MXPA99003454A - Ultrasonic fluid meter with improved resistance to parasitic ultrasonic waves - Google Patents

Abstract

The invention concerns a fluid meter (10) comprising ultrasonic transducers (26, 28), means for attenuating parasitic ultrasonic waves, of wavelength&lgr;, constituted by at least one passage (36, 38, 40) in which said waves propagate along a main direction corresponding to a longitudinal dimension (a) of said passage, said passage having a transversal dimension (b) perpendicular to (a) and much less than the wavelength&lgr;of the parasitic waves in the propagating medium, said passage comprising a plurality of consecutive passage portions (46, 48) each having a part (46) with a reduction of the transversal cross-section of the passage along the dimension (b) of the passage, the longitudinal dimension of each passage portion being substantially equal to&lgr;/2.

Description

ULTRASONIC FLUID METER WITH IMPROVEMENTS TO IMMUNITY TO PARASITE ULTRASOUND WAVES.

DESCRIPTION OF THE INVENTION The present invention relates to an ultrasonic fluid meter that includes ultrasonic transducers that define between them an ultrasonic measurement path and that emits and receives ultrasonic waves in the fluid along said measurement path to at least one frequency of ultrasound. It has been known for several years that the velocity of a flowing fluid can be measured by emitting ultrasound waves into the fluid from ultrasound transducers both in the direction of the fluid and in the opposite direction, and by measuring the respective propagation times of the waves emitted in both directions. Starting from the measurement of the fluid velocity, it is easy to determine its flow rate and also the volume of the fluid that has flowed over a certain amount of time. However, in such fluid meters, the applicant has observed the propagation of "parasitic" ultrasound waves that disturb the reception at. one of the transducers of the ultrasound waves emitted by the other transducer along the measurement path. Two different types of parasitic ultrasound waves can be mentioned: ultrasound waves generated by a source external to the fluid meter, and ultrasound waves emitted by the transducers themselves. The first type is found, for example, when installing a pressure regulator upstream of a gas meter. The pressure regulators are used, for example, to reduce the gas pressure from several atmospheres to approximately 20 bars upstream of the ultrasonic gas meters. Unfortunately, the pressure drop in the regulator is a source of a considerable amount of noise and it has been observed that such a pressure drop can cause stray ultrasound waves of high pressure amplitude and frequency (s) corresponding to the frequency (s). ) of the transducers in the meter. This causes considerable measurement errors that are totally unacceptable. The first type of ultrasound parasites can also be found in ultrasonic meters for liquids that are placed downstream from a sudden reduction of the flow section and that can cause the phenomenon called "cavitation" with the appearance of bubbles in the liquid at a frequency close to that used by ultrasound transducers. The second type of parasitic ultrasound waves correspond to the case when the ultrasonic measurement path defined between the two transducers lies inside a conduit (tube, ...) that leads to the fluid whose flow rate must be determined and when the conduit is made with a material not rigid enough to prevent acoustic coupling between the fluidic medium and said material. This may be the case, for example, when the conduit is metallic (steel, ...) and the fluidic medium is water, or indeed when the conduit is made of plastic material and the fluid is a gas. Under such circumstances, when the ultrasound waves are emitted from one transducer to the other within the measurement conduit, a portion of these waves, referred to as "parasitic" ultrasound waves, propagate through the material constituting said measurement conduit. and reaches the other transducer either before or together with the ultrasonic waves propagated through said fluid medium. It thus becomes very difficult to distinguish between the ultrasound waves received from the other transducer between those that have effectively propagated in the fluidic medium and those that have propagated in the medium that constitutes the measurement conduit. EP-A-0, 457, 999 describes an ultrasonic flow comprises a conduit in which the fluid is poured where the expense is to be determined, and two transducers located on the outside of the conduit. The ultrasonic waves generated or received by the transducers are transmitted to the fluid or received from the fluid respectively, by means of discs associated with the transducers and wall portions located in front of the transducers. The device described comprises absorbers or pairs of grooves / projections whose objective is to uncouple between them the wall portions located in front of the transducers. On the one hand, such a device uses a measuring principle that seeks to measure in resonance the wall of the tube where the fluid is spilled, and on the other hand it does not solve the problem of the parasitic ultrasonic waves of the first type. FR-A-2, 357, 69 anticipates means for attenuating the sound waves generated outside the ultrasonic fluid meter and which are implemented in the form of a sleeve of acoustically insulating material located coupled to the fluid inlet of the meter. Unfortunately, such attenuating means are insufficient and, additionally, are incapable of attenuating the parasitic ultrasound waves of the second type. EP-A-0, 048, 791 shows a device for eliminating the ultrasound waves emitted by the transducers outside the measuring tube. However, said device does not allow the ultrasonic waves propagating in the wall of the measuring tube to be attenuated, nor does it attenuate the parasitic ultrasound waves of the first type. The present invention therefore seeks to remedy this problem by attenuating in a simple and effective way the parasitic ultrasonic waves propagating in an ultrasonic fluid medium and disturbing the reception in one of the transducers, of the ultrasonic waves emitted by the other transducer along the measurement path. The present invention therefore provides an ultrasonic fluid meter comprising ultrasonic transducers that define between them an ultrasonic measurement path capable of emitting and receiving ultrasound waves in the fluid along said measurement path to at least one frequency of ultrasound, and means to attenuate wavelength "parasitic" ultrasound waves? which disturb the reception of one of the transducers of ultrasound waves emitted by the other transducer, characterized in that the attenuation means are constituted by at least one passage in which said parasitic waves propagate in a principal direction corresponding to a dimension "longitudinal" a of said passage, said passage having a transverse direction b perpendicular to the dimension of a and much smaller than the magnitude? of the parasitic waves in the propagation medium, said passage comprising a plurality of consecutive portions of passage each having a part that presents a reduction in the propagation cross section along the dimension b of the passage, with the dimension longitudinal of each portion of passage being substantially equal to * _? . In this way, the ultrasonic parasitic waves propagating in the propagation medium along the longitudinal dimension of the passage find in its path reductions of the propagation section alternated with "normal" propagation sections, thus creating a discontinuity of acoustic impedance in the medium, which reflects a portion of the energy contained in these waves, attenuating therefore the amplitude of these waves. Parasitic ultrasound waves that do not propagate along the longitudinal dimension of the passage but nevertheless find small sections of propagation in their path are also attenuated. According to a characteristic of the invention, the passage is defined by at least two longitudinal surfaces facing each other and spaced from each other along the dimension b, and on which at least one of them has a plurality of mutually parallel consecutive slots formed alternately with projectionseach portion of passage having a pair constituted by a slot and a projection.

