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

Abstract

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.

Thermocouple Response in Fires, Part 2: Validation of Virtual Thermocouple Model for Fire Codes

“…A thermal model of a MIMS thermocouple with geometry representative of an actual thermocouple to determine the bead temperature was reported by Nakos [10]. This model included the air gap around the sheath, assumed lumped masses for the bead and sheath, and considered radiation and conduction as the only (dominant) modes of heat transfer.…”

A joint computational and experimental study to evaluate Inconel-sheathed thermocouple performance in flames.

“…5) to determine the bead temperature was reported by Nakos [12]. This model included the air gap around the sheath, assumed lumped masses for the bead and sheath, and considered radiation and conduction as the only (dominant) modes of heat transfer.…”

Detailed multiphysics modeling and validation of thermocouple readings in fires

“…Uncertainty and systematic errors of specifically MIMS thermocouples were studied by Nakos. 2 One of the most dominant sources of uncertainty was associated with thermocouple installation. In another study by Gill and Nakos, 3 thermocouple error due to thermal shunting of the thermocouple assembly was addressed.…”