Validation of a soil interface model to account for apparent aberrations in capacitive soil moisture sensors

Abstract

In this paper the operation of capacitive soil moisture sensors are modeled using an electrical circuit analogue. This model aims to predict the response of capacitive sensors for a variety of soil types, moistures, soil conductivity and sensor operating frequencies. The model is extensively validated under a variety of conditions for a variety of sensor circuits and measurement techniques. The deposition of a conducting film composed of clay-like soil material over the sensing surface of a soil moisture sensor is shown to be the cause of hysteresis when the sensor is operated at low frequencies (10KHz). As the frequency is increased (10MHz) the effect of the conducting film becomes insignificant. Surface chemistry analysis techniques were used to identify the soil deposits on the conducting film. This research is motivated by the design of a small disposable sensor printed on a flexible plastic substrate measuring soil moisture as a function of the number of point contacts terminating on the insulated sensor electrode. In controlled conditions the sensor exhibits a linear response across most of its range to water content changes, but in some soils the reading becomes "stuck" on a high reading and does not return to a lower reading until the soil has dried considerably.