PUBLICATIONS ^B^EREfCE

NIST SPECIAL PUBLICATION 260-139

U. S. DEPARTMENT OF COMMERCE/Technology Administration

National Institute of Standards and Technology

Standard Reference Materials

Standard Reference Material 1750:

Standard Platinum Resistance Thermometers,

13.8033 K to 429.7485 K

W.L. Tew and G.F. Strouse

El

Institute of Standards and Technology QC f Administration, U.S. Department of Commerce

100

.U57

#260-139

2001

NIST CENTENNIAL
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NIST Special Publication 260-139

Standard Reference Materials

Standard Reference Material 1750:

Standard Platinum Resistance Thermometers,

13.8033 K to 429.7485 K

W. L. Tew

G. F. Strouse

Process Measurements Division

Chemical Science and Technology Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8363

U.S. DEPARTMENT OF COMMERCE, Donald L. Evans, Secretary

TECHNOLOGY ADMINISTRATION, Phillip J. Bond, Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Karen H. Brown, Acting Director

Issued November 200
Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended

to imply recommendation or endorsement by the National Institute of Standards and

Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 260-139

Natl. Inst. Stand. Technol. Spec. Publ. 260-139, 53 pages (November 2001)

CODEN: NSPUE2

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WASHINGTON: 2001

For sale by the Superintendent of Documents, U.S. Government Printing Office

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Mail: Stop SSOP, Washington, DC 20402-0001
Table of Contents

Page

1. Introduction 2

1.1 Specifications 2

1.2 Reference Functions 3

1.3 Deviation Functions 4

2. Calibration Procedures 7

2.1 Fixed-Point Methods 7

2.2 Comparison Methods 10

3. Results 13

3.1 Deviations at Fixed-Point Temperatures 13

3.2 Uncertainties 21

3.3 Supplementary Data 25

4. Summary 35

5. References 36

Appendix: Certificate for SRM 1750 .... 39

in
List of Figures

Figure No. Page

1. ITS-90 Reference Function for the range 13.8 K to 273.16 K 6

2. ITS-90 Reference Function for the range 273.15 K to 1234.93 K 6

3. The immersion profile for a SRM 1750 SPRT in a NIST Hg TP cell 8

4. The deviations for the SRM 1750 at various fixed-point temperatures 15

5. The deviations for the SRM 1750 at the Hg TP vs. Ga MP 15

6a. The deviations for the SRM 1750 at the Ar TP vs. Ga MP 16

6b. The deviations for the SRM 1750 at the 02 TP vs. Ga MP 16

6c. The deviations for the SRM 1750 at the Ne TP vs. Ga MP 17

6d. The deviations for the SRM 1750 at the e-H2 VP2 vs. Ga MP 17

6e. The deviations for the SRM 1750 at the e-H2 VPt vs. Ga MP 18

6f. The deviations for the SRM 1750 at the e-H2 TP vs. Ga MP 18

7. The deviations of the lowest seven fixed points vs. the Ga MP expressed in mK 19

8. Individual deviation functions for the SRM 1750 vs. W value 20

9. Individual deviation values for the SRM 1750 vs. temperature 20

10. Uncertainty propagation curves for the range 13.8 K to 273.16 K 23

11. Uncertainty propagation curve for the range 273.15 K to 429.7485 K 24

12. Uncertainty propagation curve for the user's TPW measurement 24

13a. The predicted and measured deviation differences for the first batch comparison.... 26

13b. The predicted and measured deviation differences for the second batch comparison 27

14. The correlation plot for the In FP vs. the Ga MP for the SRM 1750 28

15a. The correlation plot for the Ar TP vs. the

4He NBP... 31

15b. The correlation plot for the 02 TP vs. the

4He NBP 32

15c. The correlation plot for the Ne TP vs. the

4He NBP 32

15d. The correlation plot for the e-H2 VP2 vs. the

4He NBP 33

15e. The correlation plot for the e-H2 VP, vs. the

4He NBP 33

15f. The correlation plot for the e-H2 TP vs. the

4He NBP 34

IV
Standard Reference Material 1750: Standard Platinum Resistance Thermometers,

13.8033 K to 429.7485 K

Weston L. Tew and Gregory F. Strouse

National Institute of Standards and Technology

Process Measurements Division

Gaithersburg, MD 20899-8363

Abstract

The Standard Platinum Resistance Thermometer (SPRT) is defined by the International

Temperature Scale of 1990 (ITS-90) as the interpolating instrument for temperatures between

13.8033 K and 1234.93 K. This SRM concerns the calibration characteristics of a group of 20

capsule-type SPRTs within the range of temperatures between 13.8033 K and 429.7485 K. The

platinum wire used for the construction of the resistor elements of this SRM was derived from

platinum bar stock with a chemical purity of 99.999 % by weight. Each of the 20 units has been

evaluated, certified, and calibrated according to the definitions of the ITS-90. The resistance

characteristics of all 20 units have been measured at 1 1 fixed point temperatures from 4.22 K to 429.7485 K as well as at least 7 other intermediate points. The procedures used in performing the

calibrations and the supplemental measurements are documented in this publication. The resulting

analysis pertaining to each individual unit, as well as the SRM sample population as a whole, is presented in detail.

Disclaimer

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

to adequately specify the experimental procedure. Such identification does not imply

recommendation or endorsement by NIST, nor does it imply that these materials or equipment are necessarily the best available for the purpose.

Acknowledgement

The authors wish to thank the Standard Reference Materials Program for their support in the

development of the SRM.

l
1. Introduction

This analysis concerns the 20 units of SRM 1750 Standard Platinum Resistance Thermometers

(SPRTs) that have all been calibrated according to the definitions of the International Temperature

Scale of 1990tl] (ITS-90). Each calibration consists of a series of resistance measurements of the

SPRT at 10 different fixed-point temperatures. These fixed points are listed in Table 1, along with

their defined temperatures, Tc,0 on the ITS-90.

Table 1. The 10 fixed points of the ITS-90 used for the SRM 1750 SPRT calibrations.

iNotation Descriptive Name i 90 ' iv e-ri2 l " equilibrium-hydrogen triple point

t> T-T VP point or ecjuiiiununi-iiquia nyarogen unaer a saturatea vapor pressure near 33.3213 kPa

1 / .UJO

e-H2 VP2 point of equilibrium-liquid hydrogen under a saturated vapor pressure near 101.292 kPa20.2714

Ne TP neon triple point 24.5561

02 TP oxygen triple point 54.3584

Ar TP argon triple point 83.8058

Hg TP mercury triple point 234.3156

H2 0 TP water triple point 273.16

Ga MP gallium melting point 302.9146

In FP indium freezing point 429.7485

The calibration data are used to produce a set of numerical coefficients for two deviation equations

for the ITS-90 sub-ranges of 13.8 K to 273.16 K and 273.15 K to 429.7485 K. When these two

deviation equations are combined with the appropriate ITS-90 reference functions, the SPRT can be

used to interpolate all temperatures within the range of calibration. Further details concerning the structure of the scale may be found in NIST Technical Note 1265.

1.1. Specifications

SPRT calibrations on the ITS-90 are parameterized in terms of the resistance ratio W(Tqo) as defined by

W(Tj S -^L (1)

^V* H2OTP / where R(Th2oTP) is the SPRT resistance at the water triple point (H2 0 TP). The most important

specification for any SPRT is the rTS-90 purity criterion for the platinum element. This criterion is specified in terms of the measured W(T<)q) value at the melting point of gallium (Ga MP) or the

triple point of mercury (Hg TP), as given by

M/(rGaMP )> 1.11807, (2) or W{THSTP )< 0.844235. (3)

One of these inequalities must be met in order for an SPRT to serve as a defining instrument of the

ITS-90. The platinum wire that was used in the construction of the SRM 1750 SPRTs was derived

2
from bar stock with a purity of 99.999+ % as reported by the supplier . However, the purity of the

platinum is verified only for wire of 0.5 mm diameter or greater. Since the SRM 1750 SPRTs are constructed primarily with wire of 0.076 mm diameter, the actual purity level for that wire is expected to be slightly lower. Nonetheless, this platinum wire purity level is still sufficient to yield

W(TGaTP)> 1.1181175 and W(THgTP) ^ 0.844165 for SRM 1750, which well exceeds the ITS-90

criterion.

The resistance value for each of the SRM 1750 SPRTs at the H2 0 TP is the customary value for

capsule SPRTs, or /?(7h-,otp) = (25.5 ± 0.1) Q, but within a somewhat smaller tolerance than is normally found in commercial devices. Other important design specifications include a strain-free

platinum element and a hermetically sealed capsule containing high-purity He gas. The design of

the element used in the case of SRM 1750 is that of the so called "bird cage" type'

41

. In these and

other respects, the specifications for the SRM 1750 do not exceed, or significantly depart from,

those that are customary for most commercial capsule SPRTs. [5]

The construction of the capsule for the SRM 1750 is typical of certain commercial types of SPRTs.

The outer sheath is made from a nickel alloy with a 5.7 mm diameter which is lined with an inner

platinum sheath. The overall length is approximately 46 mm and each unit is engraved on the outer

sheath with the lettering "NIST SRM 1750," together with a four digit serial number (4450 through

4459 for the first calibration batch; and 4462, 4463, 4486 through 4493 for the second calibration

batch). The four Pt lead wires are 0.3 mm in diameter and 10 cm long, insulated with a combination

of polypyromelitimide (PPMI) and polytetraflouroethylene (PTFE), and terminated with 0.75 mm

diameter gold-plated beryllium-copper contact pins. Each SRM 1750 SPRT is provided with 1.) a borosilicate glass adapter probe, 2.) a nylon tee connector for a purge gas connection and 3.) a polarized four-wire electrical connection with a lead-wire harness. The probes dimensions are 7.5 mm in outer diameter and approximately 60 cm- in length.

1.2. Reference Functions

The ITS-90 specifies two reference functions that cover the overall SPRT temperature range, and

we refer to them here as the lower range reference function and the upper range reference function.

One reference function spans the range 13.8033 K to 273.16 K ("lower range"), and the other

reference function spans the range 273.15 K to 1234.93 K ("upper range"). The reference functions,

denoted as WT (T9o), are representations based on two real SPRTs of very high purity and are designed to closely approximate any real SPRT resistance ratio, W(T90 ). Figures 1 and 2 are plots of

the lower range and upper range reference functions respectively.

The lower range reference function is given by

12 ln(r90 /273.16 K) + 1.5

1.5 (4)

where the values for the A, are given in Table 2A and 13.8033 K < 790 < 273.16 K.

The upper range reference function is given by

(5)

3
where the values for the Q are given in Table 2B and 273.15 K < T90 < 1234.93 K.

Table 2A. Lower range reference function. Table 2B. Upper range reference function.

A0 -2.135 347 29x10^ Co 2.781 572 54x10"°

Ai 3.183 247 20x10^ c, 1.646 509 16x10^

A2 -1.801 435 97x10^ c2 -1.371 439 00x10"'

A, 7.172 720 40x10"' c3 -6.497 670 OOxlO"3 A4 5.034 402 70x10"' c4 -2.344 440 OOxlO"3 A5 -6.189 939 50x10"' C5 5.118 680 OOxlO"3 A6 -5.332 322 OOxlO"2 c6 1.879 820 OOxlO"3 A7 2.802 136 20x10"' c7 -2.044 720 OOxlO"3 A8 1.071 522 40x10"' c8 -4.612 200 OOxlO"4 A9 -2.930 286 50x10"' c9 4.572 400 OOxlO"4 Aw 4.459 872 OOxlO"2

A,, 1.186 863 20x10"' A12 -5.248 134 OOxlO"2

1.3. Deviation Functions

The extent to which an SPRT does not perfectly conform to the ideal representation given by the

reference function is referred to as the deviation, or deviation function, which is defined as the

difference between the observed resistance ratio and the reference function value, or AW(Tj^W(T90 )-Wr (T90 ). (6)

The SPRT deviation is that part of its resistance-temperature relationship which is considered

unique to each SPRT and must be included in the form of a specified deviation function as part of

the calibration process for each SPRT. The form of the deviation depends on which of the different

calibration sub-ranges is being used. In all cases there are one or more unspecified coefficients in

the deviation function that are uniquely determined for the SPRT from the fixed-point calibration

data.

1.3.1. Calibration Sub-range 13.8033 K to 273.16 K

The SPRT resistance is measured at the equilibrium hydrogen triple point (e-H2 TP), the

equilibrium hydrogen vapor pressure point (e-H2 VPj) near 33.32 kPa (T90=17.035 K), the

equilibrium hydrogen vapor pressure point (e-H2 VP2 ) near 101.29 kPa (T90=20.27 K), the neon

triple point (Ne TP), the oxygen triple point (02 TP), the" argon triple point (Ar TP), the mercury

triple point (Hg TP), and the HiO TP. The deviation function for this calibration sub-range is given

by

AW, (r90 ) = a, [w(T90 )-l] + bx [W(T90 ) - 1]

2 + £c, {\n[w{T9Q )]f

2

. (7)

The coefficients a u b\, C\, c2 , C3, C4 and C5 are determined by solving a system of seven simultaneous equations using the fixed-point calibration data. These data must include the results of

4
measurements of W(7,c -h2tp), VK(re .H 2 vp,), W(re .H 2 vp2

), W(TNeTP), W(To2tp) , W(7aitp) and WTh«tp)

for the SPRT. This sub-range covers the entire span of the lower range reference function and is the

most complex of all the SPRT calibration sub-ranges.

1.3.2. Calibration Sub-range 273,15 K to 429.7485 K

The SPRT resistance is measured at the indium freezing point (In FP) and at the H2 0 TP. The

deviation function for this calibration sub-range is given by

AWw (Tj = aJw(T90 )-\}. (8)

The coefficient a\o is determined by solving the equation using the fixed-point calibration data.

These data must include the results of measurements of W(Tin¥p) for the SPRT. The gallium melting

point (Ga MP) is a redundant point within the calibration sub-range that is used as a checkpoint for

the calibration.

5
Figure 2. The SPRT reference function for the range 273.15 K to 1234.93 K. The values of the function at the defining fixed-point temperatures are also shown.
2. Calibration Procedures

As mentioned above, the SRM 1750 capsule SPRTs have been calibrated on the ITS-90 over the

following two sub-ranges: 1) 13.8033 K to 273.16 K and 2) 273.15 K to 429.7485 K. This

calibration involved the NIST realization of 10 different fixed points, although only nine are required (the Ga MP is redundant). Five of these fixed points (In FP, Ga MP, H20 TP, Hg TP, and

Ar TP) were realized directly with fixed-point cells at the time of the calibration of each SPRT. [6] The other five fixed points (e-H2 TP, e-H2 VP,, e-H2 VP2 , Ne TP, and 02 TP) were realized at an

earlier time using fixed-point cells and NIST reference capsule SPRTs. [1] The lower five fixedpoint temperatures were then transferred to the SRM SPRTs via comparison to the reference

SPRTs.181 This is essentially the same method that is normally used at NIST to calibrate any

customer SPRT over this same range of temperatures.

