Understanding The Systematic Error of a Mineral-Insulated, Metal Sheathed (MIMS) Thermocouple Attached to a Heated Flat Surface

Uncertainty assessments of temperature measurements performed at Sandia National Laboratories fire test facilities typically focus on measurements using mineral-insulated, metal sheathed (MIMS), ungrounded junction, chromel-alumel (Type K) thermocouples (TCs). These TCs are used to observe the temperatures of both heat sources and test objects in hydrocarbon fuel fires and simulated fires (typically up to 1200°C). Among the sources of uncertainty, errors associated with TC installation often prove to be dominant. For example, ungrounded junction, MIMS TCs have a systematic error when mounted on a flat steel plate (a commonly used configuration) when attempting to measure the plate temperature. A (relatively simple) model of an ungrounded junction MIMS TC mounted on a flat steel plate was developed. The purpose of this model is not to correct TC readings. Rather, it is to qualitatively understand the systematic error associated with the measurement and find ways to reduce the error through more effective mounting procedures or use of different junction types (e.g., grounded junction). Experimental data showing the errors are presented, as are details of the model and model versus experimental data comparisons.