As an example, each groove has a V-profile or a U-profile which causes that surface to be knurled. In an execution variant, the parasitic waves are of a wavelength that varies within a certain range, and the longitudinal dimension of the passage portions vary in increasing or decreasing form to cover the determined range of wavelengths. In a first aspect of the invention, the fluid meter comprises a shell provided with an inlet orifice and an outlet orifice, a measurement block provided with ultrasonic transducers and provided with at least two openings, respectively allowing the fluid to reach the Ultrasonic measuring path and leaving it, the passage (s) in which parasitic ultrasonic waves propagate formed between the measurement path and at least one of the fluid inlets and fluid outlets. In a first embodiment of the invention, the measurement block is arranged inside said envelope in such a way as to form between them the passage (s) in which the parasitic ultrasound wave propagates, and along which the fluid flows before to penetrate into the measurement block or after leaving it. For example, the surface on which the slots are formed is the surface of the measurement block. In a second embodiment, the measurement block includes the passage (s) that allow the parasitic ultrasonic waves to be attenuated and formed between at least one of said openings and said measurement path, if any. passages also to drive the fluid. According to other features of the invention: # the measurement path is formed inside the measuring conduit; # the passage (s) is / are formed between the walls of the housing and the measuring conduit; # the passage (s) is / are formed around the measuring conduit; # the passage (s) is / are formed on one side only of the measuring conduit; # the measurement block includes a "spacer" wall on the side where the passage (s) is / are formed to separate the measurement conduit from the passage (s); # the measurement block includes another wall which is arranged facing the partition wall in such a way that the facing surfaces of these two walls define the passage (s); # the other wall is a separate part installed in the measurement block; # the pair of grooves / projections are formed on the external surface of the measuring conduit; # the (s) passage (s) is / are formed along a portion of the measurement path; and # the (s) passage (s) is / are arranged within the measuring conduit.

In a second aspect of the invention, the fluid meter comprises a measuring conduit constituting at least a portion of the ultrasonic measurement path and having at least one peripheral wall corresponding to the passage in which parasitic ultrasonic waves propagate. The surface on which the slots are formed is the external surface of the measuring passage or passage, the reduction of the propagation section being located in each passage portion in each slot of said wall. For example, the measuring conduit is a tube. The grooves are annular and arranged along the tube. In a variant embodiment, a helically shaped groove is formed on the external surface of the measuring tube. Advantageously, the measuring tube can be screwed into a housing provided in the measuring block. Other features - and advantages will be apparent on reading the following description given purely by way of non-limiting example and made with reference to the accompanying drawings in which: Figure 1 is a view of a fluid meter of the present invention with a cover portion removed to facilitate understanding of the invention; Figure 2 is a view of the interior of the measurement block shown in Figure 1; Figure 3 is a section in the section A-A of the meter of Figure 1; Figures 4a, 4b and 4c are fragmentary diagrammatic views of various embodiments of the attenuation means shown in Figure 3; Figure 4d is a graph having three curves A, B. and C representing the respective capacities R of the attenuation means of the invention in three different gases as a function of the frequency of the parasitic waves; Figure 5 is a fragmentary diagrammatic view of the attenuation means shown in Figure 3 of a further embodiment variant; Figure 6 is an analogous view of Figure 3 but in which the fluid flows in the opposite direction; Figure 7 is a view of a fluid meter that constitutes a second embodiment of the invention; Figure 8 is a sectional sectional view of the measurement block shown in Figure 7; Figure 9 is a first variant embodiment of the measurement block of Figures 7 and Figure 10 is a view of a second variant embodiment of the measurement block of Figures 7 and 8; Figure 11 is a fragmentary perspective diagramatic view of a measurement block constituting a third execution variant; Figure 12 is a longitudinal sectional view of the measurement block of Figure 11; Figure 13 is a longitudinal fragmentary sectional view of a measurement block constituting a fourth execution variant; Figure 14 is a longitudinal fragmentary sectional view of a measurement block constituting a fifth variant of execution; Figure 15 is a longitudinal fragmentary sectional view of a measurement block constituting a sixth execution variant; Figure 16a is a sectional view of the mediation block seen in Figure 15; Figure 16b shows a variant of the measurement block shown in Figure 16a; Figure 17 is a longitudinal sectional view of a measurement block constituting a seventh variant of execution; Figure 18 is an enlarged sectional view of the measurement block shown in Figure 17; Figure 19 is a fragmentary view in longitudinal section of a measurement block that constitutes an eighth variant of execution; Figure 20 is a fragmentary view in longitudinal section of a measurement block that constitutes a ninth variant of execution; Figure 21a shows a measurement block for a fluid meter which constitutes another embodiment of the invention; Figure 21b is an enlarged view of the measurement conduit shown in Figure 21a; Figure 21c is a diagrammatic view of a variant of the measurement conduit shown in Figure 21a; Figure 22 is a view of a measurement conduit that constitutes another variant of the measurement conduit shown in Figure 21a: and Figure 22b is a view showing the measurement conduit of Figure 22a integrated in a measuring block of a meter of fluid of the invention.

