Permeability relationships and units

The foregoing derivations have mostly avoided specific assumptions about units. Instead, the equations used may be described as having consistent units. For instance, the units of diffusivity or permeability must be consistent with the units of the other terms. 

The degree of separation and recovery, however, is independent of the units and depends only on relative permeability values or selectivity, although actual membrane size (area) depends on the particular units used. 

Therefore, if the actual numerical values for permeability, pressure, and flow rate are to be used, then specific units are necessarily involved, which in turn, determine membrane area, given the membrane thickness, or the overall membrane permeability. 

For a pure gas, of a single component only, commonly used units for the membrane pointwise permeability P, for a pure component i are 

Other units used for the permeability of various solids to gases are presented in the International Critical Tables (vol. V, pp. 76-77), as follows  

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