Method for producing measuring tube for fluid measuring device

Abstract

In a method for producing a measuring tube (10) of a fluid measuring device, a tubular semi-finished product (14) is provided which has a through-hole (32) extending in a longitudinal direction (L) of the semi-finished product, a circumferential wall (22) of the semi-finished product has at least one waveguide region (34) which forms a waveguide (24) for surface acoustic waves of an acoustic measuring device after completion of the measuring tube, and the waveguide region extends along the longitudinal direction of the semi-finished product and extends from the outer surface to the inner surface of the semi-finished product. The semi-finished product is inserted into the forming tool (43) and at least one mandrel (36) is inserted into the through-hole, the outer geometry (38) of the mandrel corresponding to the inner geometry of the fluid channel (16) of the measuring tube to be formed by the through-hole. In a forming step of applying a stamping force (F) to at least one axial central section (30) of the semi-finished product, at least the central section is plastically deformed, and the through-hole is formed as a fluid channel. Finally, the mandrel is removed.

Description

Technical Field

[0001] The invention relates to a method for producing a measuring tube of a fluid measuring device having at least one waveguide region. Background Art

[0002] Fluid measuring devices are known which use surface acoustic waves (SAW) to determine the properties of a fluid flowing through a fluid channel. For this purpose, the surface acoustic waves are excited by means of an acoustic signal converter in an acoustic waveguide and are partially coupled out into the fluid as volume acoustic waves and coupled back from the fluid into the waveguide. The interference between the surface acoustic waves and the recoupled volume waves produces a characteristic signal which is evaluated. For example, the time course and intensity of the characteristic signal (in particular the time-intensity curve including the time delay) allow conclusions to be drawn about the speed of sound, temperature, homogeneity, flow velocity, flow rate, concentration or viscosity, etc.

[0003] The measuring tube used in such fluid measuring devices has a continuous fluid channel, and the surface acoustic waves to be coupled in or out travel in a waveguide located in the wall of the measuring tube. Summary of the invention

[0004] The object of the present invention is to specify a method by which such a measuring tube can be produced in a simple and cost-effective manner, but nevertheless with a high degree of precision.

[0005] The object is achieved by a method for manufacturing a measuring tube of a fluid measuring device, the method comprising the following steps:

[0006] - providing a tubular semi-finished product having a through hole extending in the longitudinal direction of the semi-finished product, the circumferential wall of the semi-finished product having at least one waveguide region which, after the measuring tube is completed, forms a waveguide for surface acoustic waves of the acoustic measuring device, the waveguide region extending in the longitudinal direction of the semi-finished product and from the outer surface to the inner surface of the semi-finished product,

[0007] - inserting the semi-finished product into the forming tool,

[0008] - introducing at least one mandrel into the through-opening, the outer geometry of the mandrel corresponding to the inner geometry of the fluid channel of the measuring tube to be formed by the through-opening,

[0009] - performing a forming step in which a punching force is applied to at least an axial central section of the semi-finished product and at least the central section is plastically deformed, the through hole being formed into a fluid channel, and

[0010] -Remove the mandrel.

[0011] In the forming step, the outer geometry of the mandrel is transferred to the inner surface of the through hole, and the fluid channel thus obtains its desired cross section. In this way, the precise inner geometry of the fluid channel can be produced in a few simple and fast process steps and in an easily reproducible manner.

[0012] The excitation, measurement, coupling-in and coupling-out of surface acoustic waves usually take place purposefully at waveguides formed on the measuring tube. The waveguides are usually a component of the measuring tube, but in the finished measuring tube they may differ in geometry from the remaining walls of the fluid channel. In particular, the waveguides have a planar outer geometry, so that the measuring tube has a flat surface in the region of the waveguides. In most cases, the waveguides are formed in a rectangular shape, the long sides of which are aligned parallel to the longitudinal direction of the fluid channel.

[0013] In the shaping step, the waveguide is preferably also manufactured or completed from the waveguide region.Preferably, the waveguide region is completely arranged in the central section.

[0014] The mandrel preferably moves in a straight line without rotating, which further simplifies the process.The mandrel is typically inserted into the central section without deforming the inner surface of the through hole.

