NIST Measurement Services: NEW NIST PUBLICATION

June 12, 1989

The Calibration of

Thermocouples and

Thermocouple Materials

NIST

Special

Publication 250-35

G. W. Burns and M. G. Scroger

U.S. Department of Commerce

National Institute of Standards and Technology
Center for Basic Standards

The Center for Basic Standards develops and maintains the scientific competencies and laboratory facilities nec- essary to preserve and continue to refine the base physical quantities upon which the Nation's measurement

system is constructed. The Center is responsible for the standards of voltage, current, resistance and

impedance, temperature, vacuum, leak rate, pressure, mass, length, time and frequency. The Center collaborates closely with the standardizing laboratories of other countries to ensure that measures used in international trade are fully compatible. The Center also performs research at the frontiers of physics in order to improve measurement capability and quantitative understanding of basic physical processes that underlie measurement

science. Electricity Division

Maintains and improves the national standards of electrical measurements; conducts experiments to realize the

electrical units in terms of the mechanical units; develops stable electrical standards and improved methods for the dissemination of the units of electrical measurements; determines fundamental physical constants related to

electrical units; develops instrumentation for precise electrical measurements directly referenced to the national standards; implements techniques for their use and puts them into practical applications; assists in applying scientific and technological development to measurement problems, especially in other areas of NIST research. Temperature and Pressure Division

Maintains and improves the national standards of temperature and pressure; conducts research towards the de- velopment of new concepts for these standards; calibrates practical standards for the U.S. scientific and technical community in terms of the primary standards; develops methods and devices to assist user groups in the assessment and enhancement of the accuracy of their measurements of temperature and pressure; prepares and promulgates evaluations and descriptions of temperature and pressure measurement processes; coordi- nates these standards and measurement methods nationally and internationally; and determines the accuracy of the national standards of temperature and pressure with respect to fundamental thermodynamic relations. Lengtti and iVIass Division

Develops and uses the competence necessary to implement a more accurate measurement system for length and mass; applies research, utilizing the most advanced technology available, to the improvement of our present

standards and devises new standards and new methods for their transfer. Time and Frequency Division

Maintains, develops, and improves the national standards for time (which are coordinated with the United States Naval Observatory) and frequency, and the time scales based on these standards; carries out research in areas

of importance to the further fundamental improvement of frequency standards and their applications focusing on

microwave and laser devices, atomic and molecular resonances, and the measurement of fundamental physical phenomena and constants; adapts time and frequency standard devices and concepts to special scientific and

technological demands; develops time and frequency measurement methods in the radio frequency, microwave,

infrared, and visible radiation regions; coordinates the national time and frequency standards, time scales, and

measurement methods nationally and internationally in conjunction with the United States Naval Observatory;

operates time and frequency dissemination services, such as radio stations and broadcasts, for the purpose of

traceability to the national standards of time and frequency. Quantum Physics Division

Engages in research in atomic and molecular physics at the forefront of the field, and performs basic, highly accurate measurements and theoretical analyses that help establish a reliable foundation for scientific and technological measurement and data. Quantum IVIetrology Division

Engages in forefront research in quantum metrology; contributes to new determinations of the fundamental

physical constants, and the extension and refinement of the electromagnetic scale and devises, where possible,

tests of basic symmetries and invariances.
NIST MEASUREMENT SERVICES:

The Calibration of Thermocouples and

Thermocouple Materials

G. W. Bums and M. G. Scroger

Temperature and Pressure Division Center for Basic Standards

National Measurement Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

April 1989

'Co. NOTE: As of 23 August 1988, the National Bureau of

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY.

Raymond G. Kammer, Acting Director

standards (NBS) t>ecame the National Institute of Standards and Technology (NIST) when President Reagan signed into law the Omnibus Trade and

Competitiveness Act.
Library of Congress Catalog Card Number: 89-600732

National Institute of Standards and Technology Special Publication 250-35

Natl. Inst. Stand. Technol. Spec. Pub!. 250-35, 201 pages (Apr. 1989)

CODEN: NSPUE2

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

to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology,

nor does it imply that the materials or equipment identified are necessarily the best

available for the purpose.

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1989

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
PREFACE Calibrations and related measurement services of the National Institute of Standards and Technology provide the means for makers and users of measuring

tools to achieve levels of measurement accuracy that are necessary to attain

quality, productivity and competitiveness. These requirements include the highest levels of accuracy that are possible on the basis of the most modern

advances in science and technology as well as the levels of accuracy that are necessary in the routine production of goods and services . More than 300 different calibrations, measurement assurance services and special tests are available from NIST to support the activities of public and private

organizations. These services enable users to link their measurements to the reference standards maintained by NIST and, thereby, to the measurement

systems of other countries throughout the world. NIST Special Publication

250, NIST Calibration Services Users Guide , describes the calibrations and related services that are offered, provides essential information for placing

orders for these services and identifies expert persons to be contacted for technical assistance. NIST Special Publication 250 has recently been expanded by the addition of supplementary publications that provide detailed technical descriptions of specific NIST calibration services and, together with the NIST Calibration

Services Users Guide , they constitute a topical series. Each technical

supplement (NIST SP 250- ) on a particular calibration service includes:

o specifications for the service

o design philosophy and theory

o description of the NIST measurement system

o NIST operational procedures

o measurement uncertainty assessment

error budget

systematic errors random errors

o NIST internal quality control procedures

The new publications will present more technical detail than the information

that can be included in NIST Reports of Calibration. In general they will

also provide more detail than past publications in the scientific and technical literature; such publications, when they exist, tend to focus upon

a particular element of the topic and other elements may have been published

in different places at different times. The new series will integrate the description of NIST calibration technologies in a form that is more readily

accessible and more useful to the technical user. The present publication, SP 250-35, NIST Measurement Services: The Calibration

of Thermocouples and Thermocouple Materials, by G. W. Burns and M. G. Scroger,

iii
is one of approximately 20 documents in the new series published or in preparation by the Center for Basic Standards. It describes calibration

technology and procedures utilized in connection with NIST Service

Identification Numbers from 32010C to 32150S listed in the NIST Calibration

Services Users Guide 1989 . pages 54 to 55. Inquiries concerning the contents

of these documents may be directed to the author (s) or to one of the technical contact persons identified in the Users Guide . Suggestions for improving the effectiveness and usefulness of the new series

would be very much appreciated at NIST. Likewise, suggestions concerning the need for new calibration services, special tests, and measurement assurance

programs are always welcome.

Joe D. Simmons, Acting Chief

Office of Physical Measurement Services

Katherine Gebbie, Acting Director

Center for Basic Standards

iv
CONTENTS

Page

1. DESCRIPTION OF SERVICES 1

2. PRINCIPLES OF THERMOELECTRIC THERMOMETRY 4

2.1 Thermoelectric Phenomena 4 2.2 Choice of the Thermocouple Materials 6 2.2.1 Temperatures from 0 K to 450 °C 8 2.2.2 Temperatures from 450 °C to 1100 'c 10 2.2.3 Temperatures above 1100 °C 11 2.3 Inhomogeneity 13 2.4 Calibration 14 2.5 Installation 15 2.6 Summary of the Laws of Thermoelectric Circuits 16 2.6.1 Law of Homogeneous Metals 16 2.6.2 Law of Intermediate Metals 16 2.6.3 Law of Successive or Intermediate

Temperatures 17 2.7 References 18

3. CALIBRATION METHODS AND PROCEDURES FOR THERMOCOUPLES

AND THERMOCOUPLE MATERIALS 25 3.1 Calibration by Comparison with a Reference Thermocouple 25 3.1.1 Type S, Type R, and Type B Thermocouples .... 25 3.1.1.1 Preliminary Examination 25 3.1.1.2 Electrical Anneal 25 3.1.1.3 Mounting for Calibration 26 3.1.1.4 Furnace Anneal 26 3.1.1.5 Calibration Data 27 3.1.1.6 Packing and Shipping 27 3.1.1.7 Calibration Report 28 3.1.1.8 Apparatus 31 3.1.2 Base Metal Thermocouples 33 3.1.3 Thermoelements versus Platinum

Thermoelectric Reference Standard, Pt-67 .... 35 3.1.4 Metal-Sheathed Thermocouples 35 3.2 Calibration by Comparison with a Standard

Platinum Resistance Thermometer 36 3.2.1 Calibration Baths 36 3.2.1.1 Water and Oil Baths 36 3.2.1.2 Tin Bath 37 3.2.1.3 Cryostat 38 3.2.1.4 Liquid Oxygen and Liquid

Nitrogen Points 38

V
Page

3.2.2 Equipment 39 3.2.2.1 Standard Platinum Resistance

Thermometer 39 3.2.2.2 AC Bridge 40 3.2.2.3 Potentiometer 40 3.2.3 Calibration Procedure 40 3.2.4 Computation of Bath Temperature 41 3.2.5 Calibration Report 44

3.3 Primary Calibration of Type S Thermocouples

in Metal Freezing- Point Cells 44

3.3.1 Preliminary Examination 46 3.3.2 Electrical Anneal 46 3.3.3 Mounting for Calibration 46 3.3.4 Furnace Anneal 46 3.3.5 Preliminary Tests 47 3.3.6 Emf Measurements at Freezing Points 47 3.3.6.1 Gold and Silver Freezing Points. ... 49 3.3.6.2 Antimony Freezing Point 50 3.3.6.3 Zinc Freezing Point 50 3.3.7 Packing and Shipping 51 3.3.8 Calibration Report 51 3.3.9 Apparatus 53 3.3.9.1 Electrical Annealing System 53 3.3.9.2 Welding Station 53 3.3.9.3 Annealing Furnace 53 3.3.9.4 Furnace For Preliminary Tests 53 3.3.9.5 Freezing-Point Cells 53 3.3.9.6 Furnace for Gold and Silver

Freezing-Point Cells 54 3.3.9.7 Furnace for Antimony and Zinc

Freezing-Point Cells 55 3.3.9.8 Ice Baths 55 3.3.9.9 Measuring Instrument 55 3.4 References 56

4. INTERNAL QUALITY CONTROL 69 4.1 Voltage Standards 69 4.2 Voltage Measuring Instruments 70 4.2.1 Potentiometers 70 4.2.2 Digital Voltmeters 70 4.2.3 Thermocouple Measuring Circuitry 70 4.3 Resistance Measuring Instrument 71 4.4 Temperature Measuring Instruments 71 4.4.1 Working- Standard Reference Thermocouples , , , . 71 4.4.2 Standard Platinum Resistance Thermometers. ... 72 4.5 Platinum Thermoelectric Reference Standard 72 4.6 Thermocouple Fixed-Point Check Standards 72

vi
Page

4.7 Thermometric Fixed Points 73 4.7.1 Metal Freezing-Point Cells 73 4.7.2 Water Triple-Point Cells 75 4.7.3 Ice Baths 75 4.8 References 76

5. ASSESSMENT OF UNCERTAINTIES 103

5.1 Uncertainties in Primary Calibrations of Type S Thermocouples 103 5.2 Uncertainties in Comparison Calibrations of Type S Thermocouples 110 5.3 References 117

6. FUTURE DIRECTIONS 122

7. BIBLIOGRAPHY 123

7.1 National Bureau of Standards Publications 123 7.2 American Society for Testing and Materials Publications. . 123 7.3 Instrument Society of America Publications 124

7.4 International Electrotechnical Commission Publications . . 124

7.5 Other Publications 124

8. APPENDICES 126

8.1 Calibration Reports 126 8.1.1 Type S Thermocouple - Fixed-Point Calibration 126 8.1.1.1 Covering Letter and Bound Calibration Report for Customer. . . . 126 8.1.1.2 Summary of Calibration Results for NIST Record 136 8.1.2 Type S Thermocouple - Comparison Calibration . . 148 8.1.2.1 Covering Letter and Bound Calibration Report for Customer. . . . 148 8.1.2.2 Summary of Calibration Results

for NIST Record 157 8.1.3 Type R Thermocouple - Comparison Calibration . . 164 8.1.3.1 Covering Letter and Bound

Calibration Report for Customer. . . . 164 8.1.3.2 Summary of Calibration Results

for NIST Record 173 8.1.4 Type T Thermocouple - Comparison with SPRT . . . 180 8.1.5 Base Metal Thermocouple - Comparison

Calibration 186

vii
LIST OF FIGURES

Figure Page

2.1. Schematic diagram of thermocouples with a junction box ... 21

2.2. Thermoelectric power of some low- temperature

thermocouples 21 2.3. Calibration emf's of three standard thermocouples

at the zinc point and at the gold point. Time at high temperature is about 5 h for each set 22

2.4. Temperature -emf relationships of some high- temperature thermocouples 22

2.5. Thermoelectric power of some high- temperature thermocouples 23

2.6. Emf (E) unaffected by third material, C 23

2.7. Emfs are additive for materials 24

2.8. Emfs are additive for temperature intervals 24

3.1. Schematic of automatic thermocouple calibration

system for comparison calibrations 57

3.2. Chromel tube furnace (50 to 1100 °C) 58

3.3. Silicon carbide tube furnace (50 to 1600. "C) 59

3.4. Oil bath 60

3.5. Oil bath and water bath 61

3.6. Tin bath 62

3.7. Tin bath 63

3.8. Cryostat 64

3.9. Cryostat 65 3.10. Metal freezing-point cell 66 3.11. Freezing-point furnace for gold and silver 67 3.12. Freezing-point furnace for antimony and zinc 68 4.1. Calibration history of saturated standard cells 77 viii
Figure Page

4.2. Calibration corrections for Guildline potentiometer 78 4.3. Calibration history of type S standard thermocouple

SC-68-2 at the gold point (1064.43 °C) 79 4.4. Calibration history of type S standard thermocouple

SC-68-2 at the silver point (961.93 °C) 80

4.5. Calibration history of type S standard thermocouple

SC-68-2 at 630.74 °C 81 4.6. Calibration history of type S standard thermocouple

SC-68-2 at the zinc point (419.58 °C) 82

4.7. Calibration history of type S standard thermocouple

SC-68-7 at the gold point (1064.43 °C) 83 4.8. Calibration history of type S standard thermocouple

SC-68-7 at the silver point (961.93 °C) 84

4.9. Calibration history of type S standard thermocouple

SC-68-7 at 630.74 °C 85 4.10. Calibration history of type S standard thermocouple

SC-68-7 at the zinc point (419.58 °C) 86 4.11. Calibration history of type S standard thermocouple

SC-71-5 at the gold point (1064.43 °C) 87 4.12. Calibration history of type S standard thermocouple

SC-71-5 at the silver point (961..93 °C) 88 4.13. Calibration history of type S standard thermocouple

SC-71-5 at 630.74 °C 89 4.14. Calibration history of type S standard thermocouple

SC-71-5 at the zinc point (419.58 °C) 90

4.15. Calibration history of type S standard thermocouple

SC-71-6 at the gold point (1064.43 °C) 91 4.16. Calibration history of type S standard thermocouple

SC-71-6 at the silver point (961.93 °C) 92 4.17. Calibration history of type S standard thermocouple

SC-71-6 at 630.74 °C 93 4.18. Calibration history of type S standard thermocouple

SC-71-6 at the zinc point (419.58 °C) 94

ix
Figure Page

4.19. Calibration history of type S standard thermocouple

SC-72-1 at the gold point (1064.43 °C) 95 4.20. Calibration history of type S standard thermocouple

SC-72-1 at the silver point (961.93 °C) 96 4.21. Calibration history of type S standard thermocouple

SC-72-1 at 630.74 °C 97 4.22. Calibration history of type S standard thermocouple

SC-72-1 at the zinc point (419.58 °C) 98 4.23. Calibration history of type S standard thermocouple

SC-72-2 at the gold point (1064.43 °C) 99 4.24. Calibration history of type S standard thermocouple

SC-72-2 at the silver point (961.93 °C) 100 4.25. Calibration history of type S standard thermocouple

SC-72-2 at 630.74 °C 101 4.26. Calibration history of type S standard thermocouple

SC-72-2 at the zinc point (419.58 °C) 102

5.1. Error per unit emf error at the fixed points

in thermocouple emfs calculated from

the Sb-Ag-Au calibration quadratic 118

5.2. Error per unit emf error at the fixed points

calculated from the 0 °C-Zn-Sb quadratic used

to represent the emf difference between the calibration values and the NBS Monograph 125 reference table values at the fixed-points 119

5.3. Emf difference between values of emf determined

in a comparison calibration using type S reference thermocouple SC-83-7 and emf values obtained in a primary calibration at the fixed points for 23 type S test thermocouples.

(Emf difference = primary - comparison) 120

5.4. Emf difference between values of emf determined

in a comparison calibration using type S reference

thermocouple SC-83-8 and emf values obtained in a primary calibration at the fixed points for 16 type S test thermocouples.

(Emf difference = primary - comparison) 121

X
LIST OF TABLES

Table Page

1.1. NIST thermocouple calibration capability 3 2.1. Compositions, trade names and letter designations

for standardized thermocouples 7 2.2. Suggested upper temperature limits for protected

thermocouples and various wire sizes 9 4.1. Freezing-point differences for gold and silver cells .... 74

4.2. Antimony cell freezing points 74

5.1. Standard deviations associated with fixed-point

cell calibrations 104

5.2. Uncertainty in the temperature of the liquidus points. . . . 108

5.3. Summary of systematic errors in primary calibrations

of type S thermocouples at the fixed points 109

5.4. Propagated uncertainties from total uncertainties at

the four fixed points Ill

5.5. Means and standard deviations for differences

between primary and comparison calibrations 113

5.6. Standard deviations for comparison calibrations 114

5.7. Total uncertainties in comparison calibrations

of type S thermocouples 116

xi

1. DESCRIPTION OF SERVICES Calibration services for all conunonly used types of thermocouples and thermocouple materials are provided by NIST from -196 to +2100 °C. The temperature range for a particular calibration depends on the type of wire or thermocouple submitted. The thermocouples are calibrated by one or more of

three general methods, depending on the type of thermocouple, the temperature

range, and the accuracy required. All three methods provide traceability to the IPTS-68. In the first method, thermocouples are calibrated by comparison

with a standard thermocouple maintained at NIST. In the second method,

thermocouples are calibrated against a standard platinum resistance

thermometer (SPRT) . In the third method, thermocouples are calibrated at 630.74 "C and at three defining temperatures on the International Practical

Temperature Scale (IPTS): the freezing points of Zn, Ag, and Au. Single-leg

thermoelements are tested against the NIST maintained platinum thermoelectric

reference standard, Pt-67, by the first or second method. Thermocouple and thermoelement calibrations below 0 °C are made in a cryostat;

those above 0 °C are made in stirred liquid baths, metal freezing-point cells,

or electric tube- type furnaces. Vacuum or inert-gas furnaces are also available if needed. Test data are processed on a laboratory computer and calibration tables giving

values of the thermocouple emf at 1 degree intervals are provided for B, S, R, and T types of thermocouples. An automatic data acquisition system is used

to record the test data for calibrations performed by the comparison method.

The temperature ranges and calibration uncertainties for standard letter- designated thermocouples calibrated by the three methods described above are given in table 1.1. While over 90% of the thermocouples calibrated at NIST

are letter-designated types, calibration services are also provided to temperatures as high as 2100 "C for non-standard types of thermocouples formed

from various W-Re, Ir-Rh, and Pt-Rh alloys. Calibrations for the non-standard

types are performed as special tests on an actual cost basis, and the temperature ranges and calibration uncertainties depend on the thermocouple

type.

Only bare wires are needed for NIST thermocouple calibrations. We would

prefer customers not to send ceramic insulating and protecting tubes since

they may get broken during shipment. If the thermocouple is shipped in a mount such as a protection tube assembly, a special dismantling fee may be charged and the parts will be returned unassembled. Leads and extension wires

need not be sent to NIST with the thermocouples.

All thermocouple calibration data furnished in reports are based on a reference junction temperature of 0 "C (32 °F) . The customer may request that calibration results be given either in degrees Celsius or Fahrenheit.

For base-metal thermocouples (types E, J, K, N, and T) and thermocouple

materials, only those that are unused will be accepted for test. A cali- bration will be undertaken only if the thermocouple is likely to yield the specified accuracy.

1
Information about the NIST thermocouple calibration services and the requirements for thermocouples to be submitted for special tests may be obtained by contacting:

Margaret G. Scroger or George W. Burns National Institute of Standards and Technology

Temperature and Pressure Division

Bldg. 221, Rm. B128 Route 270 and Quince Orchard Road

Gaithersburg, MD 20899 Telephone (301) 975-4818 or 975-4817 Inquiries about thermocouple calibrations performed by comparison with a standard platinum resistance thermometer should be directed to: Jacquelyn A. Wise

National Institute of Standards and Technology

Temperature and Pressure Division

Bldg. 221, Rm. A242 Route 270 and Quince Orchard Road

Gaithersburg, MD 20899 Telephone (301) 975-4822 Shipment of thermocouples for calibration should be made to one of the above

addresses. Shipping costs are paid by the customer. NIST assumes no respon- sibility for damage in shipment. A formal purchase order for the calibration

or test should be sent to NIST at or before the time the thermocouple material

is shipped. The purchase order should provide:

1. Clear identification of the types of thermocouple materials

submitted;

2. The calibration procedure to be followed;

3. The name and telephone number of the person responsible

for the procurement;

4. The name and telephone number of a technical contact

familiar with the calibration request; and

5. Instructions for return shipment including the shipping

and billing addresses and the customer's choice of carrier.

2
Table 1.1. NIST thermocouple calibration capability

Thermocouples tested Temperature Calibration

Range Method* Uncertainties***

Type Material ("C) (°C)

NOBLE-METAL

Pt - 10% Rh

vs

Pt

0 to 1450 0.2 at fixed pts. 0.3 (0 to 1100 °C)

2 at 1450 "C 0.5 (0 to 1100 "C)

2 at 1450 °C Pt - 13% Rh

vs

Pt

0 to 1450 0.5 (0 to 1100 ="C) 2 at 1450 =C

PtPt

30% Rh

vs

6% Rh

0 to 1750 0.5 (600 to 1100 °C)

2 at 1450 °C

3 at 1750 °C BASE-METAL

E Ni - 10% Cr vs Constantan

196 to 538

0 to 1000

0.1 to 0.2

1

Fevs Constantan196 to 538

0 to 760

0.1 to 0.2

1

Nl - 10% Cr vs

Ni - 5% (Al.Si)** Ni - 14% Cr -lh% Si vs

Ni - 44% Si - 1/10% Mg

Cu

vs Constantan

196 to 538

0 to 1100

196 to 538

0 to 1100

196 to 300

0 to 400

0.1 to 0.2

1

0.1 to 0.2

1

0.1 to 0.2

1

1 Comparison with a reference thermocouple

2 Comparison with a SPRT

3 Comparison with metal fixed points

** Silicon or aluminum and silicon may be present in combination with other elements

**The uncertainties quoted here are those presently stated in NIST reports

of calibration

3
2. PRINCIPLES OF THERMOELECTRIC THERMOMETRY

2 . 1 Thermoelectric Phenomena

A thermocouple consists of two dissimilar conductors or "thermoelements" joined to form a circuit. T. J. Seebeck [1]^ first discovered that a ther- mocouple would produce a current in a closed circuit when one junction is at a different temperature from the other. The electromotive force (emf) that produced the current is referred to as the thermocouple emf or as the Seebeck

voltage, E; its temperature derivative, dE/dT, is known as the thermoelectric

power or the Seebeck coefficient, S. One of the conductors, A, is said to be positive with respect to the other, B, if current would flow from A to B at

the cooler of the two junctions (fig. 2.1).

Peltier [2] discovered the reversible phenomenon, now known as the Peltier

effect. He found that a flow of electricity introduced into a thermocouple

circuit causes an exchange of heat between a junction and its surroundings

that can be reversed by reversing the direction of the current. The effect at one junction is independent of the temperature of the other junction or of the size of the wire, and is directly proportional to the current, I, i.e.,

q = Il^gl, where IIab i-S the Peltier coefficient. The mathematical relationships between these phenomena and thermodynamic temperature were developed by

W. Thomson (Lord Kelvin) by the application of the principles of thermodynamics that were being established at that time [3]. His first efforts clearly failed to describe the actual behavior of the Seebeck coefficient with

temperature, so that Thomson was led to postulate, and was subsequently able

to demonstrate, a third effect, also reversible, that became known as the Thomson heat. Thomson heat is evolved or absorbed reversibly when a current

flows in a single homogeneous conductor in a temperature gradient, in proportion to the size and direction of both the current and the gradient, i.e., q =

±al dT, where a is the Thomson coefficient. Thomson could then show from the first and second laws of thermodynamics that Il^^g = TS^^g , where T is thermodynamic temperature and S^g is the Seebeck coefficient, and that o^-a-^ = -T(d2EAB )/dT2 , where E^g is the Seebeck voltage

The absolute thermoelectric power of a single thermoelement is defined as

Of course, the emf of the thermocouple can only be measured as a combination

of a pair of thermoelements. Thus the thermoelectric power of a thermocouple

is

^Figures in brackets indicate the literature references given at the end

of each section.

4
dE .T a,

B.

- a

^AB A dT. dT T

The thermoelectric power of every thermocouple is zero at 0 K; therefore

The net thermocouple emf is a line integral around the path of the thermoelements from the positive to the negative terminal, i.e.,

If the thermoelements are physically and chemically homogeneous, so that the values of S are unique as a function of temperature, the emf of the thermo- couple is independent of the path and is unique. Otherwise, if the thermoele- ments are inhomogeneous , as is always the case to some extent , the emf observed depends upon the location and form of the temperature gradient, ie upon the path. The laws of thermoelectric circuits [4] are rules summarizing idealized

thermocouple behavior. (See sec. 2.6 for summary of laws.) The "second law" of thermocouple circuits is a consequence of the impossibility of perpetual

motion of the second kind, i.e., that work cannot be derived from a heat

engine operating in a system all at one temperature (and is the sine qua non

for thermocouple measurement) , namely that the Seebeck voltage of a circuit

comprising any number of materials at uniform temperature is zero. The "first

law" of thermocouple circuits is a special case derivable from the line integral for E^g , namely that heat applied to a circuit consisting of a single homogeneous conductor will not produce a thermal emf. The "third law" of thermocouple circuits is a consequence of the first law of thermodynamics,

namely that the thermal emf developed by a homogeneous thermocouple with its junctions at temperatures T^^ and T3 is equal to the sum of the emf's developed

by the same thermocouple with its junctions at Tj^ and T2 and subsequently at

T2 and T3 . The first and third "laws" assume homogeneity with emphasis on the junctions, when in fact a real thermocouple is never completely homogeneous

and the thermocouple emf is developed only in those portions of the thermoelements that pass through a temperature gradient.

It is an important application of the "second law" that the exact manner of making thermocouple junctions is not critical. For reliable measurements,

some portions of the thermocouple near and including the junctions should be

isothermal. Consequently, when the thermocouple is correctly installed, there

is no contribution to the Seebeck voltage from the junction region.

5
2 . 2 Choice of the Thermocouple Materials

Of the approximately 300 different types of temperature measuring

thermocouples that have been identified and studied [5], only a few types,

having the more favorable characteristics, are in general use. There are eight types of thermocouples that have been standardized, because they are the ones most commonly used industrially. In the United States each type is identified by a letter. This practice was originated by the Instrument

Society of America (ISA) and adopted in 1964 as an American Standard to eliminate the use of proprietary names. The standards of the American

National Standards Institute (ANSI-MC96 . 1 , 1982) and the American Society for Testing and Materials (ASTM 230-87) utilize the reference tables from National

Bureau of Standards Monograph 125 [6] as the basis for standardization. As noted in the ANSI and ASTM standards, the letter designations actually

identify the tables and may be applied to any thermocouple that has a temperature -emf relationship agreeing within the tolerances specified in the standards with that of the table, regardless of the composition of the thermocouple. Substantial variations in composition for a given letter type

do occur, particularly for types J, K, and E [6,7,8]. Nevertheless, the reference tables were determined for actual thermoelements, which, so far, are representative of the type capable of conforming to the requirements of the standards. The nominal compositions, representative trade names and corresponding letter designations of the standardized thermocouple materials

are given in table 2.1.

6
Table 2.1, Compositions, trade names and letter designations for standardized thermocouples

Type designation Materials

Thermocouple combinations

B platinum- 30% rhodium/platinum- 6% rhodium

E nickel -chromium alloy/a copper -nickel alloy

J iron/another slightly different copper -nickel alloy

K nicke

- chromium alloy/nickel - aluminum alloy

N nickel -chromium- silicon alloy

/

nickel -silicon alloy R platinum- 13% rhodium/platinum

S platinum- 10% rhodium/platinum

T copper/a copper -nickel alloy

Single- leg thermoelements

. . ,N Denotes the negative thermoelement of a given

thermocouple type

...P Denotes the positive thermoelement of a given

thermocouple type

BN platinum- nominal 6% rhodium

BP platinum-nominal 30% rhodium

EN or TN a copper -nickel alloy, constantan: Cupron^

Advance'' ; ThermoKanthal JN'' , nominally 55% Cu, 45% Ni

EP or KP a nickel -chromium alloy: Chrome I'* , Tophel^ , T-l° , ThermoKanthal KP^ ; nominally 90% Ni, 10% Cr JN a copper -nickel alloy similar to but usually not interchangeable with EN and TN

JP iron: ThermoKanthal JP^ ; nominally 99.5% Fe KN a nicke 1 - aluminum alloy: Alumel*^ , Nial^ , , ThermoKanthal KN*' ; nominally 95% Ni, 2% Al,

2% Mn, 1% Si NN a nickel -silicon alloy; nominally 95% Ni,

4-1/2% Si, 1/10% Mg

NP a nickel -chromium- silicon alloy; nominally 84% Ni, 14% Cr, 1-1/2% Si

RN, SN high-purity platinum

RP platinum- 13% rhodium

SP platinum- 10% rhodium

TP copper, usually Electrolytic Tough Pitch

Registered trade marks: ^Carpenter Technology Co.; ''Kanthal Corp.; ''Driver- Harris Co. ; "^Hoskins Manufacturing Co. Note: An underlined word indicates the primary constituent of an alloy and

all compositions are expressed in percentages by weight. All materials manu- factured in compliance with the established thermoelectric voltage standards

are equally acceptable.

7
The standardized thermocouples are available commercially with thermoelectric

properties that agree with the reference tables of temperature versus emf within specified tolerances. Tolerances differ in different countries. Those

of the United States can be found in the standards of the ANSI and the ASTM

previously cited. The same tables for thermocouples have been published by

the International Electrotechnical Commission as Publication 584-1 [9]. In addition, international tolerances for the standardized thermocouples have been formulated by the lEC Subcommittee 65B/WG5, and they appear as Part 2 of

the lEC Publication 584 [10]. The choice of thermocouple is strongly influenced by the temperature range of

the measurement. We shall consider the best possibilities for three temper- ature ranges, viz. from 0 K to 450 °C, from 450 "C to 1100 "C, and above

1100 °C. 2.2.1 Temperatures from 0 K to 450 °C All of the base-metal types (E, J, K, and T) have been used at cryogenic

temperatures, but, as shown in figure 2.2, their Seebeck coefficients become

too small below 20 K for accurate measurement.

