Liquid level controller

Abstract

Disclosed is a liquid level controller that uses a pair of vertically spaced thermistors to control a switching means to turn a pump on when a sufficient depth of liquid is present and to turn the pump off before the pump runs dry. Such a device is ideally suited for applications such as 'stripper' oil wells. The present invention uses thermistors in the self heat mode and senses the changes in the resistance between a thermistor exposed to air and one submerged in liquid, the latter increasing resistance because it is far more efficient in transferring away heat generated by the thermistor. The change of resistance when the liquid level covers the higher of the thermistors is detected by the use of comparators which selectively turn on a driving transistor. Turning the transistor on activates a coil which throws a relay switch and activates the control circuitry of the pump motor. The change in resistance when the liquid level drops below the lower of the thermistors is similarly detected by the use of comparators which turn off the driving transistor, deactivating the control circuitry of the pump. Further, provisions are disclosed to automatically turn the pump off in the event that the thermistor or their leads are damaged.