Method and device for measuring a tubular strand

A device for measuring a tubular strand emerging from an extruder and conveyed in a longitudinal direction, the device comprising:

• a first radiation source configured to emit terahertz radiation in a first measurement region from an inside onto an inner surface of the tubular strand;
• a first radiation receiver configured to receive terahertz radiation reflected by the tubular strand in a second measurement region;
• a first evaluation apparatus configured to determine at least one geometric parameter of the tubular strand in the first measurement region from the received terahertz radiation;
• a second radiation source configured to emit terahertz radiation in the second measurement region from an outside onto an outer surface of the tubular strand;
• a second radiation receiver configured to receive terahertz radiation reflected by the tubular strand in the second measurement region;
• a second evaluation apparatus configured to determine at least one geometric parameter of the tubular strand in the second measurement region from the received terahertz radiation;
• a third evaluation apparatus configured to determine a change in at least one geometric parameter of the tubular strand between the first measurement region and the second measurement region, and further configured to determine at least one adjustment instruction for the extruder based on a determined change in the at least one geometric parameter;
• and
• at least one sensor configured to measure a temperature of the first and second measurement regions,
• wherein the second measurement region is located downstream of the first measurement region in the conveying direction of the strand,
• wherein at least one of:
• (i) the first evaluation apparatus;
• (ii) the second evaluation apparatus;
• (iii) the third evaluation apparatus;
• (iv) a control apparatus;
• and (v) a regulating apparatus is configured to determine a refractive index of the tubular strand from the terahertz radiation and from a difference in the temperature between the first and second measurement regions,
• wherein the first evaluation apparatus is configured to use the refractive index established in the second measurement region to determine the at least one geometric parameter in the first measurement region.