An automated probe for thermal conductivity measurements
Dougherty, Brian P.
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A transient technique was validated for making thermal conductivity measurements. The technique incorporated a small, effectively spherical, heat source and temperature sensing probe. The actual thermal conductivity measurements lasted 30 seconds. After approximately 15 minutes of data reduction, a value for thermal conductivity was obtained. The probe yielded local thermal conductivity measurements. Spherical sample volumes less than 8 cm² were required for the materials tested. Thermal conductivity (and moisture) distributions can be measured for relatively dry or wetted samples. The technique employs an encapsulated bead thermistor. A thermistor, more commonly used as a temperature transducer, has the inherent feature of being readily self-heated. A computer-based data acquisition and control system regulates the power supplied to the thermistor such that its self-heated temperature response approximates a step change. Thermal conductivity is deduced from the transient measurement of the power dissipated by the probe as a function of time. The technique was used to measure the thermal conductivity of fifteen liquids and five insulation materials. Two different thermistor types, glass-encapsulated and Teflon-encapsulated, were evaluated. Capabilities and limitations of each probe type and the measurement technique, in general, were observed.
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