DE102017006181A1 - Measuring device and method for determining a fluid size - Google Patents

Abstract

Measuring device (1) for determining a fluid size with a measuring tube (3) and a first and a second piezoelectric vibration transducer (5, 6), wherein the first and / or the second vibration transducer (5, 6) are controllable to one through a side wall (7) of the measuring tube (3) to excite guided wave, wherein the guided wave stimulates compression oscillations of the fluid, which are feasible and detectable via the fluid to the respective other vibration transducer (5, 6), wherein either the first and / or the second Vibration converter (5, 6) each have exactly one measuring tube side electrode (9) and exactly one measuring tube facing away from electrode (10), or wherein the first and / or the second vibration transducer (5, 6) a plurality of measuring tube side electrodes (9) and at least one electrode away from the measuring tube (10, 25, 26) or vice versa, wherein the control device (2) is adapted to specify the voltages between the electrodes (9, 10, 25, 26) such that the Sign of the voltage between a respective measuring tube side electrode (9) and a respective measuring tube facing away electrode (10, 25, 26) for all pairs of measuring tube side and measuring tube facing away electrodes (9, 10, 25, 26) of the respective vibration converter (5, 6) is the same ,

Description

The invention relates to a measuring device for determining a fluid quantity concerning a fluid and / or a fluid flow of the fluid with a control device, a measuring tube receiving the fluid and / or flowed through by the fluid, and a first and a second piezoelectric vibration transducer arranged at a distance from one another on the measuring tube. the first and / or the second vibration converter being controllable by the control device in order to excite a shaft guided through a side wall of the measuring tube, wherein the guided wave excites compression oscillations of the fluid, which can be guided via the fluid to the respective other vibration converter and there by the control device can be detected to determine measurement data, wherein the fluid size can be determined as a function of the measured data. In addition, the invention relates to a method for determining a fluid size.

One way to measure a flow through a measuring tube, are ultrasonic counters. In these, at least one ultrasonic transducer is used to couple an ultrasonic wave into the fluid flowing through the measuring tube, whereby it is guided on a straight path or after several reflections on walls or special reflector elements to a second ultrasonic transducer. From the transit time of the ultrasonic wave between the ultrasonic transducers or from a transit time difference at a permutation of transmitter and receiver, a flow rate through the measuring tube can be determined.

The publication US 4,735,097 A proposes to use ultrasonic transducers, which are fastened to the outside of the measuring tube, to simplify the measurement setup. These are used to induce guided waves in the measuring tube, whereby a lower accuracy in the arrangement of the ultrasonic transducers on the measuring tube is required. For coupling the guided waves, a wedge-shaped element is used whose longest side is pressed onto the pipe wall and at the shortest side of a piezoelectric element is arranged. This is vibrated to induce a guided wave in the tube wall via the wedge-shaped element. The disadvantage here is that the measurement setup used is relatively complex and large-scale. It is thus in many measurement situations in which a flow measurement is desired, not or only with great effort available. In addition, due to the use of the additional wedge-shaped element only a low degree of efficiency of the vibration coupling is achieved, so that the exciting piezoelectric element must be relatively large.

From the article G. Lindner, "Sensors and actuators based on surface acoustic waves propagating along solid-liquid interfaces", J. Phys. D: Appl. Phys. 41 (2008) 123002 , it is known to use for excitation of guided waves so-called interdigital transducers, in which a piezoelectric element is used, the one-sided comb-like intermeshing control lines to achieve excitation of certain excited modes of guided waves. Since shear modes of the piezoelectric element are necessarily excited, typically high excitation efficiencies are not achieved. In addition, a relatively complex, high-precision lithography is required to apply the required electrode structure with sufficient accuracy, although often sufficient mode purity of the excitation is not achieved.

However, excitation of a model pure guided wave is highly relevant for use in an ultrasound counter because the angle at which compression vibrations are radiated into the fluid depends on the phase velocity of the guided wave, which is typically different in different excitation modes at the same excited frequency. Thus, if different modes are excited, then different propagation paths for the compression vibrations in the fluid, which can possibly be excluded by a complex signal evaluation result.

The invention is therefore an object of the invention to provide a measuring device, in particular an ultrasonic meter, which uses guided waves for the determination of measured data, with a small space requirement and a simple structure to be realized and preferably as pure as possible excitation of guided waves to be achieved.

