Partially polished tube arrangement by forced electrolytic plasma polishing, in particular measuring tube arrangement, measuring sensor having such measuring tube arrangement, and method for plasma polishing measuring tube arrangement

Abstract

The invention relates to a measuring tube arrangement (10), comprising: two support bodies (21, 22), each having at least one duct (23, 24, 25, 26); and at least one tube, in particular a measuring tube (11, 12) having a lumen for conducting a medium, the tube, in particular the measuring tube (11, 12), having a first end portion and a second end portion, each of which is connected to one of the support bodies (21, 22), wherein the ducts (23, 24, 25, 26) of the support body communicate with one another via the lumen of the tube, in particular of the measuring tube, and wherein the tube arrangement, in particular the measuring tube arrangement (10), has a tube section (41, 42) polished by forced electrolytic plasma polishing, the tube section (41, 42) extends at least three and in particular at least four diameters of the at least one tube, in particular the measuring tube, in a region in which the flow cross-section comprises at most the cross-section of the at least one tube, in particular the cross-section of the measuring tube, the pipe section (41, 42) polished by the forced electrolytic plasma polishing is followed by an etched section (43, 44), wherein the surface structure of the etched section (43, 44) differs from the surface structure of the pipe section (41, 42) polished by the forced electrolytic plasma polishing.

Description

Technical Field

[0001] The invention relates to a tube arrangement, in particular a measuring tube arrangement, which is polished in sections by forced electrolytic plasma polishing, a measuring sensor having such a measuring tube arrangement, and a method for plasma polishing a tube arrangement, in particular a measuring tube arrangement. Background Art

[0002] A general tube arrangement, in particular a measuring tube arrangement, comprises: two support bodies, each of which has at least one channel; and at least one tube, in particular a measuring tube, which has a lumen for guiding a medium, wherein end sections of the at least one measuring tube are each connected to one of the support bodies, wherein the tube, in particular at least one end section of the measuring tube, is subjected to plasma polishing.

[0003] In the published patent application DE 2016 100 558 A1 a method and a polishing head for plasma polishing of a tube are described, wherein the polishing head is arranged coaxially with the tube and serves as a cathode and is continuously supplied with an electrolyte, and the polishing head moves through the tube in order to perform the polishing. Chinese utility model CN ​​2020 72789 U discloses a device for electropolishing the inner wall of a thin-walled tube, in which the cathode wiring is guided along the tube axis and the electrolyte is pumped through the tube to generate a plasma. However, these methods prove to be problematic, in particular for tubes of small diameter or for curved measuring tubes, since it is difficult to center the cathode and, in addition, the available volume for forming the plasma layer and supplying the electrolyte is further limited. As long as the bending of the measuring tube or the engagement of the end section of the measuring tube with the support body results in an impairment of the surface properties of the measuring tube in the affected area, it may be advisable to post-treat the surface in order to restore the surface quality. The object of the present invention is therefore to remedy this situation, wherein the solution sought should be suitable for standardization of industrial processes regardless of the shape of the tube, ie not only for curved tubes or measuring tubes. Summary of the invention

[0004] According to the invention, this object is achieved by a pipe arrangement according to claim 1 , a measuring sensor according to claim 6 and a method according to claim 8 .

[0005] The tube arrangement, in particular the measuring tube arrangement, according to the invention comprises two support bodies, each having at least one bore, and at least one tube, in particular the measuring tube, having a lumen for conducting a medium, wherein end portions of the at least one tube, in particular the measuring tube, are each connected to one of the support bodies, wherein the bores of the support bodies communicate with one another via the lumen of the tube, in particular the measuring tube, The tube arrangement, in particular the measuring tube arrangement, has a section which is polished by forced electrolytic plasma polishing and which extends in a region where the flow cross section includes at most a cross section of the at least one tube, in particular a cross section of the measuring tube, by at least three diameters and in particular at least four diameters of the at least one tube, in particular the measuring tube, wherein the section polished by forced plasma polishing is followed by an etched section, wherein the surface structure of the etched section differs from the section polished by forced plasma polishing.

[0006] In a development of the invention, the etched portion comprises an electropolished surface having a roughness greater than that of the forced plasma polished surface of the first portion.

