Differentiate Dirt from Dust
by Evan Zabawski, CLS
Business Development/Reliability Specialist

Particulate contamination is often incorrectly referred to interchangeably as dirt or dust, even though oil analysis can differentiate the two while providing evidence as to severity of the problem.

It is commonly known that dust appears as silicon in spectrometric analysis, but it is not always appreciated that spectrometric analysis typically does not measure particles larger than 5 µm accurately, and is essentially blind to particles larger than 10 µm. Understanding this basic limitation draws more information from the data.

As does understanding dirt is not dust. Dirt has a different chemical composition than dust, though they both are silicon-based. Dirt is also made up of aluminum, with a ratio of around 3-4 times more silicon than aluminum. The presence of silicon should always be compared to the aluminum levels, and only when the ratio exists can the contamination be described as dirt rather than dust.

The distinction is important because dust is a much smaller particle and is always present in the atmosphere, and therefore is more readily breathed into a system than dirt. Dust particles may be present even in new oil, hence the persistent suggestion to pre-filter oils.

Though dirt particles may be less common, they are frequently more damaging than dust particles and typically correlate with other wear metals. One common misinterpretation, however, is that a stable level of contamination and wear metals signifies a stable system. Further contamination and wear should come with more usage, therefore stable values over time may indicate the contaminant and wear particles may have become too large to be detected by typical spectrometry.

In fact, even a decrease may be viewed as an indication of a problem. As an example, engines are typically only sampled at drain, and are typically drained in equal intervals; therefore the results from one sample to another are often expected to be very similar. If the level of contaminant and wear particles suddenly decreased given the same interval and no preventive maintenance performed, this might also be an indication of an increase in particle size outside the range of detection.

The rules of thumb become very simple:

  1. Silicon is viewed as a sign of contamination, commonly dust, but also common as gasket/ seal material, from coolant in the form of Silicates, and perhaps grease
  2. Silicon should be compared to aluminum to verify if 3-4:1 ratio exists, signifying dirt and not dust
  3. Silicon and/or aluminum will likely correlate against the common wear metals, especially Iron.
  4. All contaminant and wear values are expected to increase given greater service; stable values may actually indicate cause for concern
  5. Sudden decreases in same levels without justification by recent maintenance should be investigated
  6. Catastrophic contamination and wear may go undetected by spectrometry; PQI, particle counting, MPE (analytical ferrography), and filter image microscopic analysis, offer excellent supplemental data to avoid missing such failures

It may be counter-intuitive to think that stable levels of contaminants, or even decreases under certain conditions, may indicate a serious problem, but it is perhaps even odder to consider aluminum as a contaminant rather than a wear metal. We must always consider all possibilities before deciding the likeliest.