[0015] During the forming step, a radial punching force advantageously acts on the semi-finished product, which radial punching force is generated, for example, by means of several radially movable punching punches. The forming step can be performed in a cold forming process or in a hot forming process.

[0016] A number of variants of the semi-finished product have proven sufficient to produce a wide variety of measuring tube variants. For example, two variants of the semi-finished product with different outer and / or inner diameters can be provided.

[0017] After the measuring tube is completed, it can be used in a fluid measuring device. One or more signal converters that generate or detect surface acoustic waves are placed on one or more waveguides.

[0018] Before the forming step, the semi-finished product can have an outer geometry that deviates from a cylindrical shape and defines damping elements and / or reflective elements for surface acoustic waves as well as a waveguide region, which outer geometry is at least partially maintained during the forming step. This includes, for example, bevels and / or modified material thicknesses at the longitudinal ends of the subsequent waveguide. This makes it possible, for example, to machine the measuring tube already before the forming step in order to prefabricate desired components that are to be provided on the measuring tube later.

[0019] In this case, at least one waveguide region can already be specified on the semi-finished product. The semi-finished product should then be aligned accordingly in the circumferential direction when it is inserted into the forming tool.

[0020] Additionally and / or alternatively, in particular the damping element and / or the reflective element can also be produced during the forming step by shaping the punch and / or the mandrel or by post-processing the outer surface after the forming step.

[0021] The semi-finished product may have regions of different wall thicknesses in the central section and / or in the axially adjacent sections, which remain at least partially unchanged during the forming step. Alternatively or additionally, such regions can also be produced during the forming step. The thickness variation of the circumferential wall of the measuring tube along the circumferential direction and / or in the axial direction can be used purposefully, for example, to confine surface acoustic waves to the waveguide. Such a component can also be easily integrated into the measuring tube in this way.

[0022] In order to obtain a flat waveguide with a constant wall thickness over the size of the waveguide, the outer geometry of the central section located in at least one waveguide area can be formed from a circular surface to a flat surface. In this way, although the semi-finished product is made of a tube with a cylindrical cross-section, the waveguide can be designed as a flat rectangular surface on the outer surface of the measuring tube, which reduces the manufacturing costs.

[0023] The long sides of the rectangular surface of the waveguide are preferably aligned in the longitudinal direction.A rectangular surface of this type can also be formed on the inner surface of the fluid channel in a molding step by using a mandrel inserted into the central section.

[0024] By means of appropriate geometry of the mandrel and the stamping punch, fluid channels with a polygonal cross section can generally be produced from a semi-finished product with a circular cross section.

[0025] For example, in order to compensate for the elastic component of the deformation (i.e. the amount by which the wall of the measuring tube rebounds after the punch has been removed), the punch can be pressed against the measuring tube again with a suitable force to obtain the desired plastic deformation as a final result. For repeated forming steps, mandrels with different outer geometries or different cross-sections can also be used.

[0026] If necessary, after removing at least one mandrel, the same mandrel or a different mandrel may be moved again through the fluid channel to improve the forming accuracy of the internal geometry of the fluid channel.

[0027] In this case, for example, a mandrel can be used which has a slightly larger cross-section than the mandrel which is inserted into the measuring tube in the forming step. In this case, by pulling the mandrel through the measuring tube, the inside of the measuring tube can be smoothed and / or expanded in a purposeful and precise manner. This allows an exact calibration of the fluid channel with exactly specified dimensions.

[0028] Typically, post-processing of the fluid channels for material removal after the molding step can be omitted.

[0029] However, if necessary, the inner surface of the fluid channel can be further smoothed by passing a suitable grinding fluid through the fluid channel. For example, in at least one method step, two mandrels are used simultaneously, which are moved from the axial ends of the semi-finished product into the through hole before the forming step and are moved out of the through hole again after the forming step. The two mandrels should contact each other in the central section so that there is no sharp transition, if possible, in order to form a smooth inner surface in the fluid channel.

[0030] In this way, for example, a constriction of the measuring tube can be produced from the axial end toward the central section, the fluid channel in the central section having a smaller cross-sectional area than the fluid connection. To this end, each mandrel has a smaller cross-sectional area at the region in the central section than at the axially adjacent region.

