High pressure capillary flow porometry is the most common method for characterizing the pore size distribution of filters. Bubble Point is based on the fact that test-liquid is held in the pores of the filter. Using capillary flow porometry, the high gas pressure is used to force a wetting liquid out through a nanoporous filter. The pressure at which pores empty is inversely proportional to the pore size, smaller pores require a higher pressure than larger pores. The resulting volumetric flow of gas through emptied pores is also measured. Pore size is then calculated using the “Young-Laplace equation”.