Ultrasonic transducer, method for assembling same and flowmeter comprising at least one such transducer

Abstract

Disclosed is an ultrasonic transducer including: at least one piezoelectric wafer having two parallel planar main faces: a front face and a posterior face; at least one posterior plate having two parallel planar main faces: an anterior face and a rear face, the anterior face of the posterior plate extending facing, and in contact with, the posterior face of the piezoelectric wafer. The posterior plate has a thickness between three and seven times the thickness of the piezoelectric wafer. The posterior plate has an acoustic impedance between 10 MPa.Math.s.Math.m−1 and 35 MPa.Math.s.Math.m−1. Also disclosed is a method for assembling such a transducer as well as a flowmeter including at least one such transducer.

Claims

1. An ultrasonic transducer comprising: at least one piezoelectric wafer having two parallel planar main faces: a front face and a posterior face, at least one posterior plate having two parallel planar main faces: an anterior face and a rear face, the anterior face of said posterior plate extending facing, and in contact with, the posterior face of the piezoelectric wafer, a front electrode at the front of, and in contact with, the front face of the piezoelectric wafer, said front electrode being electrically connected to the outside of the transducer, a posterior electrode at the rear of, and in contact with, the rear face of the posterior plate, said posterior electrode being electrically connected to the outside of the transducer, an outer shell having a front wall extending in front of the front electrode and in front of the front face of the piezoelectric wafer, this front wall having a thinned-down portion, named window, superimposed in front of, and facing, the front electrode and the front face of the piezoelectric wafer, wherein, the posterior plate has a thickness between three and seven times the thickness of the piezoelectric wafer, the posterior plate has an acoustic impedance between 20 MP a.Math.s.Math.m.sup.−1 and 30 MP a.Math.s.Math.m.sup.−1, and wherein the piezoelectric wafer has an acoustic impedance between 20 MP a.Math.s.Math.m.Math..sup.−1 and 30 MP a.Math.s.Math.m.Math..sup.−1, and wherein said acoustic impedance of said posterior plate is substantially similar to said acoustic impedance of said piezoelectric wafer, and wherein a synthetic resin having a thickness greater than or equal to half the thickness of the piezoelectric wafer is disposed between the piezoelectric wafer and an inner face of the window of the outer shell.

2. The transducer according to claim 1, wherein the posterior plate is formed of at least one material adapted to transmit ultrasound waves.

3. The transducer according to claim 1, wherein the piezoelectric wafer has an acoustic impedance of substantially 25 MP a.Math.s.Math.m.Math..sup.−1.

4. The transducer according to claim 1, wherein the thickness of the piezoelectric wafer is less than 2 mm.

5. The transducer according to claim 1, wherein the thickness of the posterior plate is between 1.5 mm and 5 mm.

6. A flowmeter comprising at least one ultrasonic transducer in a flow of fluid, wherein each ultrasonic transducer is a transducer according to claim 1.

7. The transducer as claimed in claim 1, wherein propagation of ultrasonic sounds coming from the piezoelectric wafer is not reflected directly at the interface between the piezoelectric wafer and the posterior plate, but occurs instead within the posterior plate as far as said rear face of the posterior plate on which the ultrasonic sounds are reflected.

8. The transducer as claimed in claim 1, wherein the posterior plate is formed from a material selected from the group consisting of metal materials, ceramic materials having metal particles, and thermoplastic or thermosetting polymer materials having metal particles.

9. The transducer according to claim 1, wherein the posterior plate has radial dimensions greater than or equal to the radial dimensions of the piezoelectric wafer in order to be entirely in contact with the posterior face of the piezoelectric wafer.

10. The transducer according to claim 1, wherein the front face of the piezoelectric wafer is in contact with the front electrode and the rear face of the posterior plate is in contact with the posterior electrodes, the posterior face of the piezoelectric wafer being in contact with the rear face of the posterior plate, wherein the piezoelectric wafer, the posterior plate, the front electrode and the posterior electrode thus form a piezoelectric stack.

11. The transducer according to claim 10, wherein the window has a shape and format which correspond at least substantially to the shape and format of the piezoelectric wafer and the piezoelectric stack.