[0007] In a development of the invention, the surface roughness Ra of the portion polished by forced plasma polishing is not greater than Ra=0.4 µm, in particular not greater than Ra=0.3 µm.

[0008] The tube, in particular the measuring tube, has a metal structure, wherein in a development of the invention the etched section has a surface structure in which grain boundaries of the metal structure are etched away.

[0009] In a development example, the tube, in particular the measuring tube, has a curved profile, which at least partially has at least one arc segment, the at least one arc segment in particular covers an angle of not less than 20°, in particular covers an angle of not less than 30°, wherein the radius of curvature of the arc segment is, for example, not greater than 10 inner diameters and in particular not greater than 6 inner diameters of the tube, in particular the measuring tube, wherein the radius of curvature of the arc segment is, for example, not greater than 10 inner diameters and in particular not greater than 6 inner diameters of the tube, in particular the measuring tube, wherein the portion polished by forced plasma polishing includes the at least one arc segment over at least half of the length of the at least one arc segment, in particular over at least two thirds of the length of the at least one arc segment.

[0010] In a development of the invention, the inner diameter of the tube, in particular the measuring tube, is not greater than 30 mm, for example not greater than 15 mm and in particular not greater than 10 mm.

[0011] The measuring sensor according to the invention comprises: a measuring tube arrangement according to the invention; a base body which rigidly connects the two supporting bodies to one another; an electrodynamic exciter for exciting bending vibration modes; and at least one vibration sensor for detecting bending vibrations of the measuring tube.

[0012] In a development example of the invention, the measuring tube arrangement of the measuring sensor comprises two parallel measuring tubes, wherein the electrodynamic exciter is designed to excite bending vibrations in at least one bending vibration mode between the measuring tubes, and wherein the at least one vibration sensor is designed to detect bending vibrations between the measuring tubes.

[0013] The method according to the invention for partial plasma polishing of a measuring tube arrangement comprises: positioning the cathode and the electrolyte supply line relative to a measuring tube arrangement, wherein the cathode has an end head which is spaced from an end face of the tube arrangement, in particular the measuring tube arrangement, by no more than two inner diameters and in particular by no more than one inner diameter of a tube of the tube arrangement, in particular the measuring tube, in particular the measuring tube; allowing electrolyte to flow through the measuring tube arrangement; and applying an electropolishing voltage between the cathode and the measuring tube arrangement, the electropolishing voltage being sufficient to maintain a plasma in the tube arrangement, in particular in a portion of the measuring tube arrangement, the portion extending from an end face of the measuring tube arrangement over a length of not less than four inner diameters, in particular not less than six inner diameters and preferably not less than eight inner diameters of the tube, in particular a measuring tube, into the measuring tube arrangement, The measuring tube arrangement comprises: two support bodies, each having at least one channel; and at least one tube, in particular a measuring tube, having a lumen for guiding a medium, wherein end portions of the at least one tube, in particular the measuring tube, are each connected to one of the support bodies, wherein the channels of the support bodies are connected to each other via the lumen of the tube, in particular the measuring tube, wherein at least one end portion of the tube, in particular the measuring tube, is polished by forced plasma polishing.

[0014] In a development of the invention, the volume flow rate of the electrolyte over each cross-sectional area through which the measuring tube arrangement can flow is not less than 0.1 l / (min•cm 2 ), for example not less than 0.2 l / (min•cm²), in particular not less than 0.3 l / (min•cm 2 ).

[0015] In a development of the invention, the volume flow rate of the electrolyte over each cross-sectional area through which the measuring tube arrangement can flow is no greater than 4 l / (min·cm 2 ), for example not more than 2 l / (min•cm²), and in particular not more than 1 l / (min•cm 2 ).

[0016] By allowing the electrolyte to flow, waste from the polishing process is effectively transported away from the forced plasma polishing surface portion of the measuring tube arrangement and plasma-supported bubbles are prevented from being washed away.

[0017] In a development of the invention, the applied polishing voltage is not less than 250 V, in particular not less than 300 V. Furthermore, according to a development of the invention, the applied polishing voltage is not more than 500 V, in particular not more than 400 V.

[0018] In a development of the invention, the electrolyte comprises ammonium sulfate and optionally sulfuric acid.

[0019] In a development of the invention, the plasma is maintained for a period of not less than 2 minutes, in particular not less than 3 minutes, while the electrolyte is flowing.

