Lamb wave ultrasonic flow meter

Abstract

A flow meter (10) is described, comprising a housing (1) with a longitudinal axis (L) and a flow channel (2), the flow meter (10) comprising a first and a second piezoelectric transducer (3.1, 3.2) arranged at an outer surface (1.3) of the housing (1) and each configured to generate an acoustic wave with a wavelength λ in the housing (1), wherein the housing (1) has a thickness d 1 at the piezoelectric transducers (3.1, 3.2) and a thickness d 2 at an opposite side (1.5) of the flow channel (2), the thicknesses d 1 and d 2 being smaller than the wavelength λ of the acoustic wave such that a Lamb wave (Lb1, Lb2, Lb3) is excitable in the housing (1), wherein the piezoelectric transducers (3.1, 3.2) are each configured to receive an acoustic wave in the form of a Lamb wave (Lb1, Lb3) from the housing, wherein the piezoelectric transducers (3.1, 3.2) each exhibit a height (H) and a width (W), wherein the height (H) is larger than the width (W).

Description

[0001] LAMB WAVE ULTRASONIC FLOW METER

[0002] Field of the invention

[0003] The present invention relates to a flow meter, in particular, to an ultrasonic flow meter .

[0004] Background of the invention

[0005] Flow meters such as ultrasonic flow meters are commonly employed to record flow signals in appliances for heating, ventilation and / or air conditioning . A flow meter may, for instance , record flow of a fluid such as water or glycol through a flux calorimeter . A quantity indicative of heat trans fer can then be derived from the recorded flow and from additional measurements of temperature . Ultrasonic flow meters can also be employed to record flow of trans former oil circulating through the ducts of power trans formers . A recorded flow value can then be used to adj ust speeds of oil pumps while the power trans former is in oil directed and air forced cooling mode .

[0006] The ultrasonic signal used for the ultrasonic flow meters is conventionally generated by ultrasonic transducers attached to the housing of the flow meter . Typically, an ultrasonic signal generated by an ultrasonic transmitter is guided into the medium flowing in the housing and detected by an ultrasonic receiver . Various reflectors arranged in the housing are thereby used to guide the ultrasonic signal along a predetermined path between the ultrasonic transmitter and ultrasonic receiver .

[0007] Such an ultrasonic flow meter is for example described in EP 0 890 826 Al , consisting of a housing, which is provided at its two ends with flanges and can be fitted into a pipeline system . The housing is provided with a screw-on cover, for accommodating the metering arrangement , which comprises ultrasonic transducers fitted to the cover, means for repeated deflection of the ultrasonic energy from the ultrasonic transmitter to the ultrasonic receiver in the flow path of the housing, and the associated electronics , wherein a troughshaped, removable insert is mounted in a matching pocket in the housing in the flow path of the housing, which insert forms with the cover a unit which contains the metering arrangement and has means for deflecting the ultrasonic energy from discs which reflect ultrasound and are inserted in preshaped pockets of the insert .

[0008] Summary of the invention

[0009] When measuring the flow of a fluid using an ultrasonic flow meter, it is des ired to optimi ze the angle under which the ultrasonic signal enters the medium for improving the measurement signal quality . This may for example be aimed for by attaching the ultrasonic transducer on a slope of a wedge arranged on an outer face of the housing . Alternatively or additionally, the ultrasonic signal may be guided by reflectors arranged in the housing . On the other hand, it is desired to reduce the structural complexity of the flow meter while ensuring a high quality of the ultrasonic signal . It is therefore an obj ect of the invention to provide a flow meter which at least partially improves the prior art and avoids at least part of the disadvantages of the prior art .

[0010] According to the present invention, this obj ect is particularly achieved by a flow meter comprising a housing with a longitudinal axis , the housing defining a flow channel for a fluid flow, a f irst and a second piezoelectric transducer arranged at an outer surface of the housing and each configured to generate an acoustic wave with a wavelength X in the housing, wherein the housing has a thickness di at the piezoelectric transducers and a thickness d2 at an opposite side of the flow channel , the thicknesses di and d2 being smaller than the wavelength X of the acoustic wave such that a Lamb wave is excitable in the housing, wherein the piezoelectric transducers are each configured to receive an acoustic wave in the form of a Lamb wave from the housing, wherein the piezoelectric transducers each exhibit a height oriented perpendicular to the outer surface of the housing at the piezoelectric transducers and a width oriented parallel to the longitudinal axis , wherein the height is larger than the width .