12. A method for measuring a flow rate of a fluid comprising: emitting a periodic signal with a first ultrasonic transductor as claimed in claim 1, where the periodic signal is emitted by the piezoelectric wafer and passing through a liquid media, delaying unwanted reflections of the periodic signal so that the first periods of the periodic signal are representatives of the passage measurement, receiving the firsts periods of the periodic signal with a second ultrasonic transductor comprising a piezoelectric wafer that receive the periodic signal, and determining the flow rate of the fluid by using the first periods of the periodic signal.

13. The method according to claim 12, wherein the periodic signal is delayed forward the piezoelectric wafer of the first ultrasonic transductor and the second ultrasonic transductor.

14. The method according to claim 13, wherein the first ultrasonic transductor and the second ultrasonic transductor respectively comprise a structure comprises a window disposed through an outer shell that includes the piezoelectric wafer, the window being disposed forward the piezoelectric wafer, wherein the thickness of the window delaying the periodic signal reflections in front of the piezoelectric wafer.

15. The method according to claim 12, wherein the periodic signal is delayed backwards the piezoelectric wafer of the first ultrasonic transductor and the second ultrasonic transductor.

16. The method according to claim 15, wherein the first ultrasonic transductor and the second ultrasonic transductor respectively comprise a structure comprises a posterior plate disposed in an outer shell that includes the piezoelectric wafer, the posterior plate being disposed backwards the piezoelectric wafer, wherein the thickness of the posterior plate delaying the periodic signal reflections backwards the piezoelectric wafer.

Description

(1) Other aims, features and advantages of the invention will become apparent upon reading the following description of one of its preferential embodiments given by way of non-limiting example and with reference to the attached figures in which:

(2) FIG. 1 is a schematic perspective view of a transducer in accordance with one embodiment of the invention,

(3) FIG. 2a is a schematic cross-sectional view of a transducer in accordance with one embodiment of the invention, FIG. 2b being a detail of FIG. 2a,

(4) FIG. 3 is a schematic cross-sectional view of a transducer in accordance with one embodiment of the invention,

(5) FIG. 4 is a schematic perspective view of a step of the method for assembling a transducer in accordance with one embodiment of the invention,

(6) FIG. 5 is a schematic axial cross-sectional view of a flowmeter in accordance with one embodiment of the invention,

(7) FIGS. 6 and 7 illustrate measurement signals coming from a transducer in accordance with one embodiment of the invention.

(8) An ultrasonic transducer 38 in accordance with the invention, illustrated in FIGS. 1 and 2a is of a shape elongated in length in a longitudinal direction 18. It also extends widthwise in a direction, named transverse direction 19, orthogonal and secant to the longitudinal direction 18.

(9) The transducer 38 comprises at least one piezoelectric wafer 31 and preferably a single piezoelectric wafer 31. It has two parallel planar main faces: a front face 48 (defining the front of the piezoelectric wafer 31 and of the transducer) and a posterior face 49 (defining the rear of the piezoelectric wafer 31 and of the transducer). These two main faces extend in planes parallel to the longitudinal direction 18 and to the transverse direction 19.

(10) A front electrode 20 is disposed in front of, and in contact with, the front face 48 of the piezoelectric wafer 31. A posterior electrode 25 is disposed at the rear of, and in contact with, the rear face 52 of the posterior plate 35.

(11) The front electrode 20 is electrically connected to the outside of the transducer by means of a front conductive rod 32 whereas the posterior electrode 25 is formed of a posterior conductive rod and is directly connected to the outside of the transducer.

(12) The front electrode 20, the piezoelectric wafer 31, the posterior plate 35 and the posterior electrode 25 form a piezoelectric stack. This piezoelectric stack is produced in a direction, named anteroposterior direction, corresponding to a thickness which extends in a direction orthogonal and secant to the longitudinal direction 18 and transverse direction 19. The anteroposterior direction defines an anterior orientation 47 (towards the front) and a posterior orientation 46 (towards the rear).

(13) The transducer 38 also comprises an outer shell 36 which protects the piezoelectric stack formed by the front electrode, the piezoelectric wafer, the posterior plate and the posterior electrode. The outer shell 36 has an internal space receiving the piezoelectric stack and a passage for each electrical conductor to the outside of the piezoelectric stack. The outer shell 36 has a front wall extending in front of the front electrode and in front of the front face of the piezoelectric wafer, this front wall having a thinned-down portion, named window 24, superimposed in front of, and facing, the front electrode and the front face of the piezoelectric wafer. The ultrasonic sounds emitted or received by the transducer, which are intended for said measurement of the fluid flow rate, pass through the window 24 substantially orthogonally to the front wall, said front wall being planar and parallel to the front face of the piezoelectric wafer 31.

