Soil moisture sensor design based on fiber Bragg grating

In order to achieve a large area of deep soil moisture monitoring, a soil moisture sensor based on fiber Bragg grating was designed. The polyimide with high water sensitivity and high linear expansion coefficient was coated to the surface of FBG. The center wavelength of the FBG will change due to moisture absorption and expansion of the polyimide material when moisture changes. Using this basis principle, the soil moisture can be measured. A stainless steel tube with a hole is used as a sensor package and wrapped in polyurethane nets to maintain consistent with the external temperature and moisture environment and prevent soil from entering the sensor. A fiber Bragg grating without a PI coating layer was packaged in the same package structure with the soil moisture sensor as a reference fiber for temperature compensation. An experimental system was set up, calibration experiments were performed on the sensors, and various performances of the sensors were tested. The experimental results show that the measuring range of moisture is 15%RH~75%RH, the sensitivity is 12.6pm/%RH, and the accuracy of the sensor is ±10.26%.