Fluid quantity indicator

Claim 1

In an apparatus responsive to the quantity of fluid within a container therefor of a predetermined volume, said apparatus comprising a capacitive sensing means disposed within said container volume so as to be immersible within said fluid and any air within said container volume, said capacitive sensing means having an associated variable dielectric constant, said dielectric constant being proportional to the proportionate amount of fluid within said container volume, said sensing means capacitance varying in accordance with the variation in said porportionate amount of fluid; a reference capacitive means disposed within said container so as to normally be immersible within said fluid and responsive to variations in the dielectric constant of said fluid due to environmental conditions; the improvement comprising AC reference means operatively connected to said sensing means for providing a signal having a predetermined phase and frequency and voltage thereto, a current proportional to the capacitance of said sensing means flowing through said sensing means in response to said provided AC reference means signal; up/down counting means for providing an output signal indicative of said fluid quantity; first computing digital to analog conversion means having a magnitude input operatively connected in parallel to said counting means output for providing an output in response to said input, said conversion means output being operatively connected to said reference capacitive means for providing a voltage thereto, said output comprising a signal having a predetermined voltage and phase and opposite to said AC reference means signal phase and a predetermined frequency identical therewith; second digital to analog conversion means operatively connected to said first conversion means for providing a reference magnitude signal thereto, said reference magnitude signal providing a full scale set for said apparatus, said second conversion means being programmable to provide a universal apparatus full scale set compensation for containers for said fluid substantially independent of said container configuration for compensating for variations in full scale capacitance of a given container; differential amplifier summing means having an input operatively connected to said capacitive sensing means and said capacitive reference means output for summing the output signals provided therefrom; means operatively connected to said summing means output for providing a directional counting signal output in response to said summing means output, said counting signal output being operatively connected to said counting means for counting in response to said counting signal, said counting means output being operatively connected to a digital display means for providing a digital display of said fluid quantity in response to said counting means output; said AC reference means, capacitive sensing means, capacitive reference means, summing means, directional counting signal providing means, counting means and first conversion means comprising a servo control loop which is nulled when said counting means output is equivalent to said fluid quantity, said servo control loop varying said voltage applied to said capacitive reference means for nulling out the current flowing through said capacitive sensing means, whereby a container configuration independent real time digital display of fluid quantity within a given container which is capacitive leakage independent is provided.