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"A recent article suggested that particles that are removed from oil reduce failure detection via oil analysis. I use an oil centrifuge on my personal Cummins engine installed in a pickup, and I use oil analysis at oil change intervals. What would happen if I sent the residue to a lab to be analyzed after cleaning the 'stuff' out?"
It’s true that particles that are removed from oil will reduce the effectiveness of oil analysis to detect failure. Oil analysis cannot detect particles that aren’t there. Nevertheless, most particles that are removed by oil filters and centrifugal separators (or centrifuges) are too large to be seen by many oil analysis instruments such as spectrometers. Spectrometers are biased toward smaller particles in the range of 3 microns or less. This is much smaller than the majority of particles trapped by these filters or centrifuges, which typically are around 10 microns or larger. Therefore, the smaller (and just as significant) particles remain in the oil for analysis.
However, larger particles that are trapped by filters or centrifuges are important as well and can be more closely associated with advanced wear. Filter debris analysis provides one possible method for this type of analysis. Several larger commercial laboratories offer this service where a swatch of contaminant-filled filter media can be analyzed.
If a centrifuge is used in the engine, the sediment inside can be gouged out and sent to the lab along with simple instructions for how it should be analyzed and information regarding its source. The best spectrometer in this case would be an X-ray fluorescence (XRF) spectrometer, which is available at many larger commercial labs.
Alternatively, the laboratory could use a solvent to break down the gouged sediment from the centrifuge and then transfer it through a membrane. This would trap the large particles on the surface of the membrane, which could then be analyzed through microscopic analysis or analytical ferrography.
Of course, the majority of sediment and residue found within the centrifuge will be soot or sludge deposits and thus will lack vital oil properties such as dispersancy to be analyzed within the engine oil. This is important to understand because filter/centrifuge debris will be limited to the characteristics of the contaminants, while oil analysis can provide information on the overall characteristics of the oil. Therefore, it is necessary to analyze the oil as well as gather information on the concentration of oil additives and any changes to the base oil.
Filtration is essential and should not be curbed for the sake of oil analysis. Failure detection via oil analysis is multifaceted. Particles trapped by filtration are only a slice of this process. Continuing to carry out oil analysis in cooperation with filter debris analysis may be the ideal solution for achieving effective analysis while maintaining your motor oil's filtration.