4.1 Test sieves are designated by the nominal clearance (side measurement or diameter) of the openings. In sieves made of perforated sheet metal and electroformed sheet, the type of openings, round or square, is also indicated. 4.2 Measurements are expressed in millimeters (mm) for those between 125 mm and 1 mm; in micrometers (μm), for those smaller than 1 mm.