Measurements of each SPRT's resistance at the respective fixed-point temperatures were made via an automatically balancing ac resistance-ratio bridge. The ratio measurements were performed at a

carrier frequency of 30 Hz, using either a 1 Q, 10 Q, or 100 Q resistance standard, Rsld , and the

various excitation currents, i\, /2 , as shown in Table 3. Other parameters shown in Table 3 are the

values of the SPRT reference function WT (T<)o), and the approximate measured resistance ratio r = R(Tgo)/Rstd.

Table 3. Measurement parameters for the 10 fixed-point temperatures used for SRM 1750.

Fixed Point 7WK Wr(T90) r 7?std / Q. i\, ij 1 mA

InFP 429.7485 1.60980185 0.410 100 1.0, 1.414

Ga MP 302.9146 1.11813889 0.285 100 1.0, 1.414

H20 TP 273.16 1.00000000 0.255 100 1.0, 1.414

HgTP 234.3156 0.84414211 0.218 100 1.0, 1.414

Ar TP 83.8058 0.21585975 0.055,0.55 100, 10 1.0, 2.0

02 TP 54.3584 0.09171804 0.234 10 1.0, 2.0

Ne TP 24.5561 0.008449736 0.215 1 2.0, 2.828

e-H2 VP2 20.2714 0.004235356 0.108 1 2.0. 2.828

e-H2 VP, 17.036 0.002296459 0.058 1 2.828, 5.0

e-H2 TP 13.8033 0.001190068 0.030 1 2.828, 5.0

2.1. Fixed-Point Methods

The use of capsule SPRTs in fixed-point cells that are designed for long-stem thermometers

requires an adapter probe for the capsule. A variety of such probes have been constructed in the

past at NIST[9] and elsewhere[10] for this purpose. For SRM 1750, NIST has chosen a precision-bore

borosilicate tube which closely fits the diameter of the SPRT. The probes are used to calibrate the

SPRTs in the five fixed-point cells referred to above. When in use, the probes are pressurized with

He gas to a level slightly above the ambient atmospheric pressure.

Any adapter probe constructed for SPRT calibration must achieve adequate immersion in the fixedpoint cells.

[U] This can be demonstrated by performing an immersion profile measurement in which

measurements are taken as the probe is step-wise lowered into the last few centimeters of the cell's re-entrant well. If the immersion is adequate over this region, the data should produce the profile of

the static pressure-head effect for the particular material in the cell.

[12]

7
The immersion characteristics of an SRM 1750 SPRT in its glass adapter probe are shown in Figure 3 for a NIST Hg TP cell.

[ 131 The data are shown for SPRT resistances extrapolated to zero power dissipation as a function of immersion depth relative to the bottom of the re-entrant well.

The theoretical immersion profile is shown according to the static pressure head generated by the

mercury, which is 7.1 mK/m or about 7.1 (LiiQ/cm. The data are in good agreement with the

expected profile and indicates the immersion of this probe is adequate for this Hg TP cell.

21.464275 n

,

21.464235

0 1.2 3 4 5 Depth / cm

Figure 3. The immersion profile for a SRM 1750 SPRT in a NIST Hg TP cell. Full immersion is 15 cm

between the element center and the top of the solid/liquid metal column, shown as "0 cm" depth in the

figure. The error bars represent type A k=l uncertainties.

The standard calibration process control employed at NIST for any SPRT calibration using

fixed-point cells involves the use of NIST check thermometers with every fixed point. For the

fixed-point cells discussed here, the check thermometers are long-stem SPRTs, each of which is dedicated for use with a particular fixed point.

2.1.1. The freezing point of indium

The serial number of the In freezing point cell used in the realizations for SRM 1750 is In 96-3.

The metal purity is 99.999 99 % as derived from SRM 1745. [14] The static pressure-head correction

applied was 0.59 mK based on an immersion depth of 18 cm from SPRT element midpoint to the

top surface of the metal column.

2.1.2. The melting point of gallium

The gallium melting point was realized by a melting plateau under saturated vapor pressure

conditions, (i.e., as a triple point). A correction of 2.01 mK was applied to correct for the absence

8
of an external pressure. The serial number of the Ga triple-point cell used in the realizations for

SRM 1750 is Ga 943. The metal purity is 99.999 995 % as derived from recent NIST results" 51 in

preparation for a future SRM. The static pressure-head correction applied was -0.22 mK based on

an immersion depth of 18 cm from the SPRT element midpoint to the top surface of the metal

column.

2.1.3. The triple point of water

The serial numbers of the water triple point cells used in the realizations for SRM 1750 are A-13-1126, A-13-1287, and A-13-1288. These cells are essentially indistinguishable in their

characteristics and were all made and purified at a local commercial facility"

61 from highly distilled

water derived from the Potomac River. The exact purity of the water in these cells is unknown,

however, they have been compared with other water triple point cells from other national

laboratories and are generally accepted as the highest quality available today[17]

. The static

pressure-head correction applied was -0.186 mK based on an immersion depth of 26.5 cm from the

SPRT element midpoint to the top surface of the water column.

2.1.4. The triple point of mercury

The serial number of the Hg triple point cell used in the realizations for SRM 1750 is Hg SS-1.

[1S|

The metal purity is 99.999 999 % as derived from SRM 743. 1191 The static pressure head correction

applied was 1.065 mK based on an immersion depth of 15 cm from the SPRT element midpoint to the top surface of the metal column (20 % melted fraction).

2.1.5. The triple point of argon

The argon triple-point cell used in the realizations for SRM 1750 is a large apparatus with six wells

designed for long-stem SPRTs and one central well designed for capsule SPRTs!20]

. The SRM 1750

SPRTs were all calibrated in this apparatus in the long-stem SPRT wells using the glass adapter

probes. For the purposes of this report, the argon apparatus is designated as "Ar-NIST-lX7." The

triple point is realized in a melting mode at approximately 20 % melted fraction. The gas purity of

the argon as supplied from a commercial source is 99.9999 %. [21] The static pressure head

correction applied was 0.36 mK based on an immersion depth of 10.9 cm from the SPRT element

midpoint to the top surface of the solidified gas column (20 % melted fraction).

9
2.2. Comparison Methods

Comparison measurements for capsule SPRTs are performed at temperatures of 83.8 K and below.

The comparisons for SRM 1750 were made in two batches of ten SPRTs each. The batch of SRM

thermometers, along with a capsule reference thermometer and three other check thermometers, were placed in a series of close-fitting wells within a high-purity copper comparison block. The

comparison block was controlled at an appropriate constant temperature under high vacuum

conditions and surrounded by a series of isothermal shields.

Each SRM batch thermometer (BT) was measured at the two excitation currents i\ and ij (see Table

3) in a symmetrical sequence, starting and ending with the reference thermometer (RT). The

sequence was a series of seven measurement records: RT(z'i), RTfe), BT(z'i), BT( i-i), BT(z'i), RT(*2),

RT(z'i). Each measurement record in the sequence consists of an average of 20 individual bridge

readings. All reference SPRT measurements were made with two currents and resistance ratios

corrected to zero-power dissipation. The average zero-power resistance of the reference

thermometer's initial and final records was then used to compute the block temperature for the

particular batch thermometer being measured. The reference thermometer was a capsule SPRT

(serial no. 1004131) previously calibrated at NIST according to the ITS-90[22] using recent

realizations of all required fixed points. These fixed-point realizations were performed according to any one of three independent techniques used at NIST

[20- 23 - 24] xhe check thermometers consisted of

two other similarly calibrated capsule SPRTs and two calibrated Rhodium-Iron Resistance

Thermometers (RIRTs). These check thermometers provided checks on the entire calibration

process through comparison of their indicated temperatures.

2.2.1. The triple point of argon

This comparison point was equivalent to and, hence, redundant with the fixed-point realization

described under section 2.1.5. It was performed primarily as a systematic check to verify the

consistencies between the two separate measurement systems which were used. Comparison

measurements at the ArTP temperature with the first batch of SRM 1750 SPRTs were performed in

October 1998. Comparison measurements at the Ar TP temperature with the second batch of SRM

1750 SPRTs were performed in April 1999. During the first batch comparison calibration,

comparisons between the reference SPRT (s/n 1004131) and check SPRTs (s/n 1842385 and s/n

1812279) indicated agreement to within 0.2 mK in indicated temperature. During the second batch

comparison calibration, comparisons between the reference SPRT (s/n 1004131) and check SPRTs

(s/n 1812282 and s/n 1812284) indicated agreement to within 0.02 mK in indicated temperature.

These levels of agreement are generally within the expanded uncertainty for the comparison process

at 83.8058 K.

The reference SPRT 1004131 was originally calibrated at the argon triple point at NIST in

December of 1994 according to the techniques given by Meyer and Reilly. [2 1 This reference SPRT

has recently undergone additional check realizations at NIST with other argon triple point cells

including Ar-NBS-1 [25] and Ar-NIST-lX7. The argon gas used in these other cells was derived from

the same source,

[21] as was the case for all NIST realizations of the Ar TP. Those check realizations

indicate consistency with the 1994 realization to within 0.2 mK.

10
2.2.2. The triple point of oxygen

Comparison measurements at the O2 TP temperature were performed in October 1998 with the first batch of SRM 1750 SPRTs, and in April 1999 with the second batch. During the first batch

comparison calibration, comparisons between the reference SPRT (s/n 1004131) and check SPRTs

(s/n 1842385 and s/n 1812279) indicated agreement to within 0.1 mK in indicated temperature.

During the second batch comparison calibration, comparisons between the reference SPRT (s/n

1004131) and check SPRTs (s/n 1812282 and s/n 1812284) indicated agreement to within 0.01 mK

in indicated temperature. These levels of agreement are generally within the expanded uncertainty

for the comparison process at 54.3584 K.

The reference SPRT 1004131 was originally calibrated at the 02 TP at NIST in February 1995

according to the techniques given by Meyer and Reilly.

[23] This reference SPRT has recently

undergone additional check realizations at NIST with one other 02 TP cell, "PO-l."[26] The oxygen

gas used in this other cell was derived from the same source

1261 as was used in the 1995 realization.

Those check realizations indicate consistency with the 1995 realization to within 0.05 mK.

2.2.3. The triple point of neon

Comparison measurements at the Ne TP temperature were performed in November 1998 with the

first batch of SRM 1750 SPRTs and in May 1999 for the second batch. During the first batch

comparison calibration, comparisons between the reference SPRT (s/n 1004131), the check SPRTs

(s/n 1842385 and s/n 1812279), and the check RIRTs (s/n B-174 and s/n B-168) indicated

agreement to within 0.32 mK in indicated temperature. During the second batch comparison

calibration, comparisons between the reference SPRT (s/n 1004131), the check SPRTs (s/n 1812282

and s/n 1812284), and the check RIRTs (s/n B-174 and s/n B-168) indicated agreement to within

0.2 mK in indicated temperature. These levels of agreement are generally within the expanded

uncertainty for the comparison process at 24.5561 K.

The reference and check SPRTs (s/n 1004131 and s/n 1842385) as well as the check RIRTs (s/n

B-174 and B-168) were originally calibrated at the Ne TP at NIST in March 1995 according to the

techniques given by Meyer and Reilly. Independent realizations of the Ne TP using sealed

cells

[24] have recently been made at NIST using RIRT B-174 which indicate agreement with the

1995 realizations to within 0.2 mK.

2.2.4. The point of equilibrium-liquid hydrogen under a saturated vapor pressure near 101.292 kPa

Comparison measurements at an e-H2 VP2 temperature of 20.2741 K were performed in November

1998 with the first batch of SRM 1750 SPRTs and in May 1999 with the second batch. During the

first batch comparison calibration, comparisons between the reference SPRT (s/n 1004131), the

check SPRTs (s/n 1842385 and s/n 1812279), and the check RIRTs (s/n B-174 and s/n B-168)

indicated agreement to within 0.21 mK in indicated temperature. During the second batch

comparison calibration, comparisons between the reference SPRT (s/n 1004131), the check SPRTs

(s/n 1812282 and s/n 1812284), and the check RIRTs (s/n B-174 and s/n B-168) indicated

agreement to within 0.13 mK in indicated temperature. These levels of agreement are generally

within the expanded uncertainty for the comparison process at 20.274 K.

The reference and check SPRTs (s/n 1004131 and s/n 1842385) were originally calibrated by

comparison to a reference RIRT (s/n B-168) at 20.2693 K in May 1996. The check RIRTs (s/n

1
B-174 and B-168) were originally calibrated at an e-H2 VP2 temperature of 20.2693 K at NIST in

June 1994 according to the techniques given by Meyer and Reilly.

[27] Additional realizations of the

e-H2 VP2 were performed at NIST in February 1999 using the check SPRTs (s/n 1812284 and s/n

1812282) . [27]

2.2.5. The point of equilibrium-liquid hydrogen under a saturated vapor pressure near 33.3213 kPa

Comparison measurements at an e-H2 VP! temperature of 17.036 K were performed in November

1998 with the first batch of SRM 1750 SPRTs and in May 1999 with the second batch. During the

first batch comparison calibration, comparisons between the reference SPRT (s/n 1004131), the

check SPRTs (s/n 1842385 and s/n 1812279), and the check RIRTs (s/n B-174 and s/n B-168)

indicated agreement to within 0.14 mK in indicated temperature. During the second batch

comparison calibration, comparisons between the reference SPRT (s/n 1004131), the check SPRTs

(s/n 1812282 and s/n 1812284), and the check RIRTs (s/n B-174 and s/n B-168) indicated

agreement to within 0.19 mK in indicated temperature. These levels of agreement are generally

within the expanded uncertainty for the comparison process at 17.036 K.

The reference and check SPRTs (s/n 1004131 and s/n 1842385) were originally calibrated by

comparison to a reference RJJRT (s/n B-168) at 17.0352 K in May 1996. The check RIRTs (s/n

B-174 and B-168) were originally calibrated at an e-H2VPi temperature of 17.0352 K at NIST in

June 1994 according to the techniques given by Meyer and Reilly.

