Patent ID: 7109729

Claim:
A method for determining the condition of a working fluid contained in a mechanical system and at a temperature above about 50° C. which comprises: disposing a pair of spaced apart electrodes in the fluid; imposing an AC signal over a range of 4 or more frequencies on the electrodes; determining the impedance or admittance data as a function of the frequencies from electrical response to the imposed AC signals; determining at least one of the following properties: resistance, capacitance, Omega max, and time constant by plotting the frequency dependent impedance data in the form of a Nyquist plot by fitting the data to a curve where, in rectangular coordinates, imaginary impedance (Z″) is plotted against real impedance (Z′) or, in polar coordinates, Z=[(Z′) 2 (Z″) 2 ) 1/2 is plotted against Θ, the phase difference between voltage and current, wherein said resistance, capacitance, Omega max, and time constant is calculated by determining the diameter of the curve; and comparing one of the properties determined to a predetermined value of the said properties whereby an indication of the condition of the fluid is determined.