22

NMI TR 11Measurement Report on the Calibration of Au/Pt Thermocouples using ITS-90 Fixed Points for a Regional Intercomparison

Ferdouse Jahan

© Commonwealth of Australia 2006

February 2006

National Measurement InstituteBradfield Road, Lindfield, NSW 2070PO Box 264, Lindfield, NSW 2070

T (61 2) 8467 3796F (61 2) 8467 3849W www.measurement.gov.au

CONTENTS

Summary..................................................................................................................................... iv

1. Introduction........................................................................................................................... 1

2. Calibration Process...............................................................................................................12.1. Annealing the Thermocouples....................................................................................12.2. Calibrating the Thermocouples...................................................................................12.3. Problems Encountered...............................................................................................12.4. Schedule of Events.....................................................................................................2

3. Results and Discussion........................................................................................................23.1. Thermoelectric Scan...................................................................................................23.2. Fixed Point Measurements.........................................................................................43.3. Effect of Change of Immersion of the IP Section in the Ice Point...............................53.4. Uncertainty Calculation...............................................................................................6

4. Conclusion............................................................................................................................ 7

5. References........................................................................................................................... 7

Appendix A. Protocol for Fixed Point Comparison of the Thermocouples....................................8

Appendix B. Preparation and Calibration of the Thermocouples................................................12

iii

SUMMARYA regional intercomparison of gold/platinum (Au/Pt) thermocouple was run among three national measurement institutes, KRISS – Korea, CSIR – South Africa and NMI – Australia. Two Au/Pt thermocouples were constructed and calibrated by KRISS (Korea) at Ga, Sn, Zn, Al and Ag fixed points from high to low temperature. The inhomogeneity of the thermocouples was measured at 200 °C in an oil bath.

The thermocouples were sent to NMISA (South Africa) in April 2003 then to NMI (Australia) in June 2003. Due to unavoidable circumstances, such as the breaking of fixed point cells and of the artefacts, NMI finished work at the end of 2004.

In the protocol it was planned that besides fixed point calibration, NMI would carry out comparison calibration against a standard platinum resistance thermometer. However, this was not done as no suitable high temperature thermometer was available.

The thermocouples were sent back to KRISS in 2005 and the final calibration was performed.

This report is based on the measurements performed on the Au/Pt thermocouples using NMI’s ITS-90 fixed points.

iv

1. INTRODUCTIONThe gold/platinum (Au/Pt) thermocouple is the most accurate thermocouple available for the range 0 to 1000 °C, with an expanded uncertainty of about 15 mK at 1000 °C [1, 2]. Relative to the most commonly used conventional type R or S thermocouple, the Au/Pt thermocouple is more homogeneous and stable by over an order of magnitude.

During an informal discussion at the Temperature Symposium 2002 in Chicago, KRISS (Korea), NMISA (South Africa) and NMI (Australia) agreed to run an international intercomparison for Au/Pt thermocouples to assess the degree of equivalence of calibration results at ITS-90 fixed points.

KRISS agreed to be the pilot lab. They wrote the protocol (Appendix A) and constructed the artefacts (Appendix B) in 2003.

This report provides details of NMI’s calibration and discusses our experiences and observations.

2. CALIBRATION PROCESS2.1. Annealing the ThermocouplesOnce received, the thermocouples (BC_0301 and BC_0302) were annealed at 1000 °C for 1 h in a horizontal furnace (ANNF-2), quickly removed from the furnace then scanned in an oil bath at 200 °C [3, 4]. The scanned lengths were from 470 mm to 100 mm.

2.2. Calibrating the ThermocouplesThe thermocouples were then calibrated against NMI’s ITS-90 fixed points Ga, Sn, Zn, Al and Ag from high to low temperature using NMI’s standard procedures [5]. During measurement the reference junction was immersed in a crushed ice point with at least 180 mm immersion. The electromotive force (EMF) was measured using a calibrated HP 34420A nanovoltmeter.

