The mean flow pore size is the pore diameter at a pressure drop at which the flow through a wetted medium is 50% of the flow through the dry medium. It is not the mean pore size because the flow through large diameter pores can be disproportionately larger than the flow through small diameter pores. Figure 6.6 diagrams the apparatus used for measuring mean flow pore size. Two setups are shown: a one holder apparatus and a two holder apparatus. In the one holder setup the dry filter specimen is placed in the holder and air pressure is applied in increasing increments. The flow is measured at each pressure and the flow-pressure relationship is plotted. Figure 6.7 illustrates the graphic procedure for determining mean flow pore size.The filter specimen is then removed t~om the holder and completely wetted with a liquid of known surface tension and density or specific gravity. The liquids listed in Table 6.5 are possibilities. The wetted specimen is then placed in the holder and the flow-pressure plot is repeated on the wetted specimen. The bubble point is determined at the pressure at which air is first seen bubbling through the bubbler displayed in Figure 6.6. The pressure at which the flow rate through the wetted specimen is exactly one half of the flow through the dry specimen is the pressure at which the mean flow pore size is determined. Equation ( 6.1 O) is applied to calculate the mean flow pore size 2.The measurement as described in ASTM F 316-861141) and illustrated by Figure 6.7 is a form of extrusion flow porometry. The extrusion flow porometry procedure utilizes air pressure to force a fluid of known surface tension from the pores of the media. The bubble point (or maximum pore size) mean flow pore size, and minimum pore size can be determined in accordance with Figure 6.7 and by using Equation (6.1 ()). The technique measures the most restricted part of the pore (see Figure 6.8). In addition to the ASTM apparatus described here, there are very sophisticated instruments tbr measuring the pore structure of filter media. At one time Beckman Coulter,Inc. marketed a Coulter-I Porometer e~' to perform the test, however the instrument is no longer available. It is mentioned here, because some organizations and companies still use the instrument. In addition, the instrument is still referenced in research reports and product data presentations. Recently, Xonics Corporation in Sunrise, Florida [tSA has introduced an updated version of the Coulter-I Porometer ~:.