Units of gas permeability constants

Gas permeability constants of polymers are generally expressed by composite units which contain\documentclass{article}\pagestyle{empty}\begin{document}$$ \frac{{\left( {{\rm amount\ of\ gas}} \right)\left( {{\rm thickness\ of\ membrane}} \right)}}{{\left( {{\rm membrane\ area}} \right)\left( {{\rm time}} \right)\left( {{\rm differential\ pressure\ of\ gas}} \right)}}. $$\end{document} Numerous combinations of various units to express each component units are used. However, permeability constants of nongaseous substances in polymer membranes are generally expressed by the units of cm 2 /sec, which are identical to the units of the diffusion constant. The relationship between constants in units of cm 2 /sec and conventional composite units for gas permeability constants is presented, and the advantage of using the units in cm 2 /sec to express gas permeability constants is discussed.