It can be shown on theoretical grounds that the thermoelectric power of any thermocouple must approach zero as absolute zero is approached. Adequate

sensitivity in the very low range of temperatures has been attained by the decrease to zero to be compressed to a very narrow range of temperature near

absolute zero. Over almost all the range below 40 K, the KP versus Au-0.07 at

% Fe thermocouple has the highest sensitivity, and is the best choice of currently available thermocouples for very low temperatures. The thermal conductivity of the KP element is the lowest of any of the more frequently

used materials; unfortunately, that of the Au-0.07 at % Fe element is high

enough to be potentially troublesome. The Au-0.07 at % Fe thermoelement was developed to replace the Au-2.1 at % Co element, which proved to be unstable because of phase changes upon cycling between room temperature and low temperatures

Above 30 K the Seebeck coefficient of the type E thermocouple is large enough

for accurate results. It has been recommended by Hust et al. [11] as the most

suitable of the standardized types for general low- temperature use, since it offers the best combination of desirable properties: high thermoelectric

power, low thermal conductivity, and good thermoelectric homogeneity and

stability. The Seebeck coefficient of type E reaches larger than usual values

(>60 K'^ for t > 15 "C) . Hust et al. point out that it is superior to type K in having a lower response to magnetic fields, and is superior to type T because of the undesirably high thermal conductivity of the TP element. They

found every thermoelement to have significant inhomogeneity , but by carefully

selecting materials, inhomogeneity voltages of the type E over the range from

0 K to 300 K can be kept within 1-2 fjM under most conditions. Typical data

for the thermoelectric inhomogeneity of commercial type E thermocouple

materials at cryogenic temperatures, as well as of other standard base-metal

types, are given by Sparks et al. [7].

8
The upper temperature limits of base-metal thermocouples for extended use in air are given in table 2.2. These are determined by the oxidation resistance

of the thermoelements and, therefore, the limits depend upon the composition

and the wire size. The limits are set to allow 'satisfactory' thermocouple

life for industrial use, when the thermocouples are operated continuously at the limits of the indicated temperatures.

Table 2.2. Suggested upper temperature limits for protected thermocouples

and various wire sizes (Values and notes extracted from ASTM

Standard E230-87.)

Upper temperature limit (°C) for various wire sizes Thermocouple AWG 14 AWG 24 AWG 30 type (1.63 mm diameter) (0.51 mm diameter) (0.25 mm diameter)

T 370 200 150

J 590 370 320

E 650 430 370 K and N 1090 870 760 R and S 1480

B 1700 Note 1: This table gives the recommended upper temperature limits for the various thermocouples and wire sizes. These limits apply to protected

thermocouples, that is, thermocouples in conventional closed-end protecting

tubes. They do not apply to sheathed thermocouples having compacted mineral

oxide insulation. Note 2: The temperature limits given here are intended only as a guide to the user and should not be taken as absolute valties nor as guarantees of satisfactory life or performance. These types and sizes are sometimes used at temperatures above the given limits, but usually at the expense of stability

or life or both. In other instances, it may be necessary to reduce the above

limits in order to achieve adequate service. ASTM STP-470 and other literature sources should be consulted for additional applications information.

Reversible thermoelectric effects that occur in the KP or EP thermoelement on heating in the 250 to 550 °C range impose an upper bound of temperature for

the use of the type K or type E thermocouple for precision measurement.

Burley [12], Fenton [13], and Kollie et al. [14] have attributed this instability to short-range ordering in the Ni-Cr atomic lattice. Fenton

showed that annealing at temperatures of 200 to 450 °C will increase the effect of short-range ordering with a larger maximum effect at increasingly

lower temperatures as annealing progresses. The effect is reversible if the wire is heated subsequently to higher temperatures . As an added difficulty

with the type K thermocouple, the KN thermoelement has a magnetic

transformation at about 170 "C which causes the curve of the Seebeck

coefficient versus T for the type K thermocouple to be somewhat tortuous [6]

9
All base-metal thermoelements of the standardized thermocouples are affected

by oxidation. Dahl [15] investigated the change in eraf of 18 gauge (1.24 mm

diameter) wire for KP, KN, JP and JN elements when they were heated in air at 427 "C for times totaling 1000 h. The drifts of the KP and JN thermoelements

(which are equivalent to the thermoelements of the type E thermocouple)

compensate well enough that in combination the net emf is nearly constant.

Thus, when its properties are compared with other thermocouples, the type E thermocouple is the most suitable for general use from 20 or 30 K to 450 °C, if the EP thermoelement has been stabilized by annealing at 400 °C for

30 days. Alternatively, the type T thermocouple is stable and reproducible up

to a lower limit of temperature. It has a lower Seebeck coefficient and a much higher thermal conductivity of the positive leg than the type E thermocouple. Under vacuum or reducing conditions, the type T thermocouple is suitable for use up to a temperature of 500 "C, except in the presence of hydrogen, for which the range has to be restricted to 370 "C because of the tendency to embrittlement

2.2.2 Temperatures from 450 °C to 1100 °C In this temperature range, the noble metal types B, R, and S, or non-standard

thermocouples should be used. Thermocouples employing platinum and platinumrhodium alloys for their thermoelements are more resistant to oxidation, have

higher melting points and at elevated temperatures in air have generally been

found to be more reproducible than base-metal types. The type S thermocouple

is the oldest [16] and has served as a standard instrument of the International Temperature Scale since the adoption of the Scale in 1927 [17]. In the IPTS-68 Amended Edition of 1975 [18], type S thermocouples must meet

strict requirements for purity and thermocouple emf to qualify as standard in- struments for interpolation in the range from 630.74 °C to the gold point

(1064.43 °C). The emf of standard type S thermocouples depends upon their thermal histories

and, therefore, upon the annealing procedures for the thermoelements. Both

the original and amended editions of the 'IPTS-68 [18, 19] recommend that the platinum thermoelement be annealed at 1100 °C and that the platinum- 10% rhodium thermoelement be annealed at 1450 °C, with a final anneal at 1100 °C until stable after assembly. Following these recommendations, Jones [20] reported a measurement uncertainty of a single determination to be ±0.2 °C at

the 99% confidence level in the IPTS-68 defining range. McLaren and Murdock [21] showed that the type S thermocouple is precise for a given depth

of immersion, but that such thermocouples as ordinarily prepared are inhomogeneous. At the copper point (1084.5 "C) McLaren and Murdock [21] found that

the thermal emf versus temperature profiles of standard type S thermocouples

might vary by amounts approaching 1 "C, and that the shape of the temperature

profile as well as the value of the Seebeck voltages depended upon the previous heat treatment of the thermocouple. They investigated the effects of different high- temperature anneals, followed by 16 h "sheathed anneals" at 450 °C. At the NIST, it has been observed that type S thermocouples prepared

in this way are subject to rapid change at high temperatures, with a sub- stantial change of thermocouple emf.

10
The stability of type S thermocouples calibrated at the NIST is demonstrated

in figure 2.3. They were calibrated in gold-point, silver-point,

antimony-point , and zinc-point cells with nearly the same immersion and

temperature gradient each time. Each data point is part of a complete set, where the mean value of 10 measurements of the emf for two different freezes

was determined at each temperature. The total time at the temperatures of the silver point and the gold point is about 5 h for each calibration.

The NIST annealing procedure is to heat the thermocouple by electric heating

to about 1450 "C for 45 minutes and then allow it to cool quickly in air to -750 °C. It is held at that temperature for 30 minutes. Then the couple is allowed to cool to room temperature in air, in a time of ~2 minutes. After

assembly, all the thermocouple that will be at high temperature or in the gradient is annealed at 1100 °C in a furnace. It is cooled slowly, taking

about 2 h to fall to 300 'C. The Seebeck emf of the type S thermocouple

annealed in this way is more nearly constant at the gold point than found by McLaren and Murdock for thermocouples annealed by one of their best

procedures. If the thermocouple is subsequently annealed at 450 °C, the readings at the fixed point for a given immersion are higher than before. The effects of quenched- in point defects and of composition change of the SP thermoelement (because of preferential oxidation of the rhodium) have been

cited as causes for the diverse results obtained. A possible alternative to the type K or type S thermocouple is the type N (Nicrosil/Nisil) thermocouple. It was developed for use in an oxidizing

atmosphere to improve on the type K thermocouple [22] . Its composition has been chosen to minimize the effect of short-range ordering on the Seebeck

coefficient. Also, the silicon content of the thermoelements was increased so that an adherent, protective oxide coating could form on the surface to minimize further oxidation. Until the protective oxide coating is formed, the composition and the thermal emf of the wire continue to change at high tem- peratures. The inhomogene i ties in sheathed type N thermocouples for which the oxygen supply is restricted may be attributed to incomplete oxidation. When

stabilized by heating in air at 1000 "C for 100 h, the thermocouple has shown

a drift rate at the same high temperatures of only about 0.1 /xV h" ^ (where S =

36 /iV K' ^ ), which is higher than the equivalent temperature drift rate for a type S thermocouple but much lower than that for a type K. The Platinel couple, a proprietary thermocouple with thermoelements containing

gold, palladium and platinum, was developed for this range and up to 1300 °C [23]. It has a sensitivity of 40 K" ^ and is relatively stable in an oxidizing atmosphere. Good performance was reported for this thermocouple

in early investigations, but further testing is needed to determine its optimum use. 2.2.3 Temperatures above 1100 °C Even though the temperature for the upper limit of use of the types R and S thermocouples is quoted as high as 1480 "C in an oxidizing atmosphere (for 0.5 mm diameter wires), there are better platinum-rhodium alloy combinations

for thermometry under oxidizing conditions above 1100 °C. Bedford [24, 25] investigated the emf-temperature characteristics of Ft- 20% Rh/Pt-5% Rh (20/5)

11
and Pt-40% Rh/Pt-20% Rh (40/20) thermocouples and also observed their emf's at

the palladium point (1552 °C) as a function of heating time at 1700 °C. After

200 h at 1700 "C in air, the emf of the 20/5 thermocouple had decreased the equivalent of about 5 °C at the palladium point; after 500 h at 1700 °C in

air, the 40/20 thermocouples exhibited changes equivalent to 4 °C at the palladium point. It is expected that the behavior of the type B (30/6)

thermocouple will be at least as good as the 20/5 thermocouple. The type B thermocouple has better tensile properties at high temperatures than the 20/5

thermocouple and it also has the additional characteristic that from 0 to

50 °C its Seebeck emf varies only between -2.5 and 2.5 /xV, so that the temper- ature of the reference junction can often be neglected. Because the melting point of platinum- rhodium thermoelements increases with

increasing rhodium content, thermocouples comprised of platinum- rhodium

elements of higher rhodium content are relatively stable to higher limits of temperature. The 40/20 thermocouple was first proposed and evaluated by

Jewell et al . [26]. Its Pt-20% Rh thermoelement melts at about 1887 °C and it

is useful for accurate measurement up to 1850 °C. Its thermoelectric power in

the range from 1700 to 1850 °C is about 4.5 /xV K' ^ , which is less than half

that of the type B thermocouple. On the basis of the currently available but

somewhat limited information, the 40/20 thermocouple is superior to the type B in stability at 1700 °C. A choice of one over the other for measurements from

1500 to 1700 °C would have to be based on the total temperature range and duration of the measurement, the availability of the thermoelements, and the importance of the size of the Seebeck coefficient in terms of the user's measuring equipment. Detailed reference tables for the 40/20 thermocouple

have been prepared by Bedford [25] and limited quantities of both thermoelements are available commercially.

For measurements in an oxidizing atmosphere up to temperatures of 2150 to 2250 °C, iridium- rhodium alloy thermocouples have been used. Such thermo- couples are expensive and have rather poor mechanical properties, relatively

low thermoelectric power, poor stability and short life [27, 28, 29] are un- satisfactory for precise thermometry. Such resistance to oxidation as iridium

and iridium- rhodium alloys possess might better be used for a protecting

sheath for a W-3% Re/W-25% Re thermocouple. Three thermocouples with thermoelements of tungsten or tungsten- rhenium alloys

are commercially available for measuring very high temperatures. Positive thermoelements of tungsten, W-3% Re, or W-5% Re are used in combination with a negative tungsten- rhenium alloy thermoelement that nominally contains between

25% and 26% Re. The temperature -emf relationship of the W-3% Re/W-25% Re thermocouple is shown in figure 2.4 and the Seebeck coefficients in figure 2.5. These figures also include the relationships for the other non- standardized thermocouples discussed in this section.

The tungsten and tungsten- rhenium alloys have melting points in excess of

3000 °C [30]. The positive thermoelements currently sold in this country

contain residual impurity dopants to control their grain growth

characteristics. Even after full recrystallization, this control results in a marked improvement in the room- temperature ductility of the dilute

alloys [31]. The ductility of the negative thermoelement is satisfactory

12
after recrystallization, so doping is neither needed nor effective [32]. The tungsten- rhenium alloy thermocouples are suitable for use in hydrogen and in pure inert atmospheres. They may also be used in high vacuum; but, when they

are exposed for long periods above 1950 °C, substantial changes in their thermal emf can occur as a result of preferential evaporation of rhenium [31]. Environments containing hydrocarbons, oxygen, and oxygen-containing gasses

such as H2O, CO and CO2 will seriously degrade the thermocouples at elevated

temperatures. Burns and Hurst [31] have shown that suitably annealed bare thermoelements exhibit no significant change in their thermoelectric

properties with exposure for 1000 h to temperatures as high as 2125 °C in high-purity argon, helium, hydrogen and nitrogen.

For precise measurements with the tungsten- rhenium alloy thermocouples, the thermoelements must be annealed to stabilize their thermoelectric properties.

Burns and Hurst [31] reported that W-3% Re thermoelements should be annealed

at 2400 K for 1 h in argon, and W-25% Re for 2 minutes under the same conditions. Later studies by Burns and Hurst [33] indicate that in the commercially annealed W-25% Re thermoelement, precipitates of the sigma phase

significantly increase the emf after long-term exposure in the 800 to 1300 °C range

The formation of the sigma phase can be retarded, however, by employing the recommended annealing procedures of Burns and Hurst. For differential

measurements at high temperatures it is good practice to use the W-25% Re element as the leg remaining totally in the high- temperature region. The W-3% Re/W-25% Re thermocouple is suitable for use only in a reducing or neutral environment, or in a vacuum. When measurements must be made in an oxidizing environment, the tungsten- rhenium pair can be used if it can be protected by a suitable sheath.

2 . 3 Inhomogeneity

Inhomogeneity is a variation in chemical composition or the physical state of

a substance. Hust et al. [11] discuss variations in emf due to variation of composition of several thermoelements from several manufacturers. Their

investigations show that the variation in composition of the thermoelements

tends to increase as the source diverges: between locations on a spool,

between spools, between lots, and even more, between manufacturers. For industrial purposes, it is probably necessary to consider this total range of variation (amounting to 50 or 60 /xV between room temperature and 4 K) , but for precise measurements exactly the opposite approach should be taken. A

thermocouple should be derived from materials of as high quality as is available, as closely related in position on the spool as possible, and the supply of thermocouple materials should be from one tested lot in a quantity

adequate for the entire job.

Thermocouple emfs are also affected by strain produced from the drawing of

the wire and from handling. Pollock and Finch [34] showed that the change of emf produced by drawing can be large. Even though thermocouple wire is ordinarily given a heat treatment in the final stage of its production, it must be carefully annealed under conditions appropriate for the particular

13
materials and then tested for homogeneity. This can be done separately for each element by observing any thermal emfs developed in a loop when short

lengths of the loop are subjected to a steep temperature gradient. More elaborate tests of the complete thermocouple are discussed by Hust

et al. [11], who observed the variation of emf as the wires were immersed in cryogenic fluids, by Kollie et al . [14], who recorded the emf while lowering

the thermocouple into a hot- liquid thermostat at a programmed rate, and by

Fenton [13], who instrumented a device that measured the emf while generating

a traveling gradient along a loop of wire. A method using a very short winding in a furnace is discussed by Ergardt [35]. The effect of work-hardening from the installation and handling of thermocouples depends upon the material, the initial condition of the wire and the severity of the deformation. Rosenbaum [36] studied the effects of cold working on Au-0.07 at % Fe thermoelements used for very low temperatures. For normal handling (defined as gently winding the wire on 1.27 cm diameter spools

and then carefully unwinding it) no change in emf was observed in the temperature range from 1 to 20 K. Fenton [13] pointed out that cold working

can change the Seebeck coefficient of the KP thermoelement by several percent

at temperatures below 400 °C, and that the effect of the cold-working, and

other, often larger, effects are removed by annealing at 450 °C. Corruccini [37] showed that forming a platinum coil of large radius (12.5 mm) followed by straightening produced very little effect, but that forming a coil

of smaller radius (2.5 mm) followed by straightening changed the emf by 3 /iV at 400 °C and by 2.5 fiV at 1200 °C. (The emf at the latter temperature should

be affected by annealing except for the colder parts of the wire.)

Burns and Hurst [38] investigated the effects of cold-working on the W-3% Re and W-25% Re thermoelements. The entire wire was coiled about a 20 mm

mandrel, straightened, then coiled in the reverse direction and again

straightened. Their tests demonstrated that annealed tungsten- rhenium

thermoelements are sufficiently ductile to be handled, but not without a significant effect on the Seebeck voltage. All tungsten- rhenium thermocouples

should be annealed after danger of deformation from handling is past.

2.4 Calibration

For accurate temperature measurements by thermocouple thermometry, the thermocouples must be calibrated. Calibration consists of measuring the thermocouple emf at a series of approximately uniformly spaced temperatures,

as established by standard instruments of the IPTS-68 or fixed points. In order to interpolate between calibration points, the coefficients of a polynomial equation are determined to express the difference of the emfs from

an accepted temperature versus emf table (e.g., from NBS Monograph 125 [6], or lEC publication 584-1 [9,10]) if they are standardized thermocouples. Over

the temperature range from 13.81 K to 630.74 °C this requires comparisons with

the values derived from platinum resistance thermometers. The best procedure

for calibration depends on the application. Most inhomogeneities are relatively stable so that the calibration is relatively stable over reasonable

periods of time. If an inhomogeneous thermocouple can be calibrated in place,

its peculiarities are accounted for, to the extent that the temperature

gradient and the inhomogeneity remain constant. When a recalibration can be

14
made in situ to account for "homogeneity changes", the difference in emf can

then be interpolated for prior measurements. Between 630.74 and 1064.43 "C, the Pt-10% Rh/Pt thermocouple (type S) is the standard instrument of the IPTS-68 and must be calibrated at the temperatures

of 630.74 °C (as determined by a platinum resistance thermometer), and the silver and gold fixed points. As there is no more accurate instrument in general use to calibrate the thermocouple against, the usual practice is to reserve one or more of the calibrated Pt-10% Rh/Pt thermocouples as standards

for checking the working instruments. High- temperature platinum resistance

thermometers, now achieving increased acceptance, are, in fact, much more precise and stable, and are very effective for improved calibration in this range

Above the temperature of the gold point, international practical temperatures

are defined by Planck's law, and usually realized with spectral pyrometers. Accurate thermocouple calibrations are much more difficult to achieve in this temperature region, so that the errors of temperature measurement grow as the temperature increases. Calibration equipment for this region of temperature

requires the construction of one or more blackbodies.

2 . 5 Installation

Thermocouples must be installed in the apparatus so that they are physically

(and if need be, chemically) protected, with adequate immersion, and with the reference junctions at a uniform and known temperature. Immersion is adequate

when the heat transfer to the thermocouple is such that the measuring junction

is brought within a chosen limit of the temperature it is desired to measure. On the other hand, adequate immersion must be achieved by means that do not invalidate the measurement sought. It is common that the design of the experiment is substantially affected by balancing the effects of these requirements

In many installations, thermocouples do not need to be sheathed, and for calibration purposes some portions or all of the thermocouple may need to be removable. They may either be inserted into wells open from the outside, or installed in an enclosed system with insulators. The choice of insulators,

which must provide electrical isolation but should not contaminate the thermocouples or the system, is an important consideration if reliable

performance is to be obtained. Factors affecting the choice of insulators

include types of thermoelements, and the temperature range and environment of

use. Suitable choices of insultors, as well as protection tubes, are discussed by Zysk and Robertson [29] and in ASTM Special Publcation 470B. To avoid any chemical action, thermocouples must not be contaminated from handling during installation, nor from the system. For instance, if the junction is made by soldering or welding, any flux must be thoroughly

eliminated.

The problem of inadequate immersion is discussed by Ginnings and West [39] . They distinguish between "continuous tempering", and "step tempering".

Continuous tempering is the process of bringing the thermocouple wires to the temperature of the object to be measured by utilizing the heat flow between

15
the thermocouple and its surroundings, whether through solid or gas or vacuum.

Step tempering is accomplished by "thermal tiedowns", i.e., thermal shunts to an object of relatively large heat capacity.

2 . 6 Summary of the Laws of Thermoelectric Circuits^ 2.6.1 Law of Homogeneous Metals

A thermoelectric current cannot be sustained in a circuit of a single

homogeneous material, however varying in cross section, by the application of heat alone. A consequence of this law is that two different materials are required for any thermocouple circuit. Experiments have been reported suggesting that a nonsyimnetrical temperature gradient in a homogenous wire gives rise to a measurable thermoelectric emf. A preponderance of evidence indicates,

however, that any emf observed in such a circuit arises from the effects of

local inhomogeneities . Furthermore, any current detected in such a circuit

when the wire is heated in any way whatever is taken as evidence that the wire

is inhomogeneous

2.6.2 Law of Intermediate Metals

The algebraic sum of the thermoelectromotive forces in a circuit composed of any number of dissimilar materials is zero if all of the circuit is at a uniform temperature. A consequence of this law is that a third homogeneous material always can be added in a circuit with no effect on the net emf of the circuit so long as its extremities are at the same temperature. Therefore, it is evident that a device for measuring the thermoelectromotive force may be introduced into a circuit at any point without affecting the resultant emf, provided all of the junctions which are added to the circuit by introducing the device are all at

the same temperature. It also follows that any junction whose temperature is uniform and which makes a good electrical contact does not affect the emf of

the thermoelectric circuit regardless of the method employed in forming the junction. (See fig. 2.6.)

Another consequence of this law may be stated as follows. If the thermal emfs of any two metals with respect to a reference metal (such as C) are known, then the emf of the combination of the two metals is the algebraic sum of their emfs against the reference metal. (See fig. 2.7.)

^The information in this section is extracted from ASTM Special

Publication 470B Manual on the Use of Thermocouples in Temperature

Measurements. American Society for Testing and Materials, 1916 Race Street,

Philadelphia, PA 19103 (1981). This ASTM publication gives a comprehensive

discussion of the use of thermocouples for industrial thermometry.

16
2.6.3 Lav of Successive or Intermediate Temperatures

If two dissimilar homogeneous metals produce a thermal emf of Ej^ , when the junctions are at temperatures and , and a thermal emf of £3 , when the junctions are at T2 and T3 , the emf generated when the junctions are at Tj^ and

T3 , will be El + E2 . One consequence of this law permits a thermocouple, calibrated for a given

reference temperature, to be used with any other reference temperature through

the use of a suitable correction. (See fig. 2.8 for a schematic example.)

Another consequence of this law is that extension wires, having the same thermoelectric characteristics as those of the thermocouple wires, can be introduced in the thermocouple circuit (say from region T2 to T3 in fig. 2.8)

without affecting the net emf of the thermocouple.

17
2.7 References

[I] Seebeck, T. J., Abh. Akad. Wiss., Berlin 265 (1822).

[2] Peltier, J. C. A., Ann. Chim. Phys . 56 (2nd series) 371 (1834).

[3] Thomson, W. , Trans. R. Soc, Edinburgh 21, 123-171, (1854).

[4] Roeser, W. F. , in Temperature: Its Measurement and Control in Science

and Industry, pp. 180-205, Volume 1 (New York: American Institute of Physics) (1941).

[5] Kinzie, P. A., Thermocouple Temperature Measurement (New York: John

Wiley) (1973).

[6] Powell, R. L. , Hall, W. J., Hyink, C. H. Jr., Sparks, L. L. , Burns, G. W. , Scroger, M, G. , Plumb, H. H. , Natl. Bur. Stand. (U.S.) Monogr. 125,

(1974)

[7] Sparks, L. L. , Powell, R. L. , Hall, W. J., Natl. Bur. Stand. (U.S.),

Monogr. 124 (1972).

[8] Burley, N. A., in Plumb, H. H. (Ed.), Temperature: Its Measurement and

Control in Science and Industry, pp. 1677-1695, Volume 4 (Pittsburgh: Instrument Society of America) (1972).

[9] lEC, Thermocouples Part 1, International Electrotechnical Commission,

Publication 584-1 (1977).

[10] lEC, Thermocouples Part 2, International Electrotechnical Commission,

Publication 584-2 (1982).

[II] Hust, J. G., Powell, R. J., Sparks, L. L. , in Plumb, H. H. (Ed.),

Temperature: Its Measurement and Control in Science and Industry,

pp. 1525-1535, Volume 4 (Pittsburgh: Instrument Society of America)

(1972)

[12] Burley, N. A., Australian Defense Scientific Service, Defense Standards

Laboratories, Report 353 (1970).

[13] Fenton, A. W. , in Plumb, H. H. , Temperature: Its Measurement and

Control in Science and Industry, pp. 1973-1990, Volume 4 (Pittsburgh: Instrument Society of America) (1972).

[14] Kollie, T. G. Horton, J. L. Carr, K. R. Herskovitz, M. B. , Mossman,

C. S., Rev. Sci. Instrum. 46, 1447-1461 (1975).

[15] Dahl, A. I., in Temperature: Its Measurement and Control in Science

and Industry, pp. 1238-1266, Volume 1 (New York: American Institute of

Physics) (1941).

[16] Le Chatelier, C. R. , Acad. Sci. 102, 819 (1886).

18
[17] Burgess, G. K. , J. Res. Natl. Bur. Stand. 11, 635-640 (1928).

[18] The International Practical Temperature Scale of 1968 Amended Edition

of 1975, Metrologia 12, No. 1, 7-17 (1976).

[19] Barber, C. R. , Metrologia 5, 35-44 (1969).

[20] Jones, T. P., Metrologia 4, 80-83 (1968).

[21] McLaren, E. H. , Murdock, E. G. , in Plumb, H. H. (Ed.), Temperature:

Its Measurement and Control in Science and Industry, pp. 1543-1560,

Volume 4 (Pittsburgh: Instrument Society of America) (1972).

[22] Burley, N. A., Powell, R. L. , Burns, G. W. , Scroger, M. G., Natl. Bur. Stand. (U.S.) Monogr. 161 (1978).

[23] Accinno, D. J., Schneider, J. F. , in Herzfeld, C. M. (Ed.),

Temperature: Its Measurement and Control in Science and Industry, Part

1, pp. 195-200, Volume 3 (New York: Reinhold) (1962).

[24] Bedford, R. E. , Rev. Sci. Instrum. 35, 1177 (1964).

[25] Bedford, R. E. , Rev. Sci. Instrum. 36, 1571-1580 (1965).

[26] Jewell. R. C. , Knowles, E. G., Land, T. , Met. Ind. (London) 87, 217

(1955).

[27] Freeze, P. D. , Thomas, D. B. , NASA CR-135055 or Natl. Bur. Stand.

(U.S.) IR 76-1026, 55 pp. (1976).

[28] Aleksaklin, I. A., Lepin, I. R. , Bregin, B. K. , Issledovaniya Splavov

dlya Termopar 22 143-158, Weight Technology Div. , Wright-Patterson Air

Force Base, Ohio AD-663573 (translation) (1967).

[29] Zysk, E. D. , Robertson, A. R. , in Plumb, H. H. (Ed.), Temperature: Its Measurement and Control in Science and Industry, pp. 697-1734, Volume 4 (Pittsburgh: Instrument Society of America) (1972).

[30] Caldwell, F. R. , in Herzfield, C. M. (Ed.), Temperature: Its Measurement and Control in Science and Industry, Part 1, pp. 81-134,

Volume 3 (New York: Reinhold) (1962).

[31] Burns, G. W. , Hurst, W. S., in Plumb, H. H. (Ed.), Temperature: Its Measurement and Control in Science and Industry, pp. 1751-1766, Volume

4 (Pittsburgh: Instrument Society of America) (1972).

[32] Pugh, J. W., Amra, L. H. , Hurd, D. T. , Trans. Am. Soc. Met. 55, 451-461 (1962).

19
[33] Burns, G. W. , Hurst, W. S., in Billing, B. F, , Quinn, T. J. (Eds.),

Temperature Measurement 1975 Institute of Physics Conference Series,

pp. 144-159 Volume 26 (London and Bristol: Institute of Physics)

(1975).

[34] Pollock, D. D., Finch, D. I., in Herzfield, C. M. (Ed.), Temperature:

Its Measurement and Control in Science and Industry, Part 2, pp. 237-241, Volume 3 (New York: Reinhold) (1962).

[35] Ergardt, N. N. , Tr. Vses. Nauchno-Issled. Inst. Metrol. 51(111) 89-96

(1961).

[36] Rosenbaum, R. L. , Rev. Sci. Instrum. 39, 890-899 (1968).

[37] Corruccini, R. J., J. Res. Natl. Bur. Stand. 47, 94-103 (1951).

[38] Burns, G. W. , Hurst, W. S., NASA Contract, C-61545-B, Report No. 3, (1970)

[39] Ginnings, D. C. , West, E. D. , Experimental Thermodynamics, Volume 1, (New York: Plenum Press) pp. 113-115 (1968).

20
Figure 2.1. Schematic diagram of thermocouples with a junction box.

Figure 2.2. Thermoelectric power of some low- temperature

Thermocouples

21
10320 _ 10318 I 10316

10314 > 10312

E 10310 3446 i 3444

,5 3442 3440

• • •(SC-68-2)

• • .(SC-71-6)

• -(5064-8)

. I O-I 'C

3 4 5 6 Years

Figure 2.3, Calibration emf's of three standard thermocouples

at the zinc point and at the gold point. Time

at high temperature is about 5 h for each set.

50

40 ? 30

0

/ Nicrosil versus Nisii

W-3%Re versus W- 25 7c Re

type S ^^^-"""^^ type B ^^^^-''''''^ Pt-4(>rcRh versus Pt -2D7rRh

800 1200 1600 2000

Temperature ("O

2400

Figure 2.4. Temperature -emf relationships of some hightemperature thermocouples

22
Nicrosil versus N'isil

lypc S

Pi--iO^;Rh versus Pl-20^•Rh

i:ui.i 160u 200U

TcnirKTaiurc ("O

Figure 2.5. Thermoelectric power of some high- temperature thermocouples

Figure 2.6. Emf(E) unaffected by third material, C.

