Temperature/Heat Flux Errors Caused by High Temperature Resistive Shunting Along Mineral-Insulated, Metal-Sheathed Thermocouples

Commercially manufactured, mineral-insulated, metal-sheathed (MIMS) thermocouples (TCs) have been used at Sandia National Laboratories (Sandia) and other experimental laboratories for many years to measure temperatures in fires and high-temperature (e.g., 1200°C) radiant-heat environments. These TCs are rugged and relatively inexpensive, which make them ideal for large-scale outdoor testing where factors such as weathering and rough handling are important. A particularly insidious source of TC error is thermal shunting of the TC assembly. This error is caused by a sharp drop in the electrical resistivity of the mineral insulation at elevated temperatures. When a TC assembly is subjected to high temperature along its length, the lower resistivity of the mineral insulation can cause shorting or shunting to occur. This paper describes several cases demonstrating the seriousness of the shunting problem at temperatures previously thought to be immune. Some preliminary experiments designed to clarify the thermal shunting problem are presented. A model developed under previous work but modified specifically for Type K TCs is discussed. Comparisons of measured and predicted temperature error data in additional thermal shunting experiments provide some degree of confidence in the model’s predictive capability. Predictions for several common field test configurations used in Sandia experiments are shown in the hopes of sensitizing other researchers in the field to this problem.