up to point 51 the yarn diameter dst from the point 51 the curve 50 rises and at point 55 passes the value of the limit diameter d gr at point 53 the predetermined yarn length l v has been covered since reaching the point 52 after a diameter increase of 15 is registered at point 52 and the exceeding of the yarn diameter g gr lasts over the predetermined length l v for example six cycles or 12 mm the point 52 is defined as the beginning of the effect the curve 50 falls below the limit diameter d gr at the point 54 the falling below lasts up to the point 55 and therefore over the predetermined yarn length l v the point 54 is therefore defined as the end of the effect the effect length l e is determined from the beginning and end of the effect between point 52 and point 54 an arithmetic average value is formed from the four largest diameters 56 inside the effect the information about the effect diameter is therefore most substantially independent of the natural diameter variations in the effect region as a result this arithmetic average value is defined as the.