23
Figure 2.7. Emfs are additive for materials.

Figure 2.8. Emfs are additive for temperature intervals.

24
3. CALIBRATION METHODS AND PROCEDURES FOR THERMOCOUPLES AND THERMOCOUPLE MATERIALS

As indicated in section 1.0, the calibrations of thermocouples and of singleleg thermoelements versus the platinum thermoelectric standard, Pt-67, are conducted by one or more of the following general methods:

a) by comparison with a calibrated reference thermocouple in an electric tube- type furnace;

b) by comparison with a standard platinum resistance thermometer in a cryostat or in a stirred liquid bath; or

c) at certain thermometric fixed points of the IPTS-68 as realized in metal freezing-point cells. The equipment and procedures employed for each of the above three calibration

methods are described in this section.

3 . 1 Calibration by Comparison with a Reference Thermocouple

3.1.1 Type S. Type R. and Type B Thermocouples

Type S, type R, and type B thermocouples may be calibrated by comparison with

reference thermocouples. The reference thermocouples used consist of a set of type S thermocouples and a set of type B thermocouples. The type S reference

thermocouples are calibrated at fixed points and are then checked periodically

for changes in calibration by comparison with other type S thermocouples that have been calibrated by fixed points. These type S reference thermocouples

are not used above 1100 °C. The type B reference thermocouples are calibrated

by comparison with type S reference thermocouples to 1100 °C and by comparison

with type B thermocouples whose calibration was determined by comparison with

an optical pyrometer using a high temperature furnace, containing a black body

enclosure, to heat the thermocouples. The type B reference thermocouples are used in the range 800 to 1600 °C. 3.1.1.1 Preliminary Examination A thermocouple submitted for calibration is carefully removed from the package

in which it was shipped. The packing material and shipping container are saved and used to return the instrument to the customer. The thermocouple is examined visually and its length, diameter, and condition (new, used, etc.)

are recorded in the laboratory notebook.

3.1.1.2 Electrical Anneal

Before undertaking a calibration of a type S, type R, or type B thermocouple,

the thermocouple wire is annealed electrically in air. The thermocouple is suspended from two spring- loaded copper clips in the electrical annealing

system (see sec. 3.1.1.8) and cleaned by wiping with an ethanol-saturated

tissue. The wire is then heated by passing a 60 Hz alternating current

through it. The annealing current is regulated with an adjustable

25
transformer. The annealing procedure is to heat the wire at about 1450 °C for

45 minutes and then allow it to cool quickly to about 750 °C. It is held at

750 °C for 30 minutes and then cooled to room temperature in a time of a few

minutes. The temperature of the wire is determined with an optical pyrometer

(Leeds and Northrup Co. Model 8622-0). It is necessary to add a correction to the observed apparent temperature to obtain the true temperature. For a true temperature of 1450 °C, the correction (based on an emissivity of 0.33) amounts to about 140 °C; hence, the optical pyrometer scale is set to 1310 °C. The electrical current required to bring the wire to 1450 °C depends upon the diameter of the wire and the type of wire. For 0.51 mm diameter thermoelements of type B, S, and R thermocouples, a current of roughly 12 to 12.5 amps is required for annealing at 1450 °C. Type S, type R, and type B thermocouples are all electrically annealed in the same manner. 3.1.1.3 Mounting for Calibration

The annealed thermocouples are inserted carefully into high purity alumina

insulating tubes (2 or 4 bore) . The insulating tubes used for type B ther- mocouples are kept separate from those used for type S and type R. A common junction is formed by welding together the measuring junctions of the test thermocouples and a calibrated reference thermocouple. A type S reference

thermocouple is used in the range 0 to 1100 °C, and a type B reference

thermocouple is used when testing type B thermocouples in the range 800 to 1600 °C. The number of thermocouples in a run is limited primarily by the inner diameter of the alumina protection tube used in the calibration furnace. Usually four to six thermocouples are tested in each calibration run. Copper

connecting wires (0.4 mm diameter) are joined electrically to the thermocouple

wires to form the reference junctions, and the reference junctions are maintained at 0 °C in an ice bath (see sec. 3.1.1.8). In this manner, the test thermocouples are connected, through a switch scanner, to one measuring

instrument, and the reference thermocouple to another measuring instrument so that measurements of the test and reference thermocouples may be made

s imul taneous ly

3.1.1.4 Furnace Anneal

TYPE S. TYPE R. AND LOW TEMPERATURE RANGE FOR TYPE B . Before the calibration

is begun, the test thermocouple assembly is annealed in the calibration

furnace. The assembly is inserted into the Chromel tube furnace (see fig. 3.2 and sec. 3.1.1.8) so that the common measuring junction is located about

30 cm from the end of the Chromel heating tube. The thermocouple assembly is protected by a closed-end alumina tube during the anneal. The furnace is heated in two steps to a temperature between 1060 and 1100 °C and held at that temperature for 30 minutes. This anneal is used to remove small mechanical

strains that may have been introduced into the wire during the mounting

operation. HIGH TEMPERATURE RANGE FOR TYPE B . Type B thermocouples in the high temperature range (800 to 1600 °C) are calibrated in the silicon carbide horizontal

tube furnace (see fig. 3.3 and sec. 3.1.1.8). Before the calibration is begun, the test thermocouple assembly is inserted in the furnace to an immersion of 30 cm from the end of the silicon carbide heating tube and then

26
annealed. The thermocouple assembly is protected by a closed-end alumina tube during the anneal. The test thermocouple assembly is heated slowly (over a period of 2 to 4 h) to about 1100 °C and held at that temperature for

30 minutes. The temperature is then reduced to about 800 °C to begin the calibration run. 3.1.1.5 Calibration Data

Data for a particular test thermocouple are obtained by measuring the emf of

the reference thermocouple and the emf of the test thermocouple

simultaneously. From these data, values for the temperature of the common measuring junction and the emf of the test thermocouple are determined.

Calibration data are obtained starting at about 1100 °C and decreasing to about 100 °C, except for the high temperature range of a type B thermocouple,

which is calibrated starting at 800 °C and increasing to 1550 or 1600 °C. Data

for automatic and semi-automatic calibrations are taken at approximately 30 °C intervals. The values of emf are measured and recorded with an automatic

digital data acquisition system. This system includes a microcomputer (IBM enhanced AT) , two precision digital voltmeters (Hewlett Packard 3456A) , two switch scanners (Fluke 2205A) , a printer (IBM Proprinter XL24) , and a color monitor) . A block schematic of the automatic calibration system is shown in figure 3.1, and the individual components are described in section 3.1.1.8. Automatic calibrations are performed with type S and type R thermocouples, and with type B thermocouples in the low temperature range. Data for the high

temperature range for type B thermocouples are normally obtained in a semi-automatic mode, in that, readings are taken at the operator's command. Before beginning the calibration, a qualitative check of the thermoelectric

homogeneity of all type S and type R thermocouples is carried out by comparing

them with the reference thermocouple at different immersions in the calibration furnace. The common measuring junction is held at about 1100 °C and the thermocouple assembly is moved to produce different temperature

gradients along the thermocouples. Test data are taken with the measuring

junction at normal immersion into the furnace (30 cm) and then at +5 . 1 , -3.8, and -7.6 cm from normal immersion. This qualitative check will indicate an inhomogeneous thermocouple only if the inhomogeneity in it happens to be located in the temperature gradient zone of our calibration furnace. If gross differences (5 microvolts or more) are found in the emf readings over the 12.7 cm change in immersion, the test thermocouple is checked again for inhomogeneity. The second check is usually made against a different reference

thermocouple, and the test thermocouple is annealed in the furnace and may

also be reannealed electrically. If the thermocouple still exhibits gross inhomogeneities , it is rejected for calibration and the results of the test

are reported to the customer. 3.1.1.6 Packing and Shipping

After the calibration is completed and the furnace has cooled to ambient

temperature, the test thermocouple assembly is removed from the furnace and

its common junction is cut off. A new measuring junction is then formed on each thermocouple by welding with the oxygen-gas torch. The thermocouples are then packed for shipment. Each of the thermocouples is coiled on a glass

27
beaker (5 cm in diameter), placed in a small envelope, and packed in the container in which it was received. A thermocouple received on a plastic spool

or in plastic tubing is returned to the customer on the same spool or in the same tubing. If the container was damaged in shipment or came with insuf- ficient packing material, the NIST Shipping and Receiving Unit provides a suitable shipping container.

3.1.1.7 Calibration Report

The calibration data are processed on a laboratory computer (see sec. 3.1.1.8.) to obtain temperature versus emf tables at 1 degree intervals, with

the emf given in millivolts to the nearest microvolt. The tables may have temperature in either °C or °F and on either the IPTS-68 or IPTS-48. The computer programs used for processing the calibration data and for calculating

and printing the tables for the type S, type R, and type B thermocouples are discussed in this section. TEMPERATURE VERSUS EMF TABLES. TYPE S . The data from an automatic calibration

of a type S thermocouple are processed using a laboratory computer to obtain a temperature versus emf table for the test thermocouple. The computer program

requires that the coefficients giving the temperature -emf relationship of the type S reference thermocouple used in the calibration be on file. The processing is divided into three temperature ranges: 0 to 630.74 °C, 630.74

to 1064.43 °C, and 1064.43 to 1450 °C. The computer calculates the temperature, tgg, for each data point from the measured emf of the type S reference thermocouple and also calculates the difference, De , between the emf readings of the test thermocouple and the reference thermocouple at that temperature. In the 0 to 630.74 °C range the corresponding values of De and tgg are fitted with a fourth degree polynomial

by the method of least squares. The polynomial is constrained to give 0 emf

at 0 °C. In the 630.74 to 1064.43 °C range, the polynomial from the lower temperature

range is used to calculate the emf difference, Dsb , at 630.74 °C. This value

is then subtracted from the values of De , and 630.74 is subtracted from the corresponding values of calculated temperature, tgg. These data are then fitted with a second degree polynomial by the method of least squares, symbolically De - Dsb = f (tgg -630. 74) . The polynomial is constrained so that the value of De - Dsb equals 0 at 630.74 °C. This polynomial is then rearranged

algebraically to yield De as a function of tgg. The polynomial for the 630.64 to 1064.43 °C range is then extrapolated

linearly above 1064.43 °C to obtain the functional relationship between De and

tgg . The slope- intercept form of a straight line is used to represent De as a linear function of tgg. The slope is determined by calculating the first

derivative with respect to tgg at 1064.43 °C of the polynomial obtained for

the 630.74 to 1064.43 °C range. The polynomials representing De = f(t) in the above three temperature ranges

are then combined algebraically with the functions that give the temperatureemf relationship of the type S reference thermocouple to obtain the

28
polynomials representing the temperature -emf relationship for the test thermocouple

TEMPERATURE VERSUS EMF TABLES. TYPE R . The data from an automatic calibration

of a type R thermocouple are processed using a laboratory computer to obtain

a temperature versus emf table for the test thermocouple. The computer program requires that the coefficients giving the temperature-emf relationship

of the type S reference thermocouple used in the calibration be on file. The processing is divided into three temperature ranges: 0 to 630.74 °C, 630.74

to 1064.43 "C, and 1064.43 to 1450 °C. The computer first calculates the temperature, tgg, for each data point from

the measured emf of the type S reference thermocouple. The coefficients for

the type R emf -temperature table given in NBS Monograph 125 [1] are used to calculate the corresponding value of the emf at each temperature. The dif- ference, De , between the measured emf value of the test thermocouple and the emf value from the reference table is then computed. In the 0 to 630.74 °C range the corresponding values of De and tgg are fitted with a fourth degree

polynomial by the method of least squares. The polynomial is constrained to give 0 emf at 0 °C. In the 630.74 to 1064.43 °C range, the polynomial from the lower temperature

range is used to calculate the emf difference, Dsb , at 630.74 °C. This value

is then subtracted from the values of De , and 630.74 is subtracted from the corresponding values of calculated temperature, tgg. These data are then fitted with a second degree polynomial by the method of least squares, symbolically De-Dsb = f (tgg -630 . 74) . The polynomial is constrained so that the value of De-Dsb equals 0 at 630.74 °C. This polynomial is then rearranged

algebraically to yield De as a function of tgg. The polynomial for the 630.74 to 1064.43 °C range is then extrapolated

linearly above 1064.43 °C to obtain the functional relationship between De and

tgg. The slope- intercept form of a straight line is used to represent De as a linear function of tgg . The slope is determined by calculating the first

derivative with respect to tgg at 1064.43 °C of the polynomial obtained for

the 630.74 to 1064.43 °C range. The polynomials representing De = f(t) in the above three temperature ranges

are then combined algebraically with the functions that give the temperatureemf relationship of the type R thermocouple reference table given in Monograph

125 to obtain polynomials representing the temperature-emf relationship for

the test thermocouple. TEMPERATURE VERSUS EMF TABLES. TYPE B . The data from an automatic calibration

of a type B thermocouple are processed using a laboratory computer to obtain a temperature-emf table for the test thermocouple. Three computer programs are used for this purpose depending on the temperature range: 1) 800 to 1750 °C,

2) 0 to 1100 °C, or 3) 0 to 1750 °C. Each of the programs requires that the coefficients giving the temperature-emf relationship of the reference

thermocouples (type S, or Type B, or both) used in the calibration be on file.

29
For a calibration in the range 800 to 1750 °C, where the temperature is measured with a type B reference thermocouple, the computer calculates the temperature, tgg, from the measured emf of the reference thermocouple and also calculates the difference, De , between the emf readings of the test ther- mocouple and the reference thermocouple at that temperature. The corresponding values of De and tgg are fitted with a quadratic equation by the method of least squares. This quadratic equation is then combined

algebraically with the functions that give the temperature -emf relationship of

the type B reference thermocouple to obtain the polynomials representing the temperature -emf relationship of the test thermocouple.

For a calibration in the range 0 to 1100 °C, where the temperature is measured

with a type S reference thermocouple, the computer calculates the temperature,

tgg, for each data point from the emf of the type S reference thermocouple.

The coefficients for the type B emf -temperature table given in NBS Monograph

125 are used to calculate a corresponding value of the emf at each

temperature. The difference, De , between the measured emf value of the test thermocouple and the emf value from the reference table is then computed. The corresponding values of De and tgg are fitted with a third degree polynomial

by the method of least squares. The polynomial is constrained to give 0 emf

at 0 °C. The third degree polynomial is then combined algebraically with the function that gives the temperature -emf relationship of the type B thermocouple reference table given in NBS Monograph 125 to obtain a polynomial

representing the emf -temperature relationship of the test thermocouple.

For a calibration over the whole range 0 to 1750 °C, the data are processed in two steps. First the calibration data obtained in the 0 to 1100 °C range, where the temperature is measured with a type S reference thermocouple, are processed in the manner described above to obtain a polynomial representing

the emf -temperature relationship of the test thermocouple in the range 0 to 1064.43 °C. In the 1064.43 °C to 1750 °C range, the polynomial from the lower temperature range is used to calculate the emf of the test thermocouple at 1064.43 °C and the difference, Dau, between the calculated emf of the test thermocouple and the emf of the type B reference thermocouple at 1064.43 °C is determined. This value, Dau, is then subtracted from the values of De, and

1064.43 °C is subtracted from the corresponding values of calculated

temperature, tgg. These data are fitted with a second degree polynomial by

the method of least squares, symbolically De-Dau = f ( tg

g

- 1064 . 43 "C) . The polynomial is constrained so that the value of De-Dau equals 0 at 1064.43 °C. This polynomial is then rearranged algebraically to yield De as a function of

tgg . It is then combined algebraically with the function that gives the temperature -emf relationship of the type B reference thermocouple to obtain

the polynomial representing the temperature -emf relationship of the test thermocouple in the 1064.43 "C to 1750 °C range. COMPUTER OPERATIONS USED TO CALCULATE AND PRINT TABLES . Tables for types S, R, and B thermocouples are calculated and printed using the laboratory

computer. The computer programs operate as outlined above, using a least- squares fitting routine to fit the data in predetermined temperature ranges. The data file is generated from the automatic calibration programs. The coefficients of the equations giving the temperature-emf relationships of the

30
reference thermocouple and the appropriate thermocouple reference table from

NBS Monograph 125 must be in files. The printed output will include the temperature versus emf table, a listing of

the measured calibration data, results of the least squares fitting of the calibration data, and a summary of the temperature versus emf table and

coefficients. The temperature versus emf table and the list of coefficients

for the equations that were used to compute the table are bound with a typed

report of calibration which gives the customer's name and location, the customer's designation of the thermocouple, the estimated calibration uncertainty, the purchase order number, the test number, and the date. The bound

calibration report, along with a typed covering letter, is sent to the customer. Samples of the above information for a comparison calibration of a type S and a type R thermocouple are given in the appendix, section 8.1.2 and

8.1.3, respectively. The calibrated thermocouple is returned under separate

cover

3.1.1.8 Apparatus WELDING STATION . In order to minimize contamination of the platinum and platinum-rhodium thermocouple wires, a laboratory work bench and special

tools are reserved for preparation and assembly of the test thermocouples. The bench top is covered with clean paper. The thermocouple measuring junctions

are prepared by welding using a small oxygen-gas torch. Although plastic

gloves are not used when handling the wires, care is taken to insure that hands are washed prior to handling the wires. ELECTRICAL ANNEALING SYSTEM . The purpose of the annealing system is to hold

the thermocouple wires during the electrical anneal and protect them from

drafts which can cause temperature variation along the wires. The annealing

system used in this laboratory is a three sided enclosure (57 cm wide by

57 cm deep and 215 cm high) and has electrical leads coming through the top to two spring- loaded copper clips that hold the thermocouple wire. The front of

the system is left open for easy access and to permit the temperature of the thermocouple wire to be determined with the optical pyrometer during the anneal. The longest thermocouple that may be annealed is about 180 cm. Longer thermocouples (up to 360 cm) can be accommodated by annealing each leg

separately. FURNACES . A horizontal tube- type furnace having a tubular heating element of Chromel (a nickel -chromium alloy) is used to heat the thermocouples during a calibration in the 0 to 1100 °C range. A simplified sectional view of the furnace is shown in figure 3.2. The heating element (22 mm i.d. and 610 mm

long) is clamped between two water-cooled copper terminals that are bolted to

a laminated copper bus that consists of 14 strips of copper, each 1 . 2 mm thick

and 100 mm wide. The copper bus supports the heating element and forms a single-turn secondary winding of an electrical transformer that has a turns

ratio of 40 to 1 . A radiation shield and a water-cooled furnace shell

(190 mm in diameter and 540 mm long) are mounted around the heating element.

To minimize the time required for heating and cooling of the furnace, no thermal insulation is used between the heating tube, the radiation shield, and

the furnace shell. The middle part of this furnace, for about 450 mm, is at

31
practically a uniform temperature and the water-cooled terminals produce a very sharp temperature gradient at each end. This furnace can be heated to about 1100 °C in about 10 minutes with about 12 kW and, if all the power is shut off, will cool from that temperature to about 300 °C in about the same

time. A large motor-driven adjustable transformer is used to supply power to the primary winding of the transformer and a smaller manually-operated

adjustable transformer is used to trim about 10% of the electrical power. During calibrations of noble metal thermocouples, a high purity alumina tube

(11 mm i.d., 16 mm o.d., and 450 mm long) is used in the furnace to protect

the test thermocouples from contamination by the Chromel heating element. The temperature of the furnace is changed during an automatic calibration by the computer, which actuates relays to control the motor-driven adjustable

transformer.

For the calibration of type B thermocouples in the 800 to -1600 °C range, a horizontal tube -type furnace with a tubular silicon carbide heating element is used. A sectional view of the furnace is shown in figure 3.3. The heating

element (29 mm i.d. and 710 mm long) is held between two water cooled terminal

assemblies that are spring loaded against the brass end plates of the furnace

shell to allow for the thermal expansion and contraction of the heating

element during heating and cooling. The outer brass shell (450 mm in diameter

and 670 mm long) is also water cooled. A refractory insulating liner formed

from ring shaped bricks and contained within an Inconel tube surrounds the heating element. The furnace may be heated to about 1100 °C in about 2 hours

and then to 1600 °C in an about four more hours. The power required to hold

the furnace at a temperature of 1600 ° C is about 4.5 kW. The temperature of

this furnace is controlled manually during a calibration and the electrical power is supplied using the large motor-driven adjustable transformer and the smaller adjustable transformer that are used with the Chromel tube furnace. A

high purity alumina tube (15 mm i.d., 20 mm o.d., and 610 mm long), closed at

one end, is used in the furnace to protect the thermocouples from contamination by the heating element. It is held at the outer end and cantilevered

into the furnace so that it is not in contact with the heater.

ICE BATHS . Thermocouple reference junctions are maintained at 0 °C in ice baths during the calibration. The ice bath consists of a mixture of finely

divided pure ice and water contained in a Dewar flask (70 mm i.d. and 340 mm

high) . The reference junctions are formed by inserting the thermocouple wires

and 0.4 mm copper connecting wires into closed end glass tubes containing a drop of mercury at the bottom. The glass tubes (4 mm i.d., 6 mm o.d., and

165 mm long) are immersed 135 mm into the ice bath through holes in the cover

of the Dewar vessel. The glass tubes must be clean and free from water, or a spurious emf may be introduced into the measurements . They are examined

visually each time they are used and replaced with clean tubes as needed. The dirty mercury is removed from the used tubes and saved. The tubes are cleaned

with dilute hydrochloric acid, then rinsed with water and with ethanol. After

the tubes have dried, they are reused. MEASURING INSTRUMENTS . During automatic calibrations, the values of emf of

the test and reference thermocouples are read simultaneously with two Hewlett

Packard 3456A Precision Digital Voltmeters (PDVM's). The computer is 32
programed to read these instruments simultaneously. The PDVM's are instructed

to take 16 readings, calculate the mean and the variance of the readings, and return these values to the computer. During a calibration, the PDVM's are checked against a 10 millivolt constant voltage source at the beginning of the test and after each six sets of readings. The 10 millivolt standard is, in turn, checked against a calibrated six-dial Guildline potentiometer (see sec. 3.3.9.9). The PDVM's are also checked periodically against the six-dial

Guildline potentiometer. The zero of each PDVM is determined each time the 10 millivolt constant voltage source is read by measuring an electrical short at the terminal board and the value of emf found is applied to subsequent PDVM

readings

COMPUTER . The computer used in the thermocouple calibration laboratory is an IBM enhanced AT computer operating under PCDOS 3.20, with 640 K memory, using

the National IEEE-488 bus interface and TransEra TBASIC. The printer is an IBM Proprinter XL24 and a color monitor is used. 3.1.2 Base Metal Thermocouples

Base-metal thermocouples are calibrated manually by comparison with a type S reference thermocouple using procedures and apparatus similar to or identical

with those used for noble metal thermocouple calibrations. After removing the thermocouple from the package in which it was shipped, it is examined

visually. Only new base -metal thermocouple wires are accepted for calibration,

since experience indicates that base-metal thermocouples exhibit various

degrees of inhomogeneity after they have been heated to elevated temperatures

for relatively short periods. If a thermocouple appears to have been

previously heated, the customer is contacted and requested to supply unused

thermocouple wire for the calibration.

The base-metal thermocouples are not given an electrical anneal before cali- bration. The wires are insulated in ceramic tubing. Normally, high purity

alumina is used, although mullite is used occasionally. The size (o.d. and

i.d.) and configuration of the tubing differ widely and depend on the diameter

of the thermocouple wire. Usually, twin-bore tubing in a single continuous

length is used, but at times short lengths or single-bore insulators are employed. If the thermocouple was received with the measuring junction

already formed, the ceramic tubing is slid over the wires starting at the reference junction end of the thermocouple, and the measuring junction is left

intact. When necessary, the measuring junction is formed by welding with an oxygen-natural gas torch, using borax flux to protect the wires from

oxidation. The flux is removed after the weld is made by mechanical means or by boiling in water or both. The type S reference thermocouple that is used to measure the temperature

during the calibration is mounted and protected in the following manner to minimize the contamination of the thermocouple wires. First the type S reference thermocouple is assembled in a twin-bore, high-purity alumina

insulator. The insulator is then inserted into a high-purity silica glass

tube that is open at both ends. The tube is positioned so that the end of the alumina tube is flush with the end of the glass tube, and the measuring

junction extends about one centimeter beyond. The ends of the glass and

33
alumina tubes are then sealed to the thermocouple wire with a small amount of borosilicate glass to prevent metal vapors from the base metal thermocouple

and from the furnace heating element from contaminating the type S reference thermocouple during a calibration. The measuring junction of the reference thermocouple is attached to the measuring junction of the base metal test thermocouple by one of the following methods:

a) It is spot welded to the junction of the base-metal test thermocouple

b) It is tied to the junction of the base-metal test thermocouple by

wrapping with a short length of platinum wire.

c) A small diameter hole is drilled in the junction of the base-metal

thermocouple and the junction of the type S reference thermocouple

is driven into this hole with a steel punch.

The first method is the one that is used most frequently. The third method is sometimes used when the base-metal thermocouple wire is very large in diameter

For the calibration of base metal thermocouples, the furnace with the Chromel heating tube is used without a closed end alumina protection tube. The thermocouple assembly is inserted into the furnace to an immersion of about

30 cm. Calibration data are taken in order of increasing furnace temperature

at temperatures specified by the customer. The highest temperature at which a particular letter-designated type thermocouple is calibrated is determined by

the recommended upper temperature limits specified in ASTM Standard E230. These temperature limits are given below.

The calibration data are taken in order of increasing temperature since the thermocouples may change even during the calibration process. The magnitude

of the change depends upon the type of thermocouple, the temperature to which

it is heated, and the time at temperature. The emf of the test thermocouple

and the emf of the type S reference thermocouple are measured simultaneously

while the furnace temperature is changing slowly, using the two precision

digital voltmeters, the switch scanners, and the automatic data acquisition

system described in section 3.1.1.5. The data for the calibration of a base-metal thermocouple are reported at the actual measured temperatures. While a calibration is in progress, the dif- ference between the measured emf of the test thermocouple and the emf given by

Thermocouple

Type

Temperature

Limit (°C)

E

JK

N

T

1000

760 1100

1100 400

34
the thermocouple reference table found in NBS Monograph 125 is calculated; measurements may be repeated if a particular calibration point appears out of line with the preceding points. After the calibration is completed, a report

listing temperature and emf of the test thermocouple is typed and sent to the customer along with a covering letter. An example of a calibration report for

a type K thermocouple and a covering letter is shown in section 8.1.5. 3.1.3 Thermoelements versus Platinum Thermoelectric Reference Standard. Pt-67 When requested by our customers, thermoelements are tested against the platinum thermoelectric reference standard, Pt-67, that is maintained by the NIST Temperature and Pressure Division. This standard is a selected portion

of NIST Standard Reference Material (SRM) 1967; its development and properties

are described in NBS Monograph 125. Both noble-metal and base-metal wires are tested against Pt-67. Platinum reference wires whose values of emf are known

relative to Pt-67 are used to form a thermocouple with the wire submitted by

the customer. This thermocouple is then calibrated using the same test procedures, calibration furnace, and measuring instruments as those described

for the comparison calibration of base-metal thermocouples in section 3.1.2. A different set of platinum reference wires is used when testing noble-metal

wires than when testing base-metal wires. The platinum reference wires used

for testing base-metal wires are insulated in a high purity alumina tube and protected from contamination in the same manner as the type S reference thermocouples used for base-metal thermocouple calibrations. There must be electrical contact, of course, between the platinum reference wire and the test wires. For base-metal wires the measuring junctions of the platinum

thermoelectric reference wire and type S thermocouple are spot welded to the test wire, but for noble-metal wires the measuring junctions of the test and reference wires and of the type S reference thermocouple are welded together

with an oxygen-gas torch. Calibration points are taken in order of increasing

temperature for the base-metal wires and in order of decreasing temperature

for the noble-metal wires. Measurements are made at up to 15 different

temperatures of the customers' choice. The maximum calibration temperature

will depend on the type of wire. In general, base-metal wires are tested to the same upper temperature limits as the corresponding thermocouple type. (See sec. 3.1.2.

)

3.1.4 Metal -Sheathed Thermocouples

Metal -sheathed thermocouples are accepted for calibration by the comparison

method if they can be accommodated in our test apparatus. The customer must

supply suitable thermocouple connectors and extension wires for making

electrical connections between the thermocouple and the copper connecting

wires going to the measuring instrument. Before a metal-sheathed thermocouple

is tested at temperatures above room temperature, it is immersed in an ice bath and the emf of the thermocouple is measured with the thermocouple connectors maintained at room temperature.

The method used above room temperature to calibrate metal-sheathed ther- mocouples is similar to that employed with thermocouples that are not

sheathed. The reference thermocouples used for testing such thermocouples are

35
the same as those used in the calibrations described in sections 3.1.1 and

3.1.2. The choice of the reference standard to be employed depends on the composition of the sheath, the temperature range, and the type of test thermocouple. The measuring junction of the reference thermocouple is attached to the outside of the metal sheath near the tip either by spot welding or by

wrapping with platinum wire. The calibration data are obtained in the same manner and with the same apparatus as used for the calibration of a thermo- couple that is not sheathed. (See sec. 3.1.1 and 3.1.2.) For sheathed type

S, R, and B thermocouples, the calibration data are measured with the automatic data acquisition system and then processed using the computer

programs described in section 3.1.1.7. A few minor changes in the programs

are required so that the temperature -emf table for the test thermocouple gives

the value of emf of the test thermocouple assembly that was measured at 0 °C.

3 . 2 Calibration by Comparison with a Standard Platinum Resistance Thermometer

3.2.1 Calibration Baths Thermocouples are calibrated by comparison with a standard platinum resistance thermometer (SPRT) in the range -110 to 540 °C and at about -183 °C (boiling

point of oxygen) and about -196 °C (boiling point of nitrogen) . Such com- parisons are carried out by placing an SPRT and the thermocouples under test

in a series of stirred liquid baths, in a cryostat, or in a copper block

immersed in liquid nitrogen or liquid oxygen. All electrically heated constant temperature comparison baths used to calibrate thermocouples at NIST have the heating coils isolated from the volume where the thermocouples will be placed. There is an unrestricted path

of flow for the bath medium which is stirred at a sufficient rate to maintain

a uniform temperature throughout the medium. Surrounding the baths is a 5 to

7 cm thickness of insulation and an insulated cover is provided to help

minimize heat loss. Fitted into the top cover is a holder containing the test thermocouples and SPRT. The comparison baths used to calibrate thermocouples above 0 °C are manually

controlled by adjusting the current to the heating coils through a rheostat.

