This invention relates to liquid level sensors, typically the sending unit for a fuel gauge. More particularly, this invention relates to liquid level sensors that employ an optical waveguide that is immersed in the fluid to provide a continuous measurement of the liquid level.
The need for continuous liquid-level measurements exists in numerous commercial and military applications, such as in the fuel, oil and water tanks of aircraft, automobiles and trucks. Less mobile applications include storage tanks or fuel dispensing, waste water, home, chemical, and food processing purposes, to name but a few. Electrical sensors are of particular concern in many cases, particularly those involving flammable fluids. Hazards are apparent from electric sparks from such sensors in these potentially explosive environments. In other environments, electromagnetic interference may overwhelm the signals generated by these sensors. Conventionally, the level of a liquid in a vessel is detected using a float with a mechanical or magnetic coupling to an external gauge, an ultrasonic or optical transducer which measures time of flight to deduce the liquid level, or a parallel wire capacitance sensor that monitors the change in the dielectric constant between the wires associated with a change in liquid level. Capacitance sensors lose accuracy with increasing amounts of water in the fuel. Sensing of fluid level by visual inspection is only viable in limited circumstances since most usage's require remote and continuous sensing of fluid levels.
A number of fluid level sensors based on fiber optics have been proposed. They are attractive because they introduce no electrical energy into the tank, are insensitive to electromagnetic interference, have no moving parts and can provide a continuous measurement of fluid level. A number of older fiber optic fluid level sensors are described in the article "Experimental Investigations on Fibre Optic Liquid Level Sensors and Refractometers," by K. Spenner et al. in IEE OFS 221 pages 96-99. These implementations employ two separate optical fibers and are primarily for the measurement of discrete levels in the vessel, e.g. to sense whether the vessel is completely full or completely empty or at some intermediate point. It would be more desirable to have a fiber optic sensor that could provide continuous measurements.
An early continuous measurement fiber optic sensor is described in "Fibre Optic Fluid Level Sensor," by M. Belderdid, N. Ghanderharioun, and B. Brennan in the Proceedings of the SPIE Conference 566 Fibre Optic and Laser Sensors IlIl (1985) pages 153-158. This sensor is based on the bending or cladding loss principle, consisting of large loops of a single fibre, the loops being of ever increasing diameter and suspended in the liquid. U.S. Pat. No. 4,870,292 to Alpert et al., teaches a fluorescent doped detector fiber to collect light reflected from a source fiber in the presence of air; the light is refracted away when a fluid of higher refractive index is present. The output signal from the fluorescent doped fiber is analogous to the fluid level.
U.S. Pat. No. 4,942,306 builds upon the '292 patent above by embedding one end of the source optical fiber into a transparent substrate such that the injected light enters the substrate at such an angle that it will refract out of the substrate when the substrate is in the liquid but will continue to be totally internally reflected when the substrate is in the air above the liquid. The detecting fiber is again a separate fluorescent doped fiber that is placed against the transparent substrate, typically being wrapped in grooves around the outside of the substrate.
Nevertheless, there remains a need in the art for a simple, low cost liquid level sensor based on this fiber optic technology, preferably realized as a single optical element immersed in the fluid.