The object is achieved by a measuring device of the type mentioned above, wherein either the first and / or the second vibration transducer each have exactly one measuring tube side electrode and exactly one Meßrohrabgewandte electrode, which are opposite to the side wall of the measuring tube parallel side surfaces of the respective vibration transducer are arranged, or wherein the first and / or the second vibration transducer a plurality of measuring tube side electrodes and at least one measuring tube facing away electrode or a plurality of measuring tube facing electrodes and at least one measuring tube side electrode, wherein the control device is adapted to specify the voltages between the electrodes such that the sign the voltage between a respective measuring tube side electrode and a respective measuring tube facing away electrode for all pairs of measuring tube side and Meßrohrabgewandten electrodes of the respective vibration transducer is the same.

According to the invention, it is proposed to use a vibration converter which, similar to the interdigital transducers discussed at the beginning, has side faces lying parallel to the side wall, ie in particular rests flat on the side wall or a coupling layer or another coupling element. As a result, a small space requirement is realized. In contrast to an interdigital transducer which is locally differently polarized via the comb-like meshing electrode structures, however, according to the invention it is proposed to use the same polarity for all electrodes on a respective side. As a result, vibration modes can be excited whose oscillation is perpendicular to the side wall.

Expansion and compression vibrations can be excited in most piezoelectric vibration transducers with higher efficiency than shearing vibrations, thus enabling a more efficient vibration excitation. In addition, due to the direction of vibration perpendicular to the side wall, an efficient entry of this vibration into the side wall is achieved. The measuring device according to the invention, which can be in particular an ultrasonic meter, can thus be particularly compact and allows a particularly efficient vibration excitation. The same applies if the vibration transducer is used to detect the vibrations. Due to the efficient vibration coupling the signal acquisition is improved.

As will be explained in detail later, it has been found in the invention that with the described very simple construction of the ultrasonic transducer already a high mode purity for the excited guided wave can be achieved. As will be explained in more detail later, a high mode purity can be achieved by selecting the dimensions of the vibration transducer, ie in particular the piezo element used, even when using a single electrode on each side. By at least one-sided adaptation of the electrode structure to the wavelength of the guided wave to be generated, the mode purity can be further improved.

The vibration transducer is disposed on the sidewall such that two of its side surfaces are substantially parallel to the sidewall. In this case, they may in particular be parallel to the outer surface of the side wall, if the outer surface and the inner surface of the side wall are shaped differently and / or are angled to one another. It is of course possible that there are production-related slight deviations from the parallelism. For example, the angle of inclination of the side surfaces of the vibration transducer to the side wall or to its outer surface may be less than 1 °, less than 3 ° or less than 5 °.

The vibration transducer may be an ultrasonic transducer. The guided wave is in particular an acoustic ultrasonic wave. The vibration transducer or the piezoelectric element which forms the vibration transducer can in particular be cuboidal. Measurements can be made on a fluid flow flowing through the measuring tube, but also on a fluid in the measuring tube. The measuring device can also have more than two vibration transducers. For example, a vibration emitted by a first vibration transducer may be detected by a plurality of second vibration transducers, e.g. to consider different propagation paths or to validate measurement data.

The use of a vibration transport for detecting fluid properties is known in principle in the prior art. For example, in ultrasound counters, transit time differences of a transit time of a vibration between a first and a second ultrasound transducer and vice versa are often detected and from this a flow velocity can be determined. However, other measurement data can also be evaluated to determine fluid properties. For example, a signal amplitude at the receiving vibration transducer can be evaluated to detect a damping of the vibration during transport through the fluid. Amplitudes can also be evaluated frequency-dependent and absolute or relative amplitudes of specific spectral ranges can be evaluated in order to detect a spectrally different damping behavior in the fluid. Phase relationships of different frequency bands can also be evaluated in order, for example, to obtain information about the dispersion relation in the fluid. Alternatively or additionally, it is also possible to evaluate changes in the spectral composition or the amplitude over time, for example within a measuring pulse.