For calibration below 0 °C, the temperature is automatically controlled.

All baths at NIST were evaluated to determine the temperature gradient

throughout the medium. This was done by placing three SPRT's at various

locations and depths in the medium. The thermometers were read simultaneously

and the maximum difference between the three readings at each temperature was considered to be caused by the thermal gradient. 3.2.1.1 Water and Oil Baths An oil bath used at NIST is shown in figure 3.4 and a drawing of both the water and oil baths is shown in figure 3.5. They consist of two cylindrical

wells of different diameters with connecting passages at the top and bottom.

The heating coil and stirrer are located in the smaller well, leaving the larger well unobstructed for the insertion of the thermocouples and SPRT. The fluid is forced past the heating coils, through the bottom opening into the

36
larger well, around the thermocouples and SPRT, and back into the smaller well through the connecting passage at the top. For calibration in the range +1 to +95 °C, distilled water is used as the bath

medium. Distilled water is preferable to tap water since no chemical deposits

will form at the top of the bath. Water is plentiful, clean, and does not produce harmful vapors. Additional distilled water is added each day the bath

is used, since some of the water will leave through the overflow tube due to the expansion of the water when heated. The level should be no more than

10 mm from the top of the thermocouple and SPRT holder. Measured temperature

gradients in the water bath are such that the temperature of the thermocouple

measuring junctions and SPRT resistor do not exceed 0.01 °C. Two oil baths are maintained at NIST. One, called the low- temperature oil bath, is used to calibrate thermocouples in the range +95 to +200 °C. An oil purchased from EXXON, know as Zerice 46, is used as the medium. The oil level must be raised to within 25 mm of the top of the bath by hand pumping oil from

a pan under the bath. (Heat from the bath will cause the oil to expand and

leave the bath through the overflow tube. It is collected in the pan below

the bath.) The temperature gradients in the low- temperature oil bath are such

that the difference in temperature between the thermocouple measuring junction

and the SPRT resistor does not exceed 0.015 °C. For calibration in the range +200 to +300 °C, the high- temperature oil bath is used. The medium is an oil called Extra Hecla from Mobil Oil Corporation.

011 coming from the overflow tube is collected in a bucket. Before the bath

is used, oil from the bucket must be poured into the bath to within 70 mm of

the top of the bath. Temperature gradients in the this oil bath are such that

the difference in temperature between the thermocouple measuring junction and

the SPRT are no greater than 0,05 °C. The calibration baths that use oil as the medium are operated under a fume hood which is not connected to the ventilation system of the building but is exhausted directly outside. 3.2,1,2 Tin Bath

The tin bath is used to calibrate thermocouples in the temperature range +300

to +540 °C. The medium is 99.9% pure tin, conforming to Federal Specification

QQ-T-371C, Grade A, and can be purchased commercially, e.g., from Nathan

Trotter in Philadelphia, Pennsylvania. A picture and drawing of the tin bath are shown in figures 3.6 and 3.7. It consists of two coaxial cylinders arranged in such a way as to permit the molten tin to circulate between the walls of the two cylinders and through the inner cylinder by means of openings at the top and bottom. The stirring

propeller is situated near the bottom of the inner cylinder, leaving the majority of the space for the reentrant tubes (not shown in the drawing) into which the test thermocouples and SPRT are inserted. The heater coils are wound on the outside of the outer cylinder. The temperature gradients in the tin bath are such that the difference between the temperature of the

37
thermocouple measuring junctions and the SPRT resistor does not exceed 0.05 °C at 300 C and 0.1 °C at 500 °C. In the evening before the tin bath is to be used, a clock is set to turn the power on to the bath in the early morning hours. This is necessary, since it takes approximately 4 h to reach a temperature of 350 °C. If a temperature of over 500 °C is to be realized, the clock should be set to turn the bath on at 0300 hours. Before the tin bath can be used, oxidized tin, which formed when the bath was

last used, must be removed. This is facilitated by removing the reentrant

tube holder. The tin oxide is at the top of the bath and can be removed

easily with special implements stored near the bath. Some residue, which

clings to the reentrant tubes, must be removed also and this must be done quickly before the tubes cool. Fresh tin must be added to bring the level of

the molten tin within 1 or 2 cm from the top of the inner cylinder. When all

this has been done, the tubes are placed back into the bath. The stirrer is turned on and the adjustment of current to the heating coils is set to control

the bath at the desired temperature.

3.2.1.3 Crvostat

Calibrations from -1 to -110 "C are made in the cryostat shown in figure 3.8. A vertical section is shown in figure 3.9. The cryostat consists of an inner Dewar flask which contains the bath liquid. This flask is surrounded by

liquid nitrogen contained in the outer Dewar flask. The rate of heat transfer between the bath liquid and the liquid nitrogen is controlled by the presence

or absence of gas or air in the space between the walls of the inner Dewar

flask. VThen the lowest temperature desired for the day is reached, the rate

of heat transfer is retarded by evacuating the gas or air through the side

tube which is connected to a vacuum system. Vigorous stirring of the bath

liquid is maintained by the propeller which circulates the liquid around the walls of the stirrer tube similar to the flow of the molten tin in the tin

bath. The temperature of the bath is thermostatically controlled by heater

coils wound on the outside of the stirrer tube. The test thermocouples and

SPRT are immersed inside the stirrer tube, thus shielding them from the heated

coils

The bath liquid is a five -component mixture containing by weight 14.5% chloroform, 25.3% methylene chloride, 33.4% ethyl bromide, 10.4% transdichloroethylene, and 16.4% trichloroethylene . This mixture freezes at approximately

-150 °C, but readily absorbs moisture and becomes cloudy at somewhat higher

temperatures. For this reason, calibrations are not performed in this bath below -110 °C. Another liquid that has been used as the medium is 2-Methylpentane . Measured gradients in the cryostat do not exceed 0.01 °C. 3.2.1.4 Liquid Oxygen and Liquid Nitrogen Points A comparison measurement can be made at approximately -196 °C (boiling point

of nitrogen) or approximately -183 °C (boiling point of oxygen). A taped

Dewar flask is used as a container for the liquid nitrogen or oxygen. A

copper block is immersed in the liquid which is agitated by bubbling nitrogen

38
or oxygen gas in the corresponding liquid through a glass tube with an outlet placed in one of the holes of the copper block.

3.2.2 Equipment

3.2.2.1 Standard Platinum Resistance Thermometer

An SPRT has a resistor made of platinum of sufficient purity that the finished

thermometer will have a value of the resistance at 100 °C divided by the resistance at 0 °C not less than 1.3925 or alpha, defined as

(^10 0 " Ro)/100Ro' l^ss than 0.003925. The typical SPRT has an ice-point

resistance of about 25.5 ohms and is called a 25 ohm thermometer; its resistor

is wound from about 61 cm of 0.075 mm diameter wire. The insulation material that supports the resistor and leads must not con- taminate the platinum during the annealing of the assembled thermometer nor when subjected for extended periods of time to temperatures to which the thermometer is normally exposed. The insulation resistance between the leads must be greater than 5X10^ ohms at 500 °C if the error introduced by

insulation leakage in the leads of a 25 ohm thermometer is to be less than the equivalent of 1 microhm. For SPRT's the most commonly used insulation is mica. The configuration of the resistor is inevitably the result of compromise

between conflicting requirements. The resistor must be free to expand and contract without constraint from its support. This characteristic is the so called "strain-free" construction. If the platinum were not free to expand,

the resistance of the platinum would not only be a function of the temperature

but would also relate to the strain that results from the differential

expansion of the platinum and its support.

The sensing elements of all SPRT's have four leads. The four leads define the resistor precisely by permitting measurements that eliminate the effect of the resistance of the leads. The resistor winding is usually "noninductive ,

often bifilar. Because the junction of the leads is electrically a part of the measured

resistor, the leads extending immediately from the resistor must also be of high purity platinum; the lengths of these leads are often as short as 8 mm. Either gold or platinum wire is employed in continuing these leads within the thermometer. Gold does not seem to contaminate the resistor and is easily

worked. Measurement of the resistor may be facilitated if the four leads are made of the same material with the length and diameter the same so that the leads have about equal resistance at any temperature. This statement is also applicable to leads that are external to the protecting envelope.

The hermetic seal through the soft glass envelope at the thermometer head is frequently made using short lengths of tungsten wire, to the ends of which

platinum lead wires are welded. The external platinum leads are soft soldered

to copper leads that are mechanically secured to the head. Coaxial leads are required when an ac bridge is used to determine the resistance of the SPRT.

39
3.2.2.2 AC Bridge

Bridges operating at 400 hertz have been built at the NIST based on a design

by Cutkosky [2]. These bridges were designed for use with SPRT's. The bridge

utilizes an inductive ratio divider that eliminates the necessity of cali- brating the bridge because the initial uncertainty of the divider is about

2 parts in 10^ and appears to be stable. Additionally, the bridge requires

only one manual resistance balance, the phase angle balance being automatic, and incorporates a built-in phase-sensitive null detector with which 1 microhm

in 25 ohms can be easily resolved. A small (usually less than 10 microhms)

error may be introduced in measuring a 25 ohm SPRT unless coaxial leads are used between the bridge and the thermometer head. (The heads of SPRT's have been modified to contain two BNC coaxial connectors . The two leads from one end of the SPRT coll were connected to the center "female" contacts of the BNC receptacles and the two leads from the other end of the SPRT were connected to the outer shells or the shield contacts.) For precision measurements the length of the pair of coaxial leads should not be greater than 15 meters to limit the dielectric losses of the shunt capacitance. Preliminary measurements on 25 ohm SPRT's indicate that, if the leads do not affect the measurements, the accuracy of the measured value in ohms of a thermometer

element is limited by the accuracy to which the reference standard resistor is known. However, in the accurate determination of the resistance ratio, R^/Rq , the stability, rather than the accuracy of the reference standard is the important requirement.

3.2.2.3 Potentiometer A Rubicon six-dial potentiometer. Cat. No. 2768, is used to measure the emf

of the thermocouples during the calibration. This instrument is calibrated in accordance with the procedure outlined in appendix I of NBS Monograph 126 [3]. 3.2.3 Calibration Procedure

In calibrating thermocouples by comparison with an SPRT, several steps are followed routinely. These include a preliminary examination, thermocouple

preparation, check of SPRT, and acquisition of data. Thermocouples are unpacked and examined. If the measuring junction is not made, this is done by

silver soldering the wires together. Fiberglass sleeving is used to insulate

the wires if it is a bare wire thermocouple. The test thermocouple is placed

in a glass tube before inserting it in the calibration baths. If the instru- ment is a sheathed thermocouple, it may be placed directly in the baths. The normal depth of immersion is 12 inches below the surface of the stirred liquid

bath. When calibrating below 0 "C, trichloroethylene is used in the glass

tube containing the test thermocouple to increase the thermal conductivity.

The measuring junction of the test thermocouple is placed adjacent to the SPRT

coil in the bath. The thermocouple wire is connected to copper extension wire and this connection is maintained at 0 "C in an ice bath. Measurements are made using

a Rubicon six-dial potentiometer and an automatic bridge. The reading

40
sequence is as follows:

SPRT, TC, SPRT, TC, SPRT. The bath temperature is determined from the three SPRT readings. The two thermocouple readings are averaged, the potentiometer correction is applied, and the data are normalized to the desired temperature. If the bath temperature changes by more than 0.05 °C during the reading sequence, then the measurement at this temperature is repeated.

Every day the SPRT is going to be used, it is necessary to check its resistance at the triple point of water to see that it is still in calibration

and to obtain the resistance at the 0 "C point for the purpose of calculating

the temperature of the bath medium. This is done by placing the SPRT in a properly prepared triple point of water cell [3]. Before inserting the thermometer in the cell, it should be checked to see that the ice mantle is moving freely around the reentrant well. A small piece of foam is placed at the bottom of the reentrant well, enabling the SPRT to have something soft on which to rest. An aluminum bushing, which is approximately 50 mm long is placed inside the reentrant well on the foam, and is made to fit snugly around

the portion of the SPRT containing the platinum coil. The SPRT is placed in the triple point of water cell and water is poured into

the reentrant well until it is full. The SPRT is covered with a black cloth

to eliminate any error in the resistance reading caused by ambient room

radiation, especially from ceiling lights. After the SPRT has been allowed to remain in the triple point of water cell for a minimum of 15 minutes, the ac bridge is balanced and the resistance at the triple point of water is recorded.

The resistance value on that particular SPRT is compared with previous

resistance values at the triple point of water. If the value has changed by

an amount equivalent to or exceeding 0.001 °C, then the SPRT is sent to the Platinum Resistance Thermometer Calibration Laboratory for recalibration.

If the SPRT is going to be used below 150 °C, the triple point of water is taken at the beginning of the day. If the SPRT is going to be used above

150 °C, the triple point of water is taken at the end of the day, after being

out of the bath medium for at least 15 minutes. 3.2.4 Computation of Bath Temperature

The first step in calculating the temperature of a stirred liquid bath is to average the two resistance values determined for the SPRT at the triple point

of water taken on the day the calibrations were performed. This average resistance value at the triple point of water, R^p, is converted to the resistance value at the ice point, Rg , by multiplying it by a factor of 0.99996101, which was derived from information given in NBS Monograph 126, Platinum Resistance Thermometry [3] and in the Metrologia article. The Inter- national Practical Temperature Scale of 1968 (Amended Edition of 1975) [4]

41
Since the derivation of tb's factor is not explicitly outlined in either of

the above publications, it is explained below. The triple point of water cell has an axial well for the thermometers and the triple point temperature is obtained whenever the ice is in equilibrium with a liquid-vapor surface. The equilibrium temperature tgg between the ice and

liquid water at a depth h below the liquid-vapor surface is given by: t^3 = A + Bh where A = 0.01 °C B = -.0007 °C/m h = height of the water column in the triple-point

cell from the middle of the SPRT coil to the water surface. The valve h varies from cell to cell. For this case, the hydrostatic head of water from the middle of the SPRT coil to the water surface of the cell used

for the calibration was 0.315 m; hence, the valve tgg of the triple point of water is calculated to be:

tgg - 0.01 °C + (-0.0007 °C/m)(0.315 m)

tgg = 0.0097795 °C. This valve for tgg must then be converted to t' . The conversion is done by

using the M(t') relation given by equation 10 in reference [4]. Near 0 °C, dM(t')/dt' is equal to -0.000450. Therefore,

t' = (0.0097795)(1. 000450) °C

t' = 0.0097839 °C. The value for the resistance at 0 "C, Rg , is calculated from the resistance at

the triple point of water according to the follo],*7ing formula:

R^/Rq = 1 + At'+ Bt'2 Since t'^ is very small, the last term is dropped, the value A is determined

from the formula:A = a(l + 6/100 °C) The a and 5 values are given with the report of calibration for each SPRT. For the resistance thermometer used in this example, the value of q was 0.003926378 °C"^ and 5 was 1.496448 °C. The value A is calculated to be: A = 0.003926378[1 + (1.496448/100 °C)] A = 0.0039851342 "C"^ Knowing the value for A and the value t' at the triple point of water, the

42
factor used to convert the R^p value to the Rq value can be determined as shown below:

Rtp/Rq = 1 + At'

Rq = Rtp/(1 + At') ohms

Rq = Rtp/[1 + (0.0039851342)(0. 0097839)] ohms

Rq = (25. 46480)(0. 99996101) ohms

Rq = 25.46381 ohms After the value Rg is determined, the calculation of the bath temperatures can

be made. The first step is to find the average of the three resistance

readings at each temperature. This value is called R^ . Since the tables computed for the SPRT's are based on a ratio of the resistance at a stated

temperature to the resistance at the ice point (the ratio of R^^/Rq), the value

R^ at each calibration point must be divided by Rq . The computation of the value Rt/Ro ^ trial temperature is given below:

Rt = 27.48510 ohms

Rq = 25.46381 ohms

R^/Rq = 1.079379

It is then determined where in the platinum resistance thermometer table this ratio falls. For the SPRT used in this example the computed value for Rt/Ro of 1.079379 falls between 19 and 20 °C. The temperature of the calibration

bath medium is, therefore, 19 and some decimal value. To obtain the decimal

portion of the temperature, the R^/Ro value at 19 "C is subtracted from the

Rt./Ro value in the example and the difference is multiplied by the inverse

difference, which is the inverse (reciprocal) of the difference between

successive values of R^^/Rg , given on the line at 20 °C. This procedure is shown below: table value for Rt/Ro at 19 "C for SPRT 445 is 1.075531

table value for R^/Rq at 20 °C for SPRT 445 is 1.079494

the value 1.079379 falls between these two ratios

the temperature of the calibration bath will be 19. XX. From Rt/Ro subtract Rt/Ro at 19 "C and multiply by the inverse difference

given for the nominal temperature of 20 °C. Rt/Ro = 1.079379

Rt/Ro at 19 "C = 1.075531 Difference = 0.003848 Decimal value = Difference X inverse difference

Decimal value = 0.003848 X 252.33 Decimal value =0.97

The calculated temperature of the calibration bath at this calibration point

is 19.97 "C. This procedure is repeated at each calibration point to determine the temperature of the calibration bath medium.

43
3.2.5 Calibration Report

The emf values determined during the calibration of a base-metal thermocouple

are reported at the measured temperatures, except for type T thermocouples.

Type T thermocouples may be calibrated at specified temperatures and a table

of temperature versus emf at 1° intervals prepared. The computer program used

for processing the calibration data and for calculating and printing the table

for a type T thermocouple is covered in this section. TEMPERATURE VERSUS EMF TABLE. TYPE T . Values of emf are measured at specified

temperatures (-183, -110, -50, +50, +100, +200, +300 °C) and a table of temperature versus emf at 1° intervals is calculated for any of seven temperature ranges within the overall range -190 to 300 "C. The data are processed using the NIST Cyber computer and the data are transmitted between

the laboratory and the computer using the NIST net. After the calculations

are completed, the table is printed in the laboratory on an Okidata Model /i83A printer.

The computer first calculates the emf difference, De, between the emf readings

of the test thermocouple and the emf value from the type T reference table in NBS Monograph 125 at each of the specified calibration temperatures. The calculations are divided into two temperature ranges: -183 or -110 °C to 0 °C and 0 °C to 100, or 200, or 300 "C. A cubic or quadratic equation, which is constrained to give 0 emf at 0 "C, is used to represent De as a function of

the temperature in each range. The polynomials representing De = f(t) in the two ranges are then combined algebraically with the functions that give the temperature -emf relationship of the type T thermocouple reference table given

in Monograph 125 to obtain polynomials representing the temperature -emf relationship of the test thermocouple.

The printed output includes the temperature versus emf table and the coefficients of the equations that were used to compute the table. One set of output is kept as the NIST record for the calibration. The second set of output is bound with a typed report of calibration which gives the customer's

name and location, the customer's designation of the thermocouple, the estimated calibration uncertainty, the purchase order number, the test number, and the date. The calibration report, along with a typed covering letter, is sent to the customer. A sample of the report and covering letter is given in

the appendix, section 8.1.4.

3 . 3 Primary Calibration of Type S Thermocouples in Metal Freezing-Point Cells

The type S thermocouple is the specified standard interpolating instrument on

the International Practical Temperature Scale of 1968 (IPTS-68) in the range 630.74 to 1064.43 °C [4]. The primary calibration of such thermocouples at NIST as IPTS standards involves determining their values of emf at four thermometric fixed points while their reference junctions are maintained at

0 "C. It is the most accurate calibration the NIST offers for type S thermocouples. The fixed points used are the liquidus points of gold, silver,

44
antimony^, and zinc. The liquidus point calibrating temperatures are realized

in metal freezing-point cells whose liquidus temperatures have been estab- lished from measurements made with platinum resistance thermometers as described in section 4.7.1. Our procedure follows the instructions given in the text of the IPTS-68 (Amended Edition of 1975) [4] to establish the temperature -emf relationship of the thermocouple over the range 630.74 to 1064.43 °C (gold point). Corresponding values of emf and temperature for the thermocouple above and below this range are obtained by extrapolation and

interpolation.

For a type S thermocouple to be a standard interpolating instrument for realizing the IPTS-68, it must meet certain requirements. The purity of the platinum wire shall be such that the ratio of its electrical resistance at

100 "C to its electrical resistance at 0 °C (Rioq/^o) not less than 1.3920. The platinum- rhodium wire shall contain nominally 10% rhodium and 90% platinum

by mass. In addition, the values of emf of the thermocouple at 630.74 °C (£530.74). the silver point (E^^^), and the gold point (E^^) must satisfy the following relations:

Eau = 10334 ± 30 /iV, Eau - = 1186 /iV + 0.17(Ea„ - 10334 /iV) ±3 ^V,

Eau - E63 0.7 A = ^782 + 0.63(Ea, - 10334 /iV) ±5 ^V. Not all type S thermocouples will meet the above requirements, but suitable

thermocouples may be obtained commercially. Reference or "premium" grade type

S thermocouple wire that meets the above requirements should be specified when

procuring thermocouples for primary calibration at NIST. Type S thermocouples

that meet the above requirements are referred to as standard thermocouples in the text of the IPTS-68. Primary calibrations are generally performed on a batch of six or more thermocouples at one time. A large batch is more economical of our time and helps to lower the costs for maintaining and providing this calibration

service. We attempt to balance the need to run a large batch of thermocouples

to hold down the calibration fee with the need to keep the calibration

turn-around time to our customers reasonably short. A special group of six thermocouples from three different lots of type S thermocouple wire have been assembled for use as NIST check standards. During

the calibration of each batch of test thermocouples, at least two thermocouples from this special group are included. The calibration history

compiled on this set of thermocouples gives an indication of the performance

of a type S thermocouple under carefully controlled conditions. Any gross change in emf measured for a particular thermocouple (or measurements during

an immersion check) at a particular freezing point from values found in previous calibrations is cause for examination of the system. The discrepancy

^The freezing point of antimony is not a defining fixed point of the IPTS-68.

45
may be caused by the measuring instrument, contamination of the metal freezing-point cell, lack of temperature uniformity in the furnace, change in the thermocouple itself or in some other part of the measuring system. This procedure can not detect small, gradual changes in the freezing point temperatures of the various cells, since the temperature -emf relationships of the thermocouples themselves change slowly with use. 3.3.1 Preliminary Examination A type S thermocouple submitted for a primary calibration is removed from its shipping container and examined visually. Its length, diameter, and condition

(new, used, good condition, poor condition, etc.) are recorded in the labora- tory notebook. A thermocouple to be calibrated by this method must be at least 100 cm long, and the diameter of the thermocouple wire must not be less than 0.4 mm. If the thermocouple does not meet these dimensional

requirements, the customer is requested to submit another thermocouple or have

the thermocouple calibrated by the comparison method (see sec. 3.1), which has

a minimum length requirement of 70 cm and no minimum diameter requirement.

3.3.2 Electrical Anneal

The type S thermocouple wire is annealed electrically in air before calibration. The electrical anneal is carried out in the same manner as for type S, R, and B thermocouples for comparison calibrations described in section

3.1.1.2.

3.3.3 Mounting for Calibration

The annealed thermocouple wires are inserted carefully into alumina insulating

tubes. The tubes are made of sintered (99.8+% purity) alumina and they are

60 cm long, 4 mm in diameter and have four 1 mm bores. These tubes are re- served for use only with thermocouples to be given primary calibrations and

the bores to be used with the pure platinum wires are notched at the cold end

of the tube. The thermocouple wires emerging from the alumina tube are insulated with flexible fiberglass tubing to within about 2 cm of their ends. The flexible fiberglass tubing is joined to the alumina tube with heat

shrinkable tubing. 3.3.4 Furnace Anneal After the type S test thermocouples are mounted in the alumina insulating

tubes, they are annealed in a horizontal tube furnace to remove small mechanical strains that may have been introduced during the mounting

operation. The silicon carbide tube furnace shown in figure 3.3 is used for

this anneal. The thermocouples are immersed approximately 55 cm into the furnace and protected by a closed-end alumina tube during the anneal. They

are heated slowly over a period of 2 to 4 h to about 1100 °C and held at that temperature for 1 h. Then the furnace is cooled slowly to near room temperature over a period of about 3 h, and the thermocouples are removed.

46
3.3.5 Preliminary Tests

Prior to calibration of the test thermocouples in the metal freezing-point

cells, they are given preliminary tests to assess their thermoelectric

homogeneity and to determine if they satisfy the purity and emf requirements

for a standard thermocouple discussed in section 3.0. For these tests, the measuring junction of a calibrated type S reference thermocouple (one of those used for comparison calibrations, see sec. 3.1.1) and the measuring junctions

of several (2 to 6) test thermocouples are welded together to form a common

junction. The test thermocouple assembly is then placed in the Chromel tube furnace shown in figure 3.2 in the same manner as for a comparison

calibration (see sec. 3.1.1). The furnace is heated to 1100 °C and held at that temperature for 30 minutes. Then a qualitative check of the thermoelectric homogeneity of each test thermocouple is made in the manner described in section 3.1.1.5. In addition, the emf of the platinum thermoelement of each thermocouple versus the platinum thermoelement of the reference thermocouple

is measured at 1100 °C. Values of emf for the test thermoelements versus

Pt-67 may be obtained from the latter measurements, since the emf of the platinum thermoelement of the reference thermocouple is known relative to

I't-B?. The test thermocouples are also calibrated by comparison with the reference thermocouple in the range 1100 to 0 °C, as described in section

3.1.1.5. After the comparison calibration is completed, the test thermocouple

assembly is removed from the furnace, its common junction is cut off, and the measuring junctions of the individual thermocouples are reformed.

The experimental data obtained in the above tests are evaluated before proceeding with a primary calibration of the test thermocouple. A positive

value for the emf of the platinum thermoelement of a test thermocouple versus

Pt-67 indicates that the purity of the thermoelement is less than that of

Pt-67. An emf for a platinum thermoelement versus Pt-67 at 1100 °C greater

than 14 microvolts indicates [5,6] that its resistance ratio (R^j^qq/Rq) is less than 1.3920; hence, the test thermocouple is rejected for a primary

calibration. Furthermore, if the value of emf at 1064,43 "C (gold point)

determined during the comparison calibration does not satisfy the IPTS-68 requirement listed in section 3.0, or if an emf variation of more than

5 microvolts is observed during the homogeneity test at 1100 °C, the test thermocouple is also rejected for a primary calibration. When a test ther- mocouple is rejected for a primary calibration, the results of the preliminary

tests are discussed with the customer to determine the action to be taken. 3.3.6 EMF Measurements at Freezing Points

The values of emf of type S test thermocouples are determined at the liquidus

point of each of the following metals: gold, silver, antimony, and zinc. The liquidus temperatures of the four metals are realized in metal freezing-point

cells of the design shown in figure 3.10 (see also sec. 3.3.9.5 for a description of the cell) . The freezing-point cells are heated in vertical

tube furnaces (see sec. 3.3.9.6 and 3.3.9.7). Before heating, the cells are pumped down to about 10"^ Torr with a rotary, oil-sealed vacuum pump and back

filled with argon three times to remove the air and then filled with argon to

a pressure slightly above ambient. This argon pressure is also maintained

when the cells are heated.

47
The method used for nucleating freezes depends on the particular metal and on

the extent to which the metal supercools. The special techniques employed to initiate freezes with each of the four metals are discussed in the following

subsections. However, the initial procedures followed for preparing the freezing-point cells for a freeze are similar with all four metals. The cells

are heated in the appropriate furnace which is set to control at a temperature

5 to 7 °C above the freezing point. When the metal is completely melted, as indicated by a monitoring thermocouple placed in the cell well, the furnace temperature is reduced and then stabilized about 2 °C above the freezing

point. The furnace is held at this temperature until the temperature of the liquid metal is nearly equal to the furnace temperature, and then the furnace temperature is again lowered to initiate the process of nucleating solid metal

on the walls of the graphite crucible. A thermocouple assembly to be tested is inserted slowly into the heated

freezing-point cell, and the thermocouple wires are joined electrically to polyvinyl insulated copper connecting wires (0.4 mm diameter). The electrical

connections form the thermocouple reference junctions and they are maintained

at 0 °C in an ice bath (see sec. 3.3.9.8) when emf measurements are made. The copper connecting wires go to a rotary, thermocouple selector switch that

is connected to a six-dial precision potentiometer (see sec. 3.3.9.9). The potentiometer, which is interfaced with the laboratory computer, is balanced

manually, and then the dial settings are read and recorded by the computer at the operators command.

The thermocouple measuring junctions are kept a few millimeters above the bottom of the glass protecting tube when measurements are made in freezing-point cells. This is accomplished by fastening an alligator clip to the thermocouple insulating tube and letting it rest on the top end of the protecting tube to hold the thermocouple assembly at the proper immersion. Experience has shown that allowing the measuring junction to touch the silica

glass can alter the thermocouple emf by a microvolt or more in the gold cell. The thermocouple emf tends to be low and shows poor repeatability from freeze

to freeze. The magnitude of this effect is smaller in the silver, antimony, and zinc cells. Measurements are made first at the gold point, and then at the silver point,

antimony point, and zinc point. After a freeze is initiated and the metal is either in or very nearly in the temperature plateau region of its freezing

curve, measurements of the thermocouple emf are taken at 30 second intervals.

Five readings are taken on the first test thermocouple, then five readings are taken on the other test thermocouple mounted in the same assembly. Such sets of measurements are then repeated until 10 successive readings are obtained

for each thermocouple that agree within 0.15 microvolts. A mean of the 10 readings is computed and is corrected by applying potentiometer dial corrections. In the cases when a cold thermocouple assembly is inserted into a cell while a freeze is in progress, the measurements are started after a delay of about 15 minutes to allow the assembly to attain temperature equilibrium with

its surroundings. Each thermocouple is tested in at least two freezes in each

of the metal freezing-point cells. If the emf values determined in the two freezes do not agree within 0.3 microvolts, the thermocouple is tested in a 48
third freeze. If there is still lack of agreement, the test thermocouple is re-annealed and tested again at each of the fixed points. A single value of emf for each test thermocouple at each calibration temperature is determined by computing the mean of the emf values from the two or more freezes. These mean values are corrected by applying small corrections

to account for the departure of the liquidus points of the gold, silver,

antimony, and zinc freezing-point cells from the IPTS-68 defining values of temperature of 1064.43, 961.93, 630.74, and 419.58 °C, respectively. As dis- cussed in section 4.0, the liquidus points of the gold-point and silver-point

cells used for thermocouple calibrations have been determined by comparing

them with gold-point and silver-point reference cells using high temperature

platinum resistance thermometers, while the liquidus points of the antimony

and zinc cells used have been determined with standard platinum resistance

thermometers calibrated at the IPTS-68 defining fixed points (freezing points

of tin and zinc, and the triple point of water),

3.3.6.1 Gold and Silver Freezing Points

The gold and silver freezing-point cells are used in the electrically heated

vertical tube furnace shown in figure 3.11. The furnace must be heated to above 600 °C before the gold or silver freezing point cell is introduced to allow the Inconel well to expand sufficiently to accommodate the silica glass

cell. The same techniques are used for nucleating freezes with gold and silver. After the metal is melted and stabilized at about 2 °C above its freezing

point, the furnace temperature is lowered to about 5 °C below the freezing

point. Since gold and silver supercool at most a few degrees, this decrease

in the furnace temperature is sufficient to cause nucleation. After recalescence is detected by observing a rapid increase in the emf of the test thermocouple, the furnace temperature is increased and set to control at about

2 °C below the freezing point. Then, after about 15 minutes, measurements of

the thermocouple emf are begun, as described above in section 3.3,6. Only one thermocouple assembly is tested during a freeze, and once sufficient data have been obtained, the metal is remelted. Remelting is accomplished in a fairly short time, since only a portion of the metal is frozen. It takes about 1 h from the beginning of one freeze to the beginning of the next

freeze

When the thermocouples are removed from the gold and silver freezing-point

cells, the rapid cooling of the wires to room temperature produces small thermoelectric inhomogeneities in them. These inhomogeneities are thought to result primarily from a complex nonequi librium concentration of vacancies that

are quenched into the wires [7]. If the thermocouples are placed in the gold

or silver point cells again, this quenching effect has very little influence

on the measured thermocouple emf since annealing for a short time at these temperatures will quickly restore an equilibrium concentration of vacancies.

