CAPACITANCE-TYPE MATERIAL LEVEL INDICATOR

摘要

A system for indicating the level of material in a vessel as a function of material capacitance comprising a resonant circuit including a capacitance probe adapted to be disposed in a vessel so as to be responsive to variations in capacitance as a function of material level. An oscillator has an output coupled to the resonant circuit and to a phase detector for detecting variations in phase angle as a function of probe capacitance. Level detection circuitry is responsive to an output of the phase detector and to a reference signal indicative of a level of material for indicating material level as a function of a difference between capacitance at the probe and the reference signal. In the preferred embodiments of the invention disclosed, an automatic calibration circuit adjusts the resonance characteristics of the parallel resonant circuit in a predetermined or preprogrammed manner during a calibration operation to a point indicative of the predetermined reference material level. A timer measures the duration of the automatic calibration operation for indicating conditions at the probe, such as the amount of material coated onto the probe. A reed switch is positioned adjacent to the cover of a housing which encloses the electronics and is responsive to variations in magnetic flux from externally of the housing for indicating a calibration operation without removing the housing cover.