Reliability study of sheathed thermocouples in helium environment

The reliability of the sheathed thermocouples used for the temperature instrumentation for the high temperature helium gas loop, HENDEL, was examined in the range from 1000 to 1200 deg C. The test was carried out in pure helium for 1000 hours at the longest in the state of contact with graphite. When Hastelloy X or Inconel 600 was used as the sheath material, the adhesion with graphite was observed at 1200 deg C, but it was prevented by the plasma spraying of Al2O3 or ZrO2 on the surface. The thermocouple wires of platinum group reacted with insulator MgO. This phenomenon was especially intensive when the sheath material was tantalum or Inconel 600, and the electromotive force dropped remarkably. On the other hand, when Pt-10% Rh alloy was used as the sheath material, the reaction was suppressed, and the electromotive force was stable even after 1000 hours at 1200 deg C. The sheathed thermocouples for the HENDEL require the compatibility with graphite, the corrosion resistance to the impurities in helium gas and the stability of electromotive force from the conditions of their use. Using various combinations of sheath materials and thermocouple wires, these properties were examined, and the combinations of R thermocouple-PR 10% sheath and WRe 5-26% thermocouple-tantalum sheath showed good results. (Kako, I.)