Abstract: A system for determining a level of a combustible fuel in an aircraft fuel tank with a thermistor bead comprises: apparatus for disposing the thermistor bead at a height in the fuel tank; a temperature sensor disposed in the fuel tank for measuring a temperature in proximity to the thermistor bead; a first circuit for conducting a constant bias current of less than thirty milliamps into the fuel tank and through the thermistor bead; a second circuit for generating a reference voltage in proportion to the measured temperature of the sensor; a third circuit for measuring a voltage across the thermistor bead in response to the bias current, and also coupled to the second circuit for detecting the dry/wet state of the thermistor bead based on the measured and reference voltages and generating a signal indicative thereof; and a fourth circuit for determining the level of fuel in the tank based on the bead height and dry/wet state signal.

Abstract: An active safety barrier circuit is disposable in series with an electrical conductor at a barrier between an adverse environment and an outside environment for passing an analog signal over the electrical conductor across the barrier while preventing potentially dangerous electrical threats from entering the adverse environment over the electrical conductor.