[0020] In a development of the invention, the plasma is maintained for a period of not more than 10 minutes, in particular not more than 6 minutes, while the electrolyte is flowing.

[0021] In a development, during the plasma polishing, the measuring tube arrangement is completely filled with the electrolyte.Complete filling with the electrolyte may also comprise gas loading of the electrolyte or solid loading of the electrolyte.

[0022] In a development of the invention, the electrolyte comprises ammonium sulfate and possibly sulfuric acid.

[0023] In a development, the conductivity of the electrolyte is not greater than 500 S / cm, in particular not greater than 400 mS / cm and preferably not greater than 350 mS / cm. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention will now be explained in more detail based on exemplary embodiments shown in the drawings.

[0025] In the figure:

[0026] Figure 1 : shows a schematic sectional view of an embodiment of a measuring sensor according to the invention, which has an embodiment of a measuring tube arrangement according to the invention;

[0027] Figure 2a : shows a cross-sectional view of a measuring tube treated by the plasma polishing process according to the present invention;

[0028] Figure 2b : shows a cross-sectional view of a measuring tube processed by a conventional plasma polishing process;

[0029] Figure 3a: shows an exemplary schematic graph of a current density distribution in the longitudinal direction of a measuring tube section during implementation of the method according to the invention;

[0030] Figure 3b : A graph showing the surface roughness of a measuring tube component processed according to the present invention compared to a measuring tube component processed by a prior art method;

[0031] Figure 4 : shows a schematic diagram of an exemplary embodiment of an arrangement for implementing the method according to the invention;

[0032] Figure 5 : A flow chart for implementing the method according to the present invention is shown.

[0033] Figure 6a : shows a schematic diagram of a second embodiment of an arrangement for implementing the method according to the invention; and

[0034] Figure 6b : Schematic diagram showing a portion of a measuring sensor with a flange mounted after plasma polishing. DETAILED DESCRIPTION

[0035] Figure 1The measuring sensor 1 shown is used to measure one or more measured variables selected from the mass flow rate, density and viscosity of a medium, and comprises a measuring tube arrangement 10, which here has two - here straight - measuring tubes 11, 12 and two support bodies 21, 22. The support bodies here serve as distributor parts, each of which comprises through-holes 23, 24, 25, 26, wherein the measuring tubes 11, 12 are each fixed with their two end portions in one of the holes 23, 24, 25, 26, for example by joining (such as welding or brazing), rolling, pressing, tightening or gluing or by a combination of the above methods. These holes are only schematically shown here. Their specific form depends on fluid considerations, but these considerations are not within the scope of the present invention. In this embodiment, process connections (here flanges 27, 28) are attached to the support bodies 21, 22 for mounting the measuring sensor in a pipeline through which the medium to be measured flows. In particular, the flanges 27, 28 are mounted after forced plasma polishing. The measuring sensor further comprises a base body 20, by means of which two support bodies 21, 22 are rigidly connected to one another in order to suppress vibrations between the support bodies at least in the range of the natural frequencies of the bending vibration modes or torsional vibration modes of the measuring tubes 11, 12, which vibrations are to be evaluated in order to determine the measured variable of the measuring sensor 1. The measuring sensor further comprises an electrodynamic exciter 30, which acts between the measuring tubes 11, 12 to excite the bending vibration modes in particular between the measuring tubes 11 and 12, and two electrodynamic vibration sensors 31, 32 for detecting bending vibrations between the measuring tubes 11, 12.

[0036] According to the invention, the measuring tube arrangement 10 has end sections 41, 42 which have been polished by forced plasma polishing and which extend from the end face of the measuring tube arrangement 10 facing away from the measuring tube through the bores 23, 24, 25, 26 of the support bodies 21, 22 into the measuring tubes 11, 12. Each forced plasma polished end section 41, 42 is followed by an etched section 43, 44 which here extends over the length of several measuring tube diameters, wherein the etched section 43, 44 merges into the electropolished section 45, 46. The surface properties of the individual sections are explained below.

