris en 1954, published by Gauthier

Villars, Editeur-Imprimeur-Libraire , Editeur du Bureau

International des Poids et Mesures, Paris, 1955 118

VI
Table of Contents (continued)

page

6.2.3 Recommendations of the CCT, concerning the

development of the ITS-90 and simplified methods

of its realization, that were presented to the

CIPM on 9 July 1987, excerpted from Proces-Verbaux

des Seances du Comite International des Poids et

Mesures , Proces-Verbaux de la 76 e session - 1987,

Tome 55, Edite par le BIPM, Pavilion de Breteuil,

F- 92312 Sevres Cedex, France 119

6.2.4 The resolution, instructing the CIPM to develop the

ITS-90, adopted by the 18th CGPM in 1987, and excerpted

from Comptes Rendus , 18 e Conference Generale des Poids

et Mesures (1987), Edite par le BIPM, Pavilion de

Breteuil, F-92312 Sevres Cedex, France 121

6.3 Examples of some typical Report of Calibration documents

from the NIST Calibration Laboratories for thermometers

calibrated over various ranges 122

6.3.1 Example of a Report of Calibration for an SPRT

calibrated over the range from 13.8033 K to

302 . 9146 K 122

6.3.2 Example of a Report of Calibration for an SPRT

calibrated over the range from 83.8058 K to

933.473 K 126

6.3.3 Example of a Report of Calibration for an SPRT

calibrated over the range from 273.15 K to

1234.93 K 129

6.3.4 Example of a Report giving the T68 to T90 conversion for a long- stem type SPRT calibrated

on the IPTS-68(75) from the oxygen condensation

point to the freezing point of zinc 132

6.3.5 Example of a Report giving the T68 to r90 conversion

for a capsule -type SPRT calibrated on the IPTS-68(75)

from the triple point of equilibrium hydrogen to

the freezing point of tin 135

6.3.6 Example of a Report of Calibration for an RIRT

calibrated over the range from 0.50 K to 30 K

on the EPT-76 138

6.3.7 Example of a Report of Calibration for an RIRT

(the RIRT of sec. 6.3.6) calibrated over the

range from 0.65 K to 25 K on the ITS-90 147

6.3.8 Example of a Report of Calibration for a thermocouple

calibrated over the range from °C to 1450 °C 154

6.3.9 Example of a Report of Calibration for a liquid- in- glass thermometer calibrated over the

range from °C to 100 °C on the IPTS-68(75) 162

VII
Table of Contents (continued)

page

6.3.10 Example of a Report of Calibration for the

liquid- in- glass thermometer of section 6.3.9

calibrated over the range from °C to 100 "C on

the IPTS-68(75) but converted to the ITS-90 164

6.4 The National Conference of Standard Laboratories (NCSL)

Ad Hoc Committee (91.3) on the Change in the International

Temperature Scale that was formed to publicize the ITS-90

and its implementation and to minimize the confusion and

inconvenience of the change from the IPTS-68(75) 166

7 . NIST Calibration Services , Thermometers 167

7 .

Laboratory Thermometers 167

7 .

Thermocouples , Thermocouple Materials , and

Pyrometer Indicators 169

7 .

3

Resistance Thermometers 173

vm
List of Tables

page

Table 1. Differences between r90 and T68 (and t90 and t68 ) , and

between T90 and T76 5

Table 2 . Defining fixed points of the ITS -90 8

Table 3. Comparison of temperatures of fixed points assigned on various scales 9

Table 4. Values of the coefficients At and of the constants B and C

for the 3 He and A He vapor pressure equations and the temperature

range for which each equation is valid 10

Table 5. Values of the coefficients Ai? BA

, C t and DL and of the

constants A , B , C , and D in the reference functions,

equations (14) and (22) , and in the functions approximating

them, given by equations (15) and (23) 15

Table 6 . The effect of pressure on the temperatures of the defining

fixed points. The reference pressure for the equilibrium

states of freezing and melting points is one standard

atmosphere (101,325 Pa). Triple points have the vapor pressure

of the material when the solid, liquid and vapor phases

are present in equilibrium 25

Table 7. Latent heats of fusion and first cryoscopic constants of

defining fixed-point materials 29

Table 8. Example of conversion of calibrations of SPRTs on the IPTS-68

to approximate calibrations on the ITS -90 66

Table 9. Values of P/(T68 ) and W(TQQ ) for various fixed-point temperatures

T68 and T90 , respectively 67

Table 10. Example of a conversion of calibration values of a type K

thermocouple on the IPTS-68 to an approximate calibration

on the ITS -90 79

IX
List of Figures

Page

Figure 1. The temperature difference (t90 - t68 )/°C in the

range between the triple point of equilibrium

hydrogen (-259.3467 °C) and the freezing point of

gold (1337 . 33 °C) 4

Figure 2. A schematic representation of the ITS -90 showing

the temperatures of the defining fixed points (or

phase equilibrium states) on the scale and temperature

ranges defined by interpolation instruments and equations ... 7

Figure 3. A schematic representation of the ITS -90 in the range

specified for the platinum resistance thermometer,

showing the various defined subranges and the

temperatures of the defining fixed points on the scale

required for calibration 13

Figure 4. A schematic drawing of the NIST argon triple -point

apparatus for calibrating seven long- stem SPRTs and

six capsule SPRTs 30

Figure 5 . A sealed cell suitable for containing cryogenic gases

at high pressures 31

Figure 6. Two types of triple point of water cells with wells

for platinum resistance thermometers 35

Figure 7 . An SPRT in a Type A triple point of water cell immersed

in an ice bath 38

Figure 8. Idealized liquid/solid equilibrium conditions inside fixed

point cells used in freezing and melting experiments 40

Figure 9 . An SPRT in a metal freezing-point cell 41

Figure 10. Two mercury triple -point cells, one constructed of

borosilicate glass and one of Type 304 stainless steel 43

Figure 11. A borosilicate glass, mercury triple-point cell in a stainless steel container 45

Figure 12. Photograph of an all-plastic indium cell and its

stainless steel container 46

Figure 13. An all-plastic gallium melting/triple-point cell 48

x
List of Figures (continued)

Page

Figure 14. An all-plastic indium freezing-point cell to be used

in a stainless steel container, such as that shown

in figure 12 50

Figure 15. A graphite freezing point cell enclosed inside a fused silica tube with tube connection to high

vacuum, purified argon gas source, and pressure gauge 53

Figure 16. A method for filling a graphite freezing-point cell by

melting the metal sample in the graphite crucible 55

Figure 17. An apparatus for installing a graphite thermometer

well and lid in a graphite crucible containing a molten metal sample 56

Figure 18. Differences between r90 and (r90 ) as calculated from IPTS _ 68 calibration

for the long-stem SPRT, S/NRS8YA-5, of table 8 68

Figure 19. Propagation of errors from errors of calibration of

SPRTs between 13 . 8033 K and 273 . 16 K 71

Figure 20. Propagation of errors from errors of calibration of

SPRTs between 273.15 K and 1234.93 K. Also included

in this figure are error curves for errors made

by the user at the triple point of water 72

Figure 21. Curve for a NIST SPRT that shows the error introduced

by extrapolating the deviation function, determined

from calibration over the range from the triple point

of argon to the triple point of water, downward from

the triple point of argon to 54 K 73

Figure 22. Curve for a NIST SPRT that shows the error introduced

by extrapolating its deviation function, determined

from calibration over the range from the triple point

of mercury to the melting point of gallium, downward

from the triple point of mercury to 84 K 74

Figure 23. Curve for the NIST SPRT of figure 22 that shows the error introduced by extrapolating its deviation function,

determined from calibration over the range from the

triple point of mercury to the melting point of gallium,

downward from the triple point of mercury to only 200 K 75

XI
List of Figures (continued)

Page

Figure 24. Curves for several NIST SPRTs that show the errors introduced by extrapolating their deviation functions,

determined from calibration over the range from the

triple point of water to the freezing point of zinc,

downward from the triple point of water to -50 °C 76

Figure 25. Curves for several NIST SPRTs that show the errors introduced by extrapolating their deviation functions

,

determined from calibration over the range from the

triple point of water to the freezing point of zinc,

upward from the freezing point of zinc to 934 K (660 °C)

Also shown are subrange inconsistencies for the subrange

triple point of water to zinc, relative to the subrange

triple point of water to aluminum 77

Figure 26 . The NCSL ITS -90 logo 81

XII
ABSTRACT

This Technical Note describes the International Temperature Scale of 1990

(ITS-90) that became the official international temperature scale on 1 January

1990, superseding the previous scales, and provides information on how the scale

may be realized at different levels of accuracy. The ITS-90 extends upward from

0.65 K, is in close agreement with the Kelvin Thermodynamic Temperature Scale,

has much improved continuity, precision and reproducibility throughout its ranges

over that of previous scales, and has subranges and alternative definitions in

certain ranges that greatly facilitate its use. In addition to a description

of the ITS-90 and how it can be realized, there are included in this document

reproductions of some articles and excerpts from documents concerned with the

ITS-90. The composition of the Comite Consultatif de Thermometrie (CCT) of the

Comite International des Poids et Mesures (CIPM) at the time of the adoption of

the ITS-90 is given. The differences between the temperatures on the ITS-90 and

those on the International Practical Temperature Scale of 1968, Amended Edition

of 1975, [IPTS-68(75)] and those on the 1976 Provisional 0.5 K to 30 K

Temperature Scale (EPT-76) are tabulated. Measurement procedures for realizing

the ITS-90 throughout the various ranges of the scale are given. Also, for the

most important temperature region, the region of the platinum resistance

thermometer (PRT) , computational examples are given for determining the

coefficients of the relevant deviation equations for PRTs calibrated at various

sets of fixed points. The effects of the introduction of the ITS-90 on electrical reference standards are addressed also.

SCOPE

The ITS-90 and, consequently, the topics discussed in this document affect all

aspects of thermometry at temperatures from 0.65 K upward. This change of scale

will affect principally those making high precision temperature measurements.

Thus, all standards and calibration laboratories will be affected and they should

implement the ITS-90 as soon as possible.

If the required minimum uncertainty of measurements over a given range of

temperature is at least three times as large as the differences between the new

and old scales in that range, as given in table 1 and shown in figure 1 of this

document, then the effect of the change in scales will be negligible and the user

will, for all practical purposes, be unaffected.

DISCLAIMER

Certain commercial equipment, instruments, or materials are identified in this

paper in order to adequately specify the experimental procedure. Such

identification does not imply recommendation or endorsement by the National

Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Xlll

Guidelines for Realizing the

International Temperature Scale of 1990 (ITS -90)

B . W . Mangum

National Institute of Standards and Technology

Gaithersburg, MD 20899

and

G. T. Furukawa

Guest Scientist

National Institute of Standards and Technology

Gaithersburg, MD 20899

1 . INTRODUCTION

The Comite Consultatif de Thermometrie (CCT) is one of eight specialized

technical subcommittees of the Comite International des Poids et Mesures (CIPM)

.

The CIPM is a committee of the Conference Generale des Poids et Mesures (CGPM)

.

These eight subcommittees are:

1. The Comite Consultatif d' Electricite (CCE) , established in 1927,

2. The Comite Consultatif de Photometrie et Radiometric (CCPR) , assigned

this name in 1971; the previous name was the Comite Consultatif de

Photometrie, established in 1933,

3. The Comite Consultatif de Thermometrie (CCT), established in 1937,

4. The Comite Consultatif pour la Definition du Metre (CCDM) , established

in 1952,

5. The Comite Consultatif pour la Definition de la Seconde (CCDS)

,

established in 1956,

6

.

The Comite Consultatif pour les Etalons de Mesure des Rayonnements

Ionisants (CCEMRI) , established in 1958,

7. The Comite Consultatif des Unites (CCU) , established in 1964, and

8. The Comite Consultatif pour la Masse et les grandeurs apparentees

(CCM), established in 1980.

The CCT is composed presently of members from the following laboratories:

1. Amt fur Standardisierung, Messwesen und Warenpriifung [ASMW] , Berlin,

DDR,

2. Bureau National de Metrologie, Paris, France : Institut National de

Metrologie [INM], du Conservatoire National des Arts et Metiers,

3. Ceskoslovensky Metrologicky Ustav [CSMU] , Bratislava, Czechoslovakia,

4. National Research Council [NRC] , Ottawa, Canada,

5. CSIRO, Division of Applied Physics [CSIRO], Lindfield, Australia,

6. D.I. Mendeleyev Institute for Metrology [VNIIM] , Leningrad, USSR;

Physico- Technical and Radio -Technical Measurements Institute [PRMI]

,

Moscow, USSR,

7. National Institute of Metrology [NIM] , Beijing, PRC

,

8. Istituto di Metrologia G. Colonnetti [IMGC] , Turin, Italy,

9. Kamerlingh Onnes Laboratorium [KOL] , Leiden, The Netherlands,

10. Korea Standards Research Institute [KSRI], Seoul, Korea,
11. National Institute of Standards and Technology [NIST] , Gaithersburg,

MD, USA,

12. National Physical Laboratory [NPL] , Teddington, UK,

13. National Research Laboratory of Metrology [NRLM] , Ibaraki, Japan,

14. Physikalisch-Technische Bundesanstalt [PTB] , Braunschweig, FRG,

15. Van Swinden Laboratorium [VSL] , Delft, The Netherlands,

16. Iowa State University, Ames, Iowa, USA, and

17. Bureau International des Poids et Mesures [BIPM] , Sevres, France.

Shortly after the adoption of the International Practical Temperature Scale of

1968 (IPTS-68) [100] , it was realized that the scale had many deficiencies and

limitations. These included its lower limit of 13.81 K, its inaccuracy relative

to thermodynamic temperatures, and its non-uniqueness and irreproducibility

especially in the temperature region from T68 = 903.89 K (630.74 °C) to

T68 = 1337.58 K (1064.43 °C) , the region in which the Pt-10%Rh/Pt thermocouple

was the standard interpolating instrument. Consequently, the CCT undertook the

development of a new scale, and completed it in accordance with Resolution 7 of

the 18th Conference Generale des Poid et Mesures [29] , which met in October 1987

(see appendices)

The CCT met 12-14 September 1989 at the Bureau International des Poids et Mesures

(BIPM) in its 17th Session [14] and completed the final details of the new

temperature scale, the International Temperature Scale of 1990 (ITS-90) [66,83].

The CCT then recommended to the CIPM, which met [84] on 26-28 September 1989 at

the BIPM, that the ITS-90 be adopted and made the official scale (see

appendices) . Upon considering this recommendation, the CIPM adopted the new

temperature scale (see appendices) , and the ITS-90 became the official

international temperature scale on 1 January 1990, the same date on which changes

affecting certain electrical reference standards were implemented [12]. The

ITS-90 supersedes the IPTS-68, the International Practical Temperature Scale of

1968, Amended Edition of 1975 [IPTS-68(75) ] [101] , and the 1976 Provisional 0.5 K

to 30 K Temperature Scale (EPT-76) [99].

The ITS-90 was implemented at the NIST on 1 January 1990. The purpose of this

document is to describe the new scale, to give some guidelines for its

realization and use, to facilitate its implementation, to give the differences

between temperatures on it and those on the IPTS-68(75) and on the EPT-76, and

to describe how the NIST realizes the scale.

The ITS-90 extends upward from 0.65 K and temperatures on this scale are in much

better agreement with thermodynamic values than are those on the IPTS-68 (75) and

the EPT-76. The new scale has subranges and alternative definitions in certain

ranges that greatly facilitate its use. Furthermore, its continuity, non- uniqueness and reproducibility throughout its ranges are much improved over the

corresponding characteristics of the previous scales. The biggest improvement

in reproducibility results from the replacement of thermocouple thermometry with

platinum resistance thermometry in the range 630 °C to the freezing-point

temperature of silver, and with radiation thermometry in the range from the

freezing-point temperature of silver to that of gold.

The change in the temperature scale affects not only technical interests involved

directly in thermometry but also those involved with other reference standards,
such as electrical standards, that are sensitive to temperature. As examples,

standard resistors and standard cells are sensitive to temperature and generally

are maintained in constant- temperature environments, at least in national

standards laboratories. At the present time, the temperatures of those

environments are normally determined with thermometers that were calibrated on

the IPTS-68(75). A given thermodynamic temperature expressed on the ITS-90,

however, has a value that is different from that expressed on the IPTS-68(75),

except at absolute zero (0 K) , at the triple-point temperature of water

(273.16 K) , and at a few other points at which the temperatures on the two scales

are fortuitously the same. This difference is shown in figure 1 [83]. A table

of differences between temperatures on the ITS-90, i.e. , r90 or t90 , and those on

the IPTS-68(75), i.e., r68 or t 68 , and those on the EPT-76, T76 , is given in the

text of the ITS-90 and is presented here in table 1. Although temperature values

expressed on the two scales are different, the change is only in the expression

of the value of temperature and not in the temperature itself . That is to say,

the Kelvin thermodynamic temperature (the hotness) of a material at any given

point is independent of the use of any of the 'practical' temperature scales.

The better the 'practical' scale is, the closer the values of temperature on that

scale are to the thermodynamic temperature values. Needless to say, the Kelvin

thermodynamic temperature values are experimentally determined, and they may have

significant error. Since temperature values expressed on the thermodynamic and

'practical' scales are different, if the temperature of the environment of a reference standard is adjusted so that its value when expressed on the ITS-90

has the same value as had been used on the IPTS-68(75), there will have been a change of the thermodynamic temperature and the value of the reference standard

will usually change. Of course, one may not want to change the thermodynamic

temperature of the reference standard. In that case, the thermodynamic

temperature, as expressed on the IPTS-68(75), can simply be expressed on the ITS90 (a numerical value different from that on the IPTS-68(75)) and the reference

standards will be unaffected. For more details on the effects of the change of

the temperature scale on electrical standards, see National Institute of

Standards and Technology (NIST) Technical Note 1263 [12].

In addition to the effect on reference standards for measurements, all

temperature -sensitive properties, e.g., tables of thermodynamic properties, that

are presently expressed on the IPTS-68(75) may require changes in values.

2. DEFINITION OF THE ITS-90

The ITS-90 was designed by the CCT in such a manner that temperature values

obtained on it do not deviate from the Kelvin thermodynamic temperature values

by more than the uncertainties of the latter values at the time the ITS-90 was

adopted. Thermodynamic temperature is indicated by the symbol T and has the unit

known as the kelvin, symbol K. The size of the kelvin is defined to be 1/273.16

of the thermodynamic temperature of the triple point of water. This definition

of the Kelvin Thermodynamic Temperature Scale (KTTS) that is based on the value

of a single finite temperature is not new; the CCT proposed it in 1954, the CIPM

recommended it, and the Tenth CGPM adopted it that same year [30]

Because temperatures on previous temperature scales were expressed relative to

the ice point (273.15 K) , temperature, symbol t, on the Celsius Temperature Scale

is defined by:
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1 O — — o — o o & en CM J

CM

e

(t,

zing

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O Q> /o. CD !-J

}-l 4-1

(1) °

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' — o 73 Tl"D

0) C

J-l cfl 3

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1 1I

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CM

1

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o" o

1 1

1

1 1

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CM

1 he

te

59.34 w 1 i w 1 1 1

1.

T

2n

(-2 \\ CMo

o

O CMO

o

o

o

o N

cT

1

1 1 0) b0 ^ O

3 H

43o/(

89J-.06

l) 90U9J9^!P 9.in \Bii)diu 91

bOT)
Table 1. Differences between T90 and r68 (and tgo and t68 ) , and between r90 and T

(T90 - r76 )/mK

-0.1 -0.2 -0.3 -0.4 -0.5

10 -0.6 -0.7 -0.8 -1.0 -1.1 -1.3 -1.4 -1.6 -1.8 -2.0

20 -2.2 -2.5 -2.7 -3.0 -3.2 -3.5 -3.8 -4.1

<r90 - r68 )/K

r90/K 1 2 3 4 5 6 7 8 9

10 -0.006 -0.003 -0.004 -0.006 -0.008 -0.009

20 -0.009 -0.008 -0.007 -0.007 -0.006 -0.005 -0.004 -0.004 -0.005 -0.006

30 -0.006 -0.007 -0.008 -0.008 -0.008 -0.007 -0.007 -0.007 -0.006 -0.006

40 -0.006 -0.006 -0.006 -0.006 -0.006 -0.007 -0.007 -0.007 -0.006 -0.006

50 -0.006 -0.005 -0.005 -0.004 -0.003 -0.002 -0.001 0.000 0.001 0.002

60 0.003 0.003 0.004 0.004 0.005 0.005 0.006 0.006 0.007 0.007

70 0.007 0.007 0.007 0.007 0.007 0.008 0.008 0.008 0.008 0.008

80 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.008

90 0.008 0.008 0.008 0.008 0.008 0.008 0.008 0.009 0.009 0.009

r90/K 10 20 30 40 50 60 70 80 90

100 0.009 0.011 0.013 0.014 0.014 0.014 0.014 0.013 0.012 0.012

200 0.011 0.010 0.009 0.008 0.007 0.005 0.003 0.001

( C90 " ^68^/ C

tgo/°C -10 -20 -30 -40 -50 -60 -70 -80 -90

-100 0.013 0.013 0.014 0.014 0.014 0.013 0.012 0.010 0.008 0.008

0.000 0.002 0.004 0.006 0.008 0.009 0.010 0.011 0.012 0.012

t 90/°c 10 20 30 40 50 60 70 80 90

0.000 -0.002

/ -0.005 -0.007 -0.010 -0.013 -0.016 -0.018 -0.021 -0.024

100 -0.026 -0.028 -0.030 -0.032 -0.034 -0.036 -0.037 -0.038 -0.039 -0.039

200 -0.040 -0.040 -0.040 -0.040 -0.040 -0.040 -0.040 -0.039 -0.039 -0.039

300 -0.039 -0.039 -0.039 -0.040 -0.040 -0.041 -0.042 -0.043 -0.045 -0.046

400 -0.048 -0.051 -0.053 -0.056 -0.059 -0.062 -0.065 -0.068 -0.072 -0.075

500 -0.079 -0.083 -0.087 -0.090 -0.094 -0.098 -0.101 -0.105 -0.108 -0.112

600 -0.115 -0.118 -0.122 -0.125 -0.08 -0.03 0.02 0.06 0.11 0.16

700 0.20 0.24 0.28 0.31 0.33 0.35 0.36 0.36 0.36 0.35

800 0.34 0.32 0.29 0.25 0.22 0.18 0.14 0.10 0.06 0.03

900 -0.01 -0.03 -0.06 -0.08 -0.10 -0.12 -0.14 -0.16 -0.17 -0.18

1000 -0.19 -0.20 -0.21 -0.22 -0.23 -0.24 -0.25 -0.25 -0.26 -0.26

t90/°C 100 200 300 400 500 600 700 800 900

1000 -0.26 -0.30 -0.35 -0.39 -0.44 -0.49 -0.54 -0.60 -0.66

2000 -0.72 -0.79 -0.85 -0.93 -1.00 -1.07 -1.15 -1.24 -1.32 -1.41

3000 -1.50 -1.59 -1.69 -1.78 -1.89 -1.99 -2.10 -2.21 -2.32 -2.43
t/°C = T/K - 273.15. (1)

The unit of temperature t is the degree Celsius, symbol °C, and it is by

definition the same size as the kelvin. A difference of temperature may be

expressed either in kelvins or in degrees Celsius.

Temperatures on the ITS -90 are expressed, in terms of the International Kelvin

Temperatures, with the symbol T90 , or, in terms of the International Celsius

Temperatures, with the symbol t90 . The unit of the temperature r90 is the kelvin,

symbol K, and the unit of the temperature t90 is the degree Celsius, symbol °C.

The relation between T90 and t90 is:

t9o/°C = r90 /K - 273.15. (2)

The ITS -90 extends upward from 0.65 K. There are alternative definitions of T90 in certain temperature ranges and they have equal status. In measurements of

the highest precision made at the same temperature, the alternative definitions

will yield detectable temperature differences. Also, at any given temperature

between defining fixed points, different interpolating thermometers that meet

the specifications of the ITS -90 will indicate different temperature values.

These latter differences are referred to as the non-uniqueness in the definition

of the ITS -90. The magnitude of the differences that result from these two sources is sufficiently small to be negligible for all practical purposes.

Temperatures on the ITS -90 are defined in terms of equilibrium phase states of

pure substances (defining fixed points) , interpolating instruments , and equations

that relate the measured property of the instruments to T90 . The equilibrium

phase states of the pure substances and the assigned temperatures used as defining fixed points of the ITS-90 are listed in table 2. Figure 2 shows

schematically the defining phase states and temperature ranges defined by the

various interpolating instruments and equations. For comparison purposes, we

give in table 3 the defining fixed points, and their assigned temperatures, of

the ITS-90 and of all the previous internationally agreed-upon scales.

2.1 BETWEEN 0.65 K AND 5.0 K: 3 He and *He VAPOR PRESSURE THERMOMETRY

The ITS-90 is defined between 0.65 K and 3.2 K by the vapor-pressure- temperature

relation of 3 He , and between 1.25 K and 2.1768 K (the A point) and between

2.1768 K and 5.0 K by the vapor-pressure- temperature relations of 4He . T90 is

defined by the vapor-pressure equations of the form:

9

T90 /K = A + I Ai {[in(p/Pa) - B]/C}\ (3)

i=l

The values of the coefficients AA and of the constants A , B and C of the vaporpressure equations for 3He and A He are given in table 4.
— CO

uw oa a

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2§ re IK >EE o 11 3 C = -o

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to •P3 <

03

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a) x, •-! 54 a w 3 t« bO 5-1 3

•H cr

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w a)
Table 2. Defining fixed points of the ITS -90

Material3 Equilib rium Stateb Temp

T90 (K)

srature

t90 (°c)

Wr (T90 ) e

He VP 3 to 5 - 270.15 to

- 268.15

e-H2 TP 13.8033 - 259.3467 0.00119007

e-H2 (or He) VP (or CVGT) » 17 « - 256.15

e-H2 (or He) VP (or CVGT) = 20.3 « - 252.85

Nec TP 24.5561 - 248.5939 0.00844974

o2 TP 54.3584 - 218.7916 0.09171804

Ard TP 83.8058 - 189.3442 0.21585975

Hgc TP 234.3156 - 38.8344 0.84414211

H2 TP 273.16 0.01 1.00000000

Gac MP 302.9146 29.7646 1.11813889

Inc FP 429.7485 156.5985 1.60980185

Snd FP 505.078 231.928 1.89279768

Zn FP 692.677 419.527 2.56891730

Alc FP 933.473 660.323 3.37600860

Ag FP 1234.93 961.78 4.28642053

Au FP 1337.33 1064.18

Cuc FP 1357.77 1084.62

a e-H2 indicates equilibrium hydrogen, that is, hydrogen with the

equilibrium distribution of its ortho and para states at the corresponding

temperatures. Normal hydrogen at room temperature contains 25% para and

75% ortho hydrogen.

b VP indicates vapor pressure point or equation; CVGT indicates constant

volume gas thermometer point; TP indicates triple point (equilibrium

temperature at which the solid, liquid and vapor phases coexist) ; FP

indicates freezing point and MP indicates melting point (the equilibrium

temperatures at which the solid and liquid phases coexist under a pressure

of 101,325 Pa, one standard atmosphere). The isotopic composition is that

naturally occurring.

c Previously, these were secondary fixed points.

d Previously, these were alternative fixed points.

e From reference functions, eqs (14) and (22).
Table 3. Comparison of temperatures of fixed points assigned on various

scales. Temperatures are expressed in kelvins on the KTTS or equivalent scales

Point NHS a ITS-27b ITS--48b IPTS--48b IPTS-68 IPTS-68(75) EPT -76 ITS-90

Au FPC _ 1336 15 1336 15 1336 15 1337 58 1337.58 _ 1337.33

Ag FP - 1233 65 1233 95 1233 95 1235 08 1235.08 - 1234.93

Al FP - - - - - - - 933.473

S BPd - 717 75 717 75 717 75 - - - - Zn FP - - - (692 655) 692 73 692.73 - 692.677

Sn FP - - - - (505 1181)(505.1181) - 505.078

In FP - - - - - - - 429.7485

H20BP 373 373 15 373 15 373 15 373 15 373.15 - - Ga TP - - - - - - - 302.9146

H2 TPe - - - (273 16) 273 16 273.16 - 273.16

H2 0FP 273 273 15 273 15 - - - - - Hg TP - - - - - - - 234.3156

2 BPf - 90 18 90 18 90 18 90 188 90.188 - - Ar TP - - - - - (83.798) - 83.8058

2 TP - - - - 54 361 54.361 - 54.3584

Ne BP - - - - 27 102 27.102 21 102 - Ne TP - - - - - - 24 5591 24.5561

H2 BP - - - - 20 28 20.28 20 2734 20.3

H2 BP* - - - - 17 042 17.042 17 0373 17.0

H2 TP - - - - 13 81 13.81 13 8044 13.8033

Pb SPh - - - - - - 7 1999 - *HeBP - - - - - - 4 2221 4.2

In SP - - - - - - 3 4145 - 3HeBP - - - - - - - 3.2

Al SP - - - - - - 1 1796 - Zn SP - - - - - - 851 - Cd SP - - - - - - 519 -

a NHS = Normal hydrogen scale [25].

b For a time, the ice point was taken to be 273.16 °K. Here, the value 273.15 K

was used to convert temperature values in degrees Centigrade or Celsius to

kelvins in order to be as consistent as possible throughout the table.

c FP = Freezing point.

d BP = Boiling point at 101,325 Pa.

e TP = Triple point.

f Changed in 1975 to the condensation point.

8 Reduced-pressure boiling point, at P = 33,330.6 Pa.

h SP = Superconductive transition point.
1 053 447

980 106

676 380

372 692

151 656

002 263

006 596

088 966

004 770

054 943

7 3 4 3

1 392 408

527 153

166 756

050 988

026 514

001 975

017 976

005 409

013 259

5 6

2 9

3 146 631

1 357 655

413 923

091 159

016 349

001 826

-0 004 325

-0 004 973

10 3 1 9

Table 4. Values of the coefficients AL and of the constants B and C for the 3He and A He

vapor -pressure equations and the temperature range for which each equation is valid

Coef. or

3He A He A He

Constant 0.65 K to 3.2 K 1.25 K to 2.1768 K 2.1768 K to 5.0 K

A

Ai

A2 A3 A,

A5 A6 A7 A8

A9 B

C

2.2 BETWEEN 3.0 K AND 24.5561 K (THE TRIPLE POINT OF Ne) : 3 He and A He CONSTANT

VOLUME GAS THERMOMETRY

Between 3.0 K and 24.5561 K, the ITS -90 is defined in terms of the 3He or A He

constant volume gas thermometer (CVGT) . The thermometer is calibrated at three

temperatures — at the triple point of neon (24.5561 K) , at the triple point of

equilibrium hydrogen (13.8033 K) , and at a temperature between 3.0 K and 5.0 K,

the value of which is determined by using either a

3 He or a

A He vapor-pressure

thermometer as described in section 2.1.

For a

A He CVGT that is used between 4.2 K and the triple point of neon

(24.5561 K) , r90 is defined by the equation:

r90 = a + bp + cp2

, (4)

where p is the CVGT pressure and a, b, and c are coefficients that are determined

from calibrations at the three specified temperatures, but with the additional

requirement that the calibration with the vapor-pressure thermometer be made at

a temperature between 4.2 K and 5.0 K.

For a

A He CVGT that is used between 3.0 K and 4.2 K, and for a

3 He CVGT that is

used between 3.0 K and 24.5561 K, the non- ideality of the gas must be accounted

for, using the respective second virial coefficients, £ 4 (X90 ) or £ 3 (r90 ) . r90 is

defined in this range by the equation:

a + bp + cp2 T90 = ~ . (5)

1 + BX (T90 )N/V

10
where p is the CVGT pressure; a, b, and c are coefficients that are determined

from calibration at the three defining temperatures; Bx (Tm ) refers to B 3 (r90 ) or

B 4 (r90 ); and N/V is the gas density, in moles per cubic meter, in the CVGT bulb.

The values of the second virial coefficients are given by the following

equations

for 3 He,

B 3 (Tg0 ) /vfmoV1 = [16.69 - 336.98 (T90 /K) -1

+ 91.04 (r90 /K)-2

- 13.82 (r90 /K)"3

] 1CT 6

, (6)

and for *He,

B 4 (r90 )/m3mol_1 = [16.708 - 374.05 (r90 /K) _1

- 383.53 (T90 /K)"2

+ 1799.2 (r90 /K)"3

- 4033.2 (r90/K)

_

* + 3252.8 (T90 /K)- 5

] 10 -6

. (7)

The accuracy of realization of Tgo by using a CVGT is dependent upon the CVGT

design and the gas density used.

2.3 BETWEEN 13.8033 K (THE TRIPLE POINT OF EQUILIBRIUM HYDROGEN) AND 1234.93 K

(THE FREEZING POINT OF SILVER) : PLATINUM RESISTANCE THERMOMETRY

Between 13.8033 K (-259.3467 °C) and 1234.93 K (961.78 °C) , the ITS-90 is defined

in terms of specified fixed points to which temperature values have been

assigned, by resistance ratios of platinum resistance thermometers obtained by

calibration at specified sets of the fixed points, and by reference functions

and deviation functions of resistance ratios which relate to Tgo between the

fixed points. (Henceforth, for convenience, the standards type platinum

resistance thermometers will be referred to by the acronym SPRT.)

2.3.1 GENERAL RELATION BETWEEN RESISTANCE RATIOS AND T90

Temperatures on the ITS-90 in the above -indicated range are expressed in terms

of the ratio of the resistance R(Tgo ) at temperature T90 and the resistance

i?(273.16 K) at the triple-point temperature of water. (Hereinafter, for

convenience, the terms triple -point temperature, freezing-point temperature and

melting-point temperature will be expressed as triple point, freezing point and

melting point, respectively.) The resistance ratio W(Tgo ) is:

W(Tgo ) = R(Tgo )/R(273.16 K).

The temperature Tgo is calculated from the resistance ratio relation:

W(Tgo ) - WX (TM) = AW(Tao),

(8)

(9)

where f/(T90 ) is the observed value, Wx (Tgo ) is the value calculated from the

reference functions, and AW(Tgo ) is the deviation of the observed W(TQ0 ) value of

the particular SPRT from the reference function value. The official version of

the ITS-90 [83] does not indicate the difference [W(T90 )

- tfr (T90 )] by Mf(TQQ )

.

11
Note that in the earlier international scales, W(T) was defined with reference

to the SPRT resistance at 273.15 K, not 273.16 K.

There are two reference functions Wr (Tgo ) , one for the range 13.8033 K to

273.16 K and another for the range 273.15 K to 1234.93 K. These reference

functions will be described in the discussion of the two ranges (sees. 2.3.3 and

2.3.4).

The deviation AW(Tg0 ) is obtained as a function of T90 for various ranges and

subranges by calibration at specified fixed points. The form of the deviation

function depends upon the temperature range of calibration.

A schematic representation of the ITS -90 in the range of temperature specified

for SPRTs is given in figure 3 . Shown in figure 3 are the temperatures of the

defining fixed points in this region of the scale and the various subranges

specified by the scale.

2.3.2 SPRT SPECIFICATIONS

The SPRT sensing element must be made from pure platinum and be strain- free.

The finished SPRT must meet one of the following criteria:

17(302.9146 K) > 1.118 07, or (10)

17(234.3156 K) < 0.844 235. (11)

These criteria are equivalent to a requirement on the slope, namely,

[aW(r90 )/dr90 ] > 3.986 x 10'3 K" 1 at 273.16 K. (12)

An SPRT that is acceptable for use to the freezing point of silver must meet the

following additional criterion:

17(1234.93 K) > 4.2844. (13)

The temperature range over which an SPRT can be used depends upon the thermometer

design, but no single design of SPRT can be used over the whole temperature range

with high accuracy. For measurements at temperatures from 13.8033 K

(-259.3467 °C) to 429.7485 K (156.5985 °C) , or perhaps to as high as 505.078 K

(231.928 °C) , capsule-type SPRTs that have a nominal resistance of 25.5 at

273.16 K may be used. Long-stem type SPRTs of the same nominal resistance may

be used in the range from about 77 K to 933.473 K (660.323 °C) . For temperatures

above about 660 °C and to as high as 1234.93 K (961.78 °C) , long-stem type SPRTs

having a nominal resistance of 0.25 (or possibly 2.5 0) at 273.16 K should be

used. When SPRTs are used at the highest temperatures, leakage currents through

the insulation supports of the platinum wire become significant and these result

in shunting of the resistor. The effect of this shunting is reduced by using

low voltages with low resistance SPRTs.

If the sheath of the long- stem type SPRT is borosilicate glass or stainless

steel, the SPRT should not be used above the upper calibration temperature limit

of 420 °C. If the sheath is Inconel, the upper temperature limit becomes about

12
dd Bv 'e6'»6ZL

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ta 4-> 4->

13
660 °C. If the sheath is fused silica, temperature measurements can be made up

to 962 °C.

For measurements up to about 630 °C, mica is just barely adequate as an electrical insulator for the temperature sensing element and leads of SPRTs.

Starting at about 500 °C, muscovite mica begins to decompose, evolving water that

electrically shunts the thermometer resistor. Phlogopite mica is adequately

stable to 630 °C. For measurements up to 962 °C, refractory materials such as fused silica and sapphire are used for electrical insulation.

2.3.3 RANGE 13.8033 K TO 273.16 K

In the range 13.8033 K to 273.16 K, the reference function Wr (Tgo ) is given by:

12

£n[Wr (TQ0 )] = A + I AiUinCTgo/273.16 K) + 1.5J/1.5} 1

. (14)

i=l

A specified, approximate inverse [83] of this equation, equivalent to within

± 0.000 1 K, is:

15

T90/273.16 K = B + I BiUt^CTgo)] 1 / 6

- . 65)/0 . 35} 1

. (15)

i=l

The values of the constants A and B , and of the coefficients AL and EL of

equations (14) and (15) are listed in table 5.

If an SPRT is to be used throughout the range from 13.8033 K to 273.16 K, it must

be calibrated at the triple points of equilibrium hydrogen (13.8033 K) , neon

(24.5561 K) , oxygen (54.3584 K) , argon (83.8058 K) , mercury (234.3156 K) , and

water (273.16 K) , and at two additional temperatures close to 17.0 K and 20.3 K.

The temperatures of calibration near 17.0 K and 20.3 K may be determined by using

either a CVGT as defined in section 2.2 or the specified vapor-pressuretemperature relation of equilibrium hydrogen.

When the CVGT is used, the two temperatures must be within the ranges 16.9 K to

17.1 K and 20.2 K to 20.4 K, respectively. When the equilibrium hydrogen vaporpressure thermometer is used, the two temperatures must be within the ranges

17.025 K to 17.045 K and 20.26 K to 20.28 K, respectively. The temperatures of

the equilibrium hydrogen vapor-pressure thermometer are determined from the

values of the hydrogen vapor pressure, p, and the equations:

T90/K - 17.035 = (p/kPa - 33 . 3213)/13. 32 (16)

r90/K - 20.27 = (p/kPa - 101.292)/30, (17)

where 13.32 and 30 are values of (dp/dT90 )/(kPa/K) at 17.035 K and 20.27 K,

respectively.

Depending upon the temperature range of use, the SPRT may be calibrated from

273.16 K down to 13.8033 K (the triple point of equilibrium hydrogen), down to

14
Table 5. Values of the coefficients Ait Ei Ci and Di and of the constants Ao» Bo> '0' and D in the reference functions, eqs (14) and (22), and in the

functions approximating them, given by eqs (15) and (23)

Constant or Coefficient

Value Constant or Coefficient

Value

2.135 347 29

3.183 247 20

1.801 435 97

0.717 272 04

Br 0.183 324 722

0.240 975 303

0.209 108 771

0.190 439 972

^5

A6 A7

An

H2

0.503 440 27

-0.618 993 95

-0.053 323 22

0.280 213 62

0.107 152 24

-0.293 028 65

0.044 598 72

0.118 686 32

-0.052 481 34

2.781 572 54

1.646 509 16

•0.137 143 90

0.006 497 67

'10

»12

»15

0.142 648 498

0.077 993 465

0.012 475 611

-0.032 267 127

-0.075 291 522

-0.056 470 670

0.076 201 285

0.123 893 204

-0.029 201 193

-0.091 173 542

0.001 317 696

0.026 025 526

439.932 854

472.418 020

37.684 494

7.472 018

0.002 344 44

0.005 118 68

0.001 879 82

0.002 044 72

2.920 828

0.005 184

0.963 864

0.188 732

0.000 461 22

0.000 457 24

0.191 203

0.049 025

24.5561 K (the triple point of neon), down to 54.3584 K (the triple point of

oxygen), or down to 83.8058 K (the triple point of argon).

The deviation function for calibration over the range 13.8033 K to 273.16 K is

given by the relation:

A^(r90 ) = ai [tf(r90 )

- l] + b 1 [t^(r90 )- l] 2 + I Ci [im/(r90 )]

i+n

, i=l

(18)

15
with n = 2. The coefficients a1( blf and the five ci

' s are obtained by

calibration at all eight of the above temperatures, including that at the triple

point of water. The values of Wr (TQ0 ) are obtained from the reference function

[eq (14)].

The official version of the ITS-90 [83] does not indicate the difference

[W(TQ0 )

- Wx (Tgo )] by AWm (TQQ )

, does not use the subscript m, where in eq (18),

m = 1, nor does it label the coefficients a and b with subscript m. However, we adopt this subscript notation to identify the subranges. Additionally, this

notation is useful when reporting calibration results.

If an SPRT is not to be used over the entire 13.8033 K to 273.16 K range, but

its use restricted to one of the subranges, the deviation functions and the

calibration points are as follows.

2.3.3.1 SUBRANGE 24.5561 K TO 273.16 K

The deviation function for calibration in the subrange 24.5561 K to 273.16 K is

given by the relation:

3

Atf2 (T90 ) = a2 [W(T9Q ) - 1] + b2 [W(T9Q )

- l] 2 + £ c± [£vW(Tgo ) } i+n

, (19)

i=l

with n = 0. The coefficients a2

, b2, and the three c^s are obtained by

calibrating the SPRT at the triple points of equilibrium hydrogen (13.8033 K)

neon (24.5561 K) , oxygen (54.3584 K) , argon (83.8058 K) , mercury (234.3156 K)

and water (273.16 K) . The values of tfr (r90 ) are obtained from the reference

function [eq (14)]. Note that for this subrange, temperatures are measured only

down to the triple point of neon, although the thermometer must be calibrated

at the triple point of equilibrium hydrogen.

2.3.3.2 SUBRANGE 54.3584 K TO 273.16 K

The deviation function for calibration in the subrange 54.3584 K to 273.16 K is

given by the relation:

W3 (Tgo ) = a3 [W(TQ0 ) - 1] + b3 [W(T30 )

- l] 2 + Cl [intf(r90 )]

1+n

, (20)

with n = 1. The coefficients a3

, b3, and c± are obtained by calibrating the SPRT

at the triple points of oxygen (54.3584 K) , argon (83.8058 K) , mercury

(234.3156 K) , and water (273.16 K) . The values of Wr (T30 ) are obtained from the

reference function [eq (14)].

2.3.3.3 SUBRANGE 83.8058 K TO 273.16 K

The deviation function for calibration in the subrange 83.8058 K to 273.16 K is

given by the relation:

Atf4 (T90 ) = ajtf(r90 )

- 1] + bA [tf(r90 )

- l]intf(r90 ). (21)

The coefficients a4 and b4 are obtained by calibrating the SPRT at the triple

16
points of argon (83.8058 K) , mercury (234.3156 K) , and water (273.16 K) . The

values of f/r (T90 ) are obtained from the reference function [eq (14)].

2.3.4 RANGE 273.15 K (0 °C) TO 1234.93 K (961.78 °C)

In the range 273.15 K to 1234.93 K, the equation for the reference function

Wr (Tgo ) is given by:

9

Wr (T90 ) = C + I CjCTgo/K - 754.15)/481] i

. (22)

i=l

A specified, approximate inverse [83] of this equation, equivalent to within

± 0.000 13 K, is:

9

T90/K - 273.15 = D + I DiU^CTgo) - 2 . 64]/l . 64} 1

. (23)

i=l

The values of the constants C and D and of the coefficients Ct and D± of eqs

(22) and (23) are listed in table 5.

If the SPRT is to be used over the entire range 273.15 K to 1234.93 K, it must

be calibrated at the triple point of water (273.16 K) and at the freezing points

of tin (505.078 K) , zinc (692.677 K) , aluminum (933.473 K) , and silver

(1234.93 K).

The deviation function is given by the relation:

W6 (T90 ) = a6 [tf(r90 )

- 1] + b6 [tf(T90 )- l] 2

+ c 6 [W(T90 )

- l] 3 + d[W(Tao ) - ^(933.473 K)] 2

. (24)

The values of a6

, b6, and c 6 are determined from the measured deviations &W(T90 ) of W(T30 ) from the reference values Wr (Tgo ) [obtained from eq (22)] at the

freezing points of tin (505.078 K) , zinc (692.677 K) , and aluminum (933.473 K)

ignoring the term involving d. Then, d is determined from these values of a6 , b6

, and c6 and the deviation AP/(r90 ) of W(TgQ ) from the reference value tfr (T90 ) at

the freezing point of silver (1234.93 K) . The coefficient d is used only for

temperature measurements in the range from the freezing point of aluminum to the

freezing point of silver. For temperature measurements below the freezing point

of aluminum, d = 0.

