A system has been developed which permits the remote non-contact sensing of the correct fill level of granular, diffuse scattering materials where electrical connections may pose a hazard. The system consists of a sensor head, a fiber optic cable containing three low-loss fibers, and a small console which contains the light source, detector and all electrical and electronic components. The system uses a pair of nearly collimated parallel light beams to define a pair of lines in space, which are intersected at a small angle by the field of view of the detection system. The midpoint between the two intersections is the position at which a scattering surface produces equal intensity signals at the detector from the two beams. The distance below the sensor head at which the intersection occurs defines the level at which valve closing signal is generated. In order to label the light beams, a chopper modulates the two beams 90° out of phase. This permits separation of an automatic gain control signal to virtually remove dependence on the absorbance of the scattering material, as well as permitting independent measures of the two backscatter signals which activate a comparator when the relative scattering intensity from the nearer beam exceeds that from the further beam. Within the defined operating range, the reproducibility of trip level is about ±0.7% at a range of about 300mm.
Fred C. Unterleitner,
"Fiber Optic Level Sensor", Proc. SPIE 0411, Electro-Optical Instrumentation for Industrial Applications, (4 November 1983); doi: 10.1117/12.935784; https://doi.org/10.1117/12.935784