Pipeline, Thick-Matter Pump and Method for Determining a Pressure and/or a Wall Thickness in the Pipeline

Abstract

A pipeline for conveying concrete determines a pressure in and/or a wall thickness of the pipeline, wherein a first length portion of the pipeline has a first wall thickness and a second length portion of the pipeline has a second wall thickness greater than the first wall thickness. The determination is made using a first strain gauge on an outer side of the first length portion and a second strain gauge on an outer side of the second length portion, wherein the strain gauges are each fixedly applied to the outer side of the length portions. By comparison of the measurements at the two strain gauges, it is possible to determine a wall thickness by determining a strain of the pipeline owing to a decrease in the wall thickness.

Claims

1.-23. (canceled)

24. A pipeline for delivery of thick matter, comprising: a first longitudinal section of the pipeline with a first wall thickness; a second longitudinal section of the pipeline with a second wall thickness which is greater than the first wall thickness; a first strain gauge on an outer side of the first longitudinal section; a second strain gauge on an outer side of the second longitudinal section, wherein the strain gauges are attached in each case fixedly on the longitudinal sections or are attached on outer sides of the longitudinal sections, and the strain gauges are used for determining a pressure in the pipeline and/or a wall thickness of the pipeline.

25. The pipeline according to claim 24, wherein the strain gauges are attached on the outer side of the longitudinal sections by way of at least one of: adhesive bonding, screwing, soldering, shrink-fitting, and fastening via a clip which engages over the gauges in a circumferential direction of the pipeline.

26. The pipeline according to claim 24, wherein the first strain gauge and the second strain gauge extend in the same direction and/or their working direction or their measuring direction is the same.

27. The pipeline according to claim 26, wherein the strain gauges extend in the circumferential direction of the pipeline and their working direction or their measuring direction extends in the circumferential direction.

28. The pipeline according to claim 24, wherein the strain gauges are selected from a group comprising: strain gage strips, electrically conductive wires, force-measuring press-fit sensors, and glass fibers.

29. The pipeline according to claim 24, wherein at least one strain gauge and/or the second longitudinal section of the pipeline are/is at a spacing from at least one end of the pipeline, which spacing corresponds to at least an internal diameter of the pipeline.

30. The pipeline according to claim 29, wherein the spacing corresponds to at least twice the internal diameter of the pipeline.

31. The pipeline according to claim 24, wherein the first strain gauge and the second strain gauge are elongate, and their length is greater than their width, the two strain gauges are attached with the same orientation on the pipeline with an extent in accordance with their longitudinal direction along the longitudinal direction of the pipeline.

32. The pipeline according to claim 24, wherein at least one strain gauge is a strain gauge strip which has a plurality of part strips which in each case form a part strain gauge strip with sensitivity along in each case precisely one direction.

33. The pipeline according to claim 32, wherein the plurality of part strips comprise two part strips which run at a right angle with respect to one another.

34. The pipeline according to claim 24, wherein an internal diameter of the two longitudinal sections are identical, the pipeline being straight.

35. The pipeline according to claim 24, wherein the pipeline has only the first longitudinal section and the second longitudinal section, and no other or further longitudinal sections with a different wall thickness and/or a different internal diameters.

36. The pipeline according to claim 24, wherein the first longitudinal section adjoins the second longitudinal section directly by way of a transition in wall thickness, the transition between the first wall thickness and the second wall thickness is abrupt as a step with a radius on the interior angle of at least 10% of the internal diameter of the pipeline.

37. The pipeline according to claim 24, further comprising: a flange at at least one end of the pipeline for connection to further lines.

38. The pipeline according to claim 24, wherein the first longitudinal section and the second longitudinal section are produced in one piece and as one part.

39. The pipeline according to claim 24, wherein the pipeline has a pipe with a continuously constant first wall thickness which forms the first longitudinal section, and an outer pipe is placed onto the pipe as a cuff with a wall thickness in accordance with the difference between the first wall thickness and the second wall thickness to form the second longitudinal section.

40. The pipeline according to claim 24, wherein the first longitudinal section and/or the second longitudinal section in each case have a constant wall thickness.

41. The pipeline according to claim 24, wherein the second wall thickness is from 10% to 30% more than the first wall thickness.

42. The pipeline according to claim 24, wherein the second wall thickness is from 75% to 250% more than the first wall thickness.

