Modelling of a Dual-Probe Heat-Pulse Soil-Moisture Sensor using COMSOL®
Measuring soil moisture content plays an important role not only for sustaining dry farming but also for soil mapping. Our work includes developing a soil moisture sensor which is based on the principle of the Dual-Probe Heat-Pulse method to measure soil moisture content. To validate and quantify the experimental results, FEM simulations using Comsol Multiphysics software has been done to model the sensor and its environment. The DPHP sensor has two probes, a heater element and a thermocouple which is placed 3mm apart. The heater element includes a Nichrome wire wound on to a copper wire and placed in a stainless steel tube. The heater and the thermocouple are encapsulated in a casing made of Verowhite material (ABS) and an epoxy resin is used to bind them together. This geometry was modeled in COMSOL® by choosing the appropriate materials from the material library. The Multiphysics module of Joules heating was incorporated to simulate the supply of constant 3.3V to heater element for a desired time duration of 120s, for which time-dependent study was incorporated. To model the different soil conditions, soil densities and moisture content, heat transfer through porous media physics was used. The maximum rise in temperature (ΔT) at a distance of 3mm was obtained through point evaluation. The 2D plots of maximum rise in temperature (ΔT) for different soil density and water content were generated and compared with the experimental results for calibration. Also, it was found that there was an alternate heat path for the heat flux from the heater element to the thermocouple through the casing instead of heat flow through soil only, which reduced the temperature rise (ΔT), in turn, reducing the resolution of the sensor at higher moisture content. To negate this effect, the positioning of the heater element was modified based on the simulation results carried out in COMSOL®.