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

There is growing interest in multifunctional sensors that expand sensing capabilities, especially for environmental measurement needs. We developed a penta-needle heat pulse probe (PHPP) and coupled it with a dielectric sensor yielding a multifunctional penta-needle thermo-dielectric sensor (MPTDS). The MPTDS consists of a central heater needle orthogonally surrounded by four thermistor needles. The thermal properties are inverse fit to the measured temperature rise at the four thermistors from an 8 second heat pulse. The dielectric permittivity was obtained from an impedance measurement made on the heater needle and two adjacent thermistor needles, leading to water content estimates. The processes of heat pulse application, temperature measurement, inverse fitting, and water content determination are automatically managed by an on-board microcontroller with SDI-12 communication. 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 Wedron sand across a range of water contents from dry to saturated, where the results demonstrated the coupled sensor’s ability to accurately determine dielectric permittivity, thermal conductivity, and thermal diffusivity, followed by accurate calculation of water content and soil heat capacity.