However, in the antimony freezing-point cell a much longer annealing time would be required to obtain an equilibrium concentration of vacancies at this temperature for a thermocouple quenched from the silver point. A thermocouple

quenched in this manner will typically yield an emf value at the antimony

49
point that is 0.5 to 0.7 microvolts lower than that of the same thermocouple

after it has annealed in the antimony cell for a long period. This condition

is alleviated by giving the test thermocouples a furnace anneal after they are tested at the silver point. The thermocouples are placed in the annealing

furnace (see sec. 3.3.9.3.), heated slowly to about 900 °C, and held at that temperature for 1 h. They are then cooled slowly in the furnace to room temperature

3.3.6.2 Antimony Freezing Point

The antimony freezing point cell is heated in the vertical tube furnace shown

in figure 3.12. The furnace must be heated to about 400 °C before the cell is inserted to allow the Inconel guide tube to expand sufficiently to accommodate

the silica glass cell. The supercooling of antimony before freezing is excessive (usually amounting

to roughly 20 to 25 °C) , and special treatment is needed, therefore, to initiate the freeze to avoid the necessity of cooling the furnace considerably

and shortening the freeze significantly. After the antimony is melted and stabilized at about 2 °C above its freezing point, the furnace is set to control at about 2 °C below the freezing point. The emf of the monitoring

thermocouple (whose emf at the freezing point of antimony is known) is observed closely as the antimony cools. When the freezing-point temperature

is reached, the freezing-point cell is completely withdrawn from the furnace and then replaced in the furnace after about 30 seconds. Experience has shown

that this cell withdrawal procedure is sufficient to cause recalescence to occur in the supercooled antimony. After nucleation of the freeze, the monitoring thermocouple is removed from the cell well, and immediately a cold

test thermocouple assembly is inserted, inducing a thin layer of antimony to freeze onto the walls of the well. After the thermocouple assembly is in the cell for approximately 15 minutes, measurements of the thermocouple emf are begun, as described in section 3.3.6. With the furnace temperature controlled

about 2 "C below the temperature of the freezing antimony, the freeze is prolonged for a few hours, allowing three or four thermocouple assemblies to be tested during each freeze. 3.3.6.3 Zinc Freezing Point

The zinc freezing-point cell is used in a vertical tube furnace of the same design as the one used with the antimony freezing-point cell. (See fig. 3.12 and sec. 3.3.9.7.) Zinc, which supercools less than 0.2 °C, requires no special treatment to nucleate a freeze. After the zinc is melted and stabi- lized at about 2 °C above its freezing point, the furnace is set to control at about 2 °C below the freezing point to begin cooling the liquid zinc. When

the monitoring thermocouple indicates the zinc has reached the freezing point,

it is removed from the cell well, and immediately a cold test thermocouple

assembly is inserted, inducing a thin layer of zinc to freeze onto the walls

of the well. Measurements of the thermocouple emf are begun after approximately 15 minutes, as described in section 3.3.6. As with antimony,

the zinc freeze is prolonged for several hours, and four to six thermocouple

assemblies may be tested during the freeze.

50
3.3.7 Packing and Shipping

After the calibration is completed, the thermocouples are packaged for shipment back to customers. The measuring junctions are cut off and the thermocouple wires are pulled gently from the alumina insulating tube. The measuring junction is reformed by welding with the oxygen-gas torch. A tag is prepared from plastic embossed tape with the NIST test number and attached to the thermocouple. The thermocouple is inserted into a polyethylene tube

(9.5 mm o.d. and 6.5 mm i.d.) that has been cleaned with alcohol. Plastic caps

are used to seal the ends of the tube. The tube is then coiled and packed in the same container in which the thermocouple was received. If the container

is inadequate or damaged, the NIST Shipping and Receiving Unit is requested to provide a suitable shipping container.

3.3.8 Calibration Report

The calibration data (mean emf value determined at each of the four fixed

points) are processed on the laboratory computer to obtain a temperature

versus emf table at one degree intervals for the type S test thermocouple.

This table gives values of the emf in millivolts to the nearest microvolt and may have temperature in either °F or °C, and on either the IPTS-68 or the IPTS-48. Computer processing of the calibration data involves deriving equa- tions to represent the emf - temperature relationship of the test thermocouple.

The processing is broken into three temperature ranges: 0 to 630.74 °C, 630.74 to 1064.43 °C, and 1064.43 to 1450 °C. From 630.74 to 1064.43 °C the temperature, tgg, is defined [4] by the equation:

E = a + btgg + Ctgg^ ^ where E is the emf of a type S standard thermocouple when its measuring

junction is at tgg and its reference junction is at 0 C. The coefficients a, b, and c in the equation are determined by the solution of three simultaneous equations using the values of emf determined at 1064.43 °C (gold point), 961.93 °C (silver point), and 630.74 °C for each test ther- mocouple. The emf -temperature relationship of the type S reference table in NBS Monograph 125 [1] is also represented by a quadratic equation in this temperature range; hence, the difference between the temperature -emf relationship of the test thermocouple and that of the reference table is given

by a quadratic equation. This equation representing the difference is extra- polated to obtain values at higher temperatures, and its value at 630.74 °C is used when determining an equation giving the difference at lower temperatures.

The difference equations, above 1064.43 °C and below 630.74 °C, are then combined algebraically with the polynomials representing the temperature -emf relationship of the type S reference table in Monograph 125 to obtain

polynomials giving the emf-temperature relationship of the test thermocouple

in the 0 to 630.74 °C and 1064.43 to 1450 °C ranges. An illustration of the method used to obtain the emf - temperature equations for

the thermocouple below 630.74 °C and above 1064.43 °C follows. In the 0 to

51
630.74 °C range, the emf deviation (De) of the thermocouple from the reference

table is represented by: De = bo + b^ t + b2 1^ , where bg , b^^ , and bg are computed from the values of De at 0, 419.58 (zinc

point), and 630.74 °C (bg = 0 since De = 0 at t = 0 °C) . The emf of the reference table (Ej^ ^ ) is given by:

Er . T . = t + a2 t^ + ^3 + a^ t"* + a^t^ + t^ , where a^ , a2 ag are from table 2.3.1 in NBS Monograph 125. The emf of

the thermocouple (E-j.^^) given by:

therefore

Et/c = (ai+b^)t + (a2+b2)t2 + t^ + a^ t^ + a^t^ + a^t^ . In the range 1064.43 to 1450 °C, the emf - temperature equation is determined in the following manner. The emf deviation (De) of the thermocouple from the reference table is represented by: De = bg + b^t, where bp and are computed from values of De and dDe/dt at 1064.43 °C, as given by the quadratic De function for the range 630.74 to 1064.43 °C: b^ = dDe/dt and

bo = De - 1064.43 X b^

The emf of the reference table (Ej^ ) is given by:

Er ^ = ao + a^L t + a2 t^ + ag t"' , where ag , a^ , a2 , and a^ are from table 2.3.1 in NBS Monograph 125. The emf of the test thermocouple (Ej^^) given by:

Ej/c = (ao+bg) + (a^+b^)t + a2 1^ + a-^t^ . From the above set of three equations, the temperature versus emf table for

the test thermocouple is calculated. The printed computer output includes the temperature versus emf table, a listing of the measured calibration data, and

the coefficients of the equations that are used to calculate the table. This information is bound with a typed report of calibration which gives the customer's name and location, the customer's designation for the thermocouple,

the test number, the date, and the estimated calibration uncertainty. The information to be kept in the NIST records includes the coefficients of the equations used to calculate the temperature versus emf table, the calibration

data, and a summary of the calculated table. The bound calibration report,

52
along with a covering letter, is sent to the customer. Samples of the above information are given in the appendix in sections 8.1.1.1 and 8.1.1.2.

3.3.9 Apparatus

3.3.9.1 Electrical Annealing System

The thermocouples are electrically annealed for primary calibrations using the same equipment outlined in section 3.1.1.8.

3.3.9.2 Weldine Station

The measuring junctions of the test thermocouples are formed by welding with a small oxygen-gas torch. This is performed at the welding station described in section 3.1.1.8. 3.3.9.3 Annealing Furnace (SiC Heating Element)

The horizontal tube furnace with the tubular silicon carbide heating element

is used to anneal the thermocouples after they are mounted in alumina 4-bore

insulators. The salient features of the furnace are described in section

3.1.1.8 and figure 3.3 gives a sectional view of the furnace. 3.3.9.4 Furnace for Preliminary Tests

The horizontal tube-type furnace having a tubular heating element of Chromel

is used to heat the thermocouples during preliminary tests. This furnace is discussed in section 3.1.1.8 and a sectional view of the furnace is shown in figure 3.2. 3.3.9.5 Freezing-Point Cells

The fixed-point calibrating temperatures are realized in metal freezing-point

cells. The longitudinal section of a typical cell is shown in figure 3.10. The freezing-point metal is contained in a high purity graphite crucible that

is 190 mm long and has a 42.5 mm o.d. and 30 mm i.d. A graphite thermometer

well (170 mm long, 12 mm o.d., and 8.5 mm i.d.) extends through the lid and

into the metal. All graphite parts are obtained from a commercial supplier

who purified the graphite after machining. The crucible contains about

100 cm'^ of metal, and when the metal is melted, the liquid surface is about

14.5 cm above the bottom of the graphite well. The freezing-point metals of

gold, silver, antimony, and zinc used in the preparation of the cells are of very high purity (typically 99.999+% to 99.9999+%).

The glass parts for the gold, silver, and antimony cells are made from high

purity silica glass. For the zinc cell, a borosilicate type glass is used. The gold and silver cells are 570 mm long, the antimony cell 500 mm long, and

the zinc cell 475 mm long. The outer diameter of all the cells is 48 mm and

the inner diameter is 44 mm.

53
The stainless steel cap at the top of the glass cell provides a means of operating the cell in an inert atmosphere to protect the freezing-point metal

and graphite parts from air oxidation. The cap is sealed to the cell with a rubber 0-ring. A thermocouple protecting tube (not shown in fig. 3.10), made

of glass and closed at one end, is inserted into each cell through the thermometer port. The protecting tube (6 mm i.d. and 8 mm o.d.) is sealed in the thermometer port with a rubber 0-ring and a compression nut, and it is held a few millimeters off the bottom of the graphite well. When the freezing-point cell is at an elevated temperature, it is filled with argon to

a pressure slightly above ambient.

The thermocouple protecting tube used in the zinc cell is made of a borosilicate type glass, while high purity silica glass tubes are used in the gold,

silver, and antimony cells. The latter tubes are roughened on the outside

with an air propelled abrasive to lessen heat losses by radiation piping up

the walls of the tubing. The loss of heat by radiation piping, if not effectively minimized, can prevent the thermocouple measuring junction from reaching the temperature of the surroundings, thus causing significant

calibration errors. 3.3.9.6 Furnace for Gold and Silver Freezing-Point Cells

The gold and silver freezing-point cells are used in an electrically heated

vertical tube furnace of the design shown in figure 3.11. The furnace, which

is 35 cm in diameter and 66 cm high, has independently heated end zones to promote temperature uniformity near the furnace center. Each of the three cylindrical heater sections is 20 cm long and is made from commercially

available resistance heaters that have helical shaped Ni-Cr alloy elements supported in groves on the inner walls of semi-cylindrical ceramic forms. Two

of the semi-cylindrical forms are used for each heater section and their

heater elements are connected in parallel. The heater sections are contained

within an Inconel tube that has a 10 cm i.d., and the space between this Inconel tube and the brass shell is filled with Fiberfrax insulation. A

rather widely spaced helical coil of 9.5 mm copper tubing (not shown in

fig. 3.11) is soldered to the outside of the brass shell to permit water

cooling; a comparable provision is used on the top and bottom end plates. An axial Inconel well for the freezing-point cell is centered inside the heaters. A 6-mm diameter Inconel rod welded to the bottom of the well is used

to support the well. Three smaller diameter Inconel tubes (not shown in fig.

3.11) are equally spaced around the outside of the Inconel well for control

thermocouples. Some Fiberfrax insulation is placed in the bottom of the Inconel well to support the freezing-point cell and the level of the insulation is adjusted to locate the bottom of the cell about 5 mm above the

top of the bottom heater. The Inconel well and tubes attached to it are connected to an electrical earth ground and serve to shield the test and control thermocouples from the heaters. Automatic temperature control of the furnace is achieved with type S ther- mocouples connected to conventional control components. The center-zone

control thermocouple, with reference junctions kept at about 22 °C in a water

bath, is connected to a K-3 potentiometer that is set to the desired control

54
point. The unbalance voltage from the potentiometer is amplified with a dc null detector, and the detector output is fed to a 3 -mode electronic

controller that operates a solid-state current regulator connected to the center heater. The top- and bottom-zone control thermocouples are of the differential type - their reference junctions are located near the midpoint of

the center heater and their control junctions are located several centimeters

inside the end zones. They are connected directly to a dc null detector.

Similarly, the outputs of the two detectors go to 3 -mode electronic

controllers that operate solid-state current regulators connected to the top and bottom heaters. Short- time (1 h) control stability of about 0.1 °C is realized with these control components.

3.3.9.7 Furnace for Antimony and Zinc Freezing- Point Cells

The antimony and zinc freezing-point cells are heated in furnaces of the design shown in figure 3.12. Each of the furnaces is 35 cm in diameter and

66 cm high. They use a commercially available heater that has a single

heating element made of Ni-Cr wire and embedded within a ceramic muffle. The heater is positioned in the center of the brass shell by transite collars secured to the top and bottom end-plates. The space between the heater and

the brass shell is filled with Fiberfrax insulation. Copper tubing (not shown in fig. 3.12) is soldered to the outside of the brass shell and end

plates to permit water cooling. A cylindrical liner (not shown in fig. 3.12) formed from 1 mm thick Inconel sheet is fitted snugly inside the ceramic

muffle. A cylindrical copper block (10 cm in diameter and 28 cm long) is used

to promote temperature uniformity near the center of the furnace. The block

is nickel plated to lessen deterioration of the copper by air oxidation. Inconel tubes (about 5 mm o.d.) used to hold a control or monitoring ther- mocouple are placed in two holes drilled through the copper block. The tem- perature of the furnace is controlled automatically. An electronic controller

is used in conjunction with a type N control thermocouple to operate a solid

state current regulator connected to the heater. 3.3.9.8 Ice Baths Thermocouple reference junctions are maintained at 0 °C in ice baths during

the calibration. The ice baths are described in section 3.1.1.8. 3.3.9.9 Measuring Instrument A six-dial precision potentiometer (Guildline model 9160D) is used to measure

the emf of the thermocouples during the freezes. Each dial of the potentiometer is equipped with an auxiliary switch that is interfaced with the laboratory computer so that the dial settings may be recorded by the computer.

This instrument is calibrated in accordance with the procedure outlined in Appendix I of NBS Monograph 126 [3]. The calibration history and accuracy of

this instrument are discussed in section 4.0.

55
3.4 References

[1] Powell, R. L. , Hall, W. J., Hyink, Jr., C. H. , Sparks, L. L. , Burns, G. W. , Scroger, M. G. , Plumb, H. H. , Thermocouple Reference Tables Based on

the IPTS-68, Natl. Bur. Stand. (U.S.) Monogr . 125, 410 pages (1974).

[2] Cutkosky, R. D., An ac resistance thermometer bridge, J. Res. Natl. Bur. Stand. (U.S.) 74C (Engr. & Instr.), Nos. 1 & 2, pp. 15-18 (Jan. -June

1970.

[3] Riddle, John L. , Furukawa, George T. , Plumb, Harmon H., Platinum

Resistance Thermometry . Natl. Bur. Stand. (U.S.), Monogr. 126, 126 pages

(1973)

[4] The International Practical Temperature Scale of 1968, Amended Edition of

1975, Metrologia 12, No. 1, pp. 7-17 (1976).

[5] Corruccini, R. J., Annealing of Platinum for Thermometry, J. Research,

NBS, 47, No. 2, 1951.

[6] Cochrane, J., Relationship of Chemical Composition to the Electric

Properties of Platinum, Temperature . Vol. 4, p. 1619 (Instrument Society

of America, Pittsburgh, PA, 1972).

[7] McLaren, E. H. , Murdock, E. G. , Properties of Some Noble and Base Metal Thermocouples at Fixed Points in the Range 0-1100 °C, Temperature . Vol.

5, Pt. 2, p. 953 (American Institute of Physics, New York, NY, 1982).

56
57
ZD

O.

<

LU

1 CQ LU 1

TA OR

CO

JU 00

o <

< CE

58

60
61
Figure 3.6. Tin bath.

62
Figure 3.7. Tin bath.

63
Figure 3.8. Cryostat.

64
Figure 3.9. Cryostat.

65
CAP

CLASS CELLGRAPHITE OISK-^

GRAPHITE WEaFREEZING-POINT- METAL

THERMOMETER PORT

GAS PORT

GLASS GUlOe

TUBE

INSULATION

4.8—

GRAPHITE LID

-GRAPHITE

CRUCIBLE

19 cm

Figure 3.10. Metal freezing-point cell.

66
BRASS

SHELL

FIBERFRAX

INSULATION

TIE ROD

SILOCEL

BRICK

INCONEL TUBE WELL

Figure 3.11. Freezing point furnace for gold and silver.

67
Figure 3.12. Freezing point furnace for antimony and zinc.

68
4. INTERNAL QUALITY CONTROL

To obtain reliable calibration results, it is necessary to maintain internal standards in the laboratory and to follow routine procedures to check equipment and standards as well as to have some equipment calibrated in other laboratories on a regular basis.

The apparatus involved in our internal quality control system include voltage

standards, voltage measuring instruments, resistance measuring instruments, standard platinum resistance thermometers, calibrated reference thermocouples,

thermocouple check standards, and various thermometric fixed-points. The maintenance and calibrations of these apparatus are discussed in the following

subsections

4 . 1 Voltage Standards

In the NIST thermocouple calibration laboratories since electromotive force is measured, it is essential to maintain dc voltage standards calibrated in terms

of the U.S. legal unit of emf.'' A primary set of four saturated standard

cells housed in a thermostatically controlled enclosure is used for this purpose in the high- temperature thermocouple calibration laboratory. They are calibrated by the NIST Electricity Division at about 4-year intervals, and

their calibration history over the past 28 years is shown in figure 4.1. The emf increase of each cell from mid- 1969 to mid- 1986 is about 10 microvolts

(10 ppm) , while the change in each cell during the past 8 years is about

2 microvolts (2 ppm)

Two additional sets of saturated standard cells are also maintained in separate temperature controlled enclosures and each set consists of two cells. One set, which is used in the low- temperature thermocouple calibration

laboratory where thermocouples are calibrated by comparison with an SPRT, is calibrated periodically by the NIST Electricity Division. The other set of cells is used in the high- temperature thermocoviple calibration laboratory as a back-up. Since mid- 1969, they have been compared with the primary set of four cells just before and just after the primary set of cells is calibrated by the NIST Electricity Division.

The reported calibration uncertainties for the standard cells in the primary

set are 0.78 microvolts and are determined by adding all estimated systematic

errors to the computed random error at the three sigma level. As stated in the report of calibration issued by the NIST Electricity Division, no allowances are made for long-term drift in the cells or for possible effects

of transporting the cells between laboratories. Considering the stability of

the cells during the past 8 years, together with the results of the comparisons made in our laboratory before and after transport, we estimate

that reasonable allowances for the calibration drift during the 4-year

interval between calibrations and for the effect of transporting the cells are

+2 or -1 microvolts and ±1 microvolt, respectively. The linear sum of these

''The U.S. legal unit of voltage is known to be consistent with the SI unit within ±10 ppm at the three sigma level.

69
allowances and the calibration uncertainty gives an overall uncertainty of +3.78 to -2.78 microvolts in our laboratory voltage standard.

4 . 2 Voltage Measuring Instruments

4.2.1 Potentiometers

The various potentiometers used in thermocouple calibrations are calibrated

periodically in terms of the U.S. legal unit of emf. In the high- temperature

thermocouple calibration laboratory the six-dial Guildline potentiometer used

during the primary calibration of type S thermocouples (see sec. 3.3) and for calibrating other emf -measuring instruments is calibrated in our laboratory at approximately 1-year intervals in accordance with the procedure given in appendix I of NBS Monograph 126. This instrument has a resolution of 0.01 microvolts and is standardized with respect to one of the saturated standard

cells in the primary set. The potentiometer dial corrections for emf measure- ments of 10.33, 9.14, 5.55, and 3.44 millivolts (which correspond to the emf

of a type S thermocouple at the gold, silver, antimony, and zinc points)

obtained in calibrations of the instrument over the past 18 years are shown in figure 4.2. The drift in the potentiometer correction between 1978 and 1986 corresponds to about 7 ppm per year of the measured voltage. The random uncertainty in determining the potentiometer correction with respect to the laboratory voltage standard at each of the above four voltages was estimated

from the residual standard deviation of a straight line fitted by the method

of least squares to the values obtained between 1978 and 1986. The estimated

uncertainty, computed at the three sigma level, is 29 ppm of the measured

voltage

The six-dial Rubicon potentiometer presently used when calibrating thermo- couples by comparison with an SPRT was obtained recently from another labora- tory and no long calibration history exists. An initial calibration of the instrument was performed using apparatus obtained from the NIST Electricity

Division. Subsequent calibrations will be made in our laboratory in the same manner used for calibrating the Guildline potentiometer.

4.2.2 Digital Voltmeters

The two Hewlett Packard 3456A digital voltmeters used in the automatic

calibration system are calibrated by comparison with the Guildline six-dial

potentiometer at 2 to 3 week intervals. This calibration is made at one millivolt intervals between 0 and 12 millivolts to check the linearity of the

in.3truments . These digital voltmeters are also compared with a 10-millivolt

constant voltage source during each thermocouple calibration run. The 10- millivolt constant voltage source, calibrated by comparison with the Guildline potentiometer, has been found to have a short- time stability (2 months) of 30 ppm computed at the three sigma level. 4.2.3 Thermocouple Measuring Circuitry

The circuitry connecting the thermocouples to the measuring instruments must

be as free as possible from parasitic thermal emfs . In the thermocouple

calibration laboratory, circuit junction pairs are located physically close

70
together to promote temperature equality and almost the entire length of the copper connecting wires is thermally and electrically shielded. However,

experience shows that the portions of the copper connecting wires located within reference-junction ice baths, as well as the electrical contacts and connections within switch scanners can be troublesome sources of residual emf

in the measuring circuits. Physical inhomogeneities are introduced in the copper connecting wire with extended use and handling due to work hardening of

the copper wire, and small unwanted emfs in the circuits result when the inhomogeneous wires are within the temperature gradient zone of the ice bath. Consequently, all thermocouple measuring circuits are checked frequently for residual emfs by electrically shorting the pairs of copper connecting wires from each selector switch position and from each channel of the switch

scanners in an ice bath and then measuring the circuit voltages with the measuring instrument. By replacing inhomogeneous copper connecting wires, the residual emfs in circuits used in the primary calibration system are kept to less than 0.1 microvolt. In the comparison calibration system, by similar

maintenance of the connecting wire circuits and the proper selection of switch scanner positions, residual emfs are kept to less than 0.3 microvolts.

4.3 Resistance Measuring Instrument During the calibration of thermocouples by comparison with an SPRT, measurements of the thermometer resistance are made with an ac bridge (see sec. 3.2.2.2) operating at 400 hertz. The bridge utilizes an inductive ratio

divider that eliminates the necessity of calibrating the bridge because the initial uncertainty of the divider is about 2 parts in 10® and has been found

to be stable. The proper operation of the resistance measuring system is monitored from measurements made with a set of three SPRT's at the triple

point of water. (See sec. 4.4.2.)

4 . 4 Temperature Measuring Instruments

4.4,1 Working Standard Reference Thermocouples

As mentioned in section 3.1.1, the type S reference thermocouples used as working standards in the comparison calibration system are calibrated

initially at fixed points and then checked periodically for changes in calibration by comparison with other type S thermocouples. Such checks of each working standard are made at 2 to 3 month intervals against customer's

type S standard thermocouples during preliminary tests for fixed- point

calibrations. (See sec. 3,3,5.) In addition, during these preliminary

tests, as well as each time the working standards are used in a comparison

calibration run, they are compared with customer's thermocouples at various

immersions in the furnace. While the latter comparisons are performed to assess the thermoelectric homogeneity of the customer's thermocouple, they

also provide a continual monitor of the thermoelectric quality of the working

standards since many of the customer's thermocouples are made from new, pre- viously unused wire. The above type S thermocouples are used to check the type B reference

thermocouples for changes in calibration. These checks with the type B

71
reference thermocouples are made in the 800 to 1100 °C range and are performed

after about every five times the type B reference thermocouples are used. The specially mounted type S reference thermocouples used in the comparison

calibration of base-metal thermocouples are monitored for calibration changes

by comparing them with other similarly mounted type S reference thermocouples

that have been calibrated at the fixed points but have not yet been used to calibrate base-metal thermocouples.

4.4.2 Standard Platinum Resistance Thermometers A set of three standard platinum resistance thermometers are maintained in the

low- temperature thermocouple calibration laboratory. A history is kept of the values of their resistances that are measured at the triple point of water

each day the thermometer is used. If this measured value deviates by more

than the equivalent of 1 mK from the value determined in the initial

calibration, the particular SPRT, along with the ac bridge, is sent to the NIST Platinum Resistance Thermometry Calibration Laboratory where it is re-calibrated. Experience has shown that the thermometers require calibration

every 2 to 4 years

The uncertainty in the calibration of the standard platinum resistance ther- mometer is given in NIST Monograph 126 as ±0.0001 "C at the triple point of water, ±0.001 °C at the freezing point of tin, and ±0.001 °C at the freezing

point of zinc. From the documentation of the NIST resistance thermometer calibration service, the uncertainty of the interpolated values between the fixed points is ±0.001 °C in the range 0 to 420 °C, increasing to ±0.003 °C at

500 °C. 4 . 5 Platinum Thermoelectric Reference Standard

The platinum thermoelectric reference standard, Pt-67, is maintained with a selected portion of NIST SRM 1967. The standard is in the form of 0.51 mm

diameter wire and has received extensive characterization under the direction

of the NIST Office of Standard Reference Materials. Three samples, each 1 meter long, taken from our reserve supply of SRM 1967 are used in the labora- tory to check the platinum thermoelectric working standards that are used in thermoelectric tests of customer's thermoelements.

4 . 6 Thermocouple Fixed- Point Check Standards A group of six type S thermocouples are used as check standards to monitor the operation of the primary calibration system, as described in section 3.3. Two

of the thermocouples are included with every set of customer's thermocouples

that are given a primary calibration. Each check standard is calibrated about

twice yearly, and data on the oldest of these thermocouples goes back to the year 1969. The calibration histories obtained with each of these check

standards in the gold-point, silver-point, antimony-point, and zinc-point

cells are shown in figures 4.3 to 4.26. Statistical analyses of these data were performed to establish limits to random error for the primary

thermocouple calibration process and are discussed in section 5.0.

72
4. 7 Thermometric Fixed Points

4.7.1 Metal Freezing-Point Cells Freezing-point cells containing zinc, antimony, silver, and gold are used in the primary calibration of type S thermocouples described in section 3.3. The values assigned to the liquidus temperatures of these cells are determined

from measurements made with platinum resistance thermometers. The most recent

tests of the gold-point, silver-point, and antimony-point cells maintained

in the high- temperature thermocouple calibration (HTTC) laboratory were carried out in mid- 1985, using NIST-made, high- temperature platinum

resistance thermometers (RT's No. 8202, 8204, and 8205). These thermometers

have toroidal resistors of nominal resistance at 0 "C of about 0.37 ohm. The design and construction of the thermometers are described in references [1] and [2] and performance of the thermometers is discussed in reference [3]. The gold- and silver-point cells were compared with similar reference cells of

the highest available quality maintained in the High Temperature Resistance

Thermometry (HTRT) laboratory. For the antimony cells, the values of the freezing points on the IPTS-68 were determined by calibrating the thermometers

at the defining fixed points (freezing points of tin and zinc, and the triple

point of water) in the HTRT laboratory.

The thermometric fixed points in the HTRT laboratory include the triple point

of water, and the freezing points of gold, silver, zinc, and tin. The metal

freezing points are contained in sealed glass cells. A description of the

cells, furnaces, and control systems may be found in reference [3]. The ref- erence gold and silver cells used in the HTRT laboratory are designated AU83-1

and AG79-1, respectively. It should be noted that cell AU83-1 is not the same gold cell reported in reference [3] . Cell AU83-1 has a total depth of immer- sion of about 17 cm, and it was found to have a freezing point about 15 mK higher than the cell reported in reference [3]. Because of the demonstrated

exceptional purity of the original metal samples, and because the sealed cell construction has ensured continued purity with use, it is believed that the freezing points of the reference cells are as close to the ideal gold and silver points as it is possible to achieve at the present time and that the departure in the freezing points from perfection probably does not exceed

2 mK. The differences found between the freezing points of two gold-point cells

(AU67-11 and AU85-1) maintained in the HTTC laboratory and the freezing point

of reference cell AU83-1 are given in table 4.1. Table 4.1 also gives the differences found between the freezing points of two HTTC laboratory silver- point cells (AG67-4 and AG65-2) and the freezing point of reference cell

AG79-1. In each case, the freezing point of the HTTC laboratory cell is lower than the freezing point of the reference cell. The freezing points of the two antimony cells (SB69-1 and SB67-1) used in the thermocouple calibration laboratory were calculated on the IPTS-68. All three platinum resistance thermometers met the requirements for standard ther- mometers according to the definition of the IPTS-68. The results are pre- sented in table 4.2. It should be noted that above 630.74 °C the IPTS-68 is not defined in terms of platinum resistance thermometry. However, the de- partures of the values in table 4.2 from IPTS-68 are negligible.

