Lamb wave ultrasonic flow meter

Abstract

A flow meter (10) is described, which comprises a housing (1) having a longitudinal axis (L) and a flow channel (2), the flow meter (10) comprising first and second piezoelectric transducers (3.1, 3.2) arranged at an outer surface (1.3) of the housing (1) and each configured to generate an acoustic wave in the housing (1) having a wavelength λ, wherein the housing (1) has a thickness d1 at the piezoelectric transducers (3.1, 3.2) and a thickness d2 at opposite sides (1.5) of the flow channel (2), the thicknesses d1 and d2 both being smaller than the wavelength λ of the acoustic wave, such that Lamb waves (Lb1, Lb2, Lb3) can be excited in the housing (1), wherein the piezoelectric transducers (3.1, 3.2) are each configured to receive an acoustic wave from the housing in the form of a Lamb wave (Lb1, Lb3), wherein the piezoelectric transducers (3.1, 3.2) each have a height (H) and a width (W), wherein the height (H) is greater than the width (W).

Description

Technical Field

[0001] This invention relates to a flow meter, and more particularly to an ultrasonic flow meter. Background Technology

[0002] Flow meters, such as ultrasonic flow meters, are commonly used to record flow signals in heating, ventilation, and / or air conditioning equipment. For example, a flow meter can record the flow rate of a fluid such as water or ethylene glycol through a heat flux calorimeter. The amount of heat transfer can then be determined from the recorded flow rate and additional temperature measurements. Ultrasonic flow meters can also be used to record the flow rate of transformer oil circulating through pipes in power transformers. The recorded flow rate values ​​can then be used to adjust the speed of the oil pump when the power transformer is in oil-guided and forced-air cooling mode.

[0003] The ultrasonic signal used in ultrasonic flow meters is typically generated by an ultrasonic transducer attached to the flow meter housing. Generally, the ultrasonic signal generated by the ultrasonic transmitter is directed into the flowing medium within the housing and detected by an ultrasonic receiver. Various reflectors arranged within the housing are thus used to guide the ultrasonic signal along a predetermined path between the ultrasonic transmitter and the ultrasonic receiver.

[0004] For example, EP 0 890 826 A1 describes such an ultrasonic flow meter, which includes a housing with flanges at both ends and can be assembled into a piping system. The housing is provided with a screw-on cover for housing a metering device, which includes an ultrasonic transducer mounted on the cover, means for repeatedly deflecting ultrasonic energy from an ultrasonic transmitter to an ultrasonic receiver in a flow path within the housing, and related electronics. A slotted removable insert is mounted in a matching recess within the housing in the flow path of the housing. This insert, together with the cover, forms a unit containing the metering device and has means for deflecting ultrasonic energy from a disk reflecting the ultrasonic waves, the disk being inserted into a pre-formed recess in the insert. Summary of the Invention

[0005] When using an ultrasonic flow meter to measure fluid flow rate, it is desirable to optimize the angle at which the ultrasonic signal enters the medium to improve the quality of the measurement signal. This can be achieved, for example, by attaching the ultrasonic transducer to the inclined surface of a wedge arranged on the outer surface of the housing. Alternatively or additionally, the ultrasonic signal can be guided by a reflector arranged within the housing. On the other hand, it is desirable to reduce the structural complexity of the flow meter while ensuring high-quality ultrasonic signals.

[0006] Therefore, the object of the present invention is to provide a flow meter that at least partially improves upon the prior art and avoids at least some of the disadvantages of the prior art.

[0007] According to the invention, this objective is achieved in particular by a flow meter comprising a housing having a longitudinal axis and first and second piezoelectric transducers, the housing defining a flow channel for fluid flow, the first and second piezoelectric transducers being disposed on the outer surface of the housing and each configured to generate an acoustic wave of wavelength λ within the housing, wherein the housing has a thickness d1 at the piezoelectric transducer and a thickness d2 on the opposite side of the flow channel, both thicknesses d1 and d2 being smaller than the wavelength λ of the acoustic wave, such that a Lamb wave can be excited within the housing, wherein each piezoelectric transducer is configured to receive an acoustic wave in the form of a Lamb wave from the housing, wherein each piezoelectric transducer has a height perpendicular to the outer surface of the housing at the piezoelectric transducer and a width parallel to the longitudinal axis, wherein the height is greater than the width.

