Patent ID: 7835875

Claim:
A method of determining the remaining useful life of a quantity of oil in use in fluid lubrication of an operating mechanism, the method comprising; continually determining resistivity and permittivity values of the oil throughout the life of the oil while the oil is at a predetermined operating temperature and during normal use in the mechanism; continually calculating normalized values of determined resistivity and permittivity values by forming ratios of the determined values with the values of resistivity and permittivity obtained for unused oil to facilitate combination of the normalized values in a single composite oil electrical property value; continually calculating an oil electrical property value, by taking the square root of one-half of the sum of the squares of the normalized resistivity and permittivity; accumulating a history of the calculated oil electrical property values so as to develop a curve of the oil electrical property values correlated with accumulated time of oil usage; computing the time derivatives of the oil electrical property values; using a predetermined variation of the time derivatives to anticipate an end of useful working life of the oil; and generating a signal indicating the remaining useful life of the oil.