43. The pipeline according to claim 24, wherein a continuous inner pipe is arranged in the pipeline with a constant internal diameter and a constant wall thickness, the wall thickness of the inner pipe being smaller than the first wall thickness, the inner pipe bearing flatly against the inner side of the two longitudinal sections.

44. The pipeline according to claim 43, wherein the inner pipe is of more wear-resistant configuration than the pipeline in respect of wear resistance on the inner side with regard to material which flows through.

45. A thick-matter pump, comprising: a delivery line for delivery of thick matter; a pipeline arranged in a path of the delivered thick matter within the delivery line, the pipeline comprising: a first longitudinal section of the pipeline with a first wall thickness; a second longitudinal section of the pipeline with a second wall thickness which is greater than the first wall thickness; a first strain gauge on an outer side of the first longitudinal section; a second strain gauge on an outer side of the second longitudinal section, wherein the strain gauges are attached in each case fixedly on the longitudinal sections or are attached on outer sides of the longitudinal sections, and the strain gauges are used for determining a pressure in the pipeline and/or a wall thickness of the pipeline.

46. A method for determining a pressure and/or a wall thickness in a pipeline comprising: a first longitudinal section of the pipeline with a first wall thickness; a second longitudinal section of the pipeline with a second wall thickness which is greater than the first wall thickness; a first strain gauge on an outer side of the first longitudinal section; a second strain gauge on an outer side of the second longitudinal section, wherein the strain gauges are attached in each case fixedly on the longitudinal sections or are attached on outer sides of the longitudinal sections, the method comprising: determining a length change of the first strain gauge and of the second strain gauge in a circumferential direction of the pipeline and/or in a longitudinal direction of the pipeline; determining a first wall thickness of the first longitudinal section with the use of at least the length change of the first and second strain gauges in the circumferential direction of the pipeline and/or in the longitudinal direction the pipeline with use of Poisson's ratio of a material of the pipeline and with use of the difference between the first wall thickness and a second wall thickness.

47. The method according to claim 46, wherein the pressure within the first longitudinal section is determined from the determined first wall thickness, at least the length changes in the circumferential direction and Poisson's ratio of the material of the pipeline, the pressure within the second longitudinal section is determined similarly with additional use of the difference between the first wall thickness and the second wall thickness, the pressure within the entire pipeline is determined by way of averaging of the first pressure and the second pressure.

48. The method according to claim 47, wherein a continuous inner pipe is arranged in the pipeline with a constant internal diameter and a constant wall thickness, the wall thickness of the inner pipe being smaller than the first wall thickness, the inner pipe bearing flatly against the inner side of the two longitudinal sections, the method further comprising: adding the wall thickness of the inner pipe to the first wall thickness and to the second wall thickness.

49. The method according to one of claim 46, wherein in the case of the calculations for the two longitudinal sections of the pipeline, the length changes of the strain gauges in the longitudinal direction of the pipeline and in the circumferential direction of the pipeline are taken into consideration by the length change in the circumferential direction being added for a longitudinal section in each case to the product of the length change in the longitudinal direction and Poisson's ratio, this sum being multiplied by the difference of the wall thicknesses, this result being divided by the difference between the product of the length change in the longitudinal direction and Poisson's ratio plus the length change in the circumferential direction for the first longitudinal section, and the product of the length change in the longitudinal direction and Poisson's ratio plus the length change in the circumferential direction for the second longitudinal section.

50. The method according to one of claim 46, wherein calculations for the two longitudinal sections of the pipeline take place by way of the following formula: $s_1=\frac{\left({\mathrm{.Math.}}_{t2}+{\mathrm{.Math.}}_{a2}*\vartheta \right)*ds}{\left({\mathrm{.Math.}}_{t1}+{\mathrm{.Math.}}_{a1}*\vartheta -{\mathrm{.Math.}}_{t2}-{\mathrm{.Math.}}_{a2}*\vartheta \right)\left({\mathrm{.Math.}}_{t2}+{\mathrm{.Math.}}_{a2}*\vartheta \right)}$ the mean diameter being made equal in the two longitudinal sections to this end.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Exemplary embodiments of the invention are shown diagrammatically in the drawings and will be described in greater detail in the following text.

