The purpose of this standard is to specify the methods of measuring the flow of air passing through the unit area of a sample of paper or cardboard, under the unit pressure difference. Measurements can be made with any device that conforms to the specifications contained in this standard. The basic requirements of the test apparatus and the general operating procedures are indicated in the UNE 57-066/1 standard. This part of the same standard refers to the determination of the air permeability of paper or cardboard with the Bendtsen apparatus.