High-frequency capacitive soil water content sensor based on detecting of true root mean square

Measurement of soil water content is the basis of implementing water-saving irrigation in precision agriculture. A high-frequency capacitive soil water content sensor which is consist of power supply filter circuit, 100MHz oscillator, XC74UL14AA, probe electrode which is made up of PCB (Printed Circuit Board) and is part of the whole PCB, and AD8361 was developed based on True RMS detection technology, by using the dielectric properties of the soil. The output of the sensor is in form of DC voltage, and the output range is 20%-70% of supply voltage, measured in air and deionized water respectively, under different supply voltages. The sensor was calibrated, and the temperature variation of the sensor was tested in the range of 5-40℃ according to the basis of 24.8℃, in a series of tested solution with different equivalent soil volumetric water content, under different supply voltages. The tested solution was prepared through 2-isopropoxy ethanol, dioxane and deionized water three kind of solution. The results show that: there is a significant dependency of the sensor’s output on the supply voltage, the output voltage of the sensor increases with the supply voltage in specific soil volumetric water content; the out voltage of the sensor has a negative linear correlation with soil volumetric water content in specific supply voltage, and the coefficient of determination R2>0.987; the greater the temperature difference, the greater the measurement error of the sensor, and the maximum error is 4.44%. Verification of the sensor was done through preparing a series of soil samples, and the maximum error is 4.95%.