[0032] FIG. 1 shows a diagrammatic illustration of a thick-matter pump according to an embodiment of the invention on a vehicle with a boom arm and a delivery line thereon;

[0033] FIG. 2 shows a section and a top view of a first embodiment of a pipeline according to the invention with two longitudinal sections of different thickness in the case of an embodiment of the pipeline in one part;

[0034] FIG. 3 shows another embodiment of the invention, similar to FIG. 2, with a wear-resistant inner pipe;

[0035] FIG. 4 shows an illustration of a pipeline according to an embodiment of the invention, similar to FIG. 1, with two different longitudinal sections with in each case a different wall thickness;

[0036] FIG. 5 shows a further pipeline according to an embodiment of the invention with four different longitudinal sections with in each case three different wall thicknesses;

[0037] FIG. 6 shows a further pipeline according to an embodiment of the invention with a thick cuff, which is attached to a continuous pipe, and a clip with strain gauge strips;

[0038] FIG. 7 shows a section through a pipeline according to an embodiment of the invention with a clip which is attached to it and on the outer side of which a strain gauge strip is arranged;

[0039] FIG. 8 shows a further pipeline according to an embodiment of the invention with a conically uniformly increasing wall thickness including a spirally wound glass fiber in the sectional illustration; and

[0040] FIG. 9 shows a view from the outside of the pipeline from FIG. 8.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0041] FIG. 1 shows a vehicle which has a tripartite boom arm 2 and a concrete pump 3. The concrete pump 3 is the thick-matter pump, and has a multiple-part delivery line 4 on the boom arm 2, which delivery line 4 opens into a line end 5. Pumped or delivered concrete can exit here. To this end, the concrete is pumped into the delivery line 4 by a pumping apparatus 6. Depending on the length of the entire delivery line 4 and, above all, also the height of the line end 5, a different pressure recognizably has to be applied by the pumping apparatus 6, possibly a very high pressure, as is known. Above all in the case of pumping of concrete, as was explained at the outset, the line is loaded greatly on its inner side and, as it were, is worn or abraded. The wall thickness therefore actually decreases. If a blockage then occurs as explained at the outset, the pressure rises greatly and there is therefore the risk that, at high pressure, the delivery line 4 can burst at a particularly thin location. This leads to the risks mentioned at the outset. The aim is therefore to avoid this, with the result that there is a desire to find out how pronounced the wear on the inside in the delivery line is and/or how greatly a wall thickness has possibly already been reduced. Furthermore, it would of course be very advantageous for it to be possible for an abovementioned sudden pronounced pressure rise to be detected very rapidly, in order to shut down or switch off the pumping apparatus 6.

[0042] A pipeline can be inserted at any desired location into the delivery line 4 which is shown in FIG. 1, which pipeline then, as it were, serves in a representative manner for the entire delivery line to detect wear with regard to a reduction in the wall thickness. Furthermore, the pressure within the delivery line 4 is also to be capable of being detected in this way.

[0043] A pipeline of this type is shown in details in FIG. 2 in a first embodiment of the invention. This pipeline 11 can be, for example, 1 m in length, and can be mounted in an easily removable manner in the lower region of the delivery line 4, for example upstream of the boom arm 2 or at the lowermost member of the boom arm 2.

[0044] The pipeline 11 is advantageously an exactly straight pipe, and can advantageously have a radially symmetrical cross section. An internal cross section is advantageously circular; the external cross sections are particularly advantageously also circular. There are recognizably two longitudinal sections 13a on the left and 13b on the right which can each case also be continued with any desired length. The left-hand first longitudinal section 13a has a wall thickness s. The right-hand second longitudinal section 13b has a wall thickness s+ds, and is somewhat more than twice as thick here as what is shown by FIG. 2. The mean diameter of the pipeline 11 in the second longitudinal section 13b is d.sub.m, this diameter d.sub.m being shown only partially here. A thick matter 12, advantageously concrete, is shown centrally in the pipeline by way of the dotting.