[0037] like Figure 2a The detailed view of the longitudinal section of half of the measuring tube 111 according to another embodiment is shown in FIG. Figure 1 In contrast to the exemplary embodiments, the measuring tube can have a curved profile in the rest position. Figure 2aThe measuring tube 111 shown has an exciter holder 130 for an electrodynamic exciter in the peak of the central measuring tube bend, wherein a measuring tube cross-sectional plane extends through the holder 130, with respect to which the measuring tube 111 has a mirror-symmetrical profile. The measuring tube arrangement also comprises two electrodynamic vibration sensors (not shown here), which are arranged symmetrically with respect to the measuring tube cross-sectional plane, and Figure 2a 1 shows a first sensor holder 131 for holding a first electrodynamic vibration sensor of the electrodynamic vibration sensor. The measuring tube 111 has an end portion with a measuring tube bend at the end, adjacent to which a short straight portion for fastening in a bore of a support body (not shown here) is attached. The measuring tube 111 has an end portion 141 polished by forced plasma polishing, which extends in the longitudinal direction from the end face of the measuring tube or the support body to approximately the middle of the end bend 141, followed by an etched portion 143, which in turn merges into an electropolished portion 145, which here extends approximately up to half of the straight portion of the measuring tube 111 between the end measuring tube bend and the middle measuring tube bend.

[0038] As a comparison, Figure 2b A portion of the measuring tube 211 is shown which has not been polished by forced plasma polishing and which is therefore not according to the invention. The measuring tube 211 has the same arrangement as the measuring tube according to the invention with regard to its components. The measuring tube 211 therefore also has an exciter holder 230 for an electrodynamic exciter in the peak of the central measuring tube bend, wherein a measuring tube cross-sectional plane extends through the holder 230 and the measuring tube 211 has a mirror-symmetrical profile with respect to this plane. The measuring tube arrangement also comprises two electrodynamic vibration sensors (not shown here) which are arranged symmetrically with respect to the measuring tube cross-sectional plane, Figure 2b 2 shows a first sensor holder 231 for holding a first electrodynamic vibration sensor of the electrodynamic vibration sensor. The measuring tube 211 has an end portion with a measuring tube bend at the end, to which a short straight portion is connected for fastening in a bore of a support body (not shown here). The measuring tube 211 has a short plasma-polished end portion 241, which extends in the longitudinal direction from the end face of the measuring tube or the support body by only about one measuring tube diameter, followed by an etched portion 243, which in turn merges into an electropolished portion 245, which here only extends approximately to the end of the end measuring tube bend.

[0039] from Figure 2a and2b As can be seen from the comparison of FIG. 1 and FIG. 2 , the forced plasma polished portion 141 of the measuring tube arrangement according to the invention extends significantly further into the measuring tube 111 than the non-forced plasma polished portion 241 of the comparative measuring tube arrangement extends into the measuring tube 211. The same applies to the extension of the electropolished portion 145 in the longitudinal direction of the measuring tube 111.

[0040] Reference Figure 3a and 3b , various aspects of forced plasma polishing are now explained. During forced plasma polishing, an electrolyte having a temperature of, for example, not less than about 80°C flows through the measuring tube arrangement to be treated, wherein a polishing voltage is applied between a central cathode and the tubes, in particular the measuring tubes, which polishing voltage is sufficient to form a plasma, for example 350V. When the plasma is formed, a bubble film is formed on the surface of the measuring tube arrangement, which limits the current density in the area of ​​the plasma due to the electrical resistance of the bubble film. The flowing electrolyte continuously carries away larger bubbles and waste generated by the polishing process, so that in the area corresponding to the forced plasma polishing area of ​​the measuring tube arrangement, the conditions for plasma polishing remain stable. As the distance from the central cathode increases, the field strength decreases, so that beyond a certain distance the plasma can no longer be maintained. This eliminates the electrical resistance of the bubble film, and as Figure 3a As shown, significantly higher current density peaks occur, which in particular leads to etching away of the microstructure in the etched section 43. As the distance from the central cathode increases, the field strength continues to decrease, which also leads to a decrease in the current density with increasing distance. In particular, this leads to a continuous transition to conventional electropolishing of the surface, wherein the quality of the electropolishing decreases with increasing distance, i.e. the roughness increases with increasing distance from the cathode. For a measuring tube arrangement polished by forced plasma polishing, in Figure 3b The final profile of the roughness of the measuring tube arrangement in the longitudinal direction is schematically shown in the form of a solid line in FIG. The low roughness in the plasma polished region 41 is followed by an etched portion 43 which is etched down to the grain boundaries of the microstructure and which merges at a certain distance from the cathode into an electro-polished portion 45 whose roughness is increased compared to the plasma polished region 41. For example, the surface roughness of the plasma polished portion is not more than Ra=0.3 µm. The electro-polished region 45 has a roughness of, for example, 0.8 µm to 1 µm.

