Measurements in GLP consist of recording the flow of gas through the wet (impregnated with wetting liquid) and the dry sample as a function of the pressure increase. A typical measurement is shown in the figure below (left image). From these two curves the porometer itself will calculate parameters, such as first bubble point or largest pore, mean flow pore, smallest pore, cumulative flow and the pore size distribution.
The first bubble point, or largest pore, is a special parameter which needs some further background information, which can be read on a dedicated page. The smallest pore represents the smallest openings inside the material. These are opened right before the filter has completely dried. The smallest pores are therefore calculated at the point where the wet curve (blue) and dry curve (red) start to coincide. The average pore size or mean flow pore size is calculated at the pressure where the wet curve and the “half-dry” curve (green) cross. The half-dry curve is obtained by the mathematical division by 2 of the data originating from the dry curve, as requested by the standard ASTM F 316-03.
From the wet and dry curves also the pore size distribution is calculated, shown with the cumulative, differential and corrected differential flow, see the figure below (right). The differential flow shows the percentage of pores present. The cumulative flow represents the sum of all these differential values between 0 and 100%. It is normally referred to as pore size distribution.
The corrected differential takes also the differences in pore size relative to its pressure step into account. More information on these parameters can be found in the application note: Calculations used in porometry.