By evaluating these variables, for example a flow rate and / or a flow volume and / or a density, temperature and / or viscosity of the fluid can be determined as fluid quantities. Additionally or alternatively, for example, a speed of sound in the fluid and / or a composition of the fluid, for example a mixing ratio of different components, can be determined. Different approaches for obtaining these fluid sizes from the Measured variables explained above are known in the prior art and are therefore not to be described in detail. For example, relationships between one or more measured variables and the fluid size can be determined empirically, and it is possible, for example, to use a look-up table or a corresponding formula to determine the fluid size.

The control device can specify the voltages for the electrodes as the respective temporal voltage curve. Alternatively, it is also possible that only the measuring tube facing away from electrodes or only the measuring tube side electrodes, a voltage waveform is supplied and the respective other electrodes are at a fixed predetermined voltage potential. The control device can act in particular on all the electrodes of a respective side surface of the vibration transducer with the same voltage.

The control device can be set up to predetermine the voltages between the electrodes in such a way that substantially no voltage drops between the measuring tube-side electrodes of the respective vibration converter and / or that substantially no voltage drops between the electrodes of the respective vibration converter facing away from the measuring tube. All electrodes of one side can thus be approximately at the same potential. Potential differences can result, for example, from slight differences in the resistance of the supply lines. The voltage difference between the electrodes of one side can be smaller by a factor of 100 or 1000 than the maximum voltage between electrodes of the different sides in the context of a voltage curve used for the control.

The first and / or the second vibration transducer may have exactly one measuring tube-side electrode and / or exactly one measuring tube facing away from the electrode. As already mentioned, even such a simple electrode structure can be sufficient for a mode-selective excitation of guided waves.

It is also possible for the first and / or the second oscillation converter to have a plurality of strip-shaped electrodes whose distance corresponds to the wavelength of the guided wave, and / or the first and / or the second oscillation converter have a plurality of strip-shaped electrodes facing away from the measuring tube Wavelength of the guided wave corresponds. Such structuring of the electrodes of at least one side face of the vibration transducer can be used to improve the mode selectivity of the excitation. The wavelength of the guided wave can in this case be predetermined by the excitation geometry and an excitation signal supplied by the control device, in particular an excitation frequency. The excitation frequency can be fixed for the measuring device or adjusted as a function of operating parameters, for example as a function of an operating temperature, which can be detected, for example, via a temperature sensor of the measuring device. As will be explained in detail later, the excitation geometry of the measuring device can be optimized to a defined frequency and wavelength of the guided wave to stimulate a guided wave with exactly these parameters as modenelektiv. The wavelength and frequency of the guided wave to be excited are thus predetermined by the structural design of the measuring device. An adaptation of an excitation frequency, as has been explained above, can only serve to change the mode structure of the vibration transducer and / or of the measuring tube or of its side wall, for. B. due to a change in temperature to compensate.

The guided wave can be a lamb wave. Excitation of Lamb waves takes place when the wall thickness is comparable to the wavelength of the transverse wave of the solid. Such guided waves are combined pressure and shear waves and thus also lead to a deflection of the inner surface of the side wall in the direction of the fluid or away from the fluid. This deflection in turn causes compression waves in the fluid. The entire area within which the guided wave propagates on the side wall can thus serve as an excitation surface for a compression oscillation of the fluid. Conversely, a compression vibration of the fluid in a wall region of the side wall can excite a corresponding Lamb wave, which is then guided in the side wall to the other vibration transducer.

As already mentioned, a mode selectivity of the excitation can be realized by an appropriate tuning of the vibration transducer and of the measuring tube or its side wall to one another. It is therefore possible that the first and / or the second vibration transducer and the measuring tube are formed such that a natural vibration of the vibration transducer has the same frequency and the same wavelength as the guided wave. If only relatively low vibration modes, ie only relatively low spatial frequencies, are taken into account, then the vibration converter only has finitely many natural mode modes. For the teaching according to the invention, in particular compression oscillation modes perpendicular to the side wall are relevant here, since they are particularly strongly coupled into the side wall. At the same time it is exploited that the measuring tube or its excited side wall for guided waves, in particular for Lamb waves or Rayleigh waves has a dispersion relation that describes a unique relationship between their frequency and wavelength for each mode. If an excitation of the vibration transducer with a frequency corresponding to the frequency of a natural vibration of the vibration transducer, so results in a standing wave with a defined frequency and wavelength. If now the dimensions and the other parameters of the vibration transducer are chosen such that this combination of frequency and wavelength lies on or close to a branch of the dispersion relation for guided waves in the measuring tube or in the excited sidewall, then almost exclusively the branch associated with it Vibration mode excited. A mode selectivity can thus be achieved solely by using a correspondingly dimensioned vibration transducer or piezoelectric element.

