A quasi-distributed liquid level sensor based on a bent side-polished plastic optical fiber

28 Jun 2007

Spanish researchers describe the design and development of a prototype liquid level sensor that exploits the changes in light transmittance in a plastic optical fiber cable.

Dr Lomer and colleagues propose a flexible liquid level sensor that consists of a plastic optical fiber (POF) cable with measurement points located along the fiber. These points are generated by removing small parts of the core, and then laterally polishing those areas. Locating a point on each full turn of a coil improves the sensitivity of the device(Meas. Sci. Technol. 18 2261-2267).

Light transmittance along the fiber is altered by changes between the refractive indices of air and liquid, generating a signal power proportional to the position and level of the liquid. The sensor is capable of high sensitivity even with several measurement points along the fiber as the lateral polishing produces only moderate losses.

The researchers have successfully demonstrated the sensor system in the laboratory and present experimental results from two prototypes with 15 and 18 measurement points in a range of 33 mm and 39 mm, respectively, and a resolution of 0.08 mm with bend radii of 5 mm and 8 mm.

Due to the flexibility of plastic optical fibers it is possible to alter the space between fiber turns from being millimeters to several meters, which gives an extensive potential measurement range. The use of POFs makes it possible to develop very cost-effective sensing systems.