The pore size in filters, membranes, and fabrics can be determined at Cambridge Polymer Group by a variety of methods, including optical and scanning electron microscopy, mercury porosimetry, and particle size exclusion. A commonly used method is ASTM F316 "Pore size characteristics of membrane filters by bubble point and mean flow pore test." This standard describes two test methods to obtain pore size in filtration media by making use of gas transmission through the filtration media. In both methods, the filtration media (in the form of a sheet) is cut into a disc, which is then placed in a filter holder. One side of the filter holder is connected to a gas line with a pressure regulator. The other side is vented to the atmosphere. In Method A, a fluid of known surface tension is placed on top of the filtration media on the vented side. The gas pressure on the other side of the filtration media is slowly increased, and the experimenter looks for the lowest pressure where gas bubbles begin to rise from the filter, indicating that the gas pressure has overcome the interfacial tension of the liquid in the pores. The maximum pore size can be calculated from this pressure and the surface tension of the fluid. In Method B, the same apparatus is used, but with the addition of a gas flow meter. In this variation, the gas pressure is also slowly increased, and the flow rates of gas through both a wet filter (using the fluid of known surface tension) and a dry filter are recorded. The percentage of filter flow rate can then be determined as a function of pressure, which in turn is related to the pore size.
Contact CPG for more information on performing this test on your filtration media.