[0008] Compared to conventional flow meters that use piezoelectric transducers to generate longitudinal waves in the housing, which are then coupled into the medium, the flow meter of the present invention can generate a Lamb wave in the housing. Since the housing surrounds the medium flowing through the flow channel, at least a portion of the Lamb wave generated in the housing can be coupled into the medium as a volume wave. In the medium, the volume wave can propagate through the flow channel to the opposite side of the flow channel, where at least a portion of the volume wave can couple into the housing, generating another Lamb wave. At least a portion of this Lamb wave can be recoupled into the medium as a volume wave and propagate through the flow channel again until it reaches the second piezoelectric transducer. At the second piezoelectric transducer, at least a portion of the volume wave can be coupled into the housing as a Lamb wave, which can be received by the second piezoelectric transducer. The first and second piezoelectric transducers can be arranged on the same side or opposite sides of the flow channel. The first and second piezoelectric transducers can also be arranged at other locations around the circumference of the flow channel. In the arrangement of the second piezoelectric transducer on the opposite side of the flow channel relative to the first piezoelectric transducer, a portion of the Lamb wave generated by the first piezoelectric transducer can be coupled into the medium as a bulk wave and propagate through the flow channel to the opposite side of the flow channel. On the opposite side, at least a portion of the bulk wave can be coupled into the housing to generate another Lamb wave, which can be received by the second piezoelectric transducer.

[0009] The advantage of using Lamb waves to measure fluid flow rate is that it allows for the coupling of volume waves into the medium at an optimal incident angle θ, relative to an axis perpendicular to the longitudinal axis of the casing. This incident angle θ is defined by the following formula:

[0010]

[0011] Where c M It is the speed of sound in the medium, c HThis refers to the velocity of sound within the housing (or corresponding housing wall). Similarly, a portion of the volume wave, after passing through the flow channel, can be coupled back into the housing as a Lamb wave at an incident angle θ. Therefore, separate structures for optimizing the angle at which the ultrasonic signal enters the flow channel, such as wedges with piezoelectric transducers or reflectors within the flow channel for guiding the ultrasonic signal, can be avoided, thus reducing the complexity of the flow meter. Instead, the incident angle θ, defined by the relative magnitude of the sound velocities involved, can be used to obtain the optimal path for the ultrasonic signal.

[0012] By reducing or eliminating additional structures within the flow channel, the linearity of the fluid flow can be improved, thereby enhancing the signal quality of the flow meter. The manufacturing of the flow meter can also be simplified, as there is no need to introduce and position a reflector within the flow channel, which can be cumbersome and, in particular, lead to inaccuracies in the expected path of the ultrasonic signal.

[0013] Preferably, the first and second piezoelectric transducers are each designed as monolithic blocks. Therefore, compared to using interdigital transducer arrays to generate surface acoustic waves, complexity and cost can be reduced.

[0014] Because the vertical orientation of the piezoelectric transducer is greater than its width, Lamb waves can be generated and detected by utilizing the oscillation of the transducer across its width. This contrasts with conventional flow meters that utilize the thickness oscillation of a horizontally oriented piezoelectric transducer, which has a width greater than its height. The widths of the first and second piezoelectric transducers can be varied according to the desired wavelength of the Lamb wave to be generated within the housing.

[0015] A further advantage of this flowmeter is that, because the wavelength of the wave generated by the piezoelectric transducer can be matched with the wavelength of the induced wave in the housing, the additional transition layer used for impedance matching can be reduced or eliminated. Therefore, wavelength mismatch between the wave in the transducer and the wave in the housing can be reduced, thereby significantly reducing energy loss when the wave transitions from one material to another.

[0016] In some embodiments, the piezoelectric transducer is attached to the outer surface of the housing by an adhesive.

[0017] By using adhesives, piezoelectric transducers can be mounted on the housing in a simple and cost-effective manner. Furthermore, additional structures for attaching the piezoelectric transducer that could potentially interfere with fluid flow can be prevented from extending into the flow channels.

[0018] Preferably, the thicknesses d1 and d2 are equal.

[0019] Therefore, the housing can be manufactured as a simple tube with a circular or polygonal cross-section, having a constant thickness in the region of the piezoelectric transducer.

[0020] Preferably, the piezoelectric transducer is configured to oscillate in a direction parallel to the longitudinal axis to generate Lamb waves in the housing.

[0021] Therefore, as described above, Lamb waves can be generated and detected by utilizing the oscillation of the piezoelectric transducer across its width, which is achieved through the vertical orientation of the piezoelectric transducer.