Figure 1 shows a gas meter located downstream from a pressure regulator (not shown in the figure) that generates parasitic ultrasonic waves at the same frequency, for example at 40 kHz, in the conduit and in the gas meter. therefore disturbing the measurements of the flow rate of the gas. As shown in the figure, the general reference gas meter (10) comprises an inlet (14) and a gas outlet (16), a casing or casing to which said inlet and outlet are connected, and a measuring block. (18) disposed within the shell (12). The measurement block (18) is organized within the shell (12) in such a way as to leave one or more passages between the block and the shell to be taken by the fluid, in order to travel from the entrance (14) to an opening (20) formed in the bottom portion of the measurement block. The measurement block (18) is maintained in position within the shell (12) by two lugs (22, 24) which are housed in recesses formed in said shell (12). As shown in Figure 2, the measurement block (18) comprises the opening (20) through which the gas penetrates together with two ultrasound transducers (26 28) each disposed facing one of the opposite ends of a measurement conduit (30) that is tubular in shape and which constitutes the ultrasonic measurement path As an example, ultrasonic transducers operate at a frequency of 40 kHz. The measuring conduit (30) passes through the wall (32) forming a solid block between two housings within which the transducers (26, 28) are arranged. The gas penetrates into one of the housings of the measuring block (18) through the opening (20), as shown by the arrow in Figure 2, penetrates into the measuring tube (30) through the end ( 30a) of said tube, flows along the inside thereof, leaves the tube through its opposite end (30b) and then exits upwards through an outlet orifice (34). The outlet orifice (34) is connected to the gas outlet (16) shown in Figure 1. In the above described ultrasonic gas meter, the ultrasonic transducers (26, 28) alternately emit and receive ultrasound waves at a frequency fixed ultrasonic, and the time of propagation of the waves, and therefore the flow rate of the fluid, are deduced from the ultrasound waves received by each of the transducers.

