Determines the air exchange differential of porous materials (Reference ASTM F2100, ISO 16603, ISO 16604, Military spec MIL36954C, ASTM F2101). This technique employs a water manometer differential upstream and downstream of the test material at a constant flow rate. For masks and respirators, the higher the delta P, the more difficult it is to breath through. Lower is better IF the BFE and fluid resistance is not compromised.