Research on the Electrical Model of a Capacitive Soil Moisture Sensor

The electrical model of a capacitive soil moisture sensor is considered in this paper. In the new model established, the contact resistor and contact capacitance are taken into account. It is pointed out that the electric double layer causes the formation of the contact resistor and contact capacitance. The electrical properties of the electric double layer are the effect of both physical electricity and electrochemistry, so the relationship between the contact capacitance and the soil relative permittivity does not follow the formula of the parallel plate capacitor. Based upon the diffuse electric double layer model, this paper successfully derives the formula of the contact capacitor , whose coefficients are determined by MATLAB simulation based on experimental data, and the soil relative permittivity. Besides, this paper has established the sensor-output-voltage-Vo -soil-moisture-θ curve and compared it to that derived from the model without considering the electric double layer. It is demonstrated that the correlation coefficient between the curve derived from the model this paper established and the experimental data is 0.9997, more accurately describing the relation between the sensor output voltage Vo and soil moisture θ.