Apparatus and method for determining the temperature within a tubular strand

Description

[0001] The invention relates to a device and a method for determining the temperature of a tubular strand conveyed from an extrusion device.

[0002] For example, plastic pipes are extruded in extrusion devices. After exiting the extrusion device, these pipes typically pass through a calibration device with a calibration sleeve, for example, a metal one, against whose inner surface the plastic pipe is pressed, for example, by suction, to define the outer geometry. Following the calibration device, the plastic pipes usually pass through one or more cooling sections, where a cooling fluid, such as cooling water, is sprayed onto the outside of the pipe to cool it.

[0003] The fundamental objective of an extrusion process for producing a tubular strand is to achieve minimal sagging, i.e., minimal wall thickness deviation of the finished strand across its circumference, minimal energy input, and maximum conveying speed of the extruded strand. Precise knowledge of the production process is crucial for optimizing these parameters. For example, WO 2016 / 139155 A1 describes the measurement of various geometric parameters, such as the wall thickness, of a tubular strand. Terahertz radiation in the range from 10 GHz to 3 THz is emitted onto the strand to be measured, and terahertz radiation reflected at the strand's interfaces is received again. For example, time-of-flight measurements can be used to reliably determine distances to the interfaces and, from these, geometric parameters such as diameter and wall thickness.In addition to the geometric parameters measured in this way, the temperature of the tubular strand is also an important process parameter. Pyroelectric sensors (PIR sensors) are known for non-contact surface temperature measurement. For example, WO 2019 / 166420 A1 and DE 20 2018 006 144 U1 propose measuring the temperature at the inner circumference of the tubular strand using a measuring device guided into the interior of the tubular strand directly after the extrusion device. However, inserting a sensor into the interior of the tubular strand is not without its complexity. Furthermore, this is essentially only possible in direct connection to the extrusion device.

[0004] DE 10 2017 116 955 B3 discloses a method and a device for production control of an extruded plastic product, in which a first actual temperature of a measuring area provided on the plastic product is recorded at a first measuring point and a second actual temperature of the measuring area is recorded at a second measuring point, which is arranged at a distance from the first measuring point in an extrusion direction of the plastic product. Furthermore, a target temperature of the measuring area is determined at the second measuring point, wherein the target temperature is calculated based on the first actual temperature using a cooling curve of the plastic product, or wherein the target temperature is determined based on a production test. Information is output indicating whether the second actual temperature lies within or outside a predetermined tolerance range of the target temperature.Deviations between the second actual temperature and the target temperature are intended to indicate a change in the wall thickness of the plastic product. This should enable continuous wall thickness monitoring, particularly inline wall thickness control.

[0005] DE 31 40 253 A1 proposes a method for measuring the thickness of hot-formed products, in which the cooling rate of the product is measured and the thickness is determined from this.

[0006] DE 10 2006 056 735 A1 discloses a measurement method for determining the wall thickness of an extruded plastic profile. It uses an ultrasonic sensor that radiates perpendicularly onto the surface of the plastic profile through the interposition of a coupling medium to detect a time-of-flight difference between the front and rear wall echoes. The wall thickness measurement is to be calibrated ultrasonically with respect to the body temperature of the extruded plastic profile by measuring the time-of-flight at least twice at different angles of incidence, and the wall thickness measurement is calibrated based on a comparison of the time-of-flight values.

[0007] EP 3 480 553 A1 discloses a terahertz measurement method and a terahertz measurement device for measuring at least one wall thickness of a tubular measurement object. A terahertz main sensor emits a terahertz main beam along a first optical axis through an axis of symmetry of the measurement object, and an additional terahertz sensor emits an additional terahertz beam along a second optical axis, which is offset by a sensor angle relative to the first optical axis and at least temporarily does not pass through the axis of symmetry. An evaluation and control device records a first signal amplitude of the terahertz main sensor and a second signal amplitude of the additional terahertz sensor, determining reflection peaks in the signal amplitudes and detecting an internal wall thickness deformation.

