Impurities in oil can always be a risk. Hard particles such as dust, coloured particles and wear metals can cause abrasive wear. Soft particles can build up from old additive components. Often they are adhesive too and cling to machine parts or filters, which they prevent from running smoothly.
Particles in oil accelerate the ageing of oil and shorten its service life. Particularly when checking hydraulic, turbine and other low-viscosity oils, the pollution level is determined based on ISO 4406 by counting the size and number of particles in the laboratory with help from automatic particle counters (APC). The level of pollution is divided up into cleanliness classes. Here, laser sensors are used to determine the number and size of the particles. After the particles are counted, the oil is classified according to cleanliness.
The processes used to determine oil cleanliness and cleanliness classes are defined under ISO 4406 and SAE 4059. ISO 4406 classification is based on particle sizes >4 µm, >6 µm and >14 µm. The ISO particle numbers are cumulative, which means that the stated particle figure for > 6 µm consists of all particles >6 µm plus particles >14 µm.