Design and Calibration of the Unilateral Sensitive Soil Moisture Sensor

Reliable and affordable moisture sensors with a simplified calibration process for obtaining the water content information of soil/soilless substrates were limited. In this paper, an unilateral sensitive capacitive soil moisture sensor, based on the improved ECH2O sensor probe structure, was presented. Through simulating the relationship between the ratio of plate space and plate width (a/b) and probe sensitivity, sensitive depth, respectively, the size of the sensor was determined with ANSYS first. Capacitance measurement circuit based on resonant principle was then designed. The relationship between the output of the sensor and the soil water content calibration model (β parameter model), was investigated. As a result, we proposed a two-step calibration β method. Experimental results showed that β parameter model appropriately described the relationship between the output and the soil water content, the minimum correlation coefficient R² = 0.9554 in the soil, peat, perlite, and vinegar residue. The measurement error of β parameter model was <;0.025 units except in perlite. The error caused by the change of bulk density was <;0.025 units in peat and vinegar residue. Therefore, the two-step calibration β method is suitable for calibrating the sensor.