I have cut 20 cm of the tube, after shrinkage, it became 18 cm. Why?
Heat shrinkable tubing has not only radial shrinkage but also longitudinal shrinkage. The more diameter of the tube is the more longitudinal shrinkage it will be subjected to. Longitudinal shrinkage ratio depends on many factors: contents of material composition, the technology of widening, length of the cut tubing, the diameter of the tubing and the way of its installation. If the surface has different heights his should be also compensated by the length of the tubing itself. If you need to know the exact ratio of longitudinal shrinkage you should better make a teat before starting the shrinkage on the surface than needs to be insulated, and only after that make a decision about the length of the tubing.