Process and apparatus for measuring false twist in thermoplastic synthetic yarn

A process for false twisting a thermoplastic synthetic yarn wherein a yarn of thermoplastic synthetic fibers is fed through a feed roller, the fed yarn is false twisted by a false-twisting means while being heated and, then, the false twisted yarn is taken up through a delivery roller, which process comprises directly measuring, in a section within the twisting zone formed between the feed roller and the false-twisting means, the speed of the yarn being processed under a normal operational condition with twisting to estimate the number of false twists actually imparted to the yarn by determining twist shrinkage of the yarn in said section from the measured yarn speed and the speed of the yarn being processed under the same condition as said normal operational condition but without twisting, which is measured in the same section as that wherein said speed of the yarn being processed under the normal operational condition with twisting has been measured and, then, estimating the number of twists from the determined twist shrinkage.