[27] Additional realizations of the

e-H? VP2 were performed at NIST in February 1999 using the check SPRTs (s/n 1812284 and s/n

1812282). [27]

2.2.6. The triple point of equilibrium-hydrogen

Comparison measurements at the e-H2 TP temperature were performed in November 1998 with the

first batch of SRM 1750 SPRTs and in May 1999 with the second batch. During the first batch

comparison calibration, comparisons between the reference SPRT (s/n 1004131), the check SPRTs

(s/n 1842385 and s/n 1812279), and the check RIRTs (s/n B-174 and s/n B-168) indicated

agreement to within 0.15 mK in indicated temperature. During the second batch comparison

calibration, comparisons between the reference SPRT (s/n 1004131), the check SPRTs (s/n 1812282

and s/n 1812284), and the check RIRTs (s/n B-174 and s/n B-168) indicated agreement to within

0.18 mK in indicated temperature. These levels of agreement are generally within the expanded

uncertainty for the comparison process at 13.8033 K.

The reference and check SPRTs (s/n 1004131 and s/n 1842385) as well as the check RIRTs (s/n

B-174 and B-168) were originally calibrated at the e-H2 TP at NIST in April 1995 according to the

techniques given by Meyer and Reilly.

[23] Additional realizations of the e-H2 TP were performed at NIST in December 1997 using the check SPRTs (s/n 1812284 and s/n 1812282).

[27]

12
3. Results

3.1. Deviations at Fixed-Point Temperatures

All results shown here are in terms of the individual deviations AW(T90) for all the SRM 1750

SPRTs that have been calibrated. A summary of the results for all 20 units is provided in Table 4. Only the zero-power values are tabulated here. The distribution of values is not at all Gaussian in shape; therefore the statistical characterization presented here is simply in terms of the maximum

and minimum values ("max." and "min." in Table 4) of the deviations at each fixed-point

temperature.

Table 4. Summary of AW(T90 ) values in for all 20 SPRTs of the SRM 1750.

Serial InFP Ga MP Hg TP Ar TP 02 TP Ne TP e-H2 VP2 e-H2 VP, e-H2 TP

no. AW/ 10"5 AW/ 10"5 AW/ 10"5 AW/ 10" 5 AW/ 10"5 AW/ 10"5 AWI 10" 5 AW/ 10'5 AW/ 10"5 4450 -8.590 -1.697 1.686 9.446 10.928 11.188 10.762 10.249 9.513

4451 -10.424 -2.056 2.162 11.824 13.746 14.196 13.715 13.122 12.288 4452 -10.472 -2.035 2.159 11.618 13.484 13.999 13.572 13.005 12.137 4453 -9.291 -1.795 1.907 10.418 11.949 12.180 11.742 11.194 10.413 4454 -10.703 -2.052 2.226 11.689 13.609 14.088 13.614 13.015 12.121 4455 -9.114 -1.762 1.835 9.668 11.314 11.691 11.295 10.785 10.029

4456 -10.007 -1.944 2.067 10.843 12.547 12.761 12.269 11.649 10.765 4457 -10.687 -2.049 2.258 11.761 13.664 14.176 13.762 13.195 12.312 4458 -10.447 -2.035 2.184 11.476 13.424 13.571 12.922 12.187 11.203 4459 -10.949 -2.130 2.277 12.051 14.099 14.816 14.441 13.924 13.096

4462 -10.493 -2.073 2.272 11.808 13.550 13.743 13.243 12.653 11.790

4463 -8.040 -1.603 1.705 8.894 10.253 10.449 10.055 9.586 8.906

4486 -10.542 -2.010 2.228 11.513 13.390 14.035 13.636 13.102 12.255 4487 -10.311 -1.994 2.167 11.238 13.028 13.552 13.114 12.561 11.719

4488 -10.619 -2.064 2.232 11.545 13.549 14.278 13.912 13.400 12.565 4489 -11.037 -2.111 2.217 12.178 14.191 14.636 14.164 13.574 12.674

4490 -11.496 -2.238 2.413 12.562 14.685 15.298 14.826 14.218 13.286

4491 -11.185 -2.151 2.317 12.241 14.282 14.822 14.338 13.721 12.790

4492 -11.407 -2.174 2.378 12.591 14.727 15.307 14.839 14.228 13.280

4493 -10.858 -2.110 2.288 11.779 13.816 14.455 14.032 13.469 12.587 min. -11.496 -1.603 1.686 8.894 10.253 10.449 10.055 9.586 8.906 max. -8.040 -2.238 2.413 12.591 14.727 15.307 14.839 14.228 13.286

3.1.1. Correlations in the Fixed-Point Deviation Data

The same data contained in Table 4 are shown graphically in Figure 4, except that the e-H2VP] and

e-H2VP2 data have been omitted for clarity. Here the AW(Tg*mp) values are plotted on the abscissa,

and all other AW(T9q) values are plotted on the ordinate. The origin of the plot represents values

equivalent to the reference function. The linear trends that are shown in Figure 4 have ordinate

intercepts that are near this origin. This feature would be expected for platinum of comparable

purity to that used in the SPRTs upon which the reference functions are based. The high degree of

correlation in the deviations between all of the fixed points is also evident. This correlation is an

indication of the uniformity in the starting material, in this case the platinum bars used to draw the

wire for the SPRT elements, as well as the uniformity of the annealing process used for the

elements after they have been formed. Furthermore, a linear correlation should always be expected

for high-purity-metal resistivity data of this type as long as the basic mechanisms responsible for

the electron scattering obey, or at least approximate, Matthiessen's rule. Similar linear relationships

13

i
in correlation plots have been found in historical studies of high purity platinum and modeled as specific forms of deviations from Matthiessen's rule.

The correlation plot of the AW(THgTP) values versus the AW(TG!lMP) values is shown in Figure 5 on two different scales. The larger-scale plot is illustrative of the purity of the SRM 1750 platinum

relative to the ITS-90 criterion for the W values of those two fixed points. In general, all SPRTs

suitable for the ITS-90 will have deviations on a plot of this type which lie within a diagonal band

approximately lxlO"

5 (2.5 mK) in width running between the origin and the intersection of the

criterion lines. The fact that the SRM 1750 SPRTs are distributed relatively close together and

closer to the origin (reference function) is again a measure of their relatively high purity and

uniformity of platinum. The fine-scale insert plot of figure 5 shows the same data with NIST

estimated uncertainties

129 ' 301 as confidence-limit bars (see section 3.2 below).

The six lowest temperature fixed-point deviations are shown in Figures 6a through 6f, again plotted

as correlations with the AW(TcaMp) deviations. The linear least-squares fits are included to help

clarify the general trend of the data in the presence of small amounts of sample-to-sample variation.

Despite this sample dependent variation, the overall limits of the deviation distributions

(maximum-minimum) remains relatively constant among these lower temperature fixed points. The

linear fit is shown in equation form on each plot. The size of the bounds on the data shown in each

plot is ±2ar where <7r is the standard deviation in the residuals of each fit as expressed in equivalent

temperature units.

It is illustrative to plot some of these same data already presented in terms of equivalent deviations

in temperature rather than resistance ratio. Figure 7 is another deviation plot of AW(Tc,0) values on a logarithmic scale versus AWiTaMp) on a linear scale except that all deviations are now converted to temperature differences and expressed in mK. In this plot the deviations now appear more

rationally ordered and are roughly proportional to the inverse temperature.

Finally, Table 5 lists a summary of all the statistical parameters derived from the correlation plots.

The linear fits shown in the correlation plots are derived according to a least-squares method. These

fits are parameterized according to the general linear fit formula

AW(X) =mAW(GaMP) + A0 , (9)

where X represents the particular fixed point of interest. The slope m has an associated standard

deviation cr„„ and the intercept A0 has an associated standard deviation <ta for each correlated pair

of fixed points. In addition, the standard deviation ar of the residuals in AW(X) for each fit are tabulated both as dimensionless deviations (/10~5

) and as the temperature deviation equivalents in

units of mK.

14
X

-2.500 -2.000 -1 500 -1.000 0.500 0.000 ..\W(Ga MP) / 10 Figure 4. The deviations for the SRM 1750 population at the fixed-point temperatures for the e-H2 TP, Ne

TP, 02 TP, Ar TP, Hg TP, and In FP versus that for the Ga MP.

10.000

9.000

8.000

7.000 ° 6.000

t- 5.000

at X

|f 4.000 <

3.000

2.000

1.000 -j

0.000

r

SRM 1750

ITS-90 Criteria

1.600

-2 300 -2.200 -2.100 -2.000 -1 900 -1.800 -1.700 -1 600 -1.500

-7.000 -6.000 -5.000 -4.000 AW(Ga MP)/ 10

-3.000

-5

-2.000 -1.000 0.000

Figure 5. The deviations for SRM 1750 at the Hg TP versus the Ga MP shown on a scale which includes the cutoff limits specified by the ITS-90, "ITS-90 Criteria." The inset plot is the same data with a smaller scale.

15
14 000

13.000

8.000 -I . 1 1 1 1 1

, , 1 -2.400 -2 300 -2 200 -2 100 -2.000 -1 900 -1 800 -1.700 -1 600 -1.500 zlW(GaMP)/10"5

Figure 6a. The correlation plot for the deviations at the Ar TP versus the Ga MP. The central curve is a simple linear least-squares fit. The dotted lines are the symmetric bounds on the data about the linear fit with

the size of the bound indicated as an equivalent temperature difference.

15.000

10 000

9.000 -i 1 1 1 1 1 1 1 ! 1 -2.400 -2.300 -2.200 -2.100 -2.000 -1.900 -1.800 -1.700 -1.600 -1.500

zl W(GaMP)/10"5

Figure 6b. The correlation plot for the deviations at the 02 TP versus the Ga MP. The central curve is a simple linear least-squares fit. The dotted lines are the symmetric bounds on the data about the linear fit with

the size of the bound indicated as an equivalent temperature difference.

16
17.000

16 000

15.000

^ 14.000 £L

I0) Z, 13.000

12.000

11.000

10000

7.1 rnK y = -8.0064X - 2.3838

-2 400 -2.300 -2.200 -2.100 -2 000 -1 900 -1.800 -1 700 -1.600 -1.500

zl W(GaMP)/10~

Figure 6c. The correlation plot for the deviations at the Ne TP versus the Ga MP. The central curve is a simple linear least-squares fit. The dotted lines are the symmetric bounds on the data about the linear fit with the size of the bound

indicated as an equivalent temperature difference.

16.000

-2.300 -2.200 -2.100 -2000 -1 900 -1.700 -1 600 -1 500 dW(Ga MP)/ 10"

Figure 6d. The correlation plot for the deviations at the e-H2 VP2 versus the Ga MP. The central curve is a simple

linear least-squares fit. The dotted lines are the symmetric bounds on the data about the linear fit with the size of the bound indicated as an equivalent temperature difference. 17
16 000

15.000

14.000

^ 13.000 CL>

CM x

a> 12.000

1 1 000

10.000

9 000

-2.400 -2 300

\ I—»—

V1

I 1

25.5 mK

1—3—

V = -7.7233X - 2.8365

-2.200 -2 100 -2.000 -1.900 -1.800 -1 700 -1.600 -1.500 AW(Ga MP)/10"!

Figure 6e. The correlation plot for the deviations at the e-H2 VP| versus the Ga MP. The central curve is a simple

linear least-squares fit. The dotted lines are the symmetric bounds on the data about the linear fit with the size of the bound indicated as an equivalent temperature difference.

15.000

14.000

8.000 -I 1 , 1 1 1

1

1 1

-2 40 -2 30 -2.20 -2 10 -2.00 -1 90 -1.80 -1.70 -1.60 -150 AW(Ga MP)/10"5

Figure 6f. The correlation plot for the deviations at the e-H2 TP versus the Ga MP. The central curve is a simple linear least-squares fit. The dotted lines are the symmetric bounds on the data about the linear fit with the size of the bound

indicated as an equivalent temperature difference. 18
1000.0

1.0 -I 1 1 1 1 1

1 1 1 1 1 -6.00 -5.80 -5.60 -5 40 -5.20 -5 00 -4 80 -4.60 -4.40 -4 20 -4 00 A W(Ga MP)/mK

Figure 7. The deviation for the e-H2 TP, e-H2 VP! , e-H2 VP2 , Ne TP, 02 TP, Ar TP, and Hg TP versus that for the Ga MP with the scales converted to equivalent temperature in mK.

Table 5. The statistical parameters for the linear fits shown in the Ga MP correlation plots of Figures 6a to

6f, and Figure 14. slope intercept resic uals

Fixed Point m AQ I 10"5 oA l 10"5 Gr i 10"5 fff /mK

InFP 5.46 0.16 0.61 0.32 0.112 0.28

HgTP -1.225 0.057 -0.31 0.11 0.040 0.10

Ar TP -6.06 0.22 -0.80 0.44 0.155 0.36

02 TP -7.32 0.22 -1.45 0.44 0.154 0.40

Ne TP -8.01 0.31 -2.38 0.62 0.217 1.77

e-H2 VP2 -7.91 0.37 -2.64 0.74 0.257 3.42

e-H2 VP, -7.72 0.42 -2.84 0.83 0.292 6.37

e-H2 TP -7.34 0.45 -2.93 0.91 0.317 13.20

3.1.2. Deviation Functions

The individual deviation functions AW(W(T90)) are shown in Figure 8 for the entire range of W(Tgo ) values spanned by the calibrations. The curves shown are the solutions to equation 7 and equation

8 for both batches of the SRM 1750 SPRTs. The complete set of fixed-point deviation data in the

lower range is shown as a function of temperature in Figure 9.

19
1 .60E-04

1 .20E-04

4450 4451

4452 4453

4454 4455 _ 4456 4457

4458 4459

4462 4463

4486 4487

4488 4489

4490 4491

4492 4493

-8.00E-05

-1.20E-04

10

W

Figure 8. The individual deviation functions for the SRM 1750 shown as a function of W value.

16000

12.000

8.000

4 000

0.000

-4 000

-8.000

-12.000

4450 4451 A 4452 4453 X4454 • 4455 + 4456 -4457 -4458 0 4459 4462 • 4463 4486 4487 4488 4489 -4490 4491 4492 4493

1000

r90 /K

Figure 9. The individual deviations for the SRM 1750 shown versus temperature.

20
3.2. Uncertainties

The NIST assessment of uncertainties in the ITS-90 calibration of a thermometer involves the

decomposition of uncertainty into Type A and Type B components. The Type A component, s, is a combined uncertainty from standard deviations of only those measurements under direct statistical process control. The Type B components, Uj, are the estimated standard uncertainties for each

known component in the measurement process that are not directly measured. 131 ' 321 In addition,

uncertainties are described by a coverage factor, k, which allows an estimated uncertainty of the

measurements to be expanded to a specific level of confidence. The resulting expanded uncertainty,

when quoted with a coverage factor of k = 2 gives a level of confidence in the measurement of

approximately 95 % when the degrees of freedom v>50, as is consistent with international

practice.

