There are several ways in which the air passes through the fabric,such as through the pores on the fabric or the pores among the fiber, and the water molecules will pass through the fabric via the fiber surface molecular adsorption. The tranmission mode of air molecules passing through the pores on the fabric is the main mode in the whole transmission system.
For the determination of fabric permeability, China is mainly based on GB/t5453-1997, which is applicable to various textile fabrics, including industrial fabrics, non-woven fabrics and other breathable textile products. There is only a slight difference on the pressure drop between the clothing fabric and industrial fabric. The pressure drop of clothing fabric is 100Pa while the pressure drop of industrial-use fabric is 200Pa. National standard GB/T 5453-1985 "fabric air permeability test method" uses air perm value (refers to the volume of air pass through the unit area per unit time under the prescribed pressure difference between the two sides of the fabric) as the index of fabric permeability. The amended standard GB/T 5453-1997 uses perm rate (refers to the air velocity of air flow vertically passing through the sample under the condition of prescribed pressure drop and time on specified sample area) to indicate the permeability of fabrics.
International standards have ISO 9237-1995 (mainly used in textile fabrics, industrial fabrics, non-woven fabrics) and ISO 7229-1997 (mainly used in rubber or plastic coated fabrics); American material testing and association standard is ASTM D 737-1996 (mainly used in textile fabrics); The British standard is BS 5636 (mainly used in textile fabrics).