Agricultural soil moisture sensor based on U-bend Plastic Optical Fibre (POF)

Soil moisture has an essential role in agriculture. The growth of specific vegetation requires a certain level of soil moisture. Therefore, monitoring soil moisture is very important for farmers. In monitoring, the first essential component is the sensor. Nevertheless, the existing proper soil moisture sensor is an electric-based system sensor with low water resistance. It easily damaged in extreme environments which can reduce sensor ability. Based on these problems, we developed a soil moisture sensor based on plastic optical fibre with a U-bend structure. Optical fibre was developed as an alternative sensor due to low prices, immunity to water, and resistance to extreme conditions. U-bend optical fibre can measure soil moisture depends on changes in the refractive index. The changes of refractive index caused by changes in external force which causes a decrease in the optical fibre output power are following the concentration of water contained in the soil. Plastic material is used because of its high durability and high responsiveness in refractive index-based measurements. In this experiment, a plastic optical fibre with a U-bend radius of 30 mm and a wavelength of 930 nm is used. The result shows that the U-Bend structure plastic optical fibre sensor is highly potential to use as a soil moisture sensor with sensor sensitivity of 2.885x10 W/%VWC, R of 97.14%, and standard deviation of 0.6%.