Patent ID: 7658104

Claim:
A liquid level sensor, comprising: a hollow member having a distal open end adapted to be positioned into a liquid reservoir to a depth below a liquid surface level, the distal open end of the hollow member allowing liquid to enter the hollow member to an equilibrium level dependent on the volume of liquid in the liquid reservoir; a sensor head unit configured for mounting at a head mount location remote from the liquid reservoir; a connection tubing system adapted to provide a closed connection between the hollow member and the sensor head unit, such that a column pressure in said hollow member dependent on said equilibrium level is transmitted to the sensor head unit; said sensor head unit comprising: a bellows member having an input stem portion adapted for connection to the connection tubing system, an expandable body portion and a distal bellows face surface, the bellows body adapted for expansion and contraction in response to differential column pressure changes so that said bellows face surface translates along a range of movement, the bellows member mounted in a sensor head housing with external surfaces of the bellows member exposed to ambient atmospheric pressure; a non-electrically-powered mechanism responsive to movement of said bellows face surface to provide an indication of said liquid surface level at each liquid level within a range of liquid levels from a full or high liquid level and a lower liquid level; and a low liquid level alarm circuit responsive to the bellows member to generate an electrical signal indicative of a low liquid level state.