ISO 16549:2021 describes a method, using capacitance measuring equipment, for determining the unevenness of linear density along the length of textile strands.

The method is applicable to tops, slivers, rovings, spun yarns and continuous filament yarns, made from either natural or man-made fibres, in the range of 4 tex (g/km) to 80 ktex (kg/km) for staple-fibre strands and 1 tex(g/km) to 600 tex (g/km) for continuous-filament yarns. It is not applicable to fancy yarns or to strands composed fully or partly of conductive materials such as metals; the latter requires an optical sensor (see A.4), and to raw silk filaments which are tested according to a specific standard.

The method describes the preparation of a variance-length curve, as well as the determination of periodicities of linear density. It also covers the counting of imperfections in the yarn, namely of neps and of thick and thin places.