The Forward Flow Test is designed to test the integrity of sterilizing and virus grade hydrophilic and hydrophobic membrane filters.  This qualitative test is based on measuring the gas flow across a completely wetted membrane at a defined constant test pressure on the upstream side.  When the downstream side of the membrane is at atmospheric pressure a diffusion flow of gas is established due to the pressure differential.  The Forward Flow test is Pall’s recommended integrity test for capsule and cartridge filters.