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Platinum Metals Rev., 1972, 16, (1), 2

Platinum Metal Thermocouples

New International Reference Tables

  • By T. J. Quinn
  • T. R. D. Chandler
  • National Physical Laboratory, Teddington, Middlesex
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Article Synopsis

As a result of a remarkable piece of international collaboration between three national standards laboratories and seven United States and United Kingdom manufacturers, new reference tables have now been completed for platinum : 10 per cent rhodium-platinum and platinum: 13 per cent rhodium-platinum thermocouples. These new tables take into account the changes in the temperature scale resulting from the introduction of the International Practical Temperature Scale of 1968 (IPTS-68) and also provide reference tables which will be common to both U.S. and U.K. manufacturers and users of thermocouples.

The changes in the temperature scale (1) when IPTS-68 was introduced highlighted the problem hitherto of the two conflicting reference tables for rhodium-platinum thermocouples. One table was based on work done in 1933 at the National Bureau of Standards (NBS) by Roeser and Caldwell (2) and was published as NBS 561, and the other was based on work done at the National Physical Laboratory (NPL) in 1950 by C. R. Barber (3) and was published in the United Kingdom as BS 1826. These tables differed from one another as a result of differences in both the realisation of the temperature scales in the original calibrations and in the compositions of the platinum and rhodium-platinum wire. Since the original measurements were made, particularly those in 1933, there have been substantial improvements in the purification of both platinum and rhodium and thus, in order to continue to meet the old tables, changes were made in the composition of the alloy arms of the thermocouples. The result was that differences between thermocouples made to meet NBS 561 and those made to meet BS 1826 have become quite substantial. It has been clear for some time that it was uneconomic for manufacturers to have to make material of nominally the same composition to these two speicifications.

In September 1967 an informal meeting took place at the NBS between representatives of the NBS, National Research Council of Canada (NRC) and the NPL. At this meeting it was agreed that the introduction of the IPTS-68 would provide an excellent opportunity to unify the reference tables for platinum thermocouples. It was agreed that a joint approach should be made by the three national laboratories to all the U.S. and U.K. manufacturers with a view to carrying out a programme of research leading to new reference tables. This was welcomed by the manufacturers, many of whom had been aware of and had encouraged the proposals discussed at this meeting, and it was agreed that the terms of reference for the project would be as follows:

  • Each of four American and three British manufacturers would contribute 24 metres of pure platinum wire and 12 metres each of 10 per cent rhodium-platinum and 13 per cent rhodium-platinum wire.

  • Each of the three types of wire would have nominal diameter 0.5 mm and be supplied in a continuous length.

  • The pure platinum wires should have a steam point to ice point resistance ratio not less than 1.3924.

  • The two alloy wires would contain as closely as possible 10 per cent rhodium and 13 per cent rhodium respectively, rather than have the rhodium contents adjusted to match particular specified e.m.f.s of the gold point. It was generally acknowledged that this would lead to slightly higher values of e.m.f. for given temperatures than in existing tables.

It was decided also to split the experimental work among the three national laboratories in the following way:

  • The materials would be collected and the thermocouples assembled by NBS, then half of the completed thermocouples would be sent to NRC.

  • NBS and NRC would perform primary calibrations on some and comparison calibrations on all of their thermocouples from 0°C to the gold point.

  • A selected number of thermocouples from each of the NBS and NRC groups would then receive primary calibrations at NPL from the gold point to the platinum point against a photoelectric optical pyrometer using a suitable black-body cavity. Enough calibrations would be done at the gold point to ensure agreement with the NBS and NRC calibrations.

  • The thermocouples retained by NBS and NRC would be intercompared from the gold point to the platinum point and also be compared with those from NPL upon their return.

  • NBS and NRC would ensure agreement with the NPL by a limited number of high temperature calibrations obtained by measuring palladium and platinum points by the wire method.

The Reference Tables

This work has now been completed and the new reference tables were presented at the 5th Symposium on “Temperature” held in Washington in June 1971.* These new reference tables, unlike the old ones, have been produced by means of agreed sets of polynomial functions fitted to the results of the experimental work. These functions are listed in Tables I and II and skeleton reference tables derived from them appear in Tables III and IV. The differences between the new tables and the old are shown in Figs. 2 and 3, which also clearly indicate the differences between the NBS 561 and BS 1826 tables.

Table I

The New Platinum : 10% Rhodium-Platinum Reference Table Defined by Sets of Polynomial Functions

Temperature RangePolynomial
−50°C to 630.74°C
  • where a0 = 0

  • a1 = 5.399578

  • a2 = 1.251977 × 10−2

  • a3 = −2.244822 × 10−5

  • a4 = 2.845216 × 10−8

  • a5 = −2.244058 × 10−11

  • a6 = 8.505417 × 10−15

630.74°C to 1064.43°C
  • where g0 = −298.245

  • g1 = 8.237553

  • g2 = 1.645391 × 10−3

1064.43°C to 1665°C
  • where t* = (t68−1365)/300

  • b0 = 13943.439

  • b1 = 3639.869

  • b2 = −5.028

  • b3 = −42.451

1665°C to 1767.6°C
  • where t*=(t68−1715)/50

  • c0 = 18113.083

  • c1 = 567.954

  • c2 = −12.112

  • c3 = −2.812

Table II

The New Platinum : 13% Rhodium-Platinum Reference Table Defined by Sets of Polynomial Functions

