Downloaded from https://www.everyspec.com ADS-79D-HDBK Sensors for Monitoring Electrical Properties of Oil: Several techniques are utilized to discern oil condition by monitoring the electrical properties of oil. Dielectric (capacitance) oil quality sensors work on the principle that the dielectric constant of oil will change as it degrades or becomes contaminated. Dielectric constant is a measurement of a substance‘s ability to resist the formation of an electric field within it. As the quality of oil deteriorates, the sensor measures its dielectric constant and outputs this information in the form of a voltage that is correlated to the quality of the oil.
Dielectric sensors have been reported to be adept at detecting the presence of water in lubricating oils due to the large difference in dielectric constant between water and oil. The dielectric constant is highly dependent on temperature, which means that lubricant temperature must also be measured and the dielectric signal compensated to make up for the temperature changes. The sensor should provide real-time qualitative analysis of oil condition and some quantitative analysis of water content to the maintainer. Some oil quality sensors also contain built-in processors that monitor the dielectric constant and provide alerts when there are substantial changes.
The conductivity (resistance) sensors measure the capacity of the oil to conduct electricity. These sensors are reported to monitor oil acidity and oil breakdown. Total electrical impedance (TEI) is the sum of resistance and capacitance. Sensors utilizing TEI are believed to provide greater discrimination of the individual oil degradation and contamination modes than either resistance or capacitance alone. Conductivity and TEI are also highly sensitive to temperature. There has not been any on-aircraft testing. Infrared Sensors:
Infrared sensors use a miniaturized infrared spectrometer to track pre- established infrared wavelength regions providing results similar to laboratory and at-line infrared instruments. These sensors are able to provide real-time data on oil condition (degradation and additive loss) and contaminants (water, fuel and incorrect oil) to the maintainer. Some applications of these sensors require extensive calibration with the desired oil characteristics. On-line applications have been tested on marine gas turbine generators but there has not been any on-aircraft testing. In summary, on-aircraft sensors provide the ability for near-real-time debris and oil monitoring.
While oil condition sensors are not available at this time for on-aircraft applications, wear debris sensors are available and in use for military and commercial applications. M.6.1.4 Wear Debris Size and a Comparison of Methods used for Detection: A comparison of wear debris size during normal and abnormal wear and a comparison of wear debris detection techniques are provided in Figure M-5 and Table M-II. Figure M-5 presents a widely used diagram to describe the progress of metallic wear debris release from normal to catastrophic failure.
91 It must be pointed out that this figure summarizes metallic wear debris observations from all the different wear modes that can range from polishing, rubbing, abrasion, adhesion, grinding, scoring, pitting, spalling, etc. 91 Toms, L and A. Toms., ―Machinery Oil Analysis - Methods, Automation & Benefits‖, 3rd Edition, STLE, Park Ridge, IL, 2008, ISBN: 978-0-9817512-0-7. 252