[0008] EP 2 153 970 A2 describes an extrusion line for hollow plastic profiles in which the entire cooling section for cooling the extruded profile is divided into individual, short cooling sections (segments) that are designed to be separately controlled. This is intended to establish an optimal sequence of cooling or tempering sections for the specific application, thus achieving a nearly linear cooling of the hollow profile, which should result in improved product properties. Furthermore, cooling is intended to be more effective, as temperature equalization in the profile wall can occur in the intermediate tempering sections.

[0009] Based on the described prior art, the invention is based on the object of providing a device and a method with which more precise information, in particular regarding the temperature of the tubular strand, can be obtained to optimize the production process. In particular, the invention is also based on the object of improving the measurement accuracy when calculating the diameter and / or wall thickness of the tubular strand at the measurement location, as well as its length, wall thickness, and diameter after the tubular strand has cooled.

[0010] The invention solves the problem by the independent claims 1 and 17. Advantageous embodiments can be found in the dependent claims, the description and the figures.

[0011] For a device of the type mentioned at the outset, the invention achieves the object in that a first temperature sensor is provided for measuring a first temperature of the outside of the tubular strand at a first position of the tubular strand, and in that an evaluation device is provided which is designed to compare the first temperature with a second temperature of the outside of the tubular strand at a second position of the tubular strand spaced from the first position in the conveying direction of the tubular strand, and to determine the temperature inside the tubular strand and / or on the inside of the tubular strand at a position of the tubular strand from the comparison of the first and second temperatures.

[0012] For a method of the type mentioned at the outset, the invention solves the problem by measuring a first temperature of the outside of the tubular strand at a first position of the tubular strand, comparing the first temperature with a second temperature of the outside of the tubular strand at a second position of the tubular strand spaced from the first position in the conveying direction of the tubular strand, and determining the temperature inside the tubular strand and / or on the inside of the tubular strand at a position of the tubular strand from the comparison of the first and second temperatures.

[0013] The device according to the invention can comprise the extrusion device and / or a conveyor device for conveying the tubular strand. The device according to the invention can also comprise the tubular strand. The tubular strand can be, for example, a plastic pipe. However, it can also be, for example, a fiberglass pipe or another strand. The device according to the invention can comprise one or more cooling sections through which the tubular strand passes after extrusion and in which, for example, a cooling liquid is sprayed onto the outside of the strand for cooling. Furthermore, the device according to the invention can comprise a calibration device, comprising, for example, a calibration sleeve, for example made of metal, against the inside of which the tubular strand is pressed, for example sucked, after leaving the extrusion device.Any cooling sections may be located downstream of the calibration device.

[0014] According to the invention, a first temperature of the outside of the tubular strand is measured at a first position of the tubular strand by means of a first temperature sensor. The measured first temperature is compared with a second temperature of the outside of the tubular strand at a second position of the tubular strand, wherein the second position is spaced from the first position in the conveying direction of the tubular strand. The first and second positions are positions along the conveying direction of the strand. The positions are spaced from one another in the longitudinal direction of the strand. The second position can, for example, be arranged upstream of the first position in the conveying direction of the strand. However, it can also be arranged downstream of the first position in the conveying direction. The temperatures at the first and second positions can, in particular, be measured or determined at the same time. However, this is not mandatory.Rather, it would also be possible to measure or determine the first and second temperatures at different times, for example, in such a way that the same strand area is measured, taking into account the conveying speed of the strand.