When a regulator is located upstream of the gas meter, the parasitic ultrasonic waves mentioned above propagate in the meter and reach the ultrasound measurement path inside the measurement block (18) where they are mixed with the ultrasound waves emitted and received by the transducers, and therefore mostly disturb the ultrasonic measurements of the flow rate. The above-mentioned passages are specially designed to exert an attenuating effect on the parasitic ultrasonic waves present in the gas flow in their running towards the ultrasonic measuring path. Each of the passages (36, 38) (see Figure 3) and (40) (Figure 1) has a "longitudinal" dimension a The gas flow propagates along this longitudinal direction as do the parasitic ultrasonic waves present in, the flow. In order to ensure that the attenuation effect on these parasitic ultrasonic waves propagated in this "principal" direction is really effective, it is necessary that each of the above-mentioned passages have a transverse dimension b perpendicular to the longitudinal direction to which it is much less than the wavelength? of parasitic waves in the fluidic medium in which parasitic waves are propagated. This condition ensures that only the plane mode of the ultrasonic waves propagates along the passage, as does the plane mode which is affected by the attenuation means of the invention. Otherwise, if the transverse dimension b of the passage and the wavelength? in the fluidic medium it is very similar, or if b is may. then the modes of propagation of parasitic ultrasonic waves, in addition to that of the planQS modes? they will appear therefore reducing the effectiveness of the means of attenuation. The passage (36) is defined by at least two longitudinal surfaces (42 and 44) arranged mutually facing each other and spaced apart by the transverse dimension b of said passage, as shown in Figure 3. At least one of these surfaces longitudinal (42, 44) is organized in such a way as to cause the passage to engage a plurality of consecutive portions of passage as marked by the broken lines of Figure 3, each comprising a portion of reduced cross-section, propagation in dimension b of the passage. The surface on which this particular arrangement is formed is the surface of the measurement block (18). On the external surface (44) of the measuring block, the projections (46) are formed, for example by overmolding. These projections (46) are parallel to each other, perpendicular to the longitudinal dimension a of the passage, and between them determine grooves (48) which are similarly parallel to one another. It should be noted that instead of forming projections on the external surface (44) of the measurement block (18), it should be equally possible to work on said surface by forming a plurality of consecutive grooves that are mutually parallel and perpendicular to the longitudinal dimension 2 of the passage. (36) Each passage portion corresponds to a projection (46) and a slot (48) disposed side by side. The projection (48) has a dimension Ll in Figure 3, and each slot (48) has a longitudinal dimension indicated by L2. The transverse dimension of the passage in each slot (48) is equal to b, and the passage dimension of each projection (46) is equal to bO. So that means of attenuation of 2? invention attenuate the desired wavelength of parasitic ultrasonic waves, it is necessary that the longitudinal dimension of each passage portion, ie Ll. + L2, is substantially equal to? . It should be noted that the dimensions Ll and L2 may vary provided that the aforementioned relationship is satisfied. As illustrated in Figure 3, each slot (48) is U-shaped in its longitudinal profile, so as to form a toothing on the longitudinal surface (44). The amplitude of the parasitic sound waves propagating along the passage (36) is attenuated each time said waves find a smaller propagation section on each projection (46). The smaller the transverse dimension of the passage on each projection (46) with respect to the dimension b on each slot (48), the greater the effectiveness of the attenuation, in any case it is also necessary to avoid exceeding certain values that can cause excessive drops in the gas flow. As shown in Figure 3, the pattern formed in this way on the longitudinal surface (44) is periodic. As a numerical example, L2, Ll = 2.5mm; the dimension bO and b of the passage is respectively equal to 2mm and 3mm, and the total length of the passage is equal to 60mm, which corresponds to 12 periods. For several gases, and in particular for a mixture of air and methane, the attenuation of more than 40 dB per decade has been obtained over a bandwidth of 12 kHz. For methane, the wavelength? for parasitic waves is equal to 11 mm which is much larger than the dimensions bO and b. The effectiveness of these attenuation means can be increased if the projections and grooves can also be formed on the surface (42), respectively facing the projections (46) and the grooves (48) on the surface (44) of the measurement block ( 18). It should be noted that for a certain effectiveness in attenuation, the grooves and the projections can be provided only on the inner surface (42) of the shell (12). Figure 4a shows a first variant embodiment of the attenuation means shown in Figure 3, in which a plurality of consecutive grooves (50) that are mutually parallel are formed transversely on the longitudinal surface (44) of the measurement block, and preferably perpendicular to the direction of flow propagation in the passage. Each slot (50) has a V-profile, and two consecutive slots (50) are separated by a passage portion (52) of essentially a flat profile constituting the portion in which the propagation section offered to the fluid and the parasitic waves of ultrasound is reduced. Figure 4b shows another embodiment variant in which the grooves (54) occupy most of each of the consecutive passage portions, and the parts in each landscape portion have a reduction in the propagation cross section, whose parts they are referenced with (56), and are reduced to mere edges. Figure 4c shows yet another variant execution of the means of the invention, in which the grooves (58) that are of parallel slopes, mutually inclined and separated from each other by inclined fronts, on which the part of reduced section is located ( 60) of passage. The longitudinal profile of the surface (44) has a profile with saw teeth.

