Porosity Versus Permeability

One of the most basic properties of filtration media is its openness or ability to flow a fluid through it. This is typically measured as permeability. Many people have this property confused with the porosity of a filter material.

Permeability is defined as the volume of a fluid that will move across a known area of filter media at a constant pressure drop. It is used to describe how easily a fluid will flow through a material and is measured by a Frasier tester.

Porosity is a 3-dimensional measurement of the void volume of a filter material. Porosity is defined as the ratio of the volume of all of the pores of a material to the volume of its mass and describes the structure of the filter material. It is measured by fluid intrusion that measures pore filling at various pressures.

A low porosity material would be a perforated film while a high porosity material would be a PTFE membrane. The film, which could have a high permeability, could be as low as 30-40% porosity while the PTFE film, having low permeability, may be well over 90% porosity.

Frazier Permeometer (Courtesy of Frazier Instrument Co.)	Liquid Intrusion Porosimeter (Courtesy of Porous Materials Inc.)
© 2004 American filtration and Separations Society