A standard platinum resistance thermometer (SPRT) or a different Au/Pt thermocouple was used to monitor the melt of the metal and a cold quartz rod was used to initiate the freeze (Ag, Al and Zn). When the flat freezing plateau started, the test thermocouple was inserted in the fixed point cell and data were recorded for at least 20 to 30 min. The thermocouples were pre-annealed in the SPRT vertical annealing furnace for 10 to 15 min before inserting in the Ag and Al cell. Each thermocouple was measured 3 to 5 times at each fixed point.

2.3. Problems EncounteredTwice the tips of both thermocouples broke during the Ag point measurements when the thin Au wire tangled with the main thermocouple wires. One reason may be that the thermocouples were annealed in a horizontal furnace (instead of a vertical furnace) and the thin soft Au wire may have tangled with the main thermocouple wires.

Figure 1 shows the broken and repaired tips of the thermocouples. The tips were repaired with Au wires supplied by KRISS and as instructed by them. Before remaking the tips, the reference junction was dismantled. After assembling, the thermocouples were annealed at 1000 °C in a vertical furnace and quenched.

In the protocol it was planned that besides fixed point calibration, NMI would carry out comparison calibration against a standard platinum resistance thermometer. However, this was not done as no suitable high temperature thermometer was available.

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Figure 1. Tips of the Au/Pt thermocouples broken (left) and repaired (right)

2.4. Schedule of EventsJune 2003 Thermocouples (BC_0301 and BC_0302) received from NMISA, South AfricaAugust 2003 Thermocouples scanned in oil bath after annealing at 1000 °C for 1 hSeptember 2003 Ag point measured on both thermocouples with cell Ag-93/1

Tips of both thermocouples broke at the end of the third realisationNovember 2003 Both tips repaired, annealed and scanned againDecember 2003 Ag point measured but the tip of BC_0301 broke at the end of the measurementApril 2004 Tip of BC_0301 was repaired, annealed and scanned

Al point was measured but the tip of BC_0302 broke at the end of the measurementApril to May 2004 Zn, Sn and Ga points measuredJuly to August 2004 Ag fixed point measurement repeated with new cell Ag-03/1October 2004 Thermocouples returned to KRISS

3. RESULTS AND DISCUSSION3.1. Thermoelectric ScanFigure 2 shows the initial inhomogeneity scan of both thermocouples measured at 200 °C in oil bath. The initial results were within ±0.1 V at 200 °C, equivalent to ±0.005%. However, during Ag point realisation, due to different thermal expansion of the individual wires, they were stressed and broke.

After the thermocouples were repaired, they were annealed in the vertical furnace CALF-4 at 1000 °C for 1 h. They were then scanned again at 200 °C. Figure 3 shows the scan of BC_0302 in different stages. The scan after repairing shows a 0.4 V change for the annealed section of the wire, which is about 300 mm from tip.

Initially the horizontal annealing furnace was used because its uniform zone was longer than that of the vertical furnace. The uniform zone of the vertical furnace was 300 mm (Figure 4). It was given another 2 h anneal at 1000 °C and scanned again. Figure 3 shows the homogeneous signature of the thermocouple after annealing.

Both thermocouples were scanned after completion of the fixed point calibration. No further change was observed in the thermoelectric signature of thermocouple BC_0302 but thermocouple BC_0301 showed similar changes in the signature for the section used in high temperatures (Figure 5).The temperature profile of the Ag and Al point furnace shown in Figure 6 indicates that about 300 mm of the thermocouples experienced high temperatures during realisation of the fixed points.

The reason for this change in inhomogeneity was not clearly understood because there should not be any reversible or irreversible changes in the Au or Pt wires up to 1000 °C [6, 7]. However it may be due to non-uniform distribution of lattice vacancies because wires exposed to high temperatures might have higher numbers of lattice vacancies. To obtain a stable Au/Pt thermocouple it is recommended that after high temperature annealing the wires are annealed at 450 °C for more than 16 h to remove quenched-in lattice vacancies [1, 8]. BC_0301 and BC_0302 did not have any vacancy anneal, the Pt wire was annealed at 1300 °C for only 30 min and the Au wire was annealed at 1000 °C for 5 h (see Appendix B).

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Figure 2. Initial scan of thermocouples after 1 h anneal at 1000 °C

150 200 250 300 350 400 450 500-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

E/

V

Immersion/mm

Figure 3. Inhomogeneity scan of BC_0302 ( initial scan, o after repair and 1 h anneal at 1000 at °C, after further anneal for 2 h at 1000 at °C, ▲ after fixed points calibration)

Figure 4. Temperature profile of the vertical annealing furnace CALF-4

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150 200 250 300 350 400 450 500-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

E/

V

Immersion/mm

Figure 5. Inhomogeneity scan of BC_0301 ( initial scan after 1 h anneal at 1000 at °C, after repair and anneal for 2 h at 1000 °C, ▲ after all fixed point measurements)

Figure 6. Temperature profile of the Ag and Al point fixed point furnace

3.2. Fixed Point MeasurementsIn accordance with the protocol, three or more realisations of fixed point measurements for Ag, Al, Zn, Sn and Ga were performed from high to low temperatures (see Table 1). The values of the measured EMF are the average during at least 10 min of realisation and are corrected for calibration correction and zero offset of the nanovoltmeter.

In February 2003, an Ag point measurement was started with cell Ag-93/1 in the three-zone furnace in room D229. The standard deviation of the measured Ag point EMF was 0.1 V, which is equivalent to 5 mK which indicates good reproducibility of the thermocouple and also the Ag cell.

Reproducibility at low-temperature fixed points is better than that of the Ag point. The standard deviation of the measured EMF was 0.05 V at Al point and less than 0.02 V at Zn, Sn and Ga points. Typical freeze curves are shown in Figure 7.

Table 1. Measured EMF at the fixed point and their uncertainty

Fixed point Mean value for 10 minBC_0301

Mean value for 10 minBC_0302

Uncertainty(U95, k =2) (V)

Uncertainty(°C)

Ag

16 117.4316 117.3616 117.3216 117.34

16 117.4616 117.4016 117.2216 117.27

1.0 0.040

Al 9 318.349 318.33

9 318.749 318.70

0.53 0.026

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9 318.31 9 318.62

Zn4 944.324 944.284 944.33

4 944.414 944.384 944.39

0.27 0.017

Sn2 235.362 235.422 235.37

2 235.462 235.452 235.46

0.16 0.012

Ga196.03196.01196.01

196.11196.09196.11

0.11 0.015

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14:40 14:50 15:00 15:10 15:20 15:30 15:40 15:5016116.5

16116.6

16116.7

16116.8

16116.9

16117.0

16117.1

16117.2

16117.3

16117.4

16117.5

run 1 done on 9/9/03 run 2 done on 11/9/03

Ag point data of TC(BC-0302)E

MF/V

Time / H:MIN

12:10 12:20 12:30 14:30 14:40 14:50 15:009318.2

9318.3

9318.4

9318.5

9318.6

9318.7

9318.8

run 1 run 2

EM

F/V

Time / H:MIN

Al point on TC(BC-0301)

12:00 12:20 13:40 14:00 14:20 14:404944.1

4944.2

4944.3

4944.4

4944.5

4944.6

4944.7

run 1 run 2

EMF/V

Time / H:MIN

run 3

Zn Point of BC-0302

15:30 15:40 15:50 16:00 16:10 16:202235.2

2235.3

2235.4

2235.5

2235.6

2235.7

Run 1 Run 2 Run 3

EM

F/V

Time / H:MIN

Sn Point of BC-0302

12:30 13:00 13:30 14:00 14:30 15:00 15:30 16:00195.8

195.9

196.0

196.1

196.2 BC-0301 BC-0302

Ga point

EMF/

V

Time / H:MIN

Figure 7. Typical fixed point freezing data

3.3. Effect of Change of Immersion of the IP Section in the Ice PointWhilst measuring low-temperature fixed points, it was observed that EMF varied with the change of immersion length of the reference junction in the ice point. The effect of immersion of the reference junction was assessed while realising Sn point and is shown in Figure 8. When immersion changed by 3 cm EMF changed by 0.3 V for BC_0301 and by 0.1 V for BC_0302. This is attributed to the inhomogeneity of the IP section of the wires.

