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NiST PUBLICATIONS NIST SPECIAL PUBLICATION 260-1 34

U. S. DEPARTMENT OF COMMERCE/Technology Administration

National institute of Standards and Technoiogy

Standard Reference Materials®

Standard Reference Material 1749:

Au/Pt Thermocouple Thermometer

Dean C. Ripple and George W. Burns

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Institute of Standards and Technology

' Administration, U.S. Department of Commerce
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NIST Special Publication 260-134

Standard Reference Materials®

Standard Reference Material 1749:

Au/Pt Thermocouple Thermometer

Dean C. Ripple and George W. Burns

Process Measurements Division Chemical Science and Technology Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8363

U.S. DEPARTMENT OF COMMERCE, Donald L. Evans, Secretary TECHNOLOGY ADMINISTRATION, Phillip J. Bond, Undersecretary of Commerce for Technology NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Arden L. Bement, Jr., Director Issued March 2002
Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended

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

Technology, nor is it intended to imply that the materials or equipment identified are

necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 260-134

Natl. Inst. Stand. Technol. Spec. Publ. 260-134, 42 pages (March 2002) CODEN: NSPUE2

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NIST Special Publication 260-134

Standard Reference Material 1749: Au/Pt Thermocouple Thermometer

Dean C. Ripple and George W. Burns

National Institute of Standards and Technology

Process Measurements Division

Gaithersburg, MD 20899

Abstract

Standard Reference Material® (SRM®) 1749 is a lot of 18 specially-constructed gold versus platinum (Au/Pt) thermocouple thermometers, each calibrated on the International Temperature Scale of 1990 (ITS-90) and supplied with integral lead wires and a protective sheath. It is the most accurate thermocouple available over the range 0 °C

to 1000 °C, with an expanded uncertainty (£=2) less than 8.3 m°C from 0 °C to 962 °C, and rising to 14 m°C at 1000 °C. We describe the fabrication and calibration methods used to prepare the SRM 1749 thermocouples. These

thermocouples are exceptionally thermoelectrically homogeneous and stable over hundreds of hours of use, making

them an excellent choice as a secondary reference thermometer. To facilitate low measurement uncertainties in applications, practical methods for emf measurements and comparison calibrations of other thermometers with the SRM 1749 thermocouple are also given. Disclaimer

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

the experimental procedure. Such identification does not imply recommendation or endorsement by NIST, nor does

it imply that the materials or equipment identified are necessarily the best available for the purpose. Acknowledgment

The authors wish to thank the Standard Reference Materials Program for their support in the development of this new SRM.

iii
Table of Contents

1. Introduction 1 2. Properties of Au/Pt thermocouples 1 3. Fabrication of the thermocouples 2

4. Calibration of the thermocouples 4

5. Uncertainty of the thermocouple calibration 6

5 . 1 Thermocouple reproducibility 7

5.2 Emf measurement uncertainties 8

5.3 Realization of the ITS-90 1

1

5.4 Thermocouple inhomogeneity 1

1

5.5 Uncertainty of the reference function 12

5.6 Uncertainty of the ice point 13

5.7 Summary of uncertainties 13

6. Properties of thermocouples prepared for SRM 1749 13

7. Mathematical methods for use with SRM 1749 17

8. Maintenance of the thermocouple 19

9. Preparation of an ice bath for the reference junctions 19

10. Emf measurements 20

1 0. 1 Digital voltmeters 20

10.2 Scanners 21

1 0.3 Junction boxes and wiring 22

1 0.4 Methods for data acquisition 23

1 1 . Preventing thermocouple contamination 24

12. Comparison measurements with the Au/Pt thermocouple 24

13. References 25 Appendix: Certificate for SRM 1749 27

iv
List of Tables

Table Page Number

1. Calibration uncertainties for the SRM 1749 thermocouples 7

2. Subcomponents of the gain uncertainty for the digital voltmeter 10

3. Sample coefficients, a{ , of a calibration function for SRM 1749 thermocouples 17

4. Temperature-emf data pairs evaluated from the coefficients in Table 3 18

5. Coefficients c, of an approximate inverse function for Au/Pt thermocouples 18

v
List of Figures

Figure Page Number

1 . Photographs of a SRM 1 749 thermocouple 3

2. Thermal expansion coil at the measuring junction of a Au/Pt thermocouple 4

3. The difference between the measured emf values for a typical SRM 1 749 thermocouple and the reference function for Au/Pt thermocouples 5

4. Calibration uncertainties for the SRM 1749 thermocouples 7

5. Non-linearity of a digital voltmeter at low voltage values 8

6. Non-linearity of a digital voltmeter on the range with full scale voltage of 1.2 V 9

7. Fractional deviation of measured voltage from value of voltage expected by scaling, as a function of

temperature change 10

8. Values of emf on insertion into and withdrawal from the aluminum freezing-point cell during a freeze ..11

9. Values of emf on insertion into and withdrawal from the silver freezing-point cell during a freeze 12

10. Differences between the measured emf values for the SRM 1749 thermocouples and the reference function 14

11. Correlation of emf values at the aluminum point, £(A1), with emf values at the

silver point, E(Ag) 14

12. Correlation of emf values at the ice point, £(Ice), with emf values at the indium point, £(In) 15

13. Ratio of the standard deviation of emf values in the lot of SRM 1749 thermocouples to the standard deviation for the reproducibility with thermal cycling 15

14. Emf value at the silver freezing point of a thermocouple fabricated at the same time and from the same wire lots as the SRM 1 749 thermocouples, and used as a check standard 1

6

15. Inhomogeneity and change in the emf measured at the silver freezing point for an SRM 1749 thermocouple that was heated at 980 °C 17

16. Schematic cross section of a junction box with low thermal emfs, constructed with commercial

binding posts 23

vi
1. Introduction

The Standard Reference Material® (SRM®) 1749 thermocouple is a specially-constructed and annealed gold versus platinum (Au/Pt) thermocouple, with integral lead wires and a protective silica glass sheath. It is the most accurate thermocouple available over the range 0 °C to 1000 °C, with an expanded uncertainty (#=2) less than 8.3 m°C from

0 °C to 962 °C, and rising to 14 m°C at 1000 °C. In this range, other common thermometer types are platinum- rhodium alloy thermocouples, and platinum resistance thermometers. Relative to platinum-rhodium alloy thermocouples, the SRM 1 749 thermocouple is over an order of magnitude more accurate and more homogenous, at the cost of being more expensive and somewhat more difficult to use. Relative to Standard Platinum Resistance Thermometers (SPRTs), the defining instrument of the International Temperature Scale of 1990 (ITS-90) [1] from

13.6 K to 962 °C, the SRM 1749 is substantially more rugged but slightly less accurate. Relative to platinum

resistance thermometers with a more rugged construction than SPRTs, the SRM 1749 thermocouple has similar

calibration uncertainties, is somewhat more resistant to loss of calibration, and has a higher upper temperature limit. The combination of ruggedness, a calibration uncertainty significantly less than almost all industrial thermometers, and long-term stability of its calibration make the SRM 1 749 thermocouple ideal for use as a secondary reference

standard.

This document serves two purposes: to provide guidance in the proper use and care of the SRM 1749 thermocouple thermometer and to document the methods used at NIST for its fabrication and calibration. Section 2 gives background on the general properties of Au/Pt thermocouples; Sections 3 through 5 describe the fabrication,

calibration, and uncertainty of the SRM 1749 thermocouples in particular, and Sections 6 through 12 give

instructions on the care and proper use of the SRM 1 749 thermocouple.

2. Properties of Au/Pt thermocouples

Although the SRM 1749 thermocouple operates on the same physical principles as all other thermocouples, advances in the fabrication, annealing, and calibration techniques result in a thermocouple thermometer that has

vastly superior accuracy and homogeneity relative to other thermocouples over the range 0 °C to 1000 °C.

It is informative to compare the SRM 1 749 with more traditional thermocouple designs. Thermocouples constructed from platinum-rhodium alloys and pure platinum are currently the predominant choice for use as a secondary

reference standard. Type S (Pt-10%Rh vs. Pt) and type R (Pt-13%Rh vs. Pt) thermocouples cover the temperature

range from 0 °C to approximately 1400 °C, and type B (Pt-30%Rh vs. Pt-6%Rh) thermocouples generally are used

over the range from 800 °C to 1700 °C [2]. Compared to thermocouples manufactured from "base" metals, such as the type K thermocouple, the platinum-rhodium alloy thermocouples have superior stability and initial calibration accuracy. However, the highest accuracy obtainable with any of the platinum-rhodium alloy thermocouples is 0.1 °C to 0.2 °C at 1 100 °C as a consequence of preferential oxidation of rhodium [3-6]. Rhodium will form a

stable oxide within a temperature band from approximately 550 °C to 900 °C. As the rhodium oxide forms, a thermoelement formed from a platinum-rhodium alloy will become depleted in rhodium, and changes in thermoelement composition will result in changes of the emf-temperature relationship and in thermoelectric inhomogeneity of the thermoelement. Thermocouples constructed from pure elements, the Au/Pt or Pt/Pd thermocouple for example [7,8], do not suffer from preferential oxidation problems. This fact has several important consequences: pure element thermocouples

are inherently more thermoelectrically homogeneous, and their thermoelectric stability is not limited by shifts in alloy composition caused by preferential oxidation. Additionally, because pure element thermocouples do not

require adjustments of alloy composition to match a reference function, the interchangeability of thermocouples

manufactured from sufficiently pure elements is excellent and the deviations of actual thermocouples from the appropriate reference function are small. In the case of Au/Pt thermocouples, homogeneity and initial calibration tolerances are superior to those of a type S or type R thermocouple by over an order of magnitude, and no long-term

drift has been detected over 1000 h of use [7].

1
Pure element thermocouples do have some limitations. Special construction techniques are necessary to minimize

mechanical strain caused by the different thermal expansion coefficients of the pure element thermoelements. Because the pure elements used have lower melting points than platinum-rhodium alloys, the upper limit of use of

the thermocouples is 1000 °C for Au/Pt thermocouples and 1500 °C for Pt/Pd thermocouples. In certain applications, the high thermal conductivity of a gold thermoelement may be a disadvantage.

3. Fabrication of the thermocouples

The SRM 1749 thermocouples were constructed from 0.5 mm diameter gold and platinum wire of the highest purity

available, typically 99.999 % mass fraction. The high purity of the platinum wire was attested to by its close thermoelectric agreement with the NIST-maintained platinum thermoelectric standard, Standard Reference Material SRM-1967, commonly referred to as Pt-67 [9]. The emf of the platinum wire versus Pt-67 at 1064 °C, with the reference junction at 0 °C, was found to be +0.6 uV. The fabrication techniques followed previous practice [8,10]. The annealing of the gold and platinum wire is a critical process in producing thermoelements that are thermoelectrically stable and uniform along the length of the thermoelements. High temperature annealing serves two purposes: First, the annealing relieves any mechanical strains in the wire. Second, annealing may reduce the quantity of impurities in the wire by either driving off volatile impurities or by oxidizing reactive impurities. Two

common methods of annealing are either to anneal the wire electrically by suspending a loop of wire from two clips and heating the wire by passing AC current through the wire [11], or to anneal the wire by placing the wire in a uniform zone of a furnace. Gold has insufficient mechanical strength to reliably anneal it by an electrical anneal, so

it was annealed only in a furnace. Platinum was annealed by both methods. Prior to furnace annealing, the platinum

wires were electrically annealed in air for 10 h at 1300 °C and 1 h at 450 °C. The wires were furnace annealed by

pulling segments of the wire into the bores of a high-purity (99.8 % mass fraction) alumina insulator tube. A

separate alumina tube is dedicated for use with each metal. The alumina tube holding the wires was placed into a

larger diameter alumina protecting tube mounted in a 1 . 1 m long horizontal tube furnace. The wire segment length was short enough that during a furnace anneal the wire was located in a zone of the furnace that was uniform in temperature to within 2 °C, ensuring that the wire will be uniformly annealed over the full length. Furnace annealing schedules were 10 h at 1000 °C and overnight at 450 °C for gold, and 1 h at 1 100 °C and overnight at 450 °C for platinum. Following annealing, pull wires were used to thread the thermoelements into the 1.6 mm bores of a twin bore, highpurity alumina tube with overall diameter of 4.7 mm and length of 76 cm. A relatively long tube length is useful for the maintenance procedures described in Section 8 below. A relatively large bore diameter allows the thermoelements to move easily through the bores as they expand with heating. Before use, all alumina tubes were baked for 50 h in air at 1 100 °C. During assembly of the thermocouple, the thermoelement segments were rejoined by butt-welding with a small hydrogen-oxygen torch. As shown in Fig. 1 , the thermocouple wires emerging from

the alumina tube were insulated with flexible fiber-glass tubing to within 1 cm of their ends, and the fiber-glass tubing was joined to the alumina tube with heat-shrinkable sleeving. A cylindrical sleeve of soft copper was

crimped over the fiber-glass tubing where the thermoelements emerged from the insulator. This crimp tube compresses the fiber-glass tubing against the thermoelements to anchor them near the end of the alumina tube. For

each thermocouple, a four or five turn coil of 1 mm diameter constructed from 0.12 mm diameter platinum wire was

used to connect thermoelements at the measuring junction, as seen in Fig. 2. A pair of insulated copper wires was

soldered to the other ends of the thermoelements to form the reference junctions. Both copper wires were cut from

one spool to minimize any thermal emf caused by slight mismatches in the copper composition. The positive lead of

the thermocouple, connected to the gold wire, is color coded with a small piece of yellow insulation. The portion of the alumina-sheathed section of the thermocouple is further protected by a silica glass sheath of 7 mm

outer diameter, as shown in Fig. 1. Sufficient room is left at the end of the sheath to allow free expansion of the thermoelements on heating to 1000 °C. The silica glass sheath may be removed by loosening the O-ring fitting at its open end, but the coil of the SRM 1 749 thermocouple is extremely vulnerable to damage in this state. Only highly

trained operators should use the SRM 1749 thermocouple without its outer sheath. The flexible portion of the thermocouple is further protected by an outer sheath of fiberglass sleeving both over the gold and platinum thermoelements and over the copper leads.

