Capacitance liquid level detector

Claim 1

An electric liquid level detector for detecting a level of liquid within a container, comprising: a pair of helically coiled electrodes extending vertically in said container and being fixed at their one ends to an upper portion of said container and at their other ends to a bottom of said container, said electrodes being positioned such that they are at least partially submerged by any liquid stored within said container and such that the amount of submersion is a function of liquid level within the container; and an oscillator circuit of the resistance capacity type including a variable condenser which is formed at least in part by said electrodes, said oscillator circuit being arranged to oscillate at a frequency that is a function of capacitance between said electrodes so as to indicate the level of the stored liquid, said oscillator circuit comprising; a first inverter connected to an electric DC power source; a second inverter connected to said electric DC power source and having an input terminal connected to an output terminal of said first inverter; a resistor connected in series with said first inverter to form a closed circuit, said variable condenser being connected at one end thereof with an output terminal of said second inverter and at the other end thereof with the output terminal of said first inverter through said resistor such that said first and second inverters are inverted opposite to each other with a time period defined by a resistance value of said resistor and an electro-static capacitance of said variable condenser; a third inverter connected to said electric DC power source and connected at its input terminal to the output terminal of said second inverter; and a second condenser connected at one end thereof with an output terminal of said third inverter and at the other end thereof with the output terminal of said second inverter through said variable condenser such that said second condenser is biased by an inverter function of said third inverter in voltage polarity opposite to that of said variable condenser.