(14) The outer shell 36 is formed of a rigid synthetic material selected from the group formed of polymer materials and composite materials with a polymer matrix. This polymer material (or the polymer matrix as applicable) is advantageously selected from the group of thermoplastic materials and thermosetting materials and especially from the group formed of polyether imide (PEI), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polysulfones (PSU), polyolefins such as polyethylenes (PE) and polyesters (PET), polystyrenes (PS), polyphenylene oxides (PPO), polyamides such as PA66, and mixtures thereof such as the Noryls®. In one particularly advantageous embodiment of a transducer in accordance with the invention, the outer shell 36 is formed of a polyether imide (PEI) sold under the name ULTEM® by the company Sabic (Riyadh, Saudi Arabia) and comprising 20% by weight of glass fibre. Therefore, the outer shell can be formed of a mould part, e.g. by injection moulding.

(15) An adhesive layer (not illustrated) is disposed between the piezoelectric wafer 31 and the front electrode 20 as well as between the piezoelectric wafer 31 and the posterior plate 35 so as to ensure good electrical contact between each of the elements of the piezoelectric stack.

(16) Advantageously and in accordance with the invention, said adhesive layer is selected from amongst electrically conductive adhesive materials (or glues). It may be e.g. a glue comprising at least one polymer material of the epoxy resin type, in which electrically conductive metal particles are dispersed. Each layer of adhesive material has a thickness which is very low with respect to the thickness of each of the elements of the piezoelectric stack. Each adhesive layer has e.g. a thickness of the order of 10 μm.

(17) A synthetic resin is disposed inside the outer shell so as to substantially fill the whole free space left by the piezoelectric stack disposed inside the outer shell. It makes it possible to hold the piezoelectric stack in position. It is also adapted to transmit the ultrasound waves coming from the piezoelectric wafer to a medium in which the transducer is disposed, such as water and which has an acoustic impedance much lower than that of the piezoelectric wafer. The synthetic resin layer thus also constitutes an impedance-adaptation layer between the piezoelectric material forming the piezoelectric wafer and the material forming the window of the outer shell then the water. The resin disposed in the internal space of the outer shell is selected from the group formed by epoxide (or “epoxy”) resins.

(18) On the other hand, in order to further optimise the reliability and the precision of the measurement, e.g. the measurement of a fluid flow rate, the synthetic resin disposed between the piezoelectric wafer and the inner face of the window of the outer shell has a thickness greater than or equal to half the thickness of the piezoelectric wafer. This also makes it possible to delay the unwanted signals linked to the reflections of the ultrasound waves towards the front of the piezoelectric wafer 31.

(19) The front face 48 of the piezoelectric wafer 31 is in contact with one of said electrodes, the front electrode 20. The rear face 52 of the posterior plate 35 is in contact with the other of said electrodes, the posterior electrode 25, the posterior face 49 of the piezoelectric wafer 31 being in contact with the rear face 51 of the posterior plate 35. These four pieces thus form the piezoelectric stack. The electrodes 20, 25 receive electrical signals (by means of the rod 32 for the electrode 20) and transmit these signals to the piezoelectric wafer 31. The latter emits, under the effect of the electric field, a mechanical constraint in the form of ultrasound waves. This first effect characterises a command of the transducer 38 in emitting mode. In a reciprocal manner, if the piezoelectric wafer 31 picks up a mechanical constraint, e.g. in the form of ultrasound waves, it is electrically polarised and thus transmits electrical signals to the electrodes 20, 25. This second effect characterises a measurement made by the transducer 38 in receiving mode.

(20) In a transducer 38 in accordance with the invention, the piezoelectric wafer 31 can have a thickness less than 2 mm and dimensions between 3 mm and 10 mm in all directions orthogonal to its thickness.

(21) The piezoelectric wafer 31 advantageously has a contour which is polygonal, in particular, square or rectangular, or is in the form of a disc. The piezoelectric wafer 31 is used in its thickness resonance mode.

