A Fiber Bragg Grating Water Level Sensor Based on the Force of Buoyancy

In this work, we propose a robust water level sensor based on a fiber Bragg grating (FBG). The sensor head is formed by a cylindrical PVC cap with 4 cm diameter. The cap was filled with silicone rubber in which two FBGs were inserted for temperature and strain measurements. The cap was placed on the top of a protective PVC pipe column, and a 2 m long float was inserted into its center. Under the action of buoyancy, the float causes a compressive stress in the silicone rubber, which results in a longitudinal strain in the fiber. The sensor was tested with the FBG embedded at 5 mm, 10 mm and 15 mm in the silicone rubber, and the obtained sensitivities were 155.7 ± 2.4 pm/m, 80.7 ± 1.2 pm/m and 43.5 ± 2.5 pm/m, respectively. Considering the maximum wavelength shift observed was 0.72 nm, the expected maximum water heights the sensor could detected are about 5 m, 10 m and 18 m, for the three silicone rubber depths considered. It was also developed a theoretical model to describe the sensor response with the silicone rubber depth and water height.