A method for determining sagging of melt of a tube extruded in an extrusion device includes structuring a measuring device to rotate about the tube. The measuring device is configured to measure a wall thickness of the tube over a circumference of the tube and determine a wall-thickness profile over the circumference of the tube from the measured wall thicknesses. The wall-thickness profile includes a frequency and an amplitude. The method further includes determining a sagging of the melt from at least one of (i) the frequency and (ii) the amplitude of the wall-thickness profile.

The invention relates to a blown film system for producing a film tube made of plastics material, comprising: an annular nozzle, out of which a plastic melt having a closed cross-section can be brought; a take-up device, by means of which the plastic melt can be drawn up from the direction of the annular nozzle so as to form the film tube; an air-provision device, which is downstream of the annular nozzle when viewed in the transport direction of the film tube and by means of which an amount of air can be provided; and a calibration device, which is downstream of the annular nozzle when viewed in the transport direction of the film tube and surrounds the film tube and by means of which the outer circumference of the film tube can be delimited. At least one detector is contained that has a plurality of detection elements, by means of which electromagnetic radiation emitted or reflected at various points of the film tube can be detected, such that at least one planar region and/or contoured region of the film tube can be detected with regard to characteristic properties, wherein the detector comprises at least 32 detection elements.