A side length of the lateral faces of the vibration transducer parallel to the sidewall of the measuring tube may be the product of the wavelength of the guided wave with a scaling factor, the scaling factor being the sum of a natural number, that is an integer, and a number between 0.05 and 0.45 , in particular between 0.15 and 0.35. This side length may in particular be the extent of the vibration transducer in a direction in which the guided wave is to be radiated, that is, for example, in a flow direction of the measuring tube.

It has been found within the scope of the invention that optimum mode selectivity is unexpectedly achieved not at the side of integer or half-integer multiples of the wavelength but at approximately five quarters, nine quarters, thirteen quarters, etc. of the wavelength. However, the concrete optimal scaling factors also depend on the specific electrode structure and the type of coupling of the vibration transducer to the measuring tube or the side wall. An optimal value in the intervals given above may be determined, for example, by a simulation of the problem or empirically. For example, at a wavelength of the guided wave of about 1.8 mm, the side length of the vibration transducer in the propagation direction of the guided wave may be 4.1 mm. A thickness perpendicular to the sidewall of the measuring tube may be chosen to match the frequency of the natural vibration of the vibrator to the frequency of the guided wave at that wavelength. For example, the thickness can be about 2 mm. The width of the vibration transducer perpendicular to the direction of propagation can be selected substantially freely and, for example, be 10 to 20 mm, for example, to extend over a large part of the width of the measuring tube.

The wavelength of the guided wave can be between 1.6 mm and 2.2 mm, in particular between 1.8 mm and 2 mm. The control device can thus be set up to excite a guided wave having said wavelength and in particular a defined frequency.

The first and / or the second vibration transducer may be coupled to the measuring tube via a viscous coupling layer. The coupling layer may have a viscosity of less than 10 ⁸ mPas (millipascal seconds), in particular a viscosity between 0.6 mPas and 10 ⁶ mPas. For example, a silicone oil can be used as a viscous coupling layer whose properties can be further adjusted by additives, such as introduced particles. The layer thickness of the coupling layer can be between 10 and 100 μm.

The use of a viscous coupling layer is advantageous in the ultrasonic transducer according to the invention for several reasons. On the one hand, this can achieve that there is no or only a slight coupling between the vibration transducer and the measuring tube or the side wall of the measuring tube with respect to shear forces. As a result, the vibration transducer and the measuring tube or its side wall with respect to their eigenmodes or the modes of the guided waves can be considered in good nutrition as separate systems, whereby the design of the vibration transducer for mode-selective excitation is facilitated.

Compared to a rigid coupling, for example, a bond, the advantage is also achieved that tensions between the vibration transducer and the measuring tube are avoided with a change in temperature. In many cases, the measuring tube, which is formed for example of metal or plastic, and the vibration transducer, which consists of a piezoceramic with applied electrodes, different thermal expansion coefficients. Due to the viscous coupling layer, these different expansions can be compensated without causing tension and thus, for example, an adhesive layer can become brittle over time.

Preferably, the coupling layer is electrically insulating. Thus, the measuring tube side electrode of the vibration transducer can be isolated, for example, by the coupling layer of the measuring tube, if this is conductive. The coupling layer may be, for example, a thin elastomeric film. It can be a film with a thickness between 5 microns and 80 microns, for example, be provided with a thickness of 20 microns. As the material, for example, a dielectric elastomer can be used. The measuring tube-side electrode can, for example be formed as an electrode sheet, which extends beyond the vibration transducer out to facilitate their contact.

Alternatively, the coupling layer could be electrically conductive. In particular, the or the measuring tube-side electrodes can be contacted via the coupling layer. For example, the coupling layer may have a conductivity of more than 1 S / m (Siemens / meter), in particular more than 10 ³ S / m. Preferably, even greater conductivities are realized. However, the said relatively low conductivities may be sufficient since no large currents have to be transported.

The coupling layer may contain metallic particles. On the one hand, this can serve to produce the abovementioned conductivity; on the other hand, the viscosity of the layer can be adjusted as required by adding particles.

