NMI TR 11
Measurement Report on the Calibration of Au/Pt Thermocouples
using ITS-90 Fixed Points for a Regional Intercomparison
Ferdouse Jahan
18
© Commonwealth of Australia 2006
February 2006
National Measurement Institute
Bradfield Road, Lindfield, NSW 2070
PO Box 264, Lindfield, NSW 2070
T
F
W
(61 2) 8467 3796
(61 2) 8467 3849
www.measurement.gov.au
CONTENTS
Summary ....................................................................................................................................... iv
1.
Introduction ............................................................................................................................ 1
2.
Calibration Process ............................................................................................................... 1
Annealing the Thermocouples .................................................................................... 1
Calibrating the Thermocouples ................................................................................... 1
Problems Encountered ................................................................................................ 1
Schedule of Events ..................................................................................................... 2
3.
Results and Discussion ......................................................................................................... 2
Thermoelectric Scan ................................................................................................... 2
Fixed Point Measurements ......................................................................................... 4
Effect of Change of Immersion of the IP Section in the Ice Point ............................... 5
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
SUMMARY
A 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.
INTRODUCTION
The 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 PROCESS
Annealing the Thermocouples
Once 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.
Calibrating the Thermocouples
The 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.
Problems Encountered
Twice 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.
Figure 1. Tips of the Au/Pt thermocouples broken (left) and repaired (right)
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Schedule of Events
June 2003
August 2003
September 2003
Thermocouples (BC_0301 and BC_0302) received from NMISA, South Africa
Thermocouples scanned in oil bath after annealing at 1000 °C for 1 h
Ag point measured on both thermocouples with cell Ag-93/1
Tips of both thermocouples broke at the end of the third realisation
November 2003
Both tips repaired, annealed and scanned again
December 2003
Ag point measured but the tip of BC_0301 broke at the end of the measurement
April 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 measurement
April to May 2004 Zn, Sn and Ga points measured
July to August 2004 Ag fixed point measurement repeated with new cell Ag-03/1
October 2004
Thermocouples returned to KRISS
3.
RESULTS AND DISCUSSION
Thermoelectric Scan
Figure 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).
Figure 2. Initial scan of thermocouples after 1 h anneal at 1000 °C
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-0.4
-0.6
E/ V
-0.8
-1.0
-1.2
-1.4
150
200
250
300
350
400
450
500
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
-0.4
-0.6
 E / V
-0.8
-1.0
-1.2
-1.4
150
200
250
300
350
400
450
500
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)
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Figure 6. Temperature profile of the Ag and Al point fixed point furnace
Fixed Point Measurements
In 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
Ag
Al
Zn
Sn
Ga
NMI TR 11
Mean value for 10 min
BC_0301
Mean value for 10 min
BC_0302
16 117.43
16 117.36
16 117.32
16 117.34
9 318.34
9 318.33
9 318.31
4 944.32
4 944.28
4 944.33
2 235.36
2 235.42
2 235.37
196.03
196.01
196.01
16 117.46
16 117.40
16 117.22
16 117.27
9 318.74
9 318.70
9 318.62
4 944.41
4 944.38
4 944.39
2 235.46
2 235.45
2 235.46
196.11
196.09
196.11
Uncertainty
(U95, k =2) (V)
Uncertainty
(°C)
1.0
0.040
0.53
0.026
0.27
0.017
0.16
0.012
0.11
0.015
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9318.8
16117.5
16117.4
Al point on TC(BC-0301)
run 1 done on 9/9/03
run 2 done on 11/9/03
Ag point data of TC(BC-0302)
16117.3
9318.7
9318.6
16117.1
EMF/V
EMF/V
16117.2
16117.0
16116.9
9318.5
9318.4
16116.8
16116.7
run 1
run 2
9318.3
16116.6
16116.5
14:40
14:50
15:00
15:10
15:20
15:30
15:40
9318.2
12:10
15:50
12:20
Time / H:MIN
12:30 14:30
14:40
14:50
15:00
Time / H:MIN
2235.7
4944.7
Sn Point of BC-0302
Zn Point of BC-0302
2235.6
4944.6
2235.5
EMF/V
EMF/V
4944.5
4944.4
2235.4
Run 1
Run 2
Run 3
4944.3
2235.3
run 1
run 2
run 3
4944.2
2235.2
15:30
4944.1
12:00
12:20
13:40
14:00
14:20
15:40
15:50
16:00
16:10
16:20
14:40
Time / H:MIN
Time / H:MIN