Whatever the means of attenuation used, these are adapted for a given gas, and if it is desired to cover a wide range of wavelengths, for example in order to allow the meter to adapt to several types of gas, it is necessary to provide a special configuration of the passages mentioned above (36, 38, and 40). In Figure 4d, three curves A, B and C determine the R ratio of amplitude of the parasitic ultrasound waves between the entrance and exit of the passage as shown in Figure 1 and 3 (36, 38 or 40), as a function of the ultrasonic frequency F. Each curve is in the form of the parabolic main lobe accompanied by a plurality of small lobes. In this way, by designing the passages of Figure 3 to attenuate parasitic ultrasound waves at a frequency of 40 kHz in a mixture of air and methane, the curve A is obtained by calculation to achieve a maximum power of attenuation at the frequency of 40kHz. However, if the mixture of air and methane is replaced by air on its own curve B. or by methane on its own curve C, it can be seen that the passage of Figure 3 is not optimal in those gases at the frequency of 40. kHz The execution variants shown in Figure 5 show a possibility for a special configuration of the passages (36, 38 and 40). In these figures, the facing surfaces (42, 44) define a passage for the gas flow and for the propagation of parasitic ultrasonic waves. In this passage, the grooves (62) and the projections (64) analogous to those shown in Figure 3 are formed on the longitudinal section (44). As can be seen in Figure 5, the longitudinal dimension of the passage portions increase from the entry of the passage to the exit of the passage in order to cover a certain range of wavelengths. For example, it is possible to cover a range of wavelengths ranging from 8.75mm (air) to llm (methane). In practice, the portion of the passages located near the entrance is / are of dimension equal to? air, and close to the exit, the dimensions of the portions of the passage (s) is equal to? methane. Between the entrance and the exit, the passage portions have a longitudinal dimension that grows, several consecutive passage portions which may have the same longitudinal dimension. It should be noted that the passage can also be provided in such a way that the portions of the passage see its longitudinal dimension vary from the entry of the passage to the exit of said passage. It should be noted that when attenuating means are conceivable that can be adapted to various gas types, the attenuation effect obtained is lower than that obtained for the attenuation means particularly adapted to a type of gas. For example, an attenuation rate or proportion of 40dB is passed for the attenuation means particularly adapted to a gas type, at an attenuation rate of 25dB for an arrangement such as that shown in Figure 5, although the slots and the projections are placed perpendicular to the main propagation direction of the parasitic ultrasonic waves in the examples cited above, it is not a mandatory condition. However, the grooves and projections must have an arrangement that is not parallel to the main propagation direction of the parasitic waves, so that they are affected by a reduction of the transverse section to the right of each outlet. Figure 6 schematically represents another configuration of the shell 120 in which the passages 121, 122 serve as means for attenuating the parasitic ultrasonic waves and which have the same characteristics as those 36, 38, 40 described with reference to figures 1 , 3, 4a-d and 5 are provided in the enclosure 124 of the fluid casing and the measurement block 126. The slots (128) and projections (130) are formed in alternation on the surfaces (132) of the measurement block. In this configuration, the noise source external to the meter is located downstream of the meter, so that the passages (121, 122) are disposed between the inlet opening (136) of the measurement block (126) and the exit orifice. (134). In this case also, the internal surface (138) of the shell (120) facing the surface (132) of the measurement block could include alternating projections and grooves to increase the effectiveness of the attenuation. A second embodiment of the invention is described below. As shown in Figure 7, a fluid meter (140), for example a gas meter, comprises a shell (142) provided with two orifices (144 and 146) respectively acting as a gas inlet and as a heat of gas A measuring block (148) is disposed within the shell and is provided with a plurality of openings (15Q-152) to allow the gas to penetrate said block, and with an opening 154 extended by a coupling (156) under the form of chimney for the outgoing gas of said block. The coupling (156) is fixed to the outlet hole (146). Within the measurement block, the ultrasonic measurement path is implemented in the form of a measuring conduit, such as a tube (158). However, the measurement conduit could, for example, be elliptical in its shape as described in EP 0 538 930 or it can have a rectangular propagation cross-section, such as that described in EP 0 580 099. The measurement block has two housings (160, 162) separated by a wall (164) through which the tube (158) passes. Two ultrasound transducers (166 and 168) are located in the chambers (160 and 16.2) respectively, facing opposite ends of the tube (158). As shown in Figures 7 and 8, the measurement block is substantially cylindrical in its external shape and the cylindrical wall of the chamber (160) cooperates with the outer wall of the tube (158) to form an annular passage (170) around the said tube. The gas penetrating the shell (142) through the orifice (144) is distributed around the measurement block (148) and penetrates therein through the openings (150 and 152), after which it flows along of the annular passage (170) before reaching the measurement path between the two transducers Consequently in the annular passage (170) between the openings (150 and 152) and the measurement path a plurality of slots (172) alternates with projections (154) ) on the cylindrical surface (170) of the wall of the shell (160).

