To select a proper filter system for the requirements of a given application, knowledge of filter efficiency and dirt capacity is required. The filter manufacturer needs to provide data, which are based on test procedures simulating real parameters on a laboratory scale. At present there is no standard for liquid filter testing (except multipass test to ISO 4572, which is for hydraulic filters).

Early cartridge filters were constructed from a square weave S.S. mesh. In this case, every hole or pore is nominally the same size. This measurement can be accomplished by evaluation of wire counts and thickness or by direct observation under microscope. As filter developed, different weaves (Photograph of woven wire mesh) for the mesh is being used and now it is difficult to measure by count and thickness of wire. Consequently, a number of methods of establishing the particle removal rating were developed.

To select a proper filter system for the requirements of a given application, knowledge of filter efficiency and dirt capacity is required. The filter manufacturer needs to provide data, which are based on test procedures simulating real parameters on a laboratory scale. At present there is no standard for liquid filter testing (except multipass test to ISO 4572, which is for hydraulic filters).

Early cartridge filters were constructed from a square weave S.S. mesh. In this case, every hole or pore is nominally the same size. This measurement can be accomplished by evaluation of wire counts and thickness or by direct observation under microscope. As filter developed, different weaves (Photograph of woven wire mesh) for the mesh is being used and now it is difficult to measure by count and thickness of wire. Consequently, a number of methods of establishing the particle removal rating were developed.

As described in ARP901 (Ref2), this is based on the observation that, if a circular hole in a flat plate is fully wetted in liquid, the minimum bubble radius that the hole will support is the radius of the hole. The differential pressure across the bubble under these conditions is controlled by the surface tension, so that

¶P = 2s             Where ¶P = differential pressur
         r                s = Surface tension

Since the minimum bubble radius corresponds to the maximum differential pressure before bubbles are released from the hole, the measurement of differential pressure can be used to establish a size for the hole.
(Picture of bubble point test)

ARP901 extends the method to triangular holes in flat plates, but it must be recognized that the measured differential pressure is also dependent on the shape of the hole.

    ¶P =  Ks                 s = surface tension     
               r                    r  =  pore size

Both the glass bead test and the bubble point test were historically reputed to provide an absolute rating for the filter media, but also implied that this was a cut-off point, in the sense that in service no particles below this size were removed, and all particles above this size were removed. Both methods were based on spherical particles, which only occur rarely in real filtration applications.

In an effort to address these difficulties, a further test was developed, and is also included in MIL-F-8815. This has been called the "nominal rating".