[0031] Preferably, a transition region is formed from one axial end of the semi-finished product to the central section, wherein the inner cross section and / or the inner geometry of the axial end and the central section merge into one another without a step. The inner cross section and / or the inner geometry can be easily predetermined by the outer geometry, i.e. the shape of the outer circumferential surface of the mandrel. The outer geometry of the mandrel then deviates in the region arranged in the central section from the outer geometry in the axially adjacent region. Since the shape of the transition region is predetermined by the mandrel, a high-quality transition region can be produced, which is evident in a stable flow profile in the fluid channel.

[0032] In particular, the inner cross section in the central section may be polygonal, whereas the cross section located axially further outward at the fluid connection may be circular.

[0033] In another variant, only one mandrel is used, which is inserted into the through hole and the semi-finished product is deformed around the mandrel. The mandrel is then pulled out of the through hole to one side.

[0034] Subsequently, the mandrel may be pulled through the through-hole once or several times in order to calibrate the internal geometry of the fluid channel in a calibration step, ie to adjust it more precisely to the predetermined dimensions.

[0035] In such a quasi-step, several mandrels, which are adapted to one another in their outer contour, can be successively guided through the fluid channel in order to obtain a precise geometry of the inner side of the fluid channel.

[0036] It is also conceivable to perform one or more method steps with two mandrels inserted in opposite directions and one or more other method steps with only one mandrel. For example, a calibration step for a fluid channel can be performed with only one mandrel.

[0037] The cross-sectional shape of the fluid connector and the fluid channel can be selected by those skilled in the art. For example, an equilateral polygon with 3 to 8 sides can be used for the fluid channel. However, the polygon can also be a rectangle. Shapes with round and flat sides are also conceivable.

[0038] In order to optimize the forming process, the shape of the semi-finished product and the forming tool can be selected so that during the forming step, the punching force is applied only in the segments along the circumference of the measuring tube. Thus, the outer circumference of the central segment remains approximately the same in the segments before and after the forming step. For example, before forming, the corners of the polygon can be located on the outer circumference of the semi-finished product. Preferably, such segments are located outside the waveguide area.

[0039] The axial end section of the semi-finished product can remain undeformed during the shaping step. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The invention will be described in more detail below with reference to several example embodiments and with reference to the accompanying drawings, in which:

[0041] Figure 1 A schematic partial sectional view shows a measuring tube of a fluid measuring device produced by the method according to the invention;

[0042] Figure 2 and Figure 3 The steps of the method according to the invention for producing a measuring tube according to a first variant are shown;

[0043] Figure 4 and Figure 5 A further step of the method according to the invention for producing a measuring tube in a forming tool is shown;

[0044] Figure 6 A measuring tube produced by the method according to the invention is shown;

[0045] Figures 7 to 9 The steps of the method according to the invention for producing a measuring tube of a fluid measuring device according to a second variant are shown;

[0046] Figures 10 to 12 Shown are measuring tubes with different geometries as produced by the method according to the invention;

[0047] Fig.13 Different internal cross-sectional shapes of the fluid channel of a measuring tube produced by the method according to the invention are shown;

[0048] Fig.14 and Fig.15 A measuring tube produced by the method according to the invention is shown with a transition region between a central section and an axially adjacent region;

[0049] Fig.16 shows a schematic partial cross-sectional view of a measuring tube having different wall thicknesses produced by the method according to the invention; and

[0050] Fig.17 The method according to the invention for producing a measuring tube according to a further variant in a forming tool is shown. DETAILED DESCRIPTION

[0051] Figure 1 Shown is a measuring tube 10 of a flow measuring device, not shown in detail, which operates according to the principle of measuring surface acoustic waves (SAW).

[0052] The measuring tube 10 is produced from a tubular semi-finished product 14 (see Figure 2 and Figure 7 ).

[0053] The measuring tube 10 has a continuous fluid channel 16 which extends between two fluid connections 18 located on opposite sides of the measuring tube 10. A suitable fluid to be measured flows through the fluid channel 16 during a measurement.

[0054] Formed on each of the fluid connections 18 is a connecting structure 20 , here in the form of two radially protruding flanges, via which connecting structure the measuring tube 10 can be connected to the fluid system.