These grooves and projections are circular and extend transversely to the propagation direction of the gas along the annular passage (170), preferably perpendicular to said measurement propagation. In a manner analogous to that described with reference to Figures 1 to 5, this set of projections and grooves serves to attenuate parasitic ultrasonic waves carried by the gas before the waves penetrate the measuring tube (158). For the attenuation to be effective it is necessary that the transverse dimension b of the passage be much smaller than the wavelength% of the parasitic waves, and for the longitudinal dimension of each portion of the passage formed by a pair of slot and projection to be on the order of ? ? Also for reasons of effectiveness the longitudinal dimension of the annular passage 170 should not be too short. The other features and advantages described with reference to Figures 1 to 5 remain valid. It should be noted that it is advantageous to arrange the passage in which the parasitic waves are attenuated along at least a portion of the length of the measuring conduit in order to achieve a measuring block with a reduced volume. In any case, when the volume of the measurement block is not a limitation, it is naturally possible to locate the passage perpendicular or at an inclined angle relative to the. longitudinal direction of the measuring conduit. It is also possible that the measurement block (148) has a square or rectangular cross section (Figure 9), its inner surface being provided with grooves and projections.

Figure 10 shows a second variant of the configuration of Figure 7, but shows only the measurement block of it. The elements already described with reference to Figure 7 are not again described and the same references are used. In this figure, the openings (150 and 152) of Figure 7 are replaced by a single opening (180) located below the measurement block, however this opening can be located above the measurement block in the course of another variant. The gas flow enters the chamber (160) through this opening and disperses around the measuring conduit. Figures 11 and 12 show a third variant of the measurement block in which the measuring conduit (182) has a cross section that is rectangular in shape and in which two passages (184, 186) are provided on at least one portion of the longitudinal dimension of the measuring conduit on both sides thereof within the measurement block (190). In Figure 11, the portion of the housing containing the ultrasonic transducer is not shown. The flow rate penetrates within these passages through two upper openings (189 and 191) as shown by the 3 arrows (Figure 11). A transducer (192) is located facing one end of the measurement conduit (182) in the measurement block (190). The measuring conduit passes through a partition (194) that subdivides the measurement block into two portions. A second ultrasound transducer (198) is located facing the opposite end of the measurement conduit. The respective surfaces (200, 202) of the lateral passages (184, 186) facing the surfaces (204, 206) of the side walls of the measuring conduit (182) are provided with grooves (208) alternating with projections (210). which have the same characteristics as those described with reference to Figures 1 to 10, with the exception of their shape which depends on the shape of the lateral passages. The grooves and projections are preferably arranged perpendicular relative to the longitudinal direction of the flow in the passage. Figure 13 shows a fourth variant in which the grooves (212) and the projections (214) are formed, on the surface (s) of the measuring conduit (218) where only one passage (219) is provided to both sides of the conduit (Figures 11 and 12). This variant is advantageous as long as it is easier to realize the projections and grooves directly on the measuring conduit that is produced separately from the remainder of the measuring block and that is substantially inserted inside the measuring block, and then they are made on the walls of the measuring unit. the chamber in which said part of the conduit is located. Figure 14 shows a variant guilloche in which the grooves (220, 222) and the projections (224, 226) are formed simultaneously on both surfaces (228, 230) of the passage (s) (232). This variant can be applied to any of the configurations shown in Figures 7 to 13. The measurement block (240) shows in Figure 15 another variant in which two openings (242, 244) respectively for the fluid inlet and outlet relative to said measurement block are substantially in alignment with each other. These openings can respectively be connected to mutually aligned inlet and outlet or, as shown in Figure 15, the measurement block can be integrated into a shell of the type shown in Figure 7. The measurement block (240) has a measuring conduit (246) and two housings (248, Z50) separated by a partition (252) through which the conduit is through. Two ultrasonic transducers (254, 256) are located in the housings (248 and 250) respectively facing the two opposite ends of the measuring conduit (246). A passage (258) is provided along one side only of the measuring conduit, first to conduct the fluid from the passage (242) towards the end of the conduit facing the transducer (254), and second to attenuate the ultrasonic waves. parasites that spread in the fluid. To this end, the passage (258) has projections (262) alternating with grooves (264) on one of its faces (260) those facing the external surface of the measuring conduit (246) / - and their characteristics are the same as those described with reference to the preceding figures. If the portion of the anchoring (248) located at the shoulder of the measuring conduit is of circular cross-section, then the grooves and protrusions (263) may be semicircular in shape (Figure 16a). Otherwise, if the portion of the housing located along the measurement conduit has a cross section that is square or rectangular, then the grooves and projections may be rectilinear in its profile (Figure 16b). Compared with the configurations shown in Figures 7 to 13, the configuration shown in Figures 15, 16a and 16b provide the advantage of providing the fluid with a greater length over which it is in contact with the grooves and projections, therefore increasing the effectiveness of the attenuation of parasitic ultrasonic waves. To * subsequently increase this effectiveness, it is also possible to provide grooves and projections on the measuring surface of the duct (246) facing the surface (260). In yet another embodiment shown in Figures 17 and 18, the measurement block (270) has two housings ( 272, 274) in which two ultrasound transducers (276, 278) are respectively located facing opposite ends of a measurement conduit (280) passing through a partition (282) separating said housings-. A fluid inlet (284) and a fluid outlet (286) are substantially aligned with each other. The measuring block (270) also carries a separating wall (288) disposed along the measuring conduit (280) and which cooperates with another wall (290) facing it to form the passage (292) taken up by the fluid to reach the measurement conduit. In this passage, at least one of the surfaces (294) of the two facing the walls (288, 290) is provided with alternating projections (296) and grooves (298) that can be arranged along the total extension of the wall. To further increase the effectiveness, the surface (300) of the wall (288) can also be provided with alternating projections (302) and slots (304) shown with dashed lines broken in Figure 17. Figure 18 is a view in extended court (on B-B) of the measurement block shown in the Figure 17, on which the thickness of the walls can be seen. In this way, the general shape of the measurement and of its wall (303) is circular in cross section. The measurement block is closed at both longitudinal ends by two respective end walls (305) and (306) (see Figure 17). In the passage (292), the wall (303) is also the partition wall (288) - Two longitudinal side walls (307, 308) are tangential to the external surface of the wall (303) and extend downward toward the wall (290) so as to form the sides of the passage (292). Advantageously, the wall (290) is a separate part installed in the measurement block (270) and forming a cap thereon. This makes it possible for the slots and projections to be easily rendered by molding one and / or the other of the two walls (288, 290) before the wall (290) is fixed on the measurement block. This also has another advantage: when the measurement block is adaptable to the different ranges of the fluid flow, it is possible to change the wall (290) and replace it with another wall having the same longitudinal dimension but having transverse dimensions that are modified in a way of changing the section of