Dielectric liquid level sensor and method

A sensor for sensing the level of a dielectric liquid such as oil, transmission fluid or the like comprising:

• a sensor body adapted to be mounted with respect to a vessel containing the dielectric liquid whose level is to be measured;
• a probe, including first and second electrodes forming a level detecting capacitor, said probe being secured to the sensor body and being adapted to be at least partially immersed in the liquid, when liquid is present, to detect the level of the liquid;
• means for measuring the capacitance of the capacitor and for deriving from the capacitance measurement a signal representative of the level of the dielectric liquid at the sensor;
• and
• said first and second electrodes being radially spaced apart and coaxial with each other, said second electrode being interior of the first, the spacing between the two electrodes forming at least two distinct regions and having at least two significantly different values as measured at different longitudinal positions along the probe so that the capacitance of the level detecting capacitor per unit length varies longitudinally along the probe, the spacing in one of the regions being sized so that the dielectric liquid in that region clings between the electrodes such that the liquid level in that region remains constant during normal operation of the probe irrespective of the level of the liquid, whereby the capacitance of that region is a function of the dielectric constant of the liquid rather than the level of the liquid.

A system for detecting the level of a dielectric liquid comprising:

• a first capacitor having a pair of electrodes adapted to be disposed so that the dielectric liquid whose level is to be measured may flow between the electrodes, whereby the capacitance of the first capacitor varies with the level of the dielectric liquid;
• a second capacitor so constructed and arranged that its capacitance is independent of the level of the dielectric liquid, said capacitance of the second capacitor being a reference generally equal to the capacitance of the first capacitor in air with no dielectric liquid present between its electrodes;
• a third capacitor having a pair of electrodes adapted to be immersed in the electric liquid for detecting the dielectric constant thereof, whereby the capacitance of the third capacitor is a function of the dielectric constant of the liquid rather than of the level of the liquid;
• a fourth capacitor so constructed and arranged that its capacitance is independent of the level of the dielectric liquid, said capacitance of the fourth capacitor being a reference generally equal to the capacitance of the third capacitor in air with no dielectric liquid between its electrodes;
• means for applying a waveform to all four capacitors in such a manner that all four capacitors are at substantially the same instantaneous voltage during charging, so that the capacitance of each capacitor is directly reflected in the charging current flowing into that capacitors;
• means for comparing the difference between the third and fourth capacitor charging currents to a reference, and for providing a comparing means output for controlling the output of the waveform applying means;
• and
• output means responsive to the charging currents for generating an output signal representative of the level of the dielectric liquid.

A system for detecting the level of a dielectric liquid comprising:

• a level detecting capacitor having a pair of electrodes adapted to be disposed so that the dielectric liquid whose level is to be measured may flow between the electrodes, whereby the capacitance of the first capacitor varies with the level of the dielectric liquid;
• a dielectric constant detecting capacitor having a pair of electrodes adapted to be immersed in the dielectric liquid for detecting the dielectric constant thereof, whereby the capacitance of the third capacitor is a function of the dielectric constant of the liquid rather than of the level of the liquid;
• reference capacitor means for providing capacitance independent of the level of the dielectric liquid and substantially equal to the capacitance of said capacitors in air with no dielectric liquid between their electrodes;
• means for applying a waveform to the capacitors and the reference capacitor means in such a manner that they are at substantially the same instantaneous voltage during charging, so that end capacitance is directly reflected in the charging current flowing into the respective capacitor or reference capacitor means;
• means for comparing the difference between the charging currents of the dielectric constant detecting capacitor and the associated reference capacitor means to a reference, the output of the comparing means controlling the output of the waveform applying means;
• and
• output means responsive to the charging currents for generating an output signal representative of the level of the dielectric liquid.

A sensor for sensing the level of a dielectric liquid in a vessel comprising:

• a sensor body adapted to be mounted with respect to the vessel containing the dielectric liquid whose level is to be measured;
• a probe including first and second electrodes forming a level detecting capacitor, said probe being secured to the sensor body and being adapted to be at least partially immersed in the liquid, when liquid is present, to detect the level of the liquid, the capacitance of the level detecting capacitor being a function of the level of the dielectric liquid in which said level detecting capacitor is immersed;
• a reference capacitor disposed in said sensor body such that its capacitance is not affected by the presence or absence of the dielectric liquid, the capacitance of said reference capacitor being substantially the same as the capacitance of the level detecting capacitor when no dielectric liquid is present between the electrodes of the level detecting capacitor;
• means for applying a waveform to each of said capacitors to charge the capacitors during a charging portion of each cycle of the waveform and to allow the capacitors to discharge during a second portion of each cycle;
• capacitance measuring means for measuring the capacitance of each capacitor whose capacitance is to be measured, said measuring means providing an output signal having a parameter proportional to the measured capacitance of each capacitor and which signal is unaffected by the charge on each such capacitor during the charging portion of the waveform;
• and
• subtracting means for effectively subtracting the signal parameter representing the capacitance of said reference capacitor with respect to the signal parameter presenting the capacitance of the level detecting capacitor to give a resulting signal representative of the level of the dielectrical liquid at the sensor.