System for detecting low liquid level and probe therefor

Abstract

A system for actuating a warning device when the level of a liquid in a receptacle is below a preselected level. This system includes a probe formed from a length of positive temperature coefficient wire whereby the resistance through the wire increases with increased temperature. This wire is mounted within the receptacle in a manner that allows the wire to be submerged in the liquid under normal conditions. A detection can be made when such wire is partially or completely out of the liquid. There is a first switching device in the system to apply a voltage across the wire and a detector for actuating the warning device in response to an electrical characteristic of the wire as determined at a preselected control point in the circuit including the probe. In accordance with the preferred embodiment, the detector includes a capacitor and a second switching device to ground the capacitor for a preselected time and then use the charged capacitor to follow the voltage of the probe. In this manner, the capacitor is charged rapidly toward a voltage determined by the temperature of the probe wire. After a selected time, the second switch is deactivated so that the voltage supplied by the first switch continues to heat the probe wire and increase the level of charge on the capacitor according to the resistance of the probe wire. If the wire is not covered sufficiently with the liquid, the voltage of the capacitor continues to increase. When it reaches a given value with respect to a reference voltage, the warning device is activated.