[0011] In contrast to conventional flow meters where piezoelectric transducers are used to generate a longitudinal wave in the housing which couples into the medium, a Lamb wave can be generated in the housing . Due to the housing surrounding the medium flowing through the flow channel , at least a part of the Lamb wave generated in the housing can couple into the medium as a bulk wave . In the medium, the bulk wave may propagate across the flow channel to the opposite s ide of the flow channel where at least a part of the bulk wave can couple into the housing generating another Lamb wave . At least a part of this Lamb wave may afresh couple into the medium as a bulk wave and propagate again across the flow channel until reaching the second piezoelectric transducer . At the second piezoelectric transducer, at least a part of the bulk wave may couple into the housing as a Lamb wave which may be received by the second piezoelectric transducer . The first and second piezoelectric transducer may be arranged at the same side of the flow channel or at opposite sides of the flow channel . The first and second piezoelectric transducer may also be arranged at other positions around the circumference of the flow channel . For an arrangement with the second piezoelectric transducer being arranged at an opposite side of the flow channel with respect to the first piezoelectric transducer, a part of the Lamb wave generated by the first pie zoelectric transducer may couple into the medium as a bulk wave , propagate across the flow channel to the opposite side of the flow channel where at least a part of the bulk wave may couple into the housing generating another Lamb wave which may be received by the second piezoelectric transducer .

[0012] Using a Lamb wave for measuring the fluid flow has the advantage that an optimal angle of incidence 6 o f the bulk wave coupled into the medium can be achieved, with the angle of incidence 6 defined by with respect to an axis being perpendicular to the longitudinal axis of the housing, where CM is the velocity of sound in the medium and CH the velocity of sound in the housing ( or the wall of the housing, respectively) . Likewise , a part of the bulk wave can in turn couple into the housing as a Lamb wave under the angle of incidence 6 after crossing the flow channel . Separate structures such as wedges with mounted piezoelectric transducers for optimi zing the angle under which the ultrasonic signal enters the flow channel or reflectors inside the flow channel for guiding the ultrasonic signal can therefore be avoided, reducing the complexity of the flow meter . Instead, the angle of incidence 6 as defined by the relative magnitudes of the involved sound velocities can be used for obtaining an optimal path of the ultrasonic signal .

[0013] Due to the reduction or avoidance of additional structures within the flow channel , linearity of the fluid flow can be improved, which in turn improves the signal quality of the flow meter . Manufacturing of the flow meter can also be simpli fied as reflectors do not have to be introduced into and positioned in the flow channel , which may be cumbersome and, in particular, lead to inaccuracies in the intended path of the ultrasonic signal .

[0014] Preferably, the first and second piezoelectric transducers are each designed as monolithic blocks . Compared to the generation of surface acoustic waves using arrays of interdigital couplers , the complexity and costs can therefore be reduced .

[0015] Due to the vertical orientation of the piezoelectric transducers with a larger height than width, an oscillation across the width of the piezoelectric transducers can be exploited for the generation and detection of Lamb waves , in contrast to conventional flow meters with a hori zontal orientation of the piezoelectric transducers with a larger width than the height where the thickness oscillation of the piezoelectric transducers is exploited . The width of the first and second piezoelectric transducers can be varied depending on the desired wavelength of the Lamb wave to be generated in the housing .

[0016] The flow meter provides the further advantage that additional transition layers for impedance matching can be reduced or dispensed with due to the possibility of matching the wavelength of the wave produced by the piezoelectric transducer to the wavelength of the induced wave in the housing . Thus , a mismatch between the wavelength of the wave in the transducer and the wave in the housing can be reduced, leading to signi ficantly lower energy loss in the transition of the waves from one material to the other .

[0017] In some embodiments , the pie zoelectric transducers are attached to the outer surface of the housing by an adhesive .

