In order to measure the pore size by capillary flow porometry it is necessary to impregnate the samples with a wetting liquid. An inert gas flow is used to displace the liquid that is in the pores and the pressure required to empty the most constricted part of the pore is measured. The most constricted part of the pore is the most challenging one because it offers the highest resistance to remove the wetting liquid. This parameter is very important in filtration and similar applications since it is essential to know the smallest diameter of the through pores.
This measured pressure permits obtaining the pore diameter, which is calculated by using the Young-Laplace formula P= 4*γ*cos θ*/D in which D is the pore size diameter, P is the pressure measured, γ is the surface tension of the wetting liquid and θ is the contact angle of the wetting liquid with the sample. 
The surface tension γ is a measurable physical property and depends on the wetting liquid used. The contact angle θ depends on the interaction between the material and the wetting liquid. In capillary flow porometry, in opposition to mercury intrusion porosimetry, the wetting liquid enters spontaneously the pores of the sample ensuring a total wetting of the material, and therefore the contact angle of the wetting liquid with the sample is 0 and the previous formula can be simplified as: P= 4*γ/D.