Monitoring the deterioration of oils

摘要

In the context of this patent the term 'fluid condition' describes the chemical material condition of the fluid. In other words, this could mean that an oil has been contaminated with coolant or it has deteriorated through ageing, or that it contains products of combustion. It is not concerned with particulate contamination which is so important to operating fluids in refined systems, although the presence of particulate may cause similar variations to the sensing arrangement. The technique which is described here covers AC electro-impedance spectroscopy. This method is already well-known but it has conventionally used high frequencies - 10 Hz to 500 kHz - and it has been costly to apply. This invention employs a low-level oscillating voltage signal which is applied across spaced electrodes of a capacitor that is immersed in the fluid at both a high frequency (for the bulk impedance of the fluid) and a low frequency (for the electrochemical properties of the surface of the electrode). The high frequency current is compared with that of an acceptable fluid, and the difference between the high and low frequency currents is compared with an acceptability threshold. If either is extreme the fluid is considered unacceptable.