(22) The piezoelectric wafer 31 is formed of a piezoelectric material. This piezoelectric material must be able to emit and receive ultrasound signals especially at a frequency of the order of 4 MHz. This material is e.g. a piezoelectric ceramic such as lead zirconate titanates (PZT). There is nothing to prevent other piezoelectric materials such as monocrystals being used.

(23) The posterior plate 35 has two parallel planar main faces: an anterior face 51 and a rear face 52. The anterior face 51 of the posterior plate extends facing, and in contact with, the posterior face 49 of the piezoelectric wafer 31. The posterior plate 35 has dimensions such that its anterior face 51 is in contact at least with the useful surface portion of the posterior face 49 of the piezoelectric wafer 31 located facing the window 24, i.e. at least in contact with the surface portion of the posterior face 49 of the piezoelectric wafer 31 through which passes at least one acoustic path which is useful for the fluid flow rate measurement of the transducer. In the embodiment illustrated in FIGS. 1 to 4, the posterior plate 35 has dimensions such that its anterior face 51 is at least in contact with the part of the posterior face 49 of the piezoelectric wafer 31 located facing the window 24. Preferably, as in the illustrated embodiment, the posterior plate 35 is such that it has radial dimensions greater than or equal to the radial dimensions of the piezoelectric wafer 31. In this way, the entirety of the anterior face 51 of the posterior plate 35 is in contact with the posterior face 49 of the piezoelectric wafer 31.

(24) The posterior plate 35 extends longitudinally and transversely to the interior of said outer shell 36. This posterior plate 35 is formed of a material adapted to transmit ultrasound waves. For example, the posterior plate 35 is formed of a material having an acoustic impedance between 10 MPa.Math.s.Math.m.sup.−1 and 35 MPa.Math.s.Math.m.sup.−1, especially between 25 MPa.Math.s.Math.m.sup.−1 and 32 MPa.Math.s.Math.m.sup.−1. The posterior plate 35 can be formed of a material having such an impedance and selected from among metal materials such as the alloys of copper and tin or of copper and aluminium, ceramic materials comprising metal particles, and thermoplastic or thermosetting polymer materials comprising metal particles. It may be e.g. CuSn8 or an epoxy resin comprising tungsten particles or an epoxy resin comprising glass fibres (such as FR-4®). Other materials are possible. The posterior plate 35 also has a thickness greater than three times the thickness of the piezoelectric wafer and in particular between three and five times the thickness of the piezoelectric wafer. In this way, the ultrasound waves emitted by the piezoelectric wafer 31 are transmitted within the posterior plate 35 in the posterior direction 46 of the transducer 38 as far as the rear face of the posterior plate 35 and are reflected thereon in the frontal direction 47 as far as the posterior face 49 of the piezoelectric wafer 31.

(25) This enables measurements, in particular fluid flow rate measurements, of great precision and reliability to be carried out, especially obtaining received signals with first periods stripped of unwanted signals and having an amplitude sufficient to permit an effective fluid measurement. In fact, it seams that the unwanted signals are delayed so that the first periods of the signal emitted and/or received are of good quality and representative of the passage measurement to be effected. In practice, the three or four first periods are stripped of unwanted signals and correspond to a maximum of energy, which makes it possible to deduce therefrom a measurement, especially a fluid flow rate measurement, which is precise and reliable. Furthermore, the unwanted signals are not only delayed: it is also necessary for these to have disappeared before the reception of the signal which corresponds to the subsequent series of waves (pulse).

(26) In one embodiment, a transducer in accordance with the invention comprises a piezoelectric wafer 31 the thickness of which is 500 μm, for a resonance frequency of the piezoelectric wafer of 4 MHz, and a posterior plate 35 the thickness of which is 2.25 mm.

(27) The electrode 20 extends longitudinally and transversely so that it respectively covers at least a part of the piezoelectric wafer 31.

