Particle Counting: Importance & Application

EP Editorial Staff | April 1, 2011


Clean oil greatly extends equipment life. Studies have indicated that over 70% of the equipment failures in circulating systems were caused by particulate contamination.

A key component in developing and maintaining an oil-cleanliness program is the ability to accurately measure that cleanliness. The most common method involves optical particle counters designed for use in the field or an oil-analysis laboratory.


Any circulating oil system where cleanliness is important should routinely have particle counts run on it. The frequency is dependent on the criticality of the equipment—normally, though, it’s on a monthly basis. Figure 1 is a diagram of a typical optical particle counter with a fluid flowing through a laser light source. As the light encounters particles, a photosensor records the resulting shadows. This provides an electrical input. The particle counter is calibrated with a sized test dust, and the results are expressed as the number of particles in specified size ranges—typically in terms of particles per milliliter. (Some labs, however, report particles per 100 milliliters.) Interestingly, the light will also count water droplets and air as particles.


Restrictions on the use of optical particle counters include:

  • Water contamination greater than 300 ppm
  • Opaque fluid
  • Contaminated with particles > 200 micron in size
  • Lube is too viscous > 460 cSt

Adjustments can be made by diluting dark samples and dehydrating them to remove water, but these are costly and introduce error in the results. Other methods of counting can be used when the above restrictions are encountered. The results are usually expressed on an oil-analysis report  as shown in Fig. 2.


Particle counts are quantitative. While they express the number of particles for a certain size range, they don’t indicate the type—meaning a particle could
be of a dirt or wear nature. Additional testing needs to be done to identify the types. (This matter will be addressed in a later “Lab Spotlight” on ferrography.)
A quick way to determine the cleanliness of a fluid is to use the ISO 4406 Cleanliness Code (which is a three-number code). The first number expresses particles ≥4µ; the second number denotes particles ≥6µ; the third number expresses particles ≥14µ. Table I is used to arrive at this code.   

lmt0304lab4Notice that for each increase in range, the number of particles doubles. This method is a shorthand way of assessing fluid cleanliness without worrying about the actual number of particles. Refer back to Fig. 2. Notice there are 1042 particles/ml ≥4µ. Referring to the chart, the range number is expressed as 17. The number of particles ≥6µ is 462/ml, which is expressed as a range number of 16. The number of particles ≥14µ is 152/ml, which is expressed as a range number of 14. Putting this three-number code together results in a fluid-cleanliness code of 17/16/14. Remember that the first number will be greater than the second, which will be greater than the third.

As for the bottom line on particle counting: The cost is minimal—the benefits are large. How can you determine the cleanliness of your fluid without particle counting? You can’t.

What’s Next?
The May/June “Lab Spotlight” will explore Viscometry Testing. LMT




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