These Buchner funnels are especially useful where filter paper would be attacked by the materials being filtered. The disc should not be subjected to pressure differential in excess of 1 atmosphere. The top edge is beaded. These funnels are available in three different porosities of Pyrex® brand fritted ware so that precipitates varying in size can be filtered at maximum speed with no sacrifice of retention. Porosity is controlled in manufacture, and discs are individually tested and graded into these classifications. The coarse porosity (40 to 60µm) is held toward the maximum pore diameter. The medium (10 to 15µm) and fine (4 to 5.5µm) are 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."