Journal Archive

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

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 Range Polynomial
−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 Range Polynomial
−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. 0 10 20 30 40 50 60 70 80 90
Absolute E.M.F. in Microvolts
0 0 −53 −103 −150 −194 −236
0 0 55 113 173 235 299 365 432 502 573
100 645 719 795 872 950 1029 1109 1190 1273 1356
200 1440 1525 1611 1698 1785 1873 1962 2051 2141 2232
300 2323 2414 2506 2599 2692 2786 2880 2974 3069 3164
400 3260 3356 3452 3549 3645 3743 3840 3938 4036 4135
500 4234 4333 4432 4532 4632 4732 4832 4933 5034 5136
600 5237 5339 5442 5544 5648 5751 5855 5960 6064 6169
700 6274 6380 6486 6592 6699 6805 6913 7020 7128 7236
800 7345 7454 7563 7672 7782 7892 8003 8114 8225 8336
900 8448 8560 8673 8786 8899 9012 9126 9240 9355 9470
1000 9585 9700 9816 9932 10048 10165 10282 10400 10517 10635
1100 10754 10872 10991 11110 11229 11348 11467 11587 11707 11827
1200 11947 12067 12188 12308 12429 12550 12671 12792 12913 13034
1300 13155 13276 13397 13519 13640 13761 13883 14004 14125 14247
1400 14368 14489 14610 14731 14852 14973 15094 15215 15336 15456
1500 15576 15697 15817 15937 16057 16176 16296 16415 16534 16653
1600 16771 16890 17008 17125 17243 17360 17477 17594 17711 17826
1700 17942 18056 18170 18282 18394 18504 18612
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. 0 10 20 30 40 50 60 70 80 90
Absolute E.M.F. in Microvolts
0 0 −51 −100 −145 −188 −226
0 0 54 111 171 232 296 363 431 501 573
100 647 723 800 879 959 1041 1124 1208 1294 1380
200 1468 1557 1647 1738 1830 1923 2017 2111 2207 2303
300 2400 2498 2596 2695 2795 2896 2997 3099 3201 3304
400 3407 3511 3616 3721 3826 3933 4039 4146 4254 4362
500 4471 4580 4689 4799 4910 5021 5132 5244 5356 5469
600 5582 5696 5810 5925 6040 6155 6272 6388 6505 6623
700 6741 6860 6979 7098 7218 7339 7460 7582 7703 7826
800 7949 8072 8196 8320 8445 8570 8696 8822 8949 9076
900 9203 9331 9460 9589 9718 9848 9978 10109 10240 10371
1000 10503 10636 10768 10902 11035 11170 11304 11439 11574 11710
1100 11846 11983 12119 12257 12394 12532 12669 12808 12946 13085
1200 13224 13363 13502 13642 13782 13922 14062 14202 14343 14483
1300 14624 14765 14906 15047 15188 15329 15470 15611 15752 15893
1400 16035 16176 16317 16458 16599 16741 16882 17022 17163 17304
1500 17445 17585 17726 17866 18006 18146 18286 18425 18564 18703
1600 18842 18981 19119 19257 19395 19533 19670 19807 19944 20080
1700 20215 20350 20483 20616 20748 20878 21006
Fig. 2

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

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

Fig. 4

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

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

Fig. 7

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

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.

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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