[0022] In some embodiments, each piezoelectric transducer is in contact with two electrodes arranged on two opposing surfaces of the respective piezoelectric transducer, wherein the two opposing surfaces are perpendicular to the longitudinal axis.

[0023] Therefore, by applying a suitable voltage to the electrodes, the piezoelectric transducer can oscillate along the longitudinal axis to generate Lamb waves in the housing.

[0024] Specifically, the two opposing surfaces can be perpendicular to the outer surface of the housing at the piezoelectric transducer.

[0025] In some embodiments, each piezoelectric transducer is in contact with two electrodes arranged on two opposing surfaces of the respective piezoelectric transducer, wherein the two opposing surfaces are parallel to the longitudinal axis.

[0026] In some embodiments, each piezoelectric transducer has a rectangular cuboid shape.

[0027] In particular, the cuboid can be designed as a single piece. Therefore, the design of the flow meter can be further simplified, especially compared to complex interdigital transducer arrays.

[0028] In some embodiments, the housing includes an intermediate wall portion disposed between piezoelectric transducers, the thickness d3 of which is less than the wavelength λ of the sound wave, such that a Lamb wave can be excited in the intermediate wall portion.

[0029] A body wave striking the intermediate wall portion at an angle θ can generate a Lamb wave within the intermediate wall portion, and another body wave from that Lamb wave can again couple into the medium at an angle -θ. Therefore, when a body wave strikes the intermediate wall portion, the intermediate wall portion can act as a reflector.

[0030] Preferably, the thickness d3 is equal to the thickness d1 and / or the thickness d2.

[0031] In some embodiments, piezoelectric transducers are arranged at a distance l from each other to generate V-shaped, W-shaped, or multi-V-shaped paths of ultrasonic signals in the flow channel.

[0032] Since the process of the housing absorbing a portion of the volume wave, exciting a Lamb wave in the housing, and then recoupled a portion of the Lamb wave back into the medium as a volume wave can be repeated along the entire path of the ultrasonic signal, various shapes of the ultrasonic signal path can be obtained as needed by properly designing the flow meter, especially the arrangement of the piezoelectric transducer and / or the thickness of one or more housings.

[0033] In some embodiments, the height of the piezoelectric transducer is 1 to 2 times the width of the piezoelectric transducer, preferably 1.2 to 1.8 times, and particularly preferably 1.3 to 1.7 times.

[0034] In particular, the height of the piezoelectric transducer can be adapted to ensure that the piezoelectric transducer operates at or near resonance.

[0035] In some embodiments, the width of the piezoelectric transducer is one-quarter to three-quarters of the wavelength λ of the sound wave generated by the piezoelectric transducer in the housing, preferably one-half of the wavelength λ.

[0036] Therefore, the width of the piezoelectric transducer can be adapted to provide optimal excitation of acoustic or Lamb waves within the housing.

[0037] Therefore, the operation of the flow meter can be optimized by adjusting the size of the piezoelectric transducer, where the height can be adjusted to ensure that the piezoelectric transducer operates at or near resonance, and the width can be adjusted to provide optimal excitation of acoustic waves within the housing.

[0038] In some embodiments, the width of the piezoelectric transducer is 0.5 mm to 8 mm, preferably 1 mm to 4 mm, and particularly preferably 1.5 mm to 2.5 mm.

[0039] In some embodiments, the height of the piezoelectric transducer is 0.3 mm to 8 mm, preferably 0.75 mm to 4 mm, and particularly preferably 1 mm to 2 mm.

[0040] By using piezoelectric transducers with these specific dimensions, optimal excitation of sound waves or Lamb waves can be achieved within the housing, thereby adapting to the desired sound wave wavelength.

[0041] The preferred excitation frequency of the piezoelectric transducer can be from 200 kHz to 4 MHz, more preferably from 500 kHz to 2 MHz, and particularly preferably from 800 kHz to 1.2 MHz.

[0042] The housing can be made of plastic (preferably injection-molded plastic) or metal, or it can be manufactured by additive manufacturing.

[0043] Plastics may include one or more of the following: epoxy polymers, polytetrafluoroethylene, polyethylene, polyethylene terephthalate, polyester, etc. Metals may include one or more of the following: steel, austenitic steel, ferritic steel, aluminum, aluminum alloys, brass, etc., or alloys thereof.

[0044] In some embodiments, the housing has a cross-section with a constant area along the longitudinal axis between the first and second piezoelectric transducers.

[0045] Therefore, the flow path can be designed to be straight along the longitudinal axis, and especially free of obstructions in the measurement-related area, thereby improving measurement conditions such as linearity, reducing or avoiding pressure loss, and reducing or avoiding dirt accumulation.