In addition to the measuring device according to the invention, the invention relates to a method for determining a fluid size concerning a fluid and / or a fluid flow of the fluid by a measuring device comprising a control device, a measuring tube receiving the fluid and / or by the fluid flow-through and a first and a second spaced apart on the measuring tube arranged piezoelectric vibration transducer, wherein the control means of the first and / or the second vibration transducer are driven to excite a guided through a side wall of the measuring tube shaft, wherein the guided wave stimulates compression oscillations of the fluid to the via the fluid in each case led to other vibration transducer and detected there by the control device for determining measurement data, wherein the fluid size is determined by the control device in dependence of the measured data, wherein as the first and / or second vibration transducer a Sch wingungswandler is used, which has either exactly one measuring tube-side electrode, and exactly one electrode away from the electrode tube, which are arranged on opposite, parallel to the side wall of the measuring tube side surfaces of the respective vibration transducer, or the plurality of measuring tube-side electrodes and at least one measuring tube facing away electrode or a plurality of measuring tube facing away electrodes and at least one measuring tube side electrode, wherein the control means predetermines the voltages between the electrodes such that the sign of the voltage between a respective measuring tube side electrode and a respective electrode facing away electrode for all pairs of measuring tube side and measuring tube facing electrodes of the respective vibration transducer is the same.

The inventive method can be further developed with those features that have been explained to the ultrasonic transducer according to the invention. Here, the advantages mentioned there are achieved.

• 1 ein Ausführungsbeispiel einer erfindungsgemäßen Messeinrichtung,
• 2 und 3 Detailansichten der in 1 gezeigten Messeinrichtung aus unterschiedlichen Blickrichtungen,
• 4 eine Detailansicht eines weiteren Ausführungsbeispiels einer erfindungsgemäßen Messeinrichtung,
• 5 beispielhaft die Abstimmung des Schwingungswandlers und des Messrohrs aufeinander zur modenselektiven Anregung der geführten Welle, und
• 6 ein weiteres Ausführungsbeispiel einer erfindungsgemäßen Messeinrichtung.

Further advantages and details will become apparent from the following embodiments and the accompanying drawings. Here are shown schematically:

• 1 an embodiment of a measuring device according to the invention,
• 2 and 3 Detailed views of in 1 shown measuring device from different directions,
• 4 a detailed view of another embodiment of a measuring device according to the invention,
• 5 For example, the vote of the vibration transducer and the measuring tube to each other for mode-selective excitation of the guided wave, and
• 6 a further embodiment of a measuring device according to the invention.

1 shows a measuring device, such as an ultrasonic counter, for detecting a fluid and / or fluid flow of the fluid fluid size. The measuring device 1 has a control device 2 , a measuring tube 3 and a first and second spaced apart on the measuring tube 3 arranged piezoelectric vibration transducer 5 . 6 on. The fluid is in an interior 4 of the measuring tube 3 longitudinal 16 of the measuring tube 3 guided.

In order to determine a flow size, in particular a flow volume, as the fluid size, the control device can 2 a transit time difference of the signal propagation time of the first vibration transducer 5 to the second vibration transducer 6 and vice versa. This exploits that this transit time of a velocity component of the fluid parallel to a propagation direction of an ultrasonic beam 15 depends on the fluid. From this runtime can thus one on the path of the respective ultrasonic beam 15 average fluid velocity in the direction of the respective ultrasound beam 15 and thus approximately an average flow velocity in that of the ultrasound beam 15 traversed volumes are determined.

On the one hand, an arrangement of the vibration transducer 5 . 6 outside the measuring tube 3 to and on the other hand to reduce sensitivity to different flow velocities at different positions of the airfoil is achieved by the first vibration transducer 5 not directly an ultrasonic beam 15 , ie a pressure wave, in which fluid is induced. Instead, it is through the vibration transducer 5 a schematic by the arrow 13 pointing its propagation direction, illustrated measuring tube led wave 3 or in particular the in 2 shown side wall 7 of the measuring tube 3 stimulated. The excitation occurs at a frequency chosen such that a Lamb wave in the sidewall 7 is stimulated. Such waves can be excited when the thickness 23 the side wall 7 is comparable to the wavelength of the transverse wave of the solid, which results from the ratio of the sound velocity of the transverse wave of the solid and the excited frequency.

