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 filterspecimen 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.