Repeatability of various types of thermocouples exposed to thermal cycling

Thermoelectric stability of some thermocouples in the range 400 to 1000 C, after exposure to air at 1000 C for periods of time up to 1000 h, was studied. Couples prepared from Pt/0.1 percent Mo-Pt/5 percent Mo; Platinel 2, Pt-Pt/10 percent Rh; Au-Pd: Ag-Pd; chromel-alumel bare wires; and Pt-Pt/10 percent Rh; chromel-alumel sheated couples were considered. In the first case, the thermal emf variation is generally not high and sometimes does not exceed the calibration error. Tables giving emfs for each degree Celsius were computed for some of them. In the other case, the errors increase with time and may amount to 100 C or more, as for Pt-Pt/10 percent Rh, MgO insulated inconel sheated thermocouples.