Self-Temperature Compensated Water Level and Velocity Sensor Based on Fiber Bragg Gratings

This work proposes a fiber Bragg gratings (FBG) sensor for simultaneous measurements of water level, velocity, and temperature in open channels. The optical fibers are placed on a rectangular aluminum rod attached to the bottom of the channel. The water flow causes a distributed load and a characteristic bend in the rod, depending on the water level and velocity. A set of four FBGs are used to monitor the rod’s bending profile. Taking the wavelength difference of two FBGs makes it possible to eliminate the temperature effect and obtain a system of nonlinear equations dependent on velocity and water level. Velocities from 0.1 ms to 0.9 m/s and water levels from 0.4 m to 0.7 m were experimentally evaluated. A semi-empirical model was obtained to describe the wavelength shift, and finally the water level and velocity are calculated. The results show that the proposed system can determine the water level, mean velocity and temperature with satisfactory accuracy. Artificial neural networks are used as a second and more straightforward approach to determine the water level and velocity from the measured wavelengths, considering mean values and standard deviation. With this method, the maximum absolute deviations were 0.02 m/s and 3.1 cm, for the mean velocity and water level, respectively.