Simple Approximation
The mass flow qm through a barrier layer can within certain pressure limits be linearized and approximated by:
qm = Perm A Δp / delta
with qm the mass flow, Perm the specific material permeability, A the surface area, Δp the pressure difference, and delta the material thickness. In Europe the permeability unit is mostly cm3.mm/(m2.Bar.day), so that the gas flow in cm3/day results when the area is given in m2, the thickness in mm, and the pressure difference in Bar.
In American units, the permeability is often given in cm3.mil/(100 inch2.Bar.day) which can be converted to the European unit by dividing by 2.54.
In SI units, it is given in s−1.
Read more about this topic: Permeation
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