transverse flow offered to the fluid, while the conditions relative to the transverse dimensions of the propagation section relative to the wavelength serve. Figure 18 shows the shape of the projections (296) that are suitable for the profile of the passageway and the partition wall (288). In this figure, the projections (302) are not illustrated for reasons of clarity. When flowing over the projections, the fluid has an M-shaped flow section. This configuration makes it possible to have a flow section that does not give rise to an excessive loss of charge while effectively attenuating the parasitic ultrasonic waves. To increase the flow rates of a meter whose measurement block has such a configuration, it is sufficient to change the wall (290) by framing the cover and replacing it with a wall such that the branches of the M in contact with the side walls ( 307, 308) are larger than those shown in Figure 18, thereby increasing the flow section offered to the fluid. Figure 19 shows a portion of a variant embodiment in which the means (310, 312) for the attenuation of parasitic ultrasonic waves are analogous to those described with reference to those described with reference to the preceding figures and are arranged between the exit opening (314) of the measurement block (316) and measuring conduit (318) for the purpose of preventing the propagation of parasitic waves in the path of ultrasonic measurements from downstream of the fluid meter. Under certain configurations of fluid gauges, the measurement conduit 320 does not constitute the ultrasonic measurement path but only a portion thereof. For example, the cross section of the measuring conduit may be circular or rectangular in shape, as described in WO 91/09280. There are at least two ultrasonic transducers (322, 324) which are mounted on the same side of the measuring conduit (320), as shown in Figure 20. They can also be mounted on diametrically opposite sides and the path of Ultrasonic measurement can therefore have different shapes (V, W ....). The transducers (324) are shown located in several locations (in interrupted lines) to indicate the possible appearance of the measurement paths. With configurations of this type, it is possible to provide alternating projections (326) and slots (328) within the conduit upstream and / or downstream of the measurement portion depending on the location of the noise source in order to attenuate the ultrasounds that originate from the outside. In order to be able to form the projections and the grooves in the conduit, it can be effected, for example, in two parts. If there is not enough available place in the measurement conduit, it is then preferable to locate the projections and grooves on the outside, that is, using one of the. configurations shown in Figures 7 to 19. When it is necessary that the external acoustic noise carried by the fluid be greatly attenuated, it may be advantageous to "combine the features shown in Figures 1 to 6 where the passage (s) is / are formed between the meter housing and the measurement block, with the characteristics shown in figures 7 to 20 where the passage (s) is / are formed in the same measurement block, at lower noise levels, the configurations shown in Figures 1 to 6 or the configurations shown in Figures 7 to 20 may themselves be sufficient .. Providing the means for attenuating ultrasonic parasitic waves in the measurement block rather than between the measurement block and the envelope in which it is placed, has its advantages.The dimensions of the envelope containing the measurement block and the location of the fluid inlet and the outlet orifice varies, depending on the requirements national healers. Consequently this makes it necessary to modify the size of the projections and grooves in order to retain the same effectiveness in the attenuation if they are located between the measurement block and the casing, however such modifications are not necessary when the projections and the grooves they are arranged in one or more passages formed within the block within the measurement block. A particularly advantageous aspect of the invention is shown and described with reference to Figures 21a and 21b. In an ultrasonic measurement block, partially shown, two ultrasound transducers are disposed at opposite ends of a measuring conduit (478) which constitutes the ultrasonic measurement path. The penetrating fluid, in the measurement block flows into the measurement conduit (478) through the end (478a) thereof, flows along the conduit, leaves it through the end (478b) and leaves the block measurement. Usually, when the ultrasound transducers are operative, the ultrasound waves are emitted at a frequency that is determined by one of the transducers, propagates within the measurement conduit (478), and reaches the other transducer, for example. (476), and the propagation time of the waves is used to determine the flow rate of the fluid. However, under certain circumstances, there are acoustic couplings between two propagation means arranged in contact, in particular between the fluid medium located within the measurement conduit (478) and the means constituting the wall of the supply conduit (478). This happens, for example, when the fluid is water and the measuring conduit is made in a metal, for example steel. The same can happen when the fluid medium is a gas and the measuring conduit is made of plastic material. Under certain circumstances, the parasitic ultrasonic waves propagated within the measuring conduit 478 from the ultrasonic transducer 474 penetrate partly into the wall of the measuring conduit 478, propagate along said parallel wall to the Ultrasonic measurement path within the conduit, and reaches the opposite ultrasonic sound transducer (476), before or at the same time as the ultrasonic waves propagated within the measurement conduit. The transducer (476) is then superimposed on the ultrasonic waves, rendering any correct measurement of the flow rate of the fluid within the measuring conduit impractical. In this example, the measuring conduit is a tube with a circular section, but it could also be a measuring conduit with a cross-section of rectangular shape, for example, as described in the European patent application no. 0 580 099. The measuring conduit (478) shown in Figures 21a and 21b has a peripheral wall (483) of longitudinal dimensions a and defines a passage in which the ultrasonic parasitic waves propagated in a principal direction coinciding with the longitudinal direction of the tube. The transverse dimension b of the passage perpendicular to its longitudinal dimension a and that is much smaller than the parasitic wavelength? in the middle under consideration, that is, steel. For example, a = lOOmm, bO = 2mm, b = 3mm, and? = 6mm. The passage is defined by two concentric longitudinal facing surfaces (484, 486), the outer surface of the measuring conduit being the surface (486). Mutually parallel grooves (488) are machined on the outer face (486) of the conduit. measurement. thus forming projections (490) between pairs of consecutive slots, each pair formed by a slot (488) and a consecutive projection (490) defines, in the wall thickness (483), a portion of passage in which the parasitic ultrasonic waves are subjected to a reduction of the propagation section in said groove projecting inside said wall, These grooves and projections are organized over the entire longitudinal dimension a of the measuring conduit and each has a length Ll for the projection (490) and L2 for the slot (488). The longitudinal dimension of each passage portion, Ll + L2, is substantially equal? ? For example Ll = 1.5mm, and L2 = 1.5mm. The conditions specified above relating to the attenuation means shown in Figures 1 and 3 remain valid in this configuration. It should be noted that the small cross-sectional dimension bO must be no less than the effects of "retaining the stiffness of the tube.In this configuration, the grooves are annular in shape, as are the projections. ultrasound (474) and (476) and the measuring conduit (492) are shown. In the variant shown in Figure 21c, the grooves (496) have a longitudinal dimension that is much larger than the projections (494) that form protrusions. Each groove is trapezoidal in shape, with a shorter parallel side located on the outer surface of the tube (492). It should be noted that the shapes shown in "Figures 4b and 4c can also be used on the external surface of the measuring conduit (492), A measuring conduit configured in this way very effectively attenuates r parasitic ultrasonic waves. Figures 22a and 22b show another variant of the attenuation means of the invention. In Figure 22a, a measuring tube of circular shape (tube) (502) is machined so as to form a helical groove (504) and fillet (506) on its external surface.

