A Multi-Functional Penta-Needle Thermo-Dielectric Sensor for Porous Media Sensing

There is growing interest in multifunctional sensors that expand sensing capabilities, especially for environmental measurement needs. This paper aims to develop a multifunctional penta-needle thermo-dielectric sensor (MPTDS) by coupling an electromagnetic (EM) sensor for determining water content with a penta-needle heat pulse probe (PHPP) for determining thermal properties. The thermal properties were inverse fit to the measured temperature rise at four surrounding thermistor needles from an 8-s heat pulse on a central heater needle. The dielectric permittivity was obtained from an impedance measurement made on the heater needle and two adjacent thermistor needles, leading to water content estimates. To determine water content and thermal properties for porous media, a two-step permittivity calibration and an in-situ effective needle spacing calibration were also made. The prototype MPTDS was evaluated in fine quartz sand across a range of water contents from dry to saturated to determine dielectric permittivity, thermal conductivity, and thermal diffusivity, followed by the accurate calculation of water content and soil heat capacity. The limited comparison here demonstrates substantially improved substrate water content determination accuracy from the EM sensor (RSME = 0.012 cm3/cm3) when compared with heat-pulse estimates (RSME = 0.042 cm3/cm3).

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