Whilst reassembling the IP section after repairing the thermocouples, the wires may be stressed by the tight fitting of the stainless tube. It has been reported [6] that strain-induced inhomogeneity in the Pt wire can be reduced by proper annealing. In this case, however, errors due to the IP section inhomogeniety were minimised by holding the immersion of the IP section constant.

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Figure 8. The effect of immersion length of the reference junction in theice point during realisation of Sn freezing point

3.4. Uncertainty CalculationThe uncertainties of the calibrations calculated according to Guide to the Expression of Uncertainty in Measurement [9] are given in Table 1 and two examples of the calculation are shown in Table 2.

Table 2. Uncertainty analysis at the Ag point and Al point

Quantity Xi

Estimatexi

ComponentsStandard

uncertaintyu(xi)

Sensitivity coefficient

ci

Probability distribution

ki

Uncertainty contribution

ui(y)Ag point

EF 16 117.8 measurement scatter 0.05 V 1 1 0.050 0tF uncertainty in fixed point 0.012 °C 24.95 2 0.149 7tHF heat loss through the TC sheath 0.005 °C 24.95 2 0.062 4EVC calibration of DVM 0.034 V 1 2 0.017 0EVD use of DVM 0.355 V 1 2 0.177 3EVR resolution/rounding 0.01 V 1 1.73 0.005 8t0 ice point measurement 0.01 °C 6.03 2 0.030 2EIH TC inhomogeneity (0.006%) 0.856 V 1 2 0.428 2EEN AC pickup 0.1 V 1 2 0.050 0

Combined uncertainty 0.497Expanded uncertainty (k = 2.0) 1.010

Al pointEF 9 318.62 measurement scatter 0.02 V 1 1 0.020tF uncertainty in fixed point 0.001 3 °C 20.14 2 0.013tHF heat loss through the TC sheath 0.004 °C 20.14 2 0.040EVC calibration of DVM 0.034 V 1 2 0.017EVD use of DVM 0.205 V 1 2 0.103EVR resolution/rounding 0.01 V 1 1.73 0.006t0 ice point measurement 0.01 °C 6.03 2 0.030EIH TC inhomogeneity 0.448 V 1 2 0.224EEN AC pickup 0.1 V 1 2 0.050

Combined uncertainty 0.258Expanded uncertainty (k = 2.0) 0.525

The components used in the calculation are described in Appendix A. One of the dominant components is the drift of the nanovoltmeter. If this term can be reduced by checking the calibration just before the

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measurements the thermocouple can be calibrated with lower uncertainty. Another dominant term is the inhomogeneity of the thermocouples, which is 0.006% of EMF, equivalent to 10 mK at Ag point.

In other works [8, 10] inhomogeneity was reported to be less than 5 mK, measured by changing the immersion in the fixed point cell up to 10 cm. In our work inhomogeneity was determined in an oil bath over a length of 45 cm from the thermocouple tip, and propagated proportionally to temperature.

4. CONCLUSIONFigure 9 plots the deviation of EMF from the reference function of the Au/Pt thermocouple given in reference [1]. As the thermocouples were constructed with high purity Au and Pt wires, the deviation from the reference function was expected to be small (within ±50 mK). However deviation was –3.2 V at Ag point equivalent to 0.13 °C which is more than expected.

Figure 9. Deviation of EMF from the reference function

The difference of EMF between the thermocouples was small at all the fixed points, except at the Al point. The different immersion of the IP section could be the reason (section 3.3).

Reproducibility of the thermocouples is less than 10 mK, which is comparable to a high temperature platinum resistance thermometer. This work indicates that the Au/Pt thermocouples can be calibrated with an uncertainty of less than 50 mK which is more than an order of magnitude better than the conventional type R and S thermocouples.

The one disadvantage of this thermocouple design is mechanical instability — it should only be used in the vertical position. NMI designed a simple Au/Pt thermocouple with no coil or bridge at the tip junction [11] which overcame this problem.