73
Table 4.1. Freezing point differences for gold and silver cells

(Test cell) - (Reference cell)

(Values in degrees Celsius)

RT

No. Test Cell:

Ref. Cell: AU67-11

AU83-1

AU85-1

AU83-1 AG67-4

AG79-1 AG65-2

AG79-1

8202 8204

8205

0.0551

0.0544

0.0497

0.0266 0.0210

0.0189

0.0442

0.0405

0.0426

0.0361 0.0307 0.0290

Mean

Std. Dev.

Std, Dev. of Mean

0.0531

0.0029

0.0017

0.0222

0.0040

0.0023

•0.0424 0.0019

0.0011

0.0319

0.0037

0.0021

Table 4.2. Antimony cell freezing points

(Values in degrees Celsius, IPTS-68) RT No. SB69-1 SB67-1

8202 8204

8205

630.7297

630.7281 630.7202

630.7474

630.7472

630.7405 Mean 630.7260

Std. Dev. 0.0051

Std. Dev. of Mean 0.0029

630.7450

0.0039 0.0023

74
The zinc freezing-point cell presently used in the HTTC laboratory was constructed in 1986 using zinc of very high purity (less than 1 ppm total impurity content) . The freezing point of the cell was determined from measurements made in the NIST platinum resistance thermometry calibration

laboratory with a 25 ohm SPRT and was found to be within 1 mK of the freezing

point of the zinc cell maintained and used in that laboratory for SPRT calibrations

4.7.2 Water Triple Point Cells

As described in section 3.2.3, a triple point of water cell is used during the calibration of thermocouples by comparison with an SPRT. The triple point is realized in a sealed glass cell containing water, ice, and water vapor. The techniques described in section 7.1 of NBS Monograph 126 are followed in the preparation, maintenance, and use of the cell. 4.7.3 Ice Baths During the calibration of thermocouples at NIST, the reference junctions are maintained at 0 °C in an ice bath. The ice bath is made from distilled water

and finely divided ice that is prepared from distilled water. The ice-water

mixture is contained in a large -mouth Dewar flask with an inside depth of

30 cm. It has been found that by following careful preparation techniques to minimize contamination, the ice point temperature of 0 °C can be repeatedly

realized to within 2 mK. Proper maintenance of the ice bath is essential during use. Care is taken to ensure that the reference junction tubes are located properly within the ice- water portion of the bath at all times. Junction tubes are used that are of sufficient length to ensure that the electrical connection between the thermoelement and the copper connecting wire very closely approaches the temperature of its surroundings. An experimental investigation [4] of thermocouple reference junction temperatures in ice baths was conducted at NIST to assess the errors caused by insufficient depth of immersion. The results of this investigation indicate that the immersion errors due to conductive heat flow into the bath along the wires and junction tube can be kept to less than 10 mK by using an immersion depth of 12.5 cm with 0.4 mm

diameter copper wire and 0.51 mm diameter platinum wire and an immersion depth

of 15 cm with 0.4 mm diameter copper wire and type K thermoelements as large

as 3.3 mm in diameter. In addition, the precautions for keeping the tubes

clean, described in section 3.1.1.8, are followed.

75
4.8 References

[1] Bass, N. , NBS Technical Note 1183, part 1 (January 1984).

[2] Evans, J. P., Tillett, S. B. , NBS Technical Note 1183, part 2 (January 1984)

[3] Evans, J. P., Evaluation of Some High-Temperature Platinum Resistance

Thermometers, J. Res. Natl. Bur. Stand. (U.S.) Vol. 89, No. 5, p. 349 (September-October 1984).

[4] Caldwell, F. R. , Temperature of Thermocouple Reference Junctions in an

Ice Bath, J. Res. Natl. Bur. Stand. (U.S.) 69C (Engr. & Instr.) 95-101

(1965).

76
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5. ASSESSMENT OF UNCERTAINTIES

In assessing the uncertainties involved in calibrating thermocouples and

thermoelements, it is necessary to take note of two features of the program.

First, a substantial variety of tests and calibrations are performed, some of which are not routine or repeated and thus defy precise analysis. Second, the recent introduction of a microcomputer into the calibration process has resulted in marked changes in the way in which calibration data are obtained

and analyzed.

Despite these cautions, we can evaluate reasonably well uncertainties

associated with our primary services -- the calibration of type S (Pt-lORh vs

Pt) standard thermocouples by the fixed-point method and by the comparison

method. We wish to acknowledge the very capable and enthusiastic assistance

of M. C. Croarkin of the NIST Statistical Engineering Division in this portion

of the project.

5 . 1 Uncertainties in Primary Calibrations of Type S Thermocouples

The random component of uncertainties associated with primary calibrations at

the fixed points of gold, silver, antimony and zinc are determined from data

on the six thermocouples that serve as check standards in the NIST primary

calibration process. Two of these thermocouples are included with every set

of customer's thermocouples that are given a primary calibration. Each check

standard is calibrated about twice yearly, and data on the oldest of these thermocouples, SC-68-2, goes back to the year 1969. The emfs as measured on each check standard in each fixed-point cell are plotted as a function of the time of measurement in figures 4.3 to 4.26. The apparent drift for some thermocouples, as measured in the gold and silver

freezing-point cells, is attributed to a slight drift in the liquidus temperature of the cells, as well as to change in the emf of the thermocouples

themselves. Cell drift is related to the age of the cell but is not necessarily linear, and changes in the thermocouples are probably related to time spent in the furnace. From the available data, it would be difficult to build a model that is sufficiently descriptive of these phenomena to allow

estimates of the model parameters and the residual standard deviation.

Therefore, we follow a model- free approach based on first differences.

Let Yj^(i=l n) be the measured emf of the thermocouple which has value

/ij (1=1 , . . . ,n) at the ith time period. Thus,

Yi = Mi + i=l, . .

. ,n (1) where the are assumed to be independent random errors which come from a distribution with mean zero and standard deviation.

We construct the (n-1) first differences

Zi = Yi - Yi + i i=l n-1 (2)

103
and compute the standard deviation of these differences by the usual formula

where

= I (Z, - Z.)2 (3) n-2

^

n-1

1 The expected value for s^ is given by the formula

2 f n-1 n-1

^ E(s2) = 2a2 + —- + I /^i' - I (5)

Thus, we can estimate the standard deviation a which is associated with the primary calibration process by substituting estimates from the data for the quantities E(s^) and ; E(s^) is estimated by s^ and is estimated by . Then

n-1 n-1

Estimates of a for the gold, silver, antimony, and zinc cells are shown in table 5.1.

Table 5.1. Standard deviations associated with fixed-point cell calibrations

(values in microvolts)

Thermocouple Au Ag Sb Zn

ID

a DF a DF o DF a DF

SC-68-2* 0,,385 11 0.,266 11 0.681 11 0 .383 11 SC-68-7 (0.,099) (31) (0.247) (30) (0 .206) (30) SC-71-5 0.,484 19 0,,443 19 0.495 19 0 .428 19 SC-71-6 0,,419 21 0,,526 21 0.386 20 0,.409 20 SC-72-1 0,,231 20 0,,307 20 0.540 19 0,,470 19 SC-72-2 (0,,141) (20) (0.476) (19) (0.,221) (19) Pooled 0,,391 71 0,,415 71 0.514 69 0,,430 69

Pooled (all cells) a = 0.439 with 280 degrees of freedom. *Data prior to last anneal deleted.

104
The blanks in table 5.1 represent negative values for the estimate of and

are explained by the fact that eq (5) and (6) depend upon the assumption that Gov ,ej ) =0

If there are covariances between adjacent measurements, then the statistics in table 5.1 approximately estimate

£7^ - a? j instead of

where erf j is the covariance between and Ci + i- Positive covariances and underestimation of a in table 5.1 may be explained by

emf drift exhibited by some of the thermocouples early in their life. Inhomogeneous behavior is expected to affect gold and silver cell determinations to a greater extent than antimony and zinc cell determinations. Thus, we conclude that thermocouples SC-68-7 and SC-72-2, which have negative or small estimates of for gold and silver fixed points, should not be used

for characterizing the measurement process. Their data are excluded from the pooled estimates.

Limits to random error for the primary calibration process are taken to be three times the pooled value over the four fixed point cells or 1.32 microvolts. Note in the analysis that the pooled random uncertainties for the Fixed-Point Check Standard thermocouples show very similar standard deviations

at all four fixed points (0.4 - 0.5 microvolt, corresponding to 0.04 - 0.05 °C) . Total uncertainties are taken as a linear combination of limits to random error plus systematic errors.

In the primary calibration system every effort is made to minimize systematic

errors. Systematic errors cannot be eliminated completely however, and the following attempts to assess known sources of error. These known sources

include: the emf measuring instrument and circuitry, the temperature assigned

to the freezing points, the thermocouple stem losses in freezing point cells, and temperature of the reference junction. As discussed in section 4.2.1, the estimated random uncertainty in determining the potentiometer correction with

respect to the laboratory voltage standard is 29 ppm of the measured voltage.

This random uncertainty is considered to be accounted for in the random error obtained from the above analysis of the check standards. However, we have decided to make allowances to the overall systematic error of +7 ppm of the measured voltage to account for the drift in the potentiometer calibration

between calibrations and +3.8 or -2.8 ppm of the measured voltage to account

for the uncertainty in our laboratory voltage standard. We consider the sum

of these errors, which amounts to +10.8 or -2.8 ppm of the measured voltage,

to be adequate allowance for systematic error caused by the emf measuring

system. As mentioned in section 4.2.3, the measuring circuity is checked

frequently for parasitic emfs and inhomogeneous connecting wires are replaced

when needed to keep these emfs to less than 0.1 microvolt. The parasitic emfs

105
tend to be random from calibration to calibration and are also considered to be part of the random error of the check standard measurements.

The values of temperature assigned to the liquidus points of the freezingpoint cells are derived from measurements made with platinum resistance

thermometers, as described in section 4.7.1. The uncertainties in determining

these values arise from random variations in the measurements, systematic

biases in the measurements, and departure of the reference cell temperature

from perfection. These uncertainties are considered to be systematic errors in

the primary calibration of type S thermocouples. For the gold and silver

freezing-point cells, which were compared with reference cells of the highest

quality, the largest contribution to uncertainty in that comparison was believed to be the random error in the measurements. Because of the electrical isolation of the bridge from ground, systematic biases due to leakage through ground were expected to be negligible. Systematic immersion

losses were known to be small in the reference cells and in the test cells; any immersion losses seen by the resistance thermometer could be expected to represent losses also seen by the thermocouple undergoing calibration. For

these reasons the uncertainties in the values assigned to the freezing points

of the gold and silver cells are based on the random variations in the comparison data given in section 4.7.1. together with an additional allowance

of 2 mK for the departure of the temperature of the reference gold and silver

cells from perfection, and no additional allowance is made for systematic

biases in the measurements.

From table 4.1, the pooled standard deviations of the mean for the measured

differences between the gold and silver cells used in the HTTC laboratory and

the reference cells are ±2 . 0 mK and ±1.7 mK, respectively. We believe that

three standard deviations of the mean plus 2 mK to allow for reference cell imperfection are adequate allowances for systematic error in the assigned

temperatures of the gold and silver freezing-point cells.

In the case of the antimony freezing-point cells we make allowances for the uncertainty in the resistance thermometer calibrations at 630.74 "C and the random variation in the antimony freezing-point temperature measurements

listed in table 4.2 to obtain an estimate of the total systematic error in the antimony freezing-point temperature. The error in the calibration of the resistance thermometer is estimated to be ±1 mK at the freezing points of tin and zinc (see sec. 4.4.2). If these errors in thermometer calibration are propagated according to figure 17 in NBS Monograph 126, they give an estimated

error of ±7 mK in the calibration of the thermometer at 630.74 °C. The pooled

standard deviation of the mean of the antimony freezing-point temperatures

given in table 4.2 is 2.6 mK. We believe that three standard deviations of the measured temperatures (7.8 mK) plus the thermometer calibration error (7 mK)

is adequate allowance for the total systematic error.

In the case of the zinc-point cell, measurements made (see sec. 4.7.1.) with a

25 ohm standard platinum resistance thermometer indicated that the IPTS-68 [1] assigned value of temperature could be used for the liquidus point of this

cell. The uncertainty of this measurement is estimated to be not more than

+1 mK or -2 mK. The uncertainties in the values of temperature assigned

106
to the liquidus points of the freezing-point cells are summarized in table 5.2. In addition to systematic errors associated with determining the liquidus

points of the freezing-point cells, we have decided to include some allowance

for possible changes in the liquidus points with use even though it is recognized that such predictions are prone to considerable uncertainty. One

of the gold-point cells, both of the silver-point cells, and both of the antimony -point cells have been in use for over 15 years, and we make use of their past performance to predict future drift. Considering the manner in which these cells are used, together with their past history, we estimate that changes in the cells during the next 6 to 8 years probably will not exceed

15 mK for the gold and silver cells and 10 mK for the antimony and zinc cells

The loss of heat up the stem of the thermocouple assembly by radiation,

conduction, or convection can prevent the measuring junction of the ther- mocouple from reaching the temperature of its surroundings; thus, errors can

be introduced during calibration. This source of error can be minimized by

providing adequate depth of immersion and by suitable thermocouple design. It has been found [2] from measurements made with fully immersed high temperature

resistance thermometers in freezing-point cells and furnaces nearly identical

to those used in the primary calibration of thermocouples described in section 3.3, that the thermometer resistors were probably within 2 mK of the liquidus point. These measurements were made with thermometers in silica

glass tubes roughened on the outer surface to lessen loss of heat by radiation

piping. The depth of immersion characteristics of thermocouples in the freezing-point cells are measured routinely during primary calibrations, but

small inhomogeneities in the thermocouple wires make it difficult to access

the magnitude of the stem loss errors in the gold and silver freezing-point

cells. However, since the silica glass tubes used with the resistance

thermometers and with thermocouples are of similar size and have had their outer surfaces similarly treated, the stem losses for the two instruments are thought to be roughly the same. We have decided, therefore, to allow -10 mK

for the contribution of stem losses to the estimated uncertainties.

We also have decided, based on information given in section 4.2., to make a small allowance of -10 mK to account for the thermocouple reference junctions

not attaining the temperature of the surroundings due to heat flow into the

ice bath. Any small variations in the ice-point temperature resulting from slight differences in preparation are considered to be reflected in the random

error. To obtain the total uncertainties in microvolts associated with this type of measurement, we must add to the limits to random error our estimates of the related systematic uncertainties. The above allowances for systematic error

that are expressed in millidegrees are converted to microvolts by multiplying

them by the Seebeck coefficient of the type S thermocouple (as taken from NBS Monograph 125) at the indicated temperature. Table 5.3 gives the estimated

systematic uncertainties and the overall uncertainties obtained by simple

addition at the four fixed-point temperatures.

107
The total uncertainties include no allowances for systematic error due to inhomogeneities in the thermocouples or for changes in the emf during subse- quent use. Such uncertainties are applicable when the thermocouples are used

in a systematic manner in the same apparatus and with nearly the same depth of immersion and temperature gradients along the thermocouple wires. However, in general use such accuracy may not be obtainable because of continually changing chemical and physical inhomogeneities in the thermocouple wires in the region of temperature gradients. The effects of quenched- in point defects and

of composition change of the type SP thermoelement (because of preferential

oxidation of rhodium) have been cited [3, 4, and 5] as the primary limitations

in the use of the standard thermocouple. The emf of the thermocouple depends

on its thermal history and in the next section we discuss allowances for in- homogeneities and drift when the standard thermocouples are used in the com- parison calibration furnace which has a much different temperature profile

than the fixed-point calibrating furnaces.

Table 5.2. Uncertainty in the temperature of the liquidus points

Gold Freezing-Point Cell Random (three sigma)

Reference cell imperfection

±6.0 mK

+0.0, -2.0 mK Uncertainty +6.0, -8.0 mK

Silver Freezing-Point Cell Random (three sigma)

Reference cell imperfection

±5.1 mK

+0.0, -2.0 mK Uncertainty +5.1, -7.1 mK

Antimony Freezing-Point Cell Random (three sigma) SPRT calibration

±7.8 mK

±7.0 mK Uncertainty ±14.8 mK

Zinc Freezing-Point Cell Uncertainty +1.0,-2.0 mK

108
Table 5.3. Summary of systematic errors in primary calibrations

of type S thermocouples at the fixed points

(Values in microvolts) Au Ag Sb Zn

emf +0.13 +0.12 +0.11 +0.10 Measuring System -0.03 -0.03 -0.03 -0.03

Temperature of +0.07 +0.05 +0.15 +0.01 Liquidus Point -0.09 -0.07 -0.15 -0.02

Change in Liquidus Point +0.00 +0.00 +0.00 +0.00 With Use -0.18 -0.17 -0.10 -0.10

Stem Losses +0.00 +0.00 +0.00 +0.00

in Thermocouple -0.12 -0.12 -0.10 -0.10

Reference Junction +0.00 +0.00 +0.00 +0.00 Temperature -0.05 -0.05 -0.05 -0.05

Total Systematic Error +0.20 +0.17 +0.26 +0.11

-0.47 -0.43 -0.43 -0.30 Limits to ±1.32 ±1.32 ±1.32 ±1.32 Random Error

Total Uncertainty +1.52 +1.49 +1.58 +1.43

-1.79 -1.75 -1.75 -1.62

109
The total uncertainties of interpolated values in the IPTS-68 defining range 630.74 to 1064.43 "C may be obtained by multiplying the total uncertainties at

the fixed points given in table 5.3 by the appropriate values taken from the curves in figure 5.1 and summing the contributions from the Sb, Ag, and Au

points. By trial we determined the maximum error caused by this propagation

and the values are listed in table 5,4.

Similarly, in the temperature range 0 to 630.74 "C, where the interpolation is accomplished with a quadratic equation fit to the difference between the emf

of the test thermocouple and that from the emf -temperature reference table in NBS Monograph 125, the uncertainties in the interpolated values may be esti- mated from the curves shown in figure 5.2. The maximum errors caused by

propagation of the uncertainties at the zinc and antimony points are also listed in table 5.4. It should be noted that the propagated errors in this temperature range do not include an allowance for possible irregularities in

the reference table.

5 . 2 Uncertainties in Comparison Calibrations of Type S Thermocouples

Whenever a primary calibration at the four fixed points is performed on a customer's thermocouple, that same thermocouple is submitted to a comparison

calibration using a NIST type S reference thermocouple to measure temperature

over the same temperature range covered by the fixed points. A table of emfs

(in millivolts) versus temperature results from each type of calibration. The emf differences between results for the same thermocouple calibrated by both

methods constitute a data base for estimating systematic differences between

the primary and comparison calibration processes and for quantifying the random error in the comparison calibration process. Calibration data obtained with two reference thermocouples (SC-83-7 and

SC-83-8) that were used during the past few years in the automatic calibration

system were examined for this purpose. During a period of about 20 months, SC-83-7 was used in about 46 calibration runs in the 1100 to 0 °C range. SC-83-8 was used in about 35 calibration runs during a period of about

17 months. Some of these calibration runs included type S test thermocouples

that were also given a primary calibration. There were 23 such thermocouples

compared with SC-83-7 and 16 such thermocouples compared with SC-83-8. The emf differences between the primary and comparison calibration values obtained

for the 23 thermocouples compared with SC-83-7 are shown in figure 5.3, and

those for the 16 thermocouples compared with SC-83-8 are shown in figure 5.4. Let and X^^ (i=l,,..,n) represent the emf values obtained with the ith test thermocouple at a given temperature from primary and comparison calibrations,

respectively, using the same reference thermocouple. Let = Yi - Xi i = 1 , . . . , n

110
Table 5.4. Propagated uncertainties from total uncertainties at the four fixed points

Temperature Uncertainty Temperature Uncertainty

degrees Celsius microvolts degrees Celsius microvolts

'-III -0 Vol

-1:11 -0 -l-.r.

350 ^^'^^ 900

-^2.73 -2.09 -2.98 - Vol

961.93 t;;^^

CCA +1.57 1^,, +1.52

550 _^^3 1064.43 _^

600 1100

""^-^^ -1.76 -3.40

630.74 +1.58

-1.75

111
At each temperature , we compute an average

and a standard deviation

^1/2

D.)2| (8)

The computed means and standard deviations are given in table 5.5. A pooled

standard deviation at each temperature is computed as

+ - 2 J

1/2

(9)

with nj^ + n2 - 2 degrees of freedom

where

n^ = sample size for standard SC-83-7

n2 = sample size for standard SC-83-8

s^ = standard deviation for standard SC-83-7

S2 = standard deviation for standard SC-83-8. Functionally, the pooled standard deviation Sp is of the form

(10)

where Sp^.^^, is the standard deviation associated with the primary calibration

process and s^j^^p is the standard deviation associated with the comparison

calibration process. Thus, based on table 5.1 we can compute the standard

deviation for the comparison process.

In order to make the best use of all comparison data, we note that the stand- ard deviations increase with temperature, and we fit the pooled standard devi- ations as a linear function of temperature by the method of least-squares.

The predicted values at the fixed point temperatures are used in the compu- tation discussed above. Again, in order to evaluate uncertainties over the temperature range 100 to 1100 °C, we fit the comparison standard deviations as computed at the four fixed points to a linear function of temperature and predict comparison values

for all temperatures. Computations are shown in table 5.6.

112
Limits to random error for the comparison calibration process are taken to be three times these standard deviations. Uncertainties assigned to values for

the working standards by the primary calibration process propagate into systematic errors for the comparison process. Total uncertainties for the comparison process are taken to be the limits to random error plus systematic

error.

Table 5.5. Means and standard deviations for differences between primary

and comparison calibrations

(values in microvolts)

Temperature Reference No. of Test Standard

°C Thermocouple Thermocouples Mean Deviation

100 CC 0 "J "7 1 1 z J -U . ZZU 0.351

O Art cr* 0 "J "7 Z Jr\ ITT 0.395

cr" Q 1 "7 bO-O J-O 1 Z J -yj . Doo r\ /. o 0

cr" Q 1 "7 0 1 Z Jr» 0 17 r\ c 1 1 U . Di

4iy . Do bL. - o J - /

r\ CO/. A t; o 0 jUU bLi - O J - / z^ 1 1 70 oUU or ft - 7 /.J - 1i . DZZ ROO n ft 0 ft U . oZo

cr ft "5 7 Z J - i . buy f\ c.ir\

vJ . D / U

~l(\t\ or" ft 7 7 0

Z J

1 171 n c/, c 800 SC-83-7 23 -1.208 0.679

900 SC-83-7 23 -1.267 0.708

961.93 SC-83-7 23 -1.406 0.740

1000 SC-83-7 23 -1.542 0.783

1064.43 SC-83-7 23 -1.832 0.923

1100 SC-83-7 23 -2.023 1.041

100 SC-83-8 16 +0.178 0.533

200 SC-83-8 16 +0.129 0.803

300 SC-83-8 16 -0.111 0.874

400 SC-83-8 16 -0.491 0.770

419.58 SC-83-8 16 -0.568 0.735

500 SC-83-8 16 -0.926 0.575

600 SC-83-8 16 -1.324 0.485 630.74 SC-83-8 16 -1 .415 0.534

700 SC-83-8 16 -0.954 0.378

800 SC-83-8 16 -0.514 0.426

900 SC-83-8 16 -0.357 0.609

961.93 SC-83-8 16 -0.395 0.739

1000 SC-83-8 16 -0.481 0.826

1064.43 SC-83-8 16 -0.704 1.002

1100 SC-83-8 16 -0.865 1.116

113
Table 5.6. Standard deviations for comparison calibrations

(values in microvolts)

Temperature Pooled Predicted Comp. Pred Comp

Std Dev DF Std Dev Std Dev Std Dev

100 0,,434 37 0.,461 0.23

200 0,. 595 37 0., 500 0.28

300 0,,647 37 0.,538 ___ 0.33 400 0,,629 37 0.,577 0. 38 419.58 (Zn) 0.,620 37 0.,585 0.387 0.39

500 0,,581 37 0.,616 0.43

600 0.,575 37 0.,655 0.48 630.74 0,.618 37 0.,666 0.501 0.50

700 0,.553 37 0.,693 0.53

800 0,.590 37 0.,732 0.58

900 0,.670 37 0.,771 0.63 961.93 (Ag) 0,.740 37 0.,795 0.663 0.66

1000 0,.801 37 0,,809 0.68 1064.43 (Au) 0,.956 37 0.,834 0.709 0.71

1100 1,.072 37 0,,848 0.73

The only known source of systematic error in the comparison calibration system

for which we make allowance is the uncertainty in the calibration of the reference thermocouple and how well its calibration transfers from the fixed- point calibration system to the comparison calibration system. No allowance

is made for systematic error in the emf measurements. As discussed in section

3.1, the values of emf of each type S test thermocouple and of the type S reference thermocouple are read simultaneously. The emf of each thermocouple

is measured with both digital voltmeters at each calibration temperature. When

the emf difference between a test thermocouple and the reference thermocouple

is analyzed during the data processing, small systematic differences that may

exist between the calibrations of the digital voltmeters will average out. As mentioned in section 4.2.3, the measuring circuits are checked frequently for parasitic emf and by replacing inhomogeneous copper connecting wires and by

proper selection of switch scanner positions the parasitic emfs are kept to less than 0.3 microvolt. They tend to be random from calibration to calibration and are considered to be reflected in the random error associated

with the automatic calibration system. Similarly, variations in the ice-point

reference junction temperature resulting from slight differences in preparation will tend to be random from calibration to calibration and are also considered part of the random error. In addition, any small departure of

the temperature of the thermocouple reference junctions from the ice point

caused by heat conduction into the ice bath would be very nearly the same for

114
the test thermocouples as for the reference thermocouple and thus produce

negligible systematic error in the measurements. We make a small allowance to account for systematic difference found between

calibrations of type S thermocouples performed by the fixed-point calibration

method and the comparison calibration method. Since 1968, we have been

calibrating all thermocouples submitted for calibration by the fixed-point

method by the comparison method as well. Experience has shown that calibrations by the comparison method usually yield slightly higher values of emf than those obtained by the fixed-point method. The systematic differences

shown in table 5.5 for the two reference thermocouples analyzed are typical

for thermocouples annealed by the present procedures. These differences are believed to be largely the result of inhomogeneities in the alloy arms of the thermocouples combined with the different temperature profiles that exist in the comparison and fixed-point calibration furnaces. For a new reference thermocouple calibrated by the fixed-point method and then used at fixed

immersion in the comparison calibration furnace we believe an allowance of approximately 0.5 microvolt at 100 °C increasing to approximately 2 microvolts over the range 400 to 1100 °C is adequate to account for inhomogeneity as well as change in the emf - temperature relationship of the reference thermocouple during the first 100 hours (equivalent to approximately

20 calibration runs) of use in the comparison calibration apparatus. Values

for this allowance over the range from 100 to 1100 °C were obtained by fitting

allowances of -1-0.5, -1-1.8, and +2.0 microvolts at 100, 419.58, and 1064.43 °C respectively with a quadratic function of temperature (constrained to give 0 microvolts at 22 °C) by the method of least-squares. The total uncertainties

in the comparison calibration process are taken as a linear combination of the limits to random error plus the allowances for systematic error and are given

in table 5.7.

115
Table 5.7. Total uncertainties in comparison calibrations of type S thermocouples

(values in microvolts)

Systematic Errors Limits Due to Reference Thermocouple

Temperature to Fixed Point Drift and Total

degrees C Random Error Calibration Inhomogeneity Uncertainty

100

200

300

400

+0.69

-0.69 +0.84

-0.84 +0.99

-0.99

-1.14 1.14

+1.28

-1.35

-1.97 2.10 +2.07

-2.24 +1.59

-1.78

+0.46

-0.00 +0.97

-0.00 +1.40

-0.00 +1.75

-0.00

+2.43

-2.04 +3.78

-2.94 +4.46

-3.23 +4.48

-2.92

419.58 (Zn)

500

+1.17

-1.17 +1.29

-1.29

-1.43 1.62 +1.53

-1.72

-1.81 •0.00 +2.02

-0.00

+4.41

-2.79 +4.84

-3.01

600

630.74

+1.44

-1.44 1.50

1.50

+1.58

-1.76

+ 1.58 -1.75

+2.21

-0.00 +2.25

-0.00

+5.23

-3.20

700

800

900

961.93 (Ag)

1000

1064.43 (Au)

+1.59

-1.59 +1.74

-1.74 +1.89

-1.89 +1.98

-1.98 +2.04

-2.04 +2.13

-2.13

+2.89

-3.09 +3.54

-3.77 +2.73

-2.98 +1.49

-1.75 +1.56

-1.82 +1.52

-1.79

+2.31

-0.00 +2.33

-0.00 +2.27

-0.00 +2.20

-0.00 +2.13

-0.00 +2.00

-0.00

+6.79

-4.68 +7.61

-5.51 +6.89

-4.87 +5.67

-3.73 +5.73

-3.86 +5.65

-3.92

1100 +2.19

-2.19 +3.10

-3.40 +1.99

-0.00 +7.28-5.59

116
5 . 3 References

[1] The International Practical Temperature Scale of 1968, Amended Edition of

1975, Metrologia 12, No. 1, 7-17 (1976).

[2] Evans, J. P., Wood, S. D., An Intercomparison of High Temperature

Platinum Resistance Thermometers and Standard Thermocouples, Metrologia

Vol. 7, No. 3, 108-130 (1971).

[3] McLaren, E. H. , Murdock, E. G. , New Considerations on the Preparations,

Properties, and Limitations of the Standard Thermocouple for Thermometry,

in Temperature: Its Measurement and Control in Science and Industry,

Vol. 4, Part 3, pp. 1543-1560 (Instrument Society of America, Pittsburgh,

PA., 1972).

[4] McLaren, E. H. , Murdock, E. G. , Properties of Some Noble and Base Metal Thermocouples at Fixed Points in the Range 0 - 1100 °C, Temperature,

Vol. 5, Part 2, pp. 953-975 (American Institute of Physics, New York,

NY, 1982).

[5] Bentley, R. E. , Jones, T. P., Inhomogeneities in Type S Thermocouples

When Used to 1064 "C, High Temperatures - High Pressures, Vol. 12, pp. 33-45 (1980).