As in 2 is shown, a relatively simple constructed vibration transducer is used to stimulate such a guided wave. The vibration converter 5 is from a block 8th made of piezoceramic, at its opposite, to the side wall 7 of the measuring tube 3 parallel side surfaces one electrode each 9 . 10 is arranged. Alternatively, blocks of other piezoelectric materials, such as a composite piezoelectric ceramic could be used. The measuring tube facing away electrode can be contacted directly. To contact the measuring tube side electrode 9 To facilitate, becomes an electrically conductive, viscous coupling layer 11 used over the the electrode 9 can be contacted. The coupling layer may for example consist of silicone oil, wherein additionally metallic particles 22 into the coupling layer 11 are introduced to achieve sufficient conductivity. To prevent drainage of the viscous coupling layer are sealing elements 12 intended. Alternatively, it would also be possible to use a sheet-metal electrode which can protrude beyond the piezoelectric main body. This is particularly advantageous if an insulating coupling layer is to be used. A sheet-metal electrode may, for example, rest loosely on the piezoelectric main body and be held there only by a contact pressure.

The coupling layer also serves to provide a coupling between the vibration transducer 5 and the side wall 7 or the measuring tube 3 with respect to a transmission of shear forces to reduce or eliminate. In essence, therefore, only forces perpendicular to the side wall 7 transfer. This extensive decoupling allows the eigenmodes of the vibration transducer 5 , which, as will be explained in more detail later, the mode-selective excitation of guided waves in the measuring tube 3 or in the side wall 7 can be used, approximately isolated from the coupling to the measuring tube 3 consider.

The excited Lamb wave is a combined shear and pressure wave. Thus results from the along the side wall 7 ongoing, guided wave local deformation of the inner surface 24 the side wall 7 , This in turn results in a radiation of a compression wave into the fluid in the entire area within which the guided wave propagates. This is in 1 schematically through the longitudinal direction 16 spaced ultrasonic beams 15 shown. At the receiving end, the incoming compression waves cause a further guided wave in the side wall 7 excited by the arrow 14 is shown and over the vibration transducer 5 from the controller 2 is detected. The radiation of the corresponding compression waves takes place in the Rayleigh angle 17 , which depends on the velocity of sound in the fluid and the phase velocity of the guided wave.

Here, it can be problematic that the dispersion relation for Lamb waves in the sidewall 7 typically has several branches in the relevant frequency range. When excited with a particular by the controller 2 given frequency would thus be possible that different modes of vibration are excited for the Lamb wave, the different phase velocities and thus different Rayleigh angle 17 exhibit. Since the ultrasonic beams involved in the measurement process 15 In this case, would cover different paths, a multipath propagation would result, the signal evaluation of the vibration transducer 6 can significantly complicate received signals. Preferably, therefore, an exclusive or at least substantially exclusive excitation of a single vibration mode should take place.

Such mode selectivity can be achieved if the in 2 and 3 shown dimensions of the vibration transducer 5 or the block 8th be selected correctly from piezoceramic and a corresponding excitation frequency for the excitation by the controller 2 is used. By the height 21 and the side length 19 in the propagation direction of the guided wave, the eigenmodes can be defined in terms of their wavelength and their frequency. This can be minor changes in the adjustment of the width 20 occur in the context of the design of the vibration transducer 5 can be considered. Also the coupling via the coupling layer 11 can, especially when using high viscosities, slightly influence the wavelengths or frequencies of corresponding eigenmodes. By an appropriate choice of Dimensions of the vibration transducer 5 can be achieved that the vibration transducer has a natural vibration having the same frequency and the same wavelength as the guided wave to be generated. In this case, the excitation dominates over the vibration transducer 5 the guided wave not only a defined frequency, but also a defined wavelength, whereby a mode-selective excitation takes place. In the example shown, the side length 19 corresponds to the vibration converter 5 about nine quarters of the wavelength of the excited Lamb wave in the sidewall 7 ,

The measurement accuracy or the robustness of the measurement can z. B. be further improved by that an additional vibration transducer 18 is provided over the signal propagation times for a portion of the path between the vibration transducers 5 . 6 are detectable. As a result, a flow variable determined from the signal propagation time can be validated and / or corrected.

