Multi-sensor system using plastic optical fibers for intrinsically safe level measurements

Abstract

A system for measuring liquid level in multiple tanks using optical fiber technology has been developed. Oil field service industry or any sector requiring liquid level measurements in flammable atmospheres can be benefited from this intrinsically safe technology. The device used a single lens for the emitting and receiving fiber and it is based on amplitude variations as a function of the liquid distance and not in time of flight or phase detection. Being the first fiber-optic liquid level sensor with those characteristics for long ranges (>200 cm). A simple model to describe their behavior has been derived and tested on two prototypes. A Monte-Carlo method is used to fit the experimental data and obtain the model parameters. High accuracy between experimental data and fitted curve is obtained. The prototypes have a good linearity, better than 1.5% FS (full scale). Sensor heads are made of plastic optical fibers (POF) that are easy to handle, flexible and economical. They are excited by 650 nm lasers, housed in ST-connectors to obtain compact and rough prototypes. Optical multiplexing is used to increase the measuring safety area. Frequency division multiplexing is used to address each sensor head. A discussion about the influence of tilts and aberrations is also included.