5. REFERENCES[1] DC Ripple and GW Burns (1998) Standard Reference Material 1749: Au/Pt Thermocouple Thermometer.

NIST Special Publication 260–134, US Government Printing Office, Washington, DC[2] M Gotoh, KD Hill and EG Murdock (1991) A gold/platinum thermocouple reference table.

Rev. Sci. Instrum. 62(11), 2778–2791[3] RE Bentley (2000) A thermoelectric scanning facility for the study of elemental thermocouples.

Meas. Sci. Technol. 11, 538–546[4] F Jahan and MJ Ballico (2002) A study of the temperature dependence of inhomogeneity in platinum-based

thermocouples. Eighth Symposium on Temperature: Its Measurement and Control in Science and Industry, vol 7, 469–474

[5] MJ Ballico and K Nguyen Calibration of Standard Resistance Thermometer. NMI Quality System PM-EADA-8.2.2

[6] RE Bentley (2001) Thermoelectric changes in Au and Pt metals used in elemental thermocouples. Meas. Sci. Technol. 12, 627–634

[7] RE Bentley (1998) The use of elemental thermocouples in high temperature precision thermometry. Measurement 23, 35–46

[8] GW Burns, GF Strouse, BM Liu and BW Mangum (1992) 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, part 1, pp 531–536, JF Schooley, ed (AIP, New York)

[9] ISO/IEC Guide 98:1993 Guide to the expression of uncertainty in measurement (GUM)[10] YG Kim, KS Gam and KH Kang (1998) Thermoelectric properties of the Au/Pt thermocouples.

Rev. Sci. Instrum. 69(10), 3577–3582[11] F Jahan and MJ Ballico (2005) Stability study of a simple design of high precision Pt/Au thermocouples.

Sixth Conference of the Metrology Society of Australia, pp 48–53

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APPENDIX A. PROTOCOL FOR FIXED POINT COMPARISON OF THE THERMOCOUPLES10 April 2003

Yong-Gyoo Kim

KRISS, PO Box 102, Yuseong, Daejeon 305–600, Republic of Korea

INTRODUCTIONThis regional comparison was initiated during the Temperature Symposium in °October 2002 by KRISS, NMI and NMISA. KRISS was invited to be the pilot lab. The procedures and instructions, which are given below, should be followed by the participants. Two Au/Pt thermocouples provided by KRISS will be used as transfer thermometers. The Au/Pt thermocouple should be compared from Ga to Ag.

PARTICIPANTSKRISS (Korea): Yong-Gyoo Kim (dragon@kriss.re.kr) Temperature-Humidity Group

NMI (Australia): Ferdouse Jahan (ferdouse.jahan@measurement.gov.au) Thermometry Group

NMISA (South Africa): Hans Liedberg (hliedber@nmisa.org) Temperature Laboratory

APPARATUSAll participating laboratories should:

have Ga, Sn, Zn, Al and Ag freezing-point cells whose thermometer wells have inner diameter larger than 8 mm;

have the annealing furnace in vertical type whose immersion depth should be longer than 50 cm and operating temperature should be high to 1100 °C;

prepare the thermocouple inhomogeneity test system operating at 150 to 200 °C; its immersion depth should be longer than 40 cm and it is recommended that the test temperature is 200 °C;

have a precise digital voltmeter having a resolution of 0.01 V.

SCHEDULEThe following schedule applies for all participants being in charge to transport the transfer thermometers to the next participating laboratory. The transfer standards can be carried by air mail in their case to the next participating laboratory. Each laboratory should complete the measurement within approximately one month.

March 2003 Fabrication of transfer standards by KRISS Measurements completed

April 2003 Moved to NMISA by air mail Measurements completed

May 2003 Moved to NMI by air mail Measurements completed

June 2003 Moved to KRISS by hand-carry Final measurements

July 2003 Completion of the comparison and preparation of the comparison report

August 2003 Submit the report to APMP

DETAILED INSTRUCTIONS FOR PARTICIPATING LABORATORIESEach participant should follow the instructions given in (B) Receiving the Thermometers as soon as possible after receiving the thermometers. After this, calibrate the specified thermometers through the given procedures at each fixed point. After the calibrations, securely pack the thermometers and transport them to the next participant. If any discussion on this protocol is necessary, the participant should share the information through e-mail to all participants.