Since a mode-selective excitation is already possible by an appropriate choice of the dimensions of the vibration transducer, electrodes 9 . 10 be used, which extends substantially over the entire side surfaces of the vibration transducer 5 or the block 8th extend from piezoceramic. However, the mode selectivity of the excitation can be further improved if a plurality of electrodes facing away from the measuring tube or electrodes on the measuring tube side are used. An example of this is in 4 shown. In this case, a measuring tube-side electrode is also used. However, two separate electrodes are located on the easier-to-contact side of the measuring tube 25 . 26 arranged. The distance 27 is between the centers of these electrodes 25 . 26 corresponds approximately to the wavelength of the Lamb wave to be excited. The electrodes 25 . 26 is supplied by the control device, a common drive signal, whereby the electrodes 25 . 26 lying on the substantially same potential. By providing corresponding spaced electrodes 25 . 26 becomes the excitation of the vibration transducer 5 imprinted a defined wavelength, so that in particular a eigenmode can be excited with corresponding wavelengths of high purity. Accordingly, a higher mode purity for the generated Lamb wave in the measuring tube 3 or in the side wall 7 be achieved.

The effect of a corresponding choice of the dimensions of the vibration transducer 5 is in 5 shown schematically. The respective deflections of the vibration transducer 5 and the side wall 7 are greatly exaggerated in order to clarify the operating principle. At an excitation of the vibration converter 5 with a correspondingly selected frequency becomes a natural vibration of the vibration transducer 5 stimulated. This means that the nodes 29 . 30 and the bellies 28 the vibration are stationary. As can be clearly seen, the page length is 19 of the vibration converter 5 in the directions of propagation of the guided wave, indicated by the arrow 13 is displayed, greater than twice the wavelength 31. By the fixed nodes 29 . 30 and bellies 28 will also be in the sidewall 7 excited Lamb wave imparted a fixed wavelength. Since an excitation with a fixed frequency and a fixed wavelength, results in a mode-pure excitation.

The previous explanations assumed that a transit time is measured after the compression wave guided through the fluid has been reflected at least once on a side wall. However, this is not necessary for the illustrated measuring principle, so that even an arrangement as shown in 6 shown can be used. The basic structure corresponds to that in 1 shown structure, wherein the first and second vibration transducer 5 . 6 are arranged here on opposite side walls. In an embodiment not shown, it would also be possible to use such an arrangement, wherein the ultrasonic waves are reflected several times on the side walls during the propagation through the fluid.

LIST OF REFERENCE NUMBERS

1

measuring device

2

control device

3

measuring tube

4

inner space

5

vibration transducer

6

vibration transducer

7

Side wall

8th

block

9

electrode

10

electrode

11

coupling layer

12

sealing element

13

arrow

14

arrow

15

ultrasonic beam

16

longitudinal direction

17

Rayleigh angle

18

vibration transducer

19

page length

20

width

21

height

22

metal particles

23

thickness

24

palm

25

electrode

26

electrode

27

distance

28

belly

29

node

30

node

31

double wavelength

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 4735097 A [0003]

Cited non-patent literature

• G. Lindner, "Sensors and actuators based on surface acoustic waves propagating along solid-liquid interfaces", J. Phys. D: Appl. Phys. 41 (2008) 123002 [0004]

Claims (11)