2
The reference junctions of the thermocouple are mounted in a single stainless steel tube, closed at one end. The recommended method for use of this reference-junction assembly is discussed in Section 9. This reference-junction assembly should never be disassembled, since disassembly and reassembly risks mechanically straining the wires.

Figure 1. Photographs of a SRM 1749 thermocouple, with key components identified.
Platinum thermoelement

High-purity alumina tube

Gold thermoelement

Welded juntion

Expansion coil, formed

from 4-5 turns of 00.12 mm platinum wire

Figure 2. Schematic drawing of the thermal expansion coil at the measuring junction of a Au/Pt thermocouple.

4. Calibration of the thermocouples

After construction but prior to full calibration, each thermocouple was furnace annealed in air for 1 h at 1000 °C, cooled to 450 °C over a period of approximately three hours, and held at 450 °C for at least 10 h. Then, the effects of inhomogeneity of the thermocouple were determined by measuring its emf on insertion into and withdrawal from

a silver freezing-point cell during a freeze of the silver. For this measurement, the test thermocouple was inserted

into the cell, after a freeze was induced, to a point where the measuring junction was held 2 cm above the surface of

the silver. After 25 min at this position, the thermocouple was inserted in steps of 2 cm every 5 min. The

thermocouple was held at full immersion (18 cm below the surface of the silver) for 25 min, and then withdrawn in steps of 2 cm every 5 min, after an initial step of 1 cm. At immersions greater than 8 cm, the thermocouple measuring junction was in thermal equilibrium with the freezing metal. The temperature of the freezing metal varies

slightly with the depth of immersion, as a consequence of the hydrostatic head, but variations in the emf from this effect are less than the equivalent of 1 m°C for immersions ranging from 8 cm to 18 cm. Measurements of very

similar freezing point cells with platinum resistance thermometers clearly indicate that for sufficient immersion into the cell, the temperature varies only slightly as the thermometer immersion is varied, as predicted by the variation in the hydrostatic head. The signal generated by a thermocouple, however, depends not only on the temperature of the measuring junction, but also on the thermoelectric properties of the thermoelements. The emf is generated in regions where the thermoelements pass through thermal gradients. If, as a consequence of chemical or physical

variations, the thermoelectric properties vary along the length of the thermoelement, the thermocouple is said to be "inhomogeneous." The emf generated by an inhomogeneous thermocouple will vary with depth of immersion into a

fixed-point cell because different sections of the thermoelements will be exposed to the regions of thermal gradients

in the furnace.

Thus, in the measured immersion curves into the aluminum or silver fixed-point cell, any deviation from a constant value of emf was taken as an indication of inhomogeneity or drift of the thermocouple (short term drift in the emf

measurements is negligible). The spread in emf values for all measurements at immersions of 8 cm and greater was

calculated and defined as the inhomogeneity. All of the SRM 1 749 thermocouples had inhomogeneity values at the

silver freezing point (961.78 °C) less than 0.17 uV, equivalent to 7 m°C, and hysteresis values less than -0.14 uV,

equivalent to 6 m°C. These values are extreme limits; the rms value for inhomogeneity for the SRM 1749

thermocouples is 0.050 uV at the silver freezing point, equivalent to 2 m°C. A reference function for Au/Pt thermocouples on the ITS-90 has been measured at NIST over the range 0 °C to 1000 °C [7]. The emf versus temperature responses of the SRM 1749 thermocouples deviate from the reference

function by no more than 16 m°C. To attain the minimum possible uncertainty with this thermometer, each

individual SRM was calibrated, and a unique calibration function was generated. Au/Pt thermocouples can readily be calibrated by measuring the thermocouple emf at fixed points defined on the ITS-90. At NIST, the fixed points used are ice (0 °C), indium (156.5985 °C), tin (231.928 °C), zinc (419.527 °C),

4
aluminum (660.323 °C), and silver (961.78 °C). Each of the fixed-point cells used had previously been compared

against the corresponding reference cell maintained in the NIST Resistance Thermometer Calibration Laboratory. At the fixed points of aluminum and silver, a full immersion curve was obtained on each thermocouple to document

the inhomogeneity of the thermocouple. The measured emf values at full immersion into each of the fixed point

cells were used to determine the calibration function for each thermocouple. Results for a typical thermocouple are shown in Fig. 3. Emf values at less than full immersions were used to determine the uncertainty component for thermocouple inhomogeneity. Between each fixed-point measurement, the SRM 1749 thermocouples were

annealed at 450 °C overnight, as described in Section 8.

-0.2

200 400 600

Temperature / °C

800 1000

Figure 3. The difference between the measured emf values for a typical SRM 1749 thermocouple and the reference function for Au/Pt thermocouples, defined as the emf deviation, is plotted versus temperature. Solid

circles indicate measured data, the solid line is a quadratic fit to the data, and the error bars indicate one

standard uncertainty at each fixed point. The dotted lines show the emf deviation equivalent to ±10 m°C. The cell design and the design of the furnaces used are discussed in Ref. [11] and [12]. The well of each freezing- point cell contained a silica glass protection tube (8 mm outer diameter, 6 mm inner diameter) closed at one end, open to air at the other end, and matte finished on the outer surface to minimize heat losses by radiation piping. At

full immersion of the thermocouple into the freezing-point cells, the measuring junction of the thermocouple was

18 cm below the surface of the metal. Freezes in the gold, silver, and aluminum cells were performed in a vertical furnace containing a 61 cm long sodium heat pipe. Freezes in the zinc, tin, and indium cells were performed in a

single zone furnace having an aluminum moderating block. For all of the freezing points, the furnace was held

0.5 °C below the freezing-point temperature after the freeze was initiated, and the freeze was induced internally by

inserting an alumina insulator, initially at room temperature, into the cell. Measurements of the thermocouples at the ice point used a properly prepared ice bath (see Section 9 of this document) contained in a cylindrical Dewar flask (7 cm inner diameter and 41 cm deep). The test thermocouple was

contained in a closed-end glass tube (7 mm outer diameter, 5 mm inner diameter) inserted in the tightly packed

mixture of ice and water in the flask. Its measuring junction was located 35 cm below the surface of the ice-water mixture when measurements were made.

5
All of the emf measurements were made with a calibrated digital multimeter (Hewlett-Packard model 345 8 A) and

scanner (Hewlett-Packard model 3495A) system. The thermocouple data were obtained automatically via a computer-controlled IEEE-488 bus and logged to a data file for later analysis. The reference junctions of the thermocouples were maintained at 0 EC in an ice bath when measurements were made. To obtain a mathematical calibration function of each thermocouple, emf values computed from the reference function were subtracted from the emf values measured at each fixed point. The resulting emf deviation was then modeled by a quadratic function of temperature. Coefficients of the quadratic function were determined by the method of least squares, and addition of these coefficients to those of the reference function gave the calibration function for the particular thermocouple under test. For the least squares fitting procedure, the calibration points were weighted by the inverse square of the reproducibility of the thermocouples, measured as the standard deviation of a set of measurements at each fixed point. The reproducibility values used had been obtained from a previous study of the reproducibility of Au/Pt thermocouples at fixed points [7]. The results of the least squares fitting of the quadratic model to the data gave values of the reduced chi-squared statistic of 0.6010.37, which is somewhat less than the expected value of one. We believe that this is primarily because the reproducibilities had been evaluated

for data including multiple thermal cycles over a period of many months, but the SRM 1 749 data included thermal

excursions to only one round of fixed-point temperatures over a briefer period of time. It is also possible that our

proficiency in fabricating, annealing, and measuring Au/Pt thermocouples has improved since the original measurement of reproducibilities at fixed points. With most thermocouples whose reference junctions are maintained at 0 °C, calibration at the ice point is not common practice—the emf value at this point is often simply assumed to equal zero. In fact, all thermocouples will have a non-zero emf value at the ice point as a result of small inhomogeneities in the thermoelements or copper

lead-wire from which the thermocouple thermometer has been constructed. For thermocouples that have no

connections or discontinuities of the thermoelements, this effect is small, and measurement of the ice point as part of

the calibration is generally necessary only for pure element thermocouples of the highest level of accuracy. In the case of SRM 1 749, there are three main sources of wire inhomogeneity. First, the copper lead wires from the ice point to the emf-measuring apparatus are neither as well annealed nor as chemically pure as the gold and platinum

thermoelements. Minor differences between the two leads will result in a small emf contribution when the lead wires pass from room temperature to 0 °C. Second, as a result of the annealing process for the gold and platinum

wires, there may be slight differences in the thermoelectric properties of the thermoelements at the reference junction end of the thermocouple relative to the properties at the measuring junction end of the thermocouple.

Third, all of the wires at the reference junctions are subject to slight mechanical strain when the reference-junction assembly is fabricated.

5. Uncertainty of the thermocouple calibration

For a thermocouple of ideal properties, the process of calibration requires the measurement of the thermocouple emf

while simultaneously maintaining the reference and measuring junctions at known temperatures. The data pairs of

temperature versus emf could then be fit by a polynomial equation that would give a temperature-emf relation. The

uncertainty of this relation at the time of test would depend only on the uncertainty of the emf measurement and the

uncertainties related to the determination of the thermocouple junction temperatures. What is of more interest to the user is knowledge of the uncertainty of the temperature-emf relation when the thermocouple is used in a different apparatus at a later date. For this, it is necessary to add components for thermocouple reproducibility and inhomogeneity to the uncertainty budget. We have chosen to construct the uncertainty budget to include these components, defining the inhomogeneity component so that the combined

uncertainty can be thought of as the uncertainty of the temperature-emf relation when used in a measurement

environment similar, but not identical, to the NIST measurement system. Cases for which the user environment can no longer be considered similar to the NIST system are discussed below, in sections 5.4 and 5.6. Table 1 lists the calibration uncertainty of the SRM 1749 thermocouples at fixed point temperatures and a number of

additional temperatures. Each of the subcomponents of uncertainty mentioned in Table 1 are discussed below, and

6
the dominant components are shown in Fig. 4. The uncertainties have been evaluated in accordance with Ref. [13]. Except for the component labeled "Thermocouple reproducibility," all of the components are Type B uncertainties.

8

0 1^ 1 1 1 1 0 200 400 600 800 1000

Temperature / °C

Figure 4. Calibration standard uncertainties and subcomponent uncertainties for the SRM 1749 thermocouples. Only subcomponents that are significant over some range of temperature are shown.

Table 1 . Calibration uncertainties for the SRM 1 749 thermocouples.

All uncertainties expressed in millidegrees Celsius trc Emf TC Realization Uncertainty ITS-90 Ice point TC Combined Expanded

measur. reproduc. of ITS-90 of reference non-unique. uncertainty inhomog. std. uncert. uncertainty

function & extrapolat. & inhomog. u U{k=2) 0 1.49 1.7 0.10 2.3 0.00 1.75 0.00 3.7 7.4 100 0.98 1.4 0.25 2.5 0.29 1.13 0.47 3.3 6.5 156.599 0.89 1.2 0.25 2.4 0.00 0.97 0.63 3.1 6.1 200 0.85 1.3 0.25 2.2 0.29 0.89 0.74 2.9 5.9 231.929 0.84 1.3 0.24 2.1 0.00 0.84 0.81 2.9 5.7 300 0.85 1.5 0.31 1.9 0.29 0.76 0.94 2.8 5.6 400 0.93 1.7 0.42 1.6 0.14 0.67 1.11 2.9 5.7 419.527 0.95 1.7 0.44 1.6 0.00 0.66 1.14 2.9 5.8 500 1.04 1.9 0.79 1.5 0.28 0.60 1.25 3.1 6.1 600 1.16 2.1 1.23 1.4 0.20 0.55 1.38 3.3 6.7 660.323 1.24 2.2 1.50 1.3 0.00 0.53 1.44 3.5 7.1 700 1.29 2.3 1.46 1.3 0.60 0.51 1.53 3.7 7.3 800 1.43 2.5 1.36 1.2 1.15 0.47 1.73 4.0 8.0 900 1.57 2.7 1.26 1.1 0.81 0.44 1.91 4.1 8.2 961.78 1.66 2.8 1.20 1.0 0.00 0.42 2.00 4.2 8.3 1000 1.71 2.9 1.16 1.6 5.77 0.41 2.06 7.3 14.5

7
5.1 Thermocouple reproducibility The reproducibility of emf measurements in fixed-point cells of a set of Au/Pt thermocouples, with construction very similar to the SRM 1749 thermocouples, was determined in the original work on the reference function for Au/Pt thermocouples [7]. This reproducibility was taken as the Type A uncertainty for the calibration of the SRM

1749 thermocouples. When expressed as an equivalent temperature uncertainty, the Type A uncertainty could be

represented by the expression «A = 0.201xl0"5 (r /°C) + 0.86 m°C for the fixed points indium, tin, cadmium, zinc, aluminum, and silver. The Type A uncertainty for the ice point was determined from repeat measurements on

several thermocouples in the SRM 1749 lot. The value found was wA(ice) = 1.74 m°C. In Table 1, this uncertainty

is termed the "Reproducibility of Au/Pt TCs", but more accurately can be considered to include also effects of the reproducibility of the emf measurements, the reproducibility of the reference junction bath, the thermoelectric

instability of the thermocouples over the course of the calibration measurements, and the reproducibility of the fixed-point realizations.