(28) As can be seen in FIGS. 3 and 4, the front electrode 20 in contact with the front face 48 of the piezoelectric wafer 31 is in a generally rectangular shape, of which a first proximal portion is solid and another distal portion is partially hollow so as to be in the form of a fork comprising two lateral branches at its distal end which is disposed at the bottom of the outer shell 36. The two lateral branches of the front electrode 20 are in contact with the front face 48 of the piezoelectric wafer 31. The thickness of said lateral branches of the front electrode 20 is important and determines the thickness of the synthetic resin which will be present between the piezoelectric wafer and the inner face of the window of the outer shell. In the particularly advantageous embodiment illustrated, this synthetic resin layer has a thickness greater than or equal to half the thickness of the piezoelectric wafer. If the thickness of the piezoelectric wafer is e.g. 500 μm, the thickness of this synthetic resin layer is e.g. of the order of 300 μm. The lateral branches of the front electrode 20 must thus also have a thickness of 300 μm.

(29) Such an electrode 20 not only ensures lateral holding of the piezoelectric wafer 31 but, by virtue of its two lateral branches, also makes it possible to form a longitudinal stop within the outer shell 36, each of these ends of the two lateral branches of the electrode 20 being adapted to come into contact with an inner surface of the outer shell 36. This makes it possible to ensure excellent holding of the piezoelectric stack in position, the lateral branches of the electrode 20 each forming a mechanical stop and permitting simultaneously positional control with respect to the window of the outer shell during placement of the piezoelectric stack. The stability of the piezoelectric stack in the anteroposterior direction is also ensured by the presence of two longitudinal grooves within the outer shell and in which the two lateral branches of the electrode 20 slide.

(30) Furthermore, the shape of the electrode 20 with its two lateral branches makes it possible to minimise the surface for contact with the front face 48 of the piezoelectric wafer 31 in order to avoid disrupting the ultrasound waves emitted by the piezoelectric wafer 31.

(31) The front electrode 20 and the rods 32 and 25, the posterior rod 25 forming the posterior electrode, permit an electrical connection between the piezoelectric stack and an outer control device such as a data processing system, especially a computer system 42 (FIG. 5) and/or an electronic circuit. These elements make it possible to transmit electrical measurement signals from the piezoelectric stack 21 to the computer system 42; and, in a reciprocal manner, to transmit control signals from the computer system 42 towards the piezoelectric stack 21.

(32) In the transducer in accordance with the embodiment illustrated in FIGS. 1 to 4, the front rod 32 extends the front electrode 20 and the posterior electrode 25 is extended as far as the outside of the outer shell.

(33) The front electrode 20 and the front rod 32 can be formed of the same electrically conductive material.

(34) This rod 25, 32 is formed of an electrically conductive material. The rods 25, 32 extend longitudinally and transversely inside said outer shell 36 and outside the outer shell 36 through a plug 50 ensuring closure of the outer shell 36.

(35) The outer shell 36 has at least one window 24 arranged in front of, and facing, the piezoelectric stack 21. Said window 24 is advantageously formed in a recessed manner with respect to a front face of the outer shell 36. The thickness of the outer shell 36 at said window 24 is advantageously between 0.5 mm and 2 mm, and is e.g. 1 mm. Moreover, the window 24 has a shape and format which correspond at least substantially to the shape and format of the piezoelectric wafer 31 and the piezoelectric stack.

(36) The ultrasound waves are thus not substantially disrupted by passage through the outer shell 36 through the window 24, i.e. the thickness of the window 24 makes it possible to delay the secondary reflections within said window but without excessively attenuating the ultrasound waves passing through it.

(37) A transducer 38 in accordance with the invention can be used in particular to form a flowmeter in accordance with the invention as shown in FIG. 5. This flowmeter comprises at least one ultrasonic transducer 38 in accordance with the invention and a computer system 42. The flowmeter preferably comprises two transducers 38 placed facing each other with the windows 24 facing each other.

(38) The flowmeter comprises a tube 41 in which a fluid 43 flows in a direction at least substantially normal to the windows 24 of the transducers 38.

(39) Each transducer 38 is fixed to the tube 41 by insertion into a hole passing through the wall thereof. The transducers 38 extend orthogonally to the longitudinal direction of the tube 41. The transducers 38 are held rigidly in position e.g. by gluing, also achieving the sealing tightness of the tube 41 and the flowmeter or even with the aid of a wafer disposed above each transducer and exerting pressure on each of then in order to hold them in place.