[0015] The second temperature can be measured or otherwise assumed or determined as known. For example, at the outlet of a calibration device, the temperature of a calibration sleeve against which the strand is pressed to define its external geometry can be assumed to be the external temperature of the strand at this position. After extrusion, the strand cools. The cooling is usually intensified or controlled by one or more cooling sections through which the strand passes. The cooling sections usually act on the outside of the strand and introduce forced cooling into the material. As a result, the strand initially has a lower temperature on its outside, especially shortly after passing through a cooling section, than in its interior, which is not directly affected by the cooling, especially on its inside.After the end of external cooling, i.e. after leaving the cooling section and the end of forced cooling, the higher temperature prevailing inside the strand spreads outwards again. As a result, the external temperature of the strand initially rises again after cooling. The extent of this temperature rise can be used to determine the temperature of the strand inside, in particular on its inside. To determine the temperature inside the tubular strand and / or on the inside of the tubular strand, in addition to the positions of the compared first and second temperatures, the conveying speed of the strand, the thermal conductivity and heat capacity of the strand material and the wall thickness of the tubular strand are known for the calculation.Based on these parameters, a temperature comparison can be used to determine the temperature difference between the outside and inside of the strand, and thus the temperature on the inside and inside of the strand. Any energy loss from the tubular strand to the environment can be neglected. In this way, the internal temperature of the strand can be reliably determined using a simple and flexible temperature measurement of the outside of the strand, even without placing a sensor inside the strand, and thus even far downstream of the extrusion device. This, in turn, can provide valuable information about the production process and use it as a control or regulation variable for the process. The optimization of the parameters mentioned above, namely minimal sagging, minimal energy input, and maximum conveying speed of the tube, is thus easier.Knowing the expansion coefficient also allows for more precise determination of the expected dimensions of the tubular strand after cooling. Knowing the temperature in the wall thicknesses of the tubular strand also allows for the temperature-dependent attenuation of terahertz radiation and the change in the refractive index to be taken into account when analyzing wall thicknesses and diameter values.

[0016] The position at which the temperature inside the tubular strand and / or on the inside of the tubular strand is determined can be a third position, which is spaced apart from the first position and the second position in the conveying direction of the tubular strand. However, it can be simpler for computational evaluation if the position at which the temperature inside the tubular strand and / or on the inside of the tubular strand is determined is equal to the first position or equal to the second position. This position can, for example, be equal to that of the first and second positions, which is arranged closer to the extrusion device in the conveying direction of the strand.

[0017] According to one embodiment, the second temperature can also be measured. A second temperature sensor can be provided for this purpose. By also measuring the second temperature, a particularly reliable determination of the internal temperature is possible.

[0018] According to a further embodiment, it is possible for the first temperature sensor to also be designed to measure the second temperature. This particularly advantageously requires only one temperature sensor. The use of only one temperature sensor can also be advantageous in terms of measurement reliability, since no distortion of the measurement result is to be expected due to different characteristics or changes in the temperature sensors.

[0019] At least the first temperature sensor can be a non-contact temperature sensor, in particular a pyroelectric sensor (PIR sensor). If a second temperature sensor is also provided, this can also be a non-contact temperature sensor, in particular a pyroelectric sensor. Such sensors offer simple and reliable non-contact measurement, even with very hot measuring objects. Furthermore, this sensor technology easily enables temperature measurement at different positions with just one temperature sensor, which then detects, for example, thermal radiation from the outer surface of the strand at different angles. It is particularly advantageous if the temperature sensor detects the thermal radiation from the strand surface at identical angles, i.e. symmetrically. This prevents falsification of the measurement result due to a different detection angle.

[0020] According to a further embodiment, at least the first temperature sensor can be arranged such that the first temperature is measured after the tubular strand has passed through a cooling section arranged downstream of the extrusion device. If a second temperature sensor is also present, this can be arranged such that the second temperature is also measured after the tubular strand has passed through a cooling section arranged downstream of the extrusion device. In the cooling section, as explained, the outside of the strand is sprayed with cooling liquid, for example cooling water, for cooling purposes. In particular, the first temperature sensor and optionally a second temperature sensor can be arranged such that the first temperature orThe second temperature is measured after the tubular strand has passed through a first cooling section arranged downstream of the extrusion device and before the tubular strand passes through a second cooling section arranged downstream of the first cooling section. A measurement is then taken between two cooling sections. As explained above, the measuring method according to the invention allows a reliable determination of the internal temperature of the strand, particularly after completion of a (first) cooling of the strand's outer surface, based on a reheating of the strand's outer surface following completion of the cooling.