5.2 Emf measurement uncertainties

Uncertainties of the emf measurements not covered in the Type A uncertainty were determined by independent measurements of the variation of the thermal emfs from the scanner relays and wiring, measurement of the voltmeter

non-linearity, measurements of the gain stability of the multimeter over extended periods of time, and by

intercomparison of the multimeter with other multimeters of the same and different manufacturer.

Figures 5 and 6 display typical results for the voltmeter nonlinearity, as measured on a Resistance Bridge Calibrator (RBC) [14]. The RBC is similar to a Hamon network of resistors [15], but is configured in such a way as to allow up to 35 independent resistance values to be obtained from serial and parallel combinations of only four resistors. Because of this large redundancy of resistance values, the four base resistance values may be obtained from a least- squares fit of the values to the data. The four resistors need not be calibrated independently.

30

20

•C 10

£ 0

-10

-20

-30

T.OX.Q.... x

. ~r*?~ ~x ~x

" ~eV

a " " " " ~~°

oX

X

:-£c: X A 0

u (offset) it (total non-linearity)

6 9

Voltage / mV

12

Figure 5. Tests of the non-linearity of a digital voltmeter at low voltage values. The four symbols represent four

separate runs on different days.

8
0 0.2 0.4 0.6 0.8 1 1.2 Voltage / V

Figure 6. Test of the non-linearity of a digital voltmeter on the range with full scale voltage of 1 .2 V.

1.4

Although the RBC is designed as an instrument for AC measurements or for DC measurements with frequent

current reversals, it can be used with DC measurement systems provided that the thermal emfs in the RBC and its associated wiring are dramatically reduced. To reduce the thermal emfs of the wiring, it is necessary to use lead wires that are very similar in thermoelectric properties to the other lead wires and to the internal wiring of the digital voltmeter. This is accomplished by using twisted pairs of untinned copper wire instead of coaxial cable for the

wiring. Thermal emfs inside the RBC are reduced dramatically by reducing the temperature gradients in the vicinity of the RBC. This is accomplished by thermally isolating the RBC inside concentric boxes of polystyrene foam, aluminum 0.8 cm thick, and polystyrene foam. The rms thermal emfs of the RBC connections when insulated in

this fashion were less than 3.5 nV. In typical operation, the RBC is used to measure the non-linearity, offset, and

gain error (these terms are defined in Section 10) of instruments that measure resistance ratios, the instrument

reading being a measurement of the ratio of a test resistance to that of a reference resistance. For the purpose of measuring the non-linearity and offset of a voltmeter, a fixed current was passed through the RBC and an external, temperature-controlled reference resistor, both of which were connected in series. Resistance ratios were

constructed by dividing the voltage measured across the RBC by the voltage measured across the reference resistor. The readings were corrected for the voltmeter zero readings either by averaging measurements with two directions of current, or by subtracting voltmeter readings made with zero current from the readings with current. As a consequence of using an external reference resistor that was not independently calibrated relative to the RBC base

resistors, it was not possible to determine the voltmeter gain error with the RBC. The rms deviation of the readings from the simple model used to fit the RBC data is expected to equal the sum of the measurement noise and the voltmeter non-linearity, added in quadrature. Sources of measurement noise include both short-term fluctuations in the current source and the inherent noise of the digital voltmeter. At low voltages, the inherent voltmeter noise dominates the measurement noise, and each data point requires approximately seven minutes of signal averaging. By

subtracting the independently-measured voltage noise from the rms deviation, we obtain an estimate of the nonlinearity. The results of the RBC on the 100 mV voltmeter range indicate that the emf measuring system has a combined offset error (5 in Section 10) and non-linearity of only 0.0085 uV at the one standard deviation level, which we round to 0.01 uV. On the 1 V range, the offset is negligible, and the non-linearity is of the same

magnitude as the measurement noise. Because we did not perform multiple runs on multiple higher voltage ranges, we chose to conservatively estimate the non-linearity on the higher ranges as the rms deviation from ideal behavior,

without any subtraction of measurement noise. Previous measurements of the same model of voltmeter against a Josephson junction array gave similar results [16]. As shown in Fig. 6, the non-linearity is 2x1

0~7 of the reading, for readings of 16 % of nominal full scale and above. We measured the gain error on the 10 V range directly by measuring a calibrated 10 V solid-state voltage reference. On the 100 mV range, which is the only range used for Au/Pt thermocouple measurements, it is possible to determine the gain error of the voltmeter by two simple methods. One straightforward method is to construct a

9
calculable 100 mV or 10 mV voltage source, based on a 1:1000 or 1:10,000 voltage divider constructed from two

independently-calibrated four-terminal resistors that are connected in series. Passing a suitable DC current through

the resistors produces voltages of nominally 10 V and 100 mV or 10 mV, whose exact ratio can be calculated from

the resistor calibrations. A second method for determining the scale error relies on the high degree of linearity of the voltmeter, as shown in Fig. 5 and 6. If the voltmeter was perfectly linear, then the scale error of one range relative

to another range can be determined by measuring the ratio of two determinations of the same voltage, on the two

scales. For example, a 100 mV voltage can be read on both the 1 V scale and the 100 mV scale. In practice, an

additional uncertainty must be added to account for the voltmeter non-linearity. We used this second method. The voltmeter gain error (a in Section 10) is dominated by changes in the gain sensitivity of the voltmeter as the voltmeter temperature varies during the course of the day. Figure 7 shows variations in the ratio of the voltmeter

readout to the true voltage, for a voltage of 100 mV, and as a function of the voltmeter internal temperature. When these effects are combined, the Type B standard uncertainty for the combined scanner and digital multimeter system can be expressed as «/uV = 2.5x1 0~6(£/uV) + 0.01, where E is the thermocouple emf. A summary of the subcomponents of this expression is listed in Table 2.

Figure 7. Fractional deviation of measured voltage at 100 mV from value of voltage expected by scaling down from

a 1 0 V reference, as a function of the temperature change from time of autocalibration.

1.5E-06

1.0E-06

c

o1 5.0E-07

A

A \ "a--.. A X i —V t

\ X

• Run 1 Run 2 a Run 3 x Run 4

\ J Slope (-1.35E-6)/K * ,

l . •

1 x1

• "*•••-. X H

•

-0.8 -0.4 0.0 0.4 0.8 [T (Measurement) - r(Calibration)] / °C

Table 2. Subcomponents of the gain uncertainty for the digital voltmeter, expressed as fractional standard

uncertainty in E. Subcomponent Fractional standard

uncertainty in E

Temperature change in voltmeter 1 .2x 1 QT

6 Voltmeter non-linearity 1 .2x 1

0

-6 Voltmeter calibration drift 1 .Ox 1 0"6 10 V dc voltage reference 1.5xlQ~6 Combined gain uncertainty 2.5xl0~6

10
5.3 Realization of the ITS-90

For measurements at each fixed point, a Type B uncertainty was included to account for deviations of our cells from

an ideal fixed point of a pure material. These deviations were determined by measurements of freezing plateaus with an SPRT or a high temperature SPRT, by comparison with the reference standard cells maintained by the Platinum Resistance Thermometry Laboratory at NIST, and by estimation of uncertainty from known impurities. In

the data analysis, corrections were made to the fixed-point cell temperature for the hydrostatic head and for the temperature difference between the cells used for this study and the reference standard cells maintained in the Platinum Resistance Thermometry Laboratory.

5.4 Thermocouple in homogeneity

As seen in the sample immersion profiles in Fig. 8 and 9, thermoelectric inhomogeneity will cause variations in the output emf of a thermocouple with changes in immersion depth even if the temperature of the measuring junction is fixed. Since it is unlikely that the SRM 1749 thermocouple will be used in a thermal environment identical to its calibration environment, an additional uncertainty w; must be included to account for inhomogeneity. At the aluminum and silver fixed points, we have taken the standard uncertainty for this effect to be equal to the rms

deviation of the measured emf values from the emf value at full immersion, for all immersions greater than 8 cm

into the fixed-point cell:

where E0 is the emf at full immersion, Ek is the emf at partial immersion, and n is the number of points taken at partial immersion. At other temperatures, we have estimated the inhomogeneity using linear interpolation and

assuming that there is no inhomogeneity uncertainly at 0 °C.

9320.4

9320.3

9320.2

9320.1 10 12 14 16 18 20

Junction immersion in freezing aluminum / cm

Figure 8. Values of emf measured for an SRM 1749 thermocouple on insertion into and withdrawal from the aluminum freezing-point cell during a freeze.

11
16120.4

16120.3

16120.2

16120.1

4m°C

• Insertion o Withdrawal

10 12 14 16 18 20

Junction immersion in freezing silver / cm

Figure 9. Values of emf measured for an SRM 1749 thermocouple on insertion into and withdrawal from the silver freezing-point cell during a freeze. The stated inhomogeneity uncertainty, u„ and combined expanded uncertainty, U, apply when the SRM 1749

thermocouple is used in apparatus where the immersion distance, L, from the coil of the measuring junction to the point at the throat of the furnace where there is a maximal thermal gradient is in the range 36 cm to 44 cm. For the two NIST furnaces used for all of the fixed-point measurements, this distance ranged from 38 cm to 44 cm. If the SRM 1749 thermocouple is used at an immersion significantly shorter than 36 cm from the point of maximum

thermal gradient to the measuring junction, it is recommended that the uncertainty be recalculated according to the following formulas: ui,L = u\ + u\ (36 cm -Z)/8cm , (2 UL =(u2 -u? + uuL

2 )'

2

, (3 where uu is the standard uncertainly subcomponent for the thermocouple inhomogeneity at an immersion depth of L

and UL is the combined expanded uncertainty at the specified immersion depth.

5.5 Uncertainty of the reference function

In the calibration procedure, the true emf-temperature relation of the thermocouple is modeled as the reference

function for Au/Pt thermocouples plus a quadratic function. This model is an approximation, with an associated

uncertainty. Attempts to fit a cubic or higher order polynomial to the emf deviation data did not result in a

statistically superior fit. If there are errors in the reference function that can be modeled with a quadratic function, such errors have no net effect on the uncertainty of the combined model of reference function plus quadratic

function. This implies that the appropriate uncertainty is the uncertainty of the reference function over temperature

ranges characteristic of the difference in temperature values of adjacent fixed points, which ranges from 75 °C to 301 °C. Because the number of degrees of freedom in the determination of the Au/Pt reference function was very

high [7], the statistical uncertainty is generally small except at 1000 °C, and a good measure of the short-range

uncertainty of the reference function is the systematic discrepancy between data obtained by fixed-point measurements and data obtained by comparison with an SPRT in the original determination of the reference

function. This uncertainty for the Au/Pt thermocouple reference function is 0.04 uV at 1000 °C, 0.026 uV from the

silver freezing point at 962 °C to the indium point at 157 °C, and decreases to approximately 0.014 uV for temperatures near 10 °C.

12
5.6 Uncertainty of the ice point An ice point as prepared according to the methods described in Section 9 has a standard reproducibility of 1 m°C

[17]. No uncertainty term has been added to account for the reproducibility of the ice point, since the Type A

uncertainty for the reproducibility of the thermocouples emf at the various fixed points already incorporates any

effects of variations in the ice points. However, we have added an uncertainty to account for any systematic errors

in the NIST ice points caused by impurities in the distilled water used for the preparation of the ice and ice baths. Measurements of ice baths prepared from the same distilled water supply as used in the SRM 1749 calibration revealed a -0.6 m°C depression below the nominal value of 0 °C. The data has been corrected for this 0.6 m°C

offset, and a square uncertainty distribution of ±1 m°C has been included for systematic variations in the ice point

temperature. Because of slight variations in the physical or chemical characteristics of the wires inside the reference-junction

probe, the emf generated by the SRM 1749 thermocouples between room temperature and the reference junctions

will vary slightly depending on the depth of immersion of the probe into the ice, even for immersion depths deep enough to ensure that the reference junctions are maintained at 0 °C. An uncertainty for this effect of reference probe inhomogeneity has been included to account for a variation of up to 1 cm in ice level or thickness of the reference bath cover in the user's laboratory compared to those at NIST. It is important to carefully adhere to the procedures of Section 9 to minimize these inhomogeneity effects. If different procedures are used, it is the user's

responsibility to determine the effect on the thermocouple emf of using an ice point of different preparation or

design. Preferably, this effect is determined by actual measurement of the emf differences obtained when the measuring junction is maintained at a fixed temperature and the bath used for the reference junction is alternated between the user's design and a bath prepared following Section 9.