[0045] As can also be seen from the top view (shown at the bottom in FIG. 2) of the pipeline 11, a first strain gauge strip 16a is applied to the first longitudinal section 13a, and a second strain gauge strip 16b is applied to the longitudinal section 13b, which strain gauge strips are identical here. The top view shows that the two strain gauge strips 16a and 16b are arranged with an exactly identical longitudinal orientation, namely along the center axis which is shown using a dash-dotted line. Here, the two strain gauge strips 16a and 16b are recognizably configured to be somewhat longer than wide, with the result that their extent in the longitudinal direction of the pipeline 11 is greater by approximately from 30% to 50% than their extent in the transverse direction with respect thereto. These two strain gauge strips are advantageously configured as explained above, namely with in each case two part strain gauge strips which run at a right angle with respect to one another. They can therefore detect an elongation in two directions at a right angle with respect to one another, and therefore possibly also an elongation in a two-dimensional plane.

[0046] The strain gauge strips 16a and 16b in each case have a relative length change ε.sub.längs1 and ε.sub.längs2 in the longitudinal direction of the pipeline 11, that is to say along two part strain gauge strips, and a relative length change ε.sub.quer1 and ε.sub.quer2 in the direction perpendicularly with respect thereto, that is to say along the two other part strain gauge strips. The strain gauge strips 16a and 16b are applied over the full surface area and fixedly to the outer side of the pipeline 11 in the longitudinal sections 13a and 13b, for example are adhesively bonded in a stable and durable manner. This is generally known for strain gauge strips. They advantageously operate with variable resistances, a corresponding contacting and electric evaluation not being shown here. This is easy for a person skilled in the art to realize, however, with standard solutions for the evaluation of strain gauge strips.

[0047] The calculation of the wall thickness s according to FIG. 2 is set forth in the following text. It is assumed here that the pressure P in the first longitudinal section 13a corresponds to the pressure P in the second longitudinal section 13b, with the result that the two can be made equal.

[0048] By way of Barlow's Formula, the tangential tension results from the internal pressure, from the mean diameter and the wall thickness of a pipe

[00001]${\sigma }_t=\frac{P*d_m}{2*s}$

TABLE-US-00001 P pipe internal pressure d.sub.m mean diameter s wall thickness of the pipe

[0049] The tangential tension is not distorted by bends or axial loads on the pipe. In order for it to be possible for it to be measured correctly, the measurement of two elongations in two directions is required in the two-dimensional tension state:

[00002]${\mathrm{.Math.}}_t+{\mathrm{.Math.}}_a*\vartheta =\frac{{\sigma }_t}{E}*{\left(1-\vartheta \right)}^2$

TABLE-US-00002 ε.sub.t elongation in tangential direction ε.sub.a elongation in axial direction θ Poisson's ratio σ.sub.t tension in tangential direction E modulus of elasticity

[0050] Reworked:

[00003]$\frac{\left({\mathrm{.Math.}}_t+{\mathrm{.Math.}}_a*\vartheta \right)*E}{{\left(1-\vartheta \right)}^2}={\sigma }_t$

[0051] Applied:

[00004]$\frac{\left({\mathrm{.Math.}}_t+{\mathrm{.Math.}}_a*\vartheta \right)*E}{{\left(1-\vartheta \right)}^2}=\frac{P*d_m}{2*s}$

[0052] If the formula is derived twice (with s1 and s2=sl+ds as wall thickness). Here, d.sub.m≈d.sub.m2 has approximately been given a constant value. It is likewise possible, however, even if it is more complex mathematically, to perform the calculation using the exact value.

[00005]$P_1=\frac{\left({\mathrm{.Math.}}_{t1}+{\mathrm{.Math.}}_{a1}*\vartheta \right)*E*2*s_1}{{\left(1-\vartheta \right)}^2*d_{m1}}{P}_2=\frac{\left({\mathrm{.Math.}}_{t2}+{\mathrm{.Math.}}_{a2}*\vartheta \right)*E*2*\left(s_1+ds\right)}{{\left(1-\vartheta \right)}^2*d_{m2}}$

[0053] Equated and resolved according to s1 and with d.sub.m1 set to be approximately equal to d.sub.m2, the result is:

[00006]$s_1=\frac{\left({\mathrm{.Math.}}_{t2}+{\mathrm{.Math.}}_{a2}*\vartheta \right)*ds}{\left({\mathrm{.Math.}}_{t1}+{\mathrm{.Math.}}_{a1}*\vartheta -{\mathrm{.Math.}}_{t2}-{\mathrm{.Math.}}_{a2}*\vartheta \right)\left({\mathrm{.Math.}}_{t2}+{\mathrm{.Math.}}_{a2}*\vartheta \right)}$

s1 can be determined with the aid of this equation. This value can then be smoothed. And P1 and P2 can then be determined with the aid of the above equations. These pressures P1 and P2 should be identical; a mean value can be used in the evaluation, and the difference between the two values can serve as a sensor control. As an alternative, a longitudinal strain gauge strip can also be dispensed with if P1=P2 is set and the missing elongation is determined from the formula. Here, the elongation in the tangential direction ε.sub.t corresponds to ε.sub.Quer, and the elongation in the axial direction ε.sub.a corresponds here to ε.sub.längs, to be precise in each case in relation to the two longitudinal sections 13a and 13b.

[0054] In FIG. 3 below FIG. 2, an alternative pipeline 111 is shown which once again has a radially symmetrical cross section and a circular cross section for the delivery of thick matter 112 therein. On the left-hand side, a first longitudinal section 113a with a first strain gauge strip 116a is provided. After this, a second longitudinal section 113b with a second strain gauge strip 116b is adjacent on the right with a hard stepped transition. On the inside, the pipeline 111 is provided with a continuous inner pipe 118. This inner pipe hundred 18 consists of a more wear-resistant material than the pipeline 111 per se. Furthermore, it can be removed as a wear part from the pipeline 111, for example because it has shrunk thermally, as soon as it wears to an excessive extent or has even worn through. It can then be replaced by a new inner pipe 118, whereas the remaining pipeline 111 can continue to be used, in particular even with the complex adhesively bonded strain gauge strips 116a and 116b.

[0055] For the above-described mathematics or calculation of a pressure or an elongation or wall thickness of the pipeline 111, a wall thickness of the first longitudinal section 113a can be used together with the inner pipe 118s as the basis taking into account the specifications from FIG. 2. In the second longitudinal section 113b, the wall thickness is then s+ds as shown. It can also be seen from this that the wall thickness of the inner pipe 118 is added as it were in full to the wall thickness of the pipeline 111. The added wall thicknesses s and s+ds can be seen in a similar manner as in FIG. 2.

[0056] As can be seen from the lower illustration of FIG. 3, the arrangement and the orientation of the strain gauge strips 116a and 116b are as shown in FIG. 2. Furthermore, these two strain gauge strips 116a and 116b are also of identical configuration with respect to one another here, and are advantageously adhesively bonded on the outside.

[0057] FIG. 4 shows a simplified illustration of a pipeline 211. It can be, for example, 1 m long and can have a diameter of approximately 20 cm. The pipeline 211 is provided at the ends in each case with flanges 220 which are integrally formed in one piece, as known per se. They serve to connect to the pipeline 211 to other pipelines or the delivery line in a known way.

[0058] The pipeline 211 largely has a first longitudinal section 213a; a longitudinal section 213b with a discernibly thicker wall thickness is provided only in the center or somewhat to the right of the center. The pipeline 211 is configured in one piece here. The transition of the wall thickness between the middle longitudinal section 213b and the adjacent longitudinal sections 213a on the left and right thereof is not stepped according to FIGS. 2 and 3, but rather is rounded somewhat. This can improve the mechanical properties of the pipeline 211 with regard to stability. The longitudinal extent of the longitudinal section 213b is approximately from 15% to 20% of the entire pipeline 211. There wall thickness is approximately 150% of that of the first longitudinal section 213a.

[0059] The illustration of strain gauge strips is also dispensed with here; they are arranged in the longitudinal section 213b and at least one of the longitudinal sections 213a, advantageously in accordance with FIGS. 2 and 3.

[0060] FIG. 5 shows a further pipeline 311 according to an embodiment of the invention with flanges 320 at the ends, as described previously in respect of FIG. 4. Even more different wall thickness regions are provided here, the wall thickness of the pipeline 311 first of all being relatively thin starting from the left. There is then a rounded rise to a wall thickness which is thicker by approximately from 50% to 70% in the longitudinal section 313a. After approximately from 15% to 20% of the length of the entire pipeline 311, there is once again a rounded rise of the wall thickness to the longitudinal section 313b, the two longitudinal sections 313a and 313b being of approximately identical length. The wall thickness of the longitudinal section 313b is approximately 50% more. There is then a rounded transition to the relatively thin wall thickness of the original pipeline 311 as far as the right-hand flange 220. In the case of this pipeline 211, strain gauge strips are advantageously attached in accordance with FIGS. 2 and 3 in the longitudinal sections 313a and 313b, but are not illustrated here. A further strain gauge strip might also be applied directly on the pipeline 311 on the left or the right of the two longitudinal sections, in a similar manner to that described for FIG. 4. Two strain gauge strips are as a rule considered to be sufficient, however.