SPRTs may be calibrated for use throughout the whole range 273.15 K to 1234.93 K

or for shorter subranges by calibrations at fixed points between 273.15 K and

the upper limit of 933.473 K (freezing point of aluminum, 660.323 °C) , of

692.677 K (freezing point of zinc, 419.527 °C) , of 505.078 K (freezing point of

tin, 231.928 °C) , of 429.7485 K (freezing point of indium, 156.5985 °C), or of

302.9146 K (melting point of gallium, 29.7646 °C)

The deviation function Afof5 (r90 ) will be discussed later in the text.

17
If an SPRT is not to be used over the entire 273.15 K to 1234.93 K range, but

its use restricted to one of the subranges, the deviation functions and the

calibration points are as follows.

2.3.4.1 SUBRANGE 273.15 K (0 °C) TO 933.473 K (660.323 °C. FREEZING POINT OF

ALUMINUM)

For an SPRT used throughout the subrange 273.15 K to 933.473 K, the thermometer

is calibrated at the triple point of water (273.16 K) , and at the freezing points

of tin (505.078 K) , zinc (692.677 K) , and aluminum (933.473 K) . The deviation

function is given by the relation:

AW7 (Tgo ) = a7 [W(Tgo )

- 1] + b7 [W(Tgo ) - l] 2 + c 7 [W(Tgo )- l] 3

. (25)

The coefficients a7

, b7, and c7, identical to a6, b6 and c 6, respectively, are determined from the deviations AW(Tgo ) of W(Tgo ) from the reference values Wr (Tgo ) [eq (22)] at the freezing points of tin (505.078 K) , zinc (692.677 K) , and

aluminum (933.473 K)

.

2.3.4.2 SUBRANGE 273.15 K (0 °C) TO 692.677 K (419.527 °C. FREEZING POINT OF

ZINC)

For an SPRT used throughout the subrange 273.15 K to 692.677 K, the thermometer

is calibrated at the triple point of water (273.16 K) , and at the freezing points

of tin (505.078 K) and zinc (692.677 K) . The deviation function is given by the

relation:

AfcT8 (r90 ) = a8 [f/(r90 )

- 1] + ba [W(Tgo ) - l] 2

. (26)

The coefficients a8 and b8 are determined from the deviations AW(Tgo ) of W(Tgo ) from the reference values Wr (Tgo ) [eq (22)] at the freezing points of tin

(505.078 K) and zinc (692.677 K)

.

2.3.4.3 SUBRANGE 273.15 K (0 °C) TO 505.078 K (231.928 °C. FREEZING POINT OF

TIN)

For an SPRT used throughout the subrange 273.15 K to 505.078 K. the thermometer

is calibrated at the triple point of water (273.16 K) , and at the freezing points

of indium (429.7485 K) and tin (505.078 K) . The form of the deviation function

is the same as that for the subrange 273.15 K to 692.677 K, i.e.,

M/9 (r90 ) = a9 [tf(r90 )

- l] + b 9 [tf(r90 )

- i] 2

. (27)

The coefficients a9 and b9 are determined from the deviations AW(Tgo ) of W(Tgo ) from the reference values Wr (Tgo ) [eq (22)] at the freezing points of indium

(429.7485 K) and tin (505.078 K)

.

2.3.4.4 SUBRANGE 273.15 K (0 °C) TO 429.7485 K (156.5985 °C. FREEZING POINT OF

INDIUM)

For an SPRT used throughout the subrange 273.15 K to 429.7485 K, the thermometer

is calibrated at the triple point of water (273.16 K) and at the freezing point

18
of indium (429.7485 K) . The deviation function is:

W10 (T90 ) = a10 [tf(T90 ) - 1]. (28)

The coefficient a10 is determined from the deviation Af/(r90 ) of W(Tgo ) from the

reference value Wr (TgQ ) [eq (22)] at the freezing point of indium (429.7485 K) .

2.3.4.5 SUBRANGE 273.15 K CO °C) TO 302.9146 K (29.7646 °C. MELTING POINT OF

GALLIUM)

For an SPRT used throughout the subrange 273.15 K to 302.9146 K, the thermometer

is calibrated at the triple point of water (273.16 K) and at the melting point

of gallium (302.9146 K) . The deviation function is:

Atfii(r90 ) = an [^(T90 )

- 1]. (29)

The coefficient an is determined from the deviation A^(T90 ) of W(Tgo ) from the

reference value Wr (TgQ ) [eq (22)] at the melting point of gallium (302.9146 K) .

2.3.5 SUBRANGE 234.3156 K (-38.8344 °C, TRIPLE POINT OF MERCURY) TO 302.9146 K

(29.7646 °C. THE MELTING POINT OF GALLIUM)

For an SPRT used throughout the subrange 234.3156 K to 302.9146 K, the

thermometer is calibrated at the triple points of mercury (234.3156 K) and water

(273.16 K) , and at the melting point of gallium (302.9146 K) . The form of the

deviation function is the same as that for the subrange 273.15 K to 692.677 K,

i.e. ,

Atf5 (r90 ) - a5 [W(T9Q ) - 1] + b5 [f7(r90 ) - l] 2

. (30)

The coefficients a5 and b5 are determined from the deviations AJ7(r90 ) of W(TQ0 ) from the reference values WZ (TQ0 ) at the triple point of mercury (234.3156 K) and

at the melting point of gallium (302.9146 K) . The reference values WT (T9Q ) must

be calculated from the appropriate reference function [either eq (14) or eq

(22)], both reference functions being required to cover this range.

2.4 ABOVE 1234.93 K (961.78 °C. FREEZING POINT OF SILVER): RADIATION THERMOMETRY

BASED ON PLANCK'S LAW OF RADIATION

At temperatures above 1234.93 K, r90 is defined by the relation:

LA (r90 ) exp[c2/AT90 (X)] - 1

LA [r90 (X)] exp[c2/Ar90 ] - 1

(31)

in which LX (TQ0 ) and LA [r90 (X)] are the spectral concentrations of the radiance

of a blackbody at wavelength A (in vacuum) at TQ0 and at T90 (X), respectively.

Tgo (X) refers to either the silver freezing point [r90 (Ag) = 1234.93 K] , the gold

freezing point [T90 (Au) - 1337.33 K] or the copper freezing point

[T90 (Cu) = 1357.77 K] . The second radiation constant, c2 (= hc/k) , of Planck's

radiation formula has the value c2 = 0.014388 m»K. Although the freezing-point

19
temperature of silver is the junction point of platinum resistance thermometry

and radiation thermometry, it is believed that the rgo values of the freezing

points of silver, gold and copper are sufficiently self -consistent that the use

of any one of them as the reference temperature T90 (X) will not result in any

significant difference in the measured values of T90 from what would be obtained

if only the silver freezing point were used.

3. REALIZATION OF THE ITS -90

3.1 VAPOR PRESSURE THERMOMETRY AND GAS THERMOMETRY

3.1.1 REALIZATION OF THE ITS -90 BELOW 273.16 K

The calibration of thermometers below the triple point of argon on the ITS -90, as defined, is relatively complex. It is expected that capsule-type SPRTs

,

rhodium-iron resistance thermometers (RIRTs) , and other stable encapsulated

resistance thermometers will be calibrated in terms of the defined ITS -90 and

then used as reference thermometers to maintain the ITS -90 below about 84 K and

used to calibrate other resistance thermometers by the comparison method. The

reference thermometers will be calibrated periodically in terms of the defined

ITS -90.

By use of the term "realization of the ITS-90," reference is made to obtaining

the equilibrium states as defined by the scale, to having thermometers in thermal

equilibrium with those equilibrium states, and to making accurate measurements

and interpretations of the requisite properties of those thermometers in terms

of the ITS-90.

Considerable effort has been expended to develop and realize the EPT-76, a scale

which covered the range 0.5 K to 30 K. This scale has been widely disseminated

among low temperature scientists. At NIST, the EPT-76 has been maintained on reference -standard RIRTs for use in calibrating customer thermometers. Upon

introduction of the ITS-90, NIST converted the EPT-76 on the reference -standard

RIRTs to the ITS-90 using the specified differences [83] between Tgo and T76 . This converted scale is being used to calibrate other thermometers until such

time that NIST realizes the ITS-90 in this temperature region directly as defined. It is recommended that those laboratories that have thermometers with

calibrations on the EPT-76 adjust their T76 values to conform to r90 values.

When NIST realizes the ITS-90 as defined in this range, the difference between

the converted scale on the reference-standard RIRTs (and where appropriate, on capsule SPRTs) and the realized scale will be determined.

3.1.2 VAPOR PRESSURE THERMOMETRY AND THE CVGT RANGE

For most measurements below about 100 K, better precision can be obtained with

capsule- type SPRTs than with the long- stem type. In the calibration of SPRTs,

however, long-stem type SPRTs (immersion- type SPRTs) can be calibrated easily

by a direct immersion process down to the triple point of argon (83.8058 K) by

moving the SPRTs from one fixed-point device to another. Unless capsule -type

SPRTs and other capsule -type thermometers are installed inside long stem- like

holders, however, they will require re- installation and re-wiring whenever

different fixed-point apparatuses are used. (In this document, the phrase

20
"capsule-type thermometers" means encapsulated resistance thermometers of small

overall dimensions.) It is desirable, therefore, to be able to calibrate

capsule-type SPRTs and other capsule-type thermometers at the argon triple point

and below (or, if possible, even at the triple point of water and below since

calibrations of SPRTs require measurements at the triple point of water) in an integrated "multi-task" (multi- fixed-point) apparatus. Such a multi-task

apparatus requires, in addition to wells for capsule thermometers (in thermal

equilibrium in a "single cryostat block"), means for calibration using 3 He, A He

and e-H2 vapor pressure thermometry, 3 He and *He CVGTs , and triple points of e-H2 , Ne, 2

, Ar, Hg, and H20. A total of 11 chambers is required if all of the

overlapping definitions of the ITS -90 are to be evaluated and if a "continuous

calibration," without re- installation and re-wiring of the SPRTs, is desired from

0.65 K to 273.16 K. In addition, unless high pressure sealed cells of the

reference gases are used, tubes to each of the chambers, except those for Hg and

H20, are required.

Since 3 He and 4 He vapor-pressure and CVGT ranges overlap to a large extent,

chambers for A He vapor-pressure measurements and for 4 He CVGT measurements could

be eliminated and the ITS -90 could still be realized. Also, chambers for e-H2 vapor-pressure and the e-H2 triple-point realizations could be combined. The

number of chambers could be reduced further if the cryostat block could be

allowed to warm to ambient temperature or higher for exchanging certain of the

fixed-point substances, or if it were permissible to perform the calibrations

of the capsule thermometers at the triple points of argon, mercury, and water

in other apparatuses, using a long- stem type holder. This procedure, however,

would require a longer time for calibration.

The number of chambers can also be reduced if suitable, highly stable capsule

thermometers are available for correlating the scales; for this purpose, the

capsule SPRTs would be calibrated in another, simpler, fixed-point apparatus.

It is expected that such a set of reference -standard resistance thermometers

would be calibrated, and that routine calibrations of customer thermometers on

the ITS -90 would be by comparison with these reference thermometers. The

reference thermometers would be checked occasionally against the defined ITS -90.

It is hoped that resistance thermometer devices will be more reproducible than

the ability to realize the defined ITS -90. Depending upon the design of the

cryostat, the defined ITS -90 may lack the desired reproducibility when realized

in a multi -task apparatus of high complexity. In order to achieve the best

realization of the ITS -90, it may be more practical to limit the number of

defining fixed points in a single cryostat block.

The greatest problem in realization of the fixed points and in calibrations of

capsule thermometers is ensuring that the multi-task or a "single-task" cryostat

block is isothermal. Depending upon the design, a thermal gradient can persist.

The presence of 3He, A He , e-H2

, Ne , 2, and/or Ar gases in their respective

chambers are expected to be beneficial in making the apparatus isothermal, but,

the gases may be a source of thermal oscillation. (In designing an apparatus

for low temperature gases, provisions should be made to avoid thermal

oscillations in the gas.) The vapor pressures of Ne and Ar are high at their

respective triple-point temperatures so that thermal equilibrium should be easily

attained at their triple points.

21
For a practical cryostat block, the 3He and 4 He CVGTs must connect thermally the

3 He and 4 He vapor-pressure scales and the fixed points of e-H2 and Ne . Such a cryostat block will require six chambers (separate 3 He and A He vapor-pressure

chambers, separate 3He and 4He CVGT chambers, e-H2 vapor-pressure and e-H2 triplepoint chamber, and a Ne chamber) to realize the ITS -90 in the most satisfactory

manner at and below the Ne triple point. This arrangement will permit a direct

comparison of the different parts of the scale and/or calibration of

thermometers. Since the combination of 4 He vapor-pressure thermometry and 4He

constant volume gas thermometry are redundant with 3He vapor-pressure thermometry

and 3He constant volume gas thermometry, the 4 He systems are not required.

Hence, the number of chambers required could be reduced to four and the ITS -90

could still be realized. It should be noted, however, that, depending upon the

CVGT filling pressure, the dp/dT of a

3He CVGT may be less sensitive than *He

vapor-pressure thermometry. Since SPRTs can be calibrated only down to the argon

triple-point temperature using the long-stem SPRT apparatus, it would be most

practical and useful to include an oxygen triple -point chamber in the lowtemperature system, thereby increasing the number of chambers to five. Also,

since it is highly desirable to overlap calibrations obtained in a long- stem type

SPRT apparatus with those obtained in a low- temperature apparatus, an argon

triple-point chamber should be included. This increases the number of chambers

for the low- temperature apparatus to six.

Although it is desirable to have as few tubes as possible going to the cryostat

block in order to minimize temperature gradients in the block, one must build

into the system enough redundant components to be able to check the system for

proper and accurate operation. For example, although the CVGT is calibrated at

only the triple -point temperatures of neon and hydrogen and at one point in the

3 He or ''He vapor pressure range between 3.0 K and 5.0 K, the system should have

the capability for the measurement of hydrogen vapor pressures at about 17 K and

20.3 K so that temperatures measured by means of vapor pressures and by the CVGT

may be compared. Of course, if the system is operating properly, the

temperatures measured by the two techniques should agree. Similarly, there

should be the capability to measure the vapor pressures of both 3He and A He so that temperatures measured with the CVGT in the range from 3 K to 5 K and by

means of 3He and A He vapor pressures may be compared for agreement. Also, it is

desirable to design the CVGT for absolute gas thermometry measurements; the CVGT can check the consistency of the ITS -90 from about 3 K to 90 K.

3.1.3 REALIZATION OF THE VAPOR PRESSURE AND CVGT SCALES AT TEMPERATURES BELOW

THE NEON TRIPLE POINT

Since measurement of pressure is common to both vapor pressure and CVGT

measurements, it will be discussed later in this section (see sec. 3.1.3.5).

In vapor pressure measurements, it is important that cold spots be absent along

the gas -pressure transmitting tube. If cold spots are present, the observed

vapor pressure will reflect the temperature of the condensation at the cold spot

instead of that of the bulk bath. A separate vacuum jacket around the tube will

maintain a continuous heat flux to the sample bulb or bath and should free the

tube of any condensation [24]. The vacuum jacket should also reduce the

occurrence of thermal oscillation in the gas -pressure sensing tube. If thermal

oscillations do occur, they may be suppressed by either one or a combination of

22
the following: increasing the volume of the external gas -pressure space at the

ambient temperature, or by inserting a wad of wool or glass fiber or a piece of

yarn in the gas -pressure sensing tube. The thermal oscillations may be

suppressed also by "tuning" a variable volume device [36] . Thermocouples should

be placed along the gas -pressure sensing tube in order to determine temperatures

along that tube, the distribution of those temperatures being required to

determine the aerostatic head correction.

The vapor pressure may be determined over the bath of bulk liquid 3He , A He, or e-H2 with which the thermometer to be calibrated is in thermal equilibrium. The

measurement can be made also by using a separate, small sample bulb with which

the thermometer is in good thermal contact. The latter method is preferred with

the rather expensive 3He and with e-H2

, which requires a catalyst for the

equilibrium ortho-para conversion of the sample [2,58,91].

3.1.3.1 3He Vapor- Pressure Measurements

Because of the relatively high cost of the sample, vapor-pressure measurements

are made with 3 He contained in a small volume of about 5 cm3

. Likewise, the gaspressure volume to the manometer should be kept relatively small, but large

enough to avoid large thermomolecular pressure effects and thermal oscillations.

Thermomolecular-pressure-effect corrections depend on the sensing tube diameter,

surface condition of the tube, temperature difference, and pressure [49,71,102].

As mentioned above, thermal oscillations can be reduced by varying the gaspressure volume at the ambient temperature or by introducing a wad of fiber or yarn (cotton, wool, or glass) in the gas-pressure tube.

In the past, 3He contained significant amounts of A He and the observed vapor

pressures of 3 He required corrections for its presence. In recent years,

however, 3He samples of 99.9995% purity have become available, eliminating the

requirement for such corrections. At 0.65 K, the vapor pressure and the

temperature derivative of the vapor pressure are 115.9 Pa and 1.08 Pa/mK,

respectively. At the upper limit of 3.2 K, the vapor pressure and the

temperature derivative of the vapor pressure are 101,662.1 Pa and 106.83 Pa/mK,

respectively. The required pressure resolution that corresponds to 0.1 mK of

vapor-pressure measurements varies from 0.108 Pa at 0.65 K to 10.7 Pa at 3.2 K.

Since the amount of 3He in the cryostat is small, and since the amount of A He

used in cooling is relatively large, every effort should be made to avoid

contamination of the sample of 3 He by the A He through diffusion, particularly

through any glass parts of the apparatus.

The sample bulb should contain enough 3 He that the liquid surface temperature and

the cryostat block temperature can be correlated with the observed vapor

pressure. The temperature of the cryostat block must be checked for consistency

with the observed dp/dT of the vapor pressure at the temperature of measurement.

Aerostatic-head corrections depend upon the density of the gas in the pressuretransmitting gas tube. Thermocouples must be distributed along the tube in order

to measure the temperatures required for calculation of these corrections.

23
3.1.3.2 He Vapor -Pressure Measurements

Since liquid 4He can be obtained easily, the vapor pressure can be determined

above a bath of the liquid in which an apparatus containing the thermometer is

immersed. Or, a technique using a small bulb of 4 He sample, with the

thermometer in good thermal contact, can be employed. The lower end of the gaspressure tube should have a small orifice in order to reduce superfluid 4 He film

flow at temperatures below 2.1768 K [61,91]. The A He sample bulb must be in

thermal equilibrium with the cryostat block. The cryostat block temperature

should be checked for consistency with the observed dp/dT of the vapor pressure

at the temperature of measurement.

At the lower temperature limit of 1.25 K, the vapor pressure of A He is 114.7 Pa

and the temperature derivative of the vapor pressure is 0.76 Pa/mK. At the

upper limit of 5 K, the vapor pressure and the temperature derivative of the

vapor pressure of 4 He are 194629.7 Pa and 146.53 Pa/mK, respectively. The

required pressure resolutions of the vapor pressure that corresponds to 0.1 mK

are 0.076 Pa at 1.25 K and 14.7 Pa at 5.0 K [37,38].

3.1.3.3 e-H2 Vapor Pressure Measurements

The equilibrium composition of the two molecular states of hydrogen (ortho and

para) is temperature dependent. The room temperature composition, about 75%

ortho and 25% para, is referred to as normal hydrogen (n-H2 ) . On liquefaction,

the composition slowly changes toward the equilibrium composition corresponding

to its temperature. In the process, the heat of transition is released,

resulting in the evaporation of some hydrogen. A catalyst, such as activated

ferric hydroxide, hastens the equilibration. The catalyst must be placed in the

sample chamber in order to ensure that the hydrogen has the appropriate

equilibrium composition. Most of the conversion must be made before collecting

the liquid in the sample chamber since the heat of conversion (1064 J/mol) from

the ortho to the para molecular state is larger than the heat of vaporization

(900 J/mol) of normal hydrogen. The normal boiling point of e-H2 (equilibrium

composition: 0.21% ortho and 99.79% para) is about 0.12 K lower than that of

n-H2

. The temperatures near 17.035 K and 20.27 K are determined from vaporpressure measurements near 33,321.3 Pa and 101,292 Pa, respectively [2,31,58].

3.1.3.4 Constant Volume Gas Thermometry

Some of the following precautions and corrections that are applicable to

absolute constant -volume gas thermometry should be included in the calibration

of the CVGT at the three specified temperatures of calibration:

1. The volume of the gas bulb should be sufficiently large relative to the gaspressure-line volume to minimize the error in correcting for the "dead space."

On the other hand, the diameter of the gas -pressure line should not be so small

as to cause large thermomolecular pressure corrections

.

2. The temperature coefficient of volume expansion and the pressure expansion

of the gas bulb should be known accurately. (It is desirable to check the

calibration by using the CVGT in the absolute mode.)

24
Table 6. The effect of pressure on the temperatures of the defining fixed

points. The reference pressure for the equilibrium states of freezing and

melting points is one standard atmosphere (101,325 Pa). Triple points have the

vapor pressure of the material when the solid, liquid and vapor phases are present in equilibrium

Material 90

e-H2 TP

Ne TP

o2 TP

Ar TP

Hg TP

H2 TP

Ga MP

In FP

Sn FP

Zn FP

Ai FP

Ag FP

Au FP

Cu FP

13.

24,

54.

83.

234.

273.

302.

429,

505.

692.

933.

1234.

1337.

1357.

8033

5561

3584

8058

3156

16

9146

7485

078

677

473

93

33

77

Pressure Effect of Fixed Point

K Pa" 1 mK/(meter of liquid)

xlO8*

34

16

12

25

5. -7.

2.0

4.9

3.3

4.3

7.0

6.0

6.1

3.3

0.25

1.9

1.5

3.3

7.1

-0.73

-1.2

3.3

2.2

2.7

1.6

5.4

10.

2.6

* Equivalent to millikelvins per standard atmosphere

3. In order to be able to calculate the aerostatic head correction, the

temperature distribution along the connecting gas -pressure transmitting tube

(capillary) must be known. That temperature distribution may be determined by

placing thermocouples along the tube.

4. The gas -bulb filling pressure should be sufficiently high to give an adequate dp/dT for measurement, but not so high as to require large corrections

for non- ideality of the gas.

5. Higher pressures reduce the thermomolecular pressure gradients in the

connecting gas -pressure tube.

6. The effect of adsorption can be reduced by designing the gas bulb so that

the volume is large relative to the surface and by polishing the inside surface

of the bulb

.

For optimizing the CVGT design, the ideal gas law may be applied.

Differentiating the ideal gas relation,

25
pV = riRT, (32)

yields

dp/dT = Rn/y, (33)

where p is the pressure of n moles of gas contained in a volume of V m

3

.

Equation (33) shows that the dp/dT sensitivity of the CVGT is directly related

to the gas density n/V. Expressing the gas constant R as 8.31441 Nm/mol»K or 8.31441 Nm3 /(m2 »mol»K) , the sensitivity dp/dT can be expressed in the units Pa/K.

Thus,

dp/dT - 8.31441(n/7) Pa/K, (34)

where n/V is given in mol/m3. If a gas bulb of 1000 cm3 is filled to four

atmospheres at 273.16 K, n/V would be approximately 178 mol/m3

, and dp/dT

becomes 1484 Pa/K. Since the resolution of many high quality mercury-manometer

systems is about 0.03 Pa to 0.2 Pa, the temperature resolution is about 0.03 mK

to 0.1 mK. It is to be noted that since

p/T - Rn/V, (35)

the sensitivity dp/dT is "constant," independent of the gas bulb volume, as long

as the gas density at the filling temperature is constant. Hence, the gas -bulb

volume and the gas filling pressure should be chosen so that errors from the

effect of dead space, gas non- ideality effects, and other effects are negligibly

small. See references [6,13,49,60].

3.1.3.5 Pressure Measurements

Efforts have been made to minimize the requirement of highly-precise and

accurate pressure measurements in the realization of the defining fixed points

of the ITS -90. The fixed points involving freezing and melting require

knowledge of the pressure only within the significance of the relatively small

pressure effect (cf. table 6). Accurate pressure measurements are required,

however, to realize the vapor -pressure -temperature scales of 3He and 4He in the

range 0.65 K to 5.0 K, and to realize the vapor-pressure -temperature scale of

e-H2 close to 17.035 K and 20.27 K. To realize the CVGT scale using 3He or

A He

gas in the range 3.0 K to the triple point of neon (24.5561 K) , only accurate

pressure -ratio measurements are required.

3.1.3.5.1 Mercury Manometry

Mercury manometry requires precise determination of the difference in height of

the two mercury surfaces in a U-tube manometer. Traditionally, cathetometers

have been used with a smallest imprecision of about 2 Pa. In recent years, the

levels have been sensed, in conjunction with length standards, by capacitive and

interferometric methods. The resolution of such mercury manometry systems is

about 0.05 Pa [19,50,52,81]. (Note: the NIST manometry resolution has been

reported [50] to be about 0.0013 Pa.) For accurate pressure measurements, it

is necessary to know the density of mercury (which is pressure and temperature

26
dependent), the capillary depression at the mercury meniscus, the vapor pressure

of mercury, the aerostatic head difference of the pressure transmitting gas or

gases, and the local acceleration due to gravity. At one standard atmosphere,

uncertainties of absolute pressure measurements of about 3 ppm and pressure

ratios of about 1 ppm have been reported. See references [19,50,52,81].

3.1.3.5.2 Oil Manometry

The techniques and the requirements of oil manometry are similar to those of

mercury manometry.

3.1.3.5.3 Piston Gauges (Pressure Balances)

The pressure balanced by a dead-weight piston gauge is obtained from the mass

of the piston and the applied weights, and the effective area of the freely

rotating piston inside a closely- fitting cylinder. For determination of the

absolute pressure, the gauge must be enclosed and evacuated by a high capacity

pumping system to minimize the back pressure from the gas leaking between the

piston and the cylinder [13,60,81,96]. The local acceleration due to gravity

must be known accurately. Corrections must be applied for the effect of the

streaming gas and for any back pressure. It is advisable to check the readings

of the piston gauge against a primary mercury manometer. Also, the variation

of the effective area with pressure must be determined with a mercury manometer.

The aerostatic head of the manometry system will change as gas leaks between the

piston and the cylinder causing the piston to sink deeper into the cylinder.

The position of the piston may be maintained by automatically pumping more gas

into the system. A resolution of 1 ppm [13] and an uncertainty of about 15 ppm

have been reported in the pressure range 2 kPa to 200 kPa [63,81].

3.1.3.5.4 Diaphragm Pressure Detector

The diaphragm pressure detector consists of a thin metal disk clamped under

tension between two flat electrodes which form two capacitors, with the disk

common to both capacitors. Any pressure differential across the metal disk

causes the disk to deflect, increasing the capacitance on one side while

decreasing the capacitance on the other side. This change is detected by

capacitance bridge techniques. Instruments for absolute pressure measurements

are available; however, they require periodic recalibrations to achieve

uncertainties of 1 to 5 parts in 10* of the readings.

The diaphragm pressure detector is used in high precision manometry as pressure

balance detectors, i.e., with the pressures equal on both sides of the

diaphragm. The diaphragm pressure balance detector separates the gas of the

apparatus (vapor pressure apparatus or GVGT) from the counter-balancing gas of

which the pressure is determined.

The resolution of diaphragm gauges has been reported [13] to be about 0.002 Pa.

Instability due to different pressures, hysteresis, temperature effects, and

other causes may decrease the resolution to 0.02 Pa in actual pressure

measurements [13].

27
3.1.3.5.5 Thermomolecular Pressure Difference

Thermomolecular pressure differences occur at low gas pressures in tubes with

temperature gradients when the tube diameter is not much larger than the mean

free path of the gas . The pressure difference depends upon the gas , the

temperature of the gas , the diameter of the tube , the tube material , and the

surface condition of the tube. The best procedure is either to use a sufficiently large tube to minimize the thermomolecular pressure difference or

to experimentally determine the difference by comparing the pressures between

the small diameter tube being used in the cryostat and a large diameter tube

[49,71,102].

3.2 REALIZATION OF THE FIXED POINTS OF THE ITS -90

3.2.1 EFFECT OF IMPURITIES

Except for the vapor -pressure -temperature points of helium and equilibrium

hydrogen, the fixed points of the ITS -90 are freezing points, melting points,

or triple points. The vapor-pressure measurements with 3He, 4 He, and e-H2 must

be performed with sufficiently pure samples to minimize the effects of

impurities. The principal components of air impurity would be frozen. Neon in

hydrogen, however, causes positive deviations from ideal behavior [93,97]. In

the vapor pressure measurements of A He, it is very likely that the 4 He will be

pure but 3 He may contain some *He. For such a circumstance, Roberts, Sherman and

Sydoriak described a procedure for correcting for the presence of 4 He in 3 He

[90].

The temperatures of freezing points (liquid-solid or liquid- solid-vapor

equilibrium points) of substances are usually lowered by the presence of an

impurity. This sometimes, however, is not the case when that impurity is

soluble in both the liquid and the solid phases of the major component. If a given impurity is known to be present or is suspected, one must consult the

literature on the heterogeneous phase data of metal and non-metal systems to

estimate the possible effect of that impurity on the freezing point [39,51,88].

(Note: often in the analysis of the effect of impurities on freezing points,

the impurity is assumed to be nonvolatile.)

Assuming that the ideal solution law holds and that the impurities remain in

liquid solution, with no concentration gradients, then as the major component

slowly freezes, the depression in the freezing point, relative to the freezing

point of the 100% pure material, is directly proportional to the overall

impurity concentration divided by the "first cryoscopic constant." This is

expressed as:

T(pure) - T(obs) = x2/A. (36)

In eq (36), T(obs) is the observed freezing point of the particular sample being

investigated, T(pure) is the freezing point of the 100% pure material, x2 is the

mole fraction impurity concentration, and A is the first cryoscopic constant.

A is given by the relation:

A = L/R[T(pure)] 2

, (37)

28
Table 7. Latent heats of fusion and first cryoscopic constants of defining

fixed-point materials

Substance Fixed Point

Temperature

T/K

Latent Heat

of Fusion

kJ/mole

•H2 13.8033 0.117

Ne 24.5561 0.335

o2 54.3584 0.444

Ar 83.8058 1.188

Hg 234.3156 2.292

H2 273.16 6.008

Ga 302.9146 5.585

In 429.7485 3.264

Sn 505.078 6.987

Zn 692.677 7.385

Al 933.473 10.79

Ag 1234.93 11.30

Au 1337.33 12.364

Cu 1357.77 13.14

First Cryoscopic Constant

K" 1

0.0739

0.0668

0.0181

0.0203

0.00502

0.00968

0.00732

0.00213

0.00329

0.00185

0.00149

0.000891

0.000831

0.000857

where L is the molar heat of fusion and R is the molar gas constant. (Note:

eq (36) is an approximation. A more complete expression includes secondary

cryoscopic constants. The term "first cryoscopic constant" is used here for

distinction. Also, in some cases, the term "cryoscopic constant" refers to the

reciprocal of eq (37) and in other cases, to the effect of impurities per liter

or kilogram of solvent.) The first cryoscopic constants of metals are relatively smaller than those of molecular substances and of the "cryogenic"

gases ( 3 He, A He , e-H2

, Ne , 2, and Ar) . Referring to eq (36), zinc, which has

a first cryoscopic constant of 0.0018/K, requires an overall impurity

concentration of approximately 2 parts in 107 for the temperature of the halffrozen sample to be depressed by 0.0001 K, relative to the liquidus point. On

the other hand, argon, with a first cryoscopic constant of 0.0203/K, requires

an impurity concentration close to 2 part in 106 for the same temperature

depression. Table 7 lists the heats of fusion and the first cryoscopic

constants of substances specified for the defining fixed points. It is the

usual practice at NIST to calibrate SPRTs during the first 50% of the freeze.

3.2.2 TRIPLE POINTS OF e-H* . Ne . g

. AND Ar

3.2.2.1 GENERAL CONSIDERATION OF APPARATUS DESIGN

The cryogenic fixed points (triple-points of pure gases: equilibrium hydrogen,

natural neon, oxygen, and argon) are best realized in a calorimetric type

apparatus designed for calibrating capsule- type thermometers (SPRTs, RIRTs

,

germanium resistance thermometers (GRTs), and others)

[1,2,17,18,20,21,41,47,58,59,78,79,80] . During calibration of the thermometers,

29
TO SAMPLE

RESERVOIR

LIQUID NITROGEN

TO HIGH VACUUM

AND ELECTRICAL

LEAD TERMINALS

PORT FOR FILLING

LIQUID NITROGEN

67 cm

LONG STEM SPRT

WELLS (7) HE GAS MANIFOLD

TO SPRT WELLS

TEMPERATURE

CONTROLLED SHIELD

PASSIVE SHIELD

SAMPLE CELL SHIELD

SAMPLE CELL

BAFFLES

CAPSULE SPRT

WELLS (6) VACUUM CAN

SUPER INSULATED

DEWAR

Figure 4. A schematic drawing of the NIST argon triple-point apparatus for

calibrating seven long-stem SPRTs and six capsule SPRTs . Six long-stem SPRT

wells surround a central SPRT well, which is large enough to accommodate a holder for calibrating a capsule SPRT. At the bottom of the sample cell, six

capsule SPRT wells are circularly arranged between the long- stem SPRT wells.

30
COPPER

CAPILLARY

AUXILIARY ISOTHERMAL

/ SHELL, HELD WITH NUT

CELL HEATER

COPPER TEMPERING

STRIPS

SAMPLE CELL

COPPER TUBES

HELICALLY GROOVED

COPPER SLEEVE

PLATINUM

RESISTANCE

THERMOMETER

THERMOCOUPLE

CLIPS

Figure 5. A sealed cell suitable for containing cryogenic gases at high

pressures. This cell of 20 cm3 volume was filled to 100 atmospheres with pure

oxygen and used to realize the triple point of oxygen. Another cell of the same outer dimensions, but of 16 cm volume and with wells for three capsule SPRTs

,

was filled to 163 atmospheres with oxygen and also used to realize the triple

point of oxygen [47]

.

31
the calorimeter system thermally isolates the vessel containing the pure gas

sample and the thermometers. The capsule thermometers should be installed in

close-fitting wells of the sample vessel, using stopcock grease to enhance the

thermal contact. The wells should be vented for easy installation and

subsequent removal of the thermometers since such thermometers are sensitive to

shock. The wells can be vented by designing the well tubes to extend completely

through the sample cell or by machining a small groove along the length of the

well wall. The number of wells in the vessel is limited only by the practical

size of the vessel and of the calorimeter system. On the other hand, for large

vessels, larger temperature gradients should be expected. The leads to the

thermometers should be tempered on the sample vessel. (Note: the term "temper"

refers to a process whereby sections of the leads or of the protective sheaths

of long- stem SPRTs are placed in "steady- state thermal equilibrium" with

selected parts of the apparatus.)

Since the triple point of argon is about 6 K above the normal boiling point

(NBP) of nitrogen and since liquid nitrogen is readily available in large

quantities, long-stem type SPRTs can be calibrated at the argon triple point in an apparatus cooled by liquid nitrogen. The apparatus should be designed to

cool the upper part of the protective sheath of the long- stem SPRTs to liquid

nitrogen temperatures, and then to heat the intermediate section of the sheath

above the sensing element to the argon triple point so that the sensing coil

will be in thermal equilibrium with the argon at its triple point [42] . See

figure 4 for a schematic of such an apparatus presently in use at the NIST.

Sample vessels for the cryogenic fixed points could be ruggedly constructed for

high pressures and sealed with a suitable amount of the pure gas [41,79]. These

"sealed-sample vessels" can be easily installed and removed from the calorimeter

for replacement of thermometers and also to be transported to other laboratories

for comparison (see fig. 5) . The amount of gas that can be sealed in such

vessels, however, is rather limited and, hence, the calorimetric system must be

operated with sufficient adiabatic control that the small amount of heat of

fusion of the sample is adequate to realize the triple point and then calibrate

the thermometers. Also, since the amount of sample gas is small, extra care must be taken to clean the vessel thoroughly before filling. The recommended

procedure is to bake the vessel at high vacuum and then purge many times with

the sample gas before finally filling and sealing the cell [41,47,79].

The vessel for a cryogenic fixed point can be installed in the calorimeter and

connected by a small diameter tube to a source of pure gas [1,2,31,32,44,58,59].

In this design, enough condensed liquid could be used to nearly fill the vessel

and, thus, to provide an abundant supply of heat of fusion for calibration of

thermometers. The tube from the vessel must be connected to an "external

expansion volume" of appropriate size so that when the system is at ambient

temperature the pressure is not excessive. The vessel could also be connected

to an external rugged container into which all or nearly all of the sample can

be transferred by cooling, and then contained by a high-pressure valve. Since

under these conditions the sample vessel would not be subjected to high

pressures, it can be constructed of thin copper parts. With appropriate gas

handling and cleaning provisions, the same vessel could be used with all of the

reference gases, stored in separate external rugged containers, and the

thermometers calibrated at the fixed points of the gases without the necessity

32
of having to remove the thermometers from the vessel between fixed-point

determinations. On the other hand, the "sample vessel" can have separate

chambers for each of the gases. In the latter design, separate tubes for the

gases must enter the calorimeter system. Although each of the chambers of a "multi- chamber sample vessel" would be relatively small, the amount of sample

that could be condensed inside each chamber would still be more than that which

is normally used with the high pressure "sealed-sample vessel." Similar to the

procedure used in filling sealed- sample vessels, a thorough baking, pumping, and

purging procedure before filling should be followed with permanently installed

vessels

.

3.2.2.2 REALIZATIONS OF THE TRIPLE POINTS AND THEIR APPLICATION TO CALIBRATION

The procedure for realizing a triple point of the cryogenic gases is to first

completely freeze the sample. If the triple point is being realized for the

first time with the apparatus, the sample should be cooled sufficiently below

the triple point to determine the heat capacity of the system (sample, vessel,

and thermometers) from about 5 K to 20 K below (depending on the gas) to about

20 K above the triple point, and to determine the heat of fusion of the sample

during the same series of measurements [41,47]. (Note: check thermometers must

be calibrated along with the test thermometers. The measurements on the check

thermometers will serve to guide the heating process during the calibration, as well as to provide measurement statistics.)

After cooling to the required low temperature, the vessel should be thermally

isolated by placing it under continuous adiabatic control. Then the following

series of measurements should be performed:

1. the equilibrium temperature should be observed with the check

thermometer,

2. a measured amount of electrical energy should be added to the

system,

3. a new equilibrium temperature should be established and measured,

4. steps 2 and 3 should be repeated until three heat capacity points

are obtained below the triple point,

5. then, the sample should be completely melted by introducing a measured amount of heat,

6. next, the equilibrium temperature just above the triple point should

be measured,

7. then, three additional heat capacity points should be obtained above

the triple point in accordance with steps 2 and 3.

From the knowledge of the eight equilibrium temperatures (four below the triple

point and four above the triple point) and the measured amounts of electrical

energies added, the heat capacities of the system below and above the triple

point, and the heat of fusion, are calculated. If Q joules are added to the

system from an initial equilibrium temperature TL just below the triple point to

heat the system to a final equilibrium temperature Tf just above the triple

point, the heat of fusion L is:

L = Q - C S (T - TO - Ct (Tf

- T ), (38)

33
where Cs and C^ are the mean heat capacities in the temperature intervals in the

solid and liquid phases, respectively, and TQ is the triple -point temperature.

For calibration of SPRTs, the sample should be completely frozen for the second

time and the temperature set at about 1 K below the triple point, the sample

thermally isolated, and the equilibrium temperature measured. From the

knowledge of the previously determined heat capacity of the system below the

triple point and of the heat of fusion, add enough electrical energy to melt

about 10% of the sample. If Ti is the initial equilibrium temperature just

below the triple point, the required amount of electrical energy Qx to melt 10%

of the sample is:

Qx = 0.1L + C8 (T - r^ (39)

Once the system comes to equilibrium, measure the resistances of all of the

thermometers. Repeat the measurements at 20%, 40%, 60%, 70%, and 80% melted.

If the sample is about 99.9999% pure, all measurements on each thermometer

throughout this melted range should agree to within 0.1 mK to 0.2 mK.

In using a temperature fixed point, one must make corrections for the

hydrostatic head of the liquid and for the gas pressure on the defined

equilibrium state. Table 6 gives the dT/dp for the defining fixed points of the

ITS-90, both in terms of the external gas pressure to which the fixed-point

material is exposed and in terms of the column of liquid.

3.2.2.2.1 TRIPLE POINT OF EQUILIBRIUM HYDROGEN. 13.8033 K (-259.3467 °C)

Hydrogen gas samples of 99.9999% and higher purity are readily available. [The

first cryoscopic constant of hydrogen is relatively high (0.0740/K).

Consequently, the liquidus point of an ideal hydrogen solution of 99.9999%

purity would be approximately 0.01 mK lower than that of 100% pure hydrogen.

Except for helium and deuterium, all other impurities would be either frozen or

in solution in very small amounts.] The commonly used catalyst for converting

ortho hydrogen to para hydrogen is hydrated ferric oxide (Fe2 3 »H2 or Fe0»0H)

Other oxides of magnetic elements, either pure or mixed-metal, such as those of

chromium, nickel, cobalt, and neodymium, also have been used as catalysts for

ortho to para hydrogen conversion. The hydrated ferric oxide catalyst is

prepared by mixing at about 30 °C relatively dilute solutions (about 2 molal)

of ferric chloride and sodium hydroxide, with only a slight excess of sodium

hydroxide, washing the resulting gelatinous Fe(0H) 3 precipitate thoroughly with

distilled water, air drying at 140 °C for 24 hours, vacuum baking at 110 °C for

16 to 20 hours, and back-filling with hydrogen while the catalyst is still hot

[7,104]. The catalyst is activated by flowing hydrogen through it for about 4

hours while the catalyst is maintained at a temperature of about 150 °C. The

sample vessel and ancillary components should be designed to permit the whole

of the hydrogen sample to come into contact with the catalyst at the equilibrium

temperature. See references [2,31,58].

3.2.2.2.2 TRIPLE POINT OF NATURAL NEON. 24.5561 K (-248.5939 °C)

Neon gas samples of 99.999% purity are commercially available. Samples of

higher purity may be obtained by special arrangement with the supplier. The

34
Type A TypeB

Figure 6. Two types of triple point of water cells with wells for platinum resistance thermometers. The cells contain pure air-free water thermometer wells are made of precision-bore tubing

The

35
principal impurities are CO, H2

, He, and N2

. The CO and the N2 impurities can

be frozen out by slowly flowing the sample through a coiled tube immersed in

liquid neon; the H2 and the He impurities can be removed by freezing the neon

sample in liquid hydrogen and pumping, with care so that the lighter isotopes

of neon are not preferentially removed. [The first cryoscopic constant of neon

is 0.0668/K. Consequently, the liquidus point of an ideal neon solution of

99.999% purity would be approximately 0.1 mK lower than that of 100% pure neon.]

The purified sample should be collected in a clean stainless -steel cylinder by

cooling the cylinder in liquid hydrogen (or cooled with liquid helium) . If

desired, the purified sample can be collected directly in the cooled sample

vessel. See references [1,79].

3.2.2.2.3 TRIPLE POINT OF OXYGEN. 54.3584 K (-218.7916 °C)

Oxygen gas samples of 99.999% purity are commercially available. Accurate

chemical analysis of oxygen is difficult and, therefore, the claimed purity may

not be correct. Unknown or undetected impurities are chemically less reactive

than oxygen, e.g, the noble gases, and, in particular, argon, which forms a peritectic with oxygen. Samples of purity greater than 99.999% may be obtained

by special arrangement with the supplier. [The first cryoscopic constant of

oxygen is 0.0181/K. Consequently, the liquidus point of an ideal oxygen

solution of 99.999% purity would be approximately . 6 mK lower than that of 100%

pure oxygen. ] Careful preparation by thermal decomposition of potassium

permanganate can yield samples of 99.9999% purity or better [47]. The oxygen

sample should be stored in clean stainless-steel cylinders. See references

[32,47,59,79,80]

.

3.2.2.2.4 TRIPLE POINT OF ARGON. 83.8058 K (-189.3442 °C)

Argon gas samples of 99.9999% purity or better are readily available. The usual

impurities are the components of air, moisture, and hydrocarbons. [The first

cryoscopic constant of argon is 0.0203/K. Consequently, the liquidus point of an ideal argon solution of 99.9999% purity would be approximately 0.05 mK lower

than that of 100% pure argon.] To fill the sample vessel, the gas may be used

directly or it may be dried first by slowly passing it through a coiled tube

immersed in either liquid oxygen or a Dry Ice/ethyl alcohol mixture. See

references [ 17 , 41 , 42 , 59 , 79 , 80 ]

.

