Patent ID: 7370514

Claim:
A method of determining remaining useful life of a quantity of oil in use in fluid lubrication of an operating mechanism, the method comprising: repeatedly determining resistivity (ρ) values of the oil at an predetermined oil temperature during operating time intervals of the mechanism; recording a resistivity-time ρ(t) history of the oil such that, as successive resistivity values starting with an initial value are accumulated with elapsed operating time, a continuous ρ(t) linear curve is formulated that includes; (1) a first portion with a slope of dρ/dt>0 up to a first time t 1 during which values of ρ increase and (2) a maxima at t 1 where dρ/dt=0 with a corresponding ρ max , comparing, from time to time, the then existing portion of the ρ(t) curve with like predetermined data on a like or functionally comparable oil composition; using the comparison to predict the remaining useful life of the oil; continuing to record a resistivity-time ρ(t) history of the oil so that a continuous ρ(t) linear curve is formulated that further includes; (3) a second portion with a slope of dρ/dt<0 during which values of ρ decrease up to a second time t 2 , (4) a minima at t 2 where dρ/dt=0 with a corresponding ρ min , and (5) a third portion where dρ/dt>0 ; continuing to compare, from time to time, the then existing portions of the ρ(t) curve with like predetermined data on a like or functionally comparable oil composition and using the comparison to predict the remaining useful life of the oil; continuing to record a resistivity-time ρ(t) history of the oil in the third portion of the ρ(t) curve until time t 3 when there is a sudden increase in the value of the time derivative of the resistivity indicating that the useful life of the oil has been depleted; and then generating a signal indicating that the useful life of the oil has been depleted.