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ISO 3668:2017 specifies a procedure for evaluating differential pressure versus flow characteristics of hydraulic filters and constitutes a basis for agreement between the filter manufacturer and user.

It also specifies a method for measurement of the differential pressure generated at different flow rates and viscosities by the relevant parts of a filter assembly, spin-on and any valves contained within the filter which are in the flow stream. The typical types of filter to be tested are as follows:

Type 1: which are spin-on filters in which the replaceable unit does not include a filter head (it might or might not include the element by-pass valve);

Type 2: which are spin-on filters in which the replaceable element is tested together with a filter head (it might or might not include the element by-pass valve);

Type 3: which are filter assembly, usually of the replacement element type, that is the housing (head and bowl) and element.