5.7 Summary of uncertainties As seen in Table 1 and Fig. 4, no single uncertainty component dominates the combined uncertainty over the full temperature range. At low temperatures, the uncertainty of the reference function is dominant. Above 962 °C, the reference function is based on an extrapolation of the ITS-90, which is not well known, and this becomes the dominant uncertainty. For intermediate temperatures, the thermocouple reproducibility, the uncertainty of the emf

measurements, and the thermocouple inhomogeneity are all significant.

6. Properties of thermocouples prepared for SRM 1749

The emf deviation, defined as the difference between the measured emf of a particular thermocouple and the

reference thermocouple, is a convenient means of comparing the emf-temperature relation of a thermocouple to the reference function. Figure 10 shows the average, maximum, and minimum emf deviation for the lot of SRM 1749

thermocouples. The maximum deviation from the reference function for all 1 8 thermocouples in the lot is 0.22 u\V

at the aluminum freezing point, equivalent to 1 1 m°C. The maximum spread in the emf values at any of the fixed

points is 0.21 uV at the silver freezing point, equivalent to 8.5 m°C, which is approximately a factor of 10 smaller

than we observe with platinum-rhodium alloy thermocouples fabricated from the same lots of wire. The small

spread in the emf values is an indication that the gold and platinum wires used to fabricate the thermocouples were

chemically homogeneous, and that the annealing procedures used in the fabrication process were highly

reproducible and uniform. The scatter at each fixed-point temperature shown in Fig. 10 is not random. Correlation plots of the emf deviation at one fixed-point temperature versus the emf deviation at another fixed-point temperature show a high degree of

correlation for the emf measurements at the lower-temperature fixed points. This can be seen in the representative

plots in Fig. 1 1 and 12. At the higher temperatures, there are correlations in the emf readings, but the correlation is relatively weak compared to the thermocouple reproducibility, as seen in Fig. 13. This result confirms that there are

statistically significant differences in the emf-temperature relationships of the 1 8 thermocouples in the lot at lower

temperatures and that individual calibration gives a smaller combined uncertainty than using an average calibration

for the lot.

13
0.2

0.1

0.0

-0.1

-0.2

-0.3

10m°C

o

Ice K...... o"

H Snof

o

1 Zn

oi

o

1 1 AloI ~T

Ag - o -10m°C

i i 200 400 600

Temperature / °C

800 1000

Figure 10. Differences between the measured emf values for the SRM 1749 thermocouples and the reference

function for Au/Pt thermocouples. Solid circles indicate the average values for all thermocouples, the

. open circles indicate maximum and minimum values, and the error bars indicate one standard deviation

variations in the emf values.

0.12 > 0.08 S 0.04 <g 0.00 5 -0.04 " -0.08

-0.12

-0.15

Fixed-point reproducibility,

1 std. deviation

•. 0... • •

•

1 • 1 1 •

: o • -

; • • ' .

o m

• •

i -0.10 -0.05 0.00 0.05 [£(Ag)-<ir(Ag)>]/uV

0.10 0.15

Figure 1 1 . Correlation of emf values at the aluminum point, E(Al), with emf values at the silver point, £(Ag). Solid

circles are for the SRM 1749 thermocouples. Open circles are from additional thermocouples fabricated from the same lots of wire as the SRM 1 749 thermocouples.

14
0.04

0.02

0.00

-0.02 — -0.04

-0.06 1 1 1 1 1 -0.04 -0.02 0.00 0.02 0.04 [£(In)-<£(In))]/nV

Figure 12. Correlation of emf values at the ice point, £(Ice), with emf values at the indium point, £(In). Solid circles are for the SRM 1 749 thermocouples. Open circles are from additional thermocouples fabricated from

the same lots of wire as the SRM 1749 thermocouples.

Ice TCs selected for SRM 1749 o Full set of 21 TCs fabricated

In Sn Zn Ag

0

200 400 600

Temperature / °C

800 1000

Figure 1 3 . Ratio of the standard deviation of emf values in the lot of SRM 1 749 thermocouples at each fixed point to the standard deviation for the reproducibility of Au/Pt thermocouples with extended thermal cycling.

Because the Seebeck coefficient is quite small near 0 °C, over a factor of four smaller than at 1000 °C, the emf

deviations of the SRM 1749 thermocouples at the ice point are equivalent to relatively large temperature deviations. The average deviation in units of temperature is -1 1 m°C at 0 °C. This result confirms the necessity of including

the ice point as a calibration point.

Figures 8 and 9 show representative immersion profiles at the aluminum and silver fixed-point cells. These results are very typical of high-quality Au/Pt thermocouples, showing little variation in emf values beyond an immersion of

8 cm of the measuring junction below the surface of the freezing metal. At the silver freezing point, there is noticeable hysteresis between measurements taken on insertion of the thermocouple into the cell, compared to measurements taken on withdrawal. This is most probably a result of reversible metallurgical changes, such as a

15
change in the number of lattice vacancies in the thermoelements, or possibly changes in the oxidation state of the platinum. When annealed at 450 °C for 10 h, the metallurgical state of the thermoelements attains thermal

equilibrium. On subsequent use at higher temperatures, the sections of the thermoelements exposed to significantly higher temperature will have an increased number of lattice vacancies. As the thermocouple is first inserted into and

then withdrawn from the furnace, the thermoelements will acquire a nonuniform distribution of lattice vacancies along the thermoelement length. Measurements were also made of the sensitivity of the thermocouple emf to immersion of the reference-junction probe. These measurements showed that immersion of at least 20 cm into the ice/water mixture was sufficient to assure thermal equilibrium of the reference junctions with the ice/water mixture. On insertion of a reference- junction probe from room temperature into an ice bath, it is necessary to wait ten minutes for the reference junctions

inside the probe to reach a steady state temperature, within the uncertainty of the calibration. Over 1 000 h of use at temperatures up to 963 °C, a Au/Pt thermocouple will maintain this stated accuracy. Figure 14 shows the emf values at the silver freezing point of one Au/Pt thermocouple that has been used as a check

standard at NIST. The data shown were obtained from measurements of 105 freezes of the silver freezing-point cell from February of 1995 to April of 1997. No long term drift has been observed with several Au/Pt thermocouples

that have been studied for periods of time exceeding 1000 h and that have been maintained in accordance with the procedures of Section 5. Measurements on one of the SRM thermocouples at the silver freezing point confirmed that the special construction of the SRM 1 749 thermocouples did not impair this long term stability. The thermocouple, with its silica glass sheath attached, was aged at a temperature of 980 °C for a cumulative period of 800 h. Periodically during this process, the thermocouple was measured at the silver freezing point. The results of this test are shown in Fig. 15. The rms deviation of all measured points from the initial measurement at the silver point is only 0.055 uV,

equivalent to 2.2 m°C, which is less than the standard reproducibility of 2.8 m°C and significantly less than the combined standard uncertainty of 4.2 m°C at this temperature

16120.7

16120.6 > 16120.5 | 16120.4

16120.3

16120.2

o TC not annealed

• TC furnace annealed

Insulator changed

4m°C

100 200 300 Time of heating above 960 °C / h 400

Figure 14. Emf value at the silver freezing point of a thermocouple fabricated at the same time and from the same

wire lots as the SRM 1749 thermocouples, and used as a check standard.

16
-0.1 -0.1 200 400 600 Time of exposure at 980 °C / h

800

Figure 15. Inhomogeneity and change in the emf measured at the silver freezing point for an SRM 1749

thermocouple that was heated at 980 °C.

7. Mathematical methods for use with SRM 1749

The standard emf tables supplied with the SRM 1749 thermocouples have a resolution of 0.1 uV. This resolution is equivalent to a temperature resolution as poor as 1 7 m°C at 0 °C, which is considerably worse than the calibration uncertainty of the thermocouples. For the highest accuracy, it is necessary to use the polynomial calibration function N E = «0+Z<W > (4

i=1 with the coefficients that are supplied in Table 3 in the SRM 1 749 Certificate. An example of these coefficients is given in Table 3. To determine the emf value of the SRM 1749 thermocouple at a particular temperature,

straightforward evaluation of the calibration function suffices. The software used must perform the calculations with an accuracy of at least eight digits. Table 4, which includes a set of temperature and emf data pairs out to a

resolution of the equivalent of 0.1 m°C evaluated from the coefficients in Table 3, may be used to check the validity of user software.

Table 3. Sample coefficients, ah of a calibration function for SRM 1749 thermocouples, (each SRM 1749

thermometer is supplied with unique values of a,.) The function gives the thermoelectric voltage, E, in

millivolts, as a function of temperature, f90 , on the ITS-90 for the temperature range 0 °C to 1000 °C.

Coefficient a0 -1.050 000 00x1

0"4 ai 6.035 698 61x10"3 a2 1.936 759 74x1

0"5 a3 -2.229 986 14x10^

a4 3.287 118 59x1

0~11 as -4.242 061 93x1

0"14 a6 4.569 270 38x1

0"17 a7 -3.394 302 59x1

0"20 a8 1.429 815 90x1

-23 a9 -2.516 727 87x1

0"27

17
Table 4. Temperature-emf data pairs evaluated from the coefficients in Table 3, to a resolution of the equivalent of

0. 1 m°C. These data pairs may be used to check the validity of user software. £/mV

0 -0.000 105 0 100 0.777 746 3 200 1 .844 884

300 3.141 542

400 4.633 170 500 6.300 671 600 8.134 800

700 10.131 941 800 12.290 580

900 14.609 001 1000 17.085 005

Table 5. Coefficients c, of an approximate inverse function for Au/Pt thermocouples. This function gives temperature, t90 , in degrees Celsius as a function of the thermoelectric voltage, E, in millivolts in the specified temperature and voltage range. The error of this inverse function is ±1 °C. See Eq. 5. Note that the error of this inverse function is much larger than the error of the inverse function in Ref. [18], which is somewhat more mathematically complex. The function below should be used only in conjunction with

the method of section 7.

Coefficient

Co 0.000 000

Ci 1.545 738x1

0

2 02 -4.219 267x1 1 c3 1.351 401x101 c4 -2.888 146

c5 3.931 653x10_1 c6 -3.366 290x1 0~: Ci 1.751 092x1

0~: c& -5.047 341x10"£ c9 6.176 037x1

0"7

To determine the temperature value of the SRM 1749 thermocouple at a measured emf value requires a more

complex method. Because there is no exact inversion of the polynomial calibration function, a numerical inversion technique must be used. The steps of this procedure are as follows:

1 . Evaluate the approximate inverse temperature /app, using the inverse function N

'app = c. + ^Ej , (5

i=l where the coefficients c, are listed in Table 5 and Em is the measured value of emf in units of millivolts. (The

approximate inverse equation given in Ref. [18] may also be used. This alternate function gives a somewhat

more accurate initial value for rapp , but it is only the tolerance specified in step 5, and not the choice of

approximate inverse function, that determines the accuracy of the final value of t.) 2. Define the function G(t) = Em - Ecai (t), where Ecal (t) is the SRM 1749 calibration function given in Table 3 of

the Certificate.

3. The derivative of G(t) is given as G\t) = -2>/-' . (6

1=1

18
4. Use the iterative technique termed Newton's method to find the value of t that will give G{t) = 0. This value of t is the desired temperature value corresponding to the measured emf value. For each iteration of Newton's method, an improved temperature value, t u is determined from a starting value of temperature, t0 , using the equation:

r, = t

0 -G{tQ )/G'(tQ ) . (7 For the first iteration, t0 = tapp . For later iterations, the value of t0 is taken as the value of t\ determined in the previous step.

5. Repeat iterations of Newton's method until the change \t0 - t t \ is less than a tolerance set by the user.

In practice, only one or two iterations will be necessary, since all of the SRM 1749 thermocouples are a very close match to the reference function. Again, for checking user software, Tables 3 and 4 are useful.

8. Maintenance of the thermocouple

Pure element thermocouples are fairly rugged devices, capable of withstanding repeated thermal cycling and small mechanical shocks. However, three maintenance procedures are necessary to ensure that there is no degradation in accuracy.

First, the platinum expansion coil at the measuring junction should be inspected periodically. At room temperature, both thermoelements should extend from the alumina tube by the same amount as when the thermocouple was first manufactured. If the protrusion of one of the thermoelements increases by more than about 1 mm after a period of

heating to elevated temperatures and then cooling to room temperature, the thermoelement may have seized slightly

in the bore of the alumina tube and may have been mechanically strained.

