Performance of Chromel versus Alumel and Nicrosil versus Nisil thermocouple assemblies in vacuum and argon between 1000 deg C and 1200 deg C

Prior work showed that Nicrosil versus Nisil (Type N) thermocouples exhibit improved thermoelectric stability at elevated temperatures in oxidizing environments greater than existing nickel base thermocouple alloys. This research evaluated the performance of Chromel versus Alumel and Nicrosil versus Nisil thermocouple assemblies in vacuum and argon between 1000(0)C and 1200(0)C. Both sheathed and bare thermocouple wires of various thicknesses were tested. The thermocouples were compared after exposure to the elevated temperatures by measuring thermoelectric drift from initial calibration data and by performing in place inhomogeneity tests. Under these test conditions, comparable behavior was exhibited by the Type K and Type N thermocouples. However, several conclusions could be made. Thick, bare thermocouples did as well as sheathed thermocouples especially at elevated temperatures where reaction with insulating materials is likely for sheathed thermocouples. Stainless steel sheathed thermocouples tended to exhibit larger maximum errors than Inconel sheathed thermocouples which is a result of contamination by diffusion of sheath component species into the thermoelements.