Calculation of permeability surface

If the viscous term predominates, the specific surface is determined using the first term of equation (1.40) and, if the compressibility factor is negligible, this takes the form of equation (1.10). When the molecular term predominates, the specific surface is obtained from the second term of equation (1.40). When the two terms are comparable the specific surface is obtained as follows. 

The specific surface using the viscous flow term is  

Substituting these equations into equation (1.40) gives  

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