A STUDY OF THE INSTABILITY OF NOBLE METAL THERMOCOUPLES IN VACUUM.
NAVAL RESEARCH LAB WASHINGTON D C
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Thermoelectric instabilities of noble metal thermocouples and individual thermoelements platinum, rhodium, iridium, and platinum-rhodium and iridium-rhodium alloys have been studied in vacuum in the temperature range 800 to 1600C. These studies are an extension of previous work performed in oxidizing and neutral atmospheres. As in that work, the principal source of instability was found to be contamination of the thermoelements by iron impurities from ceramic protection tubes. Iron contamination had a maximum effect on instability in Pt and the alloys but was less important for Ir and Rh. Thermocouple errors associated with this contamination in vacuum were compared with the previous data in air and argon, and the instabilities in vacuum were found to be comparable with those found in argon. The instabilities found for couples in argon and vacuum environments were of much greater magnitude than those found in air. Criteria have been set forth for the use of noble element thermocouples. Important among these are the use of low-iron-content sheathing and large-diameter elements. Author