Probes for measuring the thermal conductivity of granular materials

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Abstract
An investigation has been made of a number of thermal probes, each consisting of a small resistive heater and a chromel-alumel thermocouple embedded in a suitably shaped piece of copper. After the probe has been immersed in the test medium (here sand of specified water content) its temperature is monitored as a function of the time from switching on the heater. At very short times the response is dominated by the thermal capacity of the probe while at longer times the temperature rise is determined by the thermal properties of the surrounding medium. Small probes with high ratios of surface area to volume minimise the time for which the heat capacity of the probe predominates. The form of the temperature against time curve is close to that expected for a spherical source of infinite thermal conductivity buried in an infinite medium, even when cylindrical or flat probes are used. By using an effective radius concept the thermal conductivity of the surrounding medium can be estimated. The technique has been applied to the determination of the variation of the thermal conductivity of sand with specific water content up to 0.1 kg kg-1.