[0055] One or more waveguides 24 are formed in the circumferential wall 22 of the measuring tube 10. The waveguide 24 is here the region of the measuring tube 10 in which transmitters and receivers (not shown) for surface acoustic waves are arranged and in which the coupling-in and coupling-out processes of the surface acoustic waves relevant for the measurement also take place.

[0056] For example, if several waveguides 24 are provided, these are distributed along the circumferential direction U, but are arranged at the same position in the axial direction. The waveguide 24 is formed integrally with the circumferential wall 22 and extends continuously from the outer surface 26 to the inner surface 28 of the measuring tube 10. The waveguide 24 is positioned in an axial center section 30 of the measuring tube 10 between two fluid connections 18. All waveguides 24 extend along the longitudinal direction L of the measuring tube 10, along which the fluid channel 16 also extends. Each of the waveguides 24 is flat and rectangular here, with the long side of the rectangle extending along the longitudinal direction L.

[0057] In the fluid measuring device, the waveguide 24 is part of the acoustic measuring device, into which the surface acoustic waves are coupled and out of which they are coupled.

[0058] A total of at least two signal transmitters are arranged on the waveguide 24 , which can excite a surface acoustic wave in the waveguide 24 or receive a surface acoustic wave from the waveguide (not shown).

[0059] The waveguide 24 forms a boundary with the fluid flowing through the fluid channel 16, and the portion of the surface acoustic wave that travels through the waveguide 24 is coupled out into the fluid, passes therethrough and is coupled again at a different location in the same waveguide 24 or a different waveguide 24 and received by one or more of the signal transmitters.

[0060] Figures 2 to 6 The production of a measuring tube 10 from a tubular semi-finished product 14 is shown with reference to a first example.

[0061] First, the initial cylindrical tubular semi-finished product 14 (see Figure 2 ) is optionally processed in a suitable manner so that a connection structure 20 is produced at the fluid connection piece 18 at the end section 31 of the semi-finished product 14, which still has a cylindrical cross section (see Figure 3 ). These connecting structures 20 can be produced, for example, by milling and / or turning.

[0062] The semi-finished product 14 has a through-opening 32 which, when finished, forms the fluid channel 16 of the measuring tube 10 .

[0063] In this example, the semi-finished product 14 has a relatively large wall thickness s 1 , so that material removal can be performed on the outer surface 26 to produce the flange.

[0064] The central section 30 remains unprocessed here, but may optionally already be provided with a suitable structure 42 in this step (see, for example, Figures 14 to 16 ), wherein, for example, the inclination of the outer surface 26 or the wall thickness of the semi-finished product 14 is varied. Such structures 42 form in particular damping elements and / or reflection elements for surface acoustic waves in order to concentrate these surface acoustic waves in the region of the subsequent waveguide 24.

[0065] After the pretreatment steps, if necessary, the semi-finished product 14 is placed in a forming tool 43 (see Figure 4 ).

[0066] On the outer surface 26 of the semi-finished product 14 , which later forms the outer surface 26 of the measuring tube 10 , one or more waveguide regions 34 are predefined, which form the waveguide 24 in the finished measuring tube 10 .

[0067] In a variant, the waveguide region 34 is physically defined on the semi-finished product 14, for example by means of a machining step already carried out beforehand, in which, for example, the outer surface 26 is specifically machined and the structure 42 is formed. In this case, the semi-finished product 14 is placed in the forming tool 43 so that the waveguide region 34 is correctly aligned in the circumferential direction U.

[0068] In another variant, the waveguide area 34 is obtained by forming the waveguides 24 at these points of the circumferential wall 22 during a subsequent forming step. Figure 2 If the cylindrical shape has not yet been deviated from, purposeful alignment can be omitted.

[0069] The forming tool 43 comprises two opposing mandrels 36 which are linearly displaceable along the longitudinal direction L and, in a variant, whose outer geometry 38 corresponds exactly in diameter and shape to the desired inner geometry of the fluid channel 16. The dimensions of the mandrels 36 are selected so that they can be inserted into the through-holes 32 along the longitudinal direction L without deforming the semi-finished product 14. This is particularly important in Figure 4 Shown in.

[0070] The two mandrels 36 are inserted into the through-holes 32 until they abut against each other substantially seamlessly in the center of the center section 30 .

