The yarn is fed from a bobbin at a fixed rate into a hot-air treatment chamber. After passing over a tensioning device it is withdrawn at a variable rate so that the tension and length of yarn in the chamber (and therefore the time of heat treatment) are held substantially constant. The tension is known and adjustable. The shrinkage under these controlled conditions is derived from the ratio of input to output rates, and is displayed numerically as a percentage. After a bobbin has been knotted in and the instrument started, a fixed length of yarn is tested automatically.