3.2.3 TRIPLE POINT OF WATER. 273.16 K (0.01 °C)

The triple-point temperature of water is assigned the value 273.16 K on the

Kelvin Thermodynamic Temperature Scale and also on the ITS -90. It is the

reference temperature for resistance ratios in platinum resistance thermometry.

The water used in preparing triple point of water cells is pure water of

naturally-occurring isotopic composition. Figure 6 shows two commonly used

types of triple point of water cells.

Triple point of water cells are usually prepared from river water that has been

purified by chemical treatment and distillation. River water is expected to

have concentrations of deuterium and the heavier isotopes of oxygen that are lower than that of ocean water. The extreme difference in the triple points of

naturally- occurring water, including polar water, is given as 0.25 mK [100].

36
It is expected that differences among water triple-point cells of river water

would be much smaller than 0.25 mK. (The isotopic composition difference

between river water and ocean water [100] has been estimated to cause no more

than a 0.050 mK difference in the triple-point temperature.) While the basic

material is plentifully available, preparation of water triple-point cells

requires a special effort [5,40]. Although the effect on the triple-point

temperature is negligible, a trace of air always remains in most sealed triple

point of water cells. When a cell at room temperature is gently inverted from

one end to the other and a sharp "click" is produced through the water hammer

action, the amount of gas in the cell will have negligible influence on the

triple -point temperature.

3.2.3.1 REALIZATION AND APPLICATION OF THE TRIPLE POINT OF WATER

In preparation for producing an ice mantle that is required for realizing the

triple -point temperature of water, the thermometer well of the cell is wiped

thoroughly dry, sealed with a rubber stopper, and the cell placed in an ice bath

to cool to a few degrees above the ice point. When the cell has been cooled in

this manner, an ice mantle of fairly uniform thickness can be obtained.

Withdraw the cell from the ice bath, set it upright on a stand, and place one drop of ethyl alcohol at the bottom of the well. Introduce small amounts of

crushed Dry Ice into the bottom of the well and continue to do so until a thick

mantle is formed at the bottom. Then, fill the well with crushed Dry Ice to the

water level of the cell. Continue to add crushed Dry Ice to the well so as to

maintain the level of Dry Ice at the water level. If the Dry Ice level becomes

low before more is added, the ice mantle may crack. If the cell were precooled

as indicated above, a solid ice bridge may form at the water level. If such a bridge forms, melt it immediately with heat from the hands while gently shaking

the cell. The solid ice bridge can completely seal the cell at the top and any

subsequent formation of ice could produce enough pressure to rupture the glass

cell. When a mantle of approximately the desired thickness (4 to 8 mm) is

formed, stop adding Dry Ice, replace the cell in the ice bath with the well

opening slightly above the water surface of the ice bath, and leave the cell

there until all of the Dry Ice evaporates. Then, fill the well with ice water

and store the cell in an ice bath or ice pack for a day before using it. When

the ice mantle is frozen by using Dry Ice, a process that usually requires less

than one hour, the strains in the ice cause the "triple -point temperature" to

be about 0.2 mK low. These are removed by letting the mantle anneal for one

day.

Other methods can be used also to prepare the ice mantle. With ethyl alcohol

in the thermometer well, any "cold finger" technique can be used. This

technique includes successively inserting liquid nitrogen cooled rods, using a closed-end tube containing crushed Dry Ice, or using a heat-pipe cooler. These

methods require more time to freeze the mantle, but the strain produced in the

ice will be less than those produced by the Dry Ice technique.

After the strains in the ice mantle have been relieved by storing the cell in an ice bath for at least one day, insert momentarily a glass rod into the well

in order to melt a thin layer of ice next to the well. This forms an ice -water

interface immediately adjacent to the thermometer well. The test for this

"inner melt" is made by giving the cell a rotatory impulse to determine whether

37
Figure 7

bath.

An SPRT in a Type A triple point of water cell immersed in an ice

A -- platinum resistance thermometer, B -- heavy black felt to shield against

ambient radiation, C -- polyethylene tube for guiding the SPRT into the

thermometer well (the tube has a small hole near the top of the thermometer well

to allow water, but not ice, to enter the tube.). D -- water vapor, E -- borosilicate glass cell, F -- water from the ice bath, G -- thermometer well

(precision bore glass), H -- ice mantle, I -- air-free water, J -- aluminum

bushing with internal taper at the upper end to guide the SPRT into its closefitting inner bore, K -- polyure thane sponge for cushioning the SPRT, L -- finely divided ice and water.

3&
the ice mantle rotates freely about the axis of the thermometer well. The outer

ice -water interface guards and thermally stabilizes the inner ice -water

interface temperature that is measured with the SPRT.

Figure 7 shows a triple point of water cell immersed in an ice bath with an SPRT

inserted into the thermometer well. An SPRT should be precooled in a glass tube

of water in an ice bath before it is inserted into the triple -point well so that

the thickness of the water layer next to the thermometer well will not become

excessive. Also, the time required for the SPRT to come into thermal

equilibrium will be shortened. Heavy felt cloth should be used to cover the ice

bath in order to prevent ambient radiation from entering the bath and reaching

the thermometer element, which otherwise would cause the thermometer to give a slightly high (erroneous) reading. A plastic foam cushion should be placed at

the bottom of the thermometer well to protect the well and the SPRT. Since

water is a poor thermal conductor, a close fitting aluminum sleeve should be

used to enhance the thermal conduction.

The thermometer current should be imposed immediately after insertion of the

SPRT into the cell so that readings can be made under conditions of steadystate self heating. Five to ten minutes or longer may be required before

steady- state conditions are reached. To avoid errors due to variations in the

self heating that arise from variations in the thermal contact of the

thermometer with its surroundings, it is best to read the SPRT at two currents

and extrapolate the readings to zero power in the SPRT.

3.2.4 FREEZING. MELTING. OR TRIPLE POINTS OF METALS: Hg. Ga. In. Sn. Zn. Al

Ag. Au. or Cu

The realization of metal fixed points requires the continuous presence of

liquid-solid or liquid-solid-vapor phases in thermal equilibrium. With SPRTs

the liquid-solid interface, i.e., the equilibrium whose temperature is measured,

must surround and must be as close to the temperature sensing element as possible. Since the first cryoscopic constants of metals are relatively low,

the fixed-point metal samples should be at least 99.9999% pure. Figure 8 shows

idealized liquid/solid equilibrium conditions inside fixed point cells used in

freezing and melting experiments. Figure 9 shows a representative arrangement

of an SPRT inserted inside a metal fixed point cell. Ideally, and similar to

the water triple-point cell, an outer liquid-solid interface, which completely

surrounds the inner interface, exists close to the container wall. This outer

interface, which has a temperature very close to that of the inner interface,

thermally protects and thermally stabilizes the inner interface. In freezing

experiments, a layer of solid is first formed at the crucible wall, then a thin

layer of solid is induced on the thermometer well by inserting cooling rods.

As freezing advances, the outer interface approaches the inner interface until

all of the material is solid. In melting experiments, a layer of liquid is

first formed next to the crucible, then a thin layer of liquid is formed next

to the thermometer well by inserting a warming rod or a long heater. As melting

advances, the outer liquid/solid interface approaches the inner interface.

Since different furnace or bath designs are required for fixed-point cells

operated at different temperatures, they will be discussed along with each of

the fixed points, or references will be made to appropriate sources of

39
/J

l-J~

*

':

:

*

mt -'/ri -K,-L

Cell Holder * »

Sample Cell

Sample

Liquid Sample

Solid Sample

Thermometer

Well

Resistance

Thermometer

ij; '\' SZi L

"38

-•<>'-; '*B

Melting Freezing

Figure 8. Idealized liquid/solid equilibrium conditions inside fixed point

cells used in freezing and melting experiments. In freezing experiments, a layer of solid is first formed at the crucible wall, then a thin layer of solid

is induced on the thermometer well by inserting cooling rods. In melting

experiments, a layer of liquid is first formed next to the crucible, then a thin

layer of liquid is formed next to the thermometer well by inserting a warming

rod or a long heater. As melting advances, the outer liquid/solid interface

approaches the inner interface.

40
Figure 9. An SPRT in a metal freezing-point cell.

A -- platinum resistance thermometer, B - - to helium gas supply and pressure

gauge, C -- thermometer gas seal with silicone rubber, D -- silicone rubber

stopper, E -- thermal insulation (washed Fiberfrax) , F -- thermometer guide tube

[precision bore tube, ground (matt finish) to uniform outside diameter], G -- heat shunt (graphite) in close contact with F and with H, H -- borosilicate

glass cell [precision bore tube ground (matt finish) to uniform outside

diameter], I -- graphite cap (lid) for the graphite crucible, J -- graphite

thermometer well, K -- metal sample, L -- graphite crucible, M -- thermal

insulation (Fiberfrax paper) between the graphite crucible and the borosilicate

glass cell.

41
descriptions.

In radiation thermometry, the liquid- solid phase of the metal fixed point must

completely surround the blackbody radiator cavity.

3.2.4.1 CONTAINER MATERIAL

Containers for the fixed-point metals must not contaminate the metal sample, and

the container material must be rugged enough to retain its integrity under

thermal cycling between the temperature of use and ambient temperature. The

container material preferably should be inert to air at temperatures of use; if

not, e.g., graphite above about 200 °C, the container plus the fixed-point

material must be protectively enclosed in an inert gas, such as nitrogen, argon, or helium, using either a borosilicate or fused silica glass envelope. It

should be assumed that the fixed-point material itself will react chemically

with air and, thus, the material must be protected.

3.2.4.2 METAL FIXED POINT DEVICES FOR CALIBRATING SPRTs

Figure 9 shows a representative fixed-point cell that can be used for

calibrating long-stem type SPRTs. In the following sections, individual types

will be described.

3.2.4.2.1 MERCURY TRIPLE POINT

3.2.4.2.1.1 MERCURY SAMPLE

On the ITS -90, the triple point of mercury (equilibrium phase state of mercury

solid, liquid, and vapor phases) is assigned the value 234.3156 K (-38.8344 °C)

.

Depending upon the choice of container material and operating procedure, it may

be more practical to realize the mercury freezing point at one standard

atmosphere, the value being 234.3210 K (-38.8290 °C) . Mercury samples with

impurity content of 1 part in 108 or less can be prepared by potassium hydroxide

and nitric acid washings, followed by triple distillation [48]. The alkali and

acid washings can be carried out by vigorously bubbling clean filtered air

through the mixture of mercury and the alkali or the acid. To remove any

remaining oxidizable impurities, the first two distillations should be carried

out under reduced pressures with a fine stream of clean filtered air bubbling

into the mercury in the distillation container. The third distillation should

be done under high vacuum to remove the noble metals. With the high-purity

mercury (99.999999%), both freezing and melting techniques give triple-point

temperatures that agree to within i 0.1 mK over most of the liquid- solid range.

[The first cryoscopic constant of mercury is 0.00503/K. Consequently, the

liquidus point of an ideal mercury solution of 99.999999% purity would be

approximately 0.002 mK lower than that of 100% pure mercury.]

3.2.4.2.1.2 CONTAINERS FOR MERCURY

Any container material can be used with mercury that is sufficiently rigid and

does not dissolve in, or chemically react with, mercury in the temperature range

of storage and application. The choice will depend upon whether the mercury

fixed-point cell is to be used at its triple point, at its freezing point, or

42
Hi

FT10 O.D.

x 1 Wall

38 O.D.

x 2 Wall

'-£.

LlU-

-TA\

Vf-r

Tm

CM

oT-CM

9.5 O.D.

x 0.9 Wall

13

h*. in to

en ^> ^ «» OJ

1 < 1 -X1

9.5 O.D.

x 0.13 Wall

r- > *. r

9.5 O.D.

x 0.9 Wall

38.1 O.D.

x 1.7 Wall

1.0 Thick

-1.6 Thick

Borosilicate GlassType 304

Stainless Steel Dimensions in mm

Figure 10. Two mercury triple-point cells, one constructed of borosilicate

glass and one of Type 304 stainless steel. The two small -diameter tubes at the

top facilitate the cleaning of the cells before filling and sealing. The glass

cell is sealed by melting the small -diameter tubes, but the stainless steel cell

is sealed by pinching flat the small -diameter tubes and electric -arc welding

them, thereby severing them at the middle of the flat.

43
at either one.

3.2.4.2.1.3 METAL CONTAINERS

Mercury is capable of dissolving most metals, at least at the low levels of

impurity content (<1 ppm) that are the permitted maxima for metal fixed points.

Iron, nickel, chromium, and tantalum have been reported to be soluble at only

1 ppm or less; hence, stainless steel is an adequate container for mercury in

most temperature standard applications [53,103]. (Note: the solubility of

nickel in mercury may be close to the limit of 1 ppm.) Mercury triple -point

cells can be prepared by pinching and welding the small diameter tubes used for

cleaning, evacuating, and filling (see fig. 10).

3.2.4.2.1.4 CONTAINERS OF GLASS

It is expected that some metal impurities in glasses [single metal oxide (e.g.,

fused silica glass) or mixed metal oxides (e.g., borosilicate glass)] or "ceramics" can be leached out by mercury when the mercury is stored in them for

many years. Traditionally, "soft glass" has been considered suitable for

storing mercury [48]; however, soft glass, without special treatment, may be

susceptible to breakage when thermally shocked. Borosilicate glass and fused

silica glass are more practical choices for mercury containers. Figure 11 shows

a borosilicate glass mercury triple-point cell inside a stainless steel holder.

3.2.4.2.1.5 CONTAINERS OF PLASTIC

Organic plastics, such as polyethylene, polytetrafluoroethylene (Teflon), or polytrifluorochloroethylene (Kel-F) , all of which are free of metals, can be

used to contain mercury and be used at the mercury freezing point. A stainless

steel holder similar to that used for the glass mercury cell and for the

stainless steel mercury cell (see fig. 11) or similar to the holder used for the

indium cell (see fig. 12) could be used as the external holder for a plastic

mercury cell. Although plastic cells have not been used yet in preparing

mercury fixed-point cells, it would be practical and desirable to use plastic

cells for realizing the mercury fixed point. Since the vapor pressure of

mercury at room temperature is sufficiently high that mercury vapors can be

transported under vacuum conditions , the vapors should be confined by an atmosphere of helium or other inert gases.

3.2.4.2.1.6 ASSEMBLY OF MERCURY CELLS

A purified mercury sample can be vacuum distilled into glass containers, with

the glass filling tube then sealed under vacuum with a flame [43] . The mercury

sample may be vacuum distilled into stainless steel containers and the filling

tube pinched, and then cut and sealed using electric-arc welding techniques

[43].

3.2.4.2.1.7 REALIZATION AND APPLICATION

When the total impurity content of a mercury sample is about one part in 108

, both freezing and melting techniques yield triple -point temperatures agreeing

to within + 0.1 mK over most of the liquid- solid range. A dual -stage

44
VALVE AND

VACUUM

B

A = "O" ring tube seal B = Thermometer well C = Ethyl alcohol in well D = Indium gasket seal E = Insulation, rolled paper tissue F = Stainless steel jacket

G = Tubular connection for cleaning and filling H = Insulation, tissue paper

rolled around (I) for centering

I = Copper foil cylinder

J = Borosilicate glass cell K = Mercury

L = Thermometer cushion

(fused quartz wool)

M = Stand for mercury cell N = Insulation (Aluminum

silicate wool)

Figure 11. A borosilicate glass, mercury triple-point cell in a stainless steel

container. A stainless-steel, mercury triple-point cell may also be mounted

inside the stainless steel container. With the high-purity mercury sample, both

freezing and melting techniques yield triple -point values agreeing to within

± 0.1 mK over most of the liquid- solid range.

45
u

CDC

•i-i n) vCoo

0) Vw

to

[fl CDwW

4-1 •rH

X)C

4->

cfl aco

p-M

bOO

4JO

x:

Pm

a)n

bO

•H

fa

46
refrigerator can yield a temperature near -40 °C and, hence, could be used for

freezing mercury, but a much simpler stainless steel vacuum enclosure placed in

a Dry Ice/ethyl alcohol mixture (-78 C C) can reduce the freezing rate of mercury

to give a freeze duration of about 10 hours or more with 2.2 kg of mercury, and

that is perfectly adequate. Figure 11 shows such a stainless steel enclosure

that has been used with a borosilicate-glass mercury cell at the NIST.

To start a freeze, fill the stainless steel enclosure that contains the mercury

triple -point cell with dry air and immerse it in a Dry Ice/ethyl alcohol bath.

Fill the thermometer well with ethyl alcohol and insert therein an SPRT for

monitoring the temperature of the cell. Usually, mercury supercools about 6 °C

in a borosilicate glass cell but only about 3 °C in a stainless steel cell.

When recalescence is observed, evacuate the stainless steel holder. Remove the

monitoring SPRT from the well and replace it with a thin-wall stainless steel

tube that contains ethyl alcohol and that has been cooled in a tube of ethyl

alcohol immersed in a Dry Ice/ethyl alcohol bath. Insert successively into the

stainless steel tube two or three liquid-nitrogen cooled glass rods, for about

5 minutes each, in order to freeze a thin layer of mercury around the thermometer

well. The purpose of the stainless steel tube is to collect the frost that forms on the rods when they are removed from the liquid nitrogen. Remove the stainless

steel tube and replace it with the monitoring SPRT, which has been cooling in

the tube of cold ethyl alcohol. Switch on the thermometer measuring current.

(Note: it may be necessary to refill the thermometer well with a small amount

of cold ethyl alcohol from the Dry- ice cooled tube before the monitoring SPRT

is inserted into the well. The well should be completely filled with ethyl

alcohol when the SPRT is in the well.) With the induced inner freeze around the

thermometer well, temperature equilibrium is reached in about 5 minutes. After

the resistance of the monitoring SPRT is read, other cooled SPRTs are successively inserted into the mercury cell and calibrated. The final reading

in a cell is made with the monitoring SPRT in order to check the extent of the

freeze. This final reading of the monitoring SPRT must agree with the initial

reading to within ± 0.1 mK. See reference [43] for more details on the

calibration procedure at the mercury triple point.

3.2.4.2.2 MELTING POINT OF GALLIUM

The melting point of gallium is assigned a temperature of 302.9146 K (29.7646 °C) on the ITS -90. Gallium of 99.99999% purity can be obtained commercially. At

such high purity, both freezing and melting techniques should yield liquid- solid

equilibrium temperatures that agree to within ±0.1 mK. Since the metal expands

about 3% on freezing, plastic containers, such as polyethylene, polypropylene,

or polytetrafluoroethylene, are the most suitable. These are sufficiently

flexible at around 30 °C to accommodate the volume change in the gallium. In

assembling the gallium fixed-point cell, the supercooled metal can be poured

directly into the container. A second more rugged container of Nylon, glass,

or stainless steel should enclose the flexible container so that the pressure

of the inert gas over the metal can be controlled at one atmosphere or be

evacuated to observe the triple po-int. A gallium fixed-point cell, consisting

of an all plastic container, that is used at the NIST is shown in figure 13.

Since gallium supercools as much as 25 °C to 70 °C, depending upon the plastic

material that is in contact with it, the most convenient method of observing its

liquid-solid equilibrium temperature is the melting technique. [The first

47
-4.5 cm-|

Figure 13. An all-plastic gallium melting/triple -point cell. The triple point

is realized by using the melting technique. The cell is periodically evacuated

through the valve

.

A -- valve (Zytel) , B -- bath lid (Plexiglas) , C - pumping tube (polyethylene), E -- cap (Nylon), F -- G -- case (Nylon), H -- thermometer well (Nylon), I of the case (Nylon)

.

support rod (Nylon), D -- sample container (Teflon)

,

-- gallium metal, J -- base

48
cryoscopic constant of gallium is 0.00732/K. Consequently, the liquidus point

of an ideal gallium solution of 99.99999% purity would be approximately 0.01 mK

lower than that of 100% pure gallium.

]

3.2.4.2.2.1 REALIZATION AND APPLICATION

In order to solidify the gallium metal in a fixed-point cell, initially in the

supercooled state (e.g., at room temperature), first insert successively two or three liquid-nitrogen-cooled copper rods into the thermometer well of the cell

to induce nucleation, and then place the cell in an ice bath for about one hour

or longer. The cell with the solidified gallium may then be placed in an oil

bath at a temperature of about 40 °C to partially melt the sample to form an outer liquid- solid interface. To form a liquid- solid interface next to the

thermometer well (an inner melt) , the bath oil may be circulated through the

thermometer well by pumping the oil through a tube placed in the well. After

about 20 minutes in the hot oil bath, about 25% of the gallium will be melted.

The inner liquid- solid interface can also be prepared by using an electric heater

in the well. The amount of electric energy required, e.g., to form about a 1 mm shell of liquid around the thermometer well, can be calculated from the outer

dimensions of the thermometer well and the heat of fusion of gallium (see table

7) . The gallium cell is then securely mounted, completely immersed so that the

thermometer well will be filled with the bath oil, in a stirred oil bath,

controlled at a temperature about 10 mK above the liquid- solid equilibrium

temperature (melting point or the triple point). Also, the cell can immersed

in a fairly close-fitting, oil-filled aluminum or copper block, controlled at

a temperature about 10 mK above the equilibrium temperature. The monitoring

SPRT is heated and then inserted into the thermometer well of the gallium cell.

Readings are taken after about 20 minutes of equilibration in the cell. The

monitoring SPRT is replaced in the cell with a preheated test SPRT and

measurements on it are made after about 20 minutes. A number of SPRTs can be

successively calibrated in the same "melt". When all of the test SPRTs have

been calibrated, a final measurement in the cell is made on the monitoring SPRT.

This reading of the monitoring SPRT should agree with the initial reading to

within ± 0.1 mK. Also, measurements with different melts should agree to within

± 0.1 mK. See references [16,26,65,68,94].

3.2.4.2.3 FREEZING POINT OF INDIUM

The freezing point of indium is assigned the value 429.7485 K (156.5985 °C) on

the ITS-90. Metal samples of 99.9999% purity and higher are commercially

available. The freezing point of indium is at a sufficiently low temperature

to permit the use of containers of high temperature plastics

[polytetrafluoroethylene (Teflon), polyimide/amide , and others], borosilicate

glass, and stainless steel [4,69,74,92]. See figure 14 for an example of a Teflon container used for indium at the NIST. As used with metals that freeze

at higher temperatures, graphite can also be used with indium. The metal is

available in the form of small pellets, wire, and rods. Suitable amounts for

a sample can be easily weighed into the container. [The first cryoscopic

constant of indium is 0.00212/K. Consequently, the liquidus point of an ideal

indium solution of 99.9999% purity would be approximately 0.5 mK lower than that

of 100% pure indium.

]

49
9 mm

1 mm

2 mm

CAP (TEFLON)

0.005" SPIRAL GROOVE ALONG

INSIDE OF THERMOMETER WELL

(1 OR 2 THREADS/in.)

10 mm

5 mm CHA. HOLE

THROUGH CAP

34.34 mm ID 44.34 mm 00

18.70 mm 00

5 mm

CONTAINER (TEFLON)

THERMOMETER WELL

(TEFLON)

5 mm

36.34 mm ID 40.34 mm OD

12.70 mm (0.500 in.) ID 14.70 mm OD

14 mm OD

mm

30 mm OD FUSED

SILICA TUBE

Figure 14. An all-plastic indium freezing-point cell to be used in a stainless

steel container, such as that shown in figure 12. The argon gas pressure inside

the stainless steel container is adjusted to one atmosphere at the freezing

point. A similar all-plastic cell and stainless steel container may be used to

realize the mercury freezing point or Belting point at one atmosphere.

50
3.2.4.2.3.1 REALIZATION AND APPLICATION

A tube furnace containing the indium-point cell is controlled about 5 °C above

the freezing point of indium until the metal is completely melted. (It is

convenient to control the furnace temperature automatically and melt the metal

sample overnight so that the freezing of the metal may be started in the

morning.) Insert the check SPRT into the cell well and when the SPRT indicates

that the sample is about 5 °C above the freezing temperature, change the furnace

temperature control settings to control at 5

C C below the freezing point. When

the check SPRT indicates recalescence, change the furnace temperature control

settings to control at 1 "C to 0,5 "C below the freezing point. Withdraw the

check SPRT from the cell well and insert successively in the well two fused

silica glass rods, each initially at room temperature, for about 5 minutes each

and then insert the cool check SPRT in order to freeze a thin mantle around the

thermometer well. (To avoid the consequence of inserting borosilicate glass

rods into the aluminum or silver point cell to form the mantle around the

thermometer well, all glass rods used for this purpose in the laboratory should

be fused silica glass.) Within 20 to 30 minutes, the readings on the check SPRT

should indicate that the cell is at temperature equilibrium. After the readings

on the check SPRT are completed, test SPRTs, that have been heated in an auxiliary furnace, are successively inserted into the cell well and calibrated.

After all of the test SPRTs have been calibrated, the preheated check SPRT is

inserted again into the cell well and read. This second reading should agree

with the first to within ± 0.1 mK. See references [4,69,74,92].

3.2.4.2.4 FREEZING POINT OF TIN

The freezing point of tin is assigned the value 505.078 K (231.928 °C) . Metal

samples of 99.9999% purity are commercially available. Graphite containers are commonly and successfully used for tin. Although the use of materials such as boron nitride (BN) has not been reported, it could be a suitable container for

tin. High purity tin has been found to supercool 25 °C or more [73,76]; hence,

the freeze is nucleated by rapid cooling outside the furnace. The metal is

available in the form of small pellets and in rods suitable for filling the

graphite container. A method for filling graphite containers and installing the

graphite thermometer wells is described in reference [46] . [The first cryoscopic

constant of tin is 0.00329/K. Consequently, the liquidus point of an ideal tin

solution of 99.9999% purity would be approximately 0.3 mK lower than that of 100%

pure tin.

]

3.2.4.2.4.1 REALIZATION AND APPLICATION

A tube furnace [46] containing the tin freezing-point cell is controlled about

5 °C above the freezing-point temperature until the metal is completely melted.

(It is convenient to control the furnace temperature automatically and melt the

metal overnight so that freezing of the metal can be started early in the

morning.) Insert the check SPRT into the cell well and, when the SPRT indicates

that the sample temperature is about 5 °C above the freezing point, change the

furnace temperature control settings to control at 1

C C to 0.5 °C below the

freezing point. When the check SPRT indicates that the cell temperature is

close to the freezing-point value, withdraw the cell and the SPRT from the

furnace. The cell will then cool rapidly and when the SPRT detects recalescence,

51
replace the cell in the furnace. Withdraw the check SPRT from the cell well.

Insert successively in the well two fused silica glass rods, each initially at room temperature, for about 5 minutes each, and then the cool check SPRT in order

to freeze a thin mantle around the thermometer well. Within about 20 to 30

minutes , the readings on the check SPRT should indicate that the cell is at

temperature equilibrium. After the readings on the check SPRT are completed,

test SPRTs, that have been heated in an auxiliary furnace, are successively

inserted into the cell well and calibrated. After all of the test SPRTs have

been calibrated, the check SPRT is heated and inserted again into the cell well

and read. This reading should agree with the initial reading to within

± 0.1 mK. See references [43,46,73,76].

3.2.4.2.5 FREEZING POINT OF ZINC

The freezing point of zinc is assigned the value 692.677 K (419.527 °C) . Metal

samples of 99.9999% purity are commercially available. High purity liquid zinc

has been found to supercool about 0.02 °C to 0.06 °C; hence, unlike the freezing

procedure used with the tin-point cell, its freeze can be initiated in the

furnace without withdrawing the cell from the furnace. Graphite containers are commonly and successfully used for zinc. Although the use of materials such as boron nitride (BN) has not been reported, it could be a suitable container for

zinc. The metal is available in the form of small pellets and in rods suitable

for filling graphite containers. [The first cryoscopic constant of zinc is

0.00185/K. Consequently, the liquidus point of an ideal zinc solution of

99.9999% purity would be approximately 0.5 mK lower than that of 100% pure zinc. ]

3.2.4.2.5.1 REALIZATION AND APPLICATION

A tube furnace containing the zinc -point cell is controlled about 5 °C above the

freezing point until the metal is completely melted. If the furnace temperature

is maintained at a higher temperature, the zinc will melt faster, but the zinc

should never be heated by more than about 5 °C above its melting point. (It is

convenient to control the furnace temperature automatically and melt the zinc

sample overnight so that the freezing of the metal can be started early in the

morning and the calibration of six or more test SPRTs can be completed during

the same day.) Insert the zinc-point check SPRT into the cell well. When the

SPRT indicates that the melt is about 5 °C above the freezing point, change the

furnace temperature control settings to control at 5 °C below the freezing point

in order to initiate rapid cooling for nucleation. When the check SPRT indicates

recalescence, change the furnace temperature control settings to control at 1 °C

to 0.5 °C below the freezing point. Withdraw the check SPRT from the cell well

and insert successively into the well two fused silica glass rods, each initially

at room temperature, for about 5 minutes each, and then insert again the cool

check SPRT. This freezes a thin mantle around the thermometer well. Within

about 20 to 30 minutes, the readings on the check SPRT should indicate that the

cell is at temperature equilibrium. After the readings on the check SPRT are completed, test SPRTs, that have been heated in an auxiliary furnace, are successively inserted into the cell well and calibrated. After all of the test

SPRTs are calibrated, the preheated check SPRT is inserted again into the cell

well and measurements made on it. This second reading should agree with the

first to within ± 0.1 mK. See references [43,73,75].

52
51.3 cm

Figure 15. A graphite freezing point cell enclosed inside a fused silica tube

with tube connection to high vacuum, purified argon gas source, and pressure

gauge

.

A -- connection to high vacuum, purified argon source, and pressure gauge, B -

- fused-silica-to-Kovar graded seal, C -- fused-silica connecting tube, outer

surface matte finished to minimize radiation piping, D -- thermometer guide

tube, E -- heat shunts (Inconel disks), F -- thermal insulation (Fiberfrax) , G -

- fused-silica outer envelope, H -- graphite lid, I -- graphite thermometer

well, J -- fused-silica thermometer well, K -- fused-silica fiber-woven tape for

cushioning the graphite freezing-point cell inside the fused-silica enclosure,

L -- metal sample, M -- graphite crucible, N -- fused-silica fiber pad for

cushioning the thermometer, -- Fiberfrax paper liner.

53
3.2.4.2.6 FREEZING POINT OF ALUMINUM

The freezing point of aluminum is assigned, the value 933.473 K (660.323 °C) . Metal samples of 99.9999% purity are commercially available. High purity

graphite containers have been used successfully with aluminum. High purity

liquid aluminum has been found to supercool about 1 °C to 2 °C; hence, its freeze can be initiated in the furnace without withdrawing the cell from the furnace.

Aluminum is highly reactive, particularly at elevated temperatures; liquid

aluminum is capable of dissolving many metals. Liquid aluminum reacts with

moisture, forming the oxide and dissolving the hydrogen. The compounds AlA C3 and

aluminum oxycarbide have been found in aluminum samples cast in graphite at

1000 °C. Because of the high chemical reactivity of aluminum, the graphite cell

containing the metal must be completely protected by enclosing the cell in a fused silica envelope (see fig. 15). The argon or helium gas that is used to

pressurize the freezing metal at one atmosphere must be thoroughly devoid of

moisture, hydrogen, oxygen, hydrocarbons, and other substances that would react

with liquid aluminum. The cell must not be heated more than 5 °C above the

aluminum freezing point. [The first cryoscopic constant of aluminum is

0.00149/K. Consequently, the liquidus point of an ideal aluminum solution of

99.9999% purity would be approximately 0.7 mK lower than that of 100% pure

aluminum.

]

3.2.4.2.6.1 ASSEMBLY OF AN ALUMINUM- POINT CELL

High purity aluminum can be obtained in the form of shots or rods . Determine

the internal volume of the graphite container, taking into account the

thermometer well. Determine the mass of liquid aluminum required to fill the

cell to within 0.5 cm of the graphite lid. Weight out aluminum shots or cut and

clean aluminum rods that correspond to this mass . The rods should be cut with

a carbide tipped tool and cleaned by etching in a hot (about 200 °C) solution

consisting (by volume) of reagent grade phosphoric acid (15 parts), sulfuric

acid (5 parts) , and nitric acid (1 part) , and then carefully rinsing many times

in distilled water. Load the graphite crucible with the aluminum sample and

then slide it into an extra-long fused-silica test tube such as that shown in

figure 16. Insert the test tube into the tube furnace and evacuate it. While

continuing to evacuate the tube, set the furnace temperature to control at about

5 °C above the melting point of aluminum. When the sample has completely melted,

cool it to room temperature, while continuing to pump the tube. If aluminum

shot, or rods of odd sizes, are used, the graphite cell will require several

loadings and meltings before the desired amount of total sample has been loaded

into the cell. When the graphite crucible is appropriately loaded with the

sample, replace the silicone rubber stopper at the mouth of the extra- long test

tube of figure 16 with the device for inserting the graphite thermometer well

and lid (see reference [45] or fig. 17). Insert the test tube into the tube

furnace, evacuate it, and then fill it with high purity argon to a pressure

slightly above ambient. Melt the aluminum sample and push the graphite well and

lid into the cell. Cool the sample to room temperature, while maintaining the

argon pressure in the test tube slightly above the ambient pressure. Finally,

assemble the graphite cell containing the aluminum sample into the desired

freezing-point cell configuration (see references [43,45,70] or fig. 15).

54
TO HIGH ~

VACUUM

TO PURIFIED

ARGON SOURCE

Figure 16. A method for filling a graphite freezing-point cell by melting the

metal sample in the graphite crucible. The required amount of sample is placed

in the graphite crucible and the crucible is inserted into the fused silica

tube. To protect the sample, the fused silica tube is evacuated or filled with

an inert gas (e.g., purified argon) before melting the metal in a tube furnace.

Depending upon the geometry of the sample, the melting of two or more batches

of sample may be required.

A -- Silicone rubber stopper, B

D -- metal sample.

fused silica tube, C -- graphite crucible,

55
560 mm

153 mm

219 mm

560 mm

244 mm

Figure 17. An apparatus for installing a graphite thermometer well and lid in

a graphite crucible containing a molten metal sample.

A -- stainless steel pusher rod, B -- silicone rubber gas seal (permits linear

motion of the pusher rod A) , C - - inlet for purified argon gas that is used in

purging and maintaining positive pressure of the gas during the assembly process,

D -- silicone rubber stopper, E -- stainless steel flange attached to the pusher

rod for pressing against the graphite lid and thermometer well during assembly,

F -- graphite lid for the crucible, G -- slit on the pusher rod (the two halves

spring outward to hold the graphite thermometer well and lid while melting the

metal sample), H -- graphite thermometer well, I -- fused-silica tube, J -- a part of the fused-silica tube where its I.D. matches closely with the O.D. of

the crucible and its lid so that the lid can be easily guided onto the opening

of the crucible, K -- graphite crucible, L -- molten metal sample.

56
3.2.4.2.6.2 REALIZATION AND APPLICATION

SPRTs that are to be calibrated at the aluminum point and higher must have fused

silica, sapphire, or ceramic insulation for the resistance element and its

extension leads. Such high temperature SPRTs should be handled by procedures

that avoid thermally shocking them.

A tube furnace containing the aluminum-point cell is controlled about 5 °C above

the freezing point of aluminum until the metal is completely melted. It is

convenient to control the furnace temperature automatically and melt the metal

overnight so that the freezing of the metal can be started early in the morning

and the calibration of two or three test SPRTs completed during the same day.

Insert the aluminum-point check SPRT stepwise into the cell well. Since the

SPRT will cool considerably between the time it is withdrawn from any auxiliary

preheat furnace and inserted into the aluminum point cell, the SPRT is heated

in the section of the thermometer guide tube that is maintained close to the

furnace temperature. The SPRT is inserted initially to a location where its tip

is about 3 cm above the graphite cell lid. After about 5 minutes, the SPRT is

inserted an additional 5 cm and after another 5 minutes, another 5 cm, and so on until the tip of the SPRT is at the bottom of the thermometer well. When the

SPRT indicates that the sample is about 5 °C above the freezing point, change

the furnace temperature control settings to control at 5 °C below the freezing

point in order to initiate rapid cooling for nucleation. When the check SPRT

indicates recalescence, change the furnace temperature control settings to

control at 1 "C to 0.5 "C below the freezing point. Withdraw the check SPRT

stepwise from the cell, first to a location about 3 cm above the graphite cell

lid. After about 5 minutes, withdraw the SPRT another 5 cm. and after another

5 minutes, another 5 cm, and so on until the SPRT is completely out of the

thermometer guide tube. Next, insert successively into the cell well two fused

silica glass rods, each initially at room temperature, for about 5 minutes each

to freeze a thin mantle of solid aluminum around the thermometer well. Again

insert the check SPRT stepwise, as described above, into the cell. Within 20

to 30 minutes, the readings on the check SPRT should indicate that the cell is

at temperature equilibrium. After the readings on the check SPRT are completed,

the SPRT is removed from the cell stepwise, as described above. The test SPRTs

are then successively inserted stepwise, calibrated, and removed stepwise as described for the check SPRT. After all of the test SPRTs have been calibrated,

the check SPRT is inserted again into the cell well and measurements made. This

second reading at temperature equilibrium should agree with the first to within

± 0.1 mK.

When SPRTs are cooled rapidly from the aluminum point to ambient temperature,

lattice vacancies are quenched in and these must be removed before the SPRTs are calibrated at the triple point of water. To relieve any quenched in lattice

vacancies, the SPRTs that have been calibrated at the aluminum point should be

heated in an auxiliary furnace at about 660 °C for about 30 minutes and then

gradually cooled to about 500 °C over 3 hours or more before withdrawing from

the furnace to cool at ambient temperature. See references [43,45,70].

3.2.4.2.7 FREEZING POINT OF SILVER

The freezing point of silver is assigned the value 1234.93 K (961.78 °C) . Metal

57
samples of 99.9999% purity are commercially available in the form of pellets.

High purity graphite containers have been used successfully with silver. Liquid

silver has been found to supercool not more than 0.5 °C; hence, its freeze can

be initiated in the furnace without withdrawing the cell from the furnace. See

references [3,15,72,98].

Oxygen is known to "dissolve" in liquid silver and lower the freezing point.

Although the dissociation pressure of Ag2 is expected to be quite high at the

freezing point of silver, the lowering of the freezing point may be a combination

of the solution of Ag2 and of oxygen in liquid silver. (The dissociation

pressure of Ag2 is given as 414 atmospheres at 507 °C.) In a graphite

environment at the freezing point of silver, a small amount of the oxygen will

eventually react with the graphite; however, a newly prepared cell should be

pumped at high vacuum at about 1000 °C for a week before back filling to one atmosphere with purified argon, nitrogen, or helium. [The first cryoscopic

constant of silver is very small (0.000891/K) . Consequently, the liquidus point

of an ideal silver solution of 99.9999% purity would be approximately 1.1 mK

lower than that of 100% pure silver.]

3.2.4.2.7.1 REALIZATION AND APPLICATION

The freezing-point cell of silver may be assembled by a procedure similar to

that used with aluminum. See section 3.2.4.2.6.1.

SPRTs that are to be calibrated at the silver freezing point must have fused

silica, sapphire, or ceramic insulation for the resistance coil and its extension

leads. Such high temperature SPRTs should be handled by procedures that avoid

thermally shocking them.

A tube furnace containing the silver freezing-point cell is controlled at about

5 °C above the freezing point until the metal is completely melted. (It is

convenient to control the furnace temperature automatically and melt the metal

overnight so that the freezing of the metal can be started early in the morning

and the calibration of two or three test SPRTs completed during the same day.)

Insert the silver-point check SPRT stepwise into the cell well. Since the SPRT

will cool considerably between the time it is withdrawn from any auxiliary

preheat furnace and inserted into the silver-point cell, the SPRT is heated in

the section of the thermometer guide tube that is maintained close to the furnace

temperature. The SPRT is inserted initially to a location where its tip is about

3 cm above the graphite cell lid. After about 5 minutes, the SPRT is inserted

an additional 5 cm, and after another 5 minutes, another 5 cm, and so on until

the tip of the SPRT is at the bottom of the thermometer well. When the SPRT

indicates that the temperature of the sample is about 5 °C above the freezing

point, change the furnace temperature control settings to control at 5 °C below

the freezing point in order to initiate rapid cooling for nucleation. When the

check SPRT indicates recalescence , change the furnace temperature control

settings to control at 1 °C to 0.5 °C below the freezing point. Withdraw the

check SPRT stepwise from the cell well, first to a location about 3 cm above the

graphite cell lid. After about 5 minutes, withdraw the SPRT an additional 5 cm,

and after another 5 minutes, another 5 cm, and so on until the SPRT is completely

out of the thermometer guide tube. Next, insert successively into the cell well

58
two fused- silica glass rods, each initially at room temperature, for about 5

minutes each in order to freeze a thin mantle of solid silver around the

thermometer well. Then, insert again the check SPRT stepwise, as described

above , into the cell . Within about 20 to 30 minutes , the readings on the check

SPRT should indicate that the cell is at temperature equilibrium. After the

readings on the check SPRT are completed, it is removed from the cell stepwise,

as described above. The test SPRTs are then successively inserted stepwise,

calibrated, and removed stepwise as described for the check SPRT. After all of

the test SPRTs have been calibrated, the check SPRT is inserted again into the

cell well and read. This second reading should agree with the first to within

± 0.2 mK.

When SPRTs are cooled rapidly from the silver point to ambient temperature,

lattice vacancies are quenched in and these must be removed before the SPRTs are measured at the triple point of water. To relieve any quenched in lattice

vacancies, the SPRTs that have been calibrated at the silver point are heated

in an auxiliary furnace at about 960 °C for 30 minutes or so and then gradually

cooled to about 500 °C over 3.5 hours or more before withdrawing from the furnace

to cool at ambient temperature. In order to protect the platinum of the SPRT

from contamination by diffusion of metals at high temperatures (above 660 °C)

the SPRTs should be enclosed in a platinum tube or other protective device.

3.2.5 CONTROL CHARTS OF CHECK THERMOMETERS

Control charts should be kept for all of the check thermometers associated with

the various fixed points. Each of these charts will usually consist of a chronological graph of the check thermometer resistance value, R(X) , obtained

from measurements in the fixed-point cell X, and of the ratio,

W(X) = R(X)/R(TPW), of the resistance value R(X) to the resistance value, R(TPW) , obtained from measurements in a triple point of water cell. Presumably, such

charts have been kept by those involved in precision thermometry. Entries on such control charts are made each time the particular fixed-point cell is used.

Since those involved in precision thermometry would have used triple point of

water cells for their work based on the IPTS-68 and they would have used W(X)

the same control chart can be continued with the ITS -90. The reason, of course,

is that the behavior of the fixed-point cells is independent of the scale. Not

all of the fixed points of the ITS -90, however, are the same as those of the

IPTS-68. The ITS -90 uses some of the IPTS-68 fixed points but it also uses other

fixed points. See reference [64].

Some metrologists may not have used W(X) as defined above, but may have used the

W(t) of the IPTS-68. In that case, there will be a discontinuity in their

control charts involving W when they implement the ITS-90 and begin using W(X)

The magnitude of the discontinuity will simply be the ratio

R(273.16 K)/R(273.15 K)

Control charts involving only R(X) as a function of time will not have any

discontinuity due to the change in the scale. Depending on the temperature of

interest, of course, there may be need to start additional control charts.

59
3.3 RADIATION THERMOMETRY

Above the freezing point of silver (1234.93 K) , the ITS-90 is defined in terms

of Planck's radiation law. The values of temperatures r90 on the ITS-90 are obtained from the observed ratios of the spectral concentrations of the radiance

LA of a blackbody at the wavelength (in vacuum) A at T90 and at the reference

temperature T90 (X) according to eq. (31).

Inside a closed cavity, the radiation densities at different wavelengths A depend

only upon the temperature of the cavity walls . When a practical radiator is

designed with a small hole in the wall to observe the radiation density at A,

there arises the question of how much the observed radiation departs from the

blackbody radiation for a radiator design of a given geometry and material of

construction. There are numerous papers on the theoretical analysis of the

emissivities associated with cavity geometry and construction materials and

descriptions of radiator designs that have been used in radiation thermometry

[8,9,34,35,54,77 ,87,89,95] . The emissivities of cavities constructed of specular

reflectors and diffuse reflectors have been analyzed [87] . It is expected that

at high temperatures many materials become oxidized and, consequently, become

diffuse reflectors. Although it is difficult to determine the actual temperature

gradients in a cavity, the effect of temperature gradients has also been treated

[10,11]. The effective emissivity of a graphite blackbody cavity has been

computed to be 0.99997 ± 0.00003 [77].

For radiation thermometer calibrations at the silver, gold, or copper fixed

point, the blackbody cavity should be constructed of graphite and surrounded by

the freezing or melting metal contained in graphite to retain the high purity

of the metal that is used. [The first cryoscopic constants of all three of these

metals are extremely low (silver: 0.000891/K; gold: 0.000831/K; Cu: 0.000857/K).

Consequently, the ideal-solution liquidus points of these metals of 99.9999%

purity would be approximately 1.1 mK to 1.2 mK lower than that of 100% pure

metals .

