A Multiprobe Heat Pulse Sensor for Soil Moisture Measurement Based on PCB Technology

Abstract

Heat dissipation sensors operate based on the temperature dependence of the transient heat conduction within the soil, which is a function of the soil characteristics and its water content. After a heat pulse with controlled energy is applied to a heater, it is possible to show that the maximum temperature rise $\Delta T_{M}$ measured in the temperature sensing element can be related to the volumetric water content of the soil $\theta _{v}$ [m3 m−3]. The design and fabrication of a low-cost soil moisture multiprobe heat pulse sensor system using conventional printed circuit boards and surface-mount devices is presented. The proposed sensor is free of the needles’ deflection problem present in conventional multiprobe sensors and is manufactured using conventional off-the-shelf electronic components. A precision low-power electronic signal conditioning circuit, using an instrumentation switched-capacitor building block, was developed and successfully used in the prototype. Due to an energy-efficient topology for the sensor and a low-power signal conditioning circuit, the average current consumption of the system (with one measurement per day) is only 3 $\mu \text{A}$ . To demonstrate the feasibility of the concept, a prototype of the sensor was tested in soils with volumetric humidity in the range from $\theta _{v} = 0.05 \,\,\rm {m^{3} \, m^{-3}}$ to $\theta _{v} = 0.41 \,\,\rm {m^{3} \, m^{-3}}$ and, with a very low heating energy pulse (3 J), showed a sensitivity, normalized by the total energy applied, $\Gamma = 211 \times \rm {10^{-3}}\,\, {^{\circ }}\rm {C} \, \rm {m^{3} \, m^{-3} \, J^{-1}}$ . Compared with a button heat pulse probe sensor which has $\Gamma = 192 \times \rm {10^{-3}}\,\, {^{\circ }}\rm {C} \, \rm {m^{3} \, m^{-3} \, J^{-1}}$ , the developed sensor shows a higher normalized sensitivity.