The particle size of the contamination in a hydraulic fluid is spread over a wide area.
In view of this fact, contamination classes have been created that set out the contamination in SAE-ASTM-AIA and NAS 1638 in five size ranges and the particle distribution that is still permissible in ISO 4406 in three ranges for >4, >6 and >14 μm.
These contamination classes also form the basis for the determination of the degree of purity of the hydraulic fluid after the filter.
Table V 1: Contamination classes according to NAS 1638
Figure V 18: Contamination classes in accordance with ISO 4406
Table V 2: Allocation of contamination classes to areas of application