Capacitive oil life sensor

An apparatus for determining the life of hydraulic fluid in a hydraulic system having a hydraulic container, comprising:

• a pair of electrodes contained within the container and being oppositely spaced, forming a capacitor;
• charging means, coupled to said capacitor, for producing a charging current of constant magnitude and charging said capacitor to a predetermined voltage;
• timing means, connected to said capacitor, for detecting the time at which said charging means begins to produce said charging current and the time at which said capacitor has been charged to said predetermined voltage, and for producing a pulse width modulated signal, the magnitude of said pulse width modulated signal being indicative of a charging time, said charging time defined as the time between the start of said constant current and the time at which said capacitor has been charged to said predetermined voltage and being indicative of the life of the hydraulic fluid, a magnitude of said charging current chosen to minimize the effects of other parameters on said charging time;
• and,
• controlling means for receiving said pulse width modulated signal and responsively determining the life of the hydraulic fluid.