[0061] FIG. 6 shows yet a further pipeline 411 according to an embodiment of the invention which likewise has flanges 420 for fastening at the left-hand end and at the right-hand end. Here, no regions with an increased wall thickness are provided on the pipeline 411 which is provided per se continuously with a constant cross section, but rather a thick reinforcing ring 422 which is shrink-fitted or adhesively bonded is provided on the left-hand side. Below the latter, advantageously in a corresponding recess, a left-hand strain gauge strip 416a can be situated which is protected, for example, by way of the reinforcing ring 422. The strain gauge strip 416a then measures per se, however, only on the external diameter of the pipeline 411 per se, with the result that the thickness of the reinforcing ring 422 does not influence this measurement or has no effects on it.

[0062] On the right-hand side next to the reinforcing ring 422, a clip 424 is placed onto the pipeline 411 and is fastened so as to bear tightly against it, as is also shown by the sectional illustration of FIG. 7. The clip 424 is configured in a customary way, and runs largely around the pipeline 411 apart from a narrow spacing in the region of projecting fastening sections 425. The latter are braced by way of a diagrammatically indicated screw 426, with the result that the clip 424 is seated fixedly on the pipeline 411. In a similar manner to the second longitudinal section 213b according to FIG. 4, it therefore also counts as an increase in the wall thickness of the pipeline in this region, because a strain gauge strip 416b is applied on the clip 424 on the outside. This strain gauge strip 416b can then carry out the measurement according to the invention directly on the pipeline 411, for example together with the abovementioned first strain gauge strip 416a.

[0063] FIGS. 8 and 9 show yet a further modification of a pipeline 511 according to an embodiment of the invention which in each case has a first longitudinal section 513b in a manner which runs inward and adjoins flanges 520 at the ends, approximately in a similar manner to FIG. 5. A second longitudinal section 513b lies in between, which has a greater wall thickness which is not constant or uniform, however. Rather, the wall thickness rises continuously here, starting at the wall thickness of the first longitudinal section 513a as far as a wall thickness on the far right which is approximately from 200% to 250% thereof, that is to say is considerably thicker. Here, a first strain gauge strip 516a can either be arranged in the first longitudinal section 513a. As an alternative, an elongate glass fiber can be wound spirally in the second longitudinal section 513b with approximately four windings, as is shown here. A plurality of reflection planes can be used in the glass fiber 517, which are produced by way of manufacturing defects which occur during the glass fiber production, or this is possible by way of artificially introduced partially reflecting mirrors. This has been explained at the outset. This results in different longitudinal elongations or a length change on the sections of the spirally wound glass fiber, which can be recorded and offset. At least two measurements can be performed by way of the specific conical shape of the pipeline 411.

[0064] This glass fiber 517 is connected to a measuring apparatus and is closed off at one end in such a way that the length of the individual sections, into which it is divided by way of the reflection planes and/or the partially reflecting mirrors, can be determined exactly by way of a light signal which passes to and fro. Since the glass fiber 517 is connected along its length in a completely firm manner to the outer side of the pipeline 511 or the second longitudinal section 513b, it stretches accordingly with the widening of the pipeline or longitudinal section 513b. This widening of the pipeline longitudinal section is different, depending on how thick they are in each case on account of the conical shape. It results after all from the wall thickness described at the outset which decreases continuously in the direction to the left in the second longitudinal section 513b, as a result of which the strength of the pipeline decreases and it can therefore be elongated to a more pronounced extent as a result of the pressure of the thick matter which is delivered therein. Here, the calculation of a respective wall thickness takes place in the case of a glass fiber 517 of this type as a strain gauge in a similar manner to that stated above, namely also by way of its changing or rising length. The glass fiber 517 is merely divided differently into individual sections with a respective determinable length and length change. The wall thickness can also be calculated therefrom in all cases, possibly on the basis of stored comparison values.