[0041] Figure 3bThe dash-dotted line in shows the roughness formed after the treatment of the measuring tube arrangement when the electrolyte is stationary, wherein this state, in combination with an electrolyte temperature of 80° C. and the applied polishing voltage, also leads to the formation of a plasma. However, the lack of electrolyte flow prevents stable conditions at voltages similar to those used in forced plasma polishing due to the elimination of gas bubbles and the transfer of removed material. The plasma can therefore only be maintained in a stable manner over relatively short distances. The sequence of the different zones is similar to the sequence after forced plasma polishing, but the extension of the plasma polishing zone in the longitudinal direction of the measuring tube arrangement is now greatly shortened, as already described according to Figure 2a and 2b As explained in the cross-sectional diagram in .

[0042] In contrast, Figure 3b The dashed line in shows the roughness variation along the measuring tube arrangement when processed by pure electropolishing. In this case, the roughness increases with increasing distance from the central cathode.

[0043] Figure 3b All examples have in common that the cathode is positioned so that its cathode end is close to the end face of the support body facing away from the tube, in particular the measuring tube, i.e. it protrudes only slightly into the channel of the support body or into the tube, in particular the measuring tube.

[0044] Figure 4 An embodiment of a suitable arrangement 350 for forced electropolishing is outlined in , wherein the arrangement 350 has a central pin-shaped stainless steel cathode 358, which is held in a cylindrical electrolyte line 354 on the longitudinal axis of the line 354 by means of a perforated disk 352. The electrolyte line 354 has a connection device 355 at its end, wherein an O-ring 356 is clamped axially in a fluid-tight manner between the end face of the connection device 355 and the end face of the support body 321 of the measuring tube arrangement 301 to be processed. The measuring tube 311 of the measuring tube arrangement is fixed in a through-going channel 323 of the support body 321. In particular, the support body can be a distributor part of a Coriolis mass flow measurement sensor. In the present embodiment, the electrolyte line and the cathode are positioned in front of the end face of the support body 323. However, this is not essential to the invention. There are designs in which the forced plasma polishing starts directly on the end face of the measuring tube. Only after forced plasma polishing is the process connection installed on the measuring tube arrangement. In this embodiment, the electrolyte line and the cathode will be positioned on the end face of the measuring tube, such as Figure 6aDuring plasma polishing of the measuring tube arrangement 501, the electrolyte line 554 and the cathode 558 are positioned on the end face of the measuring tube 511 protruding from the support body 521. After the plasma polishing according to the invention, the process connection flange 527 is mounted on the measuring tube arrangement.

[0045] Now we turn again Figure 4 During forced plasma polishing, the measuring tube arrangement is connected to circuit ground, wherein a polishing voltage Up of, for example, 350 V is applied to the cathode, wherein an electrolyte preheated to, for example, 80° C. is conveyed through the electrolyte line at a rate of, for example, 0.3 l / (min•cm ) per cross-sectional area of ​​the measuring tube arrangement. 2 ) to fill one or more measuring tubes of the measuring tube arrangement, through which the flow can pass. Since the cathode 358 is positioned in front of the measuring tube arrangement 301, the field distribution, the formation of the plasma and the transfer of polishing waste are not influenced in any way by solid bodies, such as the cathode or its power supply lines. This is a significant advantage, in particular in the case of small diameter measuring tubes or curved measuring tubes. In addition, the forced plasma polishing covers the area of ​​the measuring tube arrangement 301 extending from the end face of the support body 321 into the measuring tube 311, thereby greatly correcting surface quality damage that may have occurred, for example by fastening the measuring tube in the channel or by bending the measuring tube.

