Characterises the number of particles present in hydraulic fluid. Purity classes are stated either in filter classes according to NAS 1638 — which specifies the permitted particle numbers in five particle size areas –– or in purity classes according to ISO 4406, where the permitted particle numbers are specified for 5μm and 15μm. For example, the marking 13/10 shows that between 4,000 and 8,000 parts more than 5μm and between 500 and 1,000 parts more than 15 μm can be contained in 100 ml of liquid (contamination classes).
Fluidlexikon