Numerical Modeling of a Thermal Conductivity Matric Suction Sensor

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Courtesy of ASTM International

Thermal conductivity matric suction sensors appear to be a promising device for the measurement of suction in soil. The technique indirectly measures matric suction by measuring the thermal conductivity of a ceramic block. This paper uses both a spherical and a cylindrical porous medium to simulate the heat flow in a thermal conductivity matric suction sensor. It is found that, although the transient temperature at the center of the sensor is heavily dependent on the material of the thermocouple and the heating device, the steadystate temperature is primarily determined by the thermal conductivity of the ceramic block. Therefore, accurate measurements can be obtained by using a longer heating period or by selecting a lower-heat-capacity material for the thermocouple and the heating device. Optimal sizes of the sensor for given heating rates and heating periods are calculated using a finite difference method. A multilayered sphere was also used to simulate the situation where the size of the sensor is not sufficiently large and an error occurs due to the influence of the surrounding soil. The numerical model is verified by comparison with the theoretical solution of a special case.

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