117
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121
6. FUTURE DIRECTIONS

We foresee marked changes in the international standard in temperature as the IPTS-68 is replaced by a new scale sometime during the next 5 years. Despite this scale modification, we expect that thermocouple thermometers will

continue to be used as working standards in a great variety of applications;

therefore, it will be necessary for NIST to continue to offer a range of thermocouple calibration services. We will continue, therefore, to update and expand the present calibration services to meet the needs of our customers.

We note that special, non-routine requests for special thermocouple tests, such as emf stability testing of thermocouples and thermocouple devices, have

grown during the past 2 years. We intend to modify existing equipment when

practical and develop new techniques as needed in order to satisfy as many

future requests of this nature as possible.

The future of the NIST thermocouple calibration laboratory thus can be outlined as follows:

o Any new temperature scale definition must be realized in

this laboratory over the whole calibration range;

o A new radiation thermometer reference standard should be installed to bring the quality of calibrations above 1000 °C into

line with the user community's needs;

o A more comprehensive check- standard program should be implemented to take advantage of our recently developed computer-based data acquisition system. New programmable controllers have been acquired to control the furnaces used

for calibrating thermocouples by fixed-point and comparison methods. They

will be put into service as soon as the final hardware is installed and testing is completed.

122
7.1

7 . BIBLIOGRAPHY

National Bureau of Standards Publications

1. Powell, R. L. , Hall, W. J., Hyink, Jr., C. H. , Sparks, L. L. , Burns, G. W. , Scroger, M. G. , and Plumb, H. H. , Thermocouple

Reference Tables Based on the IPTS-68, Natl. Bur. Stand. (U.S.)

Monogr. 125, 410 pages (1974).

2. "Accurate Thermocouple Thermometry", Guildner, L. A., and Burns,

G. W. , High Temperatures - High Pressures . Vol. 11, p. 173 (1979)

3. "Methods of Testing Thermocouples and Thermocouple Materials",

Roeser, W. R. , and Lonberger, S. T. , Natl. Bur. Stand. (U.S.)

Circular 590, 13 pages (1958).

4. Burley, N. A., Powell, R. L. , Burns, G. W. , and Scroger, M. G., "The Nicrosil versus Nisil Thermocouple: Properties and Thermoelectric Reference Data", Natl. Bur. Stand. (U.S.) Monogr.

161, 167 pages (1978).

5. Dahl, A. J., "The Stability of Base-metal Thermocouples in Air from 800 to 2200 °F", in Temperature. It's Measurement and Control in Science and Industry . Vol. 2 (Reinhold Publishing

Corp., New York, NY 1941).

6. Burns, G. W. and Hurst, W. S., "Studies of the Performance of W- Re Type Thermocouples", in Temperature. Its Measurement and Control in Science and Industry . Vol. 4, Part 3 (Instrument

Society of America, Pittsburgh, PA, 1972), p. 1751.

7. Burns, G. W. , Hurst, W. S., and Scroger, M. G. , "High Reliability

Sheathed, Beryllia Insulated, Tungsten-Rhenium Alloy Thermocouple

Assemblies - Their Fabrication and emf Stability", Report No. NASA CR-134549, NBSIR 74-447 (1974).

7 . 2 American Society for Testing and Materials Publications

1. ASTM Standard E230-87, Temperature-Electromotive Force (EMF) tables for Thermocouples, 1988 Annual Book of ASTM Standards,

Vol. 14.03, p. 101 (American Society for Testing and Materials,

Philadelphia, PA, 1988).

2. ASTM E220-86, Standard Method for Calibration of Thermocouples by

Comparison Techniques, 1988 Annual Book of ASTM Standards

Vol. 14.03, p. 90, (American Society for Testing and Materials,

Philadelphia, PA, 1988).

3 . Manual on the Use of Thermocouples in Temperature Measurement . ASTM STP 470B, (American Society for Testing and Materials,

Philadelphia, PA, 1981).

123
7.2 American Society for Testing and Materials Publications (cont'd)

4. Pollock, D. D., The Theory and Properties of Thermocouple

Elements . ASTM STP 492, (Omega Press, Ithaca, NY, 1979).

5. Pollock, D. D. , Thermoelectricity: Theory Thermometry Tool . ASTM

STP 852, (American Society for Testing and Materials,

Philadelphia, PA, 1985).

6. ASTM Standard E452-83, Standard Method for Calibration of Refractory Metal Thermocouples Using an Optical Pyrometer, 1988 Annual Book of ASTM Standard, Vol. 14.03, p. 241 (American

Society for Testing and Materials , Philadelphia, PA, 1988).

7 . 3 Instrument Society of America Publications

1. American National Standard, Temperature Measurement

Thermocouples, ANSI-MC96 . 1-1982 (Instrument Society of America,

Research Triangle Park, NC , 1982).

2. Kerlin, T. W. , and Shepard, R. L. , Industrial Temperature Measurement . ISBN 0-87664-622-4 (Instrument Society of America,

Research Triangle Park, NC , 1982).

7 .4 International Electrotechnical Commission Publications

1. International Electrotechnical Commission Standard,

Thermocouples, Part 1: Reference tables, lEC Publication 584-1 (Bureau Central de la Commission Electrotechnique Internationale,

Geneve, 1977).

2. International Electrotechnical Commission Standard,

Thermocouples, Part 2: Tolerances, lEC Publication 584-2 (Bureau Central de la Commission Electrotechnique

Internationale, Geneva, 1982).

7 .5 Other Publications

1. Thermocouple Temperature Measurement . Kinzie, P. A., John Wiley

and Sons, Inc., New York, NY (1973).

2. Barnard, R. D., Thermoelectricity in Metals and Alloys . (Halsted

Press, John Wiley and Sons, Inc., New York, NY, 1972).

3. MacDonald, D. K. C, Thermoelectricity: An Introduction to the Principles . (John Wiley and Sons, Inc., New York, NY, 1962).

4. Walker, B. E., Ewing, C. T. , and Miller, R. P., "Thermoelectric

Instability of Some Noble Metal Thermocouples at High

Temperatures, Rev. Sci. Instrum. 33, 1029-1040 (1962).

124
7.5 Other Publications (Cont'd)

5. Burley, N. A., and Acklund, R. G., "The Stability of Thermo- emf/Temperature Characteristics of Nickel-base Thermocouples", J. of Australian Inst, of Metals, 12 (1967).

6. Burley, N. A., "Solute Depletion and Thermo-emf Drift in Nickel- base Thermocouple Alloys", J. Inst. Metals 97, 252-254 (1969).

7. Potts, Jr., J. F. , and McElroy, D. L. , "The Effects of Cold

Working, Heat Treatment, and Oxidation on the Thermal emf of Nickel-base Thermoelements", in Temperature. Its Measurement and Control in Science and Industry . Vol. 3, Part 2, p. 243,

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

8. Darling, A. S., and Selman, G. L. , "Some Effects of Environment

on the Performance of Noble Metal Thermocouples", in Temperature

Its Measurement and Control in Science and Industry . Vol. 4, Part 3, p. 1633, (Instrument Society of America, Pittsburgh, PA, 1972)

9. Glawe, G. E., and Szaniszlo, A. J., "Long-Term Drift of Some Noble- and Refractory-Metal Thermocouples at 1600 K in Air,

Argon and Vacuum" , in Temperature. Its Measurement and Control in Science and Industry . Vol. 4, Part 3, p. 1645, (Instrument

Society of America, Pittsburgh, PA, 1972).

10. McLaren, E. H., and Murdock, E. 0., "Properties of Some Noble and

Base Metal Thermocouples at Fixed Points in the Range 0 - 1100

°C" , in Temperature. Its Measurement and Control in Sciences and Industry . Vol. 5, p. 953, (American Institute of Physics, New

York, NY, 1982).

11. Anderson, R. L. , Lyons, J. D., Kollie, T. G., Christie, W. H., and Eby, R. , "Decalibration of Sheathed Thermocouples", in Temperature

Its Measurement and Control in Science and Industry . Vol. 5, p. 977, (American Institute of Physics, New

York, NY 1982)

12. Wang, T. P., and Starr, C. D., "Oxidation Resistance and Stability of Nicrosil-Nisil in Air and in Reducing Atmosphere",

in Temperature. Its Measurement and Control in Science and Industry . Vol. 5, p. 1147, (American Institute of Physics, New

York, NY, 1982).

13. Burley, N. A., Cocking, J. L. , Burns, G. W. , and Scroger, M. G., "The Nicrosil versus Nisil Thermocouple: The Influence of Magnesium on the Thermoelectric Stability and Oxidation

Resistance of the Alloys", in Temperature. Its Measurement and Control in Science and Industry . Vol. 5, p. 1129, (American

Institute of Physics, New York, NY, 1982).

125
8. APPENDICES

8 . 1 Calibration Reports A variety of reports are issued for thermocouple calibrations depending upon

the type of thermocouple or thermoelement, the type of calibration, the temperature range, and the temperature scale (IPTS-68 or IPTS-48, "C or °F) . The five calibration reports listed in the following subsections are samples

of the reports issued most frequently by the thermocouple calibration

laboratory

8.1.1 Type S Thermocouple - Fixed-Point Calibration

8.1.1.1 Covering Letter and Bound Calibration Report for Customer

126
UNITED STATES DEPARTMENT OF COMMEPCE

National InaCituCa of Standarda and Technology

[fornnarly National Bureau of Sfeandarda]

GaiChersburg, Maryland 20899

January 10, 1989

In reply refer to: 586/888765

WWXX

Hometown, U. S. A.

Attn: Jane Doe / purchasing agent

Subject : Thermometric Test

Order No: XX5556

Enclosed are results of the test which you requested in the above reference.

Please refer to the above file number in any later communication, and if you

have any questions concerning this test, contact Margaret G. Scroger,

telephone number (301) 975-4818. Sincerely.

B. W. Mangum, Group Leader Thermometry Research & Calibration Group

Temperature and Pressure Division

Material tested:

1 Thermocouple

Enclosure:

1 Report of Calibration

127
DRITED STATES DEPARTHEIT OF COHMERCE ATIOBAL IRSTITOTE OF STAIDASDS AID TECHMOLOGT

GAITHERSB0R6. HARTLAID REPORT OF CALIBRATION

TYPE S THERMOCOUPLE

Submitted by

WWXX Corporation

Hometown, U. S. A. The emf of the thermocouple was determined at 1064.43 °C (gold point), 961.93 °C (silver point), 630.74 °C, and 419.58 °C (zinc point) with the reference junctions at 0 "C. The values of emf found are given in Table 1. The uncertainties in these values are estimated not to exceed 2 microvolts.

Table 2 gives values of emf from 0 °C to 1450 °C. The uncertainties in the values given are estimated not to exceed 3 microvolts in the range 0 ^C to 1100 °C and then increase to not more than 20 microvolts at 1450 °C. These uncertainties are discussed in National Bureau of Standards Circular 590, Methods of Testing Thermocouples and Thermocouple Materials. Table 3 gives the coefficients of the equations that were used to compute the values given in Table

2. Each equation is valid only within the specified temperature range. All temperatures in this report are given in degrees Celsius(IPTS-68) . The International Practical Temperature Scale of 1968, IPTS-68, was adopted by the International Committee of Weights and Measures at its meeting in October 1968, and is described in "The International Practical Temperature Scale of 1968, Amended Edition of 1975", Metrologia 12, No. 1, 7-17 (1976). The calibration of a thermocouple is subject to change during use. The magnitude

of the change depends upon such factors as the temperature, the length of time, and the conditions under which it is used. Factors affecting the stability of platinum- rhodium versus platinum thermocouples are discussed in Thermocouple and Radiation Thermometry Above 900 °K, Proceedings of Symposium on Measurement of Thermal Radiation Properties of Solids (1963). (Reprints are available from NBS, Temperature and Pressure Division, Gaithersburg, MD 20899). The thermocouple was electrically annealed in air before testing. For the Director National Institute of Standards and Technology

B. W. Mangum, Group Leader Thermometry Research & Calibration Group

Temperature and Pressure Division

P.O. No. XX5556

Test No. 888765

Date : January 10, 1989

128
JANUARY 10, 1989 NIST TEST NO. 888765

TABLE 1 TEMPERATURE IN DEGREES CELSIUS ( IPTS-68

EMF IN MICROVOLTS

REFERENCE JUNCTIONS AT 0 DEGREES CELSIUS

TEMPERATURE EMF

1064.43 (Gold Point) 10316.7

961.93 (Silver Point) 9134.4

630.74 5544.9

419.58 (Zinc Point) 3443.2

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135
8.1,1.2 Summary of Calibration Results for NIST Record

136
uEChMBER 28, 19SS

FREEZING POINT CALIBRATION AT GOLD

388765 TIME

1 10317 65 10 10 29 9

2 10317 65 10 11 01 2 3 10317 66 10 11 30 6

4 10317 66 10 11 59 8

5 10317 66 10 12 29 6 6 10317 68 10 15 30 1 7 10317 .69 10 16 00 7 8 10317 68 10 16 31 2 9 10317 .68 10 17 :00 .2 10 10317 68 10 17 30 1 VALUES 1 to 10 AVERAGE 10317.669

POT CORRECTION -1.150

CELL CORRECTION 0.260

CORR AVERAGE 10316.7 8

137
DECEMBER 28, 1988

FREEZING POINT CALIBRATION AT AU

838765 TIME

1 10317

. 54 11 :20 :30 . 2

2 10317

, 55 11;21;:00,. 1 3 10317

.55 11 :: 21 :29 .9

4 10317., 57 11 ::22;:00,.3

5 10317

.57 11: 22 :30 .1

6 10317

,59 11

:25;:31 ,,6

7 10317.60 11 :26 :00 . 1 8 10317

,60 11;26;:30,. 5 9 10317

.61 11:27 :00 .5 10 10317

,61 11: 27 ;:30,. 2 VALUES 1 to 10 AVERAGE 10317.579

POT CORRECTION -1.150

CELL CORRECTION 0.260

CORR AVERAGE 10316.69

CM EMF

ABOVE

BOTTOM 242301

1 10317.68

2 10317.71

4 10317.63

6 10317.45

IMMERSION CHECK

Microvolts

TIME DELTA

11:30:02 0.10

11:32:31 0.13

11:35:01 0.05

11:37:32 -0.13

138
JANUARY 3, 19 89 FREEZING POINT CALIBRATION AT SILVER

888765 TIME

1 9135 .07 13 ;; 51:00,. 5 2 9135 .07 13 : 51 :29 .7 3 9135 .07 13;; 52 ;:00,.0 4 9135 .07 13::52 :29 .8

5 9135 .08 13;; 52 ;; 59 ,.7 6 9135 .08 13;:56::00 . 1 7 9135 .08 13;;56;;30,.0 8 9135 .08 13;:57;:00 .0

9 9135 .08 13:;57 :;29.,9 10 9135 .08 13;:58:: 00 . 1

VALUES 1 to 10 AVERAGE 9135.076

POT CORRECTION -1.020

CELL CORRECTION 0.360

CORR AVERAGE 9134.42

139
JANUARY 3 . 19 89 FREEZING POINT CALIBRATION AT AG

888765 TIME

1 9135.12 14:53:00.4

2 9135.12 14:53:30.1

3 9135.12 14:53:59.9

4 . 9135.12 14:54:29.8

5 9135.12 14:54:59.5

6 9135.12 14:57:59.7

7 9135.12 14:58:29.7

8 9135.12 14:58:59.8

9 9135.12 14:59:30.2

10 9135.12 15:00:00.0

VALUES 1 to 10 AVERAGE 9135.120

POT CORRECTION -1.020

CELL CORRECTION 0.360

CORR AVERAGE 9134.46

IMMERSION CHECK

CM EMF, Microvolts

ABOVE

BOTTOM 24 2 301 TIME DELTA

1 9135.15 15:02:57 0.03

2 9134.98 15:05:31 -0.14

4 - 9134.74 15:08:01 -0.38

6 9134.71 15:10:38 -0.41

140
JANUARY 5, 19 89 FREEZING POINT CALIBRATION AT ANTIMONY

8887(55 TIME

1 5545 ,.57 9:;04;:30,.6 2 5545 .57 9;:05::00 .3

3 5545 ,.57 9:;05::30..0 4 5545 .57 9 ;:06::00 .3

5 5545 ,, 57 9 :;06:;30.,0 6 5545 ,.57 9;:09:: 29 .9

7 5545 ., 57 9::10:;01 ., 2 8 5545 ,. 57 9;: 10;:29 . 8

9 5545.,57 9 :: 10:;59.,8 10 5545,.57 9;; 11 :29,. 1 VALUES 1 to 10 AVERAGE 5545.570

POT CORRECTION -0.590

CELL CORRECTION -0.050

CORR AVERAGE 5544.93

141
JANUARY 5, 19 89 FREEZING POINT CALIBRATION AT

3SS765 TIME

1 5545.51 15:34:59.6

2 5545.51 15:35:30.1

3 5545.51 15:36:00.1

4 5545.51 15:36:30.0

5 5545.52 15:37:00.2

6 5545.50 15:39:59.4

7 5545.50 15:40:31.1

8 5545.50 15:41:00.4

9 5545.50 15:41:30.2

10 5545.51 15:42:00.0

VALUES 1 to 10 AVERAGE 554 5.507

POT CORRECTION -0.590

CELL CORRECTION -0.050

CORR AVERAGE 5544.87

142
JANUARY 9, 19 89

FREEZING POINT CALIBRATION AT ZN4

888 76 5 TIME

1 3443 . 54 12 : 26 :30 .3

2 3443,.54 12::27;:00.,4 3 3443 .54 12 : 27 :30,.4 4 3443,, 54 12 ;: 28;:00.,0 5 3443 . 54 12 :28 :30,.0 6 3443.,56 12 ;;34;;36 . 8 7 3443,.56 12;:34 :59,.8 8 3443.,56 12:; 35 :;29. 7 9 3443,.56 12;:36;:00.,3 10 3443 .,56 12 ::36;; 31

4 VALUES 1 to 10 AVERAGE 3443.5 50

POT CORRECTION -0.350

CELL CORRECTION 0.000

CORR AVERAGE 3443.20

143
JANUARY 9, 19 89 FREEZING POINT CALIBRATION AT ZN5

888765 TIME

1 3443 .59 17 : 00 :00. 3

2 3443 ,. 59 17: 00;;30. 1 3 3443 . 59 17 : 00 :59. 6 4 3443 ,, 59 17: 01

;30. 5 5 3443 .59 17 : 01 :59. 5 6 3443 ,.60 17 : 05;;00. 2 7 3443 .60 17: 05 :29. 8 8 3443,.60 17 : 06;:00. 3

9 3443 .60 17: 06 :30. 5 10 3443 ,. 59 17 : 07 ;:00. 7

VALUES 1 to 10 AVERAGE 3443.594

POT CORRECTION -0.3 50 CELL CORRECTION 0.000

CORR AVERAGE 3443.24

144
JANUARY 10, 1989

FOR TEST FOLDER

NIST TEST NO. 888765

{ 'AT COMPUTER'

TABLE 1 TEMPERATURE IN DEGREES CELSIUS ( IPTS-68

EMF IN MICROVOLTS

REFERENCE JUNCTIONS AT 0 DEGREES CELSIUS

TEMPERATURE EMF Table - Test

1064.43 (Gold Point) 10316.7 +17.6

961.93 (Silver Point) 9134.4 +13.8

630.74 5544.9 +-7.2 419.58 (Zinc Point) 3443.2 +4.7

Data book PTC-29

Page 150

DEGREES C TABLE GIVEN

145
( 'AT COMPUTER'

THERMOCOUPLE CALIBRATION JANUARY 10, 1989

NIST TEST NUMBER 888765 IPTS- ; 68 FIXED POINT EMF VALUES IN MICROVCOLTS

1064.43 (gold point) - 10316.74 961.93 (silver point) - 9134.44

630.74 - 5544.90 419.58 (zinc point) - 3443.22

EMF(table) - EMF(TC)

microvolts

+ 17 . 6 + 13.8

+ 7.2 +4.7 COEFFICIENTS FOR EMF EQUATIONS

Temperature Range 0 TO 630.74 630.74 TO 1064.43 1064.43 TO 1450.0

0 . 0 + 5 . 3891567E + 00

+ 1 . 2518195E-02

-2 . 2448218E-05

+2 . S452165E-08

-2 . 2440580E-11

+ 8 . 5054170E-15

-3 . 1710621E + 02 +8.2811519E+00

+1.6055796E-03

+1 . 3028746E+03

+3 .4559370E+00

+ 6 . 3824649E-03

-1 . 5722425E-06

IPTS CRITERIA

1) E(AU) -10334 . 0 = -17.26

2) E(Au)-E(Ag)-1186-.17(E(Au)-10334) = -0.765800000001

3) E(Au)-E(630.74)-4782-.63(E(Au)-10334) = 0.7138

EMF of the platinum wire vs Pt-67 at 1100 C in microvolts was 1.9 THIS THERMOCOUPLE DOES NOT MEET THE CONDITIONS FOR A STANDARD THERMOCOUPLE

CRITERIA 1) IS GREATER THAN 30 CRITERIA 2) IS GREATER THAN 3 CRITERIA 3) IS GREATER THAN 5 0 0 . 0 + 0 . 0 100 644 .,3 + 1 .. 1 200 1437 . 9 + 2 . 1 300 2319 ,,4 + 3 .. 3 400 3255 .3 + 4 .4 500 4228.,0 + 5 ., 6 600 5230 . 5 + 5 . 8 700 6266 ..4 + 7 ., 8 800 7335 .4 + 9

c 900 8436..4 + 11 ., 9 1000 9569 .6 + 15 . 1 1100 10734

.5 + 19 .,0 1200 11923 . 9 + 23 . 1 1300 13127

, 7 + 27 . 1400 14336 . 6 + 31 .4 1450 14939

. 9 + 33 .,4 DEGREES C TABLE GIVEN

146
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147
8.1.2 Type S Thermocouple - Comparison Calibration

8,1.2.1 Covering Letter and Bound Calibration Report for Customer

148
UNITED STATES DEPARTMENT OF COMMERCE

National Institute of Standards and Technology

(formerly National Bureau of Standards]

Gaicnersburg, Maryland 20B99 January 10, 1989

In reply refer to: 586/999888A

XYZ Corporation

Hometown, U. S. A.

Attn: Jane Doe / purchasing agent

Subject : The rraome trie Test

Order No: 12345

Enclosed are results of the test which you requested in the above reference. Please refer to the above file number in any later communication, and if you

have any questions concerning this test, contact Margaret G. Scroger,

telephone number (301) 975-4818. Sincerely,

B. W. Mangum, Group Leader Thermometry Research & Calibration Group

Temperature and Pressure Division

Material tested:

1 Thermocouple

Enclosure

1 Report of Calibration

149
OHITED STATES DEPARTMEHT OF COMMERCE

HATIOHAL IHSTITUTE OF STAHDARDS AHD TECHHOLOGY GAITHERSBURG , MARYLAHD REPORT OF CALIBRATION

TYPE S THERMOCOUPLE

(Tagged: ZZZ999) Submitted by XYZ Corporation

Hometown, U.S.A. The thermocouple was calibrated by comparison with a standard type S thermocouple in the range 0° to 1100 °C. Values above 1100 °C were obtained by

extrapolation. The calibration procedure is described in Section 4.1 of NBS Circular 590, Methods of Testing Thermocouples and Thermocouple Materials. The thermocouple was electrically annealed in air before testing. Table 1 gives corresponding values of the eraf of the thermocouple in millivolts

and the temperature of its measuring junction in °C(IFTS-68) when the reference

junctions are at 0 °C. The uncertainties in the values given in the table are estimated not to exceed the equivalent of 0.5 degree in the range 0° to 1100 °C and then increase to not more than the equivalent of 2 degrees at 1450 °C. These uncertainties are discussed in National Bureau of Standards Circular 590. Table

2 gives the coefficients of the equations that were used to compute the values

given in table 1. Each equation is valid only within the specified temperature

range

All temperatures in this report are given in degrees Celsius (IPTS-68) . The International Practical Temperature Scale of 1968, IPTS-68, was adopted by the International Committee of Weights and Measures at its meeting in October 1968, and is described in "The International Practical Temperature Scale of 1968 Amended Edition of 1975", Metrologia 12, No. 1, 7-17 (1976). The calibration of a thermocouple may change during use. The magnitude of the change depends upon such factors as the temperature, the length of time, and the conditions under which it is used. Factors affecting the stability of platinum- rhodium vs. platinum thermocouples are discussed in Thermocouple and Radiation Thermometry Above 900 °K, Proceedings of Symposium on Measurement of Thermal Radiation Properties of Solids (1963). Reprints are available from NBS, Temperature and Pressure Division, Gaithersburg, MD 20899. For the Director National Institute of Standards and Technology

B. W. Mangum, Group Leader Thermometry Research & Calibration Group

Temperature and Pressure Division

P.O. No. 12345 Test No. 999888A

Date : January 10, 1989 150
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156
8.1.2.2 Summary of Calibration Results for NIST Record

157
TEST THERMOCOUPLE NUMBER 999888A

COEFFICIENTS FOR EMF EQUATIONS FOR STANDARD THERMOCOUPLE SC-83-13

Temperature range 0 to 630.74 630.74 to 1064.43 1064.43 to 1450 A = B = C = D = E = F = G =

+0 . OOOOOOOE+00

+5 . 3893631E+00

2544059E-02

2448218E-05

8452165E-08

2440585E-11

5054170E-15

+ 1

-2

+ 2 , -2

+ 8 ,

-3 , + 8

+ 1 . 1875966E+02

2906905E+00

6208047E-03

+1 . 2839709E+03

+3.4978878E+00

+6.3824649E-03

-1.5722425E-06

JANUARY 10, 1989 TYPE S THERMOCOUPLE NIST TEST NO. 999888A

TEMPERATURE

DEG C STANDARDSC-83-13

TEST 999888A DELTA

(TEST - STD)

76.,612 477 ,. 343 478.,631 1

, 288

77 ,.020 480 . 187 479 ,.731 -0,.456

91.,034 579 .456 579 .,669 0..213

91.341 581 .663 582 . 100 0,.437

110.,937 725 ,.562 725.,294 -0., 268

Ill . 243 727 .850 728,.087 0 . 237

136.,647 922 ,.300 922.,256 -0.,044 137 ,.047 925 .425 924 .994 -0 .431 166 .,836 1163 ,325 1163 ,,119 -0.,206 167 ,.311 1167 . 194 1166 .756 -0 .438 200 .,677 1444 ,.656 1444 ., 188 -0.,468 201

.098 1448 .225 1447 .662 -0 . 563

236.,645 1754 ,425 1754 .,056 -0.,369 237 .159 1758 .919 1757 .950 -0 .969

274.,869 2093 ,.790 2092 ,,516 -1

,274 275 .340 2098 .026 2096,.950 -1 .076

312,,280 2434 ,.460 2433 .,416 -1.,044 312 .739 2438 . 686 2437 ,. 560 -1

. 126 349 .,301 2778,.610 2776 ,, 506 -2 ., 104 349 .724 2782 . 586 2781 .340 -1

. 246

158
385 .818 3123 ,. 980 3121 .74b -2 . ^J4

386 . 300 3128 . 566 3126 . 240 -2 . 326

421 . 330 3464 ,.750 3462 . 376 -2 .374

421 .755 3468 .846 3466 . 040 -2 .806

458 . 144 3822 .. 570 3819 . 976 -2 . 594

458 . 593 3826 . 966 3824 . 680 -2 . 286

495 ,.478 4189 ,.796 4186 . 674 -3 ,.123

495 .867 4193 . 644 4190 . 256 _ o

. 387

531.272 4545..756 4542 ,. 024 _ o .733

531 . 682 4549 . 854 4546 . 166 -3 . 688

566 ,. 522 4899 ,.976 4895 ,.744 -4 ,. 233

566 .923 4904 .034 4899 .806 -4 . 227

599 ,.061 5230 ,.326 5225 .424 -4 ,. 903

599 . 434 5234 . 134 5229 . 596 -4 . 538

631 ,. 947 5567 .,796 5562 .604 -5 ,. 193

632 . 256 5570,.984 5565 .636 -5 .348

663 .. 576 5896 .,426 5891 ,.094 -5 ,, 333

663 . 936 5900 ,. 194 5893 .886 -6 .308

697 .,641 6254 .,010 6247 ,. 540 -6 ,. 470

697 . 963 6257 ,.410 6251 .360 -6 . 050

731 ., 594 6614 ., 160 6607 ,, 360 -6 ,, 800

731 .887 6617 ,.280 6610 .760 -6 . 520

764 ,,647 6968.,350 6960 ,710 -7 ,,640

764 . 907 6971.150 6963 .940 -7 . 210

797 ,, 511 7324 .,030 7316 ,, 190 -7 ,, 840

797 .752 7326 ,.650 7319 .360 -7 . 290

828,, 179 7659 ., 090 7651 ,, 130 -7 ., 960

828 .403 7661. 550 7653 . 960 -7 ,. 590

859 .,778 8007 .,520 7998,,400 -9 ., 120

859 . 999 8009 ,. 960 8001 ,.490 -8 ,.470

893 ., 134 8378 .,824 8369 ,,424 -9 ., 400

893 . 345 8381 ,. 184 8371 ,. 654 -9 ,. 530

922.,780 8711

,854 8702 ..444 -9 .,410

922 .942 8713 ,. 684 8704 ,. 544 -9 ,. 140

952 ., 887 9052 . 984 9043 .,354 -9 .,630

953 . 157 9056 ,.054 9045 ,.874 -10 ,. 180

986 .,397 9436 . 124 9425 ., 444 -10 . 680

986 ,. 569 9438,.094 9427 ,.434 -10 ,.660

1011 ., 841 9729 . 474 9719 . 024 -10 . 450

1012 ,. 005 9731 ,, 374 9720 ,, 444 -10 ,. 930

1035 ., 874 10008

504 9997 . 254 -11

250

1035 ,.999 10009

,954 9998 .,704 -11 .,250

1062 . 721 10322

410 10311

251 -11 . 159

1062 ,. 846 10323

. 881 10311

,760 -12 .,121

1078.,479 10507 . 750 10496

041 -11 . 709

1078 ,. 702 10510

,381 10498

,450 -11 .. 931

159
TEMPERATURE

76 .6118

77 . 0200

91 . 0343

91 . 3410

110 .9373

111 . 2430

136.6473

137 . 0471

166 . 8361

167 .3106 200.6770

201 . 0984

236 .6454

237 . 1589

274 . 8693

275.3399

312 . 2800

312 .7389 349.3006

349 . 7244

385 .8184 386 . 2996

421 . 3305

421 .7545 458 . 1438

458 . 5933

495 .4782 495 . 8671

531 . 2723

531 .6821

566 . 5216

566 .9232 599.0609

599 .4340

DELTA EMF

+ 1 . 2878

-0 .4558

+0 .2127 +0.4373

-0.2683

+0.2373

-0.0443

-0.4307

-0.2063

-0 .4377 -0.4682

-0. 5627

-0.3693

-0 . 9687

-1 . 2737 -1.0763

-1.0437

-1 . 1263

-2 . 1037

-1 . 2463

-2.2337

-2 .3263 -2.3737

-2 . 8063

-2. 5937

-2 . 2863

-3 . 1225

-3 .3875 -3.7325

-3.6875

-4 . 2325

-4 .2274

-4 . 9025

-4 . 5376

CALC DELTA

+0 . 1853

+0. 1847

+0 .1556

+0 . 1548

+0 . 0896

+0 . 0884

-0.0318

-0.0339

-0 . 2153

-0. 2184

-0.4589

-0.4622

-0.7468

-0 .7510

-1.0714

-1 . 0755

-1.3988

-1.4028

-1 .7284 -1.7322

-2 .0606

-2 . 0650

-2 . 3961

-2.4002

-2 .7679

-2 .7726

-3 . 1863

-3 . 1909

-3.6468

-3 .6525

-4 . 1805

-4 .1872

-4,. 7674

-4 .7748

DEVIATION WEIGHTS

+1.1025 1 -0.6405 1 +0.0571 1 +0.2825 1 -0.3579 1 +0.1489 1 -0.0125 1 -0.3968 1 +0.0090 1 -0.2193 1 -0.0093 1 -0.1005 1 +0.3775 1 -0.2177 1 -0.2023 1 -0.0008 1 +0.3551 1 +0.2765 1 -0.3753 1 +0.4859 1 -0.1731 1 -0.2613 1 +0.0224 1 -0.4061 1 +0.1742 1 +0.4863 1 +0.0638 1 -0.1966 1 -0.0857 1 -0.0350 1 -0.0520 1 -0.0402 1 -0.1351 1 +0.2372 1

COEFFICIENTS FOR DELTA EQUATION

0 TO 630.74 deg C COEFFICIENT STD. DEV

A = B = C = D =

+ 7 .0567866E-03

-7 .0901970E-05

+ 1 . 4385087E-07

-1 . 1240066E-10

+ 3 . 1108755E-03

+ 2 . 8337852E-05

+7 . 9296834E-08

+ 6 . 8894828E-11

STANDARD DEVIATION = 0.34 3

160
p mmimmmmmimmi ------ f-7777?????TTTTT2222;:nnn^'^ III

! I?!

ss2

1

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1? ii2§Eiiii|Sij^iii;i|||ii5i2^ii^ !^ :^ .