[0021] According to a particularly practical embodiment, the evaluation device can be designed to determine the temperature inside the tubular strand and / or on the inside of the tubular strand using a finite element method, in particular an iterative one, from the comparison of the first and second temperatures. Such finite element methods, known per se to those skilled in the art, represent numerical calculation methods in which the body to be calculated, in this case the tubular strand, is divided into a finite number of sub-regions. The physical behavior of these so-called finite elements can be easily calculated using known approach functions due to their generally simple geometry. The physical behavior of the entire body, in this case the propagation of the higher internal temperature to the outside of the tubular strand, can be well simulated in this way.For small wall thicknesses compared to the diameter of the tubular strand, a linear temperature curve between the inside and outside of the tubular strand can be assumed to be a good approximation. This facilitates the mathematical determination of the internal temperature. As an iterative calculation method for calculating the internal temperature, for example, an internal temperature can be assumed in the first step and the finite element method can be used to calculate whether this internal temperature matches the measured external temperature. If this is not the case, the assumed internal temperature is changed and the mathematical test is performed again. In this way, the actual internal temperature can be determined iteratively.

[0022] According to a further embodiment, it can be provided that the first temperature sensor measures the first temperature at a plurality of locations distributed over the circumference of the tubular strand at the first position, and that the evaluation device is designed to compare the temperature measured at a plurality of locations distributed over the circumference of the tubular strand with the second temperature at a plurality of locations distributed over the circumference of the tubular strand at the second position, and to determine from the comparison the temperature inside the tubular strand and / or on the inside of the tubular strand at a plurality of locations distributed over the circumference of the tubular strand at the position of the tubular strand. The second temperature can also be measured at a plurality of locations distributed over the circumference of the tubular strand at the second position, be this by the first temperature sensor or a second temperature sensor that may be provided.By measuring and determining the temperature around the circumference, further important information about the production process can be obtained, in particular an uneven temperature distribution around the circumference, which on the one hand provides an indication of the degree of sagging of the tubular strand and on the other hand indicates a lack of cooling in discrete areas of the circumference of the tube.

[0023] It can further be provided that at least the first temperature sensor is or is rotated at least in sections over the circumference of the tubular strand. If a second temperature sensor is also provided, this can also be or be rotated at least in sections over the circumference of the tubular strand. However, it would also be conceivable, for example, for one temperature sensor to be rotatable and the other temperature sensor to be stationary. The rotatability or rotation of the first and / or second temperature sensor can of course be present or take place over the entire circumference. The first and / or second temperature can be measured at discrete locations over the circumference or essentially continuously over the circumference.By determining the temperature over the circumference of the tubular strand, the factors mentioned above, such as minimal sagging, i.e. minimal undesirable wall thickness deviation between an upper and a lower wall section, minimal energy input and maximum conveying speed of the strand, can be optimized even better.

[0024] According to a further embodiment, the evaluation device can be designed to determine the temperature of the tubular strand at several locations within the tubular strand at the position of the tubular strand at which the temperature inside the tubular strand and / or on the inside of the tubular strand is determined. A temperature profile in the radial strand direction can therefore be determined, from which further important information about the production process can be obtained. The determination of the temperature inside the tubular strand can be calculated numerically using a finite element method in a particularly practical manner, as explained, since with this method the tubular strand is divided into several sub-regions anyway, for example in the radial direction. The temperature can then be determined for these individual sub-regions, and thus a radial temperature profile of the tubular strand.