(A) Preparation of Standard Thermocouples1. KRISS should make two set of Au/Pt thermocouples as transfer standards. KRISS should

provide the preparation method to participants in detail.

2. KRISS should present the source of the thermocouple wires and their nominal purities.

3. KRISS should present the annealing temperature and time spent.

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(B) Receiving the Thermometers1. Upon receiving the transfer thermocouples, the host laboratory must inspect them for damage.

The host laboratory must report the condition of the thermocouples to KRISS. If there is damage, KRISS will give instructions on how to proceed.

2. If there is no damage, the host must anneal the thermocouples at 1000 °C for 1 h before measurement in order to eliminate the stress which would be caused during transfer. The temperature profile of the annealing furnace should be examined before inserting the thermocouples. The temperature profile should be checked from the furnace surface to its maximum immersion depth with an interval of 2 cm.

3. After annealing, pull out the thermocouple to ambient rapidly in order to prohibit the oxidation of Pt.

(C) Calibration of Au/Pt Thermocouples at the Ga, Sn, Zn, Al and Ag Fixed PointsAu/Pt thermocouples should be calibrated at the Ga, Sn, Zn, Al and Ag points. The realisation sequence is from high temperature to low temperature. The general procedures are referred to in supplementary information of the ITS-90.

1. Before measurement at the freezing points, the temperature profile of the enclosure should be examined from the bottom of the cell to the enclosure surface (5 °C below the melting temperature except for Ga). It is recommended to use another Au/Pt or Pt/Pd thermocouple as a measuring sensor (SPRT can be used also).

2. Insert the monitoring thermocouple (thermometer) into the freezing cell. During melting, record the melting EMF of the monitoring thermocouple. After completion of melting, let the melt stabilise at 2 °C above freezing temperature. And then cool down to 5 °C below freezing temperature with a cooling rate of 0.5 °C/min. If the temperature increases after supercool, ramp the furnace set temperature to 1 to 2 °C below the freezing temperature.

3. Remove the monitoring thermocouple and insert the cool quartz rod for 2 min. This chill rod induce will make a flat freezing plateau. After removing the quartz rod, insert the test thermocouple (BC_0301) into the cell and monitor its thermal EMF. It is highly recommended to use the computer-interfaced data logging system. Record the EMF for about 30 min.

4. Replace the test thermocouple with another Au/Pt (BC_0302) followed by about 30 min measurement. After measurement, cool down the furnace 20 °C below the freezing temperature and then let the melt freeze completely. The test thermocouple should be removed from the cell, and the monitoring thermocouple used to record the next melt and supercool. Repeat the realisation three times at each fixed point.

5. In the next realisation, insert BC_0302 and then BC_0301.

6. In the third realisation, get the freezing curve about 1 h for each thermocouple. After the final measurement remove the test thermocouple from the furnace to ambient rapidly.

7. In the case of Ga realisation it is not necessary to use the monitoring thermocouple. It is recommended that the set temperature to melt is 1 °C above the melting temperature.

8. After completion of the fixed point measurements the Au/Pt thermocouples should be tested at the inhomogeneity test system operating at 200 °C. Inhomogeneity test should be conducted during insertion process. The test length should be larger than 40 cm from the tip of the thermocouple. This inhomogeneity test should be conducted three times.

