Robust electronic liquid level gauge

Claim 1

Apparatus for sensing and indicating liquid level in a container, comprising: (a) a plurality of electrical probe means each at a different level and each responsive to presence of liquid in its immediate vicinity; (b) a plurality of sensing means, one each responsive to one of said plurality of electrical probe means, each comprising an operational amplifier having a pair of differential inputs and having a single one of said plurality of said electrical probe means coupled to a first of said pair of differential inputs; (c) a signal source supplying a detectable continuous signal having predetermined level and frequency to each probe means and to each sensing means, said continuous signal being applied in phase to a second of said pair of differential inputs of each of said sensing means, a common terminal of said signal source being in electrical contact with said liquid; (d) a plurality of detector means one each coupled to one of said plurality of sensing means, each detector means comprising a signal level detector and a frequency selective detector responsive to the predetermined frequency, each detector means yielding a logic signal only when both said signal level detector and said frequency selective detector detect signals; and (e) a plurality of logic means each sensing the logic signals supplied by two of said plurality of detector means to indicate presence of a logic signal corresponding to presence of liquid at two electrical probe means associated with said two of said plurality of detector means.