Temperature RangePolynomial
−50°C to 630.74°C
  • where d0 = 0

  • d1 = 5.289139

  • d2 = 1.391111 × 10−2

  • d3 = −2.400524 × 10−5

  • d4 = 3.620141 × 10−8

  • d5 = −4.464502 × 10−11

  • d6 = 3.849769 × 10−14

  • d7 = −1.537264 × 10−17

630.74°C to 1064.43°C
  • where h0 = 264.180

  • h1 = 8.046868

  • h2 = 2.989229 × 10−3

  • h3 = −2.687606 × 10−7

1064.43°C to 1665°C
  • where t* = (t68−1365)/300

  • e0 = 15540.414

  • e1 = 4235.777

  • e2 = 14.693

  • e3 = −52.214

1665°C to 1767.6°C
  • where t* = (t68−1715)/50

  • f0 = 20416.695

  • f1 = 668.509

  • f2 = −12.301

  • f3 = −2.786

Table III

Skeleton Reference Table for Platinum : 10 per cent Rhodium-Platinum (Type S) Thermocouples

Temperatures in Degrees Celsius (IPTS–68)Reference Junction at 0°C
Temp.0102030405060708090
Absolute E.M.F. in Microvolts
00−53−103−150−194−236
0055113173235299365432502573
10064571979587295010291109119012731356
2001440152516111698178518731962205121412232
3002323241425062599269227862880297430693164
4003260335634523549364537433840393840364135
5004234433344324532463247324832493350345136
6005237533954425544564857515855596060646169
7006274638064866592669968056913702071287236
8007345745475637672778278928003811482258336
9008448856086738786889990129126924093559470
10009585970098169932100481016510282104001051710635
110010754108721099111110112291134811467115871170711827
120011947120671218812308124291255012671127921291313034
130013155132761339713519136401376113883140041412514247
140014368144891461014731148521497315094152151533615456
150015576156971581715937160571617616296164151653416653
160016771168901700817125172431736017477175941771117826
170017942180561817018282183941850418612
Table IV

Skeleton Reference Table for Platinum : 13 per cent Rhodium-Platinum (Type R) Thermocouples

Temperatures in Degrees Celsius (IPTS–68)Reference Junction at 0°C
Temp.0102030405060708090
Absolute E.M.F. in Microvolts
00−51−100−145−188−226
0054111171232296363431501573
10064772380087995910411124120812941380
2001468155716471738183019232017211122072303
3002400249825962695279528962997309932013304
4003407351136163721382639334039414642544362
5004471458046894799491050215132524453565469
6005582569658105925604061556272638865056623
7006741686069797098721873397460758277037826
8007949807281968320844585708696882289499076
9009203933194609589971898489978101091024010371
100010503106361076810902110351117011304114391157411710
110011846119831211912257123941253212669128081294613085
120013224133631350213642137821392214062142021434314483
130014624147651490615047151881532915470156111575215893
140016035161761631716458165991674116882170221716317304
150017445175851772617866180061814618286184251856418703
160018842189811911919257193951953319670198071994420080
170020215203502048320616207482087821006

Fig. 2

Differences between the new reference table and NBS 561 and BS 1826 (adjusted to IPTS-68) for Pt: 10% Rh-Pt

Differences between the new reference table and NBS 561 and BS 1826 (adjusted to IPTS-68) for Pt: 10% Rh-Pt

Fig. 3

Differences between the new reference table and NBS 561 and BS 1826 (adjusted to IPTS-68) for Pt : 13% Rh-Pt

Differences between the new reference table and NBS 561 and BS 1826 (adjusted to IPTS-68) for Pt : 13% Rh-Pt

The Proceedings of the 5th Symposium on “Temperature Measurement and Control in Science and Industry” are to be published in 1972.

Experimental Work

The experimental work carried out at NPL that provided the data for the new reference tables above the gold point was undertaken using the NPL photoelectric pyrometer (Figure 1). This was used to measure the temperature of a black-body cavity which could accommodate up to four thermocouples at a time. The cavity used from the gold point up to 1748°C was made from solid platinum and was loaned to NPL for this work by Johnson Matthey. It is illustrated in Figs. 4 and 5. Using a furnace wound with pure rhodium ribbon and 40 per cent rhodium-platinum wire internal end heaters, a temperature uniformity, at about 1500°C, of within 0.3 deg C was achieved over the whole length of the block.

