Abstract: Yarn 1 fron an extruder is stretched by heated rollers (3,5) and passes to a texturizer unit 2 where it is plasticized and crimped, leaving in the form of a crimp plug along a stack (8). The yarn is heated in the unit (2) by air passing along a pipe (7) and heated by a base heating element (9) in conjunction with a booster element (11). The plug of crimped yarn (6) passes to the surface of a rotating cooling drum (4), around which it makes a number of turns before crimped yarn (1') is withdrawn from the leading end of the plug at (6'). The plug length is monitored by a sensor unit comprising a light source (16) which directs light through holes 10 in the drum onto a receiver unit (14) including three diodes A, B and C. When the plug end (6') is at the datum position, the diode A is covered, the diode (C) is uncovered and the plug end flutters backwards and forwards over the diode (B).