[0071] Compared to the axially adjacent section in the region of the fluid connection 18, the end of the mandrel 36 facing the center of the semi-finished product 14 is conical in cross section (see Figure 4 and Figure 5 ). In addition, these conical ends here have a cross-sectional shape that is different from the axially adjacent region. In the transition region, the cross-sectional shapes merge into each other continuously and without steps.

[0072] The forming tool 43 comprises several punching punches 40 which can be radially displaced relative to the longitudinal direction L. The semi-finished product 14 is held in the forming tool 43 so that the punching punches 40 act only on the central section 30. Figure 5 In the forming step of applying a punching force F to the axial center section 30 of the semi-finished product 14, the center section 30 is plastically deformed. The punching punch 40 is radially displaced so that the circumferential wall 22 in the center section 30 is pressed against the mandrel 36 and thus plastically deformed. This forming step can be performed as a hot forming step or a cold forming step.

[0073] Thus, the through hole 32 is formed as the fluid channel 16 .

[0074] The geometry of the central section 30 is predetermined at the outer surface 26 by the contour of the stamping punch 40 and at the inner surface 28 of the through hole 32 by the outer geometry 38 of the mandrel 36. The fluid channel 16 thus produced is endowed with the outer geometry 38 of the mandrel 36 as its inner geometry.

[0075] Depending on the elastic component of the deformation, for example, the forming tool 43 is closed only once or several times in order to produce the desired inner geometry of the fluid channel 16 .

[0076] In another variant, the outer geometry 38 of the mandrel 36 is selected to be slightly smaller than the desired cross section of the fluid channel 16 in order to take into account the springback of the material of the measuring tube 10 after the forming step.

[0077] In a further variation, several successive forming steps are performed with several mandrels 36 having different outer geometries until the fluid channel 16 is completed with the desired dimensions.

[0078] In this shaping step or these shaping steps, the waveguide region 34 is also formed into the final waveguide 24. The axial region in which the waveguide 24 extends forms the measuring region 44 in the finished measuring tube 10.

[0079] In the axial section of the measuring tube 10 in which the waveguide 24 is arranged, a constriction 46 is also produced here, wherein the cross section of the fluid channel 16 is conical compared to the cross section at the fluid connection 18 ( Figure 6 ).

[0080] A transition region 48 extends axially between the connection structure 20 and the waveguide 24, in each case, in which the cross section from the measuring region 44 to the fluid connection 18 widens continuously and without steps (see, for example, Figure 1 and Figure 5 ).

[0081] After the forming step, the mandrel 36 is pulled out of the through-opening 32 again in the longitudinal direction L and the forming tool 43 is opened, so that the finished measuring tube 10 can now be removed.

[0082] The axial end section 31 in the region of the connecting structure 20 and the fluid connection 18 remains deformed here.

[0083] The structures 42 , such as damping elements and / or reflective elements for surface acoustic waves, including circumferential walls 22 of varying thickness, which have been formed on the semi-finished product 14 before the shaping step, remain at least substantially unchanged during the shaping of the semi-finished product 14 .

[0084] If necessary, before the measuring tube 10 is removed from the forming tool 43, the mandrel 36 is pushed back into the through hole 32 (which has now been formed into the fluid channel 16) and the mandrel is pulled out again to smooth the inner surface 28 of the fluid channel 16. It is also conceivable to use other mandrels 36 for this step, which are similar in shape to the mandrel 36 used for forming, but which define the inner surface 28 of the fluid channel 16 more accurately, for example in a calibration step.

[0085] Optionally, further post-processing can take place, for example in order to form further elements on the outer surface 26 of the measuring tube 10. The inner surface 28 of the fluid channel 16 is not post-processed here.

[0086] However, it is conceivable, for example, to further smooth the inner surface 28 using a grinding fluid.

[0087] In the example shown, the waveguide region 34 which is rounded in the circumferential direction U is formed as a flat rectangular waveguide 24 (see, for example, Figure 1 and Figure 6 ).

[0088] In the measurement region 44, the circular cross-sectional shape of the through hole 32 is formed into a substantially square cross-sectional shape of the fluid channel 16 having four planes adjacent to each other at right angles and rounded corners (see, for example, Figure 6 ). The waveguide 24 is formed here on each side of the square cross section.

