Studies of the performance of W-Re type thermocouples.

The effect of exposure of bare-wire and BeO-insulated commercial W-Re thermocouple materials to high temperatures in gaseous environments was investigated. The temperature range of interest was primarily 2000 to 2400 K, and the investigations were confined to thermocouple wires of 0.25 mm diameter. Thermoelements were examined for metallurgical structural changes and chemical changes by conventional methods. In the temperature range of interest, grain growth was inhibited in the chemically doped W-3% Re alloy, and excellent room-temperature ductility was retained subsequent to the exposure. The compatibility of high purity (in excess of 99.8%), sintered BeO insulators with the thermoelements differed, depending upon whether the BeO-insulated thermoelement assemblies were self-heated electrically or heated in a furnace; in tests in argon environments, highly reliable performance occurred when the assemblies were heated in a furnace. BeO-insulated W-3% Re vs W-25% Re thermocouples, constructed with degassed and aged materials, exhibited drifts equivalent to about 3 mK/hr during 1000 hours of exposure at 2073 K in argon while in the presence of tantalum.