[0018] By using an adhesive , the piezoelectric transducers can be provided at the housing in a simple and cost-ef fective fashion . Further, additional structures reaching into the flow channel for attaching the piezoelectric transducers which may interfere with the fluid flow can be prevented .

[0019] Preferably, the thicknesses di and d2 are equal .

[0020] The housing may therefore be manufactured as a simple pipe with circular or polygonal cross-section with a constant thickness in the region of the piezoelectric transducers .

[0021] Preferably, the piezoelectric transducers are configured to oscillate in a direction parallel to the longitudinal axis for exciting a Lamb wave in the housing . An oscillation across the width of the piezoelectric transducers can therefore be exploited for the generation and detection of Lamb waves which, as described above , is facilitated by the vertical orientation of the piezoelectric transducers .

[0022] In some embodiments , the piezoelectric transducers are each contacted by two electrodes arranged at two opposite faces of the respective piezoelectric transducer, wherein the two opposite faces are perpendicular to the longitudinal axis .

[0023] By applying a suitable voltage to the electrodes , the piezoelectric transducers may therefore oscillate along the longitudinal axis in order to excite a Lamb wave in the housing .

[0024] In particular, the two opposite faces may be perpendicular to the outer face of the housing at the piezoelectric transducers .

[0025] In some embodiments , the piezoelectric transducers are each contacted by two electrodes arranged at two opposite faces of the respective piezoelectric transducer, wherein the two opposite faces are parallel to the longitudinal axis .

[0026] In some embodiments , the piezoelectric transducers each have a rectangular cuboid shape .

[0027] In particular, the cuboids may be designed as a monolithic block . The design of the flow meter can therefore further be simpli fied, in particular, compared to complex arrays of interdigital couplers .

[0028] In some embodiments , the housing comprises an intermediate wall portion arranged between the piezoelectric transducers with a thickness ds being smaller than the wavelength X of the acoustic wave such that a Lamb wave is excitable in the intermediate wall portion .

[0029] A bulk wave impinging on the intermediate wall portion under an angle 0 may excite a Lamb wave in the intermediate wall portion, from which Lamb wave another bulk wave may couple again into the medium under an angle -0. Thus , the intermediate wall portion can serve as a reflector i f a bulk wave impinges on the intermediate wall section .

[0030] Preferably, the thickness ds is equal to the thickness di and / or ds .

[0031] In some embodiments , the pie zoelectric transducers are arranged at a distance 1 from each other to generate a V- shaped, W-shaped or multiple-V-shaped path of an ultrasonic signal in the flow channel .

[0032] Since the process of taking up of a part of a bulk wave by the housing, exciting a Lamb wave therein and re-coupling of a part of the Lamb wave as bulk wave into the medium can be repeated along the overall path of the ultrasonic signal , various shapes of the path of the ultrasonic signal can be obtained, as desired, by a suitable design of the flow meter and in particular, of the arrangement of the piezoelectric transducers and / or one or more thicknesses of the housing .

[0033] In some embodiments , the height of the piezoelectric transducers is between 1 to 2 times , preferably 1 . 2 to 1 . 8 times , particularly preferably 1 . 3 to 1 . 7 times , larger than the width of the piezoelectric transducers . In particular, the height of the piezoelectric transducers may be adapted to provide operation of the piezoelectric transducers on resonance or near resonance.

[0034] In some embodiments, the width of the piezoelectric transducers is between a quarter and three quarter, preferably one half, of the wavelength X of the acoustic wave generated by the piezoelectric transducers in the housing.

[0035] Therefore, the width of the piezoelectric transducers can be adapted to provide optimal excitation of an acoustic wave or Lamb wave, respectively, in the housing.

[0036] Operation of the flow meter can therefore be optimized by adjusting the dimensions of the piezoelectric transducers where the height can be adjusted to provide operation of the piezoelectric transducers on or near resonance while the width can be adjusted to provide optimal excitation of an acoustic wave in the housing.

[0037] In some embodiments, the width of the piezoelectric transducers is between 0.5 mm and 8 mm, preferably between 1 mm and 4 mm, particularly preferably between 1.5 mm and 2.5 mm.

[0038] In some embodiments, the height of the piezoelectric transducers is between 0.3 mm and 8 mm, preferably between 0.75 mm and 4 mm, particularly preferably between 1 mm and 2 mm.

