The instability sometimes encountered in metal-sheathed noble metal thermocouples used for long periods at temperatures above 1300°C is attributable to the presence within the sheath of mixed platinum and rhodium oxide vapours from which the pure platinum limb can take up rhodium.
Such instability can be avoided by removing residual air from the sheath and substituting an inert gas before the assembly is hermetically sealed. Couples sheathed and sealed up in this way have thermoelectric stabilities comparable to those of the normal unsheathed thermocouple where natural
convection prevents the accumulation of dangerous concentrations of rhodium oxide vapour. In the absence of oxygen, the deleterious effects of rhodium metal vapour migration upon the thermoelectric output of these devices can be reduced to negligible levels by suitably selecting the alloys
from which the thermocouple and its sheath are constructed. This article is based upon a paper presented to the Fifth Symposium on Temperature, Its Measurement and Control in Science and Industry, held last month in Washington, D.C.
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