These PYREX^® Buchner funnels have a fine porosity fritted disc with a porosity range of 4 to 5.5µm which is held toward the minimum pore diameter. The porosity for the pore diameter of the filter is determined in the same manner as specified in ASTM E-128 “Maximum Pore Diameter and Permeability of Rigid Porous Filters for Laboratory Use.” The disc should not be subjected to pressure differential in excess of 1 atmosphere. The top edge is beaded.