Second, pure element thermocouples should be given a periodic maintenance anneal especially if they are removed

quickly from high temperature environments or if they are used in different thermal environments. The procedure

for Au/Pt thermocouples is to furnace anneal the thermocouple at 450 °C overnight at the fullest possible immersion

of the thermocouple into the furnace. This procedure reduces the number of lattice vacancies quenched into the thermoelements when the thermocouple is removed rapidly from a high-temperature environment. At NIST, the furnace used for furnace annealing is a three zone, horizontal tube furnace. During a heat treatment, the temperature along the portion of the thermocouple extending from its measuring junction to approximately 62 cm from the junction was uniform within 2 °C.

Third, the copper leads of the SRM 1749 thermocouple are not plated, and will become oxidized with time. Although the use of a junction box similar to the one described in Section 10 will minimize the thermal gradients near the terminals, it is still recommended that the copper oxide be removed occasionally by pulling the short, exposed sections of bare copper lead wire gently through a folded abrasive pad. With extensive use, these copper

wires may become work-hardened and fragile. In this event, the user may cut off the short affected length of copper

wire and strip off a short section of wire insulation without affecting the calibration of the thermocouple.

9. Preparation of an ice bath for the reference junctions

The low uncertainty of the SRM 1749 thermocouples requires a carefully prepared ice bath to maintain the temperature of the reference-junction probe at 0 °C. A properly prepared ice bath will have an expanded uncertainty of 2 m°C. The ice for the ice bath should be finely-crushed or shaved ice that has been prepared from distilled water. The ice should be saturated with distilled water, and then packed gently into an insulating Dewar flask, such that ice fully

fills the volume of the flask with no large voids. At NIST, a cylindrical flask (7 cm inner diameter and 30 cm deep) having a polyethylene-foam cover, 2.5 cm thick, is used. Other flask geometries are allowable, provided the flask is at least 6 cm inner diameter and 30 cm deep, and the thickness of the cover is in the range 1.5 cm to 3 cm. The level of the ice-water mixture should be within 5 mm of the bottom surface of the cover. The cover should have a hole of

approximate diameter 6.3 mm in the center, to allow insertion of the reference-junction probe of the SRM 1749 into

19
the ice point. The reference-junction probe should be inserted until the stop on the probe is butted against the flask cover. Since the ice in the Dewar flask will tend to float as the ice melts, a rubber band should be used to secure the cover onto the flask. The use of electronic ice-point compensators, extension wires, and automated ice points cooled with thermoelectric modules is not recommended unless a careful analysis of the additional uncertainties is performed. These devices,

in general, contribute additional errors to the measurement that are large relative to the calibration uncertainty of the SRM 1749.

It is possible to use a triple point of water (TPW) cell to maintain the reference junctions at a temperature of 0.01 °C.

If this is done, a quantity of 0.056 uV should be added to the measured emf to compensate for the reference temperature differing from the nominal 0 °C. The enclosure used to maintain the TPW cell should be configured so

that the thermal profile along the reference-junction probe of the SRM 1749 is similar to the profile that the probe would be exposed to in a ice bath as described above.

10. Emf measurements

The calibration uncertainty of the SRM 1 749 thermocouples, when expressed in units of voltage, ranges from an expanded uncertainty of 0.04 uV at a thermocouple signal of 0 u.V (0 °C) to 0.37 U.V at a thermocouple signal of

17,085 uV (1000 °C). In comparison, many voltmeters have expanded uncertainties significantly greater than this. In order to achieve the inherent accuracies possible with the SRM 1749 thermocouple, it is necessary to select the equipment for the voltage measurements carefully and to take the measurements using proper procedures. This

section describes considerations for equipment selection and describes the methods used at NIST to measure

voltage. Although it is possible to connect the SRM 1 749 thermocouple directly to a digital voltmeter, lower measurement

uncertainties and greater convenience are possible by connecting the digital voltmeter to a scanner, which is an

instrument that allows sequential connection of a voltmeter (or other instrument) to any one of a number of voltage

sources. A single set of terminals to be used for reading one voltage source consists of a positive-polarity terminal,

a negative-polarity terminal, and possibly a guard terminal, and is commonly termed a "channel" on the scanner. Because the terminals for each channel are often not conveniently placed for attaching leads of the thermocouple, it is desirable to extend the terminal connections of the scanner to a specially-constructed junction box. Consequently

there are three pieces of equipment needed for high accuracy voltage measurements: a digital voltmeter, a scanner, and a junction box. Each of these items will be considered below, and then the data acquisition procedures to be

used for the combined system will be described.

10.1 Digital voltmeters

Manufacturers typically state the specifications for DC voltage measurements with a digital voltmeter as a fraction of the voltage E that is measured, plus a fraction of the range. For example, on a 100 mV range, the specifications may be quoted as 4H10-6 of reading plus 3H10"6 of range, which is mathematically expressed as a tolerance a =(4H10"6£' + 0.3 uV). If the user does no further characterization of the voltmeter, then the standard uncertainty

contributed by the voltmeter may be estimated by assuming that the manufacturer's tolerance sets bounds Va for a

rectangular distribution of measurement errors. The standard uncertainty of such a distribution is a/v3, or for the example above, u(E) = (4H10"6£ + 0.3 uV)/V3.

In practice, we have found that it is possible with proper measurement techniques to greatly reduce the component

of u(E) that is independent of E. To achieve a smaller value of u(E) it is necessary to understand the various effects

that can cause voltmeter measurement uncertainties. The sources of voltage measurement errors include the following effects:

1 . Gain error of the voltmeter, resulting in R = (1 + a)E, where R is the reading of the voltmeter and a =

constant. Although a is independent of E, it will in general vary with the temperature of the voltmeter and may change over periods of time from several days to several months.

20
2. Offset error of the voltmeter, resulting in R = E + 6. Typical sources of 5 are differences in the thermal emfs

inside the voltmeter relative to the thermal emfs that existed when the voltmeter was calibrated, and possibly

other noise contributions that vary at such a low frequency that signal averaging is ineffective. The value of 5

varies over time scales ranging from several minutes to several months.

3. Nonlinearity of the voltmeter, resulting in higher order terms in the R(E) relation than can be modeled with a simple linear relation. In most modern voltmeters, the non-linearity is of the same order as the magnitude of

the least significant digit. Of these three components, only the offset error is amenable to reduction by the user. Non-linearity is difficult to measure and tends to be a small effect in any case. The gain error in general is different for each measurement range and may vary substantially as the internal temperature of the voltmeter varies. One could correct for gain error by

measurements of a reference voltage every hour or so on the same range as the thermocouple measurements, but it is difficult to build or obtain a reference voltage of sufficient accuracy at the low voltages generated by thermocouples.

Fortunately, the offset error is often a large contribution to the overall uncertainty, and demonstrated reduction in the

offset error can justify a reduction in u(E). A procedure that is effective in both reducing the offset error of the voltmeter and in correcting for thermal emfs contributed by the scanner is described in the Sections 10.2 and 10.4. In selecting a voltmeter for use with the SRM 1749, there is no perfect choice. So called "nanovoltmeters" have low

offset errors but relatively large gain errors. These units are ideal when the manufacturer's specifications for the term in the DC voltage accuracy proportional to E is acceptably low. Other voltmeters without input stages optimized for low voltages, but with a high number of significant digits in the readout, typically have larger offset errors and greater noise, but substantially smaller gain errors than nanovoltmeters. With careful measurement

techniques and with sufficient signal averaging to reduce the measurement noise, this type of meter has given the most accurate results at NIST.

10.2 Scanners

At NIST, we have had success with a variety of scanners that have been optimized for low-level DC voltage measurements. Some scanners have been designed with extremely low thermal emfs (less than 50 nV), and these

are highly appropriate for use with the SRM 1749 thermocouple. Surprisingly, scanners with specifications that are significantly less stringent will serve well provided that the scanner is appropriately selected and that the data

acquisition procedure described below is followed. As a general rule, avoid scanners that route analog signals via multipin connectors, which can be a significant source of extraneous thermal emf. As an example of a connector

type that should be avoided for measurements of the highest accuracy, some scanners require that the signal leads be

terminated at a multipin connector, which then is plugged into a mating connector on a circuit board of the scanner. The signal path from the thermocouple should pass through only screw connectors, copper wires or circuit board

traces, and the relays. Multipin connectors that provide power or that route digital control signals are acceptable. If the scanner uses electromechanical relays that are continuously energized while a particular channel is being read, the heat from the relay coil may cause significant changes in the thermal emfs. This effect can be minimized by a) energizing each relay for no more than 1 minute at a time, b) using each relay at a duty cycle of no more than 20%,

and c) setting the scanner to a specially-designated "hold" channel when data is not being acquired. To achieve the low duty cycle while still utilizing all available measurement time, it is convenient to route the thermocouple signal

to several pairs of terminals by using bare copper wires between terminals on the junction box. Even with the above precautions, we have found that most commercial scanners still have extraneous thermal emfs

that are unacceptably large for use with the SRM 1749 thermocouple. These emfs are relatively reproducible, though, and it is straightforward to correct for them. The procedure is as follows:

1 . Each channel of the junction box is shorted with bare copper wire, the top of the box is covered with a layer or box of insulating foam to reduce temperature gradients, and the wiring is allowed to come to thermal

equilibrium for 15 minutes.

2. One channel, or a set of channels, is designated as the "short" channel. The copper shorting wires will remain on these channels for all measurements, including measurements of the thermocouple emf. The "short" channel or channels is the reference against which other channels are compared.

3. Using the identical software used for acquisition of thermocouple emf data, each of the junction box channels

is measured several times.

21
4. The readings for each channel are averaged. The difference between the measured emf of each channel and

the average of the short channel(s) is computed. This difference is then used to correct all later readings on

that channel. For example, assume that channel 1 will be the short channel and channel 2 will be used for the thermocouple readings. If channel 2 reads an average of 0.1 uV higher than channel 1, when both are shorted, then subsequent readings of the thermocouple on channel 2 should be reduced by 0.1 uV to compensate for this difference.

5. Note that the absolute value of voltage read on each channel is not used in the computation of this channel

correction. Adjustment of the data for a voltage reading on the short channels not equaling zero will be

discussed below.

It is recommended that this procedure be done several times, at different times during the day, to evaluate the fluctuations in the channel corrections. A plot of the channel corrections versus time will show random fluctuations due to measurement noise and possibly drift over periods of several hours or days due to changes in the extraneous thermal emfs of the wiring and scanner card. The frequency of the measurement of the channel corrections can be

determined from the observed drift rate of the corrections. At NIST, to achieve the highest possible accuracy, we

determine the corrections as often as twice a day. Connection of the scanner to a junction box and the construction ofjunction boxes are discussed in the next section.

If the scanner has mechanical relays, circuit heating by the relay coil may be minimized by closing the relay on each

channel only long enough to make the measurement, then proceeding to a different channel. If there is an extended

period between measurements, the scanner may be set to a channel that is dedicated to monitoring purposes only, and that is never used in the acquisition of calibration data. When evaluating a new scanner, it is also desirable to

test all scanner channels by placing copper wire shorts across each set of terminals on the isothermal block, and then measuring the emf for each channel. Channels that give outlying values of emf should not be used, or used for noncritical purposes.

In regular use, one of the scanner channels should remain shorted at the isothermal box. Periodic readings of the voltmeter on this channel will allow correction for the voltmeter "zero". This procedure is superior to use of any

"null" button on the voltmeter because the readings of the voltmeter on the shorted channel can be monitored.

10.3 Junction boxes and wiring

The isothermal junction boxes used at NIST consist of an aluminum box with a set of labeled terminals on top of the box. The terminals are assemblies of gold-plated copper washers and nylon screws that fasten into a 6.4 mm

aluminum plate. One of the washers for each screw is a specially made washer with a "pigtail" that passes through an insulated hole in the aluminum plate. Each of the labeled terminals is connected to the corresponding screw

terminal inside the scanner, using bare copper wire insulated with polyvinylchloride polymer. For the NIST boxes,

the copper lead wires are fastened to the pigtails with low thermal emf solder (Cd-Sn eutectic). However, cadmium

vapors generated during soldering are quite toxic and Cd-Sn is hard to obtain. Alternatively, the connection between the terminal and the wire can be made by one of three techniques: a) solder with pure indium, b) establish a

tight mechanical connection by clamping between two nuts, or c) establish a tight mechanical connection by

crimping a soft pure copper tube over the terminal and wire. The thermal emf of indium is quite low relative to copper, and so the soldered joints will not generate significant thermal emf. (However, copper-indium joints may

become weak over many years due to atomic diffusion at the interface.) Wiring from the scanner to the junction box is fabricated from bare copper wire insulated with polyvinyl chloride polymer. Such wire is sold as telephone wire or as specialized wiring for computer network applications. For immunity from electromagnetic pickup, the wires for the positive and negative lead of each channel should be a

twisted pair. It is possible to wire the junction box using the multicolored twisted pairs of the as-supplied wire, but

if the scanner terminals in use are at a hotter temperature than ambient, small differences in the copper compositions of the different-colored wires will result in small extraneous thermal emfs. At NIST, all scanners with terminal temperatures more than several degrees hotter than ambient are wired with twisted pairs fabricated from a single length of wire folded back on itself and then twisted. Fabrication of this twisted pair and the work of identifying the

positive and negative leads of each pair are time consuming, but the reduction in stray thermal emf is significant.