]

References [27,55,56,57,62,77] give some details of construction of suitable

graphite fixed-point blackbody cavities. The metal should be protected from air

using an inert gas, such as argon, nitrogen, or helium, at a pressure slightly

above ambient. The graphite container or auxiliary graphite scavengers can

remove small amounts of oxygen impurities. For the blackbody cavity at the

platinum point, pure alumina has been used in an oxidizing atmosphere to avoid

the reaction between platinum metal and alumina in which oxygen gas is formed

and metallic aluminum is dissolved in the platinum [33,86].

Usually, optical pyrometers or photoelectric pyrometers are used to determine

the ratio of the radiances of a source of unknown temperature with that of the

reference source. The optical system of the instrument is designed to focus a nearly monochromatic image of the radiation source onto a photodetector , which

until about the mid- 1950 's was only the human eye; now the eye has been replaced

in high precision measurements by photoelectric detectors because of their

greater accuracy and their suitability for automation of the measurements. Two

methods are commonly used to determine the ratios of spectral radiances. Either

the photoelectric pyrometer is designed for comparing the two radiation sources by null-detection operation, similar in principle to the disappearing filament

60
optical pyrometers using suitable neutral filters or sectored discs for

attenuating the radiation of one source, or for measuring directly the radiation

density in terms of the detector output, e.g., photocell current. The latter

requires high stability and linearity of signal processing [34,77]. The optics

of the system may comprise refracting components (lenses) or reflecting

components (mirrors) [28,34,54,57,87].

Equation (31) requires the ratio of monochromatic radiances. Usually,

interference filters are used for this purpose. The bandwidth should be narrow

with high transmittance while completely blocking out wavelengths outside the

desired band. The temperature error is smaller the narrower the bandwidth. The

temperature of the filters should be controlled, since they are sensitive to

temperature changes. The photoelectric detector should be protected from

undesired radiations from outside the solid angle defined by the aperture of the

blackbody cavity. Where the output of the photocell is used to determine the

ratio of the radiances, the linearity of the detector should be carefully checked

or calibrated.

For details of optical pyrometer operation and attendant sources of errors, see references [ 28 , 34 , 54 , 57 , 77 , 87 ]

4. CALIBRATION OF THERMOMETERS ON THE ITS -90 AT VARIOUS LEVELS OF UNCERTAINTY

AND SOME APPROXIMATIONS OF THE SCALE

In a standards laboratory, the design of apparatus and equipment for calibration

of thermometers on the ITS -90 should be based on the desired accuracy, the number

of thermometers and thermometric instruments that must be calibrated per year,

the cost of realizing the ITS-90 (the fixed points and the measurement

equipment), the cost of applying and maintaining the ITS-90, and the cost of

research to maintain and make necessary improvements on the realization of the

ITS-90. In a national standards laboratory, the efforts are directed toward the

accurate realization of the ITS-90.

On 1 January 1990, no laboratory was able to calibrate thermometers over the

complete range of the ITS-90 in accordance with the strict definition of the

scale. Also, it is thought that on that date there was no immediate, widespread requirement for "experimental calibration conversion" from the IPTS-68(75)

to ITS-90 over the complete range. Since the differences between IPTS-68(75)

and ITS-90 were known, "arithmetical conversions" should have met most of the

immediate requirements. Also, where stable thermometers have been used to

maintain the EPT-76 or parts of the IPTS-68(75), the scales on those reference

thermometers could be converted to ITS-90, using the published approximate

differences between the scales, and then those thermometers can be used to

calibrate other thermometers on the ITS-90. To realize the ITS-90 as defined

and for international traceability , however, it is essential for the national

calibration laboratory to have all of the fixed-point apparatus and measurement

equipment. Furthermore, without continued research and comparison with other

standards laboratories, the question regarding the accuracy of the realization

of the scale will remain. The ITS-90 temperature calibrations are based on the

thermal equilibrium states (vapor- liquid or liquid- solid equilibrium at known

pressures, or vapor-liquid-solid triple points) of pure substances. Substances,

however, have some impurity content; the amount must be small enough to have

61
negligible effect on the measurement of temperature. Obviously, the fixedpoint device and the experimental procedure must be designed so that during

calibration, the thermometer will be in thermal equilibrium with the equilibrium

state of the defining fixed point. A method for checking whether or not the

thermometer is in thermal equilibrium with a metal fixed-point standard device

is to reduce the immersion in the device a known amount or vary the experimental

conditions. The observed temperature change of the thermometer must correspond

with the hydrostatic head effect of the liquid metal in the device, or there must

be no observed temperature change with experimental conditions (such as changing

furnace temperatures of metal fixed-point cells)

.

In order to determine the precision of the calibration process, it is essential

to use check thermometers with every calibration. The results of the check

thermometers will show whether the calibration process is "under statistical

control" or not. The accumulated results show the precision of the "gross"

calibration process.

Since some parts of this section deal with approximations of the ITS -90, and

will make reference to the scale differences given in table 1, the methods by

which the table was constructed will be described. The differences (T90 - T76 ) between 5 K and 27 K were obtained using the same relation [99] as that used for

( rNPL-75 " T76). namely,

(T9Q - r76 )/mK = - 0.0056(r90/K) 2

. (40)

The differences (r90 - T68 ) between 14 K and 100 K were obtained by Working Group

4 of the CCT by graphical interpolation of data from the published literature.

The differences (T90 - T68 ) , or (t90 - t68 ) , between -200 °C and 630 °C were obtained by Working Group 4 of the CCT from published data on two SPRTs, one SPRT covering the range below °C, and the other covering the range from °C

to 630 °C. A polynomial of the form:

8

(t90 - t68 )/°C = I a^t^A^O) 1

. (41)

i=l

was fitted to the data from -200 °C to 630 °C and the coefficients are:

a. x = -0.148759 a5 = -4.089591

a2 = -0.267408 a6 = -1.871251

a3 = 1.080760 a7 = 7.438081

aA = 1.269056 a8 = -3.536296

The polynomial with these coefficients reproduce the tabulated differences [83]

to within 1 mK above C C and to within 1.5 mK below °C. The differences

(t90 - t68 ) between 630 °C and 1064 °C were obtained by/ Working Group 4 by

graphical interpolation from published data [14]. The differences (t90 - t68 ) above 1064 °C were obtained from the equation:

(tgo - t68 )/°C = - 0.25 [<t90/°C + 273.15)/(1337.33)] 2

. (42)

In section 3, we discussed the direct realization of the ITS -90, using the

62
standard instruments of the scale, i.e., the realization of the scale at the

lowest level of uncertainty. Of course, even the standard interpolating

instruments used at the same thermodynamic temperature will indicate temperatures

that differ slightly due to the devices having nonideal behavior and the scale

being expressed in as simple a form as possible. The differences in indicated

temperatures, however, are negligible for all practical purposes, being of the

order of < ± 0.5 mK for temperatures above about 5 K (i . e ., assuming no errors

in calibration). The realization of the ITS-90 in the liquid helium range of

temperatures (0.65 K to 5.0 K) through helium vapor pressure -temperature

relations can be accurate to about ± 0.1 mK or ± 0.2 mK.

For nonstandard types of thermometers used to approximate the ITS-90, the level

of uncertainty is higher than the numbers just given because of the inherent

instability of these thermometers. In all cases, these types of thermometers

are calibrated by comparison with one or more standard instruments of the scale,

e.g. , vapor -pressure thermometry; vapor -pressure thermometry and gas thermometry;

vapor-pressure thermometry, gas thermometry, and platinum resistance thermometry;

gas thermometry and platinum resistance thermometry; platinum resistance

thermometry; or pyrometers or spectral radiometers.

The NIST offers calibration services for various thermometers and pyrometers

covering the range from 0.65 K to 4200 °C (see NIST SP 250). Of this range, the

Chemical Process Metrology Division offers calibrations for contact thermometers

covering the range from 0.65 K to 2400 K, and the Radiometric Physics Division

offers calibrations for non- contact thermometers (radiation pyrometers) covering

the range from 1234.93 K (961.78 °C) to 4200 °C. Calibrations of only contacttype thermometers will be discussed here. The types of contact thermometers

calibrated include rhodium-iron resistance thermometers (RIRTs) , germanium

resistance thermometers (GRTs) , standard platinum resistance thermometers

(SPRTs) , thermocouples (t/c) , liquid- in-glass thermometers, thermistor

thermometers, industrial platinum resistance thermometers (IPRTs) , digital

thermometers, and other special thermometers that are compatible with the NIST

calibration equipment.

4.1 RHODIUM- IRON RESISTANCE THERMOMETERS

At temperatures below 13.8033 K, RIRTs and GRTs are, at the present time, the

only thermometers that are suitable for precision temperature measurements.

Also, RIRTs (and to a lesser extent GRTs) are suitable for use at temperatures

up to the triple point of neon (24.5561 K) . In the range from 0.65 K to about

25 K, RIRTs have reproducibilities of about ±0.2 mK. Consequently, RIRTs don't

degrade the realization of the ITS-90 significantly.

When the ITS-90 is realized, as defined, at NIST, some NIST RIRTs will be

calibrated at many temperatures through the use of vapor-pressure thermometry

and gas thermometry to produce reference -standard RIRTs, which will be

periodically recalibrated. The resistance -temperature data of these RIRTs will

be represented by a polynomial.

Customer RIRTs are calibrated by comparison with reference -standard RIRTs. A

polynomial is fitted by the method of least squares to the RIRT resistancetemperature data so obtained and the results are reported in terms of the

63
polynomial that is selected.

Until the NIST completes the development of the CVGT and vapor-pressure

thermometry apparatus with which the reference -standard RIRTs will be calibrated,

calibrations of customer RIRTs are performed by comparison against referencestandard RIRTs that have been calibrated on the NPL-75 Scale [13] or on the

EPT-76 and converted to the ITS-90.

To convert a calibration of an RIRT on the EPT-76 to an approximate calibration

on the ITS-90, use the EPT-76 calibration resistance -temperature data, change

the T76 values to T90 values using the (r90 - T76 ) differences given in table 1, or calculated with eq (40), to produce a new set of resistance-temperature

values, and then fit a polynomial of the required degree to these data. Using

the coefficients of the polynomial so determined, produce the desired calibration

table. A typical calibration report on the EPT-76 and one on the ITS-90 are given in appendix 3, sections 6.3.6 and 6.3.7, respectively.

4.2 GERMANIUM RESISTANCE THERMOMETERS

GRTs are comparable with, but not quite as stable as, RIRTs. They are calibrated

in a manner similar to that of the RIRTs and their results similarly reported.

At NIST, customer GRTs are calibrated by comparison with reference -standard

RIRTs

.

Anyone with a calibration on the EPT-76 may convert it to an approximate ITS-90

calibration by the same procedure as just outlined for RIRTs.

4.3 STANDARD PLATINUM RESISTANCE THERMOMETERS

Both capsule and long- stem type SPRTs are calibrated at the NIST. They will be

discussed separately.

4.3.1 CAPSULE SPRTs (13.8033 K TO 429.7485 K OR 505.078 IP

For temperatures in the range from 13.8033 K to 273.16 K, capsule SPRTs are the

most suitable thermometers. NIST has reference capsule SPRTs that have been,

or will have been, calibrated at the defining fixed points in this range. Those

SPRTs are used in calibrating customer thermometers by the comparison technique

over the range from about 13 K to 84 K. The temperatures at which comparisons

are made are at, or within a few mK of, the defining fixed-point temperatures

of the ITS-90 and at temperatures approximately mid-way between the fixed-point

values. Data at the temperatures intermediate to the fixed-point values are incorporated as a check on the calibration process. At and above the argon

triple point (83.8058 K) , customer capsule SPRTs are calibrated by the fixedpoint method. See section 6.3 for an example of how to calculate the

coefficients of the deviation functions.

4.3.2 LONG-STEM TYPE SPRTs (83.8058 K TO 1234.93 K)

Long- stem type SPRTs are used in the range from 83.8058 K to 1234.93 K. Two

different long- stem type SPRTs are required to cover this whole range, one type

being the customary SPRT having a nominal 25.5 resistance at °C and used in

64
the range from 83.8058 K to 692.677 K or to 933.473 K, and the second type having

a somewhat longer stem and having a nominal 0.25 Q or 2.5 resistance and used

in the range from 273.15 K to 1234.93 K. Such thermometers are very stable if

handled carefully. In this range of temperature, SPRTs are calibrated by the

fixed-point method, different sets of fixed points being required for different

temperature subranges. The required fixed points for the different subranges were specified in section 2 of this document. In the fixed-point method,

corrections for hydrostatic heads and gas pressures over the metals of the

freezing-point and melting-point cells are made (see table 6). Similarly,

corrections for hydrostatic heads present in metal and in gas triple -point cells

are made. As a check on the accuracy of the calibration, measurements are made

at one or more "redundant" defining fixed points lying within the temperature

range of calibration or at a well characterized secondary fixed point, such as the cadmium freezing point, that lies within the temperature range of

calibration. The value of the temperature of a check point calculated from the

calibration constants should agree closely with the accepted value of that point.

If not, then either an error was made in the calibration, one or more fixedpoint cells are defective, or the thermometer is defective.

The procedures indicated in section 3.2 for realizing the various fixed-point

temperatures and for handling SPRTs are followed carefully during calibration

of platinum thermometers, especially so for calibration at the aluminum and

silver freezing points (see sec. 3.2.4.2.7.1 for a discussion of lattice

defects). See section 6.3 for an example of how the coefficients of the

deviation functions are calculated from the data for the two sets of fixed

points

4.3.3 CONVERSION OF THE IPTS-68 CONSTANTS AND W(Tei ) TABLES TO APPROXIMATE

ITS-90 CONSTANTS AND W(Toq ) TABLES

For SPRTs that have been calibrated recently on the IPTS-68(75) , their

calibrations may be converted to approximate calibrations on the ITS-90 at

temperatures between the triple point of equilibrium hydrogen (13.8033 K) and

the freezing point of zinc (692.677 K)

To make the conversion, first obtain values of W(T68 ) , i.e., R(T6a )/R(0 °C) , from

the IPTS-68 (75) calibrations at values of T68 corresponding to the temperatures

of the relevant fixed points of the ITS-90 in the appropriate range in which the

conversion is desired. [Note: for a fixed point, the temperature values r68 and

T90 are different, but the "hotness" is the same and so the resistance of a given

SPRT remains unchanged. The temperature values on the two scales are defined

to be the same only at the triple point of water and at the absolute zero of the

temperature scales. Due to the nature of the scales, however, there are other

temperature values of these scales that are also the same, which are fortuitous.

See fig. 1, which shows the difference of t90 and t68 as a function of t90 .] Using these values of W(T6B ) at the appropriate ITS-90 fixed-point temperatures,

calculate values of W(TQQ )

, i.e., R(T90 )/R(273 . 16 K) , by dividing the values of

W(T68 ) at the appropriate fixed-point temperatures by the value of P/(273.16 K)

,

i.e., the value of W(T68 ) at the triple point of water. Table 8 shows samples

of such conversions for a capsule- type SPRT and for a long- stem type SPRT

calibrated on the IPTS-68(75). The calibration constants of the IPTS-68(75)

equations for the two SPRTs are given also in table 8. The values of W(TQQ ) so

65
Table 8. Example of conversion of calibrations of SPRTs on the IPTS-68 to

approximate calibrations on the ITS -90

Capsule SPRT. S/N 1812284 : e-H2 TP to Sn FP

IPTS-68 CALIBRATION CONSTANTS

R(0 °C) = 25.4964808

Aj. = -1.6314209xl0"4 Dx = 1.4525327xl0"8 A2 = -2.1616630xl0"4 D2 = -2.2572375xl0"10 A3 = -4.5449999xl0"4 A4 = 3.0860000xl0"7

a = 3.9262754xl0"3 Bx = -8.7377446xl0"7

B2 = -1.7113512xl0"6

7.7242433xl0"6

S = 1.4964667

Cj = -4.0321350xl0"7

C2 = 6.6163769xl0"8

C3 = -3.9028516xl0"8 CA = -1.0880791x10"*

ITS -90 CALIBRATION CONSTANTS CONVERTED FROM THE IPTS-68 CONSTANTS

R(273.16) = 25.4974973

ax = -2.5239001x10"*

a9 = -2.5287142x10"*

b 1 = -1.2277862x10"*

bQ = -1.1130131xl0" 5 Ci =

C2 =

c 3 =

c A =

C, =

= -2.3783015xl0-6 = -4.3892024xl0"6 = -1.5608728xl0"6 = -2.1374663xl0"7 = -1.0344171xl0"8

Long- Stem SPRT. S/N RS8YA-5 : Ar TP to Zn FP

IPTS-68 CALIBRATION CONSTANTS

R(0 °C) = 25.5086208

A4 = 2.6581418xl0_1 *

A = 3.9856609xl0"3 a = 3.9268986xl0-3 = -5.8762238xl0"7 = 1.49640322

= 9.3183900xl0"7

ITS -90 CALIBRATION CONSTANTS CONVERTED FROM THE IPTS-68 CONSTANTS

R(273.16 K) = 25.5096386

a4 = -9.3225823xl0"5 a8 = -9.1058813xl0" 5 b4 = -9.9914440xl0'6 bo = -7.6061559xl0"6

obtained may then be used with the appropriate relations described in section

2 to obtain the constants of the ITS -90 deviation equations for the SPRTs. In

table 8, we list the values of the deviation constants that were calculated from

the sample data presented there. For comparison, values of the conversion from

&f(r68 ) to W(T90 ) , as well as measured values of W(TQ0 ) , for the various relevant

fixed-point temperatures T68 and Tgo , respectively, are given in table 9. The

deviations in the values of the last two columns of table 9 for the capsule SPRT

reflect the inconsistency between the NBS-IPTS-68(75) wire scale and the

difference between the IPTS-68 (75) and the ITS -90 as given in table 1. Note

that for the long- stem SPRT, the zero deviations for tin and zinc are a consequence of those fixed points having been used in both cases and the

66
Table 9. Values of W(T68 ) and W(Tgo ) for various fixed-point temperatures T6B rgo , respectively

and

Capsule SPRT (S/N 1812284)

Fixed Point T68/K W(T6q) meas

.

r90 /K W(T90 ) calc* W(T90 ) meas.

e-H2 TP 13 ,81 0.00119822** 13.8033 0.00119817 0.00119721

e-H2 BP 17 .04200 0.00231120** 17.0357 0.00231111 0.00231049

e-H2 BP 20 .280 0.00425962** 20.2711 0.00425945 0.00425815

Ne TP 24 .5616 0.00848337** 24.5561 0.00848303 0.00848391

2 TP 54 .361 0.09182547** 54.3584 0.09182180 0.09182102

Ar TP 83 .79723 0.21598575** 83.8058 0.21597714 0.21597795

Hg TP 234 .3086 0.84421212** 234.3156 0.84417846 0.84417781

H2OTP 273 .16 1.00003987 273.16 1.00000000 1.00000000

Ga MP 302 .9218 1.11815352** 302.9146 1.11810895 1.11810896

In FP 429 .7848 1.60970773** 429.7485 1.60964355 1.60964566

Sn FP 505,.1181 1.89263856

Lone- Stem SPRT

505.078

CS/N RS8YA-5^

1.89256311

i

:

1.89256311

Ar TP 83.,79723 0.21592943** 83.8058 0.21592084 0.21592281

Hg TP 234.,3086 0.84418993** 234.3156 0.84415637 0.84415625

H20TP 273.,16 1.00003976 273.16 1.00000000 1.00000000

Ga MP 302. 9218 1.11817175*** 302.9146 1.11812729 1.11812699

In FP 429.,7848 1.60980450*** 429.7485 1.60974050 1.60974062

Sn FP 505.,1181 1.89278557 505.078 1.89271033 1.89271033

Zn FP 692.,73 2.56885786 692.677 2.56875573 2.56875573

Values of W(TQ0 ) calc. were obtained by conversion of corresponding values of

W(T68 ) meas.

These values were calculated from NBS-IPTS-68 calibration constants, based on

the NBS wire scale and calibration at the triple point of water and the freezing

point of tin (and the freezing point of zinc for the long-stem SPRT)

.

**These values were calculated from IPTS-68 calibration constants determined

from fixed points.

conversion process having been consistent. The deviations for the other fixed

points (Ar, Hg, Ga, and In) reflect the non- uniqueness of this SPRT and possible

errors incurred in measurements at those fixed points.

The results of tables 8 and 9 for the long- stem SPRT, with regard to the

differences between temperatures determined from an actual ITS -90 calibration

and those determined from an IPTS-68 (75) calibration but then calculated for

approximate ITS -90 values, are shown in figure 18. Note that at temperatures

above 273.16 K, the zero deviation of figure 18 is a consequence of the fact that

the same fixed points (tin and zinc) have been used in both cases and the

conversion process having been consistent.

67
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68
4.3.4 UNCERTAINTIES OF CALIBRATIONS AND THEIR PROPAGATION

Both systematic and random errors of measurements introduced in a calibration

are propagated throughout the temperature range of the calibration. It is the

total uncertainty arising from both of these types of errors, however, that is

of interest to the customer and user of a calibrated thermometer. International

comparisons of fixed-point cells below 90 K [79] and other data suggest that the

uncertainties (at the la level) in the realizations of the defining fixed points

of the ITS -90 are about ± 0.2 mK for the triple points of hydrogen and neon,

± 0.1 mK for the triple point of oxygen through the melting point of gallium,

± 0.7 mK at the freezing point of indium, ± 1 mK from the freezing point of tin

through the freezing point of aluminum, and i 2 mK for the freezing point cf

silver.

The uncertainty of temperature measurements in the liquid helium range (0.65 K

to 5.0 K) results from the uncertainty of the helium vapor-pressure measurements.

The uncertainty (at the la level) throughout this range of temperature is

estimated to be approximately ±0.1 mK to ± 0.2 mK.

For the CVGT, the uncertainty in the measured temperature over its temperature

range (3.0 K to 24.5561 K) arises from uncertainties of realizations of Che

triple points of neon and of equilibrium hydrogen, of the measurement of the

CVGT gas pressure, and of the measurement of the vapor pressure of helium. The

uncertainty (at the la level) throughout this range of temperature is estimated

to be approximately ± 0.1 mK to ± 0.2 mK. Uncertainties introduced by a particular CVGT design may add to these uncertainties.

In the calibration of SPRTs over any given subrange, the possible error in the

realization of each of the fixed points and any error of measurement must be

considered and they will be propagated independently of that incurred at the

other fixed points involved. The total uncertainty of measurements at a given

temperature is then the root-mean- square of the appropriate contributing

uncertainties. Curves showing the propagation of a i (unit error) incurred at

each of the defining fixed points of the two major ranges are shown in figures

19 and 20. Figure 19 shows the propagation of errors associated with the fixed

points below 273.16 K; and figure 20 shows curves for the fixed points used in

the calibration of SPRTs in the range from 273.15 K to 1234.93 K. The labels on the various curves indicate the fixed-point in which there is the unit error

in its temperature, and the other fixed points without error. As an example,

consider the curve labelled Sn(Ag, Al, Zn) of figure 20. The symbol Sn indicates

that the unit error occurs in the Sn freezing-point temperature; that is clearly

indicated by a unit offset of the curve at that point. The symbols in

parenthesis, i.e., (Ag, Al , Zn) , indicate that the Ag, Al , and Zn freezing points

were the other fixed points involved in the calibration and that measurements

were made at those fixed-point temperatures without error. Also, it is assumed

that the measurements at the triple point of water were without error. The other

labelled curves are similarly interpreted. The straight lines labelled TPW show

the errors propagated for an error of ± 0.1 mK made by the user in the triple

point of water.

69
4.3.5 ESTIMATES OF POSSIBLE ERRORS INTRODUCED BY EXTRAPOLATIONS BEYOND THE

RANGE OF CALIBRATION

It is unwise and is poor practice to use any thermometer beyond the temperature

range over which it was calibrated. Nevertheless, some users persist in doing

just that. In some cases, especially if the extrapolation is for only a few

kelvins, the error introduced may be rather small. The errors of some typical

extrapolations, calculated and depicted for NIST SPRTs , are shown in figures 21,

22, 23, 24, and 25.

The curve in figure 21 shows the error introduced for a NIST SPRT by

extrapolating the deviation function, determined from calibration over the range

from the triple point of argon to the triple point of water, downward from the

argon triple point to 54 K. Extrapolating downward to about the boiling point

of nitrogen (77 K) results in a fairly insignificant error in this case and,

thus, can be done with the usual caution that some thermometers may not be as good as that of figure 21.

Figure 22 depicts the results for another NIST SPRT. The curve shows the error introduced by extrapolating the deviation function, determined from calibration

over the range from the triple point of mercury to the melting point of gallium,

downward from the mercury triple point to 84 K.

Figure 23 displays the results for the same NIST SPRT as used for figure 22, but

the curve in this case shows the error introduced by extrapolating the deviation

function, determined from calibration over the range from the triple point of

mercury to the melting point of gallium, downward from the mercury triple point

to only 200 K. A sizable error is shown for observations that would be made at

Dry Ice temperatures (-78 °C)

.

Figure 24 depicts the results for several NIST SPRTs, but the curves in this case show the errors introduced by extrapolating their deviation functions,

determined from calibration over the range from the triple point of water to the

freezing point of zinc, downward from the triple point of water to -50 °C.

Figure 25 displays the results for several NIST SPRTs, but in this case the curves show the errors introduced by extrapolating their deviation functions,

determined from calibration over the range from the triple point of water to the

freezing point of zinc, upward from the freezing point of zinc to 934 K (660 °C) .

As a general rule and good practice, one should never extrapolate any of the

ITS -90 deviation functions beyond their range of application. If, however, the

estimated uncertainty introduced by extrapolating a deviation function beyond

the range of calibration is acceptable, then the user may do so, but with the

knowledge that his particular thermometer may yield results with a larger

uncertainty than that estimated. The user that makes such extrapolations should

realize that not all SPRTs will behave as indicated in figures 21 through 25.

The results depicted in these figures are examples only and are valid for only

those SPRTs indicated. Other SPRTs may be better or worse.

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r-l

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CO i-l •h td COC o o *J

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•uc

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ex o ®

«-i 5 < u

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td <"

a)>uuo

a) S

T3

m to cm C

N

CD

0)

)-l tw

0) ,e

4J6o

tw

-d

4-> c

•r4o

ex

bO

C

•-H 3

CX

•HH

4J

0)bOG

td td & u

to o <u C 4=

•r-l 4J N M

tw

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O "4-1

77
4.4 THERMOCOUPLES (77 K TO 2400 K)

There are numerous letter- designated types of thermocouples. The Type S thermocouple was the standard instrument of the IPTS-68(75) in the range from

630.74 °C to 1064.43 °C, but it is not a standard instrument of the ITS-90.

Customer thermocouples are calibrated at NIST by using a set of temperature

fixed points, by comparison with SPRTs, or by comparison with reference -standard

thermocouples that have been calibrated either by comparison with an SPRT or a radiation pyrometer, or through the use of fixed points. For details of the

calibration procedures and of the uncertainties involved, see NIST SP 250-35

[22] and NIST Monograph 175 [23] (or Monograph 125 [82]).

Usually, the calibration data for most types of thermocouples are analyzed

relative to reference tables, such as those given in NBS Monograph 125 [82].

Monograph 125, of course, has reference tables for thermocouples based on the

IPTS-68(75). This monograph has been revised and updated to give reference

tables for all letter-designated thermocouples based on the ITS-90. The revised

version of Monograph 125 is Monograph 175 [23] and it supersedes Monograph 125.

The electromotive -force -temperature data for a thermocouple calibrated on the

IPTS-68(75) can be converted to an approximate ITS-90 calibration through the use of the differences (t90 - t 68 ) given in table 1 of this document and the S values in mV/°C for the relevant thermocouple given in Monograph 175 or Monograph

125. An example of this conversion is given in table 10. A typical calibration

report is presented in appendix 3 (see sec. 6.3.8).

4.5 LIQUID- IN -GLASS THERMOMETERS

Liquid- in- glass thermometers have uncertainties of realization as small as ± 30 mK in the temperature range from °C to about 100 °C, but deteriorates at

lower and higher temperatures. Liquid- in- glass (primarily, mercury- in- glass)

thermometers are calibrated at NIST by comparison with SPRTs in liquid baths of

various kinds that cover different temperature ranges. For details of the

calibration procedures and of the uncertainties involved, see NIST SP 250-23

[105]. An example of a calibration report, based on the IPTS-68(75), of a liquid- in-glass thermometer is given in appendix 3 (see sec. 6.3.9). A

calibration report for the same thermometer, with the IPTS-68(75) calibration

converted to an approximate ITS-90 calibration through the use of (t90 - t68 ) differences given in table 1 is given also in appendix 3 (see sec. 6.3.10).

4.6 INDUSTRIAL PLATINUM RESISTANCE THERMOMETERS

Industrial platinum resistance thermometers (IPRTs) are designed primarily for use in the temperature range from about 77 K (approximate liquid nitrogen boiling

point) to 500 °C. Typically, the manufacturer of IPRTs quotes minimum

instabilities of the IPRTs at the ± 0.1 K level over this range of temperatures.

Some IPRTs may be somewhat better than this but others may be considerably worse. As seen from table 1, the maximum difference of (r90 - r68 ) below 500 C C is about

0.08 K, and therefore the difference in temperature due to the change in

temperature scales is within the instability of many IPRTs. Continued use of

the IPTS-68(75) and of equations and standards [American Society for Testing and

Materials (ASTM) Standard, E 1137, and International Electrotechnical Commission

78
Table 10. Example of a conversion of calibration values of a type K thermocouple

on the IPTS-68 to an approximate calibration on the ITS -90

Calibration Values Calibration Values

on IPTS -68 on ITS -90

t68 (°C)

emf68 (mV)S (mV/°C)

Monograph

125

^90" ^68

(°C)

Table 1

A

[-S-.(t90 -t88 )]

(mV)

^90 (°C)

emf90

a (mV)

0.0 0.000 0.0395 0.000 0.000 0.0 0.000

100.0 4.092 0.0414 -0.026 -0.001 100.0 4.093

200.0 8.130 0.0400 -0.040 -0.002 200.0 8.132

300.0 12.195 0.0415 -0.039 -0.002 300.0 12.197

400.0 16.383 0.0422 -0.048 -0.002 400.0 16.385

500.0 20.633 0.0426 -0.079 -0.003 500.0 20.636

600.0 24.904 0.0425 -0.115 -0.005 600.0 24.909

700.0 29.136 0.0419 0.20 0.008 700.0 29.128

800.0 33.288 0.0410 0.34 0.014 800.0 33.274

900.0 37.338 0.0400 -0.01 0.000 900.0 37.338

1000.0 41.281 0.0389 -0.19 -0.007 1000.0 41.288

1100.0 45.118 0.0378 -0.26 -0.010 1100.0 45.128

emf,90 emf6a - A

(IEC) Standard, Publication 751] referenced to the IPTS-68(75), therefore, would

result in an increase in uncertainty of temperature of only about 0.1 K if the

temperature were expressed as being on the ITS -90. (Note: the ASTM and the IEC are converting their respective IPRT tables from the IPTS -68 (75) to the ITS -90.

ASTM Committee E-20 on Temperature Measurements is responsible for this

conversion for the ASTM)

.

When IPRTs are calibrated on the ITS -90, of course, they may be calibrated in

the same manner as is used for SPRTs . A better method of calibrating IPRTs,

however, is to obtain resistance -temperature data by comparison with a calibrated

SPRT at numerous temperatures over the range of interest and then fit a polynomial in t90 to £(t90 ), to R(t90 )/R(0 °C) , or to R(t90 )/R(0 . 01 °C) data by a least squares technique.

4.7 THERMISTOR THERMOMETERS. DIGITAL THERMOMETERS. AND OTHER TYPES OF

THERMOMETERS

Thermistor thermometers, digital thermometers (with resistance, thermocouple,

or diode sensors) , and other types of thermometers are calibrated at NIST by

comparison with SPRTs in liquid baths. The calibration procedures followed are similar to those used with liquid- in-glass thermometers. The uncertainties of

calibration range from as small as ± 2 mK for thermistor thermometers to tenths

79
of kelvins for the others . The temperatures of calibration for these types of

thermometers usually lie somewhere within the range from about 77 K to 850 K.

Bead- in- glass probe type thermistors used in the moderate temperature range are quite stable and they may be used to approximate the ITS -90 at a level of about

+ 1.5 mK to ± 2.0 mK [67]. In their case, a polynomial, the degree of which

depends on the temperature range of the calibration, is fitted to resistancetemperature data and the results reported in terms of that polynomial. A

calibration on the IPTS-68(75) may be converted to an approximate ITS -90

calibration by the same procedure as outlined for RIRTs

4.8 THE LOGO OF THE NATIONAL CONFERENCE OF STANDARDS LABORATORIES FOR THE ITS -90

The National Conference of Standards Laboratories (NCSL) formed an Ad Hoc

Committee on the Change of the Temperature Scale at the beginning of 1988 in

order to publicize the new temperature scale (ITS-90) and to facilitate its

implementation. At the NCSL meeting in July 1989, the Ad Hoc Committee adopted

a logo, available from the NCSL [1800 30th Street, Suite 305B, Boulder, CO 80301,

Tel. (303) 440-3339], that may be affixed to thermometers that have been

calibrated on the ITS-90. The purpose of the logo, illustrated in figure 26,

is to indicate at a glance, without having to refer to documentation, those

thermometers in a laboratory that have been calibrated on the new scale. This

is particularly useful for those laboratories that have their various

thermometers calibrated on a prescribed schedule, with different thermometers

being calibrated at different times.

80
ITS-90

Figure 26. The NCSL ITS-90 logo

81
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[90] Roberts, T. R. , Sherman, R. H. , and Sydoriak, S. G. , The 1962 3He Scale of

Temperatures III. Evaluation and Status, J. Res. Natl. Bur. Stands. 68A . 567-578

(1964).

[91] Rusby, R. L. and Swenson, C. A., A New Determination of the Helium Vapour

Pressure Scales Using a CMN Magnetic Thermometer and the NPL- 75 Gas Thermometer

Scale, Metrologia 16, 73-87 (1980).

[92] Sawada, S., Realization of the triple point of Indium in a sealed glass

cell, Temperature . Its Measurement and Control in Science and Industry . Edited

by J . F. Schooley, Vol. 5, Part 1, pp. 343-346 (American Institute of Physics,

New York, 1982)

.

[93] Simon, M. , On the Phase Separation in the Liquid System Ne-pH2

, Phys

.

Letters, 5, 319 (1963).

[94] Sostman, H. E. , Melting point of gallium as a temperature calibration

standard, Rev. Sci. Instrum. 48, 127-130 (1977).

88
[95] Sparrow, E. M. , Albers , L. U. , and Eckert, E. R. G. , Thermal Radiation

Characteristics of Cylindrical Enclosures, J. Heat Transfer C84 . 73-81 (1962).

[96] Steur, P. P. M. and Durieux, M. , Constant -Volume Gas Thermometry Between

4 K and 100 K, Metrologia 23, 1-18 (1986).

[97] Streett, W. B. and D. H. Jones, Liquid Phase Separation and Liquid-vapor

Equilibrium in the System Neon- Hydrogen, J. Chem. Phys . 42, 3989-3994 (1965).

[98] Takiya, M. , Precise Measurement of the Freezing Point of Silver with a Platinum Resistance Thermometer, Comitee Consultatif de Thermometrie. 12th

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(1979).

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(1969).

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89
APPENDICES

In these appendices, we present information on or copies of some of the relevant

documents or meetings concerning the ITS -90, typical NIST calibration reports

of thermometers, and membership of the National Conference of Standards

Laboratories (NCSL) Ad Hoc Committee on the Change of the Temperature Scale

6.1 In this appendix, copies of, or excerpts from, some papers related to the

work of the international bodies concerned with the ITS -90 are presented.

6.1.1 Reproduction of the article by H. Preston-Thomas, entitled, "The

International Temperature Scale of 1990 (ITS-90)," published in Metrologia 27,

3-10 (1990), with corrections . Reprinted with permission of the BIPM.

90
Metrologia 27, 3-10(1990) metrologia

© Springer-Verlag 1990

This copy incorporates textual corrections detailed in Metrologia 27, 107 (1990)

The International Temperature Scale of 1990 (TTS-90)

H. Preston-Thomas

President of the Comite Consultatif de Thermometrie and Vice-President of the Comite International des Poids et Mesures

Division of Physics, National Research Council of Canada, Ottawa, K1A0S1 Canada

Received: October 24, 1989

Introductory Note

The official French text of the ITS-90 is published by the BIPM as part of the Proces-verbaux of the Comite Inter- national des Poids et Mesures (CIPM). However, the English version of the text reproduced here has been authorized by the Comite Consultatif de Thermometrie

(CCT) and approved by the CIPM.

The International Temperature Scale of 1990

The International Temperature Scale of 1990 was adopt- ed by the International Committee of Weights and Measures at its meeting in 1989, in accordance with the re- quest embodied in Resolution 7 of the 18th General

Conference of Weights and Measures of 1987. This scale supersedes the International Practical Temperature Scale of 1968 (amended edition of 1975) and the 1976 Provisional 0,5 K to 30 K Temperature Scale.

1. Units of Temperature

The unit of the fundamental physical quantity known as thermodynamic temperature, symbol T, is the kelvin, symbol K, defined as the fraction 1/273,16 of the thermo- dynamic temperature of the triple point of water '. Because of the way earlier temperature scales were

defined, it remains common practice to express a temperature in terms of its difference from 273,15 K, the ice point. A thermodynamic temperature, T, expressed in this way is known as a Celsius temperature, symbol t, defined

by:

t/°C=T/K-273,15. (1)

The unit of Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the kelvin. A difference of temperature may be expressed

in kelvins or degrees Celsius. The International Temperature Scale of 1990 (ITS-90)

defines both International Kelvin Temperatures, symbol

T90 , and International Celsius Temperatures, symbol f 90 . The relation between T90 and t 90 is the same as that between Tand t, i.e.:

t 90 /°C = T90 /K-273,15 (2)

1 Comptes Rendus des Seances de la Treizieme Conference Generale des Poids et Mesures (1967- 1968), Resolutions 3 and 4. p. 104

The unit of the physical quantity Tgo is the kelvin, symbol K, and the unit of the physical quantity r 90 is the degree Celsius, symbol °C, as is the case for the thermodynamic temperature T and the Celsius temperature t.

2. Principles of the International Temperature Scale of 1990 (ITS-90)

The ITS-90 extends upwards from 0,65 K to the highest

temperature practicably measurable in terms of the Planck radiation law using monochromatic radiation. The ITS-90 comprises a number of ranges and sub-ranges

throughout each of which temperatures T90 are defined. Several of these ranges or sub-ranges overlap, and where

such overlapping occurs, differing definitions of T90 exist: these differing definitions have equal status. For measure- ments of the very highest precision there may be detect- able numerical differences between measurements made

at the same temperature but in accordance with differing

definitions. Similarly, even using one definition, at a temperature between defining fixed points two acceptable

interpolating instruments (e.g. resistance thermometers)

may give detectably differing numerical values of T90 . In

virtually all cases these differences are of negligible practical importance and are at the minimum level consistent with a scale of no more than reasonable complexity: for further information on this point, see "Supplementary

Information for the ITS-90" (BIPM-1990).

Reprinted with permission of BIPM,
J i L -200 200 400 600 800 1000

Fig. 1. The differences (t 9Q — t 6S ) as a

function of Celsius temperature t 90

The ITS-90 has been constructed in such a way that, throughout its range, for any given temperature the numerical value of T90 is a close approximation to the numerical value of T according to best estimates at the time

the scale was adopted. By comparison with direct measurements of thermodynamic temperatures, measure- ments of T90 are more easily made, are more precise and

are highly reproducible.

There are significant numerical differences between

the values of T90 and the corresponding values of T6S measured on the International Practical Temperature

Scale of 1968 (IPTS-68), see Fig. 1 and Table 6. Similarly

there were differences between the IPTS-68 and the Inter- national Practical Temperature Scale of 1948 (IPTS-48),

and between the International Temperature Scale of 1948

(ITS-48) and the International Temperature Scale of 1927

(ITS-27). See the Appendix and, for more detailed infor- mation, "Supplementary Information for the ITS-90".

3. Definition of the International Temperature Scale of 1990

Above the freezing point of silver (961,78

C C) T90 is defined in terms of a defining fixed point and the Planck

radiation law. The defining fixed points of the ITS-90 are listed in Table 1 . The effects of pressure, arising from significant

depths of immersion of the sensor or from other causes, on the temperature of most of these points are given in Table 2.

3.1. From 0,65 K to 5.0 K: Helium Vapour-Pressure

Temperature Equations

In this range T90 is defined in terms of the vapour pressure p of 3 He and 4He using equations of the form: T90/K = A + £ A-, [(In ( p/Pa) - B)/C][

. (3)

The values of the constants A , A{ , B and C are given in Table 3 for 3 He in the range of 0,65 K to 3,2 K, and for

4 He in the ranges 1,25 K to 2,1768 K (the A point) and

2,1768 K to 5,0 K.

Between 0,65 K and 5.0 K T90 is defined in terms of the vapour-pressure temperature relations of 3 He and 4 He. Between 3,0 K and the triple point of neon

(24,5561 K) T90 is defined by means of a helium gas ther- mometer calibrated at three experimentally realizable temperatures having assigned numerical values (defining

fixed points) and using specified interpolation procedures. Between the triple point of equilibrium hydrogen

(13,8033 K)and the freezing point of silver (961,78 °C) T90

is defined by means of platinum resistance thermometers

calibrated at specified sets of defining fixed points and

using specified interpolation procedures.

3.2. From 3,0 K to the Triple Point ofNeon (24,5561 K)

:

Gas Thermometer

In this range Tgo is defined in terms of a

3 He or a

4He gas thermometer of the constant-volume type that has been

calibrated at three temperatures. These are the triple point of neon (24,5561 K), the triple point of equilibrium

hydrogen (13,8033 K), and a temperature between 3,0 K

and 5,0 K. This last temperature is determined using a

3 He or a

4 He vapour pressure thermometer as specified in Sect. 3,1.
Table 1. Defining fixed points of the ITS-90 Num- Temperature

ber Sub- State" Wr (T90 ) stance 3 T90 /K t,o/°C

56

7

8

910

1112

1314

1516

17

3 to 5

13,8033 wl7 :

•270,15 to -268,15 -259,3467

-256,15

*20,3 a -252,85

He V

e-H 2 T

e-H 2 V

(or He) (or G)

e-H 2 V

(or He) (or G)

24,5561 54,3584

83,8058 234,3156

273,16 302,9146

429,7485

505,078

692,677

933,473

1234,93 1337,33

1357,77

-248,5939 -218,7916

-189,3442 -38,8344

0,01 29,7646

156,5985 231,928 419,527

660,323

961,78

1064,18

1084,62

Neo2 ArHg

H2 Ga

InSnZn

AlAgAu

Cu

TT

T

TTM

F

F

FF

F

F

F

0,001 190 07

0,008 449 74

0,091 718 04

0,215 859 75 0,844 142 11

1,000 000 00

1,118 138 89

1,609 801 85

1,892 797 68

2,568 917 30

3,376 008 60

4,286 420 53

' All substances except 3 He are of natural isotopic composition, e-H 2

is hydrogen at the equilibrium concentration of the ortho- and

para-molecular forms

b For complete definitions and advice on the realization of these various states, see "Supplementary Information for the ITS-90". The

symbols have the following meanings: V: vapour pressure point; T: triple point (temperature at which the solid, liquid and vapour

phases are in equilibrium); G: gas thermometer point; M, F: melting

point, freezing point (temperature, at a pressure of 101 325 Pa, at which the solid and liquid phases are in equilibrium)

Table 2. Effect of pressure on the temperatures of some defining

fixed points *

Substance Assigned Temperature Variation value of with pressure, p with depth, / equilibrium (dT/dp)/ (dT/d/)/ temperature (10" 8 KPa" 1 )* (10~ 3 K- m

-1)** T90 /K

e-Hydrogen (T) 13,8033 34 0,25 Neon (T) 24,5561 16 1,9 Oxygen (T) 54,3584 12 1,5 Argon (T) 83,8058 25 3,3 Mercury (T) 234.3156 5,4 7,1 Water (T) 273,16 -7,5 -0,73

Gallium 302,9146 -2,0 -1.2 Indium 429,7485 4,9 3,3 Tin 505,078 3,3 2 2 Zinc 692,677 4.3 2,7 Aluminium 933,473 7,0 1,6 Silver 1234,93 6,0 5,4 Gold 1337.33 6,1 10 Copper 1357,77 3.3 2,6

* Equivalent to millikelvins per standard atmosphere

** Equivalent to millikelvins per metre of liquid

* The Reference pressure for melting and freezing points is the standard atmosphere (p =101 325 Pa). For triple points (T) the pressure effect is a consequence only of the hydrostatic head of

liquid in the cell

Table 3. Values of the constants for the helium vapour pressure

Eqs. (3), and the temperature range for which each equation, identified by its set of constants, is valid

3 He 4 He 4He

0,65 K to 3,2 K 1,25 K to 2,1768 K 2,1768 K to 5,0 K

A

A,

*2A,

A,K

An

A,A9 BC

1,053 447

0,980 106

0,676 380

0,372 692

0,151 656

-0,002 263

0,006 596

0,088 966

-0,004 770

-0.054 943

7,3 4,3

1,392 408

0,527 153

0,166 756

0,050 988

0,026 514

0,001 975

-0,017 976

0,005 409

0,013 259

5,6 2,9

3,146 631

1,357 655

0,413 923

0,091 159

0,016 349

0,001 826 -0,00 4325 -0,00 4973

10,3

1,9

3.2.1. From 4,2 Kto the Triple Point ofNeon (24,5561 K)

with ""He as the Thermometric Gas. In this range Tgo is defined by the relation: a + bp + cp

2 (4) where p is the pressure in the gas thermometer and a, b and c are coefficients the numerical values of which are obtained from measurements made at the three defining

fixed points given in Sect. 3.2, but with the further restric- tion that the lowest one of these points lies between 4,2 K

and 5,0 K.