The uncertainties reported here do not include any estimates for: 1) the uncertainties that may exist

in the possible different realizations of the ITS-90 between national standards laboratories in other

countries and NIST; 2) the non-uniqueness of the ITS-90; 3) any effects that may be introduced by

transportation of the thermometer between NIST and the user's laboratory; 4) long-term drift of the

thermometer and; 5) any measurement uncertainties that exist in the user's laboratory.

3.2.1. Expanded Uncertainty

The Type A and Type B uncertainties at each of the SPRT calibration points are combined in

quadrature and expanded by the coverage factor, k, to form the expanded uncertainty given by

NIST expanded uncertainties are quoted with a coverage factor of k=2 unless stated otherwise.

Table 6 lists the values of Uj in mK according to equation 10 for all of the ITS-90 calibration

points relevant to SRM 1750. The values quoted from the Ar TP and higher are the latest estimates

of uncertainty incorporating the most recent fixed-point material now in use at NIST. The estimates

given here for some of the low-temperature comparison points are slightly larger than values given

in reference [29] due to additional information on reference thermometer agreement obtained since

the preparation of that earlier report.

(10)

21
Table 6. NIST expanded uncertainties for SRM 1750.

Calibration Point T90 (K) Expanded Uncertainty, Uj (mK)

e-H2 TPf 13.8033 0.25

e-H2 VP+ 17.0 0.19

e-H2 VP+ 20.3 0.17

Ne TP1 24.5561 0.31

02 TPf 54.3584 0.14

ArTPf 83.8058 0.21

Ar TP 83.8058 0.15

Hg TP 234.3156 0.20

H20 TP 273.16 0.04

Ga MP 302.9146 0.04

InFP 429.7485 0.32

1 Comparison Calibration

3.2.2. Total Calibration Uncertainty

The total calibration uncertainty at any temperature within any temperature sub-range is determined

from the expanded uncertainties as propagated from each of the relevant defining fixed points. The

uncertainty propagated from each defining fixed point is calculated by assuming the appropriate

uncertainty at that fixed point but with no uncertainty at the other fixed points, and determining

mathematically how that uncertainty propagates. The total uncertainty from a calibration is then

determined in part by calculating the root-sum-square (RSS) uncertainty arising from all of the

defining fixed points used in that calibration. Figures 10 and 11 show uncertainty propagation

curves for the two ITS-90 temperature sub-ranges relevant to SRM 1750, using the uncertainties of

the fixed points given in Table 6. The thick line represents the RSS uncertainty for the sub-ranges

based on those uncertainties.

The propagated-uncertainty curve for the triple point of water (H2O TP) (assuming 0.1 mK

uncertainty at the H2O TP), as shown in Figure 12, is the uncertainty incurred by the user, not an

uncertainty in the NIST calibration. During the NIST calibration of a thermometer, any uncertainty

from a measurement at the H2O TP is incorporated into the definition of W(T9o). If the uncertainty

made by the user at the H2O TP is not 0.1 mK, a corrected propagated uncertainty at any

temperature can be calculated by using the appropriate multiplicative factor.

22
Figure 10. These curves show the uncertainties as propagated over the range 13.8 K to 273.16 K from the

uncertainties for the calibration of SRM 1750 SPRTs (see Table 6) for each of the calibration fixed points

within that range. The individual fixed-point uncertainty curves are added in quadrature to form the curve

labeled "RSS" with only one (positive) sign shown in the figure which represents the total calibration

uncertainty as reported by NIST.

23
0.35

270 290 310 330 350 370 390 410 430 t90 /k

Figure 11. This curve shows the uncertainty as propagated over the range 273.15 K to 429.7485 K from the uncertainty

for the calibration of SRM 1750 SPRTs (see Table 6) at the In FP. The curve is also equivalent to the "RSS," total calibration uncertainty in this range because the In FP is the only fixed point which contributes to the deviation

equation in that sub-range.

0.18 -i — — 1

10 60 110 160 210 260 310 360 410

Figure 12. A separate uncertainty propagation curve for the triple point of water is shown to illustrate the calculation of

the user's uncertainty if a user's measurement uncertainty at the triple point of water is 0.1 mK.

24
3.3. Supplementary Data

In addition to the data shown above for the 10 fixed-point temperatures, a number of supplementary

data points were also obtained for SRM 1750. These data were obtained by comparison methods

and include measurements of the resistance ratio at temperatures intermediate to some of the fixedpoint temperatures (between 13.8 K and 83.8 K), as well as data below the lower temperature limit

of the normal calibration (13.8033 K). The exact temperatures included for the supplementary data

are somewhat variable, and the coverage is not entirely complete, depending on from which

comparison batch the data were derived. Table 7 gives a breakdown of all the supplementary data

for each SPRT in the SRM 1750 by serial number.

Table 7. Supplementary data point temperatures (in kelvin) for the 20 SPRTs of the SRM 1750.

Serial Below 13.8 K 17.0 K 20.3 K to 24.56 K to 54.36 K 54.36 K to 83. 8 K

no. 13.8 K to 17.0 K to 20.3 K 24.56 K

4450 4.2211 15.3946 185.6892 22.6002 — — 61.7000 69.1006 77.3997

4451 4.2211 15.3946 185.6892 22.6002 — 39.4561 61.7000 69.1006 77.3997

4452 4.2211 15.3946 185.6892 22.6002 — 39.4561 61.7000 69.1006 77.3997

4453 4.2211 15.3946 U,.6892 22.6002 39.4561 61.7000 69.1006 77.3997

4454 4.2211 15.3946 185.6892 22.6002 39.4561 61.7000 69.1006 77.3997

4455 4.2211 15.3946 18,.6892 22.6002 39.4561 61.7000 69.1006 77.3997

4456 4.2211 15.3946 18..6892 22.6002 39.4561 61.7000 69.1006 77.3997

4457 4.2211 15.3946 18.6892 22.6002 39.4561 61.7000 69.1006 77.3997

4458 4.2211 15.3946 18,.6892 22.6002 39.4561 61.7000 69.1006 77.3997

4459 4.2211 15.3946 18,.6892 22.6002 39.4561 61.7000 69.1006 77.3997

4462 4.2209 15.3936 18,.6887 22.5995 31.9969 39.4578 61.7001 69.0996 77.3985

4463 4.2209 15.3936 18,.6887 22.5995 31.9969 39.4578 61.7001 69.0996 77.3985

4486 4.2209 15.3936 18,.6887 22.5995 31.9969 39.4578 61.7001 69.0996 77.3985

4487 4.2209 15.3936 18 .6887 22.5995 31.9969 39.4578 61.7001 69.0996 77.3985

4488 4.2212 15.3936 18 .6887 22.5995 31.9969 39.4578 61.7001 69.0996 77.3985

4489 4.2209 15.3936 18 .6887 22.5995 31.9969 39.4578 61.7001 69.0996 77.3985

4490 4.2209 15.3936 18 .6887 22.5995 31.9969 39.4578 61.7001 69.0996 77.3985

4491 4.2209 15.3936 18 .6887 22.5995 31.9969 39.4578 61.7001 69.0996 77.3985

4492 4.2209 15.3936 18 .6887 22.5995 31.9969 39.4578 61.7001 69.0996 77.3985

4493 4.2209 15.3936 18 .6887 22.5995 31.9969 39.4578 61.7001 69.0996 77.3985

Temperatures below 13.8 K were determined by the reference RIRT (s/n B-174) and check RIRT

(s/n B-168) which were originally calibrated at NIST in 1995 [33] according to the

4He vapor

pressure definition of the ITS-90. The estimated uncertainty on the comparison measurements of

SRM 1750 SPRTs at 4.221 K was approximately 0.1 mK. All other supplementary data were

obtained using the same reference thermometers and associated uncertainties as already described

in sections 2.2 and 3.2.

3.3.1 Interpolation Charactenstics (13.8 K to 83.8 K)

The accuracy of interpolation of temperatures between the fixed-point temperatures can be tested

directly by performing comparison measurements of the SRM 1750 SPRTs at a set of such

intermediate points. The deviation that is predicted for each SPRT on the basis of the reference

thermometer temperature and the SPRT's individual deviation equations can then be compared

with the actual measured deviation at those intermediate temperatures.

Figure 13a shows a plot from 13.8 K to 90 K of the difference between the predicted and the

measured deviations for the first batch of the SRM 1750 SPRTs. The data include two additional

25
check SPRTs (1842385 and 1812279) as well as two check RIRTs (B174 and B168). Figure 13b

shows a plot from 13.8 K to 90 K of the difference between the predicted and the measured

deviations for the second comparison batch of the SRM 1750 including two additional check

SPRTs (1812282 and 1812284) as well as two check RIRTs (B174 and B168). The plots show all the comparison data at intermediate temperatures as well as at the Ar TP. At the fixed-point

temperatures from the O? TP and below, however, only the check thermometers are shown because

the SRM 1750 comparison data will by definition agree with the reference thermometer at these

points. The comparison data at the Ar TP is with respect to the independent fixed-point data. The

broad black lines represent the k=2 limits of uncertainty for the comparison measurements. These

limits are somewhat larger than the limits on the actual calibration of the SRM 1750 SPRTs as shown in Figure 10 due to additional uncertainties imposed on the comparison data at 83.8 K and to an additional uncertainty on the reference thermometer's calibration at the Hg TP and Ar TP. These

additional contributions to the comparison uncertainty affect only the accuracy of the comparison

with the reference thermometer at the intermediate temperatures and do not affect the accuracy of

the SRM 1750 calibrations.

0.80 t 1

0.60

0 40

5£ 0 20

000

-020

-0.40

-0.60

-0.8010 00 20.00 8000 90.00

Figure 13a. The difference between the "as calibrated" deviation predicted by 10 individual SRM 1750

calibrations and the observed deviation according the NIST reference thermometer over the range 13.8 K to 83.8 K. The black line curves ("RSS") are the comparison uncertainty bounds (k=2) for the intermediate

temperatures of the supplementary data from the first comparison batch of the SRM 1750.

26
080

E

CO

T—o

o

is

I

80 00 90.00

Figure 13b. The difference between the "as calibrated" deviation predicted by 10 individual SRM 1750

calibrations and the observed deviation according the NIST reference thermometer over the range 13.8 K to 83.8 K. The black line curves ("RSS") are the comparison uncertainty bounds (k=2) for the intermediate

temperatures of the supplementary data from the second comparison batch of the SRM 1750.

27
3.3.2 Non-Uniqueness (302.9146 K)

The Ga MP is included in all NIST SPRT calibrations above 273.16 K regardless of whether or not

it is required for the particular ITS-90 sub-range. This fixed point is then considered as redundant

as far as the calibration is concerned and can be used instead as a checkpoint for sub-range

inconsistencies. 1381 These inconsistencies are also referred to as "type I non-uniqueness"(39] in the

scale. For the sub-range 10 (273.15 K to 429.7485 K), the calibration coefficient a\o is calculated

based on the W(T\n F?) value only. The Ga MP, however, is within this range of temperatures and is defined by the ITS-90 to be 302.9146 K. The agreement between the deviation for an SPRT at 302.9146 K as predicted from the sub-range 10 calibration and that determined via a Ga TP

realization, is therefore a measure of this type I non-uniqueness.

For the SRM 1750 SPRTs, this degree of non-uniqueness is practically insignificant. This can be

illustrated via the correlation plot shown in figure 14. Here the deviations observed at the In FP are plotted versus those observed at the Ga MP. The linear fit to the complete data is shown along with

another fit to the points as predicted by the sub-range 10 (In FP) calibration alone. While it is possible to distinguish these two lines, their difference is not statistically significant. In addition, no SPRT in the SRM 1750 sample population exhibits a disagreement of this type greater than

0.12 mK at 302.9146 K. This disagreement is not completely due to non-uniqueness, however,

because there is approximately 0.06 mK calibration uncertainty (see Figure 11) at 303 K as propagated from the In FP.

-7.0 -| .

-7.5

-12.0 -I , ,

, , , ,, 1 1 -2.4 -2.3 -2.2 -2.1 -2.0 -1.9 -1.8 -17 -16 -1.5 A W(Ca MP)

Figure 14. The correlation plot of the In FP and Ga MP as measured for the SRM 1750 population sample.

The lighter line is a fit to the complete data (shown in equation form), and the darker line is a fit using the

sub-range 10 calibration only (see text).

28
3.3.3. Residual Resistance Ratios (4.221 K)

Supplementary resistance ratio data at or very near the normal boiling point of liquid

4He ( 4 He

NBP) is of interest from the point of view of characterizing the purity of any nearly pure metal.

Historically, the term "residual resistivity ratio," or "RRR"[U] has been used to denote the

following ratio of a metal's electrical resistivity p(T),

p(273.15 K)

p(4.22 K)

'

however, the exact temperatures used have not always been precisely reported and/or entirely

consistent. The RRR is usually calculated by taking the simple ratio of measured resistances with

the assumption that the specific shape factors cancel in the ratio, leaving only a bulk resistivity

ratio. The other inherent assumption made is that the temperature of 4.2 K is low enough to measure the true residual term in the resistivity, which is meant to be a purely temperatureindependent term, dependent only on the impurity concentration of the individual sample. This

assumption is true only if R(4.22 K) = R(0 K), which is generally satisfied for sufficiently high

impurity concentrations. For very high purity material (low impurity concentration), however, such

as the platinum in the SRM 1750, this is not necessarily the case. In fact, previous measurements at NIST[ ] on platinum of comparable purity to the SRM 1750, have indicated that the true RRR,

when derived from measurements close to 1 K, can be as much as 8 % higher than what would be

otherwise inferred from the definition given in equation 11.

The raw measurements reported here at 4.221 K are parameterized in the same basic manner as is done for the other fixed-point temperatures, namely, W(4.221 K) is calculated according to equation 1. However, the deviation AW(4.22l K) must be calculated in a different manner from that

of other fixed points. This is because the ITS-90 reference function Wt(T()q) is essentially

meaningless for temperatures below 13.8 K and returns an absurdly low value of approximately

1.25xl0"

27 at 4.221 K. Rather, for the purposes of the presentation of the data here, we adopt a "conventional value" for Wr(T<)o) —> W*T(4.22l K) = 0.000348. This conventional value is based on

data

[36] from the same capsule SPRT (s/n 217894) at NPL that was chosen as the basis of the

ITS-90 reference function between 13.8033 K and 273.16 K.

[37] Data are then presented in terms of

the conventional deviations AW*(4.221 K)= W(4.221 K)- W*r (4.221 K)= W(4.221 K)-0.000348.

For the purposes of comparison of the SRM 1750 platinum to historical data on other samples of

platinum found in the literature, we calculate the RRR starting from the definition of equation 11,

and use the following approximation,

^S ^f315K)

= °f"

60

\ . (12)

W(4.221K) W(4.221K)

which accounts for the 10 mK difference between the ice point and triple point of water. This

approximation assumes that R(4.22 K)=fl(4.221 K)=/?(4.222 K), or that the difference is negligible.

