10. Filtrability

For applications requiring the use of very fine filters (down to 3 µ m absolute), such as servo-valve hydraulics, it is essential that the fluids used have excellent filterability characteristics, even in the presence of water (up to 2%). This mainly depends on the cleanliness of the base oils and additives, and above all on the hydrolysis stability of the functional additives.

Several standardized methods have recently been developed to assess this property: NF E 48-690 and E 48-691 for hydraulic fluids. Older tests are also available: DENISON (procedure TP 02 100) with 2% distilled water on a 1.2 µ m membrane, PALL, etc.