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The terminals on the top of the box are designed to generate minimal thermal emfs. At NIST, we use home-made

terminals made of gold-plated copper, secured to the aluminum top with nylon screws and insulated from the top with nylon washers. Commercial binding posts made of tellurium copper will also suffice. The aluminum top itself

is either painted or anodized to prevent electrical shorts by accidental contact of thermocouple wires with the top of

the box. The aluminum top is approximately 6 mm thick, to provide effective shunting of any thermal gradients in the vicinity of the box. A schematic diagram of a junction box is shown in Fig. 16.

Figure 16. Schematic cross section of a junction box with low thermal emfs, constructed with commercial binding

posts.

10.4 Methods for data acquisition For the highest accuracy results, emf measurements should be taken using an automated system. This is especially important in two cases: 1) when using voltmeters that have not been optimized for very low voltage measurements, automated signal averaging of multiple measurements is necessary to reduce the measurement uncertainty, and 2) when a scanner with electromagnetic relays is used, any relay heating is predictable and can be corrected for, provided that the data acquisition is automated to give highly repeatable timing of the data acquisition. At NIST, we have achieved excellent results by setting the internal filters or time constants of the voltmeter at an equivalent time constant of approximately 0.5 s, and then performing additional averaging of the data after computer

acquisition. An example of a typical data acquisition cycle follows: Channel 1 Short, 10 readings spaced at 0.5 s intervals Channel 3 Thermocouple, 10 readings spaced at 0.5 s intervals Channel 4 Thermocouple, 10 readings spaced at 0.5 s intervals Channel 5 Thermocouple, 10 readings spaced at 0.5 s intervals Channel 8 Short, 10 readings spaced at 0.5 s intervals Repeat above cycle five times Channel 12 Test thermocouple ... no calibration data acquired on this "hold" channel

After each channel is selected, there is a short wait (less than one second) for the voltages to settle, and then 1

readings are acquired at 0.5 s intervals. Note that some channels may not be used (channels 2, 6, and 7 above), after determination by the procedure in Section 10.2 that some channels have larger than desired emf corrections or poor

repeatability of the emf corrections. The parallel connection of one test thermocouple to multiple channels enables almost continuous data acquisition while minimizing the self-heating of any one channel. Additionally, any errors

23
in the determination of the scanner emf corrections is averaged out somewhat. Once a data acquisition procedure has been chosen, the same procedure should be used to measure the scanner emf corrections as is used to acquire the thermocouple emf data.

Following acquisition of the data from the cycle above, the scanner emf corrections are applied and the average of

the "short" channels is subtracted from the readings on the "thermocouple" channels. The resulting values are corrected both for offset errors of the voltmeter and for emf offsets of the scanner channels.

11. Preventing thermocouple contamination

At high temperatures, metal ions can readily diffuse through silica glass and metals or metal oxides can become

volatile. If the gold or platinum thermoelements of the thermocouple are contaminated by other metals, significant degradation of the thermocouple properties will occur.

Unlike thermometers such as platinum resistance thermometers, a thermocouple generates emf only in regions where the thermoelements pass through a thermal gradient. In proper usage, the measuring junction of a thermocouple is placed in a relatively isothermal environment, deep inside a uniform zone of a furnace or isothermal

block, for example. As a consequence of the small thermal gradients, the portion of the thermocouple near the measuring junction contributes only a small portion of the total emf generated by the thermocouple, and

contamination of the measuring junction region of the thermocouple will have negligible effect. In contrast, it is very important to protect from chemical contamination the portion of the thermocouple that passes from room

temperature to a temperature close to that of the measuring junction. Experience with type S thermocouples has shown that sintered alumina provides an effective barrier to thermocouple contamination, provided that the alumina

is not cracked. We do not recommend that the inner alumina insulator of the SRM 1749 thermocouple be directly inserted into environments that could potentially contaminate the alumina. Instead, the supplied glass sheath or a customer-supplied alumina or glass sheath should be used when inserting the thermocouple into high-temperature metal blocks or similar environments.

For Au/Pt thermocouples mounted in two-bore alumina insulators and then placed in a large diameter, high-purity alumina tube, we have seen no evidence of drift after 1000 h of heating at 962 °C. For Au/Pt thermocouples mounted in two-bore alumina insulators and then placed in a silica glass tube that is surrounded by a copper

isothermal block, we have seen no evidence of drift after approximately 100 h of heating at 1000 °C.

If the Au/Pt thermocouple is used in an Inconel equilibration block with a silica glass sheath around the thermocouple, the drift in the thermocouple should be no worse than the case above with the copper block, since copper is both more volatile and more mobile than the constituents of Inconel.

12. Comparison measurements with the Au/Pt thermocouple A common application of a Au/Pt thermocouple is to calibrate a second thermometer by comparison against the Au/Pt thermocouple. The success of this measurement requires careful attention to 1) the rate of change of

temperature with time and 2) the effects of temperature non-uniformity.

If the temperature of the furnace or bath used for the calibration varies with time, there are two effects:

First, the diffusion of heat into (or out of) the measuring junction of the Au/Pt thermocouple and into the test thermometer is not instantaneous, and there will be dynamic temperature gradients in the vicinity of both thermometers, in addition to the static gradients that exist when the furnace is at a constant temperature. This effect can be measured by performing calibrations at different temperature drift rates of the furnace or bath. Data should be obtained for both rising and falling temperatures. A plot of the temperature reading of the Au/Pt thermocouple minus the reading of the test thermometer versus temperature drift rate will indicate the sensitivity of the calibration

to the drift rate. The sensitivity to temperature drift will depend not only on details of the furnace or bath environment, but also on the thermal characteristics of the test thermometer.

24
Second, if the test thermometer and the Au/Pt thermocouple are read sequentially instead of simultaneously, there

will be an error equal to the furnace drift rate multiplied by the time difference of the two readings. When the temperature is changing approximately linearly with time, this second effect can be minimized by taking readings in the order: Au/Pt thermocouple, test thermometer, Au/Pt thermocouple at equal time intervals. Averaging the two

readings of the Au/Pt thermocouple gives the effective thermocouple reading at the time of the reading of the test thermocouple.

If the temperature drift of the furnace or bath is zero, the temperatures of the Au/Pt thermocouple and the test thermometer will still differ, in general, because of static temperature gradients in the furnace. We have had good

results using Au/Pt thermocouples as reference thermometers in stirred-liquid baths of a variety of types. Use of Au/Pt thermocouples as a reference thermometer in a furnace is much more problematic, since typical spatial temperature variations in a furnace can be much higher than the uncertainty of Au/Pt thermocouples. To attain the highest accuracy possible in calibrating a test thermocouples against a Au/Pt thermocouple in a

furnace, a measuring junction common to all of the thermocouples is formed. Unfortunately, this method is labor

intensive, requires skillful manipulation of the thermocouple wires, and can only be used in the calibration of other noble metal thermocouples. The method should only be attempted by those highly skilled in welding noble metal

wires. First, any expansion coils or measuring junctions are removed. Second, each thermoelement is extended by

welding to it a 1.5 cm length of high purity, 0.12 mm diameter wire of the same nominal composition as the thermoelement. Finally, the ends of these fine wires are welded together to form a common measuring junction, leaving sufficient slack in the wires to minimize the effects of the different thermal expansions of the three metals. The common measuring junction ensures that the measuring junctions of all thermocouples are at virtually the same

temperature. This method has been used in the determination of a highly accurate reference function for Pt/Pd thermocouples, for example [8]. An alternate method of reducing temperature gradients is to place an isothermal block inside the furnace. Such

blocks are commercially available, fabricated out of Inconel, graphite, or nickel. Prior to use in regular calibrations, the performance of an isothermal block should be tested by performing calibrations at varying rates of heating and

cooling, and by interchanging positions of test and reference thermocouples, and by varying the immersion depths of

the test and reference thermocouples. At the time of preparation of this document, there is little guidance available on the performance of various isothermal block designs with Au/Pt thermocouples. This is an active area of research.

13. References

[1] H. Preston-Thomas, The International Temperature Scale of 1990 (ITS-90), Metrologia 27, 3-10 (1990); Erratum, ibid., 107.

[2] Manual on the Use of Thermocouples in Temperature Measurement, 4

th ed., ASTM Manual Series: MNL 12, Philadelphia, ASTM ( 1993).

[3] T. P. Jones, The Accuracies of Calibration and Use of I.P.T.S. Thermocouples, Metrologia 4, 80-83 (1968).

[4] E. H. McLaren, E. G. Murdock, New Considerations on the Preparation, Properties, and Limitations of the Standard Thermocouple for Thermometry, in Temperature: Its Measurement and Control in Science and

Industry, Vol. 4 (Edited by H. H. Plumb), Pittsburgh, PA, Instrument Soc. America (1972) 1543-1560.

[5] E. H. McLaren and E. G. Murdock, Properties of Some Noble and Base Metal Thermocouples at Fixed Points

in the Range 0-1000 °C, in Temperature: Its Measurement and Control in Science and Industry, Vol. 5 (Edited by J. F. Schooley), New York, American Institute of Physics (1982) 953-975.

[6] R. E. Bentley and T. P. Jones, Inhomogeneities in Type S Thermocouples When Used to 1064 °C, High

Temperatures - High Pressures 12, 33-45 (1980).

25
[7] G. W. Burns, G. F. Strouse, B. M. Liu, and B. W. Mangum, Gold versus Platinum Thermocouples:

Performance Data and an ITS-90 Based Reference Function, in Temperature: Its Measurement and Control

in Science and Industry, Vol. 6 (Edited by J. F. Schooley), New York, AIP, 1992, p. 531-536.

[8] G. W. Burns, D. C. Ripple, and M. Battuello, Platinum versus Palladium Thermocouples: an EmfTemperature Reference Function for the Range 0 °C to 1500 °C, Metrologia 35, 761-780 (1998).

[9] R. L. Powell, L. L. Sparks, and J. G. Hust, Standard Reference Materials: Standard Thermocouple Material,

Pt-67: SRM-1967. Natl. Bur. Stand. (U.S.) Special Publ. 260-56, Washington, D.C., U. S. Govt. Printing

Office, 1978, 50 p.

[10] E. H. McLaren and E. G. Murdock, The Pt/Au Thermocouple, Part I: Essential Performance, Part II: Preparatory Heat Treatment, Wire Comparisons and Provisional Scale, Ottawa, National Research Council of Canada Publication NRCC/27703, 1987.

[11] G. W. Burns and M. G. Scroger, NIST Measurement Services: The Calibration of Thermocouples and Thermocouple Materials, NIST Special Publication 250-35, Washington, D.C., U. S. Government Printing

Office, 1989, 201 p.

[12] D. Ripple, G. W. Burns, and M. G. Scroger, Assessment of Uncertainties of Thermocouple Calibrations at NIST, NISTIR 5340, Gaithersburg, MD, NIST, 1 994, 34 p.

[13] Guide to the Expression of Uncertainty in Measurement, Intern. Org. for Standardization (ISO), Switzerland,

1993, 101 p.

[14] D. R. White and J. M. Williams, Resistance Network for Verifying the Accuracy of Resistance Bridges, IEEE

Trans. Instr. Meas. 46 (2), 329-332 (1997).

[15] B. V. Hamon, A 1-100 Q build-up resistor for the calibration of standard resistors, J. Sci. Instr. 31, 450-453

(1954).

[16] M. E. Cage, D. Yu, B. M. Jeckelmann, R. L. Steiner, and R. V. Duncan, Investigating the Use of Multimeters

to Measure Quantized Hall Resistance Standards, IEEE Trans. Instr. Meas. 40 (2), 262-266 (1991).

[17] B. W. Mangum, Reproducibility of the Temperature of the Ice Point in Routine Measurements, NIST

Technical Note 141 1, Washington, D.C., U. S. Government Printing Office, 1995, 24 p.

[18] Standard Guide for Temperature Electromotive Force (EMF) Tables for Non-Letter Designated Thermocouple Combinations, ASTM E 1751-95, West Conshohocken, PA, ASTM (1995).

26
^Institute ai Btnnbzcvbs 8c W,ec

Standard Reference Material® 1749

Gold versus Platinum Thermocouple

Certified Thermometer for the Range 0 °C to 1000 °C

on the International Temperature Scale of 1990

Serial No.