The resulting data for the SRM 1750 SPRTs from data obtained at 4.221 K are given in Table 8.

RRR =

29
Table 8. The residual ratio data at 4.221 K as parameterized in three different ways (see text).

Serial no. W(4.221 K) AW*(4.221 K)/ 10"5 RRR

4450 0.0004 324l

> 8.449 2312

4451 0.00045960 11.160 2176

4452 0.00045415 10.615 2202

4453 0.00044288 9.488 2258

4454 0.00045323 10.523 2206

4455 0.00043541 8.741 2297

4456 0.00044387 9.587 2253

4457 0.00045544 10.744 2196

4458 0.00044384 9.589 2253

4459 0.00046285 11.485 2160

4462 0.00045432 10.632 2201

4463 0.00043037 8.237 2324

4486 0.00045452 10.652 2200

4487 0.00045028 10.228 2221

4488 0.00045774 10.974 2185

4489 0.00045819 11.019 2182

4490 0.00046376 11.576 2156

4491 0.00045960 11.160 2176

4492 0.00046329 11.529 2158

4493 0.00045717 10.917 2187

The degree of purity and absence of strain for each individual sample (SPRT) of SRM 1750 is then

most easily accessed by inspection of the RRR values in Table 8. The highest RRR value of 2324 is only 7 % higher than the lowest value of 2156, which again is an indication of the relative

uniformity of the SRM as a population. By contrast, the RRR for the NPL capsule SPRT (s/n

217894) mentioned above is approximately 2873, but this was perhaps the highest-purity sample of

platinum wire of small (d<0.l mm) diameter ever produced for any capsule-type SPRT. The

expanded uncertainty in W(4.221 K) is l.lxlO"

7 or approximately 0.025 % for the SRM 1750.

3.3.4. Low-Temperature Correlations

The data at 4.221 K are also suitable to illustrate another set of resistance ratio correlations that exist for the SRM 1750 platinum. These correlations are derived from the same fixed-point

temperature deviations presented already in section 3.1, but now with the AW*(4.22\ K) values

treated as the independent variable. These correlations are expected to be at least partially

independent of those presented earlier using the Ga MP, as the mechanisms responsible for electron

scattering in the low-temperature limit are dominated by impurities and other lattice defects.

Figures 15a through 15f are the correlation plots of the deviations observed for SRM 1750 at the

lowest six fixed points of e-H2 TP, e-H2 VP,, e-H2 VP2 , Ne TP, 02 TP, and Ar TP respectively, to their observed deviations at 4.221 K.

Several general features can be seen in these low-temperature correlation plots. First, an overall

linear trend is evident in all the plots, with some significant sample-to-sample departures from the

linear fit These sample dependent variations are bounded for the SRM 1750 population according

to the limits shown on each of the figures, converted to their equivalent values in mK. These limits are set by ±2<rr for the residuals of the fits and are relatively constant for these lowest six fixedpoint temperatures when expressed as pure (dimensionless) deviations: being approximately (0.3 to 0.2) xlO"5 for the temperature range studied. When expressed as equivalent temperature variations,

the lowest temperatures appear to have wider limits for these variations, due to the rapidly

30
decreasing value of dW/dT with decreasing temperature below about 30 K. When these sample

variations are compared to the analogous variations shown in Figures 6, showing the correlations

with the Ga MP, it is evident that these low-temperature fixed points are equally well or better

correlated with the

4He NBP deviations in the case of the e-H2 TP, e-H2 VP], and e-H2 VP2 , but less so for the Ne TP and other higher-temperature fixed points. This should be expected to be the case simply due to the relative proximity of the respective temperatures, and to the increasing relative

importance of the impurity scattering mechanism as temperature decreases. The linear fits shown in the Figure 15 series of correlation plots for the

4He NBP are summarized in Table 9 in manner

exactly analogous to that of Table 5 for the case of the Ga MP correlations.

Overall, the SRM 1750, as a sample population of platinum, exhibits correlations over the entire

range of calibration as well as into a range of temperatures below 13.8 K. Deviation data from any

one of the fixed-point temperatures for the SRM 1750 will in fact show a correlation with data from

any of the other temperatures. The choice made in this analysis, to parameterize all such

correlations in terms of only the two independent variables (Ga MP and 4He NBP). is based

partially on convenience and partially on the relatively low uncertainties that are placed on the data

at those two fixed points. Moreover, the two temperatures (302.9 K and 4.221 K ) are far enough

apart that they serve to best separate the two predominate mechanisms, point defects and lattice vibrations, which are known to be responsible for the observed resistivity over the temperature

range studied.

14.0

13.0

8.0 -I 1

1

1 1 1 1 1 8.0 8.6 0.0 0.6 100 10.6 11 0 11.6 12.0 A 1^(4.221 K)/ 10 5

Figure 15a. The correlation plot for the deviations at the Ar TP versus the

4He NBP. The central curve is a simple linear least-squares fit. The dotted lines are the symmetric bounds on the data about the linear fit with

the size of the bound indicated as an equivalent temperature difference in mK.

31
16.0

O

i—

ICM O

<

15.0

14.0

13 0

12.0

11.0

10

A W(4.221 K)/10"

12 0

Figure 15b. The correlation plot for the deviations at the 02 TP versus the

4He NBP. The central curve is a simple linear least-squares fit. The dotted lines are the symmetric bounds on the data about the linear fit with

the size of the bound indicated as an equivalent temperature difference in mK.

16.0

15.0

14.0

Q.

I- 13.0

12.0

11.0

8 0

.y = 1.2881x -r 0.3107. 9.9 mK

8 5 9.0 9.5 100 10.5 11.0 11.5 12 0 A 1^(4.221 K)/ 10

Figure 15c. The correlation plot for the deviations at the Ne TP versus the

4He NBP. The central curve is a simple linear least-squares fit. The dotted lines are the symmetric bounds on the data about the linear fit with

the size of the bound indicated as an equivalent temperature difference in mK.

32
16.0

15.0

10.0

9.0 -i 1 1 1

1 1 1 1 1 8.0 8.5 9.0 95 100 105 11.0 11.5 120 A W(4.221 K)/10"5

Figure 15d. The correlation plot for the deviations at the e-H2 VP2 (20.2714 K) versus the

4He NBP. The

central curve is a simple linear least-squares fit. The dotted lines are the symmetric bounds on the data about

the linear fit with the size of the bound indicated as an equivalent temperature difference in mK.

15.0 n

9 0 \ 1 1

1

1 1 1 1

1 O.U O.O MU a. 3 lUU 1U.3 11 .U 11.3 l^.U A IV(4.221 K)/10 5

Figure 15e. The correlation plot for the deviations at the e-H2 VP, (17.036 K) versus the

4He NBP. The

central curve is a simple linear least-squares fit. The dotted lines are the symmetric bounds on the data about

the linear fit with the size of the bound indicated as an equivalent temperature difference in mK.

33
140

9 0

8 0 1 1 1

1 1 1 1 1

1 8.0 8.5 9 0 9.5 10.0 105 11.0 11.5 12.0 A W(4.221 K)/10"5

Figure 15f. The correlation plot for the deviations at the e-H2 TP versus the

4He NBP. The central curve is a simple linear least-squares fit. The dotted lines are the symmetric bounds on the data about the linear fit with

the size of the bound indicated as an equivalent temperature difference in mK.

Table 9. The statistical parameters for the linear fits shown in the

4He NBP correlation plots of Figures 15a

to 15f. slope intercept Residuals

Fixed Point m <ym AQ I 10"5 (7J 10° cr,/ 10" 5 <jr l mK

Ar TP 0.959 0.066 1.41 0.69 0.287 0.66

02 TP 1.154 0.079 1.26 0.83 0.342 0.88

Ne TP 1.288 0.070 0.31 0.73 0.303 2.46

e-H2 VP2 1.287 0.062 -0.13 0.65 0.267 3.55

e-H2 VP, 1.270 0.055 -0.52 0.57 0.237 5.18

e-H2 TP 1.220 0.047 -0.86 0.49 0.202 8.40

34
4. Summary

The SRM 1750 is both a sample population of high-purity platinum wire elements as well as a

collection of highly accurate reference thermometers calibrated according to the definitions of the

ITS-90. The population has resistance characteristics which are both highly uniform and highly

correlated over the entire range of temperatures studied. The SRM 1750 W(Hg TP) and W(Ga MP)

values significantly exceed the ITS-90 criterion for acceptance as a defining instrument of the

scale.

The starting material for the platinum wire elements is know to be 99.999 % pure, but the actual

purity obtained in the final SPRT units is expected to be slightly lower. Based on the residual

resistivity ratio data, the actual impurity concentrations within the platinum wire elements probably

vary only on the order of 7 % throughout the SRM 1750 sample population. The lowest RRR

observed for the SRM 1750 was only 25 % smaller than that of the reference function SPRT.

Each individual unit of the SRM is fully certified by NIST as suitable to serve as a defining

instrument of the ITS-90 and is calibrated at the highest accuracy level available at NIST. All

certified values are determined by reference to ITS-90 defined fixed points and the ITS-90 SPRT

reference function. The values which are NIST certified are the deviation function coefficients of

each individual SPRT for sub-range 1 and sub-range 10 on the ITS-90. These coefficients form the

SPRT calibration for use at either 1 mA or 2 mA excitation current or by extrapolation to 0 mA.

Each unit of SRM 1750 is provided with a glass adapter probe for use in suitable fixed-point cells

at temperatures between 83.8 K and 430 K. The user can then periodically check the SPRT in a H20 TP cell using the same probe adapter in which it was originally calibrated, in order to maintain

the highest quality assurance.

35
5. References

1. H. Preston-Thomas, "The International Temperature Scale of 1990 (ITS-90)," Metrologia 27, 3

(1990); ibid, 107 (1990).

2. B. W. Mangum and G. T. Furukawa, Guidelines for Realizing The International Temperature

Scale of 1990 (ITS-90), NIST Technical Note 1265, (U.S. Government Printing Office,

Washington, DC, 1990).

3. Sigmund Cohn Corporation, Mount Vernon, NY, Platinum bar lots 240-203 and 240-206.

4. J. L. Riddle, G. T. Furukawa, and H. H. Plumb, Platinum Resistance Thermometry, NBS

Monograph 126, p. 8, (U.S. Government Printing Office, Washington, DC, April 1973).

5. B. W. Mangum, Platinum Resistance Thermometer Calibrations, NBS Special Publication 250-

22, pp. 14-19 (U.S. Government Printing Office, Washington, DC, October 1987).

6. G. F. Strouse, "NIST implementation and realization of ITS-90 over the range 83 K to 1235 K,"

in Temperature: Its Measurement and Control in Science and Industry, Vol. 6, pi 69, J. F. Schooley, ed., American Institute of Physics, New York (1992).

7. W. L. Tew, G. F. Strouse, C. W. Meyer, and G. T. Furukawa, "Recent Advances in the

Realization and Dissemination of the ITS-90 Below 83.8058 K at NIST," Advances in

Cryogenic Engineering, Vol. 43B, P. Kittel, editor (Plenum Press, New York, 1998).

8. W. L. Tew and B. W. Mangum, "New Procedures and Capabilities for the Calibration of

Cryogenic Resistance Thermometers at NIST," in Advances in Cryogenic Engineering, Vol.

39B, p. 1019, P. Kittel, editor, (Plenum Press, New York, 1994).

9. J. L. Riddle et al., op. cit., p. 29.

10. F. Pavase and G. Molinar, "Modern Gas-Based Temperature and Pressure Measurements,"

p. 92, Plenum Press, New York (1992).

11. J. L. Riddle et al., op. cit., p. 13. 12. B. W. Mangum and G. T. Furukawa, op. cit., p. 25.

13. B. W. Mangum and G. T. Furukawa, op. cit., pp. 42-47.

14. G. F. Strouse, NIST Special Publication 260-132.

15. G. F. Strouse, "NIST Realization of The Gallium Triple Point," submitted for publication,

Proceedings of TEMPMEKO '99, Delft, The Netherlands, June 1999.

16. Jarrett Instrument Co., Inc., Wheaton, MD, USA. All the ITO TP cells listed were produced in

the Wheaton facility and are less than 6 years old. As of 1997, the Jarrett name and production

process has been owned by Isothermal Technology, Ltd., Pine Grove, Southport, England, U.K.

17. G. T. Furukawa, B. W. Mangum, and G. F. Strouse, "Effects of different methods of

preparation of ice mantles of triple point of water cells...," Metrologia, Vol. 34, pp. 215-233

(1997).

18. G.T. Furukawa, "Realization of the mercury triple point," in Temperature: Its Measurement and

Control in Science and Industry, Vol. 6, p. 281, J. F. Schooley, ed., American Institute of Physics,

New York (1992).

19. G. T. Furukawa, J. L. Riddle, W. R. Bigge, E. R. Pfeiffer, Application of Some Metal SRM's as Thermometric Fixed Points, National Bureau of Standards Special Publication 260-17, pp. 87-

102, U.S. Government Printing Office, Washington, DC (1982).

20. G. T. Furukawa, "Argon triple point apparatus with multiple thermometer wells," in Temperature:

Its Measurement and Control in Science and Industry, Vol. 6, p. 265, J. F. Schooley, ed.,

American Institute of Physics, New York (1992).

21. Airco Rare and Specialty Gas, Riverton, NJ, USA.

22. W. L. Tew et al (1998), op. cit.

36
23. C. W. Meyer and M. L. Reilly, "Realization of the ITS-90 Triple Points From 13,80 K to 83,8 K at NIST," Proceedings of the IMEKO International Seminar on Low Temperature

Thermometry and Dynamic Temperature Measurement, pp. L-l 10 - L-l 15, IMEKO, Wroclaw,

Poland, (September 1997).

24. W. L. Tew, "Sealed-cell devices for the realization of the triple point of neon at the National

Institute of Standards and Technology," Proceedings of TEMPMEKO '96, P. Marcarino,

editor, p. 81, Levrotto and Bella, Torino, Italy (1997).

25. G. T. Furukawa, "Reproducibility of the triple point of argon in sealed transportable cells,"

in Temperature: Its Measurement and Control in Science and Industry, Vol. 5, p. 239, J. F. Schooley, ed., American Institute of Physics, New York (1982).

26. G. T. Furukawa, "The triple point of oxygen in sealed transportable cells," J. Res. Nat. Bur. Stand.

(U.S.), 91, 255 (1986).

27. C. W. Meyer, G. F. Strouse, and W. L. Tew, "Non-Uniqueness of the ITS-90 from 13.8033 K

to 24.5561 K," submitted for publication, Proceedings of TEMPMEKO '99, Delft, The

Netherlands, June 1999.

