Why Don’t My Oil Analysis Results Correlate?
A common issue related to report interpretation deals with a perceived necessity for the results from two or more different tests to indicate the same potential problem. This expectation, which may be based on a misconception, often leads to greater confusion. Therefore, it is important to understand which tests do or do not correlate, and under which circumstances. Consider these two examples.
Elemental Spectroscopy & Particle Count
Elemental spectroscopy is performed by diluting a sample with solvent and injecting into a plasma where it is ionized (essentially burned at a temperature of nearly 10,000 K (hotter than the surface of the sun). Each element on the periodic table emits a unique color of light as it is ionized, and the instrument measures the intensity of these colors to determine the concentration (reported in part per million – ppm).
A noteworthy limitation to this test is that the sample must completely ionize within a very small, measured area of the plasma. As such, only particulate within 0-5 microns is accurately measured, and particles larger than 10 microns are essentially not measured at all. Wear particles generated under normal conditions and airborne contaminants easily fall within this range, however severe wear and/or contamination may produce particles too large for detection and would require supplemental testing such as particle counting or analytical ferrography.
Particle counting, on the other hand, quantifies particles larger than 4 µm. As such, there is very little overlap between the particles measured by elemental spectroscopy and particle counting, therefore one should not necessarily expect a high particle count to manifest itself as high silicon (from dust) or elevated wear metals.
Elemental Spectroscopy and Ferrous Wear Concentration
Unlike elemental spectroscopy or particle counting, ferrous wear concentration has no limitation on particle size, and may therefore correlate with either result. When the iron by elemental spectroscopy is elevated, but ferrous wear concentration remains low, it may be surmised that the wear particles are small (<10 microns) and therefore from normal wear modes. When results from both tests are elevated, then the wear mode is likely transitioning from normal to abnormal; and if iron by elemental spectroscopy is low or consistent, but ferrous wear concentration is elevated or increasing, then the wear particles are likely large (>10 microns) and considered to be due to abnormal or severe wear modes.
Similarly, when the particle count is low or unchanged, but ferrous wear increases, it can be assumed that the wear particles are small (<4 µm) and likely from normal wear modes. When both results increase the wear mode is likely transitioning from normal to abnormal; and if particle count is elevated or increasing, but the ferrous wear concentration remains low or unchanged, then the particulate are likely contaminants and not wear metals.