REPORTING RESULTSThe participating laboratories must send the following to all participants: information on the measuring devices (Table A1); information on the inhomogeneity test system (Table A2); graphs of furnace temperature gradient and their data; measurement data and their electronic files (Table A3); uncertainty analysis according to the Guide to the Expression of Uncertainty in Measurement,

ISO 1993 (Table A4); the EMF across the test thermocouple at the fixed point can be written as:

Ex(tx) = E(tF) + (tF + tHF)CF + EVC + EVR + EVD + ESC + t0C0 + EIH + EEN

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where E(tF) is EMF at the fixed point temperature (Type A)tF is correction due to the fixed point calibrationtHF is correction due to the heat loss through the thermocouple sheathCF is sensitivity at the fixed point (V/oC)EVC is correction due to the voltmeter calibrationEVR is correction due to the voltmeter resolutionEVD is correction due to the voltmeter drift (long term accuracy in manual)ESC is correction due to the scanner (if used)t0 is correction due to the ice temperature (use the measurement result)C0 is sensitivity at the ice temperature (V/oC)EIH is correction due to the inhomogeneity (use the measurement result)EEN is correction due to the electric noise

Table A1. Information on the measuring devices used in this comparison

Devices Manufacturer Model (type) Serial no Remarks

DMM

Ice point

Scanner (if used)

AgCell

Enclosure

AlCell

Enclosure

ZnCell

Enclosure

SnCell

Enclosure

GaCell

Enclosure

Table A2. Information on the inhomogeneity test system

Items

Measuring DMM

Temperature enclosure

Scanning method

Reference thermometer (if used)

Test temperature

Stability

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Table A3. Measurement data

Fixed point

Mean value for 10 min ± standard deviation (mean values for last 10 min in

V)File names

(NMI name-fixed point-t/c ID-run number.txt)Au/Pt 1 Au/Pt 2

Ag

Data 1 Data 1

Data 2 Data 2

Data 3 Data 3

Al

Data 1 Data 1

Data 2 Data 2

Data 3 Data 3

Zn

Data 1 Data 1

Data 2 Data 2

Data 3 Data 3

Sn

Data 1 Data 1

Data 2 Data 2

Data 3 Data 3

Ga

Data 1 Data 1

Data 2 Data 2

Data 3 Data 3

Table A4. Uncertainty analysis sheet

QuantityXi

Estimatexi

Standard uncertainty

u(xi)

Probability distribution

Sensitivity coefficient

ci

Uncertainty contribution

ui(y)

Degree of

freedomEF

tF

tHF

EVC

EVR

EVD

ESC

t0

EIH

EEN

Ex

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APPENDIX B. PREPARATION AND CALIBRATION OF THE THERMOCOUPLESApril 2003

Yong-Gyoo KimKRISS, PO Box 102, Yuseong, Daejeon 305–600, Republic of Korea

B1. Preparation of the ThermocouplesB1.1 WiresAu, Pt and Pd wires were obtained from the Heraeus Company. Their nominal purity was 99.999% for Au, 99.999 9% for Pt and 99.997% for Pd. In Table B1, chemical impurities are presented.

Table B1. Impurity analysis of test wires

Au Pt PdElements Content (ppm) Elements Content (ppm) Elements Content (ppm)

Ag 1.6 Au 130 Au <1Cu <1.0 Ag <20 Ag <1Si <1.0 Pd 145 Pt 2Fe <2.0 Ir 88 Ir 2Mg <1.0 Rh 255 Rh 3Pb <1.0 Other <350 Other <4

B1.2 Annealing of Wires and Alumina InsulatorsPt and Pd wires were electrically annealed at about 1300 °C for 30 min. The wire temperature was lowered to about 450 °C and then annealed overnight. 70 cm of Au wires were annealed at 1000 °C for 5 h followed by furnace cooling. Two wires were welded together after annealing.

Twin-bore alumina insulators were obtained from the McDanel (AXD-1008). They were 60 cm long, had a diameter of 3.18 mm and the bore size was 1.02 mm. The insulators were pre-baked at 1500 °C for1 h before use.

B1.3 Preparation of Measuring JunctionThin Au and Pt wires of 0.1 mm diameter were used to make a hot junction. For Au/Pt thermocouples Au wire was used, and Pt wire was used for Pt/Pd thermocouples. The junction was a bridge-type to which thin wire was connected to lead wires.

B1.4 Assembly of Test ThermocouplesAfter inserting the thermocouple wires into the alumina tube, a ploy-vinyl sleeve was used to insulate the thermocouple wire exposed to air. The thermocouples were inserted into the quartz protecting tube with inner diameter of 5 mm and length of 59 cm. The quartz tube was sand-blasted to protect the heat-piping effect. The test thermocouples were named: BC_0301 and BC_0302 for the Au/Pt thermocouple; and BC_0303 for the Pt/Pd thermocouple.