(40) A flowmeter in accordance with the invention can comprise a number of transducers 38, and thus receiving holes in the tube 41, which is not two. Moreover, there is nothing to prevent the provision of holes which are non-aligned with the orientation of the flow rate of the fluid 43, as long as the windows 24 of the transducers 38 are, at least partly, facing each other.

(41) The rods 25, 32 of each transducer 38 extend outside the tube 41 and are connected to the computer system 42 in order to connect this system to the piezoelectric stack. These rods 25, 32 make it possible at the same time to supply the transducers 38 and to transmit electrical measurement or control signals.

(42) The computer system 42 has at least the function of transmitting control signals to at least one transducer 38 and of receiving measurement signals coming from at least one transducer 38. Said signals thus circulate through the rods 25, 32 and as far as, or within, the electrode 20 and as far as the posterior plate respectively. In this way, the electrodes can, in one sense, transmit the electrical control signals to a first piezoelectric wafer of a first transducer. The latter reacts and deforms (mechanical constraint) under the constraint of the electrical signal received owing to the piezoelectric properties thereof. This mechanical constraint is propagated in the fluid 43 in the form of ultrasound waves 53. These waves 53 reach a second piezoelectric wafer of a second transducer positioned facing the first piezoelectric wafer, and mechanically constrain this second piezoelectric wafer. This mechanical constraint allows this second piezoelectric wafer to be electrically polarised and thus to transmit electrical signals, named measurement signals, to the electrodes 20, 25 in order to transmit them to the computer system 42.

(43) The time difference between the control signals transmitted to the first piezoelectric wafer and the measurement signals transmitted to the second piezoelectric wafer corresponds to the propagation of the wave in the fluid 43. This difference is thus lined to the speed of the fluid and thus to its flow rate which can be calculated by the computer system 42. It is thus by measuring these propagation times that the flow rate of the fluid 43 is measured by the flowmeter. The distance separating the two transducers 38 is thus an important parameter in the measurement of the flow rate of the fluid 43. It must not be too small in order to permit correct measurement of the propagation times of the ultrasound waves; and must not be too large in order for the second transducer to pick up a non-zero signal.

(44) The ultrasound waves 53 emitted by a transducer 38 are more directional with a lower attenuation by virtue of said window 24, said posterior plate 35 and the shape of said piezoelectric wafer 31. For the same reasons, these ultrasound waves 53 are also more precise and more reliable upon reception thereof by a transducer 38. Thus in a flowmeter in accordance with the invention, the distance between the two transducers 38 can be increased with respect to a flowmeter comprising prior art transducers. The fact of increasing this distance makes it possible to obtain a measurement of the flow rate which is more precise because the propagation time of the ultrasound waves is measured over a greater time range. This distance separating two transducers 38 in a flowmeter in accordance with the invention is advantageously between 1 cm and 1 m, and is e.g. of the order of 5 cm to 20 cm.

(45) Moreover, this embodiment of the invention makes it possible to measure the flow rate of the fluid 43 directly by the piezoelectric stacks facing the transducers 38 in accordance with the invention, the direction passing via the two transducers 38 being parallel to the axis of the tube 41 and to the flow direction. Consequently, the measurement is carried out without an intermediate device such as reflectors. In fact, a transducer 38 in accordance with the invention is of a reduced volume, permitting insertion thereof into the tube 41 while minimising the disruption to the flow of the fluid 43. The flowmeter can thus measure the flow rate of the fluid 43 directly in its orientation of flow. Since the sound is transmitted more rapidly in the orientation of the flow of the fluid 43, this makes it possible to increase the reliability and the rapidity of the measurement of the flow rate. This embodiment is particularly advantageous since it does not necessitate an intermediate device such as reflectors and the ultrasound waves propagate in parallel with the direction of the flow and not at an angle with respect thereto, which improves the sensitivity and/or avoids reflection of the ultrasound waves on the walls of the tube.

(46) Moreover, since the transducers 38 are directly inserted into the tube 41, they are also directly in contact with the fluid 43. The materials which form the transducers 38, and in particular the materials forming the outer shell, must thus be resistant to corrosion in order to achieve sufficiently long service life for said transducers 38 and the flowmeter.

