Uncertainty of thermal conductivity measurement at high temperatures using guarded hot plate apparatus

Abstract

The guarded hot plate (GHP) is a primary method to measure the thermal conductivity (λ) of low-to-medium λ materials. Precision measurement of λ using the GHP method is challenging because the deviations from the basic assumptions in the principles of the method should be minimized, and the effect of such deviations on λ measurements must be quantified for uncertainty assessment. In the present study, the measurement uncertainty in λ of insulation materials when using a GHP apparatus was assessed at high temperatures of up to 700 °C. Both high-temperature environment and low-λ specimen make the measurement of λ difficult compared to the near-room-temperature environment or medium-λ specimen; thus, the assessed uncertainty tends to be large. A simple model that accommodates stray heat flow was described in this study and subsequently validated by measurements in various thermal configurations around the specimen. To reduce the stray heat flow, a new set of thermocouples at various parts in the GHP apparatus was specifically calibrated to accurately measure the temperature difference across the air gap before their installation, and proper auxiliary set temperatures were found in a systematic search. By employing all of these techniques, the relative expanded uncertainty (with a coverage factor of 2) in λ measurements at 200–700 °C was assessed to be 4.6%–6.8%. The present work provided a complete procedure from the measurement of λ and its uncertainty assessment for low-λ material at high temperature, where the precision measurement and uncertainty assessment of λ have been considered problematic.

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