Quantitative SIMS analysis of decalibrated Chromel versus Alumel thermocouples using indexed sensitivity factors

Secondary ion mass spectrometry (SIMS) is used to determine the failure mode responsible for the large decalibrations observed in sheathed, MgO-insulated Chromel versus Alumel, compacted thermocouple assemblies, exposed to temperatures above 11000C. Thermocouples sheathed in Inconel-600 and type 304 stainless steel were studied in this work. Quantified SIMS data showed that the observed decalibrations were due to significant alterations that took place in the Chromel and Alumel thermoelements. The amount of alteration was different for each thermocouple and was influenced by the particular sheath material used in the thermocouple construction