Measuring device (1) for determining a fluid size concerning a fluid and / or a fluid flow of the fluid with a control device (2), a measuring tube (3) receiving the fluid and / or flowed through by the fluid and a first and a second spaced apart on the Measuring tube (3) arranged piezoelectric vibration transducer (5, 6), wherein by the control device (2) of the first and / or the second vibration transducer (5, 6) are driven to one through a side wall (7) of the measuring tube (3) guided Excite wave, wherein the guided wave stimulates compression oscillations of the fluid which can be guided via the fluid to the respective other vibration transducer (5, 6) and there by the control device (2) for determining measurement data, wherein the fluid size by the control device (2 ) can be determined as a function of the measured data, characterized in that either the first and / or the second vibration converter (5, 6) each g a measuring tube side electrode (9) and exactly one measuring tube facing away from electrode (10), which are arranged on opposite, to the side wall (7) of the measuring tube (3) parallel side surfaces of the respective vibration transducer (5, 6), or that the first and / or the second vibration transducer (5, 6) has a plurality of measuring tube side electrodes (9) and at least one measuring tube facing away electrode (10, 25, 26) or a plurality of measuring tube facing away electrodes (10, 25, 26) and at least one measuring tube side electrode, wherein the control device (2) is adapted to specify the voltages between the electrodes (9, 10, 25, 26) in such a way that the sign of the voltage between a respective measuring tube side electrode (9) and a respective electrode facing away electrode (10, 25, 26) for all pairs of measuring tube side and measuring tube facing away electrodes (9, 10, 25, 26) of the respective vibration transducer (5, 6) is the same. Measuring device according to Claim 1 , characterized in that the control device (2) is adapted to specify the voltages between the electrodes (9, 10, 25, 26) such that between the measuring tube side electrodes (9) of the respective vibration converter (5, 6) substantially none Voltage drops and / or that between the Meßrohrabgewandten electrodes (10, 25, 26) of the respective vibration transducer (5, 6) substantially no voltage drops. Measuring device according to Claim 1 or 2 , characterized in that the first and / or the second vibration transducer (5, 6) exactly one measuring tube side electrode (9) and / or exactly one measuring tube facing away from electrode (10). Measuring device according to one of the preceding claims, characterized in that the first and / or the second vibration transducer (5, 6) have a plurality of strip-shaped measuring tube side electrodes whose distance corresponds to the wavelength of the guided wave, and / or that the first and / or the second Vibration converter (5, 6) a plurality of strip-shaped measuring tube facing away from electrodes (25, 26), whose distance (27) corresponds to the wavelength of the guided wave. Measuring device according to one of the preceding claims, characterized in that the guided wave is a Lamb wave. Measuring device according to one of the preceding claims, characterized in that the first and / or the second vibration transducer (5, 6) and the measuring tube (3) are formed such that a natural vibration of the vibration transducer (5, 6) the same frequency and the same Wavelength, as the guided wave. Measuring device according to one of the preceding claims, characterized in that a side length (19) of the parallel to the side wall of the measuring tube side surfaces of the vibration transducer (5, 6) is the product of the wavelength of the guided wave with a scaling factor, wherein the scaling factor is the sum of a natural Number and a number between 0.05 and 0.45, in particular between 0.15 and 0.35. Measuring device according to one of the preceding claims, characterized in that the first and / or the second vibration transducer (5, 6) via a viscous coupling layer (11) are coupled to the measuring tube (3). Measuring device according to Claim 8 , characterized in that the coupling layer (11) is electrically insulating. Measuring device according to Claim 8 or 9 , characterized in that the coupling layer (11) contains metallic particles (22). Method for determining a fluid quantity concerning a fluid and / or a fluid flow of the fluid by a measuring device (1), a measuring device (2), a measuring tube (3) receiving the fluid and / or flowed through by the fluid, and a first and a second The piezoelectric vibration converter (5, 6) arranged at a distance from one another on the measuring tube is controlled by the control device (2) of the first and / or the second vibration converter (5, 6) in order to move through a side wall (7) of the measuring tube (3 ) guided wave, with the guided wave Excites the compression vibrations of the fluid, which are guided via the fluid to the respective other vibration transducer (5, 6) and there detected by the control device (2) for determining measurement data, wherein the fluid size is determined by the control device (2) in dependence of the measured data , characterized in that as the first and / or second vibration transducer (5, 6), a vibration transducer (5, 6) is used, which has either exactly one measuring tube side electrode (9), and exactly one measuring tube facing away electrode (10, 25, 26) which are arranged on mutually opposite side surfaces of the respective vibration converter (5, 6) which are parallel to the side wall (7) of the measuring tube (3), or of a plurality of measuring tube-side electrodes (9) and at least one electrode tube (10, 25, 26) or a plurality of Meßrohrrabgewandte electrodes (10, 25, 26) and at least one measuring tube-side electrode (9), wherein the control device (2) the voltages zwis to the electrodes (9, 10, 25, 26) such that the sign of the voltage between a respective measuring tube side electrode (9) and a respective measuring tube facing away electrode (10, 25, 26) for all pairs of measuring tube side and measuring tube facing away electrodes (9 , 10, 25, 26) of the respective vibration converter (5, 6) is the same.

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