Fiber-optic strain-gage tank level measurement system for cryogenic propellants

Abstract

I. BACKGROUNDAlthough much has been done to apply fiber optic technology to design a variety of sensors, few are truly commercially available. Classical temperature, pressure, and strain sensors have not been displaced by this new technology to any significant extent.There could be important advantages to using fiber optic (FO) sensors over resistance, piezoelectric, or thermoelectric sensors, but these cannot be realized until FO sensors are demonstrated to be "better" than classical sensors, if only for some applications.Foil Gages are widely used to measure strain. They are small, and have gained wide acceptability across the scientific and engineering community. However, FG have some shortcomings where FO sensor technology may offer, perhaps, a better alternative.A change in ambient temperature produces four effects on a foil gage and specimen [2]: (1) These effects produce a thermally induced mechanical strain in the gage that does not occur in the specimen. In contrast, the fiber optic sensing element is simply an empty cavity that strictly follows dimensional changes in the specimen. A combination of specially engineered materials and circuit designs have been developed to deal with the apparent strains in FG, but they have the effect of reducing the operating temperature range and the sensitivity of the strain gage-Wheatstone bridge. The temperature effects exhibited by FG may also be produced by self-heating of the resistance element.

Fiber-Liquid-Level Sensor Based on A Fiber Bragg Grating

“…Tank level sensors are composed of a multimode optical fiber that transports white light and has white-light Fabry-Perot fiber optic strain gauges (FPFOSG) at the tip. 5) A fiber Bragg grating (FBG) liquid level sensor based on bending cantilever beam has also been demonstrated. 6) Two Bragg gratings combined with a bourdon tube have been used for constructing a water level sensor.…”