I m facing problem with Mp filtri particle counter fixed in fitration trolley (for offline operations) the display always showing NAS15 class whereas the oil we are using is AW68/ NAS7 Clean. probably it was counting more air particles. to fix it I have added 1.5 bar check valve in the outlet of Particle counter and It worked out. It had then shown NAS 9 but in after test run it gone to NAS 4, NAS 2 and sometimes NAS00.

inlet for particle counter taken before filter and after pump. oil in the outlet doesnt seem aerated
 
as well, any one having comparison chart showing NAS class for specific ISO code??

It does seem as though the particle counter is seeing air bubbles in the fluid.

One of the best ways around it is to get a de-gasing unit to remove any air before the fluid enters the particle counter.

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