28. H. Van Dijk, "On the Use of Platinum Thermometers for Thermometry Below 90 K...,"

in Temperature: Its Measurement and Control in Science and Industry, Vol. 3, p. 365, H. Plumb,

ed., Reinhold Publishing Corp., New York (1962).

29. W. L. Tew, G. F. Strouse, and C. W. Meyer, A Revised Assessment of Calibration Uncertainties

for Capsule Type Standard Platinum and Rhodium-Iron Resistance Thermometers, NISTIR

6138 (U.S. Government Printing Office, Washington, DC, April 1998).

30. G. F. Strouse and W. L. Tew, Assessment of Uncertainties of Calibration of Resistance

Thermometers at the National Institute of Standards and Technology, NISTIR 5319, (U.S.

Government Printing Office, Washington, DC, January 1994).

31. B. N. Taylor and C. E. Kuyatt, Guidelines for Evaluating and Expressing the Uncertainty of

NIST Measurement Results, NIST Technical Note 1297, 1994 Edition, 20 pp. (September

1994).

32. ISO, Guide to the Expression of Uncertainty in Measurement, International Organization for

Standards, Geneva, Switzerland (1993).

33. C. W. Meyer and M. L. Reilly, "Realization of the ITS-90 at the NIST in the range 0,65 K to 5,0 K using

3 He and 4He vapour pressure thermometry," Metrologia, Vol. 33, p. 383, (1996).

34. F. R. Fickett, "Electrical Properties," in Materials at low temperatures, R. P. Reed and A. F. Clark, eds., pp. 163-201, American Society for Metals, Metals Park, OH (1983).

35. E. R. Pfeiffer, NIST, unpublished data on SPRT 1812279, Gaithersburg, MD (1992).

36. R. L. Rusby, private communication, NPL, Teddington, England, UK (Jan. 13, 1999).

37. R. C. Kemp, "The Reference Function for PRT Interpolation Between 13.8033 K and 273.16 K

in the ITS-90," Metrologia, Vol. 28, pp. 327-332, (1991).

38. G. F. Strouse, "NIST assessment of ITS-90 non-uniqueness for 25.5 Q SPRTs at gallium,

indium, and cadmium fixed points," in Temperature: Its Measurement and Control in Science

and Industry," Vol. 6, p. 175, J. F. Schooley, ed., American Institute of Physics, New York (1992).

39. B. W. Mangum et al., "On the International Temperature Scale of 1990 (ITS-90), Part I: Some

Definitions," Metrologia, Vol. 34, pp. 427-429 (1997).

37

Certificate

Standard Reference Material 1750

Standard Platinum Resistance Thermometer

Certified Thermometer for the Range 13.8033 K to 429.7485 K

on the International Temperature Scale of 1 990

Serial No. 4450

This Standard Reference Material (SRM) is intended for use as a defining instrument of the International Temperature Scale of 1990 (ITS-90) [1] that was officially adopted by the Comite International des Poids et Mesures (CIPM) in September 1989. One unit of SRM 1750 [2] consists of a calibrated capsule-type Standard Platinum

Resistance Thermometer (SPRT) and a 60 cm long borosilicate glass adapter probe. The SPRT, which is a four- wire device with an electrical resistance at the water triple point (TP) of /?(273.16 K) = (25.5 ± 0.1) Q, has been

calibrated at NIST over the combined ITS-90 sub-ranges of 13.8033 K to 273.16 K and 273.15 K to 429.7485 K. The resistance element is wound in an unconstrained fashion, insulated by a ceramic multi-bore tube, and fully annealed. The element is hermetically sealed with a high purity helium gas atmosphere into a cylindrical capsule of

5.7 mm outer diameter and an external sheath of INCONEL X-750®.

Certified Values and Uncertainties: Table 1 contains the calibration fixed-point uncertainties in the specified

temperature range. Table 2 contains the values of the coefficients of the two deviation functions of the ITS-90

appropriate for this calibration. For the convenience of the user, coefficients are given for three different currents, 0 mA, 1.0 mA, and 2.0 mA, as derived from measurements at the two currents I\ and I2 , given on page two of this certificate. Tables 3, 4, and 5 contain values for the resistance ratio, W(T90 ) = R(T90 )/R(273.\6 K), for temperatures

Tgo in one kelvin increments and were generated using the three sets of coefficients listed in Table 2. Tables 3 and 4 were generated using the 0 mA calibration and 1.0 mA calibration, respectively, for the entire range of the

calibration. Table 5 was generated using the 2.0 mA calibration for the lower sub-range, 13.8 K to 273.16 K, only.

Expiration of Certification: The certification of this SRM is valid indefinitely to within the stated uncertainties over the temperature range specified in Table 1, provided that the SRM is used in accordance with the Notice and Warning to Users section of this certificate. The certification is nullified if the SRM is damaged, mechanically

shocked, chemically contaminated, overheated, or modified. Such severe treatment by rough handling or by

accident is generally accompanied by large and abrupt upward shifts in the SPRT resistance, #(273.16 K), at the water TP. Abrupt changes in #(273.16 K) of more than 100 u.Q may indicate this condition. Monitoring of #(273.16 K) by following the procedures outlined under "Periodic Checks at Water TP" given below is recommended. In many cases, it may still be possible to recertify SRM 1750 even in the event that severe treatment has been verified. For recertification, contact W.L. Tew by telephone at 301-975-4811, fax at 301-548-0206, or e-mail at weston.tew@nist.gov.

Preparation and calibration of this SRM were performed by W.L. Tew and G.F. Strouse of the NIST Process Measurements Division.

The support aspects involved in the preparation, certification, and issuance of this SRM were coordinated through

the NIST Standard Reference Materials Program by J. W.L. Thomas.

James R. Whetstone, Chief

Prooooo Mooaurcmcnta Division

Gaithersburg, MD 20899

Certificate Issue Date: 26 June 2001

Nancy M. Trahey, Chief

Standard Reference Materials Program

SRM 1750 Page 1 of 12 39
Maintenance of SRM Certification: NIST will monitor this SRM over the period of its certification. If substantive technical changes occur that affect the certification, NIST will notify the purchaser. Return of the attached registration card will facilitate notification.

Discussion of Calibration Fixed-Point Uncertainties: The NIST calibration uncertainty for this SRM for each of

the ten fixed-point temperatures is shown in Table 1. For calculation of the calibration uncertainty at all other temperatures in the calibration range, refer to the discussion under Appendix A, "Total Calibration Uncertainty".

All measurements of the SPRT resistance have been made with an AC resistance bridge operating at a frequency of

30 Hz and with continuous measuring currents of A and I2 as given below. The calibration points at the argon (Ar)

TP, mercury (Hg) TP, water (H2 0) TP, gallium (Ga) melting point (MP), and indium (In) freezing point (FP) were

measured in fixed-point cells [3] with expanded uncertainties as given below for the particular cells used in conjunction with this type of SPRT [4]. The calibration points at the oxygen (02 ) TP, neon (Ne) TP, equilibriumhydrogen (e-H2 ) TP, and e-H2 vapor pressure (VP) points were measured by comparison [5] to a reference SPRT, s/n 100413 1, that has been calibrated according to the ITS-90 using recent realizations of these fixed points at NIST [6]. The comparison measurements were performed under isothermal conditions, in vacuum, and the expanded

uncertainties given below [7] were combined from NIST uncertainties due to the comparison measurements and the fixed-point realizations

Fixed Point

Table I. Calibration Fixed-Point Uncertainties

Temperature Currents Uncertainty, (k = 2) T90 (K) /90 (°C) 7,(mA) /2 (mA) (mK)

e-H2 TP 13.8033 -259.3467 2.828 5.0 0.25 e-H2 VP 17.036 -256.1 143 2.828 5.0 0.19 e-H2 VP 20.271 -252.8799 2.0 2.828 0.17 Ne TP 24.5561 -248.5939 2.0 2.828 0.31 o2 TP 54.3584 -218.7916 1.0 2.0 0.14 Ar TP 83.8058 -189.3442 1.0 2.0 0.15 Hg TP 234.3156 -38.8344 1.0 1.414 0.15 H2 0 TP 273.16 0.01 1.0 1.414 0.04 Ga MP 302.9146 29.7646 1.0 1.414 0.04

In FP 429.7485 156.5985 1.0 1.414 0.32

This thermometer is satisfactory as a defining instrument of the ITS-90 in accordance with the criteria that ^(302.9146 K)> 1.1 1807 or ^(234.3156 K) < 0.844235.

Source of Material: The platinum wire in this SRM was drawn from bars (Bars 240-203 and 240-206) having a

reported purity as a mass fraction of 99.999+ % and was obtained from Sigmund Cohn Corporation, Mount Vernon,

NY1. The SPRT element and capsule were prepared and fabricated by Rosemont Aerospace, Inc., Eagan, MN. The

precision-bore borosilicate tubing was supplied by Richland Glass, Richland, NJ.

NOTICE AND WARNING TO USERS

Storage: For each unit of SRM 1750, the SPRT is made from a strain-free platinum element that is susceptible to mechanical shock. When not in use, the SPRT should be stored in the wooden case provided and this case should,

in turn, be kept in a secure and protected location. Avoid long term exposure of the SPRT to high levels of

mechanical vibration and to corrosive gases and fluids. The glass tube adapter should be kept clean and dry and

stored in its original container when not in use.

Handling: Due to the relatively delicate nature of the platinum element, the handling of the SPRT should be limited

to specially trained personnel. The degree of handling should also be kept to a minimum in order to reduce the risk of accidental mechanical shock. Avoid excessive bending or kinking of the platinum lead wires of the SPRT.

Certain commercial equipment, instruments, or materials are identified in this certificate 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. SRM 1750 Page 2 of 12 40
Follow the recommended procedures given in Appendix B, "Use of the Glass Adapter Probe", for loading and

unloading the SPRT into and out of the glass tube.

INSTRUCTIONS FOR USE

The SPRT capsule alone can be used over the entire range of its calibration (13.8 K to 430 K). The glass adapter

probe allows the SPRT capsule to be used in the same way as a conventional long-stem SPRT within the temperature limits of 83.8 K to 430 K. The resistance of the SPRT can be measured with any high resolution four- wire resistance measurement system capable of sourcing excitation currents between 1 mA to 5 mA. To achieve the highest level of accuracy, the measurement system should be able to resolve voltages of 0.01 uV or less. The

measurement current should be chosen to match or otherwise be extrapolated to one of the three currents listed in Table 2 for the deviation function coefficients. The 2.0 mA current is most appropriate for measurements below

about 30 K, while 1.0 mA is normally adequate for all other temperatures. The 0 mA case is used when

extrapolating to zero power dissipation to eliminate self-heating errors.

When using the SPRT capsule alone, it is customary to place it into a close fitting well in a copper block for comparison to other thermometers. The copper block may be held under vacuum, in a low pressure (< 100 kPa)

inert gas or in an inert heat transfer fluid which is electrically insulating. The lead wires going between the SPRT

and the ambient temperature regions should be thermally anchored near to or on the copper block itself to minimize

heat flow into the SPRT through the leads. The SPRT is not designed to be directly immersed into water or any

other electrically conductive fluid. Use of the SPRT in water baths is only possible when using the adapter probe.

The primary role of the adapter probe is to facilitate periodic checks of the SPRT resistance at the triple point of

water 7?(273.16 K). Such checks are important to make in order to (a) properly calculate the resistance ratio W{T) = R(T)/R(273.\6 K) that is used in the ITS-90 deviation equations and (b) verify the stability of the SPRT (see "Periodic Checks at water TP", below). In this way, the user need not maintain a strict resistance scale calibration in 1990 ohms, as long as the same resistance scale or reference resistance standard is used in both of the measurements

required to calculate the W{T) value. These checks are also necessary to accommodate the inevitable trend of any SPRT to gradually increase in resistance over time from handling. The user should first measure R(21i.\6 K) for the SRM 1750 SPRT with the user's measurement system before using it in any measurements to determine

any other temperatures.

During use of the adapter probe in any fixed-point cell or bath for which the temperature is below ambient, the adapter should be pressurized with a slightly positive pressure (I kPa to 3 kPa above ambient) of an inert gas, preferably helium. Maintaining this positive pressure is necessary to avoid condensation of atmospheric water vapor

inside the probe. This can be accomplished by connecting a regulated gas supply to the small nipple at the top of the nylon tee fitting with a flexible hose of approximately 3 mm inner diameter. The connectors found in the nylon tee are not perfectly gas tight and some small gas leakage will occur in normal operation. Care must be taken not to over-pressurize the adapter tube, as this could result in personal injury, damage to the tube, SPRT, and/or contiguous

equipment.

When used alone, the SPRT capsule can support external pressures from vacuum up to 6.8 MPa without gross mechanical failure. However, this certification is restricted in pressure range from vacuum to 0.3 MPa.

DO NOT attempt to bring any portion of the SRM adapter probe assembly to a temperature above 160 °C, as this can cause the bonding film on the surface of the wiring harness to soften and stick to the inner surface of the borosilicate tube. In the event this does occur, follow the instructions given in Appendix B, "Use of the Glass Adapter Probe", to safely remove the adhered section of wire. The upper temperature limit for the SPRT capsule is also 160 °C. While the capsule may survive if exposed to temperatures higher than this limit, the accuracy of the NIST calibration will no longer be certified.

Periodic Checks at the Water TP: The stability of this SRM should be verified periodically by the user by

measuring its resistance at the water TP. The interval of time between verification measurements will depend on the frequency of use; however, once per year should be considered the maximum allowable interval. The best possible

reproducibility will be achieved by following a few special procedures Huring these check""?' (11 always make sure the ice mantle of the water TP cell is free to rotate about the re-entrant well and not stuck to the well; (2) always take measurements at two different currents and calculate the zero-power resistance for the most accurate comparison to the previous measurements; (3) fabricate an aluminum bushing to fit loosely over the outside of the glass adapter

tube and inside the water TP cell re-entrant well; then use this bushing during water TP measurements with the adapter probe to minimize local heating effects in the SPRT; and (4) allow approximately 30 minutes for the adapter

SRM 1750 Page 3 of 12 41
probe to come into equilibrium with the water TP cell before taking measurements. Further information on the preparation and use of the water TP can be found in Reference [3].

REFERENCES

[1] Preston-Thomas, H., "The International Temperature Scale of 1990," Metrologia, 27, pp. 3-10, (1990) and

Metrologia, 27, p. 107, (1990).

\2\ l ew. W.L., "Standard Reference Material 1750: Standard Platinum Resistance Thermometer, 13.8033 K to 429.7485 K," NIST Special Publication 260-139, U.S. Government Printing Office, Washington DC, (2001).