Fig. 1

Experimental work at the National Physical Laboratory to provide the data for new reference tables for platinum : 10 per cent rhodium-platinum and platinum : 13 per cent rhodium-platinum thermocouples above the gold point was carried out using the NPL photoelectric pyrometer. The standard lamps and furnace are seen here also and the furnace containing a gold point black-body as a reference is on the right

Experimental work at the National Physical Laboratory to provide the data for new reference tables for platinum : 10 per cent rhodium-platinum and platinum : 13 per cent rhodium-platinum thermocouples above the gold point was carried out using the NPL photoelectric pyrometer. The standard lamps and furnace are seen here also and the furnace containing a gold point black-body as a reference is on the right

Fig. 4

The platinum black-body constructed by Johnson Matthey before assembly and use

The platinum black-body constructed by Johnson Matthey before assembly and use

Fig. 5

The platinum black-body after prolonged use at temperatures up to 1748°C. One of the thermocouples used for this work is also shown

The platinum black-body after prolonged use at temperatures up to 1748°C. One of the thermocouples used for this work is also shown

It was found that the reproducibility of platinum : 13 per cent rhodium-platinum thermocouples was significantly better than that of platinum : 10 per cent rhodium-platinum thermocouples over the whole temperature range. For example, the mean gold point e.m.f. determined by NPL for eight platinum : 13 per cent rhodium-platinum thermocouples was 0.4 microvolts above the NBS and NRC mean ingot value, while that of the platinum:10 per cent rhodium-platinum thermocouples was 2 microvolts higher. A difference in behaviour of this sort can be reasonably accounted for by the fall in slope, between 10 per cent rhodium and 13 per cent rhodium, of the e.m.f./composition curve for rhodium-platinum alloys. It would seem reasonable therefore to hope that in due course the platinum:13 per cent rhodium-platinum thermocouple would supersede the platinum:10 per cent rhodium-platinum thermocouple in general use, particularly if the IPTS-68 between 630.74°C and 1064.43°C is eventually defined in terms of the platinum resistance thermometer rather than the platinum: 10 per cent rhodium-platinum thermocouple.

To cover the range between 1748°C and the melting point of platinum further measurements were made using a black-body cavity made from alumina. It was found with this cavity that there is a significant drop in the thermoelectric power both of platinum:10 per cent rhodium-platinum and platinum:13 per cent rhodium-platinum thermocouples above 1700°C. Figures 6 and 7 show the results of these high temperature measurements of thermoelectric power. The change in slope of the thermoelectric power/temperature curve above about 1100°C can be accounted for qualitatively by the effects of the increasing concentration of lattice vacancies at high temperatures. The drop above 1700°C, however, seems too steep to be accounted for solely by lattice defects; there must be another factor which is becoming important. One such factor could be the conductivity of the alumina refractory which is increasing at a significant rate at these temperatures.

Fig. 6

Experimental measurements of the thermoelectric power of platinum: 10% rhodium-platinum thermocouples

+ A5 □ A8 × D4 ▵ D5

Experimental measurements of the thermoelectric power of platinum: 10% rhodium-platinum thermocouples+ A5 □ A8 × D4 ▵ D5

Fig. 7

Experimental measurements of the thermoelectric power of platinum: 13% rhodium-platinum thermocouples

+ A14 □ A17 × D14 ▵ D15

Experimental measurements of the thermoelectric power of platinum: 13% rhodium-platinum thermocouples+ A14 □ A17 × D14 ▵ D15

Fig. 8

An ingot of platinum after melting. Neither the alumina crucible nor black-body were damaged during heating or cooling from room temperature to the melting point. The crucible was broken after the measurement so that the melted platinum could be examined

An ingot of platinum after melting. Neither the alumina crucible nor black-body were damaged during heating or cooling from room temperature to the melting point. The crucible was broken after the measurement so that the melted platinum could be examined

The Freezing Point of Platinum

It became apparent during the course of this work that a temperature of 1772°C (IPTS-68) for the freezing point of platinum would not be consistent with the results of measurements made in the two black-bodies from the gold point upwards. The e.m.f./temperature curve thus obtained showed that the temperature at which the platinum arm of the thermocouple melted was some 4 deg C below 1772°C. A similar result was obtained from platinum wire-point measurements made at NRC. That the freezing point of platinum was lower than the previously accepted value was subsequently confirmed at NPL by measurements made with the photoelectric pyrometer using substantial ingots of pure platinum (4). Three series of measurements were made, two ingots being supplied by Engelhard (U.K.) and one by Johnson Matthey. There was no significant difference found between the results from the three ingots, nor between the melts and the freezes. The final value for the freezing point of platinum was found to be 1767.6±0.3°C (IPTS-68).

The authors are pleased to acknowledge the generous assistance given by Johnson Matthey & Co Ltd, throughout this work by the supply of the platinum and rhodium-platinum wire, the construction and loan of the platinum black-body, the machining and loan of one of the ingots of platinum used for the melting point work, and for spectrographic analysis of pieces of the platinum before and after melting.

Much of the impetus behind this work, together with invaluable advice and encouragement during its execution, came from the late C. R. Barber of NPL.

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References

  1. 1
    C. R. Barber, Nature, 1969, 222, 929
  2. 2
    W. F. Roeser and H. T. Wensel, J. Res. Nat. Bur. Stds., 1933, 10, 275
  3. 3
    C. R. Barber, Proc. Phys. Soc., 1950, B63, 492
  4. 4
    T. J. Quinn and T. R. D. Chandler, Metrologia, 1971, 7, 132

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