[0025] According to a further embodiment, a diameter and / or wall thickness measuring device can be provided, which measures the diameter and / or the wall thickness of the tubular strand at the first position and / or the second position of the tubular strand. The evaluation device can then be designed to take the measured diameter and / or the measured wall thickness into account when determining the temperature inside the tubular strand and / or on the inside of the tubular strand. The evaluation device can also be designed to take the temperature inside the tubular strand and / or on the inside of the tubular strand and, if applicable, the temperature of the outside of the tubular strand at the first and / or second position into account when determining the diameter and / or the wall thickness. The temperature of the strand material is thus an important value for determining geometric parameters of the strand.For example, the refractive index of the strand material is temperature-dependent. At the same time, the refractive index is an important parameter when determining the wall thickness of the strand, e.g., using a terahertz radiation measuring device. According to a particularly practical embodiment, the diameter and / or wall thickness measuring device can comprise a terahertz radiation measuring device. As explained above, diameter and wall thickness are important parameters of the production process. Furthermore, precise knowledge of the diameter or wall thickness can be important for accurately determining the temperature inside or on the inside of the tubular strand, since the diameter and, in particular, the wall thickness influences the extent to which a higher internal temperature of the strand spreads to the outside. The diameter and / or wall thickness could, in principle, be assumed to be known as parameters for the calculation.However, with the aforementioned embodiment, the accuracy of temperature determination is increased because the actual diameter or wall thickness is measured, thus taking into account any deviations from an expected diameter or wall thickness. The diameter and / or wall thickness measurement can, for example, be carried out as a transit time measurement of terahertz radiation reflected at boundary surfaces of the strand, in particular the outer and inner sides of the strand. For this purpose, the terahertz radiation measuring device can, for example, comprise a terahertz transceiver, which therefore comprises a terahertz transmitter and a terahertz receiver. The terahertz radiation measuring device can be rotated around the strand so that the diameter or wall thickness can be determined at several locations distributed around the circumference of the strand.In a particularly simple manner, the first and / or second temperature sensor can be integrated into the terahertz radiation measuring device. It can then rotate together with the device, if necessary. This ensures, in a particularly simple manner, that the diameter or wall thickness and the temperature are measured at the same locations around the circumference of the strand. The diameter and / or wall thickness measuring device can be configured, for example, as explained in WO 2016 / 139155 A1.

[0026] According to a further embodiment, the evaluation device can further be configured to determine, based on the temperature-dependent expansion coefficient of the material of the tubular strand, an expected shrinkage of the tubular strand starting from the position of the tubular strand at which the temperature within the tubular strand and / or on the inside of the tubular strand is determined, until its final shape is reached. The strand material shrinks during its cooling until it reaches its final shape, for example, when the strand has reached room temperature.If the temperature-dependent expansion coefficient of the strand material is known, the shrinkage expected until the strand reaches its final shape can be determined according to the aforementioned embodiment based on the temperature of the strand determined according to the invention within the tubular strand and / or on the inside of the tubular strand, and optionally also taking into account the measured temperature on the outside. This, in turn, can be advantageously used to predict certain geometric parameters of the strand after its final shape has been reached.Thus, according to a further embodiment, the evaluation device can further be designed to determine a diameter and / or a wall thickness of the tubular strand after reaching its final shape, taking into account the diameter and / or the measured wall thickness of the tubular strand measured at the first position and / or the second position of the tubular strand and taking into account the determined expected shrinkage.

[0027] According to a further embodiment, the evaluation device can be designed to determine the refractive index of the material of the tubular strand based on the determined temperature. The refractive index is known to be temperature-dependent. This means that the temperature measurement can be used to determine the refractive index of the strand material at the position or location of the temperature determination. This can be taken into account, for example, in a wall thickness measurement that uses the refractive index as a parameter. The wall thickness measurement is thus possible more precisely. For example, the refractive index can be determined at several locations distributed over the wall thickness of the strand. A refractive index distribution can therefore be determined. The refractive index can also be determined at several locations distributed over the circumference of the strand, and a refractive index distribution can also be determined from these.These refractive index distributions provide additional valuable information for the production process, such as the composition of the material and its consistency throughout the production process. Furthermore, the temperature-dependent absorption of terahertz radiation can be determined, and conversely, by measuring the average tube temperature, a prediction of the expected absorption can be derived. If tubes are manufactured from materials that exhibit a strong temperature-dependent increase in absorption, measuring and limiting the temperature of the material can ensure that reliable wall thickness and diameter measurements can be generated. Knowing the temperature-dependent expansion coefficient of the material can also be used to determine the extent to which the material will shrink from the measurement location at higher room temperature until it finally cools down.