[0089] In the transition region 48, not only does the diameter of the fluid channel 16 change, but also the cross-sectional shape of the fluid channel changes from polygonal to circular.

[0090] For fluid channels 16 whose internal geometry is not conical compared to the fluid connection 18, it is also possible to use only a single mandrel 36, which is then inserted into the through hole 32 over the entire central section 30 (see Fig.17 ). The cross section of the mandrel 36 is no larger than the narrowest cross section of the formed fluid channel 16, so that the mandrel 36 can be completely pushed into the central section 30 and pulled out again after forming. The figure shows the step of pulling the mandrel 36 out of the fluid channel 16 after forming.

[0091] In this case, as also described above, several mandrels 36 with slightly different outer geometries are optionally used to compensate for elastic deformations of the measuring tube 10. The mandrel 36 whose outer geometry corresponds exactly to the cross section of the fluid channel 16 is finally optionally moved through the through hole 32 once or several times in order to complete the fluid channel 16 exactly with the desired cross section.

[0092] Fig.13A number of possibilities are shown for the cross-sectional area 50 of the fluid channel 16 located in the measuring region 44, for example a polygon with 3 to 8 faces, wherein the polygon can be regular or irregular, or a mixed shape with circular and flat surfaces. Of course, substantially elliptical or circular cross-sectional shapes are also conceivable.

[0093] Figures 7 to 9 The shaping of a semi-finished product 14 is shown with reference to a second example.

[0094] In contrast to the first example just described, the semi-finished product 14 has a wall thickness s less than 1 Wall thickness s 2 .

[0095] As described above, the axial end 31 of the fluid connector 18 is machined to produce a suitable connection structure 20 (see FIG. Figure 8 ).

[0096] The semi-finished product 14 is then placed in the forming tool 43 and plastically deformed by the punching punch 40 and the inserted mandrel 36 . Fig. 9 The result of the forming step is shown.

[0097] In this example, the measuring region 44 has a square cross-sectional shape, as in the first example, including the fluid channel 16 which also has a square cross-sectional shape. However, as mentioned above, other cross-sectional shapes may also be realized.

[0098] In contrast to the first example, the forming tool 43 is designed so that the punching force is applied only in sections along the circumference. In this example, these are the subsequent flat side surfaces. No direct force is applied to the intermediate regions (here the corners of the square cross section). In these regions, the diameter d of the finished measuring tube 10 corresponds approximately to the diameter d of the semi-finished product 14 (e.g. Figure 7 and Fig. 9 ), and the original outer surface 26 and the original outer circumference of the semi-finished product 14 are at least partially retained.

[0099] During the shaping step, the previously processed connection structure 20 is not deformed at all or only slightly. This also applies to any structures 42 that may be present, as in the first example described above.

[0100] Figures 10 to 12 Further possible exemplary embodiments of the finished measuring tube 10 are shown.

[0101] exist Fig.10 In the embodiment, the measuring region 44 is formed as a rectangular cross section. Therefore, in the circumferential direction U, the lengths of the sides of the cross section are not equal. For example, the waveguide 24 is formed only on the two longer sides.

[0102] In addition, the connection structure 20 is not designed as a flange here, but is designed as a thread. As described above with respect to the flange, the thread can also be prefabricated before the forming step and will not be deformed in the forming step.

[0103] Fig.11 A measuring tube 10 is shown which combines a square cross section of the fluid channel with a different type of flange at the fluid connection 18 .

[0104] at last, Fig.12 A measuring tube 10 is shown which has a circular cross section of the fluid channel 16 in the measuring region 44 and a flange at the fluid connection 18 .

[0105] Of course, any other design and combination of the elements shown is possible according to the judgment of a person skilled in the art. 1 、s 2 With several variations in the aspect of FIG. 1 , a large number of variations of the measuring tube 10 can be produced by changing the stamping punch 40 and the mandrel 36 .

[0106] Figures 14 to 16 The structure 42 already mentioned above is shown in more detail.

[0107] In the transition region 48 , a damping and / or reflecting element which tapers towards the measuring region 44 is formed for each of the waveguides 24 on the measuring tube 10 , wherein the curvature of the fluid connection 18 which is circular in the peripheral direction U merges steplessly into the planar surface of the waveguide 24 .