(47) In another alternative of the flowmeter illustrated in FIG. 5 of one embodiment of the invention, two transducers 38 in accordance with the invention can be used to form a flowmeter in accordance with the invention in which the tube 41 and the outer shells of each transducer form a single piece. In this embodiment, said tube 41 comprises two inner housings adapted to be able to each receive a piezoelectric stack so as to form two transducers in accordance with the invention. The liquid synthetic resin is thus injected inside each of these two housings before or after insertion of the piezoelectric stacks.

(48) On the one hand, the transducers 38 can extend in a direction non-parallel to the longitudinal direction of the tube 41 but not necessarily orthogonal thereto, i.e. the two transducers 38 can be placed facing each other with the windows 24 facing, forming an angle which is non-zero but less than 90° with respect to the direction in which the fluid 43 flows (variation not illustrated).

(49) FIG. 6 illustrates a signal (amplitude in mV as a function of time in ms) emitted by a first transducer 38 and received by a second transducer 38 when the liquid contained in the tube in which the transducer is inserted is at a temperature of 10° C. FIG. 7 illustrates a signal (amplitude in mV as a function of time in ms) emitted by a first transducer 38 and received by a second transducer 38 when the liquid contained in the tube in which the transducer is inserted is at a temperature of 75° C. As can be seen in each of FIGS. 6 and 7, the first periods of the signal have a maximum amplitude and are thus stripped of unwanted signals. Moreover, by comparing FIGS. 6 and 7 it is seen that a variation in temperature does not cause a change in the profile of the signal, in particular the profile of the first periods of the signal. A transducer in accordance with the invention thus has very little sensitivity to the temperature, which is particularly advantageous for numerous uses, especially in water conditions where the temperature is likely to vary. In particular, the measurement of fluid flow rate using a transducer in accordance with the invention is facilitated and is much precise and reliable than with a known transducer in that the first periods of the start of the signal emitted or received (from the first crossing of the zero of the ordinate axis corresponding to the amplitude of the signal) are of high amplitude and permit calculation of the flow rate of the fluid, the unwanted ultrasound waves being delayed especially by virtue of the great thickness of the posterior plate 35, a thickness between three and seven times, and in particular between three and five times, the thickness of the piezoelectric wafer, actually being sufficient while limiting the spatial volume of the posterior plate and thus that of the ultrasonic transducer. The ultrasound signal received can thus be used to deduce therefrom the measurement of the fluid flow rate from the first crossing of the zero of the ordinate axis in a reliable and sure manner without having to effect sorting among the first peaks of the signal. In particular, at least the first period of the signal emitted and received by the transducer is always usable to deduce therefrom a fluid measurement.

(50) Furthermore, the inventors have demonstrated that such a transducer makes it possible to carry out reliable measurements for highly variable fluid flow rates, especially from 0.1 L/mn to 4 m.sup.3/hr with a very low relative error (close to zero for a flow rate less than 1 L/mn and less than 1% beyond 1 L/mn). In a method for assembling such a transducer 38: firstly said piezoelectric stack is prepared then the interior of said outer shell 36 is filled using a liquid synthetic resin, said piezoelectric stack is inserted into the outer shell 36 containing said synthetic resin, and a step of polymerising the synthetic polymer resin is carried out so that, after polymerisation, said resin encloses said piezoelectric stack so as to hold the electrodes, the piezoelectric wafer and the posterior plate in mechanical and electrical contact with each other.

(51) It is thus not necessary to use pads to hold the piezoelectric wafer inside the transducer in order to ensure that the elements of the transducer are held in position.

(52) Once polymerized, the polymer resin disposed inside the outer shell permits the electrodes, the piezoelectric wafer and the posterior plate to be held in mechanical and electrical contact with each other. It also makes it possible to ensure the sealing tightness around the piezoelectric stack.

(53) In the embodiment illustrated, the shape of the transducer 38 inserted in a flowmeter is generally cylindrical—permitting revolution—with an overall diameter which is as small as possible in order to minimise the disruptions induced in the flow of the fluid 43. However, there is nothing to prevent a transducer 38 of a different shape being produced, e.g. one which is optimised hydrodynamically (especially at least substantially in a droplet shape with a straight transverse cross-section) in order to promote the flow of fluid about the transducer 38.

(54) There is nothing to prevent the use of a transducer 38 in applications other than a flowmeter, e.g. in a heat sensor, a level sensor, a distance sensor, a position sensor . . . .