Bendtsen Porosity (ml/min) ISO 5636-3
In a paper sheet the fibres and filler particles are agglomerated together to a strong and dense packing, but between the fibres and fillers is still some included air, with the process of calendering the porosity is significantly reduced by compressing the sheet and by collapsing the fibres.
The measurement of porosity describes the potential air flow trough a sheet in z-direction, the porosity is measuring the air flow which is able to pass this porous structure of the sheet. The measurement is done with a defined air pressure (1,47 kPa) to the surface of the sheet and with a defined measurement area (10 cm²).