[0108] Furthermore, in the transition region 48 and in the structure 42 , the cross section of the measuring tube 10 decreases from the cross section of the fluid connection 18 to the cross section of the fluid channel 16 .

[0109] Such structures 42 may be prefabricated on the semi-finished product 14 prior to the forming step, produced by the stamping punch 40 and the mandrel 36 during the forming step, or produced using a combination of these two methods.

[0110] Fig.16 It is shown that in this example the wall thickness of the measuring tube 10 also changes from the waveguide 24 in the measuring region 44 to the fluid connection 18 and in this example from the wall thickness t 1 Through the wall thickness t 2 Increase to wall thickness t 3 .

[0111] Furthermore, by way of example, the wall thickness can optionally also vary in different circumferential regions of the measuring region 44, for example for different waveguides 24 or within a waveguide 24 and outside a waveguide 24 (here indicated by t 1 and t4 express).

[0112] These different wall thicknesses are produced, for example, during a forming step by which the semi-finished product 14 is formed.

[0113] All features of the individual embodiments and variants can be freely exchanged or combined with one another at the discretion of a person skilled in the art.

[0114] For the sake of clarity, identical components are not always provided with reference numerals.

Claims

1. A method for manufacturing a measuring tube (10) of a fluid measuring device, the method comprising the following steps: A tubular semi-finished product (14) is provided, the tubular semi-finished product having a through hole (32) extending in the longitudinal direction (L) of the semi-finished product (14), the circumferential wall (22) of the semi-finished product (14) having at least one waveguide region (34), the at least one waveguide region forming a waveguide (24) for surface acoustic waves of an acoustic measuring device after the measuring tube (10) is completed, the waveguide region (34) extending along the longitudinal direction (L) of the semi-finished product (14) and extending from an outer surface (26) to an inner surface (28) of the semi-finished product (14), inserting the semi-finished product (14) into a forming tool (43), at least one mandrel (36) is introduced into the through-opening (32), the outer geometry (38) of the mandrel (36) corresponding to the inner geometry of the fluid channel (16) of the measuring tube to be formed by the through-opening (32), performing a forming step, in which a punching force (F) is applied to at least one axial central section (30) of the semi-finished product (14), and at least the central section (30) is plastically deformed, the through hole (32) being formed into the fluid channel (16), and The mandrel (36) is removed.

2. The method according to claim 1, wherein: Prior to the shaping step, the semi-finished product (14) has an external geometry deviating from a cylindrical form and defining a damping element and / or a reflective element for surface acoustic waves, a waveguide (24), which external geometry is at least partially maintained during the shaping step.

3. A method according to any one of the preceding claims, wherein: The semi-finished product (14) has regions with different wall thicknesses (t1, t2, t3, t4) in the central section (30) and / or in axially adjacent sections, and wherein these different wall thicknesses (t1, t2, t3, t4) are at least partially maintained during the forming step.

4. A method according to any one of the preceding claims, wherein: The outer geometry of the central section (30) located in the at least one waveguide region (34) is formed from a circular surface in the circumferential direction (U) to a flat surface in the circumferential direction (U).

5. A method according to any one of the preceding claims, wherein: After the at least one mandrel (36) is removed, the same mandrel (36) or another mandrel (36) is moved again through the through hole (32).

6. A method according to any one of the preceding claims, wherein: Two mandrels (36) are used simultaneously, which are moved from the axial ends of the semi-finished product (14) into the through hole (32) and out of the through hole.

7. The method according to claim 6, wherein: Each mandrel (36) has a smaller cross-sectional area (50) at a region arranged in the central section (30) than at an axially adjacent region.

8. The method according to claim 7, wherein: In the forming step, a transition region (48) is formed accordingly from an axial end of the semi-finished product (14) to the central section (30), wherein the inner cross-sections and / or inner geometries of the axial end and the central section (30) merge into one another without steps.

9. The method according to any one of claims 1 to 5, wherein: Precisely one mandrel (36) is introduced into the through-hole (32).

10. A method according to any one of the preceding claims, wherein: In the forming step, a punching force (F) is applied only in sections along the circumference (U) of the measuring tube (10).

11. A method according to any one of the preceding claims, wherein: The axial end section (31) of the semi-finished product (14) remains deformed during the forming step.

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