Plastic fiber optic sensor for continuous liquid level monitoring

Abstract

The study demonstrates the development of a novel low-cost plastic optical fiber (POF) based sensor for liquid level measurement. The sensor works on the principle of reduction in scattering-based optical losses (in contrast to the increase in optical losses for evanescent wave-based sensing) in a straight decladded fiber with an increase in the liquid level due to refractive index changes in the medium surrounding the fiber. A compact optical setup is developed in-house consisting of two U-bent fiber probes coupled to a single LED on one end and two photodetectors on the other ends to measure the optical intensity changes. One of the U-bent probes is decladded over the length of the fiber to act as a test probe to measure the liquid level, while the other probe acts as a reference to compensate for the light intensity fluctuations due to light source instability and ambient conditions. The difference in voltage responses of the two photodetectors gives a measure for liquid level. We have investigated the fiber optic level sensor response to rising and falling liquid levels over 55 cm in presence of aqueous liquids of different refractive index values (1.33 to 1.38) and 95% ethanol as well as DI water at various temperatures from 16 °C up to 70 °C. A level sensitivity of 1.4 and 3.3 mV/mm for water level changes was obtained for the liquid level below and above 45 cm respectively. In addition, the fiber optic level sensor shows a stable and reproducible response over several cycles of 30 min duration at different liquid levels. The results demonstrate that this fiber optic level sensor is not only easy-to-make, cost-effective and robust but also offers sensitive, stable and reproducible instantaneous level measurements.