[0046] In some embodiments, the housing includes a flat outer surface portion, on which the first and second piezoelectric transducers are disposed.

[0047] By providing a flat outer surface portion, it is easier to attach the piezoelectric transducer. Furthermore, the thickness of the adhesive used to attach the piezoelectric transducer can be reduced, and the acoustic coupling between the piezoelectric transducer and the housing can be improved.

[0048] Preferably, the piezoelectric transducer has a width perpendicular to the longitudinal axis, which is equal to or greater than the width perpendicular to the longitudinal axis of the flat outer surface portion.

[0049] Therefore, the piezoelectric transducer can extend across a flat outer surface portion of the housing in a direction perpendicular to the longitudinal axis. By designing the width of the piezoelectric transducer in the direction perpendicular to the longitudinal axis to be equal to or greater than the width of the flat outer surface portion, the ultrasonic signal can be improved. Attached Figure Description

[0050] The invention will now be explained in more detail with reference to exemplary embodiments and schematic drawings, in which:

[0051] Figure 1 A perspective view of an embodiment of the flow meter is shown;

[0052] Figure 2 It shows Figure 1 A vertical cross-sectional view of the flow meter along the longitudinal axis of the housing;

[0053] Figure 3 It shows Figure 2 A magnified view of a detail of one of the piezoelectric transducers. Detailed Implementation

[0054] Figure 1A perspective view of an embodiment of the flow meter 10 is shown. The flow meter 10 includes a housing 1 having a longitudinal axis L. A flow channel 2 for fluid flow along the longitudinal axis L is defined in the housing 1. First and second piezoelectric transducers 3.1, 3.2 are arranged on the outer surface of the housing 1. The housing 1 has a cylindrical shape and two flanges 1.1 and 1.2 for connection with other piping components. The housing also includes a flat outer surface portion 1.3 to which the first and second piezoelectric transducers 3.1, 3.2 are attached by adhesive.

[0055] In other embodiments, the housing may have other shapes, such as those with a rectangular cross-section.

[0056] Piezoelectric transducers 3.1 and 3.2 are vertically oriented, and the height of the top surface portion of piezoelectric transducers 3.1 and 3.2 perpendicular to their attachment is greater than their width parallel to the longitudinal axis L. The width of piezoelectric transducers 3.1 and 3.2 perpendicular to the longitudinal axis L is greater than the width of the flat outer surface portion 1.3. In other embodiments, the width of piezoelectric transducers 3.1 and 3.2 may be equal to the width of the flat outer surface portion 1.3 of the housing.

[0057] Figure 2 It shows Figure 1 A vertical sectional view of the flow meter 10 along the longitudinal axis L of the housing 1. Figure 2 In the configuration shown, the piezoelectric transducer 3.1 generates an acoustic wave with wavelength λ within the housing 1. At the piezoelectric transducer 3.1, the housing 1 has a thickness d1, which is smaller than the wavelength λ of the acoustic wave, such that a Lamb wave Lb1 is excited within the housing 1. Then, at least a portion of the Lamb wave Lb1 is coupled as a bulk wave Bk1 into the medium F flowing in the flow channel 2. The incident angle θ of the bulk wave Bk1 coupling into the medium F is determined by the sound velocity c in the medium. M With the speed of sound c in the shell H The ratio may be determined by the speed of sound c in the medium. M The speed of sound c in the wall of the shell H The ratio (i.e., through arcsin(c)) M / c H ))definition.

[0058] The bulk wave Bk1 then propagates through the flow channel 2 until it impacts the wall portion 1.5 of the housing 1 on the opposite side of the piezoelectric transducers 3.1 and 3.2 relative to the flow channel 2. The wall portion 1.5 of the housing 1 on the opposite side of the piezoelectric transducers 3.1 and 3.2 has a thickness d2, which is also less than the wavelength λ. Therefore, at least a portion of the bulk wave Bk1 is coupled into the housing 1 at an incident angle -θ, generating another Lamb wave Lb2. Then, at least a portion of this Lamb wave Lb2 is coupled again into the medium F as the bulk wave Bk2 at an incident angle θ. The bulk wave Bk2 then propagates through the flow channel 2 until it reaches the second piezoelectric transducer 3.2. At the second piezoelectric transducer 3.2, at least a portion of the bulk wave Bk2 is coupled into the housing as a Lamb wave Lb3, which is received by the second piezoelectric transducer 3.2. The described paths of the Lamb wave and bulk waves Lb1, Bk1, Lb2, Bk2, Lb3 between the first and second piezoelectric transducers 3.1, 3.2 can be reversed, such that the second piezoelectric transducer 3.2 acts as a transmitter and the first piezoelectric transducer 3.1 acts as a receiver. In the illustrated embodiment, the thickness of the housing (or corresponding housing wall) is uniform in the region between the first and second piezoelectric transducers 3.1, 3.2, such that d1 = d2.