[3] Mangum, B.W. and Furukawa, G.T., "Guidelines for Realizing The International Temperature Scale of 1990

(ITS-90)," NIST Technical Note 1265, U.S. Government Printing Office, Washington DC, (1990).

[4] Strouse, G.F. and Tew, W.L., "Assessment of Uncertainties of Calibration of Resistance Thermometers at the National Institute of Standards and Technology," NISTIR 5319, p. 16, (1994).

[5] Tew, W.L. and Mangum, B.W., "New Procedures and Capabilities for the Calibration of Cryogenic Resistance

Thermometers at NIST," in Advances in Cryogenic Engineering, 39B, p. 1019, P. Kittel, editor, Plenum Press, New York, (1994).

[6] Tew, W.L., Strouse, G.F., Meyer, C.W., and Furukawa, G.T., "Recent Advances in the Realization and

Dissemination of the ITS-90 below 83.8058 K at NIST," Advances in Cryogenic Engineering, 43B, P. Kittel, editor, Plenum Press, New York, (1998).

[7] Tew, W.L., Strouse, G.F., and Meyer, C.W., "A Revised Assessment of Calibration Uncertainties for CapsuleType Standard Platinum and Rhodium-Iron Resistance Thermometers," NISTIR 6138, p. 36, (1998).

Users of this SRM should ensure that the certificate in their possession is current. This can be accomplished by

contacting the SRM Program at: telephone (301) 975-6776; fax (301) 926-4751; e-mail srminfo@nist.gov; or via

the Internet http://www.nist.gov/srm.

SRM 1750 Page 4 of 12 42
Serial No. 4450

Table 2. Certified values of the ITS-90 deviation function coefficients for the sub-ranges 1 and 1 0 using 0 mA and

1 mA excitations, and sub-range 1 using a 2.0 mA excitation. The most recent (as of 9-Aug-01 ) values of the triple point of water resistance

3 for those same currents are also shown in ohms.

0 mA Calibration 1 .0 mA Calibration 2.0 mA Calibration -1.044357907E-04 a\ = -1.040971 885E-04 a, = -1.031219354E-04

bv = 2.567946933E-05 bt = 2.924072463E-05 b t = 3.977980386E-05

C\ = 2.369852739E-06 C\ = 3.446201 830E-06 C\ = 6.6304644 17E-06

Cl = 1.418234293E-06 Cl = 2.072774788E-06 Cl = 4.007001081E-06

C3 = 3.501 131336E-07 c3 = 5.063623545E-07 Cl = 9.675855506E-07

C4 = 3.96496445 1E-08 c4 = 5.663556387E-08 c4 = 1.067214836E-07

Cs = 1.716391648E-09 c5 = 2.418339775E-09 c5 = 4.485994497E-09

<3\0 = -1.406229563E-04 <3|0 = -1.414004738E-04

.16K)= 25.42666 ohms a /?(273.16 K)= 25.42668 ohms a /?(273.16 K)= 25.42674 ohms 3

' The resistance values are not certified values and are provided for information purposes only.

The coefficients au b\, c u c2j) cjj, c4, an;d 273.16 K) as given by, 1 .

c5 arq used in ttye deviation equation for sub-range 1 (13.8033 K to

A Wx {T90 ) = a, [W(T90 )- 1] + \ [W{T90 ) - l]

2 + fc'MH^o )]}' +2

• (

1 )

The coefficient a w is used in the deviation equation for sub-range 10 (273.15 K to 429.7485 K) as given by,

AWw(Tgo ) = aw [w(T90 )-i\. (2)

Equations 1 and 2 are used in conjunction with the ITS-90 definitions,

A^(r90 )-^(r90 )-^r (r90 ), (3) and

72(273.16 K)'

(4)

where IV^Too) is the upper range reference function in the case of sub-range 10, or the lower range reference

function in the case of sub-range 1. Given a measured value of W(T90), the equations may be solved by iteration to find the temperature T90 . For additional information on the ITS-90 reference functions and a full description of the

scale see NIST Technical Note 1265.

SRM 1750

43

Page 5 of 12
Serial No. 4450

Table 3. Certified values of W(T90 ) versus Temperature in kelvins (K) for the SRM 1750 SPRT at 0 mA.

T1 90 0 1 2 3 4 5 6 7 8 QV

10 0.00133413 0.00161734 0.00196426 0.00238290 0.00288090 0.00346544 20 0.00414317 0.00492017 0.00580185 0.00679289 0.00789724 0.00911812 0.01045797 0.01191852 0.01350078 0.01520509 30 0.01703116 0.01897814 0.02104462 0.02322874 0.02552820 0.02794035 0.03046220 0.03309052 0.03582185 0.03865255 40 0.04157886 0.04459692 0.04770283 0.05089263 0.05416239 0.05750820 0.06092618 0.06441253 0.06796352 0.07157553

50 0.0752450 0.0789686 0.0827429 0.0865648 0.0904313 0.0943394 0.0982863 0.1022694 0.1062861 0.1103341 60 0.1144110 0.1 185147 0.1226431 0.1267943 0.1309664 0.1351578 0.1393669 0.1435920 0.1478319 0.1520850 70 0.1563503 0.1606264 0.1649124 0.1692072 0.1735098 0.1778193 0.1821349 0.1864559 0.1907814 0.1951109 80 0.1994438 0.2037793 0.2081171 0.2124566 0.2167973 0.2211388 0.2254809 0.2298229 0.2341648 0.2385061 90 0.2428466 0.2471860 0.2515241 0.2558606 0.2601955 0.2645284 0.2688593 0.2731880 0.2775144 0.2818383

0.2861596 0.3292168 0.3719659

0.4144071 0.4565614

0.4984560

0.5401 178 0.5815699

0.6228317

0.6639186

0.7048427 0.7456139

0.7862399

0.8267275

0.8670823

0.2904783 0.3335058

0.3762236- 0.4186350

0.46Q7620

0.5026323

0.5442721 0.5857044

0.6269481

0.6680181

0.7089266

0.7496829

0.7902948

0.8307689

0.8711 106

0.2947943

.0.3377917 0.3804783

0.42286tJO 0.4649602

0.5068062

0.5484243 0.5898370

0.6310627

0.6721161

0.7130089 0.7537504

0.7943483 0.8348089

0.8751377

0.2991075

0.3420745 0.3847299

0.4270823 0.4691558

0.5109779

0.5525745 0.5939677

0.6351755 0.6762124

0.7170897 0.7578166

0.7984005 0.8388476

0.8791635

0.3034178

0.3463541 0.3889784

0.4313017;

0 4733488

0.5151473; 0.5567226

0.5980965 0.6392867

0.6803072

0.7211690

0.7618813 0.8024512

0.8428851 0.8831880

0.3077252 0.3506306 0.3932239

0.4355184

0.4775394

0.5193145 0.5608688

0.6022236

0.6433962

0.6844003

0.7252469

0.7659446

0.8065006

0.8469212

0.8872113

1 0.3 120296

; 0.3549039

0.3974665

0.4397323 0.4817276

0.5234795 0.5650129

0.6063488 0.6475040 0.6884919

0.7293232

0.7700065 0.8105487

0.8509560

0.8912334

0.3163310

0.3591741 0.4017060

: 0.4439436

0.4859133

0.5276423

0.5691551 0.6104722

0.6516101 0.6925819

0.7333981 0.7740669 0.8145954

0.8549895

0.8952541

0.3206293 0.3634412

0.4059426

0.4481522

0.4900966

0.5318029

0.5732953

0.6145938 0.6557146

0.6966704

0.7374715 0.7781260

0.8186408 0.8590217

0.8992736

1.3028625 1.3417553 1.3805287 1.4191828

1.4577179

1.4961341 1.5344318

1.5726109 1.6106719

1.3067571 1.3456380

1.3843995

1.4230417 1.4615649

1.4999692 1.5382550

1.5764223

1 .3106506

1.3495195 1.3882690

1.4268993 1.4654106

1.5038031

1.5420771

1 5802326

1.3145429

1 3533999

1.3921374

1.4307558 1.4692552

1.5076359

1.5458979

1.5840416

SRM 1750

1 3184340

1.3572790

1.3960046

1.4346111 1.4730986

1.51 14674

1.5497176

1.5878495

44

1.3223238

1.3611569. 1.3998706

1.4384652 1.4769409

1.5152978

1.5535361 1.5916562

1.3262125 1.3650337

1.4037355

1.4423181 1.4807819

1.5191269

1.5573535 1.5954617

1.3301000

1.3689092

1.4075991 1.4461699

1.4846217

1 .5229549

1.5611696 1.5992660

1.3339863 1.3727836

1 4114615

1.4500204

1.4884604

1.5267817 1.5649846

1.6030691

0.3249246

0.3677051 0.4101763

0.4523581 0.4942775

0.5359614

0.5774336

0.6187136

0.6598174

0.7007573

0.7415434 0.7821836

0.8226848 0.8630526

0.9032919

250 0.9073089 0.9113247 0.9153392 0.9193525 0.9233646 0.9273754 0.9313850 0.9353934 0.9394006 0.9434065 260 0.9474112 0.9514147 0.9554170 0.9594180 0.9634179 0.9674165 0.9714139 0.9754101 0.9794051 0.9833989 270 0.9873915 0.9913828 0.9953730 0.9993619 1.0033495 1.0073358 1.0113209 1.0153048 1.0192875 1.0232689 280 1.0272492 1.0312282 1.0352060 1.0391826 1.0431580 1.0471322 1.0511051 1.0550768 1.0590474 1.0630167 290 1.0669848 1.0709516 1.0749173 1.0788818 1.0828450 1.0868070 1.0907679 1.0947275 1.0986859 1.1026431

300 1.1065991 1.1105538 1.1145074 1.1184598 1.1224109 1.1263609 1.1303096 1.1342571 1.1382034 1.1421485 310 1 1460925 1.1500352 1.1539767 1.1579169 1.1618560 1.1657939 1.1697306 1.1736660 1 .1776003 1.1815334 320 1.1854652 1.1893959 1.1933253 1 .1972536 1.2011806 1.2051065 1.2090311 1.2129546 1.2168768 1.2207978 330 1.2247177 1.2286363 1.2325537 1.2364700 1.2403850 1.2442988 1.2482115 1.2521229 1.2560331 1.2599422 340 1.2638500 1 .2677566 1.2716621 1.2755663 1.2794694 1.2833712 1.2872719 1.2911713 1.2950696 1.2989666

1.3378714

1.3766567 1.4153228 1.4538697

1.4922979

1.5306073 1.5687983

1.6068711

Page 6 of 1
Serial No. 4450

Table 4. Certified values of W(T90 ) versus Temperature in kelvins (K) for the SRM 1750 SPRT at 1 mA.

T1 90 n 1 2 3 4 f.\ > 7 8<> Q

10 0.00133413 0.00161736 0.00196427 0.00238290 0.00288090 0.00346543 20 0.00414317 0.00492018 0.00580186 0.00679289 0.00789724 0.00911811 0.01045794 0.01191847 0.01350070 0.01520498 30 0.01703103 0.01897798 0.02104444 0.02322853 0.02552797 0.02794010 0.03046194 0.03309025 0.03582157 0.03865226 40 0.04157857 0.04459664 0.04770255 0.05089236 0.05416214 0.05750796 0.06092596 0.06441232 0.06796334 0.07157536

50 0.0752449 0.0789685 0.0827428 0.0865648 0.0904312 0.0943394 0.0982863 0.1022694 0.1062862 0.1103342 60 0.1144111 0.1185148 0.1226432 0.1267945 0.1309666 0.1351581 0.1393671 0.1435923 0. 1478322 0.1520853 70 0.1563506 0.1606268 0.1649128 0.1692076 0.1735102 0.1778197 0.1821354 0.1864563 0.1907819 0.1951114 80 0.1994443 0.2037798 0.2081 176 0.2124571 0.2167978 0.2211394 0.2254814 0.2298235 0.2341654 0.2385067 90 0.2428472 0.2471866 0.2515247 0.2558613 0.2601961 0.2645291 0.2688600 0.2731886 0.2775150 0.2818389

0.2861603 0.3292174

0.3719665 0.4144077

0.4565619

0.4984565 0.5401182

0.5815703 0.6228320

0.6639188

0.2904790 0.2947950

0.3335065 0.3377924

0.3762243- . 0.3804,789 0.4186356 0.4228606

0.46Q7626 i 0.4649607

0.5026328

0.5442725

0.5857048

0.6269483 0.6680184

0.5068067 0.5484247

0.5898373 0.6310629

0.6721 163

0.2991082

0.3420752

0.3847305 0.4270829

0.4<p9I563

0.5109784 0.5525749

0.5939680

0.6351758

0.6762126

0.3034185

0.3463548 0.3889790

0.4313023

0.4733494!

i | 1 j

0.5151478; 0.5567230

0.5980969 0.6392870

0.6803074

0.3077259,

0.3506312, 0.3932246 j 0.4355190

0.4775399

0.5193149

0.5608692

0.6022239 0.6433964

0.6844005

0.3120303 0.3549046

0.3974671 0.4397329

0.4817281

0.3163317

0.3591748 0.4017066

0.4439442

0.4859138

0.3206300

0.3634418 0.4059432

0.4481527

0.4900971

0.5650133

0.6063491 0.6475042

0.6884921

0.5691555 0.6104725

0.6516104

0.6925821

0.5732957

0.6145941 0.6557148

0.6966706

1.6106714

0.3249253 0.3677057 0.4101769

0.4523586

0.4942780

0.5234799 0.5276427 0.5318033 0.5359618 0.5774339

0.6187139

0.6598176

0.7007575

200 0.7048429 0.7089267 0.7130091 0.7170899 0.7211692 0.7252470 0.7293234 0.7333982 0.7374716 0.7415435 210 0.7456140 0.7496830 0.7537506 0.7578167 0.7618814 0.7659447 0.7700065 0.7740670 0.7781261 0.7821837 220 0.7862400 0.7902949 0.7943484 0.7984005 0.8024513 0.8065007 0.8105488 0.8145955 0.8186408 0.8226849 230 0.8267275 0.8307689 0.8348090 0.8388477 0.8428851 0.8469212 0.8509560 0.8549895 0.8590217 0.8630526 240 0.8670823 0.8711106 0.8751377 0.8791635 0.8831881 0.8872113 0.8912334 0.8952541 0.8992736 0.9032919

250 0.9073089 0.9113247 0.9153392 0.9193525 0.9233646 0.9273754 0.9313850 0.9353934 0.9394006 0.9434065 260 0.9474112 0.9514147 0.9554170 0.9594180 0.9634179 0.9674165 0.9714139 0.9754101 0.9794051 0.9833989 270 0.9873915 0.9913828 0.9953730 0.9993619 1.0033495 1.0073358 1.0113209 1.0153048 1.0192875 1.0232689 280 1.0272492 1.0312282 1.0352060 1.0391826 1.0431580 1.0471321 1.0511051 1.0550768 1.0590473 1.0630166 290 1.0669847 1.0709516 1.0749173 1.0788817 1.0828450 1.0868070 1.0907678 1.0947274 1.0986858 1.1026430