This Standard Reference Material (SRM) is intended for use as a highly accurate secondary reference thermometer in the range from 0 °C to 1000 °C [1]. SRM 1749 consists of a specially constructed gold versus platinum (Au/Pt) thermocouple, individually calibrated on the International Temperature Scale of 1990 (ITS-90) [2]. This SRM has an overall length of 1.9 m, as measured from the tip of the silica-glass sheath (7.0 mm outer diameter, 71 cm long) to the tip of the stainless steel sheath of the reference-junction probe (6.3 mm outer diameter, 28 cm

long). The copper leads from the reference-junction probe of the SRM are 2. 1 m long. In use, the reference- junction probe of the SRM is inserted into a properly prepared ice point bath and the copper leads are connected

to a high-accuracy voltmeter. The relation of voltage readings of the SRM to the temperature of the measuring

junction is provided in this certificate in tabular and mathematical forms.

Certified Values and Confidence Limits: The electromotive force (emf) of the SRM was measured in fixed- point cells at 961.78 °C (silver point), 660.323 °C (aluminum point), 419.527 °C (zinc point), 231.928 °C (tin point), and 156.5985 °C (indium point), and in an ice bath at 0.000 6C, with the reference junctions at the ice point. The measured values of emf are given in Table 1. The emf values computed from the reference function

[3] were subtracted from the measured values, and the resulting emf deviations were modeled by a quadratic function of temperature up to 1000 °C. Coefficients of the quadratic function were determined by the method of

least squares, and addition of these coefficients to those of the reference function gave the coefficients of the calibration function for this SRM. Table 2 gives corresponding values of the emf of the SRM in millivolts and the temperature of its measuring junction in degrees Celsius when the reference junctions are at 0 °C. Table 3 gives

the coefficients of the equation that was used to compute the emf values given in Table 2. Figure 1 shows the deviations of the measured values of emf from values obtained from the Au/Pt reference function. The

thermoelectric inhomogeneity of the thermocouple was assessed by measuring its immersion characteristics in the silver and aluminum fixed-point cells [1]

The uncertainty of an emf value can be expressed as an expanded uncertainty, U — kuv with U determined from a combined standard uncertainty, ue and a coverage factor, k = 2. The expanded uncertainties of the emf values given in Tables 1 and 2 are estimated not to exceed the equivalent of 7 m°C in the range 0 °C to 660 °C, the equivalent of 8 m°C in the range 660 °C to 962 °C, and the equivalent of 14 m°C in the range 962 °C to 1000 °C. These uncertainties include a contribution for the inhomogeneity of the thermocouple.

Construction and calibration of the Au/Pt thermocouple were performed by G.W. Burns and D.C. Ripple of the NIST Process Measurements Division. The support aspects involved in the preparation, certification, and issuance of this SRM were coordinated through

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

Gaithersburg, MD 20899 Thomas E. Gills, Chief

Certificate Issue Date: 18 March 1998 Standard Reference Materials Program SRM 1749 Page 1 of 10
Expiration of Certification: The certification of this SRM is valid within the measurement uncertainties

specified, for at least 1000 h of use at temperatures up to 963 °C, provided the SRM is used in accordance with

the Notice and Warning to Users section of this certificate. No long term drift has been observed with several Au/Pt thermocouples that have been studied for periods of time exceeding 1000 h and that have been maintained

in accordance with the procedures described in the Instructions for Use. Recertification may be arranged by

contacting D.C. Ripple of the NIST Process Measurements Division by phone (301) 975-4801, fax (301) 548-0206,

or e-mail dean.ripple@nist.gov. Certification is invalid if the SRM is damaged, comaminated, or modified. Maintenance of SRM Certification: NIST will monitor this SRM over the period of its certification. If substantive technical changes occur that affect the certification, NIST will notify the purchaser. Return of the attached registration card will facilitate notification. Source of Material: The gold wire (Bar 100-431) and the platinum wire (Bar 240-183) for the SRM have a

reported purity as a mass fraction of 99.999+ % and were obtained from the Sigmund Conn Corporation, Mount

Vernon, NY. The alumina insulator tubes have a reported purity of 99.8+ % and were obtained from Vesuvius McDanel, Beaver Falls, PA. NOTICE AND WARNING TO USERS

Storage: SRM 1749 is a thermocouple thermometer, packaged in foam in a wooden case. When not in use, the thermocouple should be stored in its original case to avoid damage.

Handling: This SRM is supplied with a protective silica-glass sheath. If this sheath is damaged, the thermocouple may be subject to contarnination with resulting degradations in performance. The thermocouple may be used with or without the protective sheath. However, use of the thermocouple without the protective sheath should be attempted only by trained personnel. Extreme care must be taken to ensure that the expansion

coil is not damaged and that the thermocouple is used only in oxidizing environments free of vapors from metals or metal oxides.

Instructions for Use: To use, the copper leads of the SRM should be connected to a high-accuracy voltmeter. To achieve the highest level of accuracy with this SRM, the user should use a calibrated voltmeter with a

resolution of 0.01 uV or better, and care must be taken to measure and correct for stray thermal emfs in the measurement circuit. The reference-junction probe is designed to be inserted through the lid of a standard Dewar flask filled with crushed ice, with distilled water filling the voids between the ice particles. The tip of the reference-junction probe

should be immersed 20 cm into the ice-water mixture. The ice should be made from distilled water, the flask should be at least 28 cm deep, and the ice should be lightly packed to the full depth of the flask. After inserting the probe into the ice bath, allow 10 min to 15 min for the probe to attain thermal equilibrium. The reference- junction probe should be kept clean of contaminants that could cause a depression of the melting point of the ice. Washing the probe in distilled water or wiping the probe with a tissue saturated in distilled water will aid in the removal of salts and other impurities. The silica-glass sheath will devitrify and eventually crack if it is used at temperatures above 500 °C after being contaminated with oils or salts from handling of the sheath with bare hands or other sources. This problem may

be avoided by wiping the sheath with a tissue saturated in ethanol or methanol prior to use at elevated

temperatures. The thermocouple should not be used at temperatures exceeding 1000 °C. Higher temperatures may cause

slippage of grain boundaries in the gold wire. During use or annealing, not more than 65 cm of the portion of the SRM sheathed in silica glass, as measured from the tip of the sheath, should be heated. Au/Pt thermocouples are fairly rugged devices, capable of withstanding repeated thermal cycling and small mechanical shocks. However, two maintenance procedures are necessary to ensure that there is no degradation in accuracy. SRM 1749 Page 2 of 10
First, the platinum expansion coil at the measuring junction should be inspected periodically. The silica-glass sheath does not have a matte finish in the vicinity of the measuring junction. This allows the user to inspect the

coil without removal of the protective sheath. At room temperature, both thermoelements should extend from the alumina tube by the same amount as when the SRM was first received from NIST. If the protrusion of one of the thermoelements increases by more than about 1 mm after a single period of heating to elevated temperatures and

then cooling to room temperature, the thermoelement may have become slightly lodged in the bore of the alumina

tube and may have been mechanically strained.

Second, Au/Pt thermocouples should be given a periodic maintenance anneal if they are removed quickly from

high temperature environments or if they are used in different thermal environments. The procedure is to anneal

the portion of the SRM sheathed in silica glass in a tube furnace at the fullest possible immersion (not more than 65 cm) into the furnace at 1000 °C for 30 min, cool slowly in the furnace to 450 °C, and maintain at 450 °C

overnight. The anneal at 1000 °C is optional and only necessary to alleviate strains introduced if the thermocouple has been rapidly cooled. PREPARATION AND CERTIFICATION MEASUREMENTS

Method of Preparation: After being annealed in air, the Au and Pt thermoelements were inserted into a high

purity alumina tube. The measuring junction of the Au/Pt thermocouple was formed by joining the thermoelements at one end with a small coil formed from Pt wire 0.12 mm in diameter. The thermocouple and

alumina insulator were mounted inside a protective silica-glass sheath, with the alumina insulator secured in the sheath with a compression fitting. To minimize heat loss by piping of thermal radiation, the silica-glass sheath was given a matte finish. The thermoelements extending from the insulator were covered with coated fiberglass

sleeving, and a crimp-type clamp was installed that compresses the sleeving against the thermoelements to anchor them near the end of the alumina insulator. At the end of the thermoelements opposite from the measuring

junction, matched copper leads were attached to form the reference junctions. The reference junctions are contained within a stainless steel sheath. To remove residual stresses introduced in the assembly process, the portion of the SRM sheathed in silica glass was given a final furnace anneal. Measurement Techniques: The emf measurements were made using a calibrated digital multimeter connected to a scanner. Stray thermal emfs from the scanner and voltmeter were carefully measured, and the data were

appropriately corrected. Data acquisition was computer controlled. The freezing-point cells used for the

calibration of this SRM were all constructed at NIST and intercompared with the reference cells maintained in the NIST Platinum Resistance Thermometer Calibration Laboratory. COMPATABDUTY WITH NIST STANDARD REFERENCE DATABASE 60

The equation in Table 3 is compatible with NIST Standard Reference Database 60, NIST ITS-90 Thermocouple

Database. For further information, contact D.C. Ripple of the NIST Process Measurements Division by phone

(301) 975-4801, fax (301) 548-0206, or e-mail dean.ripple@nist.gov.

SRM 1749 Page 3 of 10
REFERENCES

[1] Burns, G.W. and Ripple, D., "Standard Reference Material 1749: Gold vs. Platinum Thermocouple," NIST

Special Publication 260-134, U.S. Government Printing Office, Washington, DC, (1998).

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

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

[3] Burns, G.W., Strouse, G.F., Liu, B.M., and Mangum, B.W., "Gold versus Platinum Thermocouples: Performance Data and an ITS-90 Based Reference Function," in Temperature: Its Measurement and Control

in Science and Industry, American Institute of Physics, New York, NY, p. 531, (1992).

It is the responsibility of users of this SRM to assure that the certificate in their possession is current. This can be

accomplished by contacting the SRM Program at: Phone (301) 975-6776 (select "Certificates"). Fax (301) 926-4751,

e-mail srminfo@nist.gov, or via the Internet http://ts.nist.gov/srm. SRM Page 4 of 10 1749
Date of Certification: October 1, 1997 S/N 95-3

TABLE 1 .Values of EMF Determined at Fixed Points for the Au/Pt Thermocouple with the Reference Junctions atO°C

Temperature EMF

(°C, ITS-90) (mV)

961.78 (silver point) 16.12048 660.323 (aluminum point) 9.32029 419.527 (zinc point) 4.94546 231.928 (tin point) 2.23605

156.5985 (indium point) 1.35079 0.000 (ice point) -0.00008

SRM 1749 Page 5 of 10
Date of Certification: October 1, 1997 S/N 95-3

TABLE 2. Values of EMF in Millivolts versus Temperature in Degrees Celsius (ITS-90)

for the Au/Pt Thermocouple with Reference Junctions at 0 °C

°c 0 1 2 3 4 5 6 7 8 9 EMF ( tnV)

o -0.0001 0.0060 0.0121 0.0182 0.0244 0.0306 0.0368 0.0431 0.0494 0.0558

10 0.0622 0.0686 0.0751 0.0816 0.0882 0.0947 0.1014 0.1080 0.1147 0.1214 20 0.1282 0.1350 0.1418 0.1487 0.1556 0.1626 0.1696 0.1766 0.1836 0.1907 30 0.1978 0.2050 0.2122 0.2194 0.2267 0.2340 0.2413 0.2487 0.2561 0.2635 40 0.2710 0.2785 0.2860 0.2936 0.3012 0.3088 0.3165 0.3242 0.3320 0.3397 50 0.3475 0.3554 0.3632 0.3711 0.3791 0.3870 0.3950 0.4031 0.4111 0.4192 60 0.4274 0.4355 0.4437 0.4519 0.4602 0.4685 0.4768 0.4852 0.4935 0.5020 70 0.5104 0.5189 0.5274 0.5359 0.5445 0.5531 0.5617 0.5704 0.5791 0.5878 80 0.5965 0.6053 0.6141 0.6230 0.6318 0.6407 0.6497 0.6586 0.6676 0.6766 90 0.6857 0.6948 0.7039 0.7130 0.7222 0.7314 0.7406 0.7498 0.7591 0.7684 100 0.7778 0.7871 0.7965 0.8060 0.8154 0.8249 0.8344 0.8439 0.8535 0.8631 110 0.8727 0.8824 0.8921 0.9018 0.9115 0.9212 0.9310 0.9409 0.9507 0.9606

120 0.9705 0.9804 0.9903 1.0003 1.0103 1.0203 1.0304 1.0405 1.0506 1.0607 130 1.0709 1.0811 1.0913 1.1016 1.1118 1.1221 1.1324 1.1428 1.1532 1.1636 140 1.1740 1.1845 1.1949 1.2054 1.2160 1.2265 1.2371 1.2477 1.2583 1.2690