196.2
Ga point
BC-0301
BC-0302
EMF/V
196.1
196.0
195.9
195.8
12:30
13:00
13:30
14:00
14:30
15:00
15:30
16:00
Time / H:MIN
Figure 7. Typical fixed point freezing data
Effect of Change of Immersion of the IP Section in the Ice Point
Whilst 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 the
ice point during realisation of Sn freezing point
Uncertainty Calculation
The 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 Estimate
Xi
xi
Components
Standard
uncertainty
u(xi)
Sensitivity
coefficient
ci
Probability
distribution
ki
Uncertainty
contribution
ui(y)
0.05 V
1
1
0.050 0
0.012 °C
24.95
2
0.149 7
Ag point
EF
16 117.8 measurement scatter
tF
uncertainty in fixed point
tHF
heat loss through the TC sheath
0.005 °C
24.95
2
0.062 4
EVC
calibration of DVM
0.034 V
1
2
0.017 0
EVD
use of DVM
0.355 V
1
2
0.177 3
EVR
resolution/rounding
0.01 V
1
1.73
0.005 8
ice point measurement
0.01 °C
6.03
2
0.030 2
0.856 V
1
2
0.428 2
0.1 V
1
2
0.050 0
t0
EIH
TC inhomogeneity (0.006%)
EEN
AC pickup
Combined uncertainty
0.497
Expanded uncertainty (k = 2.0)
1.010
Al point
EF
9 318.62 measurement scatter
tF
uncertainty in fixed point
0.02 V
1
1
0.020
0.001 3 °C
20.14
2
0.013
tHF
heat loss through the TC sheath
0.004 °C
20.14
2
0.040
EVC
calibration of DVM
0.034 V
1
2
0.017
EVD
use of DVM
0.205 V
1
2
0.103
EVR
resolution/rounding
0.01 V
1
1.73
0.006
ice point measurement
0.01 °C
6.03
2
0.030
0.448 V
1
2
0.224
0.1 V
1
2
0.050
t0
EIH
TC inhomogeneity
EEN
AC pickup
NMI TR 11
Combined uncertainty
0.258
Expanded uncertainty (k = 2.0)
0.525
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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
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.
CONCLUSION
Figure 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 THERMOCOUPLES
10 April 2003
Yong-Gyoo Kim
KRISS, PO Box 102, Yuseong, Daejeon 305–600, Republic of Korea
INTRODUCTION
This 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.
PARTICIPANTS
KRISS (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
APPARATUS
All 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.
SCHEDULE
The 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 LABORATORIES
Each 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 Thermocouples
1.
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 Thermometers
1.
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 Points
Au/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 RESULTS
The 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
tHF is correction due to the heat loss through the thermocouple sheath
o
CF is sensitivity at the fixed point (V/ C)
EVC is correction due to the voltmeter calibration
EVR is correction due to the voltmeter resolutionEVD 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)
o
C0 is sensitivity at the ice temperature (V/ C)
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)
Cell
Ag
Enclosure
Cell
Al
Enclosure
Cell
Zn
Enclosure
Cell
Sn
Enclosure
Cell
Ga
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
Ag
Al
Zn
Sn
Ga
Mean value for 10 min ± standard deviation
(mean values for last 10 min in V)
Au/Pt 1
Au/Pt 2
Data 1
Data 1
Data 2
Data 2
Data 3
Data 3
Data 1
Data 1
Data 2
Data 2
Data 3
Data 3
Data 1
Data 1
Data 2
Data 2
Data 3
Data 3
Data 1
Data 1
Data 2
Data 2
Data 3
Data 3
Data 1
Data 1
Data 2
Data 2
Data 3
Data 3
File names
(NMI name-fixed point-t/c ID-run
number.txt)
Table A4. Uncertainty analysis sheet
Quantity
Xi
Estimate
xi
Standard
uncertainty
u(xi)
Probability
distribution
Sensitivity
coefficient
ci
Uncertainty
contribution
ui(y)
Degree
of
freedom
EF
tF
tHF
EVC
EVR
EVD
ESC
t0
EIH
EEN
Ex
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APPENDIX B. PREPARATION AND CALIBRATION OF THE THERMOCOUPLES
April 2003
Yong-Gyoo Kim
KRISS, PO Box 102, Yuseong, Daejeon 305–600, Republic of Korea
B1.
Preparation of the Thermocouples
B1.1 Wires
Au, 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
Elements
Ag
Cu
Si
Fe
Mg
Pb
Au
Content (ppm)
1.6
<1.0
<1.0
<2.0
<1.0
<1.0
Elements
Au
Ag
Pd
Ir
Rh
Other
Pt
Content (ppm)
130
<20
145
88
255
<350
Elements
Au
Ag
Pt
Ir
Rh
Other
Pd
Content (ppm)
<1
<1
2
2
3
<4
B1.2 Annealing of Wires and Alumina Insulators
Pt 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 for
1 h before use.
B1.3 Preparation of Measuring Junction
Thin 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 Thermocouples
After 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 Annealing
Three 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 Points
B2.1. Calibration at Ag Point
Initially 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 Point
The calibration process was just same to the case of Ag point realisation.
B2.3 Calibration at Zn, Sn, Ga Points
At 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 Point
At 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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