Specific Surface - Air Permeability Formula

Equation for calculate specific surface - air permeability is,

$S=\frac{7d}{\rho (1-\epsilon )}\times \sqrt{\frac{{\epsilon }^3\mathrm{\pi } \mathrm{\delta P}}{l \eta Q}}$

where:
d is the cylinder diameter
ρ is the sample particle density
ε is the volume porosity of the bed
δP is the pressure drop across the bed
l is the cylinder length
η is the air dynamic viscosity
Q is the flowrate
S is specific surface