B1.5 Final Furnace AnnealingThree thermocouples were annealed at 1000 °C in the vertical annealing furnace (ATF-V-01) before being removed from the furnace quickly to be cooled rapidly. The: Au/Pt wires were annealed for 10 h; and Pt/Pd thermocouple was annealed for 34 h.

Figure B1 shows the temperature profile measured at the set temperature of 1000 °C. The measuring thermometer was another Pt/Pd thermocouple. Most of the temperature gradient was formed in range of 10 cm from the furnace entrance.

B2. Calibration of Au/Pt Thermocouples at the Ag, Al, Zn, Sn and Ga Fixed PointsB2.1. Calibration at Ag PointInitially Ag point was realised in the FPF-H-01 furnace with LSAG1 cell. The temperature gradient near the Ag freezing temperature is shown in Figure B2. Five times of realisation were done for both thermocouples. Another Pt/Pd thermocouple was used to monitor the cell temperature. After the

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recovery of the temperature followed by supercooling, the monitoring thermocouple was removed from the cell and a cool quartz rod was inserted into the cell to induce uniform freezing along the cell. But the freezing EMF was found to be very unstable and irreproducible. It seemed to be caused by the electric noise. During the experiment, thermocouples were placed in the annealing furnace(ATF-V-01) operating at 1000 °C before insertion into the freezing cell. The total holding time was about 4 h 30 min in the freezing cell and about 10 h in the annealing furnace.

After this experiment, I changed the experimental system which was located in other laboratory. The system was used for the calibration of the SPRT for industry. The final Ag point realisation was performed in the FPF-Ag-1 furnace with AG-1 sealed-type cell. Figure B3 shows the temperature gradient in the furnace.

Figure B1. Temperature gradient profile for ATF-V-01 furnace

Figure B2. Temperature gradient profile for HPF-H-01 furnace

Figure B3. Temperature gradient profile for HPF-AG-01 furnace

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Three times of realisation were performed. The freezing-induce temperature was about 2 °C below the freezing temperature. At the first realisation, BC_0301 was inserted followed by BC_0302. After measurement of about 30 min, the furnace temperature was lowered to freeze completely. All thermocouples were pulled out from the furnace to the room temperature after measurement. At the second realisation, BC_0302 was firstly inserted and the next was BC_0301. For the third measurement, Ag point was realised separately, so one thermocouple one realisation. The overall process is: first realisation: BC_0301 then BC_0302 second realisation: BC_0302 then BC_0301 third realisation: BC_0301 fourth realisation: BC_0302

B2.2 Calibration at Al PointThe calibration process was just same to the case of Ag point realisation.

B2.3 Calibration at Zn, Sn, Ga PointsAt these points, the freezing or melting time duration were long enough to finish three times of measurements for both thermocouples. So, during one realisation all thermocouples were tested three times. The overall process is first realisation: BC_0301 -> BC_0302 -> BC_0301 -> BC_0302 -> BC_0301 ->BC_0302.

B3. Calibration of Pt/Pd Thermocouple at the Cu PointAt about 800 °C, the test thermocouple was inserted into the cell and it was placed in the cell until the experiment finished. The freezing-induce temperature was 5 °C below the freezing temperature. After supercool, the furnace temperature was set to 2 °C below the freezing temperature. After about 1 h measurement, the furnace temperature was lowered to 50 °C below to induce the complete freezing. In the midnight and holiday, the furnace temperature was set to 1000 °C. For the calibration of Pt/Pd thermocouple, ten times of realisation were done because the freezing EMF was found to increase slightly with realisation. Figure B4 shows the temperature gradient of Cu furnace, and Figure B5 shows the variation of the freezing EMF with the holding time in the furnace.

Figure B4. Temperature gradient profile for Dynatherm furnace

Figure B5. Freezing EMF versus holding time in the Cu furnace

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