Removing index of refraction constraints in the optical measurement of liquid level

Continuous and discrete liquid level measurements made with fiber optic sensors generally depend on refraction of light in the liquid to be measured. Problems, including erratic or incorrect readings, arise when the index of refraction of the liquid is near that of the outer surface of the optical probe. This is often the case for common optical probe materials and many organic compounds including fuels and lubricants. This paper describes a continuous liquid level sensor which operates over a wide range of indices. The lower limit of the range is determined by an intermediate optical material in the probe, rather than the material of the outer surface; there is no upper limit. The liquid being measured may have any index equal to or greater than that of the intermediate material. Water is used as the intermediate material in the sensor to be described; it surrounds a lambertian fiber optic emitter and detector pair. The sensor is linear within +/- 5 percent over a five-inch measurement span, for liquids of any index equal to or greater than 1.33, even though the outer surface of the probe is fused quartz, with an index of 1.46. Since most liquids have an index greater than that of water, and many liquids of interest have an index near that of quartz or glasses, this method greatly extends the range of liquids that can be measured with a given sensor.