[0039] By using piezoelectric transducers with these specific dimensions, optimal excitation of an acoustic wave or Lamb wave, respectively, in the housing adapted to desired wavelengths of the acoustic wave can be achieved. The preferred excitation frequencies for the piezoelectric transducers may be between 200 kHz and 4 MHz , preferably between 500 kHz and 2 MHz , particularly preferably between 800 kHz and 1 . 2 MHz .

[0040] The housing may be made from plastics , preferably inj ection- molded plastics , or metal or by additive manufacturing .

[0041] The plastics may comprise one or more of : an epoxy polymer, polytetrafluoroethylene , polyethylene , polyethylene terephthalate , polyester, etc . The metal may comprise one or more of : steel , austenitic steel , ferritic steel , aluminum, aluminum alloy, brass etc . or an alloy thereof .

[0042] In some embodiments , the housing has along the longitudinal axis between the first and second piezoelectric transducers a cross-section with a constant area .

[0043] Therefore , the flow channel can be designed to be straight along the longitudinal axis and, in particular, without obstacles in the measurement-relevant region, thereby improving measurement conditions such as linearity, reduction or avoidance of pressure loss , reduction or avoidance of dirt accumulation, etc .

[0044] In some embodiments , the housing comprises a flattened outer surface portion, wherein the first and second piezoelectric transducers are arranged on the flattened outer surface portion .

[0045] By providing a flattened outer surface portion, the attachment of the piezoelectric transducers can be facilitated . Furthermore , the thickness of adhesive used to attach the piezoelectric transducers can be reduced and the acoustic coupling between the piezoelectric transducers and the housing be improved .

[0046] Preferably, the piezoelectric transducers have a breadth perpendicular to the longitudinal axis which is equal or larger than the breadth of the flattened outer surface portion perpendicular to the longitudinal axis .

[0047] The piezoelectric transducers may therefore extend in the direction perpendicular to the longitudinal axis across the flattened outer surface portion of the housing . By dimensioning the breadth of the piezoelectric transducers perpendicular to the longitudinal axis to be equal or larger than the breadth of the flattened outer surface portion, the ultrasonic signal can be improved .

[0048] Brief description of the drawings

[0049] The present invention will be explained in more detail , by way of exemplary embodiments , with reference to the schematic drawings , in which :

[0050] Fig . l shows a perspective view of an embodiment of a flow me ter ;

[0051] Fig . 2 shows the flow meter of Fig . l in a vertical cut view along the longitudinal axis of the housing;

[0052] Fig . 3 shows a magni fication of a detail of Fig . 2 at one o f the piezoelectric transducers . Detailed description of exemplary embodiments

[0053] Figure 1 shows an embodiment of a flow meter 10 in a perspective view. The flow meter 10 comprises a housing 1 with a longitudinal axis L. The housing 1 defines therein a flow channel 2 for a fluid flow along the longitudinal axis L. A first piezoelectric transducer 3.1 and a second piezoelectric transducer 3.2 are arranged at an outer surface of the housing 1. The housing 1 has a cylindrical shape with two flanges 1.1 and 1.2 for connection with other duct components. The housing further comprises a flattened outer surface portion 1.3 on which the first and second piezoelectric transducers 3.1, 3.2 are attached by an adhesive.

[0054] In other embodiments, the housing may have other shapes, such as for example comprising a rectangular cross section.

[0055] The piezoelectric transducers 3.1, 3.2 have a vertical orientation, exhibiting a height perpendicular to the top surface portion on which they are attached, being larger than the width parallel to the longitudinal axis L. The breadth of the piezoelectric transducers 3.1, 3.2 perpendicular to the longitudinal axis L is larger than the breadth of the flattened outer surface portion 1.3. In other embodiments, the breadth of the piezoelectric transducers 3.1, 3.2 may be equal to the breadth of the flattened outer surface portion 1.3 of the housing .

