 |
Figure 1: X100 Micrograph image of dirt particles and fibers present in an oil sample. Polarized light is used to highlight material of interest. Fibers will be counted as particles, but not show up on emission spectroscopy. |
 |
Figure 2: X100 micrograph of coal particles from a pulverizer gearbox in a coal fired powerplant. Metals analysis may show some elements present in the coal, such as silicon and calcium, but in low levels relative to the particle count. |
Correlation of metals analysis and particle count data - The comparison myth
Q: Reading my oil analysis report, I notice my particle count is high, even though metals analysis shows very low to zero concentration levels (ppm). How can this be?
A: You are trying to make a comparison of two tests that do not have a direct correlation with each other. Let’s first clarify what the tests look for:
Metals analysis refers to elemental analysis using emission spectroscopy. We look for typical wear metals, typical inorganic contaminants, and lube oil additives. This testing answers the question "What’s it made of ?" and "How much is there?" in relative terms of each other.
Particle counting is a measure of fluid cleanliness. It counts all solid material encountered- metallic, inorganic and organic, and classifies according to size and quantity. It answers the questions "How big and how many?" and nowadays thanks to new technology* can answer the question "What shape?"
If you have high amounts of wear and contaminants showing up in the emission spectroscopy, you will usually see a corresponding increase in particle counts. However the converse is not true; a high particle count can occur with neglible results for spectroscopy! When this happens, we look for organic material such as gels, polymers pellets, seal material (nitrile rubber, BUNA-N, etc), filter media (cellulosic or synthetic fiber), varnish flakes, coal dust etc that may be present but cannot be detected elementally.
Typically we find that fibers and filter media to be the most common reason for high counts in elementally clean samples. There can be other reasons for the variation, but in summary the situation is quite common.
* Newer particle counters such as the Laser net fines (which we have) can give you shape classification information.
-DPW