3.2.2. From 3.0 Kto the Triple Point ofNeon (24,5561 K)

with 3 He or

A He as the Thermometric Gas. For a

3 He gas thermometer, and for a

4 He gas thermometer used below

4,2 K, the non-ideality of the gas must be accounted for

explicitly, using the appropriate second virial coefficient B3 {Tgo) or Bx (T90 ). In this range Tgo is defined by the

relation:

a + bp + cp2 90 \+Bx (T90 )N/V r

}

where p is the pressure in the gas thermometer, a, b and

c are coefficients the numerical values of which are obtained from measurements at three defining temperatures

as given in Sect. 3.2, N/V is the gas density with N being

the quantity of gas and V the volume of the bulb, x is 3 or 4 according to the isotope used, and the values of the second virial coefficients are given by the relations:

For 3 He, S(T90)/m3 mor' = {16,69-336.98 (T90/K)- ' (6a) + 91.04(T9O /K)- 2 -13.82(T9O /K)- 3

] 1(T 6

. For 4He. B4 (T90 )/m 3 mo! -1 = (16,708-374,05 (T90 /K)' (6b) -383,53 (T90 /K)- 2 + 1799,2 (T90 /K)- 3 -4033.2 (

r

90 /K)- 4 + 3252,8 (T90 /K)- 5

} 10" b
Table 4. The constants A , At \ B , Bx ; C , C,; D and D-t in the

reference functions of equations (9a); (9 b); (10a); and (10b) respectively

A

A,A2

A*

A,A6 A,

n9 Alt A l2 Co

C,C2 C3 Q

c5 c6 c7 c8 C,

-2,135 347 29

3,183 247 20

-1,801 435 97

0,717 272 04

0,503 440 27

-0,618 993 95

-0,053 323 22

0,280 213 62

0,107 152 24

-0,293 028 65

0,044 598 72

0,118 686 32

-0,052 481 34

2,781 572 54

1,646 509 16 -0,137 143 90

-0,006 497 67

-0,002 344 44

0,005 118 68

0,001 879 82

-0,002 044 72 -0,000 461 22

0,000 457 24

B 0,183 324 722 B 13 -0,091 173 542

B, 0,240 975 303 B 14 0,001317 696 B2 0,209 108 771 B 15 0,026 025 526 B3 0,190 439 972 B4 0,142 648 498 B5 0,077 993 465 B6 0,012 475 611 B7 -0,032 267 127 B8 -0,075 291 522 B9 -0,056 470 670 B10 0,076 201 285

B,, 0,123 893 204 B12 -0,029 201 193 D 439,932 854 D, 472,418 020 D, 37,684 494 D\ 7,472 018 04 2,920 828 D5 0,005 184 D6 -0,963 864 D1 -0,188 732

0,191 203

0,049 025

The accuracy with which T90 can be realized using Eqs. (4) and (5) depends on the design of the gas thermometer and

the gas density used. Design criteria and current good

practice required to achieve a selected accuracy are given

in "Supplementary Information for the ITS-90".

3.3. The Triple Point of Equilibrium Hvdrogen

(13.8033 K) to the Freezing Point of Silver (961,78°C):

Platinum Resistance Thermometer

In this range T90 is defined by means of a platinum resis- tance thermometer calibrated at specified sets of defining

fixed points, and using specified reference and deviation

functions for interpolation at intervening temperatures. No single platinum resistance thermometer can

provide high accuracy, or is even likely to be usable, over

all of the temperature range 13,8033 K to 961,78 °C. The

choice of temperature range, or ranges, from among those

listed below for which a particular thermometer can be used is normally limited by its construction. For practical details and current good practice, in particular concerning types of thermometer available,

their acceptable operating ranges, probable accuracies,

permissible leakage resistance, resistance values, and

thermal treatment, see "Supplementary Information for the ITS-90". It is particularly important to take account

of the appropriate heat treatments that should be fol- lowed each time a platinum resistance thermometer is subjected to a temperature above about 420 °C. Temperatures are determined in terms of the ratio of

the resistance R (T90 ) at a temperature T90 and the resistance R (273,16 K) at the triple point of water. This ratio, W(T90), is 2

: W(T90 ) = R(T90 )/R (273,16 K)

.

(7) An acceptable platinum resistance thermometer must

be made from pure, strain-free platinum, and it must satisfy at least one of the following two relations: W(29,7646 °C)> 1,1 18 07,

If(-38,8344 o C)<0,844 235.

(8 a)

(8 b) An acceptable platinum resistance thermometer that

is to be used up to the freezing point of silver must also

satisfy the relation: W(961,78 °C)> 4,2844. (8 c)

In each of the resistance thermometer ranges, T90 is obtained from Wr (T90 ) as given by the appropriate refer- ence function {Eqs. (9b) or (10b)}, and the deviation W(T90)— Wr (T90). At the defining fixed points this deviation is obtained directly from the calibration of the ther- mometer: at intermediate temperatures it is obtained by means of the appropriate deviation function {Eqs. (12), (13) and (14)}.

(i) - For the range 13,8033 K to 273,16 K the following

reference function is defined:

\n[WT (T90 )] = AQ +Y. A, 'ln(T90 /273,16K) + l,5' 15

(9 a)

An inverse function, equivalent to Eq. (9 a) to within

0,1 mK, is: T9O /273,16K = B +I B-, Wr (T90)

1 16 -0,65

0.35

(9 b)

The values of the constants A , A{ , B and B, are given in Table 4. A thermometer may be calibrated for use throughout

this range or, using progressively fewer calibration points,

for ranges with low temperature limits of 24,5561 K,

54,3584 K and 83,8058 K, all having an upper limit of 273,16 K.

(ii) - For the range 0°C to 961,78 °C the following refer- ence function is defined: WAT90) = C +X C, T90 /K- 754,1

5'

481

(10a)

An inverse function, equivalent to equation (10 a) to within 0,13 mK is: T9O/K-273,15 = D +X; D, X(T90)-2,64

"

1,64 (10b)

The values of the constants C , Cx , D and D, are given in Table 4.

2 Note that this definition of W{Tqo) differs from the corresponding

definition used in the ITS-27, ITS-48. IPTS-48, and IPTS-68: for all of these earlier scales W{T) was defined in terms of a reference temperature of 0°C, which since 1954 has itself been defined as 273,15 K
A thermometer may be calibrated for use throughout

this range or, using fewer calibration points, for ranges with upper limits of 660,323°C, 419,527°C, 231,928°C,

1 56,5985 °C or 29,7646 °C, all having a lower limit of °C.

(iii) - A thermometer may be calibrated for use in the range 234,3156 K (- 38,8344 °C) to 29,7646 °C, the cali- bration being made at these temperatures and at the triple point of water. Both reference functions {Eqs. (9) and

(10)} are required to cover this range. The defining fixed points and deviation functions for the various ranges are given below, and in summary form

in Table 5.

3.3.1. The Triple Point of Equilibrium Hydrogen

(13,8033 K) to the Triple Point of Water (273,16 K).

The thermometer is calibrated at the triple points of equi- librium hydrogen (13,8033 K), neon (24,5561 K), oxygen

(54,3584 K), argon (83,8058 K), mercury (234,3156 K), and water (273,16 K), and at two additional temperatures

close to 1 7,0 K and 20,3 K. These last two may be deter- mined either: by using a gas thermometer as described in

Sect. 3.2, in which case the two temperatures must lie within the ranges 16,9 K to 17,1 K and 20,2 K to 20,4 K

respectively; or by using the vapour pressure-temperature

relation of equilibrium hydrogen, in which case the two

temperatures must lie within the ranges 17,025 K to 17,045 K and 20,26 K to 20,28 K respectively, with the

precise values being determined from Eqs. (11a) and (lib)

respectively:

T90 /K -1 7,035 =(p/kPa- 33,321 3)/l 3,32 (11a) T90 /K- 20,27 =(p/kPa-101,292)/30. (lib)

The deviation function is

3

:

Table 5. Deviation functions and calibration points for platinum

resistance thermometers in the various ranges in which they define

W{T90 )-Wr (Tqo) = a[W(T90)-\] + b[W(T90)-\]2

+ 1 Ci [\nW(T90)r" (12)

with values for the coefficients a, b and cs being obtained from measurements at the defining fixed points and with

« = 2.For this range and for the sub-ranges 3.3.1.1 to 3.3.1.3 the required values of Wr (T90 ) are obtained from Eq. (9a) or from Table 1.

3.3.1.1. The Triple Point of Neon (24,5561 K) to the

Triple Point of Water (273,16 K). The thermometer is calibrated at the triple points of equilibrium hydrogen

(13,8033 K), neon (24,5561 K), oxygen (54,3584 K), argon

(83,8058 K), mercury (234,3156 K) and water (273,16 K). The deviation function is given by Eq. (12) with values

for the coefficients a, b, c l

, c 2 and c 3 being obtained from measurements at the defining fixed points and with

c4 = c 5 = « = 0.

3.3.1.2. The Triple Point of Oxygen (54.3584 K) to the

Triple Point of Water (273,16 K). The thermometer is

a Ranges with an upper limit of 273,16 K

Sec- Lower Deviation functions

tion temperature limit T/K.

Calibration

points

(see Table 1)

3.3.1 13,8033 a[W(T90 )-i] + b[W(T90)-l] 2 2-9 + £ Ci nnW(T90)r", n = 2

i= i

3.3.1.1 24,5561 As for 3.3.1 with c4 = c 5 = n = 2,5-9

3.3.1.2 54,3584 As for 3.3.1 with 6-9

C2 =C3=C4. = C5=0, H=l

3.3.1.3 83,8058 a[W(T90)-\] 7-9 + b[W(T90)-\]\nW(T90)

b Ranges with a lower limit of0°C

Sec- Upper Deviation functions Calibration

tion temper- points ature limit (see Table 1

)

r/°C

3.3.2* 961,78 a[W(r90 )-i]+fe[w(r90)-i]2 + c[W(T9Q)-\f + d[W(T90) -W(660,323 °C)] 2

9,12-15

3.3.2.1 660,323 As for 3.3.2 with d=0 9,12-14

3.3.2.2 419,527 As for 3.3.2 with c = d = 9,12,13

3.3.2.3 231,928 As for 3.3.2 with c = d = 9,11,12

3.3.2.4 156,5985 As for 3.3.2 with b = c = d = Q 9,11

3.3.2.5 29,7646 As for 3.3.2 with fe = c = <f = 9,10

3 This deviation function {and also those of Eqs. (13) and (14)} may

be expressed in terms of Wr rather than W\ for this procedure see "Supplementary Information for ITS-90"

c Range from 234,3156 K ( -38,8344° C) to 29. 7646° C

3.3.3 As for 3.3.2 with c = d=0 8-10

* Calibration points 9, 12-14 are used with d=0 for ; 90 <; 660.323°C; the values of a. b and c thus obtained are retained for

/ 90 > 660.323 °C, with d being determined from calibration point 1

calibrated at the triple points of oxygen (54,3584 K), argon (83,8058 K), mercury (234,3156 K) and water

(273,16 K). The deviation function is given by Eq. (12) with values

for the coefficients a, b and c l being obtained from measurements at the defining fixed points, with c2 =c3 =

c4 = c 5 = and with n = 1

3.3.1.3. The Triple Point of Argon (83.8058 K) to the

Triple Point of Water (273,16 K). The thermometer is calibrated at the triple points of argon (83,8058 K), mercury (234,3156 K) and water (273,16 K). The deviation function is: W(T90 )- WT (T90 ) = a [W{T90)-\] + b[W(T90 )-\] In W(T90 ) (13) with the values of a and b being obtained from measure- ments at the defining fixed points.

3.3.2. From 0°C to the Freezing Point of Silver (961,78°C). The thermometer is calibrated at the triple
point of water (0,01 °C), and at the freezing points of tin (231,928 °C), zinc (419,527 °C), aluminium (660,323 °C)

and silver (961,78 °C). The deviation function is: W(T90 )-Wr (T90) = a[W(T90 )-\] + b[W(T90 )-\] 2 (14) + c[W(T90 )-\]° + d[W(T90 )-W (660,323 °C)]2

. For temperatures below the freezing point of aluminium

d =0, with the values of a, b and c being determined from

the measured deviations from Wr (T9Q) at the freezing

points of tin, zinc and aluminium. From the freezing point of aluminium to the freezing point of silver the above

values of a, b and c are retained and the value of d is determined from the measured deviation from Wr (T90 ) at the freezing point of silver. For this range and for the sub-ranges 3.3.2.1 to 3.3.2.5 the required values for WT (Tgo ) are obtained from

Eq. (10a) or from Table 1.

3.3.2.1. From 0°C to the Freezing Point of Aluminium

(660.323° C). The thermometer is calibrated at the triple point of water (0,01 °C), and at the freezing points of tin (231,928 °C), zinc (41 9,527 °Q and aluminium (660,323 °C). The deviation function is given by Eq. (14), with the values of a, b and c being determined from measurements

at the defining fixed points and with d = 0.

3.3.2.2. From 0°C to the Freezing Point of Zinc (419,527°C). The thermometer is calibrated at the triple point of water (0,01 °C), and at the freezing points of tin (231,928°C) and zinc (419,527°C).

The deviation function is given by Eq. (14) with the values of a and b being obtained from measurements at the defining fixed points and with c = d = 0.

3.3.2.3. From 0°C to the Freezing Point of Tin (231.928°C). The thermometer is calibrated at the triple point of water (0,01 °C), and at the freezing points of indi- um (156.5985°C) and tin (231,928 °C). The deviation function is given by Eq. (14) with the values of a and b being obtained from measurements at the defining fixed points and with c = </ = 0.

3.3.2.4. From 0°C to the Freezing Point of Indium

(156.5985°C). The thermometer is calibrated at the

triple point of water (0.01 °C), and at the freezing point of indium (1 56,5985 "C). The deviation function is given by Eq. (14) with the value of a being obtained from measurements at the defin- ing fixed points and with b = c = d = 0.

3.3.2.5. From 0°C to the Melting Point of Gallium (29.7646°C). The thermometer is calibrated at the triple point of water (0,01 °C), and at the melting point of galli- um (29,7646 °C). The deviation function is given by Eq. (14) with the value of a being obtained from measurements at the defin- ing fixed points and with b = c = d = 0.

3.3.3. The Triple Point of Mercury ( - 38.8344 J C) to the Melting Point of Gallium (29. 7646° C). The thermometer

is calibrated at the triple points of mercury ( — 38,8344 °C), and water (0,01 °C), and at the melting point of gallium

(29,7646 °C). The deviation function is given by Eq. (14) with the values of a and b being obtained from measurements at the defining fixed points and with c = d = 0. The required values of WT (T90) are obtained from

Eqs. (9 a) and (10 a) for measurements below and above

273,16 K respectively, or from Table 1.

3.4. The Range Above the Freezing Point of Silver (961. 78° C): Planck Radiation Law

Above the freezing point of silver the temperature T90 is defined by the equation:

LX (T9I ^ (15) exptcJAT^tX)]- 1 )-!

Lx [T90 (X)] exp(c2 [*T90]~ l )-\

'

where T90 (X) refers to any one of the silver {

T

90 ( Ag) = 1234,93 K}, the gold {

T

90 (Au)=l 337,33 K} or the copper {T90 (Cu) = 1357,77 K} freezing points* and in which LX {T90 ) and L X [T90 (X)] are the spectral concentrations of the radiance of a blackbody at the wavelength

(in vacuo) k at T90 and at T90 (X) respectively, and

c2 =0,014388m-K.

For practical details and current good practice for optical pyrometry, see "Supplementary Information for the ITS-90" (BIPM-1990).

4. Supplementary Information and Differences from Earlier Scales

The apparatus, methods and procedures that will serve to

realize the ITS-90 are given in "Supplementary Information for the ITS-90". This document.also gives an account

of the earlier International Temperature Scales and the numerical differences between successive scales that in- clude, where practicable, mathematical functions for the

differences T90 — T68 . A number of useful approximations

to the ITS-90 are given in "Techniques for Approximating

the ITS-90". These two documents have been prepared by the Comite Consultatif de Thermometrie and are published

by the BIPM; they are revised and updated periodically.

The differences T90 — T68 are shown in Fig. 1 and

Table 6. The number of significant figures given in Table 6 allows smooth interpolations to be made. However, the reproducibility of the IPTS-68 is, in many areas, substantially worse than is implied by this number.

4 The T90 values of the freezing points of silver, gold and copper are believed to be self consistent to such a degree that the substitution of any one of them in place of one of the other two as the reference temperature Tqo (X) will not result in significant differences in the measured values of Ton .
Table 6. Differences between ITS-90 and EPT-76, and between ITS-90 and IPTS-68 for specified values of T90 and t 90

(T90 -T76 )/mK T90 /K 1 2 3 4 5 6 7 8 9 -0,1 -0,2 -0,3 -0,4 -0,5

10 -0,6 -0,7 -0,8 -1,0 -1,1 -1,3 -1,4 -1,6 -1,8 -2,0

20 -2,2 -2,5 -2,7 -3,0 -3,2 -3,5 -3,8 -4,1

(T9Q

- -T6S )/K r90 /K 1 2 3 4 5 6 7 8 9 10 -0,006 -0,003 -0,004 -0,006 -0,008 -0,009

20 -0,009 -0,008 -0,007 -0,007 -0,006 -0,005 -0,004 -0,004 -0,005 -0,006

30 -0,006 -0,007 -0,008 -0,008 -0,008 -0,007 -0,007 -0,007 -0,006 -0,006

40 -0,006 -0,006 -0,006 -0,006 -0,006 -0,007 -0,007 -0,007 -0,006 -0,006

50 -0,006 -0,005 -0,005 -0,004 -0,003 -0,002 -0,001 0,000 0,001 0,002 60 0,003 0,003 0,004 0,004 0,005 0,005 0,006 0,006 0,007 0,007 70 0,007 0,007 0,007 0,007 0,007 0,008 0,008 0,008 0,008 0,008 80 0,008 0,008 0,008 0,008 0,008 0,008 0,008 0,008 0,008 0,008 90 0,008 0,008 0,008 0,008 0,008 0,008 0,008 0,009 0,009 0,009 T90 /K 10 20 30 40 50 60 70 80 90

100 0,009 0,011 0,013 0,014 0,014 0,014 0,014 0,013 0,012 0,012 200 0,011 0,010 0,009 0,008 0,007 0,005 0,003 0,001

('90 -t68)/°c

t 9o/°C -10 -20 -30 -40 -50 -60 -70 -80 -90

-100 0,013 0,013 0,014 0,014 0,014 0,013 0,012 0,010 0,008 0.008 0,000 0,002 0,004 0,006 0,008 0,009 0,010 0,011 0,012 0,012

'9o/°C- 10 20 30 40 50 60 70 80 90

0,000 -0,002 -0,005 -0,007 -0,010 -0,013 -0,016 -0,018 -0.021 -0,024

100 -0,026 -0,028 -0,030 -0,032 -0,034 -0,036 -0,037 -0,038 -0,039 -0,039

200 -0,040 -0,040 -0,040 -0,040 -0,040 -0,040 -0,040 -0,039 -0,039 -0,039

300 -0.039 -0,039 -0,039 -0,040 -0,040 -0,041 -0,042 -0,043 -0,045 -0,046

400 -0.048 -0,051 -0,053 -0,056 -0,059 -0,062 -0,065 -0,068 -0,072 -0,075

500 -0,079 -0,083 -0,087 -0,090 -0,094 -0,098 -0,101 -0,105 -0,108 -0,112

600 -0.115 -0,118 -0,122 -0,125* -0,08 -0,03 0,02 0,06 0,11 0,16 700 0,20 0,24 0,28 0.31 0,33 0,35 0,36 0,36 0,36 0,35 800 0,34 0,32 0,29 0.25 0,22 0,18 0,14 0,10 0,06 0,03 900 -0,01 -0,03 -0,06 -0,08 -0.10 -0,12 -0,14 -0,16 -0,17 -0,18

1000 -0,19 -0.20 -0,21 -0,22 -0.23 -0,24 -0,25 -0,25 -0,26 -0,26 ho/°C 100 200 300 400 500 600 700 800 900

1000 -0,26 -0.30 -0,35 -0,39 -0,44 -0,49 -0.54 -0,60 -0.66

2000 -0,72 -0,79 -0.85 -0.93 -1,00 -1,07 -1,15 -1.24 -1,32 -1,41 3000 -1.50 -1,59 -1.69 -1,78 -1.89 -1,99 -2,10 -2,21 -2,32 -2,43

A discontinuity in the first derivative of (r 90 —

1

68 ) occurs at a temperature of t 90 = 630,6°C,at which (t 90 — f 6s)= — 0,125

e C

Appendix

The International Temperature Scale of 1927 (ITS-27) The International Temperature Scale of 1927 was adopted by the seventh General Conference of Weights and Measures to overcome

the practical difficulties of the direct realization of thermodynamic

temperatures by gas thermometry, and as a universally acceptable replacement for the differing existing national temperature scales. The ITS-27 was formulated so as to allow measurements of temperature to be made precisely and reproducibly, with as close an ap- proximation to thermodynamic temperatures as could be deter- mined at that time. Between the oxygen boiling point and the gold

freezing point it was based upon a number of reproducible temperatures, or fixed points, to which numerical values were assigned, and

two standard interpolating instruments. Each of these interpolating instruments was calibrated at several of the fixed points, this giving

the constants for the interpolating formula in the appropriate temperature range. A platinum resistance thermometer was used for the lower part and a platinum rhodium/platinum thermocouple for

temperatures above 660 °C. For the region above the gold freezing point, temperatures were defined in terms of the Wien radiation law:

in practice, this invariably resulted in the selection of an optical pyrometer as the realizing instrument.

The International Temperature Scale of 1948 (ITS-48)

The International Temperature Scale of 1948 was adopted by the ninth General Conference. Changes from the ITS-27 were: the lower limit of the platinum resistance thermometer range was changed

from — 190°C to the defined oxygen boiling point of — 182.97°C. and the junction of the platinum resistance thermometer range and

the thermocouple range became the measured antimony freezing point (about 630°C) in place of 660"C; the silver freezing point was defined as being 960.8 C instead of 960.5'C; the gold freezing point replaced the gold melting point (1063"C); the Planck radiation law replaced the Wien law: the value assigned to the second

radiation constant became 1.438 x I0~ : m K. in place of 1.432 x 10"" m • K; the permitted ranges for the constants of the interpola-
10

tion formulae for the standard resistance thermometer and thermocouple were modified; the limitation on XT for optical pyrometry (A7"<3 x 10~ 3 m K.) was changed to the requirement that "visible" radiation be used.

The International Practical Temperature Scale of 1948 ( Amended Edition of 1960) (IPTS-48)

The International Practical Temperature Scale of 1948, amended

edition of 1960, was adopted by the eleventh General Conference:

the tenth General Conference had already adopted the triple point of water as the sole point defining the kelvin, the unit of thermody- namic temperature. In addition to the introduction of the word

"Practical", the modifications to the ITS-48 were: the triple point of water, defined as being 0,01 °C, replaced the melting point of ice as the calibration point in this region; the freezing point of zinc, defined as being 419,505 °C, became a preferred alternative to the sulphur boiling point (444,6 °C) as a calibration point; the permitted

ranges for the constants of the interpolation formulae for the stan- dard resistance thermometer and the thermocouple were further modified; the restriction to "visible" radiation for optical pyrometry was removed. Inasmuch as the numerical values of temperature on the IPTS48 were the same as on the ITS-48. the former was not a revision of

the scale of 1948 but merely an amended form of it.

The International Practical Temperature Scale of 1968 (IPTS-68)

In 1968 the International Committee of Weights and Measures

promulgated the International Practical Temperature Scale of 1968, having been empowered to do so by the thirteenth General Conference of 1967-1968. The IPTS-68 incoporated very extensive changes from the IPTS-48. These included numerical changes, de- signed to bring it more nearly in accord with thermodynamic temperatures, that were sufficiently large to be apparent to many users. Other changes were as follows: the lower limit of the scale was

extended down to 13,81 K: at even lower temperatures (0,5 K to 5.2 K), the use of two helium vapour pressure scales was recom- mended: six new defining fixed points were introduced - the triple point of equilibrium hydrogen (13,81 K), an intermediate equilibri- um hydrogen point (17,042 K.). the normal boiling point of equilib- rium hydrogen (20,28 K), the boiling point of neon (27.102 K), the

triple point of oxygen (54,361 K), and the freezing point of tin (231.9681 °C) which became a permitted alternative to the boiling point of water; the boiling point of sulphur was deleted; the values assigned to four fixed points were changed - the boiling point of oxygen (90.188 K), the freezing point of zinc (419,58°C), the freezing point of silver (961,93 °C), and the freezing point of gold

( 1064,43 °C); the interpolating formulae for the resistance ther- mometer range became much more complex; the value assigned to the second radiation constant c2 became 1,4388 x 10

~ 2 m • K; the permitted ranges of the constants for the interpolation formulae for the resistance thermometer and thermocouple were again modified.

The International Practical Temperature Scale of 1968 (Amended Edition of 1975) (IPTS-68)

The International Practical Temperature Scale of 1968, amended

edition of 1975, was adopted by the fifteenth General Conference in 1975. As was the case for the IPTS-48 with respect to the ITS-48,

the IPTS-68 (75) introduced no numerical changes. Most of the extensive textural changes were intended only to clarify and simplify

its use. More substantive changes were: the oxygen point was defined as the condensation point rather than the boiling point; the

triple point of argon (83,798 K) was introduced as a permitted

alternative to the condensation point of oxygen; new values of the isotopic composition of naturally occurring neon were adopted; the recommendation to use values of Tgiven by the 1958 4He and 1962

3 He vapour-pressure scales was rescinded.

The 1976 Provisional 0,5 K to 30 K Temperature Scale (EPT-76)

The 1976 Provisional 0.5 K. to 30 K. Temperature Scale was intro- duced to meet two important requirements: these were to provide means of substantially reducing the errors (with respect to corre- sponding thermodynamic values) below 27 K that were then known

to exist in the IPTS-68 and throughout the temperature ranges of

the

a He and 3 He vapour pressure scales of 1958 and 1962 respectively, and to bridge the gap between 5,2 K and 13,81 K in which there had not previously been an international scale. Other objectives in devising the ETP-76 were "that it should be thermodynamically smooth, that it should be continuous with the IPTS-68 at 27,1 K. and that is should agree with thermodynamic temperature T as

closely as these two conditions allow". In contrast with the IPTS-68. and to ensure its ranid adoption, several methods of realizing the ETP-76 were appr ved. These included: using a thermodynamic

interpolation instrument and one or more of eleven assigned refer- ence points: taking differences from the IPTS-68 above 13.81 K;

taking differences from helium vapour pressure scales below 5 K: and taking differences from certain well-established laboratory

scales. Because there was a certain "lack of internal consistency" it was admitted that "slight ambiguities between realizations" might

be introduced. However the advantages gained by adopting the EPT-76 as a working scale until such time as the IPTS-68 should be

revised and extended were considered to outweigh the disadvantages.
6.1.2 Reproduction of the article by T. J. Quinn, entitled "News from the BIPM"

,

published in Me tro 1 ogi a 26, 69-74 (1989).

99
Metrologia 26, 69-74 (1989) metrologia

© Springer-Verlag 1989

News from the BIPM

T.J. Quinn

Bureau International des Poids et Mesures, Pavilion de Breteuil, F-92312 Sevres Cedex, France

Received: October 14, 1988

Comite International des Poids et Mesures

77th Meeting

The Comite International des Poids et Mesures (CIPM) at its 77th Meeting, held at the Pavilion de

Breteuil on the 4th, 5th and 6th of October 1988,

adopted two important Recommendations concerning the use of the Josephson effect and the quantum

Hall effect for maintaining reference standards for the measurement of emf and resistance. These Recommendations are based upon proposals made to the CIPM

by its Comite Consultatif d' Electricite (CCE) which met in September 1988. The CCE proposals were the

result of a great deal of work and discussion that had

taken place among representatives of the national standards laboratories and the BIPM, particularly over the past twelve months.

New determinations of the SI volt and ohm, directly by realizations of the SI definitions and indirectly

through determinations of the relevant fundamental

physical constants, have established the values of the Josephson constant K, and the von Klitzing constant RK of the quantum Hall effect with uncertainties of a few parts in 107

. The constants K, and RK were as- sumed by the CCE to be equal to 2 e/h and h/e2 respectively for the purpose of including determinations of fundamental constants in their evaluation. The reproducibility, however, of the emf across a Josephson

junction and the resistance of a quantum Hall device

is much better than a few parts in 10 7 and approaches one part in 108

. Furthermore, the value of 2 e/h suggested by the CCE in 1972 is now known to be in error by about 8 parts in 106

, although not all national standards laboratories are using this value. In order to unify world reference standards of emf

and allow users to take advantage of the great reproducibility of the Josephson effect and the quantum

Hall effect, the CIPM acted on the advice of the CCE

and adopted particular values of K, and RK , desig- nated Kj_ qo and RK -Q < f° r use by all laboratories beginning on 1st January 1990. For those countries Reprinted with, permission of BIPM.

that have based their reference standard of emf on the Josephson junction and use the 1972 CCE value for 2 e/h, the adoption of the new value for K, will lead to a change in their reference standards of about 8 parts

in 106

, i. e. 8 uV per volt. For the countries using values of K, which differ from the 1972 CCE value, the changes will be within the range of about 9 to 3.5 parts

in 106. Also 1st January 1990 it is expected that a new

international temperature scale, the International Temperature Scale of 1990 (ITS-90), will come into

effect, which will replace the International Practical Temperature Scale of 1968 (IPTS-68). The differences between ITS-90 and IPTS-68 are significant; not only

are the scales substantially different in definition but

temperatures measured on the ITS-90 differ signifi- cantly from those measured on the IPTS-68. For ex- ample, near room temperature: for t 68 ^ 20 °C the dif- ference T90 - T68 ~ — 5 mK and for t 68 ~ 100 °C the

difference T90 — T68 ^ — 25 mK. The normal boiling

point of water thus will be about 99.97 °C and not

100.00 °C as in the past.

It is important that users be made aware of these changes in electrical and temperature reference stan- dards well in advance of their introduction. For this reason the 18th Conference Generale des Poids et Measures asked the CIPM to announce one year in advance, namely at the beginning of 1989, the magnitude of the changes consequent upon their coming into

effect. These official announcements are now made in the following three CIPM recommendations:

Representation of the volt by means

of the Josephson effect

Recommendation 1 (CI-1988)

The Comite International des Poids et Mesures,

acting in accordance with instructions given in Resolution 6 of the 18th Conference Generale des Poids et
Mesures concerning the forthcoming adjustment of

the representations of the volt and the ohm,

considering

-that a detailed study of the results of the most recent determinations leads to a value of 483 597.9 GHz/V for the Josephson constant, K,, that

is to say, for the quotient of frequency divided by the potential difference corresponding to the n = 1 step in the Josephson effect, - that the Josephson effect together with this value of ATj can be used to establish a reference standard of

electromotive force having a one-standard-deviation

uncertainty with respect to the volt estimated to be 4 parts in 10 7

, and a reproducibility which is signifi- cantly better,

recommends - that 483 597.9 GHz/V exactly be adopted as a conventional value, denoted by K^90 , for the Joseph- son constant, Klt -that this new value be used from 1st January 1990, and not before, to replace the values

currently in use, - that this new value be used from this same date by all laboratories which base their measurements of

electromotive force on the Josephson effect, and - that from this same date all other laboratories adjust the value of their laboratory reference stan- dards to agree with the new adopted value,

is of the opinion - that no change in this recommended value of the Josephson constant will be necessary in the foresee- able future, and

draws the attention of laboratories to the fact that the new value is greater by 3.9 GHz/V, or about 8 parts in 106

, than the value given in 1972 by the Comite Consultatif d 'Electricite in its Declaration E-72.

Representation of the ohm by means

of the quantum Hall effect

Recommendation 2 (CI- (988)

The Comite International des Poids et Mesures,

acting in accordance with instructions given in Resolution 6 of the 18th Conference Generale des Poids et Mesures concerning the forthcoming adjust- ment of the representations of the volt and the ohm,

considering - that most existing laboratory reference standards

of resistance change significantly with time.

- that a laboratory reference standard of resistance based on the quantum Hall effect would be stable and

reproducible, - that a detailed study of the results of the most

recent determinations leads to a value of 25 812.807 Q. for the von Klitzing constant, RK , that is to say, for the quotient of the Hall potential difference divided by current corresponding to the plateau i = 1 in the quantum Hall effect, - that the quantum Hall effect, together with this value of RK , can be used to establish a reference stan- dard of resistance having a one-standard-deviation

uncertainty with respect to the ohm estimated to be

2 parts in 107

, and a reproducibility which is signifi- cantly better, recommends

- that 25 812.807 Q. exactly be adopted as a conventional value, denoted by RK-9o> f°r tne von Klit- zing constant, RK , -that this value be used from 1st January 1990, and not before, by all laboratories which base their measurements of resistance on the quantum Hall effect, - that from this same date all other laboratories

adjust the value of their laboratory reference stan- dards to agree with RK-90 , - that in the use of the quantum Hall effect to

establish a laboratory reference standard of resistance, laboratories follow the most recent edition of the

technical guidelines for reliable measurements of the quantized Hall resistance drawn up by the Comite

Consultatif d 'Electricite and published by the Bureau

International des Poids et Mesures, and

is of the opinion - that no change in this recommended value of the von Klitzing constant will be necessary in the foresee- able future.

Preparation of the International Temperature Scale of 1990 (ITS-90)

Recommendation 3 (CI-1988)

The Comite International des Poids et Mesures,

acting in accordance with instructions given in Resolution 7 of the 18th Conference Generale des Poids et Mesures concerning the preparation of the new international temperature scale, announces that the differences between the ITS-90 and

the IPTS-68 will be approximately those indicated in the graph attached to this Recommendation,

recommends that national laboratories take note of

these differences with a view to the implementation of

the ITS-90 on 1st January 1990.
0.02-

J> -0.02 -

*? -0.04

2

3

2

<5Q.E -0.2

-i 1 1 1 1 r

- 0.4

0.2

J_ J_ _L

0.2

-200

Fig. 1. Graph of r 90 - t68

200 400

f/°C

600 800 1000

Comite Consultatif d'Electricite

18th Meeting

The CCE at its meeting in September 1988 consid- ered very carefully the way in which the recommended

values for K, and RK(defined above) be used. Al- though no formal recommendation was made on this point, the following four statements were drafted and

unanimously approved by the CCE and subsequently

supported by the CIPM '

/./. Recommendations 1 (CI-1988) and 2 (CI-1988)

do not constitute a redefinition of SI units The conventional values K,-go and RK - go cannot be

used as bases for defining the volt and ohm (meaning

the present volt and ohm units in the Systeme Inter- national d'Unites). To do so would change the status of n from that of a constant having an exactly defined value (and would therefore abrogate the definition of

the ampere) and would also produce electrical units which would be incompatible with the definition of the kilogram and units derived from it.

Because the original draft CCE recommendations were slighly modified by the CIPM, I have chosen to refer here only to the CIPM designations, i.e., CI-1988.

1.2. Concerning the use of subscripts on symbols

for quantities or units The CCE considers that symbols for electromotive

force (electric potential, electric potential difference)

and electric resistance, and for the volt and the ohm,

should not be modified by adding suffixes to denote

particular laboratories or dates. The principal reasons for this viewpoint with re- spect to the physical quantities are that: - until now, temperature being one of the very few

exceptions, it has not been necessary to introduce explicitly the concept of a system of conventional physical quantities differing from the traditional quantities,-it would be difficult to make such a concept

widely understood and accepted, - the concept, if introduced for electromotive force and electrical resistance, would propagate to other

quantities.

The principal reasons for this viewpoint with re- spect to the units are that: - the appearance of creating a unit system other than SI should be avoided, particularly as this would

propagate to units for other quantities.
- the new reference standards will be completely

satisfactory representations of the volt and the ohm

for the great majority of applications, - any disagreement between those laboratories

that realize the new reference standards will be negligible from the point of view of the great majority of

users,- many countries are in any case constrained by

their existing legislation concerning physical quantities and units to use the SI names and symbols.

1.3. Concerning the practical implementation of the Recommendations 1 (CI-1988) and 2 (CI-1988)

The CCE having carefully considered the three ap- proaches listed in the reports of the Working Groups,

documents CCE/88-34 and CCE/88-35 2

, is of the opinion that a rigorous solution to this problem has been identified which avoids

(i) defining new units "V90" or "ft90'\

(ii) defining new physical quantities "£90" or "i?90 "

.

(iii) the use of subscripts or other distinguishing symbols of any sort on either unit symbols or quantity

symbols.

The preferred approach is indicated in the following

example of a statement that may be communicated to users of standard-cell calibration certificates

:

The measured emf, £, or electric potential differ- ence, U, of the unknown source may be rigorously

expressed in terms of the SI volt, V, as: £ = (1.018 xxx xx)V±e

The symbol e represents the total uncertainty, at the

level of one standard deviation, and is given by

£ = [(A£)2 +(£- S)

2 ]" 2

where A£ is the combined uncertainty in volts (at one

standard deviation) associated with the calibration it- self and with the realization of the Josephson-effect

reference standard at the particular national standards

laboratory, and 5 represents the relative uncertainty with which the ratio /Cj_ 90//C, is known. At present 8

is estimated to be 4 x 10" 7 (one standard deviation)

according to Recommendation 1 (CI-1988).

Since, by international agreement, 5 is common to

all laboratories, it may be indicated separately and the above expression for £ may be rewritten £ = (1.018 xxx xx) V±A£

for all practical purposes of precision electrical metrol- ogy and trade. However, when this is done, 8 should

Note to reader: these two Documents dealt with the Joseph- son effect and the quantum Hail effect, respectively.

appear separately on the certificate where the preci- sion is such as to require it. When A£/£ is significantly greater than 4 x 10" 7

, 8 may be omitted. The treatment of resistance measurements [Rec- ommendation 2 (CI-1988)] is strictly analogous.

1.4. Example of the wording to be used on calibration certificates The values of emf below are based on ... [a description of the calibration procedure may be placed

here] ... using the new conventional value of the Josephson constant internationally adopted for use from 1st January 1990 (see Note A).

cell number emf in volts uncertainty in volts

1 1.018 123 4 A£

[other data related to the calibration may be placed

here] Note A

The value of the Josephson constant used in this cali- bration is K,_ 90 = 483 597.9 GHz/V and is that adopted, by international agreement, for implementation on 1st January 1990 by all national standards

laboratories that use the Josephson effect as a refer- ence standard of the volt. By international agreement,

all such laboratories now use the same value of the Josephson constant whereas until recently they did

not. National standards laboratories of those countries that do not use the Josephson effect as a reference standard can maintain their own reference standards

so as to be consistent with the above value of the Josephson constant, by periodic comparisons with a laboratory that does use the Josephson effect. An ideal reference standard of emf based on the Josephson ef- fect and Kj_ 90 is consistent with the SI volt within an

assigned fractional one-standard-deviation uncertainty of 4 x 10" 7 (0.4 uV for an emf of one volt). Because this uncertainty is the same for all national standards laboratories, it has not oeen formally in- cluded in the uncertainties given in the table. However,

its existence must be recognized when the utmost

consistency between electrical and mechanical measurement is required.

Cornice Consultatif pour la Masse

et les grandeurs apparencies

3rd Meeting

The Comite Consultatif pour la Masse et les gran- deurs apparentees (CCM) met at the Pavilion de Breteuil on the 26th and 27th of May 1988. The President,

Prof. Bray, being indisposed the meeting was chaired by Dr. Giacomo, Director of the BIPM. The CCM

covers a wide range of activities in the fields of mass.
force and pressure standards and much of the meeting was devoted to the examination of reports drawn up

by the nine Working Groups. Most of these Working

Groups had met in the days immediately preceding the meeting of the CCM. The fields of work covered by

the Working Groups were: 1, the measurement of air density; 2, the conservation and behaviour of Pt/Ir mass standards; 3, the conservation and behaviour of

stainless-steel mass standards; 4, the measurement of

the density of liquids and solids; 5, force measurement;

6, 7, 8 and 9, the measurement of high, medium, low

and very low pressures, respectively. In many of these fields, in addition to the individual and collaborative projects under way, international comparisons had taken place or were in progress and

reports were presented. The CCM also discussed

recent advances in high-accuracy weighing and bal- ance design and the third verification of national prototypes of the kilogram now in its preliminary

stages at the BIPM. It is expected that the first part of

this operation, the comparison of the international prototype with its copies at the BIPM, will have been

completed by the end of 1988. Over the next three years or so, national prototypes will come to the BIPM in groups for verification. Since the last time such a large scale verification took place, some forty years ago, the accuracy of the best balances has im- proved by at least a factor of ten. The present verifi- cation is being carried out using the NBS-2 balance which is a single-pan, knife-edge balance that has an

accuracy of about 1 ug in the comparison of Pt/Ir mass

standards, i.e. a relative uncertainty at the level of one

standard deviation of about 1 part in 109

.

Comite Consultatif pour les Etalons de Mesure

des Rayonnements Ionisants

Section I (X-rays, y-rays and electrons) and Section III (Neutrons) of the Comite Consultatif pour

les Etalons de Mesure de Rayonnements Ionisants (CCEMRI) met from 8th to 11th and 18th to 20th of April respectively. During the meeting of Section I, the work of the BIPM was reviewed and the results of

recent comparisons of national standards were dis- cussed. The need for comparisons of absorbed-dose

standards for radiation of energy above 1 MeV was

stressed. Two Recommendations were adopted and

sent to the CCEMRI for approval at its next meeting. At the meeting of Section III, the BIPM work on

neutron measurements was reviewed. The published

results of completed neutron-emission-rate and flu- ence-rate comparisons, the NPL neutron-dosimetry

comparison and the preliminary report on the BIPM

neutron-dosimetry comparison were all discussed.

Work in the BIPM Laboratories

Among the principal activities in the laboratories of the BIPM over the past year have been the following:

- Electricity section : a limited comparison of 1 Q

national reference standards of resistance was carried out to check the consistency among the various real- izations of the quantum Hall effect now being made in

1 1 national laboratories and the BIPM. Expressed in terms of the BIPM's representation of the ohm,

J2 69 _ BI , on the central date of the comparison, the measured results for the quantized Hall resistance in

five of the six laboratories claiming uncertainties of 4 parts in 108 or less are within a range of 7 parts in 108

. The overall spread of all results was 6 parts in 107

. Refinements in the BIPM quantum-Hall-effect system, based upon a cryogenic current comparator, now al- low Q69 -si to be monitored with an uncertainty of 1.5 parts in 108

. Using Josephson junctions made from the new

high-Tc superconducting material a measurement of 2e/h was made having an uncertainty of 3 parts in 106 and which differed from the value obtained from a

metallic Josephson junction by about 5 parts in 106.

- Mass section: as already mentioned, work began on the third international verification of national prototypes of the kilogram. The BIPM flexure-strip bal- ance has been used to carry out an experiment to search for a nuclear-isospin-coupled "fifth-force". No

evidence for the existence of such a force was found, but during the experiment weighings of 2.3 kg masses were carried out that showed remarkably small stan- dard deviations of about 5 parts in 10 12

.

-Time section: as part of the regular analysis of clock comparisons made using the GPS satellites which go to make up International Atomic Time (TAI)

and Universal Coordinated Time (UTC), it was found

that for laboratories within about 1000 km of each

other errors in coordinates could be discerned and

corrected for with an accuracy of about 1 5 cm in the geocentric coordinates x, y and z. As a result, time comparisons between these (European) laboratories can now be made to within a few nanoseconds.

- Laser section: important international comparisons of laser wavelengths and frequencies have been

carried out between a total of 8 countries, mostly at wavelengths in the visible (A = 633 mm) but also one in the infrared (X = 3.39 urn). Uncertainties in these comparisons are at level of about 1 part in 10 11 and differ- ences between lasers from participating countries were

found, in many cases, to be hardly greater than this.
- Ionizing radiations section : participation in the

international comparison of the measurements of activity of iodine- 125 has begun. Two methods of spec- trum analysis are being used in order to understand

better the results obtained earlier during a limited comparison of measurement of activity of the same

nuclide. The long-term work on counting methods

and counting statistics continues with much emphasis now being placed on the method of 'generalized dead

times'. - Radiometry: new work has begun in radiometry.