[0056] Figure 2 shows the flow meter 10 of Figure 1 in a vertical cut view along the longitudinal axis L of the housing 1. In the configuration as shown in Figure 2, the piezoelectric transducer 3.1 generates an acoustic wave with a wavelength X in the housing 1. At the piezoelectric transducer 3.1, the housing 1 has a thickness di being smaller than the wavelength X of the acoustic wave such that a Lamb wave Lbi is excited in the housing 1. At least a part of the Lamb wave Lbi then couples as a bulk wave Bki into the medium F flowing in the flow channel 2. The angle of incidence 6 of the bulk wave Bki coupled into the medium F is defined by the ratio of the velocity of sound CM in the medium and the velocity of sound CH in the housing or the wall of the housing, respectively (i.e. by arcs in ( CM / CH) ) .

[0057] The bulk wave Bki then propagates across the flow channel 2 until impinging on a wall portion 1.5 of the housing 1 on the opposite side to the piezoelectric transducers 3.1, 3.2 with respect to the flow channel 2. The wall portion 1.5 of the housing 1 has a thickness d2 at the opposite side to the piezoelectric transducers 3.1, 3.2 which is also smaller than the wavelength X. Therefore, at least a part of the bulk wave Bki couples into the housing 1 under an angle of incidence - 6 generating another Lamb wave Lbi. At least a part of this Lamb wave Lbi then couples afresh into the medium F under the angle of incidence 6 as a bulk wave Bki. The bulk wave Bki then propagates across the flow channel 2 until reaching the second piezoelectric transducer 3.2. At the second piezoelectric transducer 3.2, at least a part of the bulk wave Bki couples into the housing as a Lamb wave Lbi, which is received by the second piezoelectric transducer 3.2. The described path of the Lamb waves and bulk waves Lbi, Bki, Lbi, Bki, Lbi between the first piezoelectric transducer 3.1 and the second piezoelectric transducer 3.2 can be reversed such that the second piezoelectric transducer 3.2 acts as an emitter and the first piezoelectric transducer 3.1 acts as a receiver. In the shown embodiment, the thickness of the housing (or the wall of the housing, respectively) is uniform in the region between the first and second piezoelectric transducers 3.1, 3.2, such that di=d2.

[0058] It can be recognized that for a steeper angle of incidence, which may be achieved by varying the material of the housing, or for a larger distance 1 between the first and second piezoelectric transducers 3.1, 3.2, the path of the ultrasonic signal comprising the waves Lbi, Bki, Lb2, Bk2, Lbs could be changed from the shown V-shape to a W-shape. In a W-shaped path, the bulk wave coupled into the medium F from the wall portion 1.5 at the opposite side to the piezoelectric transducers 3.1, 3.2 may impinge on an intermediate wall portion 1.4 arranged between the piezoelectric transducers 3.1, 3.2. The intermediate wall portion 1.4 has a thickness d3 being smaller than the wavelength such that a Lamb wave would be excited in the intermediate wall portion 1.4. From this Lamb wave, at least a part would be coupled into the medium F and propagate again to the opposite side of the flow channel 2. After reflection on the opposite side of the flow channel 2 by exciting a Lamb wave at the respective wall portion of the housing 1 and re-coupling as another bulk wave into the medium F, the second piezoelectric transducer 3.2 may be reached where the bulk wave can couple into the housing 1 as a Lamb wave which is received by the second piezoelectric transducer 3.2. Likewise, a multiple-V-shaped path of the ultrasonic signal may be achieved by further varying the housing material or the distance 1 between the first and second piezoelectric transducers 3.1, 3.2.

[0059] Figure 3 shows a magnification of a detail of Figure 2 at 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 to the outer surface of the housing 1 on which the piezoelectric transducer 3.1 is attached to. The piezoelectric transducer 3.1 is attached to the outer surface of the housing 1 by a layer of an adhesive 44.

[0060] The height H of the piezoelectric transducer 3.1 is 1.6 times larger than the width W of the piezoelectric transducer 3.1. The height H of the piezoelectric transducer is adapted to the wavelengthpiezo of the excitation of the piezoelectric transducer 3.1 whereas the width W of the piezoelectric transducer 3.1 is adapted to half of the wavelength X of the acoustic wave to be generated in the housing 1. The piezoelectric transducer 3.1 therefore has a vertical orientation with respect to the longitudinal axis L.

