In any measurement the pressure drop, Ap, and the viscosity of air # is fixed. Themeasured airflow Po then reflects on the permeability constant k and the thickness Lof the medium. The permeability constant relates to the structure of the media.Traditionally the air permeability is expressed in units of ft3/min/ft 2 at Ap = 0.5 inH20 and simply reported as CFM. The equivalent in SI units is m3/s/m 2 at Ap =12.7 mm H20. Often it is reported as m3/5/m 2 at Ap = 20ram H20. On their website,Frazier Instrument Company (14o1 lists the following conversions for the various units ofmeasurement (Table 6.2).