[0059] It can be recognized that for a steeper angle of incidence achievable by changing the material of the housing, or for a greater distance l between the first and second piezoelectric transducers 3.1, 3.2, the path of the ultrasonic signal containing waves Lb1, Bk1, Lb2, Bk2, Lb3 can be changed from the V-shape shown to a W-shape. In the W-shaped path, the bulk wave coupled from the wall portion 1.5 on the opposite side of the piezoelectric transducers 3.1, 3.2 into the medium F can impact the intermediate wall portion 1.4 arranged between the piezoelectric transducers 3.1, 3.2. The intermediate wall portion 1.4 has a thickness d3, which is smaller than the wavelength, such that a Lamb wave will be excited in the intermediate wall portion 1.4. At least a portion of this Lamb wave will be coupled into the medium F and propagate again to the opposite side of the flow channel 2. By exciting a Lamb wave at a corresponding wall portion of the housing 1, the Lamb wave is reflected on the opposite side of the flow channel 2 and recoupled into the medium F as another volume wave. This volume wave can reach the second piezoelectric transducer 3.2, where it can be coupled back into the housing 1 as a Lamb wave and received by the second piezoelectric transducer 3.2. Similarly, by further changing the housing material or the distance l between the first and second piezoelectric transducers 3.1 and 3.2, a multi-V-shaped path for the ultrasonic signal can be achieved.

[0060] Figure 3 It shows Figure 2A detailed enlarged view of the first piezoelectric transducer 3.1. The piezoelectric transducer 3.1 has a rectangular cuboid shape, with a width W along the longitudinal axis L and a height H perpendicular to the longitudinal axis L and perpendicular to the outer surface of the housing 1 to which the piezoelectric transducer 3.1 is attached. The piezoelectric transducer 3.1 is attached to the outer surface of the housing 1 by adhesive 44 layers.

[0061] The height H of piezoelectric transducer 3.1 is 1.6 times the width W of piezoelectric transducer 3.1. The height H of the piezoelectric transducer is adapted to the excitation wavelength λ of piezoelectric transducer 3.1. piezo The width W of the piezoelectric transducer 3.1 is adapted to be half the wavelength λ of the sound wave to be generated in the housing 1. Therefore, the piezoelectric transducer 3.1 is vertically oriented relative to the longitudinal axis L.

[0062] The piezoelectric transducer 3.1 is connected via a first electrode 41 and a second electrode 42, which are arranged on two opposing surfaces of the piezoelectric transducer 3.1. These two opposing surfaces are perpendicular to the longitudinal axis L of the housing. Electrodes 41 and 42 are connected via a wire 43. By applying a suitable voltage to electrodes 41 and 42, the piezoelectric transducer 3.1 oscillates parallel to the longitudinal axis L, as indicated by the double arrows in the piezoelectric transducer 3.1. Specifically, the piezoelectric transducer 3.1 can be excited at a suitable frequency such that the wavelength of the sound wave generated in the housing 1 is equal to twice the width W of the piezoelectric transducer 3.1.

[0063] The oscillation of the piezoelectric transducer 3.1 excites acoustic waves within the housing 1. The thickness d1 of the housing is less than the wavelength λ of the acoustic waves, resulting in the generation of a Lamb wave Lb. In the example shown, the thickness d1 of the housing is within the width W of the piezoelectric transducer 3.1. At least a portion of the Lamb wave Lb is coupled into the medium F as a volume wave Bk at an incident angle θ, which propagates through the flow channel 2.

