Abstract: Apparatus for monitoring the level of liquid in a tank or reservoir comprises a probe formed by a resistive metal element having a high temperature coefficient, and a supply means for the probe. The apparatus further comprises a voltage-frequency converter which is connected in parallel with the probe, two counters which are connected in parallel to the output of the converter, and a subtracting means connected to the outputs of the counters. A time base controls operation of the supply means during a complete cycle to effect a counting operation of the first counter for a given period of time at the beginning of a cycle, a counting operation of the second counter for the same period of time but at the end of the cycle, and operation of the subtracting means at the end of the cycle.

Abstract: A hot-wire level detecting apparatus includes a micro-processor which registers a look-up table of digital level values associated with digital values of a differential parameter representing the change in the voltage or current in the hot-wire probe. The micro-processor controls a switch for initiating the application of power to the probe. In one embodiment a constant voltage supply is connected across the emitter and collector of a transistor with the probe in the emitter circuit and a resistor in the collector circuit. The voltage across the resistor, measured at switch-on and after a period for stabilization, is fed to the micro-processor by an analog-digital converter.