Equipment has been built and is now in operation for the comparison of calibrations of silicon diodes and

thermopiles. The method of self-calibration of silicon diodes has been used and experience is being gained so

that international comparisons and, in due course,

calibrations can be carried out at the BIPM.

Other News

During the meeting of the CIPM in October 1988

a new building (of some 900 m

2 in surface) was inaugurated. It will house a library and offices for scientific

staff, the secretariat and the Director. This completes

the second stage of a long-term plan for buildings at the BIPM, the first stage of which was a new laboratory building for laser work opened in 1984. On 31st July 1988 Dr. P. Giacomo, who had been

Director of the BIPM since 1978, retired and was suc- ceeded by Dr. T.J. Quinn, previously Deputy Director.

Publications

Since October 1987 the BIPM has published:

18e Conference Generate des Poids et Mesures

(1987), Comptes Rendus, 108 pages. Proces-verbaux des seances du Comite International des Poids et Mesures, tome 55 (76e session, octobre

1987), 178 pages. Comite Consultatif de Photometrie et Radiometrie,

ll e session (1986), 183 pages. Comite Consultatif de Thermometrie, 16e session

(1987), 162 pages. Rapport Annuel du BIHpour 1987 (about 72 pages

covering the contribution of the BIPM on time scales). Circulaire D du BIH (monthly) (contribution of

the BIPM on time scales). Circulaire T (monthly), circular of the BIPM

taking over the part "Temps" of the circular D of the BIH (Circular Tl for January 1988 was published on March 1st, 1988). Le BIPM et la Convention du Metre, illustrated brochure in French and in English, 47 pages, widely

circulated through various national laboratories. One should also mention some 40 articles published by staff members in scientific journals and a dozen BIPM reports; the complete list of these publi- cations may be found in the Proces-Verbaux du

CIPM, 77th meeting (1 988), which will appear in 1 989. Copies of these documents may be obtained upon

application to: Mr le Directeur du BIPM, Pavilion de

Breteuil, F-92312 Sevres Cedex, or from the Librairie

Offilib, 48 rue Gay-Lussac, F-75240 Paris Cedex 05.
6.1.3 Three recommendations were adopted by the CCT at its 17th Session [14].

These recommendations were considered by the CIPM and Recommendation Tl (1989)

of the CCT was adopted as Recommendation 5 (CI -89) of the CIPM. Recommendations

T2 (1989) and T3 (1989) of the CCT were noted by the CIPM as CCT recommendations.

The CCT recommendations were as follows:

6.1.3.1 Recommendation Tl (1989)

The International Temperature Scale of 1990

The Comite Consultatif de Thermometrie (CCT) acting in accordance with

Resolution 7 of the 18e CGPM has generated the International Temperature Scale

of 1990 (ITS -90) in order to supersede the International Practical Temperature

Scale of 1968 (IPTS-68).

The CCT notes that, by comparison with the IPTS-68, the ITS -90

- extends to lower temperatures, down to 0.65 K, and hence also supersedes

the EPT-76,

is in substantially better agreement with corresponding thermodynamic

temperatures

has much improved continuity, precision and reproducibility throughout

its range and

- has subranges and alternative definitions in certain ranges which greatly

facilitate its use. The CCT also notes that, to accompany the text of the ITS -90 there will be

two further documents, the Supplementary Information for the ITS -90 and

Techniques for Approximating the ITS -90. These documents will be published by

the BIPM and periodically updated.

The CCT recommends

that on 1 January 1990 the ITS -90 come into force and

- that from this same date the IPTS-68 and the EPT-76 be abrogated.

106
6.1.3.2 Recommendation T2 (1989)

Reference tables for thermocouples and industrial platinum resistance

thermometers

The Comite Consultatif de Thermometrie,

considering

that the introduction of the International Temperature Scale of 1990

(ITS-90) will lead to an urgent requirement for new reference tables for both

thermocouples and industrial platinum resistance thermometers,

requests its Working Group 2 to collaborate with national laboratories in the rapid preparation of

new reference tables taking into account not only the change from IPTS-68 to

ITS-90 but also new information on the behavior of thermocouples and industrial

platinum resistance thermometers,

recommends

that these new tables be used as the basis for new national and

international reference tables for thermocouples and industrial platinum

resistance thermometers and

that meanwhile the existing reference tables based upon IPTS-68 should be

used in conjunction with the table of differences r90 - T68 which appears in the

ITS-90.

6.1.3.3 Recommendation T3 (1989)

The uncertainty inherent in the realization of the International

Temperature Scale of 1990

The Comite Consultatif de Thermometrie,

considering the requirement for assigning an uncertainty to the numerical

value of any temperature on the International Temperature Scale of 1990 (ITS-90),

encourages national laboratories to

a) quantify the uncertainties in the fixed point realizations,

b) quantify the uncertainties resulting from the use of the specified

interpolating instruments of ITS-90,

c) develop the mathematical procedures describing the propagation of these

uncertainties to any intermediate temperature

.

107
6.1.4 Reproduction of article by B. W. Mangum, entitled "Special Report on the

International Temperature Scale of 1990; Report on the 17th Session of the

Consultative Committee on Thermometry," and published in J. Res. Natl. Inst.

Stand. Technol. 95, 69-77 (1990).

108
Volume 95, Number 1, January-February 1990 Journal of Research of the National Institute of Standards and Technology

[J. Res. Natl. Inst. Stand. Technol. 95, 69 (1990)]

Special Report on the International Temperature Scale of 1990

Report on the 1 7th Session of the

Consultative Committee on Thermometry

Volume 95 Number 1 January-February 1990

B. W. Mangum

National Institute of Standards

and Technology,

Gaithersburg, MD 20899

This article summarizes the results of

the 17th Session of the Consultative Committee on Thermometry of the In- ternational Committee of Weights and

Measures (Comite Consultatif de Ther- mometrie of the Comite International des Poids et Mesures) that met in Sevres, France, September 12-14, 1989. That session was devoted exclusively to the completion of the International Temperature Scale of 1990, described

herein, and to the implications of its adoption.

Key words: CCT; Comite Consultatif de Thermometrie; Consultative Committee

on Thermometry; International Temperature Scale of 1990; temperature; temperature scale; thermodynamic temperature; thermometry.

Accepted: November 4, 1989

1. Introduction

The Consultative Committee on Thermometry

(Comite Consultatif de Thermometrie, CCT) is one

of eight Consultative Committees (Comites Consultants) of the International Committee of

Weights and Measures (Comite International des

Poids et Mesures, CIPM). The CIPM is a committee of the General Conference of Weights and

Measures (Conference Generale des Poids et Mesures, CGPM). The eight Consultative Committees (Comites Consultatifs) of the CIPM are:

1. The Comite Consultatif d'Electricite (CCE),

established in 1927,

2. The Comite Consultatif de Photometrie et Radiometric (CCPR), assigned this name in 1971; the previous name was the Comite Consultatif de

Photometrie, established in 1933,

3. The Comite Consultatif de Thermometrie

(CCT), established in 1937,

4. The Comite Consultatif pour la Definition du

Metre (CCDM), established in 1952,

5. The Comite Consultatif pour la Definition de

la Seconde (CCDS), established in 1956,

6. The Comite Consultatif pour les Etalons de

Mesure des Rayonnements Ionisants (CCEMRI),

established in 1958,

7. The Comite Consultatif des Unites (CCU), es- tablished in 1964, and

8. The Comite Consultatif pour la Masse et les grandeurs apparentees (CCM), established in 1980. The CCT is composed presently of members

from the following laboratories:

1. Amt fur Standardisierung, Messwesen und

Warenprufung [ASMW], Berlin, DDR,

2. Bureau National de Metrologie, Paris, France: Institut National de Metrologie, [INM] du

Conservatoire National des Arts et Metiers,

3. Ceskoslovensky Metrologicky Ustav [CSMU],

Bratislava, Czechoslovakia,

4. National Research Council [NRC], Ottawa,

Canada,

69
Volume 95, Number 1, January-February 1990 Journal of Research of the National Institute of Standards and Technology

5. CSIRO, Division of Applied Physics

[CSIRO], Lindfield, Australia,

6. D. I. Mendeleyev Institute for Metrology

[VNIIM], Leningrad, USSR; Physico-Technical

and Radio-Technical Measurements Institute (PRMI), Moscow, USSR,

7. National Institute of Metrology [NIM], Beijing, PRC,

8. Istituto di Metrologia G. Colonnetti [IMGC],

Turin, Italy

9. Kamerlingh Onnes Laboratorium [KOL],

Leiden, The Netherlands,

10. Korea Standards Research Institute [KSRI],

Seoul, Korea,

11. National Institute of Standards and Technology [NIST], Gaithersburg, MD, USA

12. National Physical Laboratory [NPL], Teddington, UK,

13. National Research Laboratory of Metrology

[NRLM], Ibaraki, Japan,

14. Physikalisch-Technische Bundesanstalt [PTB],

Braunschweig, FRG,

15. Van Swinden Laboratorium [VSL], Delft,

The Netherlands,

16. Iowa State University, Ames, Iowa, USA,

and

17. Bureau International des Poids et Mesures

[BIPM], Sevres, France. The CCT met September 12-14, 1989 at the Bureau International des Poids et Mesures (BIPM) in

its 17th Session [1] and completed the final details of the new temperature scale, the International

Temperature Scale of 1990 (ITS-90) [2]. The CCT

then recommended to the CIPM at its meeting on

September 26-28, 1989 at the BIPM that the ITS90 be adopted and made the official scale. The

CIPM did adopt the new temperature scale at their meeting [3] and the ITS-90 became the official in- ternational temperature scale on January 1, 1990,

the same date on which changes affecting certain

electrical reference standards were implemented

[4]. The ITS-90 supersedes the International Practical Temperature Scale of 1968, Amended Edition

of 1975 [IPTS-68(75)] [5] and the 1976 Provisional

0.5 K to 30 K Temperature Scale (EPT-76) [6]. The CCT undertook the development of the

ITS-90 because of the deficiencies and limitations

of the IPTS-68(75) and completed the scale in ac- cordance with Resolution 7 of the 18th Conference

Generate des Poid et Mesures [7], which met in October 1987. The deficiencies and limitations of

the IPTS-68(75) included its lower limit of 13.81 K,

its inaccuracy relative to thermodynamic temperatures, and its non-uniqueness and irreproducibility,

especially in the temperature region from

r68 = 903.89 K (r68 = 630.74 °C) to T58 = 1337.58 K

(t6S = 1064.43 °C), the region in which the Pt- 10%Rh/Pt thermocouple was the standard inter- polating instrument.

The ITS-90 extends upward from 0.65 K and

temperatures on this scale are in much better agree- ment with thermodynamic values than are those on

the IPTS-68(75) and the EPT-76. The new scale has subranges and alternative definitions in certain

ranges that greatly facilitate its use. Furthermore,

its continuity, precision, and reproducibility

throughout its range are much improved over the corresponding characteristics of the previous

scales. The replacement of the thermocouple with

the platinum resistance thermometer at temperatures in the range from 630.74 to 961.93 °C of the

IPTS-68(75) resulted in the biggest improvement in reproducibility. Also, improvements in radiometric

techniques have allowed using the silver freezing

point as the reference point for radiation ther- mometry. This is a lower temperature reference

point than was used in the IPTS-68(75).

The change in the temperature scale affects not only technical interests involved directly in ther- mometry but also those involved with other refer- ence standards, such as electrical standards, if those

standards are sensitive to temperature. As examples, standard resistors and standard cells are sensi- tive to temperature and generally are maintained in constant-temperature environments, at least in national standards laboratories. At the present time,

the temperatures of those environments are nor- mally determined with thermometers that have

been calibrated on the IPTS-68(75). A given ther- modynamic temperature expressed on the ITS-90,

however, has a value that is different from that ex- pressed on the IPTS-68(75), as indicated [2,8] in figure 1. A table of differences between temperatures on the ITS-90, Tw or t90, and those on the

IPTS-68(75), Tb& or tbS , and those on the EPT-76,

Tlb, is given in the text of the ITS-90. Since temperature values expressed on these scales are different,

if the temperature of the environment of a refer- ence standard is adjusted so that its value when ex- pressed on the ITS-90 has the same value as had been

used on the IPTS-68(75), there will have been a change of the thermodynamic temperature and the

value of the reference standard will usually change.

Of course, one may not want to change the ther- modynamic temperature of the reference standard.

In that case, the thermodynamic temperature, as expressed on the IPTS-68(75), can simply be ex- pressed on the ITS-90 (a numerical value different

70
Volume 95, Number 1, January-February 1990 Journal of Research of the National Institute of Standards and Technology

0.02

o

00 -0.02

1O

en -0.04

©

ocow

0) 4

«*- * o

«b.3

<0«

Q.E -0.2

H1 1 r-" 1 1 i

— 11

1

1

1

~*

•

1

i -200 /

t / /

/

200 f/°C

»

<

) 1 100 X

i 1 I.I.I

-10 mX

-20mK

\

\

\

1 ' 1 ..

- 0.4

- 0.2

--0.2

200 200 400

r/°C

600 800 1000

Figure 1. Differences between rTO and r6s as a function of fTO (expressed as t).

from that on the IPTS-68(75)) and the reference

standards will be unaffected. For more details on

the effects of the change of the temperature scale on electrical standards, see National Institute of

Standards and Technology (NIST) Technical Note

1263 [4]. In addition to the effect on reference standards

for measurements, all temperature-sensitive properties that are presently expressed on the IPTS-68(75)

may be affected and may require changes in values. For details on realizations and approximations of

the ITS-90, see NIST Technical Note 1265 [9].

2. CCT's 17th Session Principal Decisions

2.1 Definition of the ITS-90

The ITS-90 was designed by the CCT in such a manner that temperature values obtained on it do

not deviate from the Kelvin thermodynamic temperature values by more than the uncertainties of

the latter values at the time the ITS-90 was

adopted. Thermodynamic temperature is indicated

by the symbol T and has the unit known as the

kelvin, symbol K. The size of the kelvin is defined

to be 1/273.16 of the thermodynamic temperature

of the triple point of water.

Temperatures on the ITS-90 can be expressed in terms of the International Kelvin Temperatures,

with the symbol T90 , or in terms of the International Celsius Temperatures, with the symbol t90. The unit of the temperature T90 is the kelvin, symbol K, and the unit of the temperature t90 is the

degree Celsius, symbol °C. The relation between

T90 and t90 is:

r90/°C=7VK-273.15. (1)

The ITS-90 has alternative definitions of T90 in certain temperature ranges and they have equal

status. In measurements of the highest precision

made at the same temperature, the alternative defi- nitions may yield detectably different temperature

values. Also, at any given temperature between

defining fixed points, different interpolating ther- mometers that meet the specifications of the ITS90 may indicate different temperature values. The

magnitude of the differences resulting from these

two sources, however, is sufficiently small to be

negligible for all practical purposes. Temperatures on the ITS-90 are defined in terms

of equilibrium states of pure substances (defining

fixed points), interpolating instruments, and equations that relate the measured property to r90. The

defining equilibrium states of the pure substances

and the assigned temperatures are listed in table 1.

71
Volume 95, Number 1, January-February 1990 Journal of Research of the National Institute of Standards and Technology

Table 1. Defining fixed points of the ITS-90 Table 2. Values of the coefficients Ait and of the constants Aa, B, and C for the 3 He and 4 He vapor pressure temperature range for which each equation is

equations and the Material3 Equilibrium state

b Tempierature valid

r*, (K) '90 (°C) Coef. or constant

3 He

0.65 to 3.2 K

4He

1.25 to 2.1768 K

4He

3 He and 4He VP 3 to 5 -270.15 to 2.1768 to 5.0 K

TP 13.8033

-268.15 e-H2 -259.3467 A 1.053 447 1.392 408 3.146 631 e-H2 (or He) VP(or CVGT) = 17 = -256.15 As 0.980 106 0.527 153 1.357 655 e-H2 (or He) VP (or CVGT) = 20.3 = -252.85 A2 0.676 380 0.166 756 0.413 923 Nec TP 24.5561 -248.5939 o2 TP 54.3584 -218.7916 A, 0.372 692 0.050 988 0.091 159 Ar TP 83.8058 -189.3442 A* 0.151 656 0.026 514 0.016 349 Hgc TP 234.3156 -38.8344 A, -0.002 263 0.001 975 0.001 826 H2 TP 273.16 0.01 Gac MP 302.9146 29.7646 A6 0.006 596 -0.017 976 -0.004 325

Inc FP 429.7485 156.5985 An 0.088 966 0.005 409 -0.004 973 Sn FP 505.078 231.928 A% -0.004 770 0.013 259 Zn FP 692.677 419.527 Alc FP 933.473 660.323 A, -0.054 943 Ag FP 1234.93 961.78 B 7.3 5.6 10.3 Au FP 1337.33 1064.18 C 4.3 2.9 1.9 Cuc FP 1357.77 1084.62

" e-H2 indicates equilibrium hydrogen, that is, hydrogen with

the equilibrium distribution of its ortho and para states. Normal

hydrogen at room temperture contains 25% para hydrogen and 75% ortho hydrogen.

b VP indicates vapor pressure point; CVGT indicates constant volume gas thermometer point; TP indicates triple point (equi- librium temperature at which the solid, liquid and vapor phases

coexist); FP indicates freezing point and MP indicates melting

point (the FP and the MP are equilibrium temperatures at which

the solid and liquid phases coexist under a pressure of 101 325

Pa, one standard atmosphere). The isotopic composition is that naturally occurring.

c Previously, these were secondary fixed points.

2.1.1 Temperature Range From 0.65 to 5.0 K

Between 0.65 and 3.2 K, the ITS-90 is defined by

the vapor pressure-temperature relation of 3 He, and

between 1.25 and 2.1768 K (the X point) and between 2.1768 and 5.0 K by the vapor pressure-temperature relations of 4He. T^ is defined by the

vapor pressure equations of the form:

value of which is determined by using either 3 He or

4He vapor pressure thermometry.

For a

4F£e CVGT used between 4.2 K and the

triple point of neon (24.5561 K), Too is defined by

the equation:

T90 =a+bp+cp: (3)

where p is the CVGT pressure and a , b , and c are

coefficients that are determined by calibration at the three specified temperatures, but with the additional requirement that the calibration with the

vapor pressure thermometer be made at a temperature between 4.2 and 5.0 K.

For a

4He CVGT used between 3.0 and 4.2 K,

and for a

3He CVGT used from 3.0 to 24.5561 K,

the non-ideality of the gas must be taken into ac- count, using the appropriate second virial coefficient, ^(Too) or BiiTw)- 7"

90 is defined in this range

by the equation:

T90/K=AQ +2 A{(\n(p/Y2L)-B)/C}\ (2)

J on —

bp+cp 1

with the values of the coefficients Ait and of the constants A , B, and C of the equations being specified, and given in table 2. 2.1.2 Temperature Range From 3.0 to 24.5561 K

Between 3.0 and 24.5561 K, the ITS-90 is defined

in terms of the 3 He or

4He constant volume gas thermometer (CVGT). The thermometer is cali- brated at three temperatures—at the triple point of neon (24.5561 K), at the triple point of equilibrium

hydrogen (see footnote a in table 1) (13.8033 K),

and at a temperature between 3.0 and 5.0 K, the

l+BAT^N/V (4)

where p is the CVGT pressure; a, b, and c are

coefficients that are determined from calibration at the three defining temperatures; BX {T<^) refers to B)(Tqo) or BtiTw); and N/V is the gas density in moles per cubic meter in the CVGT bulb. The val- ues of the second virial coefficients at any given

temperature are to be calculated according to equations specified in the official document of the ITS90 (and also in the NIST Technical Note 1265 [9]).

72
Volume 95, Number 1, January-February 1990 Journal of Research of the National Institute of Standards and Technology

2.1.3 Temperature Range From 13.8033 to 1234.93 K Between 13.8033 K (-259.3467 °C)

and 1234.93 K (961.78 °C), the ITS-90 is defined in terms of the specified fixed points given in table 1

,

by resistance ratios of platinum resistance ther- mometers (PRTs) obtained by calibration at specified sets of the fixed points, and by reference

functions and deviation functions of resistance ratios which relate to Tw between the fixed points.

Temperatures on the ITS-90 are expressed in terms of the ratio W(Tw) of the resistance R (T^) at temperature Tw and the resistance R (273.16 K) at the triple point of water, i.e., W(T90)=R(T90)/R(213.16 K). (5)

For a PRT to be an acceptable instrument of the

ITS-90, its coil must be made from pure platinum

and be strain-free. Additionally, the finished PRT

must meet one of the following criteria:

W(302.9146 K)> 1.118 07

^(234.3156 K) < 0.844 235.

(6)

(7) An acceptable PRT that is to be used to the freez- ing point of silver must meet the following requirement also:

^(1234.93 K) > 4.2844. (8)

The temperature T9Q is calculated from the resis- tance ratio relation: W(T90)-JVt {T90) = MV(T90), (9)

where W{T<^) is the observed value, Wr(r90) is the

value calculated from the reference function, and

AW(r9o) is the deviation of the observed W{T^

value of the particular PRT from the reference

function value at T^.

There are two reference functions Wr{T<^), one

for the range 13.8033 to 273.16 K and the second

for the range 273.15 to 1234.93 K. The deviation

A W^Tcio) is obtained as a function of Tn for various

ranges by calibration at specified fixed points. The

form of the deviation function depends upon the

temperature range of calibration.

2.1.4 Temperature Subrange From 13.8033 to 273.16 K In the range 13.8033 to 273.16 K, the

equation for the reference function Wt{T<^) as a function of T^ is given by:

ln[Wt(T90)]=AQ +2 /t{[ln(7V273.16 K)

1=1 + 1.5]/1.5}'. (10)

The specified inverse of this equation, equivalent to within ±0.000 1 K, is:

7V273.16 K=5, £ n /[^r(r90)]'

/6 -0.65

0.35

)'

(11)

The values of the constants A and B and values of

the coefficients At and Bt of the two equations are

listed in table 3.

Table 3. Values of the coefficients A,, Biy C,, and D, and of the constants A , B , Co, and A> in the reference functions, eqs (10) and (18), and in the inverse functions approximating them, given

by eqs (11) and (19)

Constant or Value Constant or Value

coefficient coefficient

A -2.135 347 29 Bo 0.183 324 722 Ax 3.183 247 20 fi, 0.240 975 303 A2 -1.801435 97 B2 0.209 108 771 A, 0.717 272 04 Bi 0.190 439 972

A, 0.503 440 27 B< 0.142 648 498 As -0.618 993 95 B, 0.077 993 465 A6 -0.053 323 22 B„ 0.012 475 611

A-, 0.280 213 62 Bi -0.032 267 127

At 0.107 152 24 B* -0.075 291 522 A, -0.293 028 65 5, -0.056 470 670

Aio 0.044 598 72 B\o 0.076 201 285 An 0.118 686 32 Bu 0.123 893 204

An -0.052 48134 Bn -0.029 201 193 Bn -0.091 173 542

B\A 0.001 317 696

5,5 0.026 025 526

Ca 2.781 572 54 Do 439.932 854

c, 1.646 509 16 £>! 472.418 020 c2 -0.137 143 90 D2 37.684 494 c3 -0.006 497 67 D, 7.472 018

c4 -0.002 344 44 Z>4 2.920 828 c5 0.005 118 68 A 0.005 184 c. 0.001 879 82 A, -0.963 864

c, -0.002 044 72 D, -0.188 732 c8 -0.000 461 22 D, 0.191 203 c, 0.000 457 24 D, 0.049 025

73
Volume 95, Number 1, January-February 1990 Journal of Research of the National Institute of Standards and Technology

If the PRT is to be used throughout the range

from 13.8033 to 273.16 K, it must be calibrated at the triple points of equilibrium hydrogen (13.8033

K), neon (24.5561 K), oxygen (54.3584 K), argon

(83.8058 K), mercury (234.3156 K), and water

(273.16 K) and at two additional temperatures very

close to 17.0 and 20.3 K. The temperatures of cali- bration at 17.0 and 20.3 K may be determined by

using either a CVGT or the vapor pressure-temperature relation of equilibrium hydrogen. When

the CVGT is used, the two temperatures must be

within the ranges 16.9 to 17.1 K and 20.2 to 20.4 K,

respectively. When the equilibrium hydrogen va- por pressure thermometer is used, the two temperatures must be within the ranges 17.025 to 17.045 K

and 20.26 to 20.28 K, respectively. The temperatures of the equilibrium hydrogen vapor pressure thermometer are determined from the values of the

hydrogen vapor pressure, p , and the equations:

7VK- 17.035=0? /kPa-33.3213)/13.32 (12)

7VK-20.27 = (p/kPa- 101.292)730. (13)

Depending upon the temperature range of application, PRTs may be calibrated from 273.16 K

down to 13.8033 K (triple point of equilibrium hydrogen), down to 24.5561 K (triple point of neon),

down to 54.3584 K (triple point of oxygen), or down to 83.8058 K (triple point of argon).

The deviation function for calibration in the

range 13.8033 to 273.16 K is given by the relation:

A^,(r90)= W(T90)- lVT (T90) =a,[W(T90)-l]

+ b ] [W(T90)-\]1 + 2cl [\nlV(T90)] i+n

, (14)

with n —2. The coefficients a u b u and the five c,'s of the deviation function are obtained by calibra- tion at all of the above eight temperatures, including that at the triple point of water. The values of

Wr (T90) are obtained from the reference function

for this range. Although the official text of the

ITS-90 does not assign subscripts to the coefficients a and b, nor does it designate the deviation equations by the symbols LWm{T9Q), where in eq (14)

m = 1, these designations will be used in this paper

for clarity and for ease of reference. In any case, some such terminology must be used in PRT cali- bration reports and this was chosen for convenience.

2.1.5 Subrange From 24.5561 to 273.16 K The

deviation function for calibration in this range is

given by the relation: &W2(T90)= W(T90)-Wt (T90) =a2 [1V(T90)- 1]

+ b2 [W(T90)-l]2 + ^ aWnWiTso)]'*", (15)

where the exponent n has the value n =0. The coefficients a2 , b2 , and c, of this deviation function are obtained by calibrating the PRT at the triple points

of equilibrium hydrogen (13.8033 K), neon

(24.5561 K), oxygen (54.3584 K), argon (83.8058

K), mercury (234.3156 K) and water (273.16 K).

The values of W^T^) are obtained from the refer- ence function.

2.1.6 Subrange From 54.3584 to 273.16 K The

deviation function for calibration in this range is given by the relation: A^3 (r9o)=a3[^(r9o)-i]+63 [^(r9o)-i]2

+c l [\nW(T9o)Y + ", (16)

where the exponent n has the value n = 1. The coefficients a3 , bi, and c, of this deviation function are obtained by calibrating the PRT at the triple points

of oxygen (54.3584 K), argon (83.8058 K), mercury

(234.3156 K), and water (273.16 K). The values of

fVr(Tgo) are obtained from the reference function.

2.1.7 Subrange From 83.8058 to 273.16 K The

deviation function for calibration in this range is given by the relation:

MV,(T90) = a,[W(T90)-\]

+ b,[W(T90)-l]\nW(T90). (17)

The coefficients a4 and bx of this deviation function

are obtained by calibrating the PRT at the triple

points of argon (83.8058 K), mercury (234.3156 K),

and water (273.16 K). The values of Wr {T9^) are obtained from the reference function.

2.1.8 Temperature Subrange From 273.15 to 1234.93 K In the range 273.15 to 1234.93 K, the

equation for the reference function W,{ T<^) is given

by:

Wr (T90) = C +^Ci 7VK48

754.15 V

1 )' (18)

The specified inverse of this equation, equivalent to within ±0.000 13 K, is: ^7*0-2-64

(19)

74
Volume 95, Number 1, January-February 1990 Journal of Research of the National Institute of Standards and Technology

The values of the constants C and D and of the

coefficients C, and Z>, for these equations are listed

in table 3. If the PRT is to be used over this entire subrange

(273.15 to 1234.93 K), it must be calibrated at the

triple point of water (273.16 K) and at the freezing

points of tin (505.078 K), zinc (692.677 K), alu- minum (933.473 K), and silver (1234.93 K).

The deviation function is given by the relation: MV6(T«>) = W(T90)- W! {T90) =ab[W{T9O)- 1] + b6 [W(T90)-\]2 +c6 [W(T90)-\]i

+d[W(T90)-lV(933A73K)]2

. (20)

The values of the coefficients ab, b6, and c6 are determined from the measured deviations &W(T90) of

WiTgo) from the reference values Wr (T90) at the

freezing points of tin (505.078 K), zinc (692.677 K)

and aluminum (933.473 K). The coefficient d is determined from these values of the coefficients a6, Z> 6, and c6 and the deviation kW(T90) of W(T90) from

the reference value WC (T90) at the freezing point of

silver. The coefficient d in this equation is used

only for those temperature measurements in the

range from the freezing point of aluminum to the

freezing point of silver. For temperature measure- ments below the freezing point of aluminum, d =0.

PRTs may be calibrated for use over the whole

range (273.15 to 1234.93 K) or for shorter ranges

by calibrations at fixed points between 273.15 K

and the upper limit of 933.473 K (freezing point of

aluminum, 660.323 °C), of 692.677 K (freezing

point of zinc, 419.527 °C), of 505.078 K (freezing

point of tin, 231.928 °C), of 429.7485 K (freezing

point of indium, 156.5985 °C), or of 302.9146 K

(melting point of gallium, 29.7646 °C).

2.1.9 Subrange From 273.15 to 933.473 K For

application in this range, the PRT is calibrated at the triple point of water (273.16 K), and at the

freezing points of tin (505.078 K), zinc (692.677 K),

and aluminum (933.473 K). The deviation function

is given by the relation: MV1 (T90) =a1 [W(T90)-l] + b1 [W(T90)-l]2

+Cl [W(T90)-l]\ (21)

The coefficients a-,, b-,, and c7 , identical to ab, bb , and

c6, respectively, are determined from the deviations

A^(r9o) of W(T90) from the reference values

WXT90) at the freezing points of tin (505.078 K),

zinc (692.677 K), and aluminum (933.473 K).

2.1.10 Subrange From 273.15 to 692.677 K

For application in this range, the PRT is calibrated

at the triple point of water (273.16 K), and at the

freezing points of tin (505.078 K) and zinc (692.677

K). The deviation function is given by the relation: MVi (T90) = as [W(T90)-l] + bi [W(T90)-\]2

. (22)

The coefficients a8 and 68 are determined from the

deviations A ^(7^) of WiT^) from the reference

values W^Tw) at the freezing points of tin (505.078

K) and zinc (692.677 K).

2.1.11 Subrange From 273.15 to 505.078 K

For application in this range, the PRT is calibrated

at the triple point of water (273.16 K), and at the

freezing points of indium (429.7485 K) and tin (505.078 K). The form of the deviation function is the same as that for the subrange 273.15 to 692.677

K, i.e., A^9(r90)=a9 [^(r90)-i]+69 [^(r90)-i]2

. (23)

The coefficients a9 and b9 are determined from the

deviations A.W(T90) of W(T90) from the reference

values Wr (T90) at the freezing points of indium

(429.7485 K) and tin (505.078 K).

2.1.12 Subrange From 273.15 to 429.7485 K

For application in this range, the PRT is calibrated

at the triple point of water (273.16 K) and at the

freezing point of indium (429.7485 K). The deviation function is: A^10(r90)=aio[^(r9o)-i]. (24)

The coefficient a w is determined from the deviation

&.W(T90) of W(T9Q) from the reference value

Wr (T90) at the freezing point of indium (429.7485

K).

2.1.13 Subrange From 273.15 to 302.9146 K

For application in this range, the PRT is calibrated

at the triple point of water (273.16 K) and at the

melting point of gallium (302.9146 K). The deviation function is: MVn(T90)=an[W(T90)-l]. (25)

The coefficient a n is determined from the deviation

AJf(r90) of W(T90) from the reference value

Wr (Tgo) at the melting point of gallium (302.9146

K).

2.1.14 Subrange From 234.3156 to 302.9146 K

For application in this range, the PRT is calibrated

at the triple points of mercury (234.3156 K) and

water (273.16 K), and at the melting point of gal75
Volume 95, Number 1, January-February 1990 Journal of Research of the National Institute of Standards and Technology

Hum (302.9146 K). The form of the deviation function is the same as that for the subrange 273.15 to 692.677 K, i.e., ^W5 (T90)=a5 [W(T90)-l] +bs [W(T90)-l]2

. (26)

The coefficients a5 and b5 are determined from the

deviations LW{T^ of W{T<^) from the reference

values Wr (Tgo) at the triple point of mercury

(234.3156 K) and at the melting point of gallium

(302.9146 K). The reference values Wt(T^) must be

calculated from the relevant reference function,

both reference functions being required to cover

this range.

2.1.15 Temperature Range Above 1234.93 K

At temperatures above 1234.93 K, T90 is defined by

the relation: Lx(Tn) _exp[c2/\rqo(X)] - 1 (27)

in which Lx(7ro) and Lx[T90(X)] are the spectral

concentrations of the radiance of a blackbody at wavelength X (in vacuum) at T^ and at T^X), re- spectively. r^X) refers to either the silver freezing

point [T90(Ag) = 1234.93 K], the gold freezing point

[7'9 (Au)= 1337.33 K] or the copper freezing point

[r9o(Cu)= 1357.77 K]. c2 =0.014388 m-K. Although

the freezing-point temperature of silver is the junction point of platinum resistance thermometry and

radiation thermometry, it is believed that the Tw

values of the freezing points of silver, gold and

copper are sufficiently self-consistent that the use of any one of them as the reference temperature

F9o(X) will not result in any significant difference in the measured values of T90 from what would be

obtained if only the silver freezing point were used.

2.2 Recommendations of the CCT

Three recommendations were adopted by the

CCT at its 17th Session. These recommendations were considered by the CIPM and Recommendation Tl (1989) of the CCT was adopted as Recommendation 5 (CI-89) of the CIPM. Recommendations T2 (1989) and T3 (1989) of the CCT

were noted by the CIPM as CCT recommendations. The CCT recommendations were as follows:

Recommendation Tl (1989)

The International Temperature Scale of 1990

The Comite Consultatif de Thermometrie (CCT)

acting in accordance with Resolution 7 of the 18e

CGPM has generated the International Temperature Scale of 1990 (ITS-90) in order to supersede

the International Practical Temperature Scale of

1968 (IPTS-68).

The CCT notes that, by comparison with the

IPTS-68, the ITS-90 —extends to lower temperatures, down to 0.65 K, and hence also supersedes the EPT-76, —is in substantially better agreement with corre- sponding thermodynamic temperatures, —has much improved continuity, precision, and

reproducibility throughout its range and —has subranges and alternative definitions in certain ranges which greatly facilitate its use. The CCT also notes that, to accompany the text of the ITS-90 there will be two further documents,

the Supplementary Information for the ITS-90 and

Techniques for Approximating the ITS-90. These

documents will be published by the BIPM and periodically updated.

The CCT recommends —that on January 1, 1990 the ITS-90 come into

force and —that from this same date the IPTS-68 and the

EPT-76 be abrogated.

Recommendation T2 (1989)

Reference Tables for Thermocouples and Industrial

Platinum Resistance Thermometers

The Comite Consultatif de Thermometrie,

considering —that the introduction of the International Temperature Scale of 1990 (ITS-90) will lead to an ur- gent requirement for new reference tables for both

thermocouples and industrial platinum resistance

thermometers,

requests its Working Group 2 —to collaborate with national laboratories in the

rapid preparation of new reference tables taking

into account not only the change from IPTS-68 to ITS-90 but also new information on the behavior of

thermocouples and industrial platinum resistance

thermometers,

recommends —that these new tables be used as the basis for new national and international reference tables for thermocouples and industrial platinum resistance

thermometers and —that meanwhile the existing reference tables based upon IPTS-68 should be used in conjunction

with the table of differences Tw — TbS which ap- pears in the ITS-90.

76
Volume 95, Number 1, January-February 1990 Journal of Research of the National Institute of Standards and Technology

(Note: the table of differences T^— Tbi referred to here may be obtained also from NIST from the

author of this article).

Recommendation T3 (1989)

The Uncertainty Inherent in the Realization of the International Temperature Scale of 1990

The Comite Consultatif de Thermometrie,

considering the requirement for assigning an uncertainty to the numerical value of any temperature on the International Temperature Scale of 1990

(ITS-90),

encourages national laboratories to

a) quantify the uncertainties in the fixed point

realizations,

b) quantify the uncertainties resulting from the use of the specified interpolating instruments of

ITS-90,

c) develop the mathematical procedures describing the propagation of these uncertainties to any

intermediate temperature.

3. Conclusion

About the author: B. W. Mangum, a physicist, is a Group Leader in the Temperature and Pressure Division of the NIST Center for Chemical Technology

and is the NIST representative to the Comite Consultatif de Thermometrie.

4. References

[1] BIPM Com. Cons. Thermometrie 17, 1989, in press.

[2] The International Temperature Scale of 1990, Metrologia

27, 3 (1990).

[3] Proces-Verbaux des seances du Comite International des Poids et Mesures, (78

= session, octobre 1989), in press.

[4] Belecki, N. B., Dziuba, R. F., Field, B. F., and Taylor,

B. N., Guidelines for Implementing the New Representations of the Volt and Ohm Effective January 1, 1990, NIST

Technical Note 1263 (June 1989).

[5] The International Temperature Scale of 1968, Amended

Edition of 1975, Metrologia 12, 7 (1976).

[6] The 1976 Provisional 0.5 K to 30 K Temperature Scale, Metrologia 15, 65 (1979).

[7] Comptes rendus des seances de la dix-huitieme Conference

Generate des Poids et Mesures, Resolution 7 (1987).

[8] Quinn, T. J., Metrologia 26, 69 (1989).

[9] Mangum, B. W., and Furukawa, G. T., Guidelines for Realizing the International Temperature Scale of 1990, NIST

Technical Note 1265 (1990), in press.

Although the uncertainties in the values of ther- modynamic temperatures above 100 °C used in the

definition of the ITS-90 were larger than desired

and larger than had been anticipated a few years

ago, the agreement of temperatures on the ITS-90

with thermodynamic temperatures is nevertheless a

significant improvement over that of previous

scales. The replacement of the thermocouple with

the platinum resistance thermometer as the stan- dard instrument of the scale at temperatures in the

IPTS-68(75) range from 630.74 to 961.93 °C has

improved the reproducibility over that of the

IPTS-68(75) significantly. Also, advances in radiometric techniques have improved the precision of

measurements in radiation thermometry. The precision of the scale, or what has been called the nonuniqueness of the scale, is significantly improved

over that of the IPTS-68(75), as is also the scale's continuity. The extension of the scale downward in temperature to 0.65 K and the use of subranges over which thermometers may be calibrated make

the ITS-90 more useful and much more flexible than were the previous scales.

77
6.2 In this appendix, excerpts from proceedings of some meetings of the CIPM

and of the CGPM that are related to the designation of the size of the kelvin

and to the ITS -90 are presented.

6.2.1 The following is an excerpt from Proces-Verbaux des Seances, Deuxieme

Series, Tome XXIV, pp. 81-82, Session de 1954 du Comite International des Poids

et Mesures, published by Gauthier Villars, Editeur-Imprimeur-Libraire, Paris,

1955, concerning the recommendation of the CIPM that the numerical value of

273.16 °K, exactly, be assigned to the triple point of water on the thermodynamic

scale

"Le Comite International des Poids et Mesures recommande que l'on ddfinisse

desormais 1'echelle thermodynamique au moyen du point triple de l'eau comme point

fixe fondamental .

"II recommande que l'on attribue par definition A la temperature du point

triple de l'eau dans 1'echelle thermodynamique la valeur numerique 273 ,16°

exactement .

6.2.2 The following is an excerpt from Comptes Rendus des Seances, Dixieme

Conference Generale des Poids et Mesures, Reunie a Paris en 1954, p. 79, published

by Gauthier Villars, Editeur-Imprimeur-Libraire, Editeur du Bureau International

des Poids et Mesures, Paris, 1955, concerning the definition of the thermodynamic

scale of temperature. This entails the assignment of the value 273.16 °K,

exactly, to the triple point of water as a fundamental fixed point.

DEFINITION DE L'ECHELLE THERMODYNAMIQUE DE TEMPERATURE.

RESOLUTION 3

"La Dixieme Conference Generale des Poids et Mesures decide de definir

1'echelle thermodynamique de temperature au moyen du point triple de l'eau comme point fixe fondamental, en lui attribuant la temperature 273,16 degres Kelvin,

exac t ement .

118
6.2.3 The following recommendations of the CCT, concerning the development of

the ITS-90 and simplified methods of its realization, that were presented to the

CIPM on 9 July 1987 were excerpted from Proces-Verbaux des Seances du Comite

International des Poids et Mesures, Proces-Verbaux de la 76e session - 1987, Tome

55, pp. T7-T8, Edite par le BIPM, Pavilion de Breteuil, F-92312 Sevres Cedex,

France

.

9 juillet 1987

Rec ommandat i ons du Comite Consultatif de Thermometrie

presentees

au Comite International des Poids et Mesures

Necessite de travaux fondamentaux en thermometrie

RECOMMANDATION T 1 (1987)

Le Comite Consultatif de Thermometrie,

tout en dSclarant que la valeur T90 de la temperature mesuree dans l'Echelle

internationale de temperature de 1990 (EIT-90) sera aussi voisine de la

temperature thermodynamique T que le permettent les connaissances actuelles,

reconnalt — que 1' adoption de 1' EIT-90 ne signifie en aucune facon que nous ayons

maintenant une connaissance definitive de la valeur des temperatures

thermodynamiques

,

— que le travail necessaire pour developper cette connaissance fondamentale

par des determinations directes des temperatures thermodynamiques est difficile,

complexe et de longue haleine, — qu'il est necessaire d'anticiper les besoins a venir d' exactitude et

de precision croissantes,

propose, en consequence, de surveiller en permanence 1' accord entre T90 et

T au fur et a mesure des progres de la science,

insiste sur 1' importance de 1' etude et de la mise en uvre de methodes

nouvelles conduisant a des determinations plus exactes des temperatures

thermodynamiques, dans tout le domaine couvert par 1' EIT-90 et aussi vers les

tres basses temperatures jusqu'a quelques millikelvins

,

et insiste sur 1' importance de la poursuite des travaux des laboratoires

nationaux dans ce but.

119
Methodes simplifiees et economiques de mesure des temperatures

RECOMMANDATION T 2 (1987)

Le Comite Consul tatif de Thermometrie,

tout en reconnaissant que sa tache princlpale est de veiller a ameliorer

la misc en pratique et a mettre a jour periodiquement l'Echelle internationale

de temperature, s'est efforce dans les annees recentes de repondre aux demandes

des chercheurs , ingenieurs et autres utllisateurs qui ont besoin de bonnes

mesures de temperature plutot que de mesures de temperature du niveau le plus

eleve,

a note

- les travaux recents sur les thermocouples or/platine et

palladium/platine indiquant que ces thermocouples peuvent presenter des avantages

appreciables , pour de nombreux usages, par rapport aux thermocouples habituels

platine/platine rhodie.

En consequence, le Comite Consul tatif de Thermometrie,

reconunande aux laboratoires nationaux de poursuivre leurs recherches sur

les methodes simplifiees et de cout reduit de mesure des temperatures, en particulier sur ces nouveaux thermocouples pour determiner leur fonction ainsi

que les regies pratiques de construction et d' utilisation qui devraient permettre

d' exploiter au mieux leurs possibilites

.

120
6.2.4 The following resolution, instructing the CIPM to develop the ITS -90, was adopted by the 18th CGPM in 1987, and is excerpted from Comptes Rendus , 18e Conference Generale des Poids et Mesures (1987), p. 101, Edite par le BIPM,

Pavilion de Breteuil, F-92312 Sevres Cedex, France.

Mise au point d'une nouvelle echelle international de temperature

RESOLUTION 7

La Dix-huitieme Conference Generale des Poids et Mesures,

considerant

que 1'uniformite mondiale et la stabilite a long terme des mesures de

temperature sont d'une importance majeure pour la science, le commerce et

l'industrie, du point de vue technique comme du point' de vue economique,

etro

que la valeur de la temperature dans 1' echelle pratique doit correspondre

itement a la valeur de la temperature thermodynamique dans le SI,

que la Quinzieme Conference Generale des Poids et Mesures, ayant pris

connaissance des ecarts qui ont ete mis en evidence entre la valeur de la

temperature dans 1' Echelle internationale pratique de temperature de 1968

(EIPT-68) et la valeur correspondante de la temperature thermodynamique, a charge

le Comite International des Poids et Mesures d'etablir une nouvelle echelle qui

devrait etre sensiblement plus exacte,

qu'une nouvelle echelle pourra etre proposee en 1989, echelle telle que

les ecarts residuels avec la temperature thermodynamique seront negligeables

pendant de nombreuses annees a venir,

invite le Comite International des Poids et Mesures et les laboratoires

nationaux a achever la mise au point de cette nouvelle echelle de temperature

et a fixer une date pour sa mise en application simultanement dans tous les pays;

la valeur des ecarts entre cette nouvelle echelle et 1' EIPT-68 devrait etre

annoncee au mo ins un an avant 1' adoption de la nouvelle echelle qui pourrait

intervenir au l

er Janvier 1990.