Claims

1. A flow meter (10) comprising a housing (1) having a longitudinal axis (L), and first and second piezoelectric transducers (3.1, 3.2), the housing (1) defining a flow channel (2) for a fluid flow (F), the first and second piezoelectric transducers (3.1, 3.2) being disposed at an outer surface (1.3) of the housing (1) and each configured to generate an acoustic wave of wavelength λ in the housing (1), wherein the housing (1) has a thickness d1 at the piezoelectric transducers (3.1, 3.2) and a thickness d2 on opposite sides (1.5) of the flow channel (2), the thickness d 1 and d2 are less than the wavelength λ of the sound wave, such that Lamb waves (Lb, Lb1, Lb2, Lb3) can be excited in the housing (1), wherein each of the piezoelectric transducers (3.1, 3.2) is configured to receive sound waves in the form of Lamb waves (Lb, Lb1, Lb3) from the housing (1), wherein each of the piezoelectric transducers (3.1, 3.2) has a height (H) perpendicular to the outer surface (1.3) of the housing (1) at the piezoelectric transducer (3.1, 3.2) and a width (W) parallel to the longitudinal axis (L), wherein the height (H) is greater than the width (W).

2. The flow meter (10) according to claim 1, wherein the piezoelectric transducer (3.1, 3.2) is attached to the outer surface (1.3) of the housing (1) by an adhesive (44).

3. The flow meter (10) according to claim 1 or 2, wherein the thicknesses d1 and d2 are equal.

4. The flow meter (10) according to any one of the preceding claims, wherein the piezoelectric transducer (3.1, 3.2) is configured to oscillate in a direction parallel to the longitudinal axis (L) to excite Lamb waves (Lb, Lb1, Lb3) in the housing (1).

5. The flow meter (10) according to any one of the preceding claims, wherein each of the piezoelectric transducers (3.1, 3.2) is in contact with two electrodes (41, 42), the two electrodes (41, 42) being arranged on two opposite surfaces of the respective piezoelectric transducers (3.1, 3.2), wherein the two opposite surfaces are perpendicular to the longitudinal axis (L).

6. The flowmeter according to any one of claims 1 to 4, wherein each of the piezoelectric transducers is in contact with two electrodes arranged on two opposing surfaces of the respective piezoelectric transducer, wherein the two opposing surfaces are parallel to the longitudinal axis.

7. The flow meter (10) according to any one of the preceding claims, wherein each of the piezoelectric transducers (3.1, 3.2) has a rectangular cuboid shape.

8. The flow meter (10) according to any one of the preceding claims, wherein the housing (1) includes an intermediate wall portion (1.4) disposed between the piezoelectric transducers (3.1, 3.2), the thickness d3 of the intermediate wall portion (1.4) being less than the wavelength λ of the sound wave, such that a Lamb wave can be excited in the intermediate wall portion (1.4).

9. The flow meter (10) according to claim 8, wherein the thickness d3 is equal to the thickness d1 and / or the thickness d2.

10. The flow meter (10) according to any one of the preceding claims, wherein the piezoelectric transducers (3.1, 3.2) are arranged at a distance l from each other to generate V-shaped, W-shaped or multi-V-shaped paths of ultrasonic signals (Bk1, Bk2) in the flow channel (2).

11. The flow meter (10) according to any one of the preceding claims, wherein the height (H) of the piezoelectric transducer (3.1, 3.2) is 1 to 2 times, preferably 1.2 to 1.8 times, and particularly preferably 1.3 to 1.7 times, the width (W) of the piezoelectric transducer (3.1, 3.2).

12. The flow meter (10) according to any one of the preceding claims, wherein the width (W) of the piezoelectric transducer (3.1, 3.2) is one-quarter to three-quarters of the wavelength λ of the sound wave generated by the piezoelectric transducer (3.1, 3.2) in the housing (1), preferably one-half of the wavelength λ.

13. The flow meter (10) according to any one of the preceding claims, wherein the width (W) of the piezoelectric transducer (3.1, 3.2) is 0.5 mm to 8 mm, preferably 1 mm to 4 mm, and particularly preferably 1.5 mm to 2.5 mm.

14. The flow meter (10) according to any one of the preceding claims, wherein the height (H) of the piezoelectric transducer (3.1, 3.2) is 0.3 mm to 8 mm, preferably 0.75 mm to 4 mm, and particularly preferably 1 mm to 2 mm.

15. The flow meter (10) according to any one of the preceding claims, wherein the housing (1) is made of plastic, preferably injection-molded plastic, or metal, or by additive manufacturing.

16. The flow meter (10) according to any one of the preceding claims, wherein the housing (1) has a cross-section with a constant area along the longitudinal axis (L) between the first and second piezoelectric transducers (3.1, 3.2).

17. The flow meter (10) according to any one of the preceding claims, wherein the housing (1) includes a flat outer surface portion (1.3), wherein the first and second piezoelectric transducers (3.1, 3.2) are arranged on the flat outer surface portion (1.3).

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