[0061] The piezoelectric transducer 3.1 is contacted by a first electrode 41 and a second electrode 42 which are arranged at two opposite faces of the piezoelectric transducer 3.1. The two opposite faces are perpendicular to the longitudinal axis L of the housing. The electrodes 41 and 42 are contacted by wires 43. By applying a suitable voltage to the electrodes 41 and 42, the piezoelectric transducer 3.1 oscillates parallel to the longitudinal axis L, as symbolized by the double arrow in the piezoelectric transducer 3.1. In particular, the piezoelectric transducer 3 . 1 may be excited by a suitable frequency such that the wavelength of the acoustic wave generated in the housing 1 amounts to twice the width W of the piezoelectric transducer 3 . 1 . Oscillation of the piezoelectric transducer 3 . 1 excites an acoustic wave in the housing 1 . The thickness d of the housing is smaller than the wavelength X of the acoustic wave such that a Lamb wave Lb is generated . In the shown example , the thickness d of the housing is in the range of the width W of the piezoelectric transducer 3 . 1 . At least a part o f the Lamb wave Lb couples into the medium F under the angle of incidence 0 as a bulk wave Bk which propagates across the flow channel 2 .

Claims

Claims1. A flow meter (10) comprising a housing (1) with a longitudinal axis (L) , the housing (1) defining a flow channel (2) for a fluid flow (F) , a first and a second piezoelectric transducer (3.1, 3.2) arranged at an outer surface (1.3) of the housing (1) and each configured to generate an acoustic wave with a wavelength X in the housing (1) , wherein the housing (1) has a thickness di at the piezoelectric transducers (3.1, 3.2) and a thickness d2 at an opposite side (1.5) of the flow channel (2) , the thicknesses di and d2 being smaller than the wavelength X of the acoustic wave such that a Lamb wave (Lb, Lbi, Lb2, Lbs) is excitable in the housing (1) , wherein the piezoelectric transducers (3.1, 3.2) are each configured to receive an acoustic wave in the form of a Lamb wave (Lb, Lbi, Lbs) from the housing (1) , wherein the piezoelectric transducers (3.1, 3.2) each exhibit a height (H) oriented perpendicular to the outer surface (1.3) of the housing (1) at the piezoelectric transducers (3.1, 3.2) and a width (W) oriented parallel to the longitudinal axis (L) , wherein the height (H) is larger than the width (W) .

2. The flow meter (10) according to claim 1, wherein the piezoelectric transducers (3.1, 3.2) are 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 di and d2 are equal.

4. The flow meter (10) according to one of the preceding claims, wherein the piezoelectric transducers (3.1, 3.2) are configured to oscillate in a direction parallel to the longitudinal axis (L) for exciting a Lamb wave (Lb, Lbi, Lbs) in the housing (1) .

5. The flow meter according (10) to one of the preceding claims, wherein the piezoelectric transducers (3.1, 3.2) are each contacted by two electrodes (41, 42) arranged at two opposite faces of the respective piezoelectric transducer (3.1, 3.2) , wherein the two opposite faces are perpendicular to the longitudinal axis (L) .

6. The flow meter according to one of the claims 1 to 4, wherein the piezoelectric transducers are each contacted by two electrodes arranged at two opposite faces of the respective piezoelectric transducer, wherein the two opposite faces are parallel to the longitudinal axis.

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

8. The flow meter according (10) to one of the preceding claims, wherein the housing (1) comprises an intermediate wall portion (1.4) arranged between the piezoelectric transducers (3.1, 3.2) with a thickness ds being smaller than the wavelength X of the acoustic wave such that a Lamb wave is excitable in the intermediate wall portion (1.4) .

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

10. The flow meter (10) according to one of the preceding claims, wherein the piezoelectric transducers (3.1, 3.2) are arranged at a distance 1 from each other to generate a V-shaped, W-shaped or multiple-V-shaped path of an ultrasonic signal (Bki, Bk2) in the flow channel (2) .

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

12. The flow meter (10) according to one of the preceding claims, wherein the width (W) of the piezoelectric transducers (3.1, 3.2) is between a quarter and three quarter, preferably one half, of the wavelength X of the acoustic wave generated by the piezoelectric transducers(3.1, 3.2) in the housing (1) .

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

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

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

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

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