Definition
Air porosity refers to the ease with which air can enter into the volume of a material. This property is typically associated with materials that contain a large number of surface holes and inter-connected internal voids. The larger the size of these voids and the higher their degree of inter-connectedness, the higher the porosity value. Materials with a high air porosity are usually termed ‘porous’ and the individual voids or holes are referred to as ‘pores’.
Figure 1: Concrete with no-fines is a good example of a material with high air porosity.
It is important to note that, whilst air porosity and air infiltration are usually directly related, this is not always the case. Air infiltration requires that the internal voids be sufficiently inter-connected for air to be able to find pathways to flow entirely through a material. Some materials can have highly porous surfaces but no pathways that pass entirely from one side to the other.
Units
Porosity can be considered in a number of ways. Technically porosity refers to the ratio of void volume to the total volume of the material, typically given as a fraction (0-1).
However, when looking at air porosity, the value of most interest is usually the volumetric air flow rate through the material, typically given in terms of air volume over time.
SI Units
The metric measure for air porosity is typically in milli-litres per minute (ml/min) or cubic centimetres per minute (cc/min). For most gasses and fluids, one cubic centimetre is equal to 1 milli-litre.
Imperial Units
The imperial measure for air porosity is typically in fluid ounces per minute (fl oz/min).
Measurement
Figure 2: A Gurley Permeometer (left) and a Gurley Densometer (right).
Air porosity or permeability can be measured using either an Air Permeometer or a Densometer. Both meters are typically measure the air-permeability or air-resistance of sheet-like materials such as papers, fabrics, plastics and membranes.
The Densometer test measures the time required for a given volume of air (25cc to 400cc) to flow through a standard area of material tested, under light uniform pressure. The air pressure is supplied by an inner cylinder of specific diameter and standardized weight, floating freely within an outer cylinder partly filled with oil to act as an air-seal. The sample material is held between clamping plates having a circular orifice area of 1.0 (standard), 0.25 or 0.1 square inch (optional).
Densometer readings may be evaluated on both a direct or indirect basis dependent upon the material and test purpose. They are a direct test of materials which are intended to either resist or permit the passage of air. Indirectly, they are used to measure other physical properties which affect the flow of air through a porous sheet.
Example Values
| MATERIAL | POROSITY |
|---|---|
| Uncoated Paper | 500 to 1500 ml/min |
| Coated Paper | 0 to 10 ml/min |
| Test Liner (186gsm) | 25 ml/min |
| Gasket Paper (186gsm) | 1 to 5 ml/min |
Table 1: Air porosity levels for different paper types.
Useful References
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