[0028] According to a further embodiment, the device can further comprise a control and / or regulating device that controls and / or regulates the extrusion device based on the determined temperature within the tubular strand and / or on the inside of the tubular strand. In this way, improved control and / or regulation of the process, in particular of the extrusion device, is possible based on the information about the production process obtained according to the invention.

[0029] The device according to the invention can be designed to carry out the method according to the invention. The method according to the invention can be carried out accordingly with the device according to the invention.

[0030] An embodiment of the invention is explained in more detail below with reference to a drawing. The single figure shows a schematic side view of a device according to the invention.

[0031] The device shown in the figure comprises an extrusion device 10 with a conveyor device. A tubular strand 12 emerging from the extrusion device 10, in this case a plastic pipe 12, is conveyed in a conveying direction 14 along its longitudinal axis. The strand 12 passes through a first cooling section 16 and a second cooling section 18, in each of which a cooling liquid is sprayed onto the outside of the strand 12 for cooling. Between the extrusion device 10 and the first cooling section 16, a calibration device (not shown) can be arranged, for example with a metallic calibration sleeve, against the inner wall of which the strand 12 is pressed, for example by suction, for external shaping. In an area between the first cooling section 16 and the second cooling section 18, the strand 12 is accessible from the outside.In this area, a diameter and / or wall thickness measuring device 20 is provided, shown in dashed lines, comprising a terahertz transceiver 22 that emits terahertz radiation onto the tubular strand 12, as can be seen in the figure at arrow 24. The terahertz radiation penetrates the tubular strand 12 and is reflected at the boundary surfaces of the strand 12, in particular its outer and inner sides. The reflected terahertz radiation is in turn received by the terahertz transceiver 22. The measuring device 20 is connected to an evaluation device 28 via a data connection 26. The evaluation device 28 uses the reflected terahertz radiation to determine at least the wall thickness of the wall of the strand 12 facing the terahertz transceiver 22 and, if applicable, also the wall thickness of the wall of the strand 12 opposite the terahertz transceiver 22, but also its diameter.

[0032] The measuring device 20 also includes a first temperature sensor 30, in the present example a pyroelectric temperature sensor 30. The first temperature sensor 30 measures the heat radiation emanating from the strand 12 along the directions indicated by arrows 32 and 34, and thus a first temperature of the outside of the strand 12 at a first position 36 in the longitudinal direction of the strand 12, as well as a second temperature of the outside of the strand 12 at a second position 38 in the longitudinal direction of the strand 12. As can be seen in the figure, the first temperature sensor 30 measures the first temperature and the second temperature along the directions 32 and 34 symmetrically at the same angles to the strand surface. The measured values ​​of the first and second temperatures are also made available to the evaluation device 28 via the connection 26.The evaluation device 28 calculates the temperature inside the tubular strand 12 and / or on the inside of the tubular strand 12, for example at the second position 38 of the tubular strand 12, from a comparison of the measured first and second temperatures and taking into account the conveying speed of the strand 12, the heat capacity and the thermal conductivity of the strand material as well as the measured wall thickness and the diameter of the strand 12. The calculation can be carried out on the basis of iterative finite element methods, as explained.

[0033] For example, it is possible to determine a radial temperature profile over, for example, the wall section of the tubular strand 12 facing the temperature sensor 30. Furthermore, it is possible for the transceiver 22 and the temperature sensor 30 to be rotated about the longitudinal axis of the strand 12, so that both a wall thickness measurement and a measurement of the first and second temperatures can be performed at the first and second positions at several locations distributed over the circumference of the strand 12. In this way, a temperature profile of the strand 12 inside or on its inner side can also be calculated over the circumference of the strand 12.