121
6.3 We present here examples of some typical Report of Calibration documents

from the NIST Calibration Laboratories for thermometers calibrated over various

range

.

6.3.1 The following report is an example of a Report of Calibration for an SPRT

calibrated over the range from 13.8033 K to 505.078 K.

122
UNITED STATES DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

NATIONAL MEASUREMENT LABORATORY

GAITHERSBURG, MARYLAND 20899

REPORT OF CALIBRATION

International Temperature Scale of 1990

Platinum Resistance Thermometer

LAN Model 8164

Serial Number 1812284

Submitted by

National Institute of Standards and Technology

Gaithersburg, Maryland

This thermometer was calibrated with an AC bridge at a frequency of 30 Hz using a continuous measuring current of 1.0 mA In accordance with the International Temperature Scale of 1990 (ITS-90) that was officially adopted by

the Comite* International des Poids et Mesures (CIPM) in September 1989, the subranges from 13.8033 K to 273.16 K

and 273.15 K to 505.078 K, with the following fixed points and their stated uncertainties, were used to calibrate the thermometer:

Fixed Point

H2 TP

17 K VP

20 K BP

Ne TP

2 TP

Ar TP

Hg TP

H2 TP

In FP

Sn FP

The following values were determined for the coefficients of the pertinent deviation functions of the ITS-90, as given

in the attached material describing the scale. The attached tables were generated using these values.

Coefficients for 1.0 mA Calibration

a, = -2.0257300E-04 b, = -2.7691 191E-05

c, - 1.3443513E-05 ^ = 5.9700519E-06

c, = 1.1044359E-06 c, = 9.7199229E-08

c5 = 3.3585947E-09

a, = -2.4194948E-04 b, = -2.3366736E-05 The resistance of this thermometer at 273.16 K was calculated to be 25.4975 ohms at 1.0 mA During calibration, the resistance at 273.16 K changed by the equivalent of 0.1 mK at 1.0 mA.

This thermometer is satisfactory as a defining instrument of the ITS-90 in accordance with the criteria that W(302.9146 K) fc 1.11807 or W(234.3156 K) < 0.844235.

For the Director, National Institute of Standards and Technology

Dr. Hratch G. Semerjian Test No. 456123

Chief, Chemical Process Metrology Division 1 January 1990 Center for Chemical Technology P.O. No. 123-456

Temperature Uncertainty

T„(K) »90(°C) (mK)

13.8033 -259.3467 ±0.2

17.0357 -256.1143 ±0.2

20.2711 -252.8799 ±0.2

24.5561 -248.5939 ±0.2

54.3584 -218.7916 ±0.1

83.8058 -189.3442 ±0.1

234.3156 -38.8344 ±0.1

273.16 0.01 ±0.1

429.7485 156.5985 ±0.7

505.078 231.928 ±1.0
1 January 1990 ITS-90 Table for PRT S/N 1812284 at mA

T(kelvin) W(T) dT/dW(T) T(kelvin)

13,.8 0..00119642

13,.9 0..00122091

14,.0 0..00124595

14..1 0.,00127155

14,.2 0.,00129771

14,.3 0.,00132445

14,.4 0,,00135178

14,,5 0.,00137970

14,.6 0.,00140822

14,.7 0.,00143735

14.,8 0.,00146709

14.,9 0.,00149747

15.,0 0. 00152847

40830

39934

39063

38217

37394,

36595,

35818,

35063,

34330,

33617,

32924,

32250,

101

422

736

450

955

625

832

944

332

370

443

944

15

15

15

15

15

15

15

15

15.8

15.9

16

16

16

16

16

16

16.6

16.7

16.8

16.9

17

17

17

17

17

17

17,

17,

17.8

17.9

18

18

18

18

18

18

18

18,

18.8

18.9

19

19

19

19

19.4

19.5

19.6

19.7

19.8

19.9

20.0

0,

0,

0,

0,

0,

0,

0,

0.

0.

0.

0.

0.

0.

0,

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

W(T)

,00152847

,00156012

,00159242

,00162538

,00165901

,00169331

,00172829

,00176396

,00180033

00183741

00187520

00191372

00195296

00199295

00203367

00207515

00211739

00216040

00220418

00224875

00229410

00234026

00238722

00243499

00248358

00253300

00258325

00263434

00268628

00273907

00279273

00284726

00290266

00295895

00301612

00307419

00313316

00319305

00325385

00331557

00337822

00344180

00350633

00357180

00363823

00370562

00377398

00384330

00391361

00398490

00405718

dT/dW(T)

31596 .276

30959 .857

30341 .117

29739 .502

29154 .471

28585 .499

28032 .076

27493 .708

26969 .914

26460 .232

25964 .212

25481 .419

25011 .434

24553 .850

24108 .276

23674 .331

23251 .652

22839 .884

22438 .686

22047,.730

21666,.698

21295,.282

20933,.188

20580,.129

20235,.830

19900,.025

19572,.457

19252.,879

18941.,049

18636.,739

18339.,725

18049.,790

17766. 728

17490. 337

17220. 423

16956. 799

16699. 283

16447. 700

16201. 882

15961. 664

15726. 888

15497. 402

15273. 058

15053. 712

14839. 227

14629. 468

14424. 306

14223. 616

14027. 276

13835. 170
1 January 1990 ITS-90 Table for PRT S/N 1812284 at 0mA

T(kelvin)

20.0

20.1

20.2

20.3

20.4

20.5

20.6

20.7

20.8

20.9

21.0

21.1

21.2

21.3

21.4

21.5

21.6

21.7

21.8

21.9

22.0

22

22

22

22

22

22

22

22.8

22.9

23

23

23

23

23

23

23

23

23.8

23.9

24.0

24

24

24

24

24

24

24

24.8

24.9

25.0

W(T)

0.00405718

0.00413045

0.00420473

0.00428001

0.00435631

0.00443362

0.00451196

0.00459133

0.00467173

0.00475317

0.00483565

0.00491918

0.00500377

0.00508942

0.00517613

0.00526391

0.00535276

0.00544270

0.00553371

0.00562581

0.00571901

0.00581330

0.00590869

0.00600519

0.00610279

0.00620151

0.00630134

0.00640230

0.00650438

0.00660758

0.00671192

0.00681740

0.00692401

0.00703177

0.00714067

0.00725072

0.00736193

0.00747429

0.00758780

0.00770248

0.00781833

0.00793534

0.00805352

0.00817288

0.00829341

0.00841511

0.00853800

0.00866207

0.00878733

0.00891377

0.00904141

dT/dW(T) T(kelvin)

13647 .183

13463 .205

13283 .130

13106 .855

12934 .279

12765 .306

12599 .842

12437 .796

12279 .080

12123 .607

11971 .296

11822 .066

11675 .839

11532 .539

11392 .094

11254 .431

11119 .482

10987 .179

10857..459

10730..257

10605..512

10483,.165

10363,.157

10245,.434

10129,,940

10016.,622

9905.,428

9796.,310

9689.,217

9584.,103

9480.,922

9379.,629

9280. 181

9182. 535

9086. 650

8992. 487

8900. 006

8809. 169

8719. 941

8632. 284

8546. 165

8461. 548

8378. 401

8296. 693

8216. 391

8137. 464

8059. 885

7983. 622

7908. 648

7834. 937

25

25

25

25.

25

25,

25.

25,

25.8

25.9

26

26

26

26

26

26

26

26

26.8

26.9

27.0

27

27

27

27

27

27

27

27.8

27.9

28.0

28

28

28

28

28

28.6

28.7

28.8

28.9

29.0

29.1

29

29

29

29

29

29,

29.8

29.9

30.0

W(T)

0.00904141

0.00917023

0.00930025

0.00943147

0.00956388

0.00969749

0.00983230

0.00996832

0.01010554

0.01024396

0.01038359

0.01052444

0.01066649

0.01080975

0.01095423

0.01109991

0.01124682

0.01139494

0.01154427

0.01169482

0.01184659

0.01199958

0.01215379

0.01230922

0.01246587

0.01262373

0.01278282

0.01294314

0.01310467

0.01326742

0.01343140

0.01359660

0.01376302

0.01393066

0.01409952

0.01426960

0.01444091

0.01461344

0.01478718

0.01496215

0.01513833

0.01531574

0.01549436

0.01567420

0.01585526

0.01603753

0.01622102

0.01640572

0.01659163

0.01677876

0.01696710

dT/dW(T)

7762 .460

7691 .191

7621 .106

7552 .180

7484 .388

7417 .707

7352 .113

7287 .585

7224 .101

7161 .639

7100 .178

7039 .699

6980 .181

6921 .605

6863 .953

6807 .205

6751,.345

6696,.354

6642,.216

6588,.913

6536,.430

6484,.750

6433,,858

6383,.739

6334,.379

6285,,761

6237.,874

6190..702

6144.,231

6098.,450

6053.,345

6008.,904

5965. 113

5921. 962

5879. 438

5837. 530

5796. 226

5755. 516

5715. 389

5675. 834

5636. 841

5598. 400

5560. 501

5523. 135

5486. 292

5449. 962

5414. 138

5378. 809

5343. 968

5309. 606
6.3.2 The following report is an example of a Report of Calibration for an SPRT

calibrated over the range from 83.8058 K to 933.473 K.

126
UNITED STATES DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

NATIONAL MEASUREMENT LABORATORY

GAJTHERSBURG, MARYLAND 20899

REPORT OF CALIBRATION

International Temperature Scale of 1990

Platinum Resistance Thermometer

Chino Model R800-2

Serial Number RS59A-8

Submitted by

National Institute of Standards and Technology

Gaithersburg, Maryland

This platinum resistance thermometer (PRT) was calibrated with an AC bridge at a frequency of 30 Hz using

continuous measuring currents of 1.0 mA and 2.0 mA. In accordance with the International Temperature Scale of 1990 (ITS-90) that was officially adopted by the Comity International des Poids et Mesures (CIPM) in September 1989, the subranges from 83.8058 K to 273.16 K and 273.15 K to 933.473 K, with the following fixed points and their stated uncertainties, were used to calibrate the thermometer:

Fixed Point

Ar TP

Hg TP

H2 TP

Sn FP

Zn FP

Al FP

Temperature Uncertainty

T,o(K) t*>(°C) (mK)

83.8058 -189.3442 ±0.1

234.3156 -38.8344 ±0.1

273.16 0.01 ±0.1

505.078 231.928 ±1.0

692.677 419.527 ±1.0

933.473 660.323 ±1.0

The following values were determined for the coefficients of the pertinent deviation functions of the ITS-90, as given in the attached material describing the scale. The attached tables were generated using these values.

Coefficients for Zero-Power Dissipation Calibration Coefficients for 1.0 mA Calibration

a< = -3.2324853E-04 b, = 1.2626274E-05 a4 = -3.2877518E-04 b4 = 1.1459514E-05

a7 = -3.64615 15E-04 b7 = -8.5363999E-06 a7 = -3.5921484E-04 b7 = -1.8940305E-05

c, = 1.4664695E-06 c, = 3.7262747E-06

The resistance of this thermometer at 273.16 K was calculated to be 25.0694 ohms at zero-power dissipation and

25.0696 ohms at 1.0 mA During calibration, the resistance at 273.16 K changed by the equivalent of 0.4 mK

at zero-power dissipation and 0.3 mK at 1.0 mA

This thermometer is satisfactory as a defining instrument of the ITS-90 in accordance with the criteria that W(302.9146 K) > 1.11807 or W(234.3156 K) < 0.844235. For the PRT to be acceptable for use between the

freezing points of aluminum and silver, it must also meet the additional criterion: W(1234.93 K) > 4.2844.

For the Director,

National Institute of Standards and Technology

Dr. Hratch G. Semerjian Test No. 456123

Chief, Chemical Process Metrology Division 1 January 1990 Center for Chemical Technology P.O. No. 123-456
1 January 1990 ITS-90 Table for PRT S/N RS59A-8 at mA

t (Celsius)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

W(t)

0.99996012

1.00394660

1.00793186

1.01191590

1.01589872

1.01988032

1.02386070

1.02783987

1.03181782

1.03579456

1.03977008

1.04374438

1.04771747

1.05168934

1.05566000

1.05962945

1.06359768

1.06756470

1.07153051

1.07549510

1.07945848

1.08342066

1.08738162

1.09134137

1.09529990

1.09925723

1.10321335

1.10716826

1.11112196

1.11507446

1.11902574

1.12297582

1.12692468

1.13087234

1.13481880

1.13876404

1.14270808

1.14665092

1.15059254

1.15453296

1.15847218

1.16241019

1.16634700

1.17028260

1.17421699

1.17815019

1.18208218

1.18601296

1.18994254

1.19387092

1.19779810

dt/dW(t)

250.8476

250.9248

251.0016

251.0784

251.1552

251.2320

251.3089

251.3857

251.4626

251.5395

251.6164

251.6934

251.7703

251.8473

251.9243

252.0014

252.0784

252.1555

252.2326

252.3097

252.3869

252.4640

252.5412

252.6184

252.6957

252.7730

252.8503

252.9276

253.0049

253.0823

253.1597

253.2371

253.3146

253.3921

253.4696

253.5472

253.6247

253.7023

253.7800

253.8576

253.9353

254.0130

254.0908

254.1686

254.2464

254.3242

254.4021

254.4800

254.5579

254.6359

t (Celsius)

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

W(t)

1.19779810

1.20172407

1.20564884

1.20957241

1.21349478

1.21741595

1.22133591

1.22525468

1.22917224

1.23308860

1.23700376

1.24091772

1.24483049

1.24874205

1.25265241

1.25656157

1.26046954

1.26437630

1.26828187

1.27218624

1.27608941

1.27999138

1.28389216

1.28779173

1.29169011

1.29558730

1.29948328

1.30337807

1.30727166

1.31116406

1.31505526

1.31894527

1.32283408

1.32672169

1.33060811

1.33449333

1.33837736

1.34226020

1.34614183

1.35002228

1.35390153

1.35777959

1.36165645

1.36553213

1.36940660

1.37327989

1.37715198

1.38102288

1.38489259

1.38876110

1.39262843

dt/dW(t)

254.7139

254.7919

254.8700

254.9481

255.0262

255.1043

255.1825

255.2607

255.3390

255.4173

255.4956

255.5740

255.6523

255.7308

255.8092

255.8877

255.9662

256.0448

256.1234

256.2020

256.2806

256.3593

256.4381

256.5168

256.5956

256.6744

256.7533

256.8322

256.9111

256.9901

257.0691

257.1481

257.2272

257.3063

257.3854

257.4646

257.5438

257.6231

257.7024

257.7817

257.8610

257.9404

258.0198

258.0993

258.1788

258.2583

258.3379

258.4175

258.4971

258.5768
6.3.3 The following report is an example of a Report of Calibration for an SPRT

calibrated over the range from 273.15 K to 1234.93 K.

129
UNITED STATES DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

NATIONAL MEASUREMENT LABORATORY

GAITHERSBURG, MARYLAND 20899

REPORT OF CALIBRATION

International Temperature Scale of 1990

Platinum Resistance Thermometer

Chino Model R800-3, 0.25 ohm

Serial Number RS78A-2

Submitted by

National Institute of Standards and Technology

Gaithersburg, Maryland

This platinum resistance thermometer (PRT) was calibrated with an AC bridge at a frequency of 30 Hz using

continuous measuring currents of 5.0 mA and 10.0 mA In accordance with the International Temperature Scale of 1990 (ITS-90) that was officially adopted by the Comite" International des Poids et Mesures (CIPM) in September 1989, the subrange from 273.15 K to 1234.93 K, with the following fixed points and their stated uncertainties, was used to calibrate the thermometer:

Fixed Point Temperature Uncertainty

TW(K) t*,(°C) (mK)

H2 TP 273.16 0.01 ±0.1

Sn FP 505.078 231.928 ±1.0

Zn FP 692.677 419.527 ±1.0

Al FP 933.473 660.323 ±1.0

Ag FP 1234.93 961.78 ±2.0

The following values were determined for the coefficients of the pertinent deviation functions of the ITS-90, as given in the attached material describing the scale. The attached tables were generated using these values.

Coefficients for Zero-Power Dissipation Calibration Coefficients for 5.0 mA Calibration

a6 = -1.1296072E-04 b6 = 1.1080496E-04 a6 = -9.2794623E-05 b6 = 8.3014954E-05

c6 = -3.5516098E-05 d = 3.6725603E-04 c6 = -2.7075707E-05 d = 2.8767080E-04

The resistance of this thermometer at 273.16 K was calculated to be 0.2553 ohms at zero-power dissipation and

0.2553 ohms at 5.0 mA During calibration, the resistance at 273.16 K changed by the equivalent of 2.0 mK at zero-power dissipation and 1.6 mK at 1.0 mA

This thermometer is satisfactory as a defining instrument of the ITS-90 in accordance with the criteria that W(302.9146 K) > 1.11807 or W(234.3156 K) < 0.844235. For the PRT to be acceptable for use between the freezing points of aluminum and silver, it must also meet the additional criterion: W(1234.93 K) > 4.2844.

For the Director,

National Institute of Standards and Technology

Dr. Hratch G. Semerjian Test No. 456123

Chief, Chemical Process Metrology Division 1 January 1990

Center for Chemical Technology P.O. No. 123-456
1 January 1990 ITS-90 Table for PRT S/N RS78A-2 at mA

t(Celsius)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

W(t)

0.99996011

1.00394760

1.00793386

1.01191891

1.01590275

1.01988537

1.02386678

1.02784697

1.03182595

1.03580372

1.03978027

1.04375561

1.04772974

1.05170266

1.05567437

1.05964487

1.06361416

1.06758223

1.07154910

1.07551476

1.07947921

1.08344246

1.08740449

1.09136532

1.09532494

1.09928335

1.10324056

1.10719656

1.11115135

1.11510494

1.11905732

1.12300850

1.12695847

1.13090724

1.13485480

1.13880116

1.14274631

1.14669027

1.15063302

1.15457456

1.15851491

1.16245405

1.16639199

1.17032872

1.17426426

1.17819859

1.18213173

1.18606366

1.18999439

1.19392392

1.19785226

dt/dW(t)

250.7845

250.8613

250.9378

251.0144

251.0909

251.1675

251.2441

251.3207

251.3973

251.4740

251.5507

251.6274

251.7041

251.7808

251.8576

251.9344

252.0112

252.0880

252.1649

252.2417

252.3186

252.3956

252.4725

252.5495

252.6265

252.7036

252.7806

252.8577

252.9348

253.0120

253.0891

253.1663

253.2436

253.3208

253.3981

253.4754

253.5528

253.6301

253.7075

253.7850

253.8624

253.9399

254.0175

254.0950

254.1726

254.2502

254.3279

254.4055

254.4833

254.5610

t(Celsius)

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

W(t)

1.19785226

1.20177939

1.20570532

1.20963005

1.21355358

1.21747592

1.22139705

1.22531699

1.22923573

1.23315327

1.23706961

1.24098476

1.24489871

1.24881146

1.25272301

1.25663337

1.26054252

1.26445049

1.26835726

1.27226283

1.27616720

1.28007038

1.28397237

1.28787316

1.29177275

1.29567115

1.29956835

1.30346436

1.30735918

1.31125280

1.31514523

1.31903647

1.32292651

1.32681536

1.33070301

1.33458947

1.33847474

1.34235882

1.34624170

1.35012340

1.35400390

1.35788321

1.36176132

1.36563825

1.36951398

1.37338853

1.37726188

1.38113404

1.38500501

1.38887480

1.39274339

dt/dW(t)

254.6388

254.7166

254.7944

254.8723

254.9502

255.0282

255.1061

255.1842

255.2622

255.3403

255.4184

255.4965

255.5747

255.6529

255.7312

255.8094

255.8878

255.9661

256.0445

256.1229

256.2014

256.2799

256.3584

256.4369

256.5155

256.5942

256.6728

256.7515

256.8303

256.9090

256.9878

257.0667

257.1456

257.2245

257.3034

257.3824

257.4614

257.5405

257.6196

257.6987

257.7779

257.8571

257.9363

258.0156

258.0949

258.1743

258.2536

258.3331

258.4125

258.4920
6.3.4 The following is an example of a Report giving the T68 to Tgo conversion

for a long- stem type SPRT calibrated on the IPTS-68(75) from the oxygen

condensation point to the freezing point of zinc.

132
UNITED STATES DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

NATIONAL MEASUREMENT LABORATORY

GAITHERSBURG, MARYLAND 20899

TEST REPORT

International Temperature Scale of 1990

Platinum Resistance Thermometer

Chino Model R800-2

Serial Number RS8YA-5

Submitted by

National Institute of Standards and Technology

Gaithersburg, Maryland

This thermometer was calibrated 1 March 1989 on the International Practical Temperature Scale of 1968 (IPTS-68)

and that calibration has been converted to the International Temperature Scale of 1990 (ITS-90). The IPTS-68

calibration was performed with a DC bridge at a continuous current of 1.0 mA The WfT1,,,) values for the triple

point of mercury and the triple point of argon were calculated from the IPTS-68 coefficients.

Fixed Point Temperature Uncertainty

T«,(K) t»(°C) (mK)

Ar TP 83.8058 -189.3442 ±0.1

Hg TP 234.3156 -38.8344 ±0.1

H2 TP 273.16 0.01 ±0.1

Sn FP 505.078 231.928 ±1.0

Zn FP 692.677 419.527 ±1.0

The following values were determined for the coefficients of the pertinent deviation functions of the ITS-90, as given

in the attached material describing the scale. The attached tables were generated using these values.

Coefficients for 1.0 mA Calibration

a 4 = -9.3225823E-05 b< = -9.9914440E-06

a8 = -9.1058813E-05 b8 = -7.6061559E-06

The resistance of this thermometer at 273.16 K was calculated to be 25.5096 ohms at 1.0 mA. During calibration, the resistance at 273.16 K changed by the equivalent of 0.5 mK at 1.0 mA

This thermometer is satisfactory as a defining instrument of the ITS-90 in accordance with the criteria that W(302.9146 K) > 1.11807 or W(234.3156 K) < 0.844235. For the PRT to be acceptable for use between the freezing

points of aluminum and silver, it must also meet the additional criterion: W(1234.93 K) > 4.2844.

For the Director,

National Institute of Standards and Technology

Dr. Hratch G. Semerjian Test No. 456123

Chief, Chemical Process Metrology Division 1 January 1990 Center for Chemical Technology P.O. No. 123-456
1 January 1990 ITS-90 Table for PRT S/N RS8YA-5 at 1mA

t(Celsius)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

W(t)

0.99996011

1.00394768

1.00793403

1.01191916

1.01590307

1.01988576

1.02386723

1.02784749

1.03182653

1.03580435

1.03978096

1.04375635

1.04773052

1.05170348

1.05567523

1.05964576

1.06361508

1.06758318

1.07155008

1.07551576

1.07948022

1.08344348

1.08740552

1.09136636

1.09532598

1.09928439

1.10324159

1.10719759

1.11115237

1.11510594

1.11905831

1.12300946

1.12695941

1.13090815

1.13485569

1.13880201

1.14274713

1.14669104

1.15063375

1.15457525

1.15851554

1.16245463

1.16639252

1.17032919

1.17426467

1.17819894

1.18213200

1.18606386

1.18999452

1.19392398

1.19785223

dt/dW(t)

250.7791

250.8562

250.9329

251.0097

251.0865

251.1633

251.2401

251.3170

251.3938

251.4707

251.5476

251.6245

251.7015

251.7784

251.8554

251.9324

252.0095

252.0865

252.1636

252.2407

252.3178

252.3950

252.4721

252.5493

252.6266

252.7038

252.7811

252.8584

252.9357

253.0131

253.0904

253.1679

253.2453

253.3228

253.4003

253.4778

253.5553

253.6329

253.7105

253.7882

253.8658

253.9435

254.0212

254.0990

254.1768

254.2546

254.3325

254.4103

254.4883

254.5662

t (Celsius)

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

W(t)

1.19785223

1.20177928

1.20570512

1.20962977

1.21355321

1.21747545

1.22139648

1.22531632

1.22923496

1.23315239

1.23706862

1.24098366

1.24489749

1.24881012

1.25272155

1.25663179

1.26054082

1.26444866

1.26835529

1.27226073

1.27616497

1.28006801

1.28396985

1.28787050

1.29176994

1.29566819

1.29956524

1.30346110

1.30735576

1.31124922

1.31514149

1.31903255

1.32292243

1.32681111

1.33069859

1.33458488

1.33846997

1.34235386

1.34623657

1.35011807

1.35399839

1.35787751

1.36175543

1.36563216

1.36950770

1.37338205

1.37725520

1.38112716

1.38499792

1.38886750

1.39273588

dt/dW(t)

254.6442

254.7222

254.8002

254.8783

254.9564

255.0345

255.1127

255.1909

255.2691

255.3474

255.4257

255.5040

255.5824

255.6608

255.7392

255.8177

255.8962

255.9747

256.0533

256.1319

256.2105

256.2892

256.3679

256.4466

256.5254

256.6042

256.6830

256.7619

256.8408

256.9198

256.9988

257.0778

257.1568

257.2359

257.3150

257.3942

257.4734

257.5526

257.6319

257.7112

257.7905

257.8698

257.9493

258.0287

258.1082

258.1877

258.2672

258.3468

258.4264

258.5061
6.3.5 The following is an example of a Report giving the T68 to r90 conversion

for a capsule -type SPRT calibrated on the IPTS-68(75) from the triple point of

equilibrium hydrogen to the freezing point of tin.

135
UNITED STATES DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

NATIONAL MEASUREMENT LABORATORY

GAITHERSBURG, MARYLAND 20899

TEST REPORT

International Temperature Scale of 1990

Platinum Resistance Thermometer

LAN Model 8164

Serial Number 1812284

Submitted by

National Institute of Standards and Technology

Gaithersburg, Maryland

This thermometer was calibrated 21 September 1989 on the International Practical Temperature Scale of 1968 (IPTS-68) and that calibration has been converted to the International Temperature Scale of 1990 (ITS-90). The

IPTS-68 calibration was performed with an AC bridge at a continuous current of 1.0 mA. The W(TW) values for the

triple point of neon, the triple point of mercury, the triple point of argon, and the freezing point of indium were

calculated from the IPTS-68 coefficients.

Fixed Point Temperature Uncertainty

T»(K) t»(°C) (mK)

H2 TP 13.8033 -259.3467 ±0.2

17 K VP 17.0301 -256.1143 ±0.2

20 K BP 20.2701 -252.8799 ±0.2

Ne TP 24.5561 -248.5939 ±0.2

o2 TP 54.3584 -218.7916 ±0.2

At TP 83.8058 -189.3442 ±0.1 Hg TP 234.3156 -38.8344 ±0.1 H2 TP 273.16 0.01 ±0.1

In FP 429.7485 156.5985 ±0.7

Sn FP 505.078 231.928 ±1.0

The following values were determined for the coefficients of the pertinent deviation functions of the ITS-90, as given

in the attached material describing the scale. The attached tables were generated using these values.

Coefficients for 1.0 mA Calibration

a, = -2.5239001 E-04 b, = -1.2277862E-04

c, = -2.3783015E-06 Cj = -4.3892024E-06

Cj = -1.5608728E-06 c4 = -2.1374663E-07

c5 = -1.0344171E-08

a, = -2.5287142E-04 b, = -1.1130131E-05 The resistance of this thermometer at 273.16 K was calculated to be 25.4975 ohms at 1.0 mA During calibration, the resistance at 273.16 K changed by the equivalent of 0.1 mK at 1.0 mA.

This thermometer is satisfactory as a defining instrument of the ITS-90 in accordance with the criteria that W(302.9146 K) 2s 1.11807 or W(234.3156 K) < 0.844235.

For the Director,

National Institute of Standards and Technology

Dr. Hratch G. Semerjian Test No. 456123

Chief, Chemical Process Metrology Division 1 January 1990 Center for Chemical Technology P.O. No. 123-456
1 January 1990 ITS-90 Table for PRT S/N 1812284 at mA

T(kelvin)

20

20

20

20

20

20

20

20.7

20.8

20.9

21.0

21.1

21.2

21.3

21

21

21

21

21.8

21.9

22

22

22

22

22

22

22

22

22.8

22.9

23

23

23

23

23

23

23

23.7

23.8

23

24

24

24

24

24

24

24.6

24.7

24.8

24.9

25.0

W(T)

0.00405854

0.00413180

0.00420605

0.00428130

0.00435757

0.00443485

0.00451315

0.00459248

0.00467284

0.00475424

0.00483668

0.00492017

0.00500471

0.00509031

0.00517698

0.00526471

0.00535351

0.00544339

0.00553435

0.00562640

0.00571954

0.00581378

0.00590911

0.00600555

0.00610310

0.00620176

0.00630154

0.00640244

0.00650446

0.00660761

0.00671189

0.00681731

0.00692386

0.00703156

0.00714041

0.00725041

0.00736155

0.00747386

0.00758732

0.00770195

0.00781774

0.00793469

0.00805282

0.00817213

0.00829261

0.00841426

0.00853710

0.00866112

0.00878633

0.00891273

0.00904032

dT/dW(T)

13651.443

13467.816

13288.054

13112.056

12939.724

12770.963

12605.681

12443.789

12285.200

12129.832

11977.602

11828.432

11682.245

11538.968

11398.528

11260.856

11125.883

10993.545

10863.776

10736.516

10611.702

10489.278

10369.186

10251.371

10135.779

10022.357

9911.055

9801.824

9694.615

9589.382

9486.079

9384.662

9285.088

9187.315

9091.302

8997.011

8904.401

8813.437

8724.080

8636.296

8550.050

8465.308

8382.037

8300.205

8219.782

8140.736

8063.038

7986.659

7911.572

7837.748

T(kelvin)

25

25

25

25

25

25

25

25

25.8

25.9

26.0

26.1

26

26

26

26

26

26

26.8

26.9

27

27

27

27

27

27

27

27

27.8

27.9

28.0

28

28

28

28

28

28

28

28.8

28.9

29.0

29.1

29

29

29

29

29

29,

29.8

29.9

30.0

W(T)

0.00904032

0.00916910

0.00929907

0.00943025

0.00956262

0.00969619

0.00983096

0.00996694

0.01010412

0.01024251

0.01038210

0.01052291

0.01066493

0.01080816

0.01095261

0.01109826

0.01124514

0.01139323

0.01154254

0.01169307

0.01184481

0.01199778

0.01215196

0.01230737

0.01246400

0.01262185

0.01278092

0.01294122

0.01310273

0.01326547

0.01342944

0.01359462

0.01376103

0.01392866

0.01409752

0.01426759

0.01443889

0.01461141

0.01478515

0.01496012

0.01513630

0.01531370

0.01549232

0.01567216

0.01585322

0.01603549

0.01621898

0.01640369

0.01658961

0.01677674

0.01696508

dT/dW(T)

7765.161

7693.784

7623.594

7554.564

7486.671

7419.891

7354.201

7289.579

7226.003

7163.451

7101.904

7041.339

6981.739

6923.083

6865.353

6808.530

6752.596

6697.534

6643.327

6589.957

6537.409

6485.667

6434.715

6384.537

6335.120

6286.448

6238.507

6191.284

6144.765

6098.936

6053.785

6009.300

5965.467

5922.275

5879.711

5837.765

5796.426

5755.681

5715.520

5675.933

5636.910

5598.439

5560.512

5523.119

5486.250

5449.896

5414.048

5378.697

5343.835

5309.452
6.3.6 The following is an example of a Report of Calibration for an RIRT

calibrated over the range from 0.50 K to 30 K on the EPT-76.

138
UNITED STATES DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

NATIONAL MEASUREMENT LABORATORY

GAITHERSBURG, MARYLAND 20899

REPORT OF CALIBRATION

1976 Provisional 0.5 K to 30 K Temperature Scale (EPT-76)

Rhodium-Iron Resistance Thermometer

Serial Number B189

Submitted by

NIST - Thermometry Group

Gaithersburg, MD 20899

This resistance thermometer was calibrated by comparison under isothermal conditions with a set of stable reference rhodium-iron resistance thermometers. A table listing thermometer resistance R as a function

of temperature T follows. This table constitutes the calibration.

In addition, as a convenience to the user, least-squares polynomial smoothings of the calibration data have

been done. A table of temperature, resistance, and the first and second derivatives of resistance with

respect to temperature has been generated from a selected polynomial.

This thermometer was calibrated using an AC bridge at a frequency of 30 Hz using excitation currrents of 0.125 mA below 1 K and 0.25 mA above 1 K. The current was maintained through the thermometer

only during the actual time when measurements of resistance were made.

For the Director,

National Institute of Standards and Technology

B. W. Mangum, Physicist Test No. 12345

Temperature and Pressure Division 29 December 1989

Center for Chemical Technology Purchase Order No. 123456789
* RHODIUH-IRON THERMOMETER B189 * Calibrated for NIST—Therao&etrY Group

* Calibrated on EPT-76, Mar-June 1988

Calibration Current = 0.125, 0.25 aA

Height = (dT/dR)«2

T(K> R(oheis) Weight I(iA)

0.43976 4.293722 2.87 0.125

0.51872 4.340174 2.86 0.125

0.64985 4.417578 2.87 0.125

0.85100 4.536304 2.89 0.125

1.02827 4.640395 2.92 0.25

1.18271 4.730482 2.96 0.25

1.30962 4.804059 3.00 0.25

1.72215 5.039635 3.15 0.25

2.17154 5.288860 3.37 0.25

2.47997 5.454826 3.55 0.25

3.41404 5.930224 4.22 0.25

4.22195 6.308779 4.93 0.25

4.50339 6.433776 5.21 0.25

5.76304 6.953086 6.64 0.25

7.19977 7.475341 8.62 0.25

8.78512 7.980605 11.2 0.25

10.45483 8.447636 14.5 0.25

12.14414 8.865576 18.4 0.25

13.80845 9.233974 22.7 0.25

15.42337 9.557786 27.3 0.25

17.03860 9.854363 32.2 0.25

18.65600 10.128928 37.3 0.25

20.27451 10.385641 42.2 0.25

21.84940 10.621898 46.6 0.25

23.30557 10.831357 49.9 0.25

24.55872 11.006661 52.1 0.25

25.54642 11.142596 53.4 0.25

26.25499 11.239309 54.0 0.25

26.69840 11.299615 54.2 0.25

26.95381 11.334286 54.3 0.25

27.10184 11.354377 54.3 0.25

27.21302 11.369470 54.3 0.25

09/08/88 -ERP

12th order R-T fit, «>.d. = 0.11 bK
05/10/90 - 11:09:50 RIB189 RESIST-841023 RHODIUM-IRON THERMOMETER B189

Calibrated for NIST—Thermometry Group

Calibrated on EPT-76, Hay-June 1988

FORTRAN POLYFIT

AN ORTHONORMAL LEAST-SQUARES FITTING OF THE POLYNOMIAL

J=N J R = SUM A T, N = P, P+l, P+2 .... N J=0 J,N

The fitting is done in double-precision arithmetic. The computer

is programed to select the best polynomial fitting from which to generate a table of temperature^ resistance* and the first and second

derivatives of resistance with respect to temperature. The page listing the coefficients of the selected polynomial is clearly marked

at the bottom.

The deviations of each fitted polynomial from the calibration

data are given in the columns headed "Delta R" and "Delta T". The

first and second derivatives of each fitted polynomial (in terms of the fitted variables) may be given also.

This analysis and the resultant generated table are provided for convenience in interpolating between the calibration points and do not constitute the calibration.

This is a fitting of EPT-76 calibration data for the range 0.5 K to 27.1 K.

Table derived from lowest order fit where the ratio

of the coefficients to their standard deviations exceeds 2.00

and the absolute value of each delta T is less than 0.25 mK. Read in 32 data Foints. 32 of which used for fit.
RHODIUH-IRON THERMOTETER B189

Order of fit = 11 Standard Coefficients Deviation Ratio AO = 0.403461344715970+01 0. 251940-03 16014.35

A 1 = 0.58409816514135D+0O 0. 642090-03 909.68 A2 = 0.15633522974138D-01 0.56019D-03 27.91 A3 =-0.125241885614870-01 0.23576D-O3 53.12 A4 = 0.22840914978526D-02 0.56075D-O4 40.73 A5 =-0.254865085790450-03 0.822090-05 31.00 A6 = 0.191971332104650-04 0.778380-06 24.66 A7 =-0.991992563931580-06 0.485050-07 20.45 A8 = 0.346403512658310-07 0.197670-08 17.52 A9 =-0.780615777385770-09 0.507010-10 15.40 AlO = 0.102395491343720-10 0.742660-12 13.79

All =-0.593574267764460-13 0.473690-14 12.53

Standard Deviation of Fit = 0.374764E-04 Oh«s

T R R Calculated Delta R Delta T Weight Kelvins Ohms Onus Onus Kelvins

0.4398 4.29372 4.29352 0.00021 -0.348 2.87000 0.5187 4.34017 4.34021 -0.00004 0.063 2.86000 0.6499 4.41758 4.41773 -0.00016 0.264 2.87000 0.8510 4.53630 4.53638 -0.00007 0.121 2.89000

1.0283 4.64040 4.64042 -0.00002 0.042 2.92000

1.1827 4.73048 4.73051 -0.00003 0.047 2.96000

1.3096 4.80406 4.80407 -0.00001 0.019 3.00000

1.7221 5.03964 5.03960 0.00003 -0.053 3.15000 2.1715 5.28886 5.28877 0.00009 -0.173 3.37000

2.4800 5.45483 5.45471 0.00011 -0.210 3.55000 3.4140 5.93022 5.93032 -0.00009 0.188 4.22000 4.2220 6.30878 6.30881 -0.00003 0.076 4.93000 4.5034 6.43378 6.43378 -0.00001 0.017 5.21000 5.7630 6.95309 6.95310 -0.00001 0.028 6.64000 7.1998 7.47534 7.47532 0.00002 -0.066 8.62000

8.7851 7.98061 7.98056 0.00004 -0.149 11.20000 10.4548 8.44764 8.44767 -0.00004 0.134 14.50000

12.1441 8.86558 8.86561 -0.00003 0.141 18.39999 13.8085 9.23397 9.23394 0.00003 -0.139 22.70000

15.4234 9.55779 9.55776 0.00003 -0.134 27.29999

17.0386 9.85436 9.85438 -0.00002 0.118 32.20000

18.6560 10.12893 10.12895 -0.00002 0.143 37.29999 20.2745 10.38564 10.38562 0.00002 -0.127 42.20000 21.8494 10.62190 10.62188 0.00002 -0.145 46.59999 23.3056 10.83136 10.83139 -0.00003 0.221 49.89999 24.5587 11.00666 11.00666 -0.00000 0.023 52.09999

25.5464 11.14260 11.14257 0.00003 -0.183 53.39999 26.2550 11.23931 11.23931 0.00000 -0.025 54.00000 26.6984 11.29962 11.29963 -0.00001 0.083 54.20000 26.9538 11.33429 11.33431 -0.00002 0.151 54.29999 27.1018 11.35438 11.35438 -0.00001 0.040 54.29999 27.2130 11.36947 11.36945 0.00002 -0.166 54.29999

Standard Deviation of R = 0.00160 Percent

Standard Deviation of T = 0.18001 nKelvins
RHODItfHRON THERMOMETER B189

Order of fit = 12 Standard

Coefficients Deviation Ratio

A = 0.40353440386808D+01 0. 181260-03 22262.26 A 1 = 0.581681 249467860+00 0.51824D-03 1122.41 A 2 = 0.183151225094050-01 0.51786D-03 35.37 A 3 =-0.13936140688461D-Ol 0.25342D-03 54.99 A 4 = 0.27019102278194D-02 0.71194D-O4 37.95

A 5 =-0.33123326008865D-03 0. 125400-04 26.41 A 6 = 0.28300891747533IH)4 0.14562D-05 19.44 A 7 =-0.172026884480610-05 0.11436D-O6 15.04 A 8 = 0.74047308522468D-07 0.61099D-08 12.12 A 9 =-0.22037748481731D-08 0.21880D-09 10.07 A10 = 0.43099007915774D-10 0.50258D-U 8.58

All =-0. 4980341471 1131D-12 0.66912D-13 7.44 A12 = 0.25757636010739D-14 0.39257D-15 6.56

***** THESE COEFFICIENTS SELECTED FOR THE GENERATED TABLE *****

Standard Deviation of Fit = 0.212764E-04 Ohas

T Kelvins 0.4398

0.5187

0.6499

0.8510

1.0283

1.1827 1.3096

1.7221 2.1715 2.4800

3.4140

4.2220

4.5034 5.7630

7.1998

8.7851 10.4548

12.1441 13.8085

15.4234

17.0386

18.6560 20.2745

21.8494 23.3056

24.5587 25.5464

26.2550

26.6984 26.9538

27.1018

27.2130

R

Ohas 4.29372

4.34017

4.41758

4.53630 4.64040

4.73048 4.80406

5.03964 5.28886

5.45483 5.93022

6.30878

6.43378

6.95309

7.47534

7.98061 8.44764

8.86558

9.23397

9.55779

9.85436

10.12893

10.38564

10.62190

10.83136

11.00666

11.14260

11.23931 11.29962

11.33429 11.35438

11.36947

R Calculated

Otns 4.29360

4.34024

4.41771

4.53631 4.64035

4.73046

4.80404

5.03964

5.28884 5.45480

5.93033

6.30876

6.43372

6.95307

7.47537 7.98059 8.44763

8.86558

9.23397 9.55779

9.85436

10.12893

10.38564 10.62189

10.83136

11.00666

11.14260

11.23931

11.29961 11.33429 11.35438

11.36947

Delta R Ohas 0.00013

-0.00007 -0.00013

-0.00001 0.00004

0.00002

0.00002

-0.00001 0.00002

0.00003

-0.00011 0.00002

0.00005

0.00001 -0.00003

0.00001 0.00001

-0.00001 0.00000

0.00000

0.00000 -0.00000

-0.00000

0.00001 -0.00000 0.00000

-o.ooooo -0.00000

0.00001 -0.00000

-0.00000

0.00000

Delta T Kelvins

-0.212

0.112 0.215

0.009 -0.070

-0.037

-0.034 0.010

-0.028

-0.053

0.217

-0.035

-0.119

-0.029

0.096 -0.043 -O.019

0.025

-0.001 -0.003

-0.008

0.003 0.019

-0.035

0.034 -0.023

0.006

0.020 -0.049

0.029 0.025

-0.023

Height

2.87000 2.86000

2.87000

2.89000

2.92000

2.96000

3.00000

3.15000 3.37000

3.55000

4.22000

4.93000

5.21000

6.64000 8.62000

11.20000

14.50000

18.39999 22.70000 27.29999

32.20000

37.29999

42.20000 46.59999

49.89999 52.09999

53.39999 54.00000

54.20000

54.29999

54.29999

54.29999

Standard Deviation of R = 0.00114 Percent

Standard Deviation of T = 0.10222 Kelvins
RHODIUM-IRON THERMOMETER B189

12th order

First Second

Temperature Resistance Derivative Derivative

Kelvins Ohms Ohnts/K Ohns/K/K

0.4398 4.29360 0.5906 0.0056

0.5187 4.34024 0.5908 0.0011 0.6498 4.41770 0.5905 -0.0057

0.8510 4.53631 0.5884 -0.0147

1.0283 4.64035 0.5852 -0.0214

1.1827 4.73046 0.5815 -0.0264

1.3096 4.80404 0.5779 -0.0299

1.7221 5.03964 0.5637 -0.0386

2.1715 5.28884 0.5450 -0.0441 2.4800 5.45480 0.5310 -0.0462

3.4140 5.93033 0.4871 -0.0469

4.2219 6.30876 0.4502 -0.0441 4.5034 6.43372 0.4379 -0.0428

5.7630 6.95307 0.3880 -0.0364

7.1998 7.47537 0.3406 -0.0298

8.7851 7.98059 0.2983 -0.0239

10.4548 8.44763 0.2625 -0.0192

12. 1441 8.86558 0.2334 -0.0155

13.8084 9.23397 0.2101 -0.0126

15.4234 9.55779 0.1915 -0.0104

17.0386 9.85436 0.1762 -0.0086

18.6560 10.12893 0.1638 -0.0069

20.2745 10.38564 0.1539 -0.0054

21.8494 10.62189 0.1465 -0.0040

23.3056 10.83136 0.1415 -0.0029

24.5587 11.00666 0.1385 -0.0019

25.5464 11.14260 0.1369 -0.0013

26.2550 11.23931 0.1361 -0.0008

26.6984 11.29961 0.1358 -0.0005

26.9538 11.33429 0.1357 -0.0003

27.1018 11.35438 0.1357 -0.0002

27.2130 11.36947 0.1357 -0.0001
RHODIUM-IRON THERMOMETER B189

12th order

First Second Tewerature Resistance Derivative Derivative Kelvins Ohns Ohms/K Ohms/K/K

0.5000 4.32918 0.5908 0.0022

1.0000 4.62380 0.5858 -0.0204

1.5000 4.91350 0.5718 -0.0344 2.0000 5.19472 0.5524 -0.0424 2.5000 5.46542 0.5301 -0.0462 3.0000 5.72463 0.5066 -0.0474 3.5000 5.97202 0.4830 -0.0467 4.0000 6.20776 0.4600 -0.0451 4.5000 6.43224 0.4381 -0.0428 5.0000 6.64602 0.4173 -0.0403 5.5000 6.84973 0.3978 -0.0377 6.0000 7.04401 0.3795 -0.0352 6.5000 7.22948 0.3625 -0.0328 7.0000 7.40672 0.3467 -0.0306 7.5000 7.57632 0.3319 -0.0285 8.0000 7.73877 0.3181 -0.0266 8.5000 7.89456 0.3052 -0.0249 9.0000 8.04414 0.2932 -0.0232 9.5000 8.18790 0.2820 -0.0217

10.0000 8.32623 0.2715 -0.0203

10.5000 8.45947 0.2616 -0.0190

11.0000 8.58794 0.2524 -0.0179

11.5000 8.71196 0.2437 -0.0168

12.0000 8.83178 0.2356 -0.0157

12.5000 8.94767 0.2280 -0.0148

13.0000 9.05985 0.2208 -0.0139

13.5000 9.16856 0.2141 -0.0131 14.0000 9.27399 0.2077 -0.0124

14.5000 9.37632 0.2017 -0.0116

15.0000 9.47575 0.1961 -0.0110

15.5000 9.57243 0.1907 -0.0103

16.0000 9.66652 0.1857 -0.0097

16.5000 9.75818 0.1810 -0.0092

17.0000 9.84755 0.1765 -0.0086

17.5000 9.93477 0.1724 -0.0081 18.0000 10.01998 0.1685 -0.0075

18.5000 10.10330 0.1648 -0.0070

19.0000 10.18486 0.1614 -0.0065

19.5000 10.26479 0.1583 -0.0061 20.0000 10.34320 0.1554 -0.0056 20.5000 10.42021 0.1527 -0.0052 21.0000 10.49592 0.1502 -0.0047 21.5000 10.57046 0.1480 -0.0043 22.0000 10.64391 0.1459 -0.0039 22.5000 10.71639 0.1440 -0.0035 23.0000 10.78799 0.1424 -0.0031 23.5000 10.85882 0.1409 -0.0027 24.0000 10.92896 0.1397 -0.0023 24.5000 10.99852 0.1386 -0.0020 25.0000 11.06759 0.1377 -0.0016
RHODIUM-IRON THERMOMETER B189

12th order

First Second

Te»perature Resistance Derivative Derivative

Kelvins Onus Ohms/K Ohns/K/K

25.5000 11.13624 0.1370 -0.0013

26.0000 11.20457 0.1364 -0.0010

26.5000 11.27264 0.1360 -0.0007

27.0000 11.34056 0.1357 -0.0003

27.5000 11.40841 0.1357 0.0003
6.3.7 The following is an example of a Report of Calibration for an RIRT (the

RIRT of sec. 6.3.6) calibrated over the range from 0.65 K to 25 K on the ITS -90.

