Good contact is necessary between the paper and the cylinders of the printing press, and this means good paper smoothness. The traditional air leak testers lightly compress the paper, thus simulating compression in the printing press, and create an air pressure difference so that air has to pass under the sides of the measuring head. As shown in the diagram of a cross-section of a tester (where the paper is lightly clamped, and also backed so that no air can pass through it), the rougher the paper, the easier it is for the air to pass. According to the tester used, either: - The time taken to pass through a fixed volume of air is measured, and the result expressed in seconds (eg: Bekk tester, Gurley tester). The larger the figure, the smoother the paper. - The volume of air passed through in a fixed time is measured, and the result expressed in mL/min (eg: Bendtsen tester).