[0034] Based on the temperature values ​​determined inside the strand 12 or on its inner side, the refractive index, absorption, and shrinkage of the strand material, which are known to be temperature-dependent, can be determined more precisely. With better knowledge of these properties, significantly more accurate wall thickness and diameter values ​​can be generated, both for the hot values ​​at the measurement location and for predictions after cooling, e.g., to room temperature.

[0035] In the example shown, the values ​​for the wall thickness and the temperature determined by the evaluation device 28 are made available to a control and / or regulating device 42 of the device via a data connection 40. On this basis, the control and / or regulating device 42 can control and / or regulate the extrusion device 10 and, for example, a conveyor device for the strand 10 included therein via a further data connection 44. List of reference symbols

[0036] 10Extrusion device 12Tubular strand 14Conveying direction 16First cooling section 18Second cooling section 20Diameter and / or wall thickness measuring device 22Terahertz transceiver 24Terahertz radiation 26Data connection 28Evaluation device 30Temperature sensor 32Temperature measuring direction 34Temperature measuring direction 36First position 38Second position 40Data connection 42Control and / or regulating device 44Data connection

Claims

1. A device for determining the temperature of a tubular strand (12) conveyed out of an extrusion device (10), comprising a first temperature sensor (30) for measuring a first temperature of the outer face of the tubular strand (12) at a first position of the tubular strand (12), and an evaluation apparatus (28), characterized in that the evaluation apparatus (28) is designed to compare the first temperature with a second temperature of the outer face of the tubular strand (12) at a second position of the tubular strand (12) that is at a distance from the first position in the conveying direction (14) of the tubular strand (12), and to determine the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) at a position of the tubular strand (12) from the comparison of the first and second temperature.

2. The device according to claim 1, characterized in that a second temperature sensor (30) is provided for measuring the second temperature or in that the first temperature sensor (30) is also designed to measure the second temperature.

3. The device according to one of the preceding claims, characterized in that at least the first temperature sensor (30) is arranged such that the first temperature is measured after the tubular strand (12) has travelled through a cooling section (16) arranged downstream of the extrusion device (10).

4. The device according to one of the preceding claims, characterized in that the evaluation apparatus (28) is designed to determine the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) from the comparison of the first and second temperature using a finite element method.

5. The device according to one of the preceding claims, characterized in that the first temperature sensor (30) measures the first temperature at multiple locations distributed over the circumference of the tubular strand (12) at the first position, and in that the evaluation apparatus (28) is designed to compare the temperature measured at multiple locations distributed over the circumference of the tubular strand (12) with the second temperature at multiple locations distributed over the circumference of the tubular strand (12) at the second position, and to determine, from the comparison, the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) at multiple locations distributed over the circumference of the tubular strand (12) at the position of the tubular strand (12).

6. The device according to claim 5, characterized in that at least the first temperature sensor (30) can be rotated at least in portions over the circumference of the tubular strand (12).

7. The device according to one of the preceding claims, characterized in that the evaluation apparatus (28) is further designed to determine the temperature of the tubular strand (12) at multiple locations inside the tubular strand (12) at the position of the tubular strand (12) at which the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) is determined.

8. The device according to one of the preceding claims, characterized in that a diameter and / or wall thickness measuring apparatus (20) is further provided, which measures the diameter and / or wall thickness of the tubular strand (12) at the first position and / or second position of the tubular strand (12).

9. The device according to claim 8, characterized in that the evaluation apparatus (28) is designed to take into account the measured diameter and / or the measured wall thickness when determining the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) and / or in that the evaluation apparatus (28) is designed to take into account the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) when determining the diameter and / or wall thickness.

10. The device according to one of claims 8 or 9, characterized in that the diameter and / or wall thickness measuring apparatus (20) comprises a terahertz radiation measuring apparatus.

