A Fiber Bragg Grating Liquid Level Sensor Based on the Archimedes' Law of Buoyancy

We present a very simple and versatile fiber optic sensor for liquid level measurements based on the Archimedes' law of buoyancy. It includes a proper mass suspended in the liquid with one end fixed to a fiber Bragg grating (FBG) strain sensor, while the remaining fiber termination is fixed to a rigid support. When the liquid level increases inside the tank, the load exerted by the suspended mass on the optical fiber is reduced as a consequence of the increase of the force acting on the immersed mass according to the Archimedes' law of buoyancy, leading to a reduction of the strain applied to the FBG. The proposed device is able to perform real time and continuous liquid level measurements, and is characterized by sensing features (such as sensitivity and operating range) that exclusively depend on the geometrical characteristics (e.g., shape, diameter, and length) of the suspended mass. Therefore, by a judicious selection of its geometrical features, the proposed device can be customized so as to suit the requirements for the specific application. In addition, the constitutive material of the attached mass can be properly selected in order to confer to the proposed platform the capability to operate in any kind of liquid. Here, as proof of principle, we exploited an aluminium cylindrical mass having a radius of 8 mm and a length of 30 cm, and experimentally demonstrated the capability of the realized device to measure the water level inside a beaker over a range of 25 cm with a sensitivity of similar to 27 pm/cm, a resolution of similar to 0.04 cm, and a repeatability error of similar to 0.1% of the full scale reading. We also demonstrated, both theoretically and experimentally, the possibility to largely tune the sensing performances by acting on the cylinder radius. Obtained results pave the way for the exploitation of the proposed platform for accurate liquid level monitoring in large-scale storage tanks, useful both for petrochemical industry and for agricultural/residential applications.