147
UNITED STATES DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

NATIONAL MEASUREMENT LABORATORY

GAITHERSBURG, MARYLAND 20899

REPORT OF CALIBRATION

International Temperature Scale of 1990 (rTS-90)

Rhodium-Iron Resistance Thermometer

Serial Number B189

Submitted by

NIST - Thermometry Group

Gaithersburg, MD 20899

This resistance thermometer was calibrated by comparison under isothermal conditions with a set of stable reference rhodium-iron resistance thermometers. A table listing thermometer resistance R as a function

of temperature T follows. This table constitutes the calibration.

In addition, as a convenience to the user, least-squares polynomial smoothings of the calibration data have

been done. The values of the various polynomial coefficients, their standard deviations, and the deviation

of the calibration values from the polynomial values are reported. Finally, a table of temperature,

resistance, and the first and second derivatives of resistance with respect to temperature has been generated

from a selected recommended polynomial.

This thermometer was calibrated using an AC bridge at a frequency of 30 Hz using excitation currrents of 0.125 mA below 1 K and 0.25 mA above 1 K. The current was maintained through the thermometer

only during the actual time when measurements of its resistance were made.

For the Director,

National Institute of Standards and Technology

Dr. Hratch G. Semerjian Test No. 12345

Chief, Chemical Process Metrology Division 7 May 1990

Center for Chemical Technology Purchase Order No. 123456789
* RHODIUM-IRON THERMOMETER B189

* Calibrated for NIST—ThernoaetrY Group * Calib. on EPT-76, converted to ITS-90

Calibration Current = 0.125. 0.25 oA

Height = <dT/dR)**2

T(K) R(ohns) Height KaA)

0.51872 4.340174 2.86 0.125

0.64985 4.417578 2.87 0.125

0.85100 4.536304 2.89 0.125

1.02826 4.640395 2.92 0.25

1.18270 4.730482 2.96 0.25

1.30961 4.804059 3.00 0.25

1.72213 5.039635 3.15 0.25

2.17151 5.288860 3.37 0.25

2.47994 5.454826 3.55 0.25

3.41397 5.930224 4.22 0.25

4.22185 6.308779 4.93 0.25

4.50328 6.433776 5.21 0.25

5.76285 6.953086 6.64 0.25

7.19948 7.475341 8.62 0.25

8.78469 7.980605 11.2 0.25

10.45422 8.447636 14.5 0.25

12.14331 8.865576 18.4 0.25

13.80738 9.233974 22.7 0.25

15.42204 9.557786 27.3 0.25

17.03697 9.854363 32.2 0.25

18.65405 10.128928 37.3 0.25

20.27221 10.385641 42.2 0.25

21.84673 10.621898 46.6 0.25

23.30253 10.831357 49.9 0.25

24.55534 11.006661 52.1 0.25

25.54277 11.142596 53.4 0.25
05/07/90 - 14:03:47 EXAMPL RESIST-841023 RHODIUM-IRON THERMOMETER B189

Calibrated for NIST—Thermoaetry Group

Calib. on EPT-76, converted to ITS-90

FORTRAN POLYFIT

ORTHONORNAL LEAST-SQUARES FITTING OF THE POLYNOMIAL

J=N J R = SUM A T, N = P, P+l, P+2

N J=0 J,N

The fitting is done in double-precision arithmetic. The computer

is programed to select the best polynomial fitting froi which to generate a table of temperature, resistance, and the first and second derivatives of resistance with respect to temperature. The pa9e

listing the coefficients of the selected polynomial is clearly narked

at the bottom.

The deviations of each fitted polynomial from the calibration

data are given in the columns headed "Delta R" and "Delta T". The

first and second derivatives of each fitted polynomial (in terns of the fitted variables) my be given also.

This analysis and the resultant generated table are provided for convenience in interpolating between the calibration points and do not constitute the calibration.

Special coments:

(1) This example includes the fitting results of only the best polynomial fitting. Normal 1y> fitting results for several lower-order polynomial fittings are included for comparison.

(2) The data are weighted by a factor <dT/dR)«2.

Table derived from lowest order fit where the ratio

of the coefficients to their standard deviations exceeds 2.00

and the absolute value of each delta T is less than 0.20 aK. Read in 26 data points, 26 of which used for fit.
RHODIUM-IRON THERMOMETER B189

11th order

First Second Tewerature Resistance Derivative Derivative Kelvins 0hn5 Ohi»s/K Otas/K/K

0.5000 4.32904 0.5915 0.0004

1.0000 4.62383 0.5859 -0.0210

1.5000 4.91353 0.5718 -0.0345 2.0000 5.19472 0.5524 -0.0423 2.5000 5.46541 0.5301 -0.0462 3.0000 5.72463 0.5067 -0.0473 3.5000 5.97205 0.4831 -0.0467 4.0000 6.20781 0.4601 -0.0451 4.5000 6.43231 0.4381 -0.0428 5.0000 6.64610 0.4173 -0.0403 5.5000 6.84981 0.3978 -0.0378 6.0000 7.04409 0.3795 -0.0352 6.5000 7.22955 0.3625 -0.0328 7.0000 7.40680 0.3467 -0.0306 7.5000 7.57640 0.3319 -0.0285 8.0000 7.73887 0.3181 -0.0266 8.5000 7.89468 0.3053 -0.0249 9.0000 8.04427 0.2932 -0.0232 9.5000 8.18805 0.2820 -0.0217

10.0000 8.32639 0.2715 -0.0203

10.5000 8.45964 0.2616 -0.0191 11.0000 8.58813 0.2524 -0.0179

11.5000 8.71215 0.2438 -0.0168

12.0000 8.83197 0.2356 -0.0157

12.5000 8.94786 0.2280 -0.0148

13.0000 9.06005 0.2208 -0.0139

13.5000 9.16877 0.2141 -0.0131 14.0000 9.27421 0.2077 -0.0123

14.5000 9.37656 0.2017 -0.0116

15.0000 9.47600 0.1961 -0.0110

15.5000 9.57269 0.1908 -0.0103

16.0000 9.66680 0.1857 -0.0097

16.5000 9.75847 0.1810 -0.0092

17.0000 9.84785 0.1766 -0.0086

17.5000 9.93507 0.1724 -0.0081 18.0000 10.02028 0.1685 -0.0075

18.5000 10.10361 0.1649 -0.0070

19.0000 10.18518 0.1615 -0.0065

19.5000 10.26511 0.1583 -0.0061 20.0000 10.34354 0.1554 -0.0056 20.5000 10.42056 0.1527 -0.0052 21.0000 10.49630 0.1502 -0.0047 21.5000 10.57085 0.1480 -0.0043 22.0000 10.64431 0. 1459 -0.0039 22.5000 10.71680 0.1441 -0.0035 23.0000 10.78841 0.1424 -0.0031 23.5000 10.85924 0.1410 -0.0027 24.0000 10.92940 0.1397 -0.0023 24.5000 10.99899 0.1387 -0.0019 25.0000 11.06809 0.1377 -0.0018
RHODIUM-IRON THERMOMETER B189

11th order

First Second

Tenperature Resistance Derivative Derivative

Kelvins Ohns Ohns/K Ohms/K/K

0.5187 4.34011 0.5915 -0.0006

0.6498 4.41765 0.5910 -0.0070

0.8510 4.53631 0.5887 -0.0156

1.0283 4.64038 0.5853 -0.0220

1.1827 4.73049 0.5815 -0.0267

1.3096 4.80407 0.5779 -0.0301

1.7221 5.03965 0.5636 -0.0386

2.1715 5.28882 0.5449 -0.0440

2.4799 5.45477 0.5310 -0.0461 3.4140 5.93032 0.4871 -0.0469

4.2218 6.30878 0.4502 -0.0441 4.5033 6.43374 0.4380 -0.0428

5.7628 6.95308 0.3880 -0.0364

7.1995 7.47536 0.3407 -0.0297

8.7847 7.98059 0.2983 -0.0239

10.4542 8.44765 0.2625 -0.0192

12.1433 8.86558 0.2334 -0.0155

13.8074 9.23396 0.2101 -0.0126

15.4220 9.55779 0.1916 -0.0104

17.0370 9.85437 0.1762 -0.0086

18.6540 10.12892 0.1638 -0.0069

20.2722 10.38564 0.1539 -0.0054

21.8467 10.62190 0.1465 -0.0040

23.3025 10.83135 0.1415 -0.0028

24.5553 11.00666 0.1386 -0.0019

25.5428 11.14260 0.1368 -0.0020
RHODIUM-IRON THERMOMETER 0189

Order of fit = 11 Standard

Coefficients , Deviation Ratio

A = Q.40345618512987D+01 0. 172030-03 23452.05

A 1 = 0.58376417022443D+00 0.42964D-03 1358.72 A 2 = 0. 16272867175907D-01 0.38192D-03 42.61 A 3 =-0.129180383967580-01 0.16723D-03 77.25 A 4 = 0.24053622765144D-02 0.41881D-04 57.43

A 5 =-0.276531060638970-03 0.65102D-05 42.48 A 6 - 0. 216099359574820-04 0.65633D-06 32.93 A 7 =-0.116478740572690-05 0.43662D-07 26.68 A 8 = 0.42603165622150D-07 0. 190260-08 22.39 A 9 =-0.100883806745190-08 0.522350-10 19.31 A10 = 0.139413592145660-10 0.81948D-S2 17.01 All =-0.853183872150180-13 0.56000D-14 15.24

Standard Deviation of Fit = 0.198756E-04 Ohis

T R R Calculated Delta R Delta T Height Kelvins Ohos Otns Ohms etKelvins 0.5187 4.34017 4.34011 0.00006 -0.105 2.86000 0.6499 4.41758 4.41765 -0.00007 0.116 2.87000 0.8510 4.53630 4.53631 -0.00001 0.018 2.89000

1.0283 4.64040 4.64038 0.00002 -0.026 2.92000

1.1827 4.73048 4.73049 -0.00001 0.019 2.96000

1.3096 4.80406 4.80407 -0.00001 0.021 3.00000

1.7221 5.03964 5.03965 -0.00001 0.019 3.15000 2.1715 5.28886 5.28882 0.00004 -0.069 3.37000 2.4799 5.45483 5.45477 0.00006 -0.104 3.55000 3.4140 5.93022 5.93032 -0.00009 0.187 4.22000 4.2219 6.30878 6.30878 0.00000 -0.005 4.93000 4.5033 6.43378 6.43374 0.00003 -0.074 5.21000 5.7629 6.95309 6.95308 0.00001 -0.017 6.64000 7.1995 7.47534 7.47536 -0.00001 0.044 8.62000 8.7847 7.98061 7.98059 0.00002 -0.054 11.20000 10.4542 8.44764 8.44765 -0.00001 0.038 14.50000 12.1433 8.86558 8.86558 -0.00000 0.020 18.39999

13.8074 9.23397 9.23396 0.00001 -0.055 22.70000

15.4220 9.55779 9.55779 -0.00000 0,015 27.29999

17.0370 9.85436 9.85437 -0.00001 0.041 32.20000

18.6541 10.12893 10.12892 0.00001 -0.035 37.29999 20.2722 10.38564 10.38564 0.00000 -0.011 42.20000 21.8467 10.62190 10.62190 -0.00001 0.039 46.59999 23.3025 10.83136 10.83135 0.00000 -0.033 49.89999 24.5553 11.00666 11.00666 -0.00000 0.015 52.09999 25.5428 11.14260 11.14260 0.00000 -0.003 53.39999

Standard Deviation of R = 0.00081 Percent

Standard Deviation of T = 0.08401 aKelvins

*«** THESE COEFFICIENTS SELECTED FOR THE (BERATED TABLE *****
6.3.8 The following is an example of a Report of Calibration for a thermocouple

calibrated over the range from °C to 1450 °C.

154
UNITED STATES DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

GAITHERSBURG, MARYLAND

REPORT OF CALIBRATION

TYPE S THERMOCOUPLE

(Tagged: S/N 2211998)

Submitted by

ZZYYXX Company

Hometown, U.S.A

All temperatures in this report are given in degrees Celsius (ITS-90). The International Temperature Scale of 1990

(ITS-90) was adopted by the International Committee of Weights and Measures at its meeting in September 1989, and is described in The International Temperature Scale of 1990", Metrologia 27, No. 1, 3-10 (1990).

The thermocouple was calibrated by comparison with a standard type S thermocouple in the range °C to 1100 °C. Values above 1100 °C were obtained by extrapolation. The calibration procedure is described in Section 3.1 of the National Institute of Standards and Technology Special Publication 250-35, entitled "NIST Measurement Services: The Calibration of Thermocouples and Thermocouple Materials". The thermocouple was electrically annealed in

air before testing.

Table 1 gives corresponding values of the emf of the thermocouple in millivolts and the temperature of its measuring junction in degrees Celsius when the reference junctions are at °C. The uncertainties in the emf values given in the table are estimated not to exceed the equivalent of 0.5 °C in the range °C to 600 "C and the equivalent of 0.7 °C in the range 600 °C to 1100 °C; the uncertainties in the emf values increase above 1100 C C

to not more than the equivalent of 2 °C at 1450 °C. These uncertainties are discussed in Section 5.2 of NIST SP 250-35. Table 2 gives the coefficients of the equations that were used to compute the emf values given

in table 1. Each equation is valid only within the specified temperature range.

The calibration of a thermocouple may change during use. The magnitude of the change depends upon such factors as the temperature, the length of time, and the conditions under which it is used. Factors affecting the performance

of platinum-rhodium versus platinum thermocouples are discussed in NIST SP 250-35 and in "Accurate Thermocouple Thermometry", High Temperatures - High Pressures 11, 173-192 (1979). Copies of these documents

are available from NIST, Chemical Process Metrology Division, Gaithersburg, MD 20899.

For the Director National Institute of Standards and Technology

Dr. Hratch G. Semerjian

Chief, Chemical Process Metrology Division Center for Chemical Technology

National Measurement Laboratory

P.O. No. 123ABC123

Test No. 998877 Date : May 4, 1990
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6.3.9 The following is an example of a Report of Calibration for a liquid- inglass thermometer calibrated over the range from °C to 100 °C on the

IPTS-68(75).

162
U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

(FORMERLY NATIONAL BUREAU OF STANDARDS)

NATIONAL MEASUREMENT LABORATORY

GAITHERSBURG, MD. 2 0899

REPORT OF CALIBRATION

LIQUID-IN-GLASS THERMOMETER

TESTED FOR: NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

DIVISION 586

MARKED: SURETY Tl

RANGE: -2 TO +102 DEGREES C IN 0.2 DEGREE

.MOMETER CORRECTION

READING (IPTS-68) ** -.10 c .10 C

20.00 .05

40.00 -.07

60.00 .01

80.00 -.04

100.00 .00

**ALL TEMPERATURES IN THIS REPORT ARE BASED ON THE INTERNATIONAL

PRACTICAL TEMPERATURE SCALE OF 1968, IPTS-68. THIS TEMPERATURE SCALE

WAS ADOPTED BY THE INTERNATIONAL COMMITTEE OF WEIGHTS AND MEASURES AT

ITS MEETING IN OCTOBER, 1968, AND IS DESCRIBED IN "THE INTERNATIONAL

PRACTICAL TEMPERATURE SCALE OF 1968 AMENDED EDITION OF 1975,"

METROLOGIA 12, NO. 1, 7-17 (1976).

ESTIMATED UNCERTAINTIES IN THE ABOVE CORRECTIONS DO NOT EXCEED 0.05

DEGREE UP TO 102 DEGREES C . FOR A DISCUSSION OF ACCURACIES ATTAINABLE WITH SUCH THERMOMETERS SEE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY SP 250-23, LIQUID-INGLASS THERMOMETER CALIBRATION SERVICE.

IF NO SIGN IS GIVEN ON THE CORRECTION, THE TRUE TEMPERATURE IS HIGHER

THAN THE INDICATED TEMPERATURE; IF THE SIGN GIVEN IS NEGATIVE, THE TRUE

TEMPERATURE IS LOWER THAN THE INDICATED TEMPERATURE. TO USE THE CORRECTIONS PROPERLY, REFERENCE SHOULD BE MADE TO THE NOTES GIVEN BELOW.

THE TABULATED CORRECTIONS APPLY FOR THE CONDITION OF TOTAL IMMERSION OF

THE BULB AND LIQUID COLUMN. IF THE THERMOMETER IS USED AT PARTIAL IMMERSION, APPLY AN EMERGENT STEM CORRECTION AS EXPLAINED IN THE ACCOMPANYING

STEM CORRECTION SHEET.

TEST NUMBER 999999

P.O. NO. 99999

COMPLETED 10-3-89
6.3.10 The following is an example of a Report of Calibration for the liquid- in-glass thermometer of section 6.3.9 calibrated over the range from °C to

100 °C on the IPTS-68(75) but converted to the ITS -90.

164
U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

NATIONAL MEASUREMENT LABORATORY

GAITHERSBURG, MD. 2 0899

REPORT OF CALIBRATION

LIQUID-IN-GLASS THERMOMETER

TESTED FOR: NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

DIVISION 586

MARKED: SURETY Tl

RANGE: -2 TO +102 DEGREES C IN 0.2 DEGREE GRADUATIONS

MOMETER CORRECTION

READING (ITS-90) ** -.10 C .10 C

20.00 .04

40.00 -.08

60.00 -.01

80.00 -.06

100.00 -.03

**ALL TEMPERATURES IN THIS REPORT ARE ON THE INTERNATIONAL TEMPERATURE

SCALE OF 1990, ITS-90. THIS TEMPERATURE SCALE WAS ADOPTED BY THE

INTERNATIONAL COMMITTEE OF WEIGHTS AND MEASURES AT ITS MEETING IN

SEPTEMBER 1989, AND IS DESCRIBED IN ' 'THE INTERNATIONAL TEMPERATURE

SCALE OF 1990", METROLOGIA 27, NO. 1, 3-10 (1990).

ESTIMATED UNCERTAINTIES IN THE ABOVE CORRECTIONS DO NOT EXCEED 0.05

DEGREE UP TO 102 DEGREES C . FOR A DISCUSSION OF ACCURACIES ATTAINABLE WITH SUCH THERMOMETERS SEE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY SP 250-23, LIQUID-INGLASS THERMOMETER CALIBRATION SERVICE.

IF NO SIGN IS GIVEN ON THE CORRECTION, THE TRUE TEMPERATURE IS HIGHER

THAN THE INDICATED TEMPERATURE; IF THE SIGN GIVEN IS NEGATIVE, THE TRUE

TEMPERATURE IS LOWER THAN THE INDICATED TEMPERATURE. TO USE THE CORRECTIONS PROPERLY, REFERENCE SHOULD BE MADE TO THE NOTES GIVEN BELOW.

THE TABULATED CORRECTIONS APPLY FOR THE CONDITION OF TOTAL IMMERSION OF

THE BULB AND LIQUID COLUMN. IF THE THERMOMETER IS USED AT PARTIAL IMMERSION, APPLY AN EMERGENT STEM CORRECTION AS EXPLAINED IN THE ACCOMPANYING

STEM CORRECTION SHEET.

TEST NUMBER 999999

P.O. NO. 99999

COMPLETED 2-2-90
6.4 The National Conference of Standard Laboratories (NCSL) Ad Hoc Committee

(91.3) on the Change in the International Temperature Scale that was formed to

publicize the ITS -90 and its implementation and to minimize the confusion and

inconvenience of the change from the IPTS-68(75) was as follows.

Dr. B. W. Mangum, Chairman

PHY B128

National Institute of Standards

and Technology

Gaithersburg, MD 20899

Mr. George W. Burns

PHY B128

National Institute of Standards

and Technology

Gaithersburg, MD 20899

Dr. Stanley Abramowitz

CHEM B348

National Institute of Standards

and Technology

Gaithersburg, MD 20899

Dr. Richard L. Anderson

Instrumentation and Controls Division

Oak Ridge National Laboratory

Oak Ridge, TN 37830

Laurel Auxier, Manager

Corporate Instrumentation and

Measurement Standards

Smith Kline -Beckman,

Beckman Instruments Division

M/S H-02-C

2500 Harbor Boulevard

Fullerton, CA 92634

Mr. Laurie H. Baker

Metrology Engineering

Autonetics Marine Systems Division

Rockwell International

P. 0. Box 4921

Anaheim, CA 92803

Mr. Jesse Berlanga

NADEP/NPSD, Code 062

North Island, Building 378

San Diego, CA 92135

Dr. George N. Bowers

Director, Clinical Laboratories

Hartford Hospital

Hartford, CT 06115

Mr. Neil L. Brown

Woods Hole Oceanographic Institution

Woods Hole, Ma 02543

Dr. George T. Furukawa

PHY B128

National Institute of Standards

and Technology

Gaithersburg, MD 20899

Mr. Paul D. Levine

Primary Standards Laboratory

0/48-75 B195A

Lockheed Missiles & Space Company

P. 0. Box 3504

Sunnyvale, CA 94808

Dr. Michael R. Moldover

PHY A105

National Institute of Standards

and Technology

Gaithersburg, MD 20899

Mr. Henry Sostmann

2307 Whitley Drive

Durham, NC 27707

Mr. Sidney Ween

Ever Ready Thermometer Co ., Inc

.

401 Park Avenue South

New York, NY 10022

Mr. John Wehrmeyer

Eastman Kodak Co.

3rd Floor, Building 69

Rochester, NY 14650-1912

Mr. James Zugel

Boeing Aerospace Co.

Mail Stop 2P-30

P. 0. Box 3999

Seattle, WN 98124

166
7. NIST Calibration Services, Thermometers

In this appendix, calibration services offered by the Thermometry Group are specified (taken in part from NIST SP 250).

7 . 1 Laboratory Thermometers

Technical Contact

Mailing Address:

Shipping Address

Jacquelyn A. Wise

Tel: 301/975-4822

B126 Physics

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

National Institute of Standards and Technology

1-270 at Quince Orchard Road

A242 Physics

Gaithersburg, MD 20899-0001

Attn: J. A. Wise

Table 1. Calibration Services

Test No Items

31010C

31020C

31030C

31040C

31050C

31060C

31070C

31080C

31100C

31110S

31120S

31130S

31140S

31150S

31160S

31200S

31250S

31260S

Laboratory Thermometers (0 to 150 °C)(32 to 300 °F)

Laboratory Thermometers (151 to 300 °C)(301 to 600 °F)

Laboratory Thermometers (301 to 538 °C)(601 to 1000 °F)

Laboratory Thermometers (-1 to -110 °C)(31 to -166 °F)

Laboratory Thermometers (Liquid N2 )(-196 °C or -320 °F)

Laboratory Thermometers (Liquid 2 )(-183 °C or -297 °F)

Calorimetric Thermometer

Beckmann Thermometer

Quantity Tests of Liquid- in-Glass Thermometers

Special Tests of Thermometers (0 to 150 °C)(32 to 300 °F)

Special Tests of Thermometers (151 to 300 °C)(301 to 600 °F)

Special Tests of Thermometers (301 to 538 °C)(601 to 1000 °F)

Special Tests of Thermometers (-1 to 110 °C)(31 to -166 °F)

Special Tests of Thermometers (Liquid N2 )(-196 °C or -320 °F)

Special Tests of Thermometers (Liquid 2 )(-183 °C or -297 °F)

Preliminary Exam or Ineligible Thermometer

Additional Copy of Report

Special Thermometry Services, by Prior Arrangement

Laboratory Thermometers (31010C-31100C)

This service provides for the calibration of a variety of thermometers covering the

range from -196 to +538 °C (-320 to +1000 °F)

Thermometers belonging to the large and varied group, which may be classed as laboratory or "chemical" thermometers, are regularly accepted. These are of the

167
liquid- in- glass type with either solid- stem or enclosed scale. Ordinary household

or meteorological thermometers will not, in general, be accepted unless the scale

is graduated on the glass stem itself and the thermometer can be readily detached

from its mounting for insertion in a liquid bath. Every thermometer submitted must

be uniquely identified by a serial number and must pass a preliminary examination

for fineness and uniformity of graduation; for cleanliness of the mercury and

capillary bore; for freedom from moisture, gas bubbles, and cracks in the glass; for

adequacy or omission of gas filling where needed; for insufficient annealing; and

for misnumbered graduations. When these or other serious defects are found, the

thermometer is returned untested.

The thermometers to be calibrated are placed in a constant temperature bath along

with a NIST-calibrated standard platinum resistance thermometer (SPRT) . The SPRT

maintains calibrations traceable to the International Temperature Scale of 1990

(ITS-90) with an uncertainty of about 1 mK. (See table 2.)

Table 2: Thermometer Calibration Uncertainties

Type of Thermometer Range Uncertainty

(Total Immersion)

Mercury- in- glass to 100 °C 0.03 to 0.05 °C

(graduations: 0.1-0.2 °C)

Mercury- in- glass to 300 °C 0.2 to 0.5 °C

(graduations: 1-2 °C) 300 to 500 °C 0.5 to 1.0 °C

Organic liquid-in-glass -200 to °C 0.2 to 0.5 °C

Special Tests of Thermometers (31110S-31160S)

Special tests may be conducted on temperature -measuring devices such as industrial

grade platinum resistance thermometers, digital thermometers, and thermistors.

Laboratory personnel should be contacted before submitting items.

168
7.2. Thermocouples, Thermocouple Materials, and Pyrometer Indicators

Technical Contacts: George W. Burns

32010C-32101C, 32150S

Tel: 301/975-4817

Margaret G. Scroger

32010C-32101C, 32150S

Tel: 301/975-4818

Jacquelyn A. Wise

32110C-32147C

Tel: 301/975-4822

Mailing Address: B126 Physics

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

Shipping Address: National Institute of Standards and Technology

1-270 at Quince Orchard Road

Gaithersburg, MD 20899-0001

B229]

For 32010C-32101C, 32150S

[Attn: G.W. Burns or M.G. Scroger, Bldg. 221, Room

For 32110C-32147C

[Attn: J. A. Wise, Bldg. 221, Room A242]

169
Comparison Calibrations , Temperature Measured with Reference Thermocouple

Test No. Items

Temp.

TC Range

Type °C

32010C S 0-1450 1

Interv. Table

32020C R 0-1450

Points

'C or 1 °F

32030C B 0-1750

32031C B 800-1750

32040C

32041C

32042C

32043C

32044C

32050C

32060C

32061C

32070C

E

J

K

N

T

0-1000

0-760

0-1100

0-1100

0-400

4 to 15

4 to 15

4 to 15

4 to 15

4 to 15

Min. Est.

Length Temp. Uncert

(mm) (°C) (°C)

700 0-600 0.5

600 to 1100 0.7

1450 2

700 to 600 0.5

600 to 1100 0.7

1450 2 1000 to 600 (3 MV)

600 to 1100 0.7

1450 2

1750 3

1000 800 to 1100 0.7

1450 2 1750 3 700 1

700 1

700 1

700 1

700 1 Comparison calibration, two point minimum, per point, for all

items above

Each additional table of results at 1 °C or 1 °F intervals, for Type S,

R, or B at later date

Each additional table of results at 1 °C or 1 °F intervals, for Type S,

R, or B at time of test

Thermocouple materials tested against Pt thermoelectric standard, 4 to 15

points , 700 mm minimum lengths

Calibration at Metal Freezing Points, Minimum Diameter 0.4 mm,

Freezing Point Determination at Au, Ag, Sb and Zn

32090C 0-1450 Table, 1 °C or 1 °F 1000

interv. and equations

to generate table

700 to 1100

at freezing 0.2

points

to 700

0.4

1450

0.3

2

32091C Type S, freezing point determination, per point, two point minimum

Calibration of Pyrometer Indicators

32100C Portable Potentiometer, first dial or range

32101C Portable Potentiometer, each additional dial or range

170
Comparison Calibration of Thermocouples or Thermocouple Materials Tested

against Pt Thermoelectric Standard, Temperature Measured with Platinum

Resistance Thermometer, Minimum Length 36 Inches, Two Point Minimum

32110C Range -110 to +300 °C and Liquid N2 (-196 °C) or 166 to 600 °F and

Liquid N2 (-320 °F)

32120C 301 to 538 °C or 601 to 1000 °F

32130C Liquid 2 (-183 °C) or (-297 °F)

Table at one degree intervals for Type T thermocouple for any of the following

options: (The cost of the table will be in addition to the calibration per point

covered under fee schedule items numbered 32110C-32130C.

32141C Option 1: Table

points at -183,

32142C Option 2: Table

points at -183,

32143C Option 3: Table

points at -110, - 32144C Option 4: Table

points at -110, - 32145C Option 5: Table

+100, +200, +300

32146C Option 6: Table

points at -110, - 32147C Option 7: Table

points at -183, - 32150S Special Tests of

from -190 to +300 °C (-310 to +572 °F) , calibration

110, -50, +100, +200, +300 °C

from -190 to +100 °C (-310 to +212 °F) , calibration

110, -50, +50, +100 °C

from -110 to +300 °C (-166 to +572 °F) , calibration

50, +100, +200, +300 °C

from -110 to +100 °C (-166 to +212 °F) , calibration

50, +50, +100 °C

to 300 °C (32 to 572 °F) , calibration points at

°C

from -110 to °C (-166 to +32 °F) , calibration

50 °C.

from -190 to °C (-310 to +32 °F) , calibration

110, -50 °C

Thermocouples and Thermocouple Materials

Thermocouples, Thermocouple Materials , and Pyrometer Indicators (32010C-32147C)

Calibration services for all commonly used types of thermocouples are provided

by NIST from -196 to 1750 °C depending upon the wire or thermocouple type. The

thermocouples are calibrated by one or a combination of three general methods,

depending on the thermocouple type , the temperature range , and the accuracy

required. All three methods provide traceability to the ITS -90. In the first

method, thermocouples are calibrated by comparison with a standard thermocouple

maintained at NIST. In the second method, thermocouples are calibrated by

comparison with a standard platinum resistance thermometer. In the third method,

thermocouples are calibrated at three defining temperatures on the ITS-90--the

freezing points of Zn, Ag, and Au, as well as at the freezing point of Sb (at

about 630.62 °C) . Below °C the thermocouple calibration is carried out in a cryostat, while above °C stirred liquid baths, metal freezing-point cells, and

electric tube-type furnaces are employed for the calibrations. Vacuum or inert

gas furnaces are also available for testing thermocouples.

Only the bare wires are required to perform the thermocouple calibrations. It

is preferable not to send insulating and protecting tubes as the rate of breakage

of these in shipment is high. If the thermocouple is furnished mounted (as in

a protection tube assembly) , a nominal charge will be made for dismantling the

mounting and the various parts will be returned to the sender without

reassembling them. Thermocouple length requirements listed in the appendix are

171
exclusive of lead wire. Lead wires (for connections) need not be sent with

thermocouples. All thermocouple calibration data furnished in reports will be on the basis of a reference junction temperature of °C (32 °F) . The

calibration results will be given in degrees Celsius or degrees Fahrenheit, as requested by the customer.

The calibration of a thermocouple will not be undertaken if it will likely not

yield the specified uncertainty or if it possesses unusual characteristics that

would prevent the calibration or test at a reasonable cost. Only unused basemetal thermocouples and thermocouple materials will be accepted for test.

Special Tests of Thermocouples and Thermocouple Materials (32150S)

For requirements not covered by calibrations described above, special arrangements may be made by contacting one of the specified staff members.

172
7.3 Resistance Thermometry

Technical Contacts Earl R. Pfeiffer

0.65 K to 84 K

Tel: 301/975-4821

Gregory F. Strouse

83 K to 962 °C

Tel: 301/975-4803

Mailing Address B126 Physics

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

Shipping Address: National Institute of Standards and Technology

1-270 at Quince Orchard Road

B04 Physics,

Gaithersburg, MD 20899-0001

Attn: E.R. Pfeiffer or G.F. Strouse

173
Test No, Items

33010C Capsule PRT (13.8 K to 30 °C)

33020C Capsule PRT (13.8 K to 157 °C)

33030C Capsule PRT (13.8 K to 232 °C)

33040C Capsule PRT (54 K to 30 °C)

33050C Capsule PRT (54 K to 157 °C)

33060C Capsule PRT (54 K to 232 °C)

33070C Capsule PRT (84 K to 30 °C) Ar to Ga

33080C Capsule PRT (84 K to 157 °C) Ar to In

33090C Capsule PRT (84 K to 232 °C) Ar to Sn

33100C Capsule PRT (0 °C to 30 °C) TPW to Ga

33110C Capsule PRT (0 °C to 157 °C) TPW to In

33120C Capsule PRT (0 °C to 232 °C) TPW to Sn

33130C Capsule PRT (234 K to 30 °C) Hg to Ga

33140C Germanium and Rhodium- Iron Resistance Thermometers

(0.65 K to 24.6 K)

33150C Long Stem PRT

33160C Long Stem PRT

33170C Long Stem PRT

33180C Long Stem PRT

33190C Long Stem PRT

33200C Long Stem PRT

33210C Long Stem PRT

33220C Long Stem PRT

33230C Long Stem PRT

33240C Long Stem PRT

33250C Long Stem PRT

33260C Long Stem PRT

33270C Long Stem PRT

33280C Long Stem PRT

33290C Long Stem PRT

33300C Long Stem PRT

33310C Long Stem PRT

33320C Additional Copy of Table from Results of 33010C-33310C

at Time of Test

33330C Additional Copy of Table from Results of 33010C-33310C

at a Later Date

33340C Minimum Charge for Unsuitable Thermometer

33350S Special Tests of Resistance Thermometers

33360S Special Tests of Thermometric Fixed-Point Devices

33370M Measurement Assurance Program for Temperature

(8 3 K to 420 °C) Ar to Zn

33380M Measurement Assurance Program for Temperature

(83 K to 661 °C) Ar to Al

33390M Measurement Assurance Program for Temperature

(0 °C to 962 °C) Ar to Ag

(83 K to °C) Ar to TPW

(83 K to 30 ° C) Ar to Ga

(83 K to 157 °C) Ar ito In

(83 K to 232 °C) Ar i to Sn

(83 K to 420 °C) Ar 1 to Zn

(83 K to 661 °C) Ar 1 to M

(234 K to 30 °C) Hg 1 :o Ga

(234 K to 157 °c; Hg to In

(234 K to 232 °c; Hg to Sn

(234 K to 420 °c; Hg to Zn

(234 K to 661 °C) Hg to Al

(0 °C to 30 ° C) TPW 1 :o Ga

(0 °C to 157 °C) TPW to In

(0 °C to 232 °C) TPW to Sn

(0 °C to 420 °C) TPW to Zn

(0 °C to 661 °C) TPW to Al

(0 °C to 962 °C) TPW to Ag

Resistance Thermometers (33010C-33310C)

NIST provides calibration services for standard platinum resistance thermometers

174
(SPRTs) from 13.8 K to 1235 K. Both long- stem and capsule -type SPRTs are calibrated, providing direct access to the International Temperature Scale of

1990 (ITS-90). The reproducibility of the calibration measurements of SPRTs is

better than 0.3 mK and the total uncertainty of calibration is ± 1 mK at

temperatures below 661 °C. The reproducibility increases to about 0.5 mK and

the uncertainty to about ± 2 mK at 962 °C. The reproducibility of triple point

of water cells used for calibration of SPRTs is ± 50 ^K or better while that for

Ar, Hg, Ga, In, Sn, Zn, and Al triple-point or freezing-point cells is 0.1 mK;

and that for Ag freezing-point cells is 0.5 mK. Presently, the NIST temperature

scale in the range 13.8 K to 84 K is based on stable reference SPRTs of the

capsule type that have uncertainties of about ± 0.3 mK down to about 20 K. Below

20 K, the uncertainty degrades to about ± 0.5 mK as the SPRTs become less

sensitive. For calibration of rhodium- iron resistance thermometers (RIRTs) and

germanium resistance thermometers (GRTs) in the temperature range from 0.65 K

to 24.6 K, the NIST scale is based presently on reference standard rhodium- iron

resistance thermometers. The uncertainty of those calibrations is about

± 0.3 mK.

To qualify for testing, either long- stem or capsule platinum resistance

thermometers must meet two conditions. They must reasonably be expected to meet

the requirements of the ITS-90 for a standard interpolating instrument (i.e., a four lead resistor of high-purity platinum hermetically sealed in a protecting

tube). Second, they must be compatible with the NIST calibration equipment.

It is important that, insofar as possible, resistance thermometers be protected

from any mechanical shock that could alter their calibration. For shipment, the

thermometer should be softly supported within a case but not be free to rattle.

This necessitates the use of packing material that does not become compacted.

The thermometer case should, in turn, be softly packed inside a shipping

container. The outside shipping container must be sufficiently rigid and strong

that it will not appreciably deform under the treatment usually given by common

carriers. Styrofoam is not sufficiently rigid to be used as an outside

container. Thermometers will not be returned in containers that are obviously

unsuitable, such as those closed by nailing. Suitable containers will be

provided, for a fee, when a thermometer shipping container is not satisfactory

for re-use.

Special Tests of Resistance Thermometers and Special Tests of Thermometric FixedPoint Devices (33350S and 33360S)

Special tests of various resistance thermometers and thermometric fixed point

devices may be obtained by prior arrangement with the specified technical

contacts

Measurement Assurance Program for Temperature (33370M, 33380M, and 33390M)

The purpose of this MAP service is to assure the accuracy of the calibration of

temperature standards (83 K to either 420 °C or 661 °C, or °C to 962 °C

temperature ranges) made by participating laboratories when using platinum

resistance thermometry.

The MAP transport standard consists of a set of three commercial glass -sheathed

SPRTs packaged in a special shock-proof shipping container (mechanical shock or

175
sudden temperature excursions may result in shifts in calibration) . These SPRTs

are used to assess both the reproducibility and the accuracy of calibrations

performed by the participating laboratory.

MAP participants should use the techniques described in NBS Monograph 126 or NBS

Special Publication 250-22 and the same fixed points as those used in the NIST

calibrations, or use an SPRT previously calibrated by NIST. In order to achieve

high accuracy, SPRTs used as standards should be of the matte -finish type to

avoid systematic errors arising from light-pipe effects in the glass sheath.

The participant must have a triple point of water cell and a calibrated

resistance bridge.

After unpacking and inspecting the SPRTs for damage, the participant should

measure the resistances of the SPRTs at the triple point of water (using the

triple-point cell). This preliminary measurement of the resistances at the

triple point of water is used as a "go/no go" check to ensure that the

thermometers have not been damaged in shipment.

These preliminary measurements are reported by telephone to NIST; if the values

are consistent with the data taken by NIST before shipment, the participant

should proceed with further measurements at the fixed points of the ITS -90. NIST

provides a worksheet on which the participant should record data. The

participant then calculates the thermometer constants from experimental data,

records them, and prepares tables of resistance ratio or resistance versus

temperature. The SPRTs are recalibrated upon return to NIST and the

participant's data are compared with NIST's calibrations. NIST provides a plot

of the participating laboratory's temperature deviation from NIST values and a written analysis of the data, including any pertinent observations. In a typical

MAP transfer, the participant makes several measurements over a period of one

to two months. A typical turnaround time from the date NIST receives the

participant's data until a test report is sent to the participant is four to six

weeks

.

The best NIST SPRT calibrations have precisions of about 0.1 to 0.2 mK. Sources

of error that may contribute to the total uncertainty include changes in the

calibration of the measurement instruments, changes in the SPRT itself, and

uncertainty of the degree of purity of the materials used as fixed-point

references (e.g. , zinc). As a result of quantifying these errors, NIST currently

assigns an uncertainty of 1 mK to the values assigned to the MAP transport

standards used below 661 °C. A standards laboratory conscientiously

participating in this MAP and having suitable equipment should be able to closely

approximate this uncertainty figure. Former participants in the MAP have had

uncertainties that ranged from about 1 mK to several tens of millikelvins

.

No rigid recommendations can be given concerning how often a participant should

utilize the temperature MAP service. Experience has indicated that when

temperature measurements are in a state of statistical control, as evidence by

in-house check standards and control charts to monitor the measurement process,

the participant (s) should be able to go at least three years between transfers

from NIST without significantly degrading the confidence in the correctness of

the measurements

.

176
filST-114A

(REV.-3-89) U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR REPORT NUMBER

NIST/TN-1265

2. PERFORMING ORGANIZATION REPORT NUMBER

3. PUBUCATION DATE August 1990

4. TITLE AND SUBTITLE Guidelines for Realizing the International Temperature Scale of 1990 (ITS-90)

5. AUTHOR(S)

B . W . Mangum

G . T . Furukawa

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

GAITHERSBURG, MD 20899

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

Same as item #6

10. SUPPLEMENTARY NOTES

DOCUMENT DESCRIBES A COMPUTER PROGRAM; SF-185, FIPS SOFTWARE SUMMARY, IS ATTACHED.

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBLIOGRAPHY OR

LITERATURE SURVEY, MENTION IT HERE.)

This Technical Note describes the International Temperature Scale of 1990 (ITS-90) that became

the official international temperature scale on 1 January 1990, superseding the previous

scales, and provides information on how the scale may be realized at different levels of

accuracy. The ITS-90 extends upward from 0.65 K, is in close agreement with the Kelvin

Thermodynamic Temperature Scale, has much improved continuity, precision and reproducibility

throughout its ranges over that of previous scales, and has subranges and alternative

definitions in certain ranges that greatly facilitate its use. In addition to a description

of the ITS-90 and how it can be realized, there are included in this document reproductions

of some articles or excerpts from articles concerned with the ITS-90. The composition of the

Comite Consultatif de Thermometrie (CCT) of the Comite International des Poids et Mesures

(CIPM) at the time of the adoption of the ITS-90 is given. The differences between the

temperatures on the ITS-90 and those on the International Practical Temperature Scale of 1968,

Amended Edition of 1975, [IPTS-68(75) ] and those on the 1976 Provisional 0.5 K to 30 K

Temperature Scale (EPT-76) are tabulated. Measurement procedures for realizing the ITS-90

throughout the various ranges of the scale are given. Also, for the most important temperature

region, the region of the platinum resistance thermometer (PRT) , computational examples are given for determining the coefficients of the relevant deviation equations for PRTs calibrated

at various sets of fixed points. The effects of the introduction of the ITS-90 on electrical

reference standards are addressed also.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

fixed points; ITS-90; International Temperature Scale of 1990: SPRTs; temperature;

temperature scale; thermometers; thermometry

13. AVAILABILITY UNLIMITED FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161.

14. NUMBER OF PRINTED PAGES

190

15. PRICE

ELECTRONIC FORM i? U.S. GOVERNMENT PRINTING OFFICE: 1990 -2 6 1-913 '20673

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