Pore size, pore structure, along with porosity are related properties that help define a nonwoven filter medium. Porosity relates to the void volume within a filter medium and its capacity to hold dirt. Pore size and pore structure relate to the particle size that can be arrested. Almost all pore size measurements assume cylindrical capillaries through the medium and express the pore size in terms of the diameters of those capillaries. Two primary pore size measurements of a filter medium are: maximum pore size (often referred to as bubble point) and mean flow pore size. Thetest methods are described in ASTM F316-86 (141). Although this particular standard is specified for membrane filter media, the technique has been used by nonwoyen and paper filter manufacturers for many years.