where P is permeability expressed in Barrer (1 Barrer = 1010 cm3
(STP) cm/cm2 S cm Hg), q is flow rate of the permeate gas passing
through the membrane (cm3/s), L is membrane thickness (cm), P1
and P2 are the absolute pressures of feed and permeate, respectively
(cmHg) and A is an effective membrane area (cm2).
Consequently, ideal selectivity for a pair of gases could be calculated
per the following equation: