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Oil, How Clean Does It Have To Be

It is clear that there is great benefit to be gained in having clean oil and that it may be well worth spending a lot of money to achieve it. This would be the case where expensive equipment was used and the cost of maintenance was high or where the equipment was costly but not highly profitable to operate. Increasing the equipment life and the period between maintenance up to ten times normal would be highly pfitable in both cases. On the other hand if the cost of replacement equipment is cheap it is unlikely to be justifiable to spend money on oil filtration.

Table 1 - Allocation of Particle Scale Numbers


Solid particle counts in oil can be done with optical equipment (microscope, light extinction), with an electron-scanning microscope (ESM) or by sifting through screens. Each procedure produces slightly different particle counts due to their varying sensitivity in detecting particles of different sizes. The ESM detects many more of the smaller particles than the optical methods.

Counting standard ISO 4406 -1999 is used internationally to rate solids contamination of oils. This standard classifies the cleanliness of oil and provides a basis to define acceptable solids contamination. It also means oil filters can be tested to prove their performance meets acceptable standards. Table No. 1 is part of the ISO 4406 method of coding the level of solid particles in an oil sample. The solid particle content of oil gets a classification that represents the number of particles of a particular size range.

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