11. The device according to one of the preceding claims, characterized in that the evaluation apparatus (28) is further designed, on the basis of the temperature-dependent expansion coefficient of the material of the tubular strand (12), to ascertain an expected shrinkage of the tubular strand (12) proceeding from the position of the tubular strand (12) at which the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) is determined until it has achieved its final shape.

12. The device according to claims 8 and 11, characterized in that the evaluation apparatus (28) is further designed to ascertain a diameter and / or wall thickness of the tubular strand after it has achieved its final shape taking into account the diameter and / or wall thickness of the tubular strand (12) measured at the first position and / or the second position of the tubular strand (12) and taking into account the ascertained expected shrinkage.

13. The device according to one of the preceding claims, characterized in that the evaluation apparatus (28) is further designed to determine the refractive index of the material of the tubular strand (12) based on the determined temperature.

14. The device according to one of the preceding claims, characterized in that an open-loop and / or closed-loop control apparatus (42) is further provided, which controls the extrusion device (10) in an open-loop and / or closed-loop manner on the basis of the determined temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12).

15. A method for determining the temperature of a tubular strand (12) conveyed out of an extrusion device (10), wherein a first temperature of the outer face of the tubular strand (12) is measured at a first position of the tubular strand (12), characterized in that the first temperature is compared with a second temperature of the outer face of the tubular strand (12) at a second position of the tubular strand (12) that is at a distance from the first position in the conveying direction (14) of the tubular strand (12), and in that the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) is determined at a position of the tubular strand (12) from the comparison of the first and second temperature.

16. The method according to claim 15, characterized in that at least the first temperature is measured after the tubular strand has travelled through a cooling section (16) arranged downstream of the extrusion device (10).

17. The method according to one of claims 15 or 16, characterized in that the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) is determined from the comparison of the first and second temperature using a finite element method.

18. The method according to one of claims 15 to 17, characterized in that the first temperature is measured at multiple locations distributed over the circumference of the tubular strand (12) at the first position, and in that the temperature measured at multiple locations distributed over the circumference of the tubular strand (12) is compared with the second temperature also at multiple locations distributed over the circumference of the tubular strand (12) at the second position, and in that the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) is determined from the comparison at multiple locations distributed over the circumference of the tubular strand (12) at the position of the tubular strand (12).

19. The method according to one of claims 15 to 18, characterized in that the temperature of the tubular strand (12) at multiple locations inside the tubular strand (12) is determined at the position of the tubular strand (12) at which the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) is determined.

20. The method according to one of claims 15 to 19, characterized in that the diameter and / or wall thickness of the tubular strand (12) is further measured at the first position and / or second position of the tubular strand (12).

21. The method according to claim 20, characterized in that the measured diameter and / or the measured wall thickness is taken into account when determining the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) and / or in that the temperature inside the tubular strand and / or on the inner face of the tubular strand is taken into account when determining the diameter and / or wall thickness.

22. The method according to one of claims 15 to 21, characterized in that an expected shrinkage of the tubular strand (12) proceeding from the position of the tubular strand (12) at which the temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12) is determined until it has achieved its final shape is ascertained on the basis of the temperature-dependent expansion coefficient of the material of the tubular strand (12).

23. The method according to claims 20 and 22, characterized in that a diameter and / or wall thickness of the tubular strand after it has achieved its final shape is ascertained taking into account the diameter and / or wall thickness of the tubular strand (12) measured at the first position and / or the second position of the tubular strand (12) and taking into account the ascertained expected shrinkage.

24. The method according to one of claims 15 to 23, characterized in that the refractive index of the material of the tubular strand (12) is determined based on the determined temperature.

25. The method according to one of claims 15 to 24, characterized in that the extrusion device (10) is controlled in an open-loop and / or closed-loop manner on the basis of the determined temperature inside the tubular strand (12) and / or on the inner face of the tubular strand (12).