Magnetic-field-response sensors have been developed for use in measuring levels of fluids under extreme conditions. The sensors work without wire connections or direct physical contact with power sources, microprocessors, data- acquisition equipment, or electrical circuitry. For fuel-level sensors, the absence of wire connections offers an important safety advantage in elimination of potential ignition sources.

Figure 1. This Liquid-Level Sensor comprises two parallel capacitor plates and an inductor, all completely encased in poly(ethylene terephthalate) that has been formulated to afford protection against acids and similar harsh liquids.
The sensors can be designed for measuring the levels of any fluids that can be stored in electrically nonconductive reservoirs. The sensors can readily be designed and built to withstand cryogenic, acidic, or caustic fluids: The sensor design and the method of powering and interrogating them makes it possible to completely encase the sensors in materials that can be chosen for their ability to endure, and to protect the sensor circuitry against, the harsh fluid environments.

Figure 2. Resonance Frequency vs. Liquid Level was measured in experiments in which the sensor of Figure 1 was immersed in several different liquids.
A fluid-level sensor of this type contains a passive resonant circuit comprising an inductor and a pair of parallel capacitor plates, all encased in a material that protects them from the fluid environment (see Figure 1). When the sensor is mounted so that the parallel capacitor plates extend downward into a dielectric fluid, the capacitance increases, and thus resonance frequency of the circuit decreases, as the level of the liquid rises.

The sensor is interrogated by use of the system described in “Magnetic-Field-Response Measurement-Acquisition System” (LAR-16908), NASA Tech Briefs, Vol. 30, No. 6 (June 2006) page 28. To recapitulate: The system includes a transmitting/receiving antenna that is placed in proximity to the inductor. The system generates a series of increasing oscillating magnetic field harmonics that powers the sensors. Once powered, the sensors respond with their own oscillating magnetic fields. The system measures the response of the sensor circuitry to excitations at different frequencies to identify the resonance frequency. Hence, once calibration data of liquid level versus resonance frequency have been acquired (see Figure 2), the sensor can be used as a fluid-level sensor.

This work was done by Stanley E. Woodard of Langley Research Center and Bryant D. Taylor of Swales Aerospace.