150 1.2797 1.2904 1.3011 1.3119 1.3227 1.3335 1.3443 1.3552 1.3661 1.3770 160 1.3879 1.3989 1.4098 1.4208 1.4319 1.4429 1.4540 1.4651 1.4762 1.4874 170 1.4986 1.5098 1.5210 1.5323 1.5435 1.5548 1.5661 1.5775 1.5889 1.6003 180 1.6117 1.6231 1.6346 1.6461 1.6576 1.6691 1.6807 1.6923 1.7039 1.7155 190 1.7271 1.7388 1.7505 1.7622 1.7740 1.7857 1.7975 1.8094 1.8212 1.8330 200 1.8449 1.8568 1.8688 1.8807 1.8927 1.9047 1.9167 1.9287 1.9408 1.9529 210 1.9650 1.9771 1.9893 2.0015 2.0136 2.0259 2.0381 2.0504 2.0627 2.0750 220 2.0873 2.0996 2.1120 2.1244 2.1368 2.1493 2.1617 2.1742 2.1867 2.1992 230 2.2118 2.2243 2.2369 2.2495 2.2622 2.2748 2.2875 2.3002 2.3129 2.3257 240 2.3384 2.3512 2.3640 2.3768 2.3897 2.4025 2.4154 2.4283 2.4413 2.4542 250 2.4672 2.4802 2.4932 2.5062 2.5193 2.5323 2.5454 2.5586 2.5717 2.5848 260 2.5980 2.6112 2.6244 2.6377 2.6509 2.6642 2.6775 2.6908 2.7042 2.7175 270 2.7309 2.7443 2.7577 2.7712 2.7846 2.7981 2.8116 2.8251 2.8387 2.8523 280 2.8658 2.8794 2.8931 2.9067 2.9204 2.9340 2.9477 2.9615 2.9752 2.9890 290 3.0027 3.0165 3.0304 3.0442 3.0581 3.0719 3.0858 3.0997 3.1137 3.1276 300 3.1416 3.1556 3.1696 3.1836 3.1977 3.2118 3.2259 3.2400 3.2541 3.2682 310 3.2824 3.2966 3.3108 3.3250 3.3393 3.3535 3.3678 3.3821 3.3964 3.4108 320 3.4251 3.4395 3.4539 3.4683 3.4827 3.4972 3.5117 3.5262 3.5407 3.5552 330 3.5697 3.5843 3.5989 3.6135 3.6281 3.6427 3.6574 3.6721 3.6868 3.7015 340 3.7162 3.7310 3.7457 3.7605 3.7753 3.7901 3.8050 3.8198 3.8347 3.8496 350 3.8645 3.8795 3.8944 3.9094 3.9244 3.9394 3.9544 3.9695 3.9845 3.9996 360 4.0147 4.0298 4.0449 4.0601 4.0753 4.0904 4.1057 4.1209 4.1361 4.1514 370 4.1667 4.1820 4.1973 4.2126 4.2279 4.2433 4.2587 4.2741 4.2895 4.3050 380 4.3204 4.3359 4.3514 4.3669 4.3824 4.3980 4.4135 4.4291 4.4447 4.4603 390 4.4760 4.4916 4.5073 4.5230 4.5387 4.5544 4.5701 4.5859 4.6017 4.6174 400 4.6333 4.6491 4.6649 4.6808 4.6967 4.7126 4.7285 4.7444 4.7604 4.7763 SRM 1749 Page 6 of 10
Date of Certification: October 1, 1997 S/N 95-3 TABLE 2 (Continued). Values of EMF in Millivolts versus Temperature in Degrees Celsius (ITS-90)

for the Au/Pt Thermocouple with Reference Junctions at 0 °C

°c 0 1 2 3 4 5 6 7 8 9 EMF (mV)

400 4.6333 4.6491 4.6649 4.6808 4.6967 4.7126 4.7285 4.7444 4.7604 4.7763 410 4.7923 4.8083 4.8243 4.8404 4.8564 4.8725 4.8886 4.9047 4.9208 4.9369 420 4.9531 4.9693 4.9855 5.0017 5.0179 5.0341 5.0504 5.0667 5.0830 5.0993 430 5.1156 5.1320 5.1483 5.1647 5.1811 5.1975 5.2140 5.2304 5.2469 5.2634 440 5.2799 5.2964 5.3129 5.3295 5.3460 5.3626 5.3792 5.3958 5.4125 5.4291 450 5.4458 5.4625 5.4792 5.4959 5.5126 5.5294 5.5462 5.5630 5.5798 5.5966 460 5.6134 5.6303 5.6472 5.6640 5.6810 5.6979 5.7148 5.7318 5.7488 5.7657 470 5.7828 5.7998 5.8168 5.8339 5.8510 5.8680 5.8852 5.9023 5.9194 5.9366 480 5.9538 5.9710 5.9882 6.0054 6.0226 6.0399 6.0572 6.0745 6.0918 6.1091 490 6.1264 6.1438 6.1612 6.1786 6.1960 6.2134 6.2308 6.2483 6.2658 6.2833 500 6.3008 6.3183 6.3359 6.3534 6.3710 6.3886 6.4062 6.4238 6.4415 6.4591 510 6.4768 6.4945 6.5122 6.5299 6.5477 6.5654 6.5832 6.6010 6.6188 6.6366 520 6.6545 6.6723 6.6902 6.7081 6.7260 6.7439 6.7618 6.7798 6.7978 6.8158 530 6.8338 6.8518 6.8698 6.8879 6.9060 6.9241 6.9422 6.9603 6.9784 6.9966 540 7.0147 7.0329 7.0511 7.0693 7.0876 7.1058 7.1241 7.1424 7.1607 7.1790 550 7.1973 7.2157 7.2341 7.2525 7.2709 7.2893 7.3077 7.3261 7.3446 7.3631 560 7.3816 7.4001 7.4186 7.4372 7.4558 7.4743 7.4929 7.5115 7.5302 7.5488 570 7.5675 7.5862 7.6049 7.6236 7.6423 7.6610 7.6798 7.6986 7.7174 7.7362 580 7.7550 7.7738 7.7927 7.8116 7.8305 7.8494 7.8683 7.8872 7.9062 7.9252 590 7.9442 7.9632 7.9822 8.0012 8.0203 8.0393 8.0584 8.0775 8.0967 8.1158 600 8.1349 8.1541 8.1733 8.1925 8.2117 8.2309 8.2502 8.2695 8.2887 8.3080 610 8.3274 8.3467 8.3660 8.3854 8.4048 8.4242 8.4436 8.4630 8.4825 8.5019 620 8.5214 8.5409 8.5604 8.5799 8.5995 8.6190 8.6386 8.6582 8.6778 8.6974 630 8.7171 8.7367 8.7564 8.7761 8.7958 8.8155 8.8352 8.8550 8.8748 8.8945 640 8.9144 8.9342 8.9540 8.9739 8.9937 9.0136 9.0335 9.0534 9.0734 9.0933 650 9.1133 9.1332 9.1532 9.1733 9.1933 9.2133 9.2334 9.2535 9.2736 9.2937 660 9.3138 9.3339 9.3541 9.3743 9.3945 9.4147 9.4349 9.4551 9.4754 9.4957 670 9.5159 9.5363 9.5566 9.5769 9.5973 9.6176 9.6380 9.6584 9.6788 9.6993 680 9.7197 9.7402 9.7607 9.7812 9.8017 9.8222 9.8428 9.8633 9.8839 9.9045 690 9.9251 9.9457 9.9664 9.9870 10.0077 10.0284 10.0491 10.0698 10.0906 10.1113 700 10.1321 10.1529 10.1737 10.1945 10.2154 10.2362 10.2571 10.2780 10.2989 10.3198 710 10.3407 10.3617 10.3827 10.4036 10.4246 10.4457 10.4667 10.4877 10.5088 10.5299 720 10.5510 10.5721 10.5932 10.6144 10.6355 10.6567 10.6779 10.6991 10.7203 10.7416 730 10.7628 10.7841 10.8054 10.8267 10.8480 10.8694 10.8907 10.9121 10.9335 10.9549 740 10.9763 10.9977 11.0192 11.0406 11.0621 11.0836 11.1051 11.1267 11.1482 11.1698 750 11.1914 11.2130 11.2346 11.2562 11.2778 11.2995 11.3212 11.3429 11.3646 11.3863 760 11.4080 11.4298 11.4516 11.4734 11.4952 11.5170 11.5388 11.5607 11.5825 11.6044 770 11.6263 11.6482 11.6702 11.6921 11.7141 11.7361 11.7581 11.7801 11.8021 11.8241 780 11.8462 11.8683 11.8904 11.9125 11.9346 11.9568 11.9789 12.0011 12.0233 12.0455 790 12.0677 12.0899 12.1122 12.1345 12.1567 12.1790 12.2014 12.2237 12.2460 12.2684 800 12.2908 12.3132 12.3356 12.3580 12.3805 12.4029 12.4254 12.4479 12.4704 12.4929

SRM 1749 Page 7 of 10
Date of Certification: October 1, 1997 S/N 95-3

TABLE 2 (Continued). Values of EMF in Millivolts versus Temperature in Degrees Celsius (ITS-90)

for the Au/Pt Thermocouple with Reference Junctions at 0 °C

°c 0 1 2 3 4 5 6 7 8 9 EMF ( tnV)

800 12.2908 12.3132 12.3356 12.3580 12.3805 12.4029 12.4254 12.4479 12.4704 12.4929 810 12.5155 12.5380 12.5606 12.5832 12.6058 12.6284 12.651

1

12.6737 12.6964 12.7191 820 12.7418 12.7645 12.7872 12.8100 12.8327 12.8555 12.8783 12.9011 12.9240 12.9468 830 12.9697 12.9925 13.0154 13.0383 13.0613 13.0842 13.1072 13.1301 13.1531 13.1761 840 13.1991 13.2222 13.2452 13.2683 13.2914 13.3145 13.3376 13.3607 13.3839 13.4070 850 13.4302 13.4534 13.4766 13.4998 13.5231 13.5463 13.5696 13.5929 13.6162 13.6395 860 13.6628 13.6862 13.7096 13.7329 13.7563 13.7798 13.8032 13.8266 13.8501 13.8736 870 13.8971 13.9206 13.9441 13.9677 13.9912 14.0148 14.0384 14.0620 14.0856 14.1092 880 14.1329 14.1566 14.1802 14.2039 14.2277 14.2514 14.2751 14.2989 14.3227 14.3465 890 14.3703 14.3941 14.4180 14.4418 14.4657 14.4896 14.5135 14.5374 14.5613 14.5853 900 14.6093 14.6332 14.6572 14.6813 14.7053 14.7293 14.7534 14.7775 14.8016 14.8257 910 14.8498 14.8739 14.8981 14.9223 14.9465 14.9707 14.9949 15.0191 15.0434 15.0676 920 15.0919 15.1162 15.1405 15.1649 15.1892 15.2136 15.2380 15.2623 15.2868 15.3112 930 15.3356 15.3601 15.3845 15.4090 15.4335 15.4581 15.4826 15.5071 15.5317 15.5563 940 15.5809 15.6055 15.6301 15.6548 15.6794 15.7041 15.7288 15.7535 15.7782 15.8030 950 15.8277 15.8525 15.8773 15.9021 15.9269 15.9517 15.9766 16.0014 16.0263 16.0512 960 16.0761 16.1010 16.1260 16.1509 16.1759 16.2009 16.2259 16.2509 16.2759 16.3010 970 16.3261 16.3511 16.3762 16.4013 16.4265 16.4516 16.4768 16.5020 16.5271 16.5524 980 16.5776 16.6028 16.6281 16.6533 16.6786 16.7039 16.7292 16.7546 16.7799 16.8053 990 16.8307 16.8561 16.8815 16.9069 16.9323 16.9578 16.9833 17.0088 17.0343 17.0598

1000 17.0853

SRM 1749 Page 3 of 10
Date of Certification: October 1, 1997 S/N 95-3

TABLE 3. Coefficients of the Equation Used to Compute the EMF Values Given in Table 2

The equation for computing values of emf as a function of temperature is of the form: E = a0 + aifoo) + a2 (t9Q)

2 + a3(t90)

3 + a4(t90)4 + a5 (t90f + a6 (t90) 6 + a7 (t90)

7 + a&(t90f + a9

(t90)9

, where E is the emf in millivolts and t90 is the temperature in degrees Celsius (ITS-90). The coefficients for the indicated temperature range are: 0°C to 1000 °C

a0 = -0.829775530E-04

fli = 0.603577729E-02

a2 = 0.193678032E-04

a3 = -0.2229986 14E-07

a4 = 0.32871 1859E-10

a5 = -0.424206 193E-1

a6 = 0.456927038E-16

an = -0.339430259E-19

a% = 0. 14298 1590E-22

a9 = -0.251672787E-26

SRM 1749 Page 9 of 10
Date of Certification: October 1, 1997 S/N 95-3

2

-2 1 1 1 1 1 1 1 1 1 1 1 0 200 400 600 800 1000

Temperature / °C

Figure 1 : This figure shows the deviation of the emf of the Au/Pt thermocouple from an emf/temperature reference

function for Au/Pt thermocouples given in Reference [3]. The deviation equals the thermocouple emf minus

the reference function emf. The dots denote measured values and the solid curve is calculated from a polynomial fitted to the data by the method of least squares. The dashed lines indicate an emf deviation

equivalent to 0.05 °C

SPJvl 1749 Page 10 of 10
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