LIQUID LEVEL SENSING APPARATUS

摘要

A liquid level sensing apparatus includes a probe having a high positive temperature coefficient of resistivity. The probe is connected in a circuit including an operational amplifier and a series transistor which maintains the voltage across the probe constant for a fixed period following switching on of the apparatus and which produces a voltage signal related to the current flowing in the probe. This voltage signal is differentiated by a differentiating circuit to provide a signal indicating the rate of change of the voltage signal from which it is determined whether or not the probe is immersed in a cooling liquid. A warning lamp is actuated by a latching switch circuit when the probe is not immersed. A second example is described in which the probe current is maintained at a fixed level and the rate of change of the voltage across it is monitored.