300 1.1065990 1.1105538 1.1 145073 1.1184597 1.1224108 1.1263608 1.1303095 1.1342570 1.1382033 1.1421484 310 1.1460923 1.1500350 1.1539765 1.1579168 1.1618559 1.1657938 1.1697304 1.1736659 1.1776002 1.1815332 320 1.1854651 1.1893957 1.1933252 1.1972534 1.2011805 1.2051063 1.2090310 1.2129544 1.2168766 1.2207977 330 1.2247175 1.2286361 1.2325536 1.2364698 1.2403848 1.2442986 1.2482113 1.2521227 1.2560329 1.2599420 340 1.2638498 1.2677564 1.2716619 1.2755661 1.2794692 1.2833710 1.2872716 1.2911711 1.2950693 1.2989664

350 1.3028622 1.3067569 1.3106504 1.3145426 1.3184337 1.3223236 1.3262123 1.3300998 1.3339860 1.3378711 360 1.3417550 1.3456377 1.3495193 1.3533996 1.3572787 1.3611566 1.3650334 1.3689089 1.3727833 1.3766564 370 1.3805284 1.3843992 1.3882687 1.3921371 1.3960043 1.3998703 1.4037351 1.4075988 1.4114612 1.4153224 380 1.4191825 1.4230413 1.4268990 1.4307555 1.4346108 1.4384649 1.4423178 1.4461695 1.4500200 1.4538694 390 1.4577175 1.4615645 1.4654103 1.4692549 1.4730983 1.4769405 1.4807815 1.4846214 1.4884600 1.4922975

400 1.4961338 1.4999688 1.5038028 1.5076355 1 5114670 1.5152974 1.5191265 1.5229545 1.5267813 1.5306069 410 1.5344313 1.5382546 1.5420766 1.5458975 1.5497172 1.5535357 1.5573530 1.5611692 1.5649841 1.5687979 420 1.5726105 1.5764219 1.5802321 1.5840412 1.5878490 1.5916557 1.5954612 1.5992655 1.6030687 1.6068706

SRM 1750 Page 7 of 12 45
Serial No. 4450

Table 5. Certified values of W(T90) versus Temperature in kelvins (K) for the SRJV1 1750 SPRT at 2 mA.

100 110

120

130 140

150 160

170

180 190

200

210

220

230

240

250

260

270

'90 0 1 i 3 4 5 6 7 8 9

10 0.00133415 A AA1 Z

1 ! lit 1 0.00 1 61 74

A AA 1 A f A "> A 0.00196430 A AA'"\ir>'\Al 0.00238291 0.00288088 0.00346542 20 0.004143 17 a aa<aiaia 0.00492019 A AA £ O A 1 O O 0.003801 88 0.00679291 A A A*"7 O A C 0.00789725 A AAA1 t O A *1 0.0091 1807 0.01045786 A A1 1 A 1 O "~> "\ 0.01 191832 0.01350047 0.01520467 30

A A t *7 A "> A t 0.0 170.>064 A AIOAT7CA 0.01897730 a mio.noo 0.02104388 0.02322791 A. A1f CTTTO 0.02352729 A ATTATAT

/

0.02793936 A ATA/I / 1 1 /" 0.030461 16 A A "> A O A /I ~> 0.03308943 0.03582073 0.03865 141 40 0.04157772 A A 4 i

/• A .T OA 0.04439580 0.047701 73 0.050891 57 0.03416139 0.05750725 A A «^ AA ^\ "\ A 0.06092529 A A S~ A 4 1 i *T'"\ 0.06441 172 0.06796279 0.07157488

50 0.0752445 0.0789681 0.0827425 0.0865646 0.0904311 0.0943393 0.0982863 0.1022695 0.1062864 0.1103344 60 0.1144114 0.1185152 0.1226437 0.1267950 0.1309673 0.1351588 0.1393679 0.1435931 0.1478330 0.1520863 70 0.1563516 0.1606279 0.1649139 0.1692087 0.1735114 0.1778210 0.1821367 0.1864577 0.1907833 0.1951129 80 0.1994458 0.2037814 0.2081192 0.2124587 0.2167995 0.2211411 0.2254831 0.2298252 0.2341671 0.2385085 90 0.2428490 0.2471884 0.2515265 0.2558631 0.2601980 0.2645309 0.2688619 0.2731906 0.2775169 0.2818409

0.2861622

0.3292194

0.3719685 0.4144095 0.4565636

0.4984580

0.5401195

0.5815714

0.6228329

0.6639195

0.7048434

0.7456143 0.7862402 0.8267277

0.8670823

0.9073089

0.9474112

0.9873915

0.2904810

0.3335084

0.3762262

0.4186374

0.4607642

0.5026342 0.5442738

0.5857058

0.6269492 0.6680190

0.7089272

0.7496833

0.7902951 0.8307690

0.8711 107

0.91 13247 0.9514147

0.9913828

0.2947969 0.3377944

0.3804808

0.4228624

0.4649623

0.5068081 0.5484260

0.5898384

0.6310638 0.6721169

0.7130095 0.7537509

0.7943486

0.8348091 0.8751377

0.9153392 0.9554170

0.9953730

0.2991101

0.3420771 0.3847324 0.4270846

0.4691579

0.3034204 0.3077278. 0.3120322 0.3163336 0.3206320 0.3249272

0.5109798

0.5525761 0.5939690

0.6351766

0.6762133

0.7170904 0.7578170

0.7984007

0.8388478

0.8791635

0.9193525 0.9594180

0.9993619

0.3463567 0.3506332 0.3889809 0.3932264

0.4313041 0.4355207 0.4733509

0.5151492

0.5567242' 0.5980979

0.6392878

0.6803080

0.7211697

0.7618817

0.8024515 0.8428852

0.8831881

0.9233646

0.9634179

0.4775415

0.5193163

0.5608703 0.6022249

0.6433972

0.6844011

0.7252475 0.7659450

0.8065009

0.8469213 0.8872114

0.9273754

0.9674165

0.3549065 0.3974689

0.4397346

0.4817296

0.5234813

0.5650145

0.6063501 0.6475050

0.6884927

0.7293238

0.7700068 0.8105489

0.8509561 0.8912334

0.9313850

0.9714139

0.3591767

0.4017085 0.4439459

0.4859153

0.5276440

0.5691566

0.6104734

0.6516111 0.6925827

0.7333986

0.7740673 0.8145956

0.8549896

0.8952541

0.9353934

0.9754101

0.3634437

0.4059451 0.4481544

0.4900986

0.5318047

0.5732968 0.6145950

0.6557155

0.6966711

0.7374720

0.7781263 0.8186410

0.8590218

0.8992736

0.9394005

0.9794051

0.3677077

0.4101787

0.4523603

0.4942795

0.5359631 0.5774350

0.6187148

0.6598183 0.7007580

0.7415439 0.7821840

0.8226850

0.8630527

0.9032919

0.9434065 0.9833989

SRM 1750

46

Page 8 of 12
APPENDIX A. Total Calibration Uncertainty

The NIST assessment of uncertainties in the ITS-90 calibration of an SPRT involves the decomposition of

uncertainty into Type A and Type B components. The Type A component, s, is a combined uncertainty from

standard deviations of only those measurements under direct statistical process control. The Type B components, u., are the estimated standard uncertainties for each known component in the measurement process that cannot be

directly measured. In addition, uncertainties are described by a coverage factor, k, for a specific level of confidence.

The Type A and Type B uncertainties at each of the SPRT calibration points, "x", are combined in quadrature and

expanded by the coverage factor, k, to form the expanded fixed-point uncertainty, Ux, given by

U x ^k^s 2 +Zi<

2

j (A-l)

In general, each fixed-point uncertainty Ux will propagate over the calibration sub-range with a specific form of

temperature dependence. The NIST total calibration uncertainty, Ucai(T90 ), is then calculated as a root-sum-square

(RSS) from the propagation of these individual fixed-point uncertainties as given on page 1 of this certificate,

(A-2)

where the summation is over all the fixed points required for the sub-range.

The curves in Figure A-l show Ucal (T90 ), expressed in units of mK, for all temperatures in the SPRT calibration range for SRM 1750. The UC3l (T90 ) uncertainty is expanded for a coverage factor of k = 2, so that the two curves as shown in Figure A-l represent a bounded interval having a confidence level of approximately 95 %.

08 t 1 1

1 1

1—,1

, 1

, ,

1—

0.6

-06

-0.8 -I 1 1 1 1- 1 1 1

1—I1 1 1 1 1 1 1 1—

10 100 1000 Tan I K

Figure A-l. The NIST total calibration uncertainty for the SRM 1750 SPRT. The curves represent a bounded

interval with a confidence level of 95 %. The interval is constrained to be zero at 273.16 K (see

text).

SRM 1750 Page 9 of 12 47
As a consequence of the definition of W(T90 ), the uncertainty of the NIST realization of the water TP is already

included in the stated uncertainties for the other fixed points. Hence, the curves for Uca\(T90), are constrained to be

zero at the water TP in order to avoid overestimating the uncertainty contribution from the NIST water TP

realization. However, in order to use the SPRT to determine temperature, the user must account for an additional uncertainty contribution from the user's realization of the water TP. This additional user's uncertainty component,

"wtp(^o), wi" propagate throughout the calibration temperature range with a temperature dependence as shown in Figure A-2. The size of wwtp(273.16 K) is assumed to be 0.1 mK in Figure A-2. For a specific user's application,

however, this will depend on a variety of factors including the resolution of the user's measurement system for a

resistance of 25.5 H.

0 18

10 60 110 160 210 260 310 360 410 r90 /K

Figure A-2. The propagation of an uncertainty of 0.1 mK at the water triple point (WTP) as incurred by the user

for the SRM 1750 SPRT. The relative location of the fixed-point temperatures are shown for reference only.

Finally, the user must determine the uncertainty with which the user's measurement system can determine the

resistance ratio W(T90 ) of the SPRT for all of the resistance values in the calibration range. This uncertainty,

expressed in units of mK, would then be added in quadrature to both «wtp(79o) and Uca \(T9Q ) in order to determine

the user's total expanded uncertainty for the determination of temperature with the SPRT.

SRM 1750

48

Page 10 of 12
APPENDIX B. Use of the Glass Adapter Probe

The glass adapter probe assembly of SRM 1750 is very simple to use with the SPRT; however, the user must read

and understand the contents of this appendix before attempting to load the probe assembly.

Referring to Figure B-l, the probe is made from a 60 cm long precision-bore borosilicate glass tube of 5.8 mm ID

and 7.5 mm OD which is custom made to fit the SRM 1750 SPRT. A nylon tee header is included to terminate the open end of the tube. This header is slightly oversized for the tube, so a teflon front-ferrule must be used for that connection. The teflon ferrule has sufficient compliance to accommodate the slight mismatch in diameters. This

connection should be made snug by finger tightening only, over-tightening the nut onto the tube will break the glass. The other two connections on the nylon tee accommodate the four-wire polarized plug and a 3 mm diameter hose

nipple.

The wiring harness consists of four 32 AWG phosphor-bronze lead wires that are polyimide insulated. The wires

are bonded together as a ribbon by a polyvinyl butral bonding film. The harness is terminated with four #22 socket

contacts made from gold-plated beryllium-copper. These contacts mate onto the pin contacts located on the ends of

the SPRT platinum lead wires. Three short pieces of polytetrafluoroethylene (PTFE) tubing serve to insulate these contacts from each other while the entire assembly is loaded inside the glass tube.

When loading and unloading the SPRT into and out of the glass adapter tube, several critical precautions must be

taken in order to minimize the risk of accidental mechanical shocks to the SPRT. Follow the procedures described below for loading and unloading the SPRT.

Loading: For loading, first prepare a horizontal working surface with approximately 1.2 m of clearance in one

direction and cover the surface with a soft cushioning foam. Disconnect the nylon tee from the glass tube and remove the wiring harness. Lay the glass tube and the SPRT on the foam horizontally in such a way that they are prevented from rolling. Next, connect the pins on the SPRT leads to the sockets on the end of the wiring harness,

keeping the SPRT fixed on the foam surface. To prevent shorting of the SPRT leads at the pin-to-socket connection, be sure to overlap a short section of PTFE tubing over the exposed sections of the connectors for three out of four of

the pin-to-socket pairs. Then insert the bottom end of the SPRT into the glass tube and begin to slowly slide the

entire assembly of the SPRT and wiring harness further inside. It should be possible to continue to slide the assembly and keep the SPRT horizontal by gently pushing the wire into the glass until the SPRT reaches the bottom

of the tube. At this point, there will be very little, if any, slack in the wiring harness left and the nylon tee may then be re-connected to the tube. Tighten the nut on the nylon tee with fingers only (DO NOT use a wrench). Once the SPRT is resting firmly at the bottom of the tube and the nylon tee reconnected, the entire assembly may then be

picked up and held in a vertical orientation suitable for insertion into a fixed-point cell. Always support the assembly by holding onto the glass tube; never hold it using the nylon tee alone.

Unloading: Unloading the SPRT can be accomplished by disconnecting the nylon tee from the glass tube and

gently pulling on the wiring harness to retract the SPRT. Once the SPRT has been removed from the tube,

disconnect the pins and sockets and place the SPRT in its wooden case for storage and/or transport. As with the loading operation, the unloading operation should be done with the glass tube held horizontally, resting entirely on

the foam cushion. Never hold the tube more than a few degrees off horizontal or otherwise allow the SPRT to accelerate freely towards either end of the tube when loading and unloading.

If the probe has been accidentally overheated during its use, a section of the wiring harness' bonding film will often

soften and then stick to the inner surface of the glass tubing upon cooling. This section of adhered wire must be mechanically dislodged from the inner glass surface before the SPRT can be safely unloaded. This can be accomplished by using a small gauge piece of tempered steel, such as piano wire, with a small hook fashioned on

one end. Use the steel wire hook to gently lift the section of adhered wiring harness off of the inner glass wall. DO

NOT pull or yank on the wiring harness until all sections are free. After dislodging the wire, all four leads in the harness should then be inspected for any permanent damage and the insulation resistance between each lead checked. If the insulation resistances between all lead wires are still greater than 100 MQ, then it should be possible

to re-use the wiring.

SRM 1750

49

Page 1 1 of 12
Nylon Header

Glass Tube

Four-wire plug

Figure B-l . The glass adapter probe shown as a cross-section with the SPRT in the properly loaded position.

SRM 1750

50

Page 12 of 12


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