Design and Calibration of Soil Water Content Sensor Based on Dual Frequency Excitation

Accuracy of capacitance moisture sensors is typically deteriorated on account of soil bulk electrical conductivity effect. In this work, a low-cost capacitance sensor with dual frequency excitation for soil water content measurement was proposed. Then, a two-step calibration method was presented to reduce soil bulk electrical conductivity effect on soil water content. Firstly, the mathematical model of output voltages ${U}$ under dual frequency (f1 and f2) excitation and dielectric permittivity Ka was established with several solution samples. Secondly, the empirical calibration equation of dielectric permittivity Ka and soil volume water content $\theta $ was determined with soil samples. Finally, the two-step calibration method was evaluated compared with empirical equations. Four different soil water content (25%, 31%, 36%, and 41%) samples were prepared with electrical conductivities in range of 0- $1100~\mu \text{S}$ /cm. The results showed that the mathematical model of $\theta $ – Ka has a better linearity which determination coefficient R2 was 0.994, the maximum measurement error was 0.012 $\text{m}^{{3}}\cdot \text{m}^{-{3}}$ , and the maximum electrical conductivity variation was 0.029 when the soil water content was 41%. In a word, the two-step calibration method is effective on reducing soil bulk electrical conductivity effect on soil water content measurement and improving its accuracy.