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# Calculations used in porometry application notes

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A porometer measures gas flow as function of an applied pressure. Typical curves for the wet and dry curve for a filter or membrane are shown in figure 1 below. The wet curve is measured to determine the pore sizes, the dry curve is needed for the calculation of both the mean flow pore size, smallest pores and the gas permeability.

As one can read in the application note about the determination of the first bubble point, there are several ways to determine this important parameter. The mean flow pore size or MFP is calculated by the intersection of the wet curve (blue line in figure 1) and the so-called “half dry” curve. This “half dry” curve (the grey dotted line in figure 1) is the mathematical half of the dry curve. Usually this coincides with the most populated fraction of pores within a filter. The smallest pore is calculated as the pressure where the dry curve (black line in figure 1) meets the wet curve.

To recalculate these pressures towards a pore size value, the Washburn equation is used. The Washburn equation expresses the mathematical link between pressure and pore diameter, using surface tension and contact angle of the wetting liquid:
D = 0.04 ã cos è / P

With D the pore diameter in ìã the contact angle between the wetting liquid and the substrate and P the pressure in bar.