This tube can be located within a measurement block of the type shown in Figure 21a, or of the type shown in Figure 22b. In Figure 22b, the measurement block (500) has an opening (5Q8) through which the fluid enters a first chamber having an ultrasonic transducer (510) and in which it faces one end of the measurement conduit ( 502), and the other end of the measuring conduit (502) has a transducer (512) facing it located inside a second chamber that is in communication with an outlet (513) through which the fluid is protruding. . In this figure, the measurement block (500) has a central portion (514) of thick walls in which a housing (516) is formed. Advantageously, the cylindrical measuring conduit shown in Figure 22a is inserted into the housing (516) by a screw coupling using the external threaded surface. In addition to this advantageous feature, the measurement conduit (502) configured in this way possesses the properties described above with respect to the other figures to effectively filter the parasitic ultrasonic waves propagating in the wall of said conduit (502).

Claims (4)

1. Ultrasonic fluid meter comprising ultrasound transducers placed in fluid contact that define between them an ultrasonic measurement path and that emits and receives ultrasonic waves in the fluid along said measurement path to at least one frequency of ultrasound, and means to attenuate wavelength "parasitic" ultrasound waves? which disturb the reception of one of the transducers of ultrasound waves emitted by the other transducer, characterized in that the attenuation means are constituted by at least one passage in which said parasitic waves propagate in a principal direction corresponding to a "longitudinal" dimension a of said passage, said passage having a transverse direction b perpendicular to the dimension of a and much smaller than the magnitude? of the parasitic waves in the propagation medium, said passage comprising a plurality of consecutive portions of passage each having a part that presents a reduction in the propagation cross section along the dimension b of the passage, with the dimension longitudinal of each portion of passage being substantially equal to * _? .

2. - Fluid Meter, according to claim 1, characterized in that the passage is delimited by at least two longitudinal surfaces facing each other and spaced from each other along the dimension b, and on which at least one of them has a plurality of mutually parallel consecutive slots formed alternately with projections, each portion of -page having a pair consisting of a slot and a projection. 3. - Fluid meter, as claimed in 2, characterized by each slot has a profile in V. 4. - Fluid meter, according to claim 2, characterized in that each slot has a U-shaped profile determining that said surface is oled. 5. - Fluid meter, according to claim 1 to 4, characterized in that the parasitic waves are of a wavelength that varies within a certain range, and the longitudinal dimension of the portions of passage vary in increasing or decreasing cover the determined range of wavelengths. 6. - Fluid meter, according to claim 1 to 5, characterized by comprising a casing provided with a fluid inlet orifice and a fluid outlet orifice, a measuring block provided with ultrasonic transducers and provided with minus two openings, respectively allowing the fluid to reach the ultrasonic measurement path and leave it, the passage (s) being n which parasitic ultrasonic waves propagate formed between the measurement path and at least one of the fluid inlets and of the fluid outlets. 7. - Fluid meter, according to claim 6, characterized in that the measurement block is arranged inside said envelope in such a way as to form between them the passage (s) in which the parasitic ultrasound wave is propagated, and length of which the fluid flows before entering the measurement block or after leaving it. 8. - Fluid meter, according to claim 2 and 7, characterized in that the surface on which the grooves are formed is the surface of the measurement block. 9. - Fluid meter, according to claim 6 or 7, characterized in that the measurement block includes the passage (s) that allow the parasitic ultrasonic waves to be attenuated and formed between at least one of said openings and said path of measurement, these passages also serving to drive the fluid. 10. - Fluid meter, according to claim 9, characterized by being the measurement path formed within a measurement conduit. 11. - Fluid meter, according to claim 10, characterized in that the measurement conduit is at least partially arranged in a housing of the measurement block. 12. - Fluid meter, according to claim 11, characterized in that the passage (s) is / are formed between the walls of the housing and the measurement conduit. 13. - Fluid meter, according to claim 12, characterized in that the passage (s) is / are formed around the measuring conduit. 14. - Fluid meter, according to claim as claimed in any of claims 10 to 12, characterized in that the passage (s) is / are formed on one side only of the measuring conduit. 15. - Fluid meter, according to claim as claimed in any of claims 10 to 12, characterized in that the passages are formed on both sides of the measurement conduit. 16. - Fluid meter, according to claim as claimed in any of claims 9 to 15, characterized in that the passage (s) is / are formed along a portion of the measurement path. 17. - Fluid meter, as claimed in any of claims 10 to 16, characterized in that the pair of grooves / projections are formed on the external surface of the measuring conduit. 18. - Fluid meter, according to claim 10, characterized in that the passage (s) is / are arranged within the measurement conduit. 19. - Meter-fluid, according to claim 10, characterized in that the measurement conduit is a tube. 20. - The fluid meter, according to claim 19, characterized in that the measurement block includes another wall that is placed with respect to the partition wall so that the surfaces in front of these two walls delimit the passage (s). 21. - The fluid meter, according to claim 20, characterized in that the other wall is a piece added on the measurement block. 22. - The fluid meter, according to claim 1 - 5, characterized in that it comprises a measurement conduit that constitutes at least partially the ultrasonic measurement path, and that has at least one peripheral wall corresponding to the passage in which propagate parasitic ultrasound waves. 23. - The fluid meter, according to claim 2, characterized in that the surface on which the slots are made is the outer surface of the measuring conduit, the reduction of the section of the