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.