Platinum Metals Rev., 1965, 9, (4), 118
Stability of Platinum Metal Thermocouples
High Temperature Tests In Vacuum
The drift in calibration of platinum: 13 per cent rhodium-platinum thermocouples when they are heated for long periods in oxidising or neutral atmospheres is due almost entirely to contamination by impurities picked up from the alumina protection tubes. Careful tests by B. E. Walker and his collaborators at the United States Naval Research Laboratory in Washington, D.C., reported in 1962 (abstracted in Platinum Metals Rev., 1963,7, 38), showed that of these impurities iron was by far the principal cause of trouble.
These investigators have now studied the behaviour of platinum, rhodium-platinum, and other platinum metal thermocouple elements in order to assess their reliability when heated to between 1000º and 1700°C in vacuum (B. E. Walker, C. T. Ewing and R. R. Miller, Rev. Sci. Instruments, 1965, 36, (5), 601-606).
Not unexpectedly, the changes after heating with various grades of alumina in vacuum are found to be not significantly different from the changes after heating in argon. Moreover, in both vacuum and in neutral atmospheres, the changes due to contamination by iron are at least one order of magnitude greater than in an oxidising atmosphere.
The change in calibration after 120 hours in a vacuum of 5 x 10–5 Torr was only significant at temperatures above 1200°C. At 1600°C the contamination of a pure platinum element in contact with a grade of alumina containing 0.07 per cent of iron resulted in a change in the calibration at 860°C of 2000 microvolts; but with alumina containing 0.03 per cent iron the change was only 400 microvolts.
Pure platinum elements were very much less stable than the alloy elements, the wires becoming rapidly more stable as the rhodium content increased. The 20 per cent rhodium alloy was apparently quite resistant to the effects of contamination at least for periods up to 120 hours at 1600ºC under the most severe conditions of test.
No evidence at all was found that preferential volatilisation of platinum or rhodium contribute at all to the instability of platinum metal thermocouples in vacuum (or, indeed, in air or any other atmosphere).