Analysis of heat paths in dual-probe-heat-pulse soil-moisture sensors for improved performance

Abstract

We analyse heat paths of a packaged Dual-Probe-Heat-Pulse (DPHP) soil-moisture sensor to improve the sensor’s performance at higher levels of volumetric water content. A DPHP sensor consists of heater and temperature probes protruding out into the soil from a casing. The two probes help estimate the soil moisture content by assessing the heat conduction through the soil in the intervening space between them. At lower soil-moisture conditions, the heat conduction through soil dominates whereas at higher soil-moisture contents heat flow through soil decreases due to the increase in volumetric heat capacity and specific heat. Inappropriate placement of the heater element could lead to erroneous measurement due to the heat conduction to the temperature probe through the casing material and the epoxy adhesive inside the casing. To study the heat flow in DPHP sensors, and to develop an improved design, thermal paths in the DPHP sensors having different heater element positions were first analysed through Finite Element Analysis. A Lumped Element Model was also developed to further examine the heat paths in the DPHP sensor and suggest design modifications. The trend exhibited by the lumped model corroborates the results of Finite Element Analysis. Laboratory experiments were performed with DPHP sensors having different heater element positions, results of which validated the modeling outcomes. The insights gained in this study would serve as the design guidelines for manufacturing DPHP soil-moisture sensors with improved accuracy at minimal power consumption.