161
JANUARY 10, 1989 TYPE S THERMOCOUPLE ... AUTO CALIBRATION NIST TEST NO. 999888A

(AT' COMPUTER) CALCULATED FIXED POINT EMF VALUES IN MICROVOLTS EMF( TABLE) - EMF(TC) MICROVOLTS

1064 .43 961.93

630.74 419.58

(GOLD POINT) - 10330.83

(SILVER POINT) - 9145.97

- 5549.87

(ZINC POINT) - 3445.48

+ 3

+ 2 + 2 + 2

52

24 COEFFICIENTS FOR EMF EQUATIONS

TEMPERATURE RANGE 0 . 0 TO 630.74 630.74 TO 1064.43 1064.43 TO 1450.0

IPTS CRITERIA

1) E(AU)-10334. = -3.46986678822

2) E(Au)-E(Ag)-1186-.17(E(Au)-10334) = -0.54 89 50378131

3) E(Au)-E(630.74)-4782-.63(E(Au)-10334) = 1.14 583423497

THIS THERMOCOUPLE DOES NOT MEET THE CONDITION S FOR A STANDARD THERMOCOUPLE IF; CRITERIA 1) IS GREATER THAN 30 CRITERIA 2) IS GREATER THAN 3 CRITERIA 3) IS GREATER THAN 5 Pt leg VS. Pt-67 = -2.2uV AT 1105.80 deg C

A = B = C = D = E = F = G =

+ 0 . OOOOOOOE + 00

+ 5 . 3964199E + 00

+ 1 . 2473157E-02

-2 . 2304367E-05 +2.8339764E-08

-2.2440585E-11

+ 8 . 5054170E-15

-3 . 1737366E + 02

+ 8 . 2819167E + 00

+1 .6175332E-03

+1 . 2890636E+03 +3.4821494E+00

+6.3824649E-03

-1 . 5722425E-06

0 0.0 + 0 . 0

+ 0.7

+ 1.5

+ 2.2

+ 2.4

+ 2.3

+ 2.2

+ 1.7

+ 1.5

+ 1.8

+ 2.6

+ 4.0

+ 5 . 5 + 7.0

+ 8 . 5 + 9.2

100 200

300 400

500 600

700

800

900 1000

1100

1200

1300

1400

1450

644 .7 1438.

2320.5

3257 .3 4231 . 3 5235 . 1 6272.6

7343 .4 8446 .6 9582 .

10749.6

11941 .6 13148 .

14359. 5 14964 . 1 DEGREES C TABLE GIVEN

162
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163
8.1.3 Type R Thermocouple - Comparison Calibration

8,1.3.1 Covering Letter and Bound Calibration Report for Customer

164
>rt$o»'' Gaithersburg. Mary'and aOBSS

UNITED STATES DEPARTMENT OF COMMEPCE

National Ina1:l«ut:a of Standards and Tachnology

[formarly National Bureau of Standards]

January 10, 1989

In reply refer to: 586/999999

AAA Company

Any Town, Any State

Attn: John Doe / purchasing agent

Subject : The rmome trie Test Order No: AB123

Enclosed are results of the test which you requested in the above reference. Please refer to the above file number in any later communication, and if you

have any questions concerning this test, contact Margaret G. Scroger,

telephone number (301) 975-4818. Sincerely,

B. W. Mangum, Group Leader Thermometry Research & Calibration Group

Temperature and Pressure Division

Material tested:

1 Thermocouple

Enclosure

1 Report of Calibration

165
DHITED STATES DEPARTMEHT OF COMMERCE

HATIOHAL INSTITUTE OF STANDARDS AND TECHROLOGT GAITHERSBDRG , HARTLAHD REPORT OF CALIBRATION

TYPE R THERMOCOUPLE

(Tagged: AAAlll) Submitted by

AAA Company

Any Tovm, Any State

The thermocouple was calibrated by comparison with a standard type S thermocouple in the range 0° to 1100 °C. Values above 1100 °C were obtained by

extrapolation. The calibration procedure is described in Section 4.1 of NBS Circular 590, Methods of Testing Thermocouples and Thermocouple Materials. The thermocouple was electrically annealed in air before testing. Table 1 gives corresponding values of the emf of the thermocouple in millivolts

and the temperature of its measuring junction in °C (IPTS-68) when the reference

junctions are at 0 °C. The uncertainties in the values given in the table are estimated not to exceed the equivalent of 0.5 degree in the range 0° to 1100 °C and then increase to not more than the equivalent of 2 degrees at 1450 °C. These uncertainties are discussed in National Bureau of Standards Circular 590. Table

2 gives the coefficients of the equations that were used to compute the values

given in table 1. Each equation is valid only within the specified temperature

range

All temperatures in this report are given in degrees Celsius (IPTS-68). The International Practical Temperature Scale of 1968, IPTS-68, was adopted by the International Committee of Weights and Measures at its meeting in October, 1968, and is described in "The International Practical Temperature Scale of 1968 Amended Edition of 1975," Metrologia 12, No. 1, 7-17 (1976). The calibration of a thermocouple may change during use. The magnitude of the change depends upon such factors as the temperature, the length of time, and the conditions under which it is used. Factors affecting the stability of platinum- rhodium thermocouples are discussed in Thermocouple and Radiation

Thermometry Above 900° K, Proceedings of Symposium on Measurement of Thermal Radiation Properties of Solids (1963). Reprints are available from NBS, Temperature and Pressure Division, Gaithersburg, Maryland 20899. For the Director

National Institute of Standards and Technology

P.O. No. AB123 Test No. 999999

Date : January 10, 1989

B. W. Mangum, Group Leader Thermometrey Research & Calibration Group

Temperature and Pressure Division

166
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171
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172
8.1.3.2 Sununary of Calibration Results for NIST Record

173
JANUARY 10, 1989 999999

( AT' COMPUTER)

TEMPERATURE STANDARD TEST

M-125

TABLE DELTA

deg C SC-83-13 999999 VALUE (TEST - Ml: 60 . 374 366 ., 52 363 ,, 96 365 ,. 20 -1 . 24 60 . 811 369 . 44 367 . 14 368 . 14 -1 .,00

79.679 498 . 78 496 ,. 89 498 , 94 -2 . 04 80 . 235 502 . 68 501., 38 502 ., 89 -1 .,51

105 .426 684 . 53 685 ,, 26 688 ,. 04 -2 .79 106 ,078 689 . 36 691., 02 692 ., 98 -1

, 96 137 .484 928 .,84 9 3 5,. 4 5 938 ,. 7 2 -3 . 27 138 . 297 935. 21 942 ., 26 94 5., 28 -3,.02 176 .112 1239 . 36 1256 ,. 61 1260 ,. 14 -3 5 3 177 .034 1246 . 96 1263 ., 98 1268 ., 05 -4..08 216 .155 1576 .,71 1609 ,.01 1612 ,. 36 -3 . 35

217 . 162 1585. 37 1617 ., 40 1621 ., 44 -4 ,.04 255 . 891 1924 ., 10 1974 ,. 41 1978 . 06 -3 . 65

256 . 896 1933 . 02 1982 .,73 1987 ,,49 -4 ,. 77 297 .001 2294 .,40 2367 ,. 14 2370 . 87 -3 .73 298 . 035 2303 . 84 2376 ., 34 2380 ., 93 -4 ,. 59 339 . 130 2683 .,43 2783 ,. 17 2786 . 54 -3 . 37 340 . 272 2694 . 09 2793 .,61 2797 ., 98 -4 ,. 37 381 . 744 3085 ., 19 3215 .47 3218 . 76 _ 2 . 29 382 .749 3094 . 74 3225 ., 00 3229 ,, 07 -4 ,.08 423 .229 3483 ., 10 3646 ,. 27 3649 .61 -3 . 35

424 . 176 3492 . 25 3656 ., 03 3659 ,. 55 -3 ,. 53 462 .006 3860 .,37 4057 . 38 4060 .60 -3 .22 463 . 025 3870 . 35 4068 ,, 57 4071 ,, 50 -2 .. 93 500 .219 4236 .,73 4470 . 25 4472 . 94 -2 .69 501 .112 4245 . 58 4479 .,61 4482 ., 66 -3 ,. 05 535 . 634 4589 .,39 4858 ,. 04 4861 .36 -3 .32 536 .539 4598. 45 4868 ,,03 4871 .,36 -3 ,.33 567 .491 4909 .,77 5212 . 97 5215 . 79 -2 . 82 568 . 136 4916 . 29 5219 ,,71 5223 .,01 -3..31 600 .517 5245 ., 19 5585 ,. 13 5588 . 13 -3 .01 601 .378 5253 . 98 5594 .,79 5597 ., 90 -3 ,. 12 640.480 5656 ., 14 6043 ,.35 6045 ,. 29 -1 . 94 641 .487 5666 . 57 6054 ., 17 6056 .,92 i.

.75 679.079 6058 .,70 6493 . 32 6494 . 59 -1 . 27 679.812 6066. 39 6501 ,,75 6503 ., 20 -1

.45 716 . 167 6450.,06 6933 . 53 6933 . 16 + 0 . 37 716 .918 6458. 03 6941 ,, 19 6942 ., 12 -0,. 93 750.053 5811 ., 53 7340 .48 7 3 3 9 . 67 f 0 . 81 750 . 654 6817 . 97 7346 ., 1 L / ; 4 6 ., 93 -0 .. 82

174
782 . 881 7165 . 26 7739 ,. 13 / B8 . 70 + 0 .43 784 . 278 7180 .39 7755 ., 76 / 755 ., 80 -0 ., 05

814 . 239 7506 .42 8125 ,.39 8124 .63 + 0,.77 814 . 680 7511 .23 8129..65 8130 .,08 -0.,44

852 . 245 7924 .17 8598 .87 8598 . 50 + 0,.37 852 . 747 7929 .71 8605 .,37 8604 .,81 + 0., 56 886. 623 8306 .07 9032 ,.98 9032 . 88 + 0 .11 887 . 081 8311 .17 9038.,99 9038.,69 + 0 ,, 29 917 . 943 8657 .33 9432 ,.90 9433 . 29 -0 . 39

918. 283 8661 .15 9437 ,,68 9437 .,66 + 0 ., 01 943. 968 8951 .62 9769 . 61 9769 . 37 + 0 . 25 944. 342 8955 .86 9774 .,55 9774 .,22 + 0 .,32

969 . 988 9248 .06 10107

.69 10108 .41 -0 .71 970. 238 9250 .91 10111

,05 10111,68 -0.,63

1009 . 205 9698 .99 10624,.47 10625.02 -0 ,.55 1009 . 635 9703 .96 10629

,36 10630.,73 -1

37

1035. 249 10001 .22 10971

.48 10971.81 -0 ,. 33

1035. 490 10004 .03 10973

,36 10975.,04 -168

1059. 217 10281 .31 11293

. 28 11293. 55 -0 ,. 27 1059 . 516 10284 .81 11295

,89 11297

, 57 -1

68

1077 . 050 10490 .87 11534

. 26 11534.48 -0 ,. 22 1077. 202 10492 .66 11534

,08 1153654 -2 . 46

1086. 955 10607 .63 11668,. 38 11668

.81 -0 ,, 43

1087 . 116 10609 .53 11668

, 24 11671 . 00 -2. 76

1088. 284 10623 .32 11687

.38 11686.87 + 0,.51 1088. 477 10625 .59 11686

,97 11689. 48 -2 . 52 1099. 369 10754 .31 11836,. 10 11837

.66 -1 ,. 56

1099. 289 10753 .36 11834

,41 11836

57 -2 . 17

175
JANUARY 10, 1989 TYPE R THERMOCOUPLE NIST TEST NO. 999999

(^AT' COMPUTER)

TEMPERATURE DELTA EMF CALC DELTA DEVIATION WEIGHTS

60 ,,374 -1 ,, 239 -1 .461 + 0 ,. 222 1 60 .,811 -1 ., 000 -1 ., 471 + 0 ., 471 1 79 ..679 -2 ,. 043 -1 . 882 -0 ,. 161 1 80 ., 234 -1

, 510 -1 ., 894 + 0 ., 384 1 105 .. 425 -2 ,. 786 -2 .398 -0 . 388 1 106 ., 077 -1

,960 -2 .,410 + 0 ., 450 1 137 .. 484 -3 .274 -2 .955 -0 .319 1 138 .,297 -3 .,018 -2 ,,967 -0 .,051 1 176 ,.111 -3 ,.532 -3 . 486 -0 .046 1 177 ., 033 -4 ,, 076 -3 .,497 -0 .,579 1 216 ,.155 -3 .346 -3 .867 + 0 .521 1 217 .,162 -4 .,036 -3 .,874 -0 .,162 1 255 ,, 891 -3 ,.650 -4 .071 + 0 .421 1 256 ., 895 -4 .,769 -4 ,, 074 -0 ,,695 1 297 ,, 000 -3 .. 730 -4 . 114 + 0 . 384 1 298 ., 035 -4 ., 594 -4 .,113 -0 ,,481 1 339 ,.129 -3 .. 370 -4 . 002 + 0 . 632 1 340 ,, 272 -4 .,373 -3 .. 997 -0

, 376 1 381 . 744 -3 ,. 287 -3 .769 + 0 . 482 1 382 .,748 -4 ., 079 -3 ,, 762 -0 ,,317 1 423 .229 -3 ,. 350 -3 .477 + 0 . 127 1 424 ., 175 -3 ,,526 _ -5 .471 -0,,055 1 462 .006 -3 ,. 221 -3 . 204 -0 .017 1 463 .,025 -2 , 932 -3 ,. 197 + 0 ,, 265 1 500 .219 -2 .690 -2 . 994 + 0 . 304 1 501 ,, 112 -3.,051 -2,,990 -0.,061 1 535 .633 -3 .323 -2 .913 -0 .410 1 536 ., 538 -3.,334 -2 ,,913 -0.,421 1 567 .490 -2,.825 -2 .984 + 0,.159 1 568.,135 -3.,309 -2 ,, 988 -0 .,321 1 600 .516 -3..010 -3 .258 + 0 ,. 248 1 601 ,,377 -3 .,118 -3 ,, 269 + 0 .,151 1

COEFFICIENTS FOR DELTA EQUATION

0 TO 630.74 Deg C COEFFICIENT STD. DEV

B = C = D = E =-2.5416699E-02

+1 . 2217191E-05

+1.4371083E-07

-1.8087732E-10

+ 2 . 3264567E-07

+ 1 .6263324E-11

+8 . 1748810E-16

+3.9510814E-20

STANDARD DEVIATION = 0.386

176
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H

17.7
COEFFICIENTS FOR DELTA EQUATION

630.74 TO 1064.43 deg C

DE-DE{630.74) = STD

FUNCTION (T-630.74) DEV DE = FUNCTION (T)

A = -4 .8622222E+01

B = +3 . 2334952E-02 +2 . 7482624E-07 +1 . 0967249E-01

C = -6.1306990E-05 +3 . 7740216E-11 -6 . 1306990E-05

STANDARD DEVIATION = 0.924

CALACULATE4D DELTA AT 630.74 = -3.73733378834 Microvolts

CALCULATED DELTA AT 1064.43 = -1.24 503740324 Microvolts

SLOPE OF DELTA AT 1064.43 = -0.0208415053908 Microvolts / deg C

178
JANUARY 10, 1989 TYPE R THERMOCOUPLE NIST TEST NO. 999999

(AT COMPUTER) Degrees C table given

TABLE 2 COEFFICIENTS USED TO GENERATE TABLE 1 TEMP RANGE — 0 TO 630.74 630.74 TO 1064.43 1064.43 TO 1450.0 A = 0 . OOOOOOOE + 00 -3 . 1270229E-01 +1 . 5111096E + 00

B = +5 . 2637228E-03 +8 . 1565406E-03 +2 . 8431452E-03

C = +1 . 3923327E-05 +2 . 9279224E-06 +8 . 082363 lE-06

D = -2 . 3861528E-08 -2 . 6876059E- 10 -1 . 9338478E-09

E = +3 . 6020533E-11

F = -4 .4645019E-14

G = +3 . 8497692E-17

H = -1.5372642E-20

The equations are of the form:

EE = A + BT + CT**2 + DT**3 + ET*-*4 + FT**5 -i- GT**6 + HT**7

Where EE is in millivolts and T is in degrees Celsius (IPTS-68).

TEMP. CALC. DELTA TEMP. CALC . DELTA

deg C (test - table) deg C (test - table)

0., 0 fO., 00 750 . 0 -0,,75 50 . 0 -1

. 22 800 ,.0 -0 .02 100.. 0 -2 ,, 29 850 .,0 + 0,,41

150 .0 -3 ,. 14 900 . 0 + 0 .52 200.,0 -3 .,73 950.,0 + 0.,34

250 .0 -4 ,.05 1000 .0 -0 .16 300..0 -4., 11 1050. 0 -0,.96 350 .0 -3 .95 1100 .0 -1 .99 400.,0 -3.,64 1150., 0 -3,.03 450 .0 -3 . 28 1200 .0 -4 .07 500,,0 -3 ,,00 1250.,0 -5,, 11 550 .0 -2 .92 1300 . 0 -6 .15 600,.0 -3,, 25 1350,,0 -7 ,. 20 650 .0 -3 . 14 1400 .0 -8 .24 700,.0 -1

.79 1450,.0 -9,. 28 Platinum leg versus Pt-67 = 15.6 microvolts at 1099.3 degrees Celsuis.

179
Type T Thermocouple - Comparison with SPRT

180
UNITED STATES DEPARTMENT OF COMMERCE

Nacional Inatsituta of Standards and Technology

(formerly National Bureau of Standards]

GaiChersburg, Maryland 20B99

Date: January 10, 1989 In reply refer to: 586/999999

National Institute of Standards and Technology

Gaithersburg, Maryland 20899

Attn: Mr. John Smith

Subject: Thermometric Test

P.O. No: 99999 Gentlemen: Enclosed are results of the test which you requested in the above reference. Please refer to the above file number in any later communication, and if you

have any questions concerning this test, contact Jacquelyn A. Wise, telephone number (301) 975-4822. For questions relating to shipping please call Robert

J. Lewis, telephone number (301) 975-6353. Sincerely yours,

B. W. Mangum, Physicist

Temperature and Pressure Division

Material tested:

1 Thermocouple

Enclosure

1 Report of Calibration

2 Tables

181
DHITED STATES DEFARTMERT OF COMMERCE ATIOBAL IISTITUTE OF STARDARDS AID TECHROLOGT

GAITHERSBORG. MARTLARD REPORT OF CALIBRATION

THERMOCOUPLE

Type T

Submitted by- National Institute of Standards and Technology

Gaithersburg, Maryland

Electromotive Force as a Function of Temperature of Measuring Junction (reference junctions at 0 C) Degrees Celsius

(IPTS-68) Microvolts

Degrees Celsius aPTS-68) Microvolts

-110

-50

0

-3620

-1802

0

100 200

300

4266

9257 14816

The uncertainty in the above values is estimated not to exceed the equivalent of

-0.1 C, A discussion of uncertainties inherent in thermocouple calibrations is given in National Bureau of Standards Ciricular 590, Methods of Testing

Thermocouples and Termocouple Materials. All temperatures in this report are given in degrees Celsius (IPTS-68). The International Practical Temperature Scale of 1968, IPTS-68 was adopted by the International Committee of Weights and Measures at its meeting in October, 1968, and is described in "The International Practical Temperature Scale of 1968, Amended Edition of 1975", Metrologia 12, No. 1, 7-17 (1976). For the Director

National Institute of Standards and Technology

B. W. Mangum, Physicist

Temperature and Pressure Division

Test No. : 999999

Completed: January 10, 1989

P. 0. No. : 99999

182
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8.1.5 Base Metal Thermocouple - Comparison Calibration

186
UNITED STATES DEPARTMENT OF COMMERCE

National lnaeitu«a of Standards and Technology

(fornnarly National Bureau of Standards]

GaiChersburg. Maryland 20S39 January 10, 1989

In reply refer to: 586/998877

LMNOP Company

Your Town, Your State

Attn: John Doe / purchasing agent

Subject : Thermometric Test

Order No: RRA123

Enclosed are results of the test which you requested in the above reference. Please refer to the above file number in any later communication, and if you

have any questions concerning this test, contact Margaret G. Scroger,

telephone number (301) 975-4818. Sincerely,

B. W. Mangum, Group Leader Thermometry Research & Calibration Group

Temperature and Pressure Division

Material tested:

1 Thermocouple

Enclosure

1 Report of Calibration

187
URITED STATES DEPARTMEHT OF COMMERCE ATIORAL IRSTITUTE OF STAHDARDS AID TECHIOLOGT

GAITHERSBORG, MARTLARD REPORT OF CALIBRATION

TYPE K THERMOCOUPLE

Submitted by

LMNOP Company

Your Town, Your State Electromotive Force versus Temperature of Measuring Junction

(reference junction at 0 °C) Degrees Celsius

(IPTS-68) Millivolts

Degrees Celsius

(IPTS-68) Millivolts

0 0.00 4.10

8.12 12.18

14.25

16.35

18.49 20.63

550 600

650

700 750 800

900

1000

22.78 24.93

27.06

29.16

31.25

33.32

37.38 41.33

100 200

300 350 400

450

500 The uncertainties in the above values are estimated not to exceed the equivalent

of 1 degree C. These uncertainties are discussed in National Bureau of Standards Circular 590, Methods of Testing Thermocouples and Thermocouple Materials.

All temperatures in this report are given in degrees Celsius (IPTS-68). The International Practical Temperature Scale of 1968, IPTS-68, was adopted by the International Committee of Weights and Measures at its meeting in October 1968, and is described in "The International Practical Temperature Scale of 1968, Amended Edition of 1975", Metrologia 12, No. 1, 7-17 (1976). The calibration of a thermocouple may change during use. The magnitude of the change depends upon such factors as the temperature, the length of time, and the conditions under which it is used. The stability and reliability of iron versus constantan and Chromel versus Alumel thermocouples are discussed in The Stability

of Base-Metal Thermocouples in Air from 800 to 2200 °F, J. Res. NBS 24, 250

(1940) RP1278. Copies are available on request from the Temperature and Pressure

Division, Gaithersburg, Maryland 20899. For the Director National Institute of Standards and Technology

B. W. Mangum, Group Leader Thermometry Research & Calibration Services Temperature and Pressure Division

P.O. No. Test No. Date :RRA123

998877 January 10, 1989 188
NBS-n4A (REV. 2.ec) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR REPORT NO. M T r*T / f no c r\ oc NIST/SP2dO-35

2. Performing Organ. Report No. 3. Publication Date Apri 1 1989

4. TITLE AND SUBTITLE NIST Measurement Services: The Calibration of Thermocouples and Thermocouple Materials

5. AUTHOR(S)

G. W. Burns and M. G. Scroger

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions; NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMENT OF COMMERCE

QAITHERSBURQ, MD 20899

7. Contract/Grant No.

S. Type of Report & Period Covered Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. Stote, ZIP) Same as Item #6

10. SUPPLEMENTARY NOTES Library of Congress Catalog Card Number: 89-600732

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant bibliography or literature survey, mention it here) This document describes the calibration services for thermocouples and thermocouple materials presently provided by the NIST at temperatures from -210

to 2100 °C. Three general calibration methods used at NIST, which provide

traceability to the International Practical Temperature Scale of 1968, are outlined in detail. Consideration is given primarily to the calibration of type S (platinum- 10% rhodium versus platinum) thermocouples at defining fixed

points of the IPTS-68 as standard instruments, and to the calibration of other letter-designated type thermocouples (types B, R, S, E, J, K, N, and T) by

comparison with a calibrated reference thermocouple and by comparison with a standard platinum resistance thermometer. The procedures followed to maintain

internal quality control of apparatus and standards in the NIST thermocouple

calibration laboratories are covered, and statistical assessments of the uncertainties involved in the calibrations of type S thermocouples by the fixed-point method and by the comparison method are presented.

12. KEY WORDS fS/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons) Calibration; calibration furnaces; calibration uncertainties; SPRT; standards

laboratory; temperature measurement; thermocouples; thermometric fixed points;

thermometry.

13. AVAILABILITY

Unlimited

I I For Official Distribution. Do Not Release to NTIS

["Yl Order From Superintendent of Documents, U.S. Government Printing Office, Washington. D.C.

20402.

QX] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

200

•fr U.S. GOVERNMENT PRINTING OFFICE: 1989-242-311/04034 USCOMM-DC «043-PS0



THE SP 250 SERIES ON NIST MEASUREMENT SERVICES^

SP 250-1 Spectral Radiance Calibrations PB871 79883 SP 250-2 Far Ultraviolet Detector Standards PB87227609 SP 250-3 Radiometric Standards in the Vacuurr)

Ultraviolet PB87227625 SP 250-4 Fricke Dosimetry in High-Energy Electron Beams

SN003-003-02816-9 $2.75 SP 250-5 Aipha-Partlcle Calibrations SN003-003-02823-1 $2.00 SP 250-6 Regular Spectral Transmittance PB881 08550 SP 250-7 Radiance Temperature Calibrations SN003-003-02827-4 $2.25 SP 250-8 Spectral Reflectance PB88 109905 SP 250-9 Calibration of Beta-Particle-Emitting Ophthalmic Applicators SN003-003-02817-7 $2.00 SP 250-10 Radioactivity Calibrations wWh the "4tt" Gamma ionization Chamber and Other

Radioactivity Calibration Capabilities SN003-003-02824-O $2.25 SP 250-11 SP 250-12

SP 250-13

Dosimetry for High Dose Applications PB88201587

Neutron Personnel Dosimetry SN003-003-028 1 1 -8 $2.50

Activation Foil Irradiation with Californium

Fission Sources SN003-003-02866-5 $2.25 SP 250-14 Activation Foil irradiation by Reactor Cavity

Fission Sources SN003-003-02861-4 $3.25 SP 250-15 Photometric Calibrations PB881 53747 SP 250-16 Calibration of X-Ray and Gamma-Ray

Measuring Instruments SN003-003-02862-2 $7.00 SP 250-17 The NBS Photodetector Spectral Response

Calibration Transfer Program PB88201595

SP 250-19 Calibration of Gamma- Ray-Emitting Brachytherapy Sources SN003-003-O2923-8 SP 250-20 Spectral Irradiance Calibrations SN003-003-02829-1 $5.50 SP 250-21 Calibration of Beta-Partlde

Radiation Instnjmentatlon PB88201579 SP 250-22 Platinum Resistance Thermometer

Calibrations PB88138367 SP 250-23 Liquid-in-G lass Thermometer

Calibration Service SN003-003-O2891-6 $6.00 SP 250-24 Standard Cell Calibrations PB88 123690 SP 250-25 Calibration Service for Inductive Voltage Dividers SP 250-26 NBS Phase Angle Calibration Sen/ices SN003-003-02871-1 $5.00 SP 250-27 AC-DC Difference Calibrations SP 250-28 Solid-state DC Voltage Standard Calibrations SN003-003-02842-8 $2.00 SP 250-29 Traceable Frequency Calibrations SN003-003-02844-4 $2.25 SP 250-30 GOES Satellite Time Code Dissemination:

Description and Operation PB88 168760 SP 250-31 Mass Calibrations SN003-003-02919-0 $4.00 SP 250-32 A Calibration Sen/ice for 30 MHz

Attenuation and Phase Shift SN003-O03-02875-4 $13.00 SP 250-33 A Calibration Service for Voltage Transformers and High-Voltage Capacitors SN003-003-02880-1 $1.25 SP 250-34 High Vacuum Standard and Its Use SP 250-35 The Calibration of Thermocouples and Thermocouple Materials SP 250-18 Neutron Source Strength Calibrations SN003-003^2863-1 $3.25

* Entries containing a stock number (003-003-) and price can be purchased from the Superintendent of Documents, U.S. Government

Printing Office, Washington, DC 20402. GPO will accept checks, money orders, VISA, and Mastercharge. For more infomation, or to place an order, call (202) 783-3238. Be sure to cite the stock number on all orders.

Entries containing PB numbers can be purchased from the National Technical Information Service, Springfield, VA 22161. NTIS will accept American Express in addition to the payment methods listed for GPO. For more information call (703) 487-4650; to place an order

call (800) 336-4700. Be sure to cite the PB number on all orders.

Titles without stock numbers are in preparation.
U.S. Department of Commerce

National Institute of Standards and Technology

(formerly National Bureau of Standards)

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300