[0046] If the arrangement has a single cathode, this can be positioned in the region of the support body depending on the symmetry of the measuring tube arrangement, i.e. in a symmetry plane, at the junction of two symmetry planes or on an axis of rotational symmetry. If a plurality of cathodes are provided, the symmetry of all cathodes should follow the symmetry of the support body of the measuring tube arrangement. For example, the cathode can in each case be arranged on the axis of the bore for receiving the measuring tube.

[0047] Finally, reference Figure 5 Let's discuss the method steps of an embodiment 400 of the method according to the present invention.

[0048] The method first positions 410 a cathode and an electrolyte supply line relative to a measuring tube arrangement, wherein the cathode has an end head which is positioned, for example, approximately in the plane of an end face of the measuring tube arrangement.

[0049] Subsequently, an electrolyte is caused to flow 420 through the measuring tube arrangement at a flow rate of, for example, 8 cm / s, wherein the electrolyte is conveyed via an electrolyte supply line and is heated to a temperature of about 80° C., and wherein the electrolyte comprises an ammonium sulfate solution to which sulfuric acid has been added. The conductivity of the electrolyte is not greater than, for example, 350 mS / cm. With this limited conductivity, it is ensured that the potential required for plasma polishing is not dissipated over too short a distance.

[0050] While the electrolyte is flowing, an electropolishing voltage Up of 430 V, for example 350 V, is then applied between the cathode and the measuring tube arrangement, which is sufficient to maintain a plasma in a portion of the measuring tube arrangement that extends over several inner diameters of the measuring tube into the measuring tube arrangement. By applying this electropolishing voltage, a plasma with a bubble layer is generated on the surface of the measuring tube arrangement, the flowing electrolyte stabilizing the plasma, wherein surface defects of the measuring tube arrangement are eliminated by plasma polishing. As described above, the plasma subsides at a certain distance from the cathode, since the field strength there is insufficient to maintain the plasma. Since the gas layer as a resistor is thus eliminated, high current densities occur in a localized confined area, which leads to the etching away of the grain boundaries of the microstructure. In adjacent areas where the residual field strength is low, a quasi-conventional electropolishing is then carried out, the effectiveness of which decreases with increasing distance from the cathode, as described above.

[0051] For measuring tube arrangements with measuring tubes having an inner diameter of no more than 1 cm, the polishing time ends after 5 minutes, for example, because a sufficiently good surface quality, for example a roughness Ra<0.4 μm, is achieved in the region polished by forced plasma polishing.

[0052] This is followed by a final treatment 450 which consists in particular in flushing the measuring tube arrangement in order to remove the electrolyte without leaving any residues.

Claims

1. Measuring tube arrangement (10), including: Two supporting bodies (21, 22), each of the supporting bodies having at least one hole (23, 24, 25, 26); and at least one tube, in particular a measuring tube (11, 12) having a lumen for conducting a medium, wherein the tube, in particular the measuring tube (11, 12), has a first end portion and a second end portion, the first end portion and the second end portion are each connected to one of the support bodies (21, 22), wherein the bores (23, 24, 25, 26) of the support bodies communicate with each other via the lumen of the tube, in particular the measuring tube, The tube arrangement, in particular the measuring tube arrangement (10), has a portion (41, 42) polished by forced electrolytic plasma polishing, and the portion (41, 42) extends in a region where the flow cross section includes at most a cross section of the at least one tube, in particular a cross section of the measuring tube, by at least three diameters, in particular at least four diameters of the at least one tube, in particular the measuring tube, wherein the portion (41, 42) polished by forced plasma polishing is followed by an etched portion (43, 44), wherein the surface structure of the etched portion (43, 44) differs from the surface structure of the portion (41, 42) polished by forced plasma polishing.

2. The tube arrangement, in particular the measuring tube arrangement (10) according to claim 1, wherein: The surface roughness Ra of the portion (41, 42) polished by forced plasma polishing is not greater than Ra=0.4µm, in particular not greater than Ra=0.3µm.

3. The tube arrangement, in particular the measuring tube arrangement (10) according to claim 1 or 2, wherein: The tube, in particular the measuring tube, has a metal structure, and the etched section has a surface structure in which grain boundaries of the metal structure are etched away.

4. A tube arrangement, in particular a measuring tube arrangement (10) according to any of the preceding claims, wherein the tube, in particular the measuring tube, has a curved profile, which at least partially has at least one arc segment, the at least one arc segment in particular sweeping over an angle of not less than 20°, in particular sweeping over an angle of not less than 30°, wherein the radius of curvature of the arc segment is, for example, not greater than 10 inner diameters and in particular not greater than 6 inner diameters of the tube, in particular the measuring tube, wherein the portion polished by forced plasma polishing includes the at least one arc segment over at least half of the length of the at least one arc segment, in particular over at least two thirds of the length of the at least one arc segment.

5. The tube arrangement, in particular measuring tube arrangement (10) according to any of the preceding claims, wherein the inner diameter of the tube, in particular the measuring tube, is not greater than 30 mm, for example not greater than 15 mm and in particular not greater than 10 mm.

6. Measuring sensor (1), comprising: A measuring tube arrangement (10) according to any one of the preceding claims; A base body (20) which connects the two support bodies (21, 22) to each other in a rigid manner; an electrodynamic exciter which is used to excite a bending vibration mode; and at least one vibration sensor which is used to detect the bending vibration of the measuring tube.

7. A measuring sensor according to claim 6, wherein the measuring tube arrangement comprises two parallel measuring tubes, wherein the electrodynamic exciter is configured to excite bending vibrations in at least one bending vibration mode between the measuring tubes, and wherein the at least one vibration sensor is configured to detect bending vibrations between the measuring tubes.

8. Method for plasma polishing a tube arrangement, in particular a measuring tube arrangement, in sections, in particular to obtain a tube arrangement, in particular a measuring tube arrangement, according to any one of claims 1 to 7, the method comprising: positioning the cathode and the electrolyte supply line relative to a measuring tube arrangement, wherein the cathode has an end head which is spaced from an end face of the tube arrangement, in particular a measuring tube arrangement, by no more than four inner diameters, for example no more than two inner diameters, and in particular no more than one inner diameter, of a tube of the tube arrangement, in particular a measuring tube; allowing electrolyte to flow through the measuring tube arrangement; as well as applying a voltage between the cathode and the measuring tube arrangement, which voltage is sufficient to maintain a plasma in the tube arrangement, in particular in a portion of the measuring tube arrangement, which portion extends from an end face of the tube arrangement, in particular the measuring tube arrangement, into the tube arrangement, in particular the measuring tube arrangement, by a length of not less than four inner diameters, in particular not less than six inner diameters and preferably not less than eight inner diameters of the measuring tube, The tube arrangement, in particular the measuring tube arrangement, comprises: two support bodies, each of the support bodies having at least one bore; and at least one tube, in particular a measuring tube, having a lumen for conducting a medium, wherein end sections of the at least one measuring tube are each connected to one of the support bodies, wherein at least one end section of the measuring tube is forced plasma polished.

9. The method according to claim 8, wherein the volume flow rate of the electrolyte per flowable cross-sectional area of ​​the tube arrangement, in particular the measuring tube arrangement, is not less than 0.1 l / (min·cm 2 ), for example not less than 0.2 l / (min•cm²), in particular not less than 0.3 l / (min•cm 2 ), and / or The volume flow rate of the electrolyte per flowable cross-sectional area of ​​the measuring tube arrangement is not greater than 4 l / (min•cm 2 ), for example, no more than 2 l / (min•cm 2 ), and in particular not less than 1 l / (min•cm 2 ).

10. The method according to any one of claims 8 to 9, wherein the applied voltage is not less than 250 V, in particular not less than 300 V, and / or the applied voltage is not more than 500 V, in particular not more than 400 V.

11. The method according to any one of claims 8 to 10, wherein the electrolyte flows through the tube, in particular the measuring tube, at a temperature of not less than 70°C, in particular not less than 80°C.

12. The method according to any one of claims 8 to 11, wherein the plasma is maintained for a period of time of not less than 2 minutes, in particular not less than 3 minutes, while the electrolyte is flowing; and / or The plasma is maintained for a period of no more than 10 minutes, in particular no more than 5 minutes, while the electrolyte is flowing.

13. A method according to any one of claims 8 to 12, wherein the electrolyte comprises ammonium sulphate and optionally sulphuric acid.

14. The method according to any one of claims 8 to 13, wherein the conductivity of the electrolyte is not greater than 500 S / cm, in particular not greater than 400 mS / cm, and preferably not greater than 350 mS / cm.

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