Air permeability

Permeability is a measure of the ability of a porous material to transmit air and fluids, normally under the situation of pressure difference between two sides of the fabric. Fabric permeability relates to its geometric structure as well as to the path of streamlines for flow through the structure. That is to say, it is the speed that airflow vertically through a prescribed test area. 

The most common measurements for air permeability are L/m2/sec@200pa; L/dm2/min@200pa and m3/m2/min@200pa.

In order to properly test a fabric’s permeability it is required a background knowledge of fluid and textile fabrics mechanics, being the air permeability testers the common gadget to measure it. 

The principle is that, in a prescribed pressure difference, the amount of airflow that passes through a certain area of fabric in a certain time is measured, so that air permeability can be calculated. 

For measuring the air permeability of a wide range of fabrics, a number of instruments are available for the purpose.

Permeability is also dependent upon the porosity of the fabric. The porosity is largely determined by the tightness of the fabric weave. The passage of air is important for a number of fabrics such as industrial filters, tents, sailcloth, parachutes, airbags and many others. Porosity is the ratio of the volume of openings to the total volume of material, representing the storage capacity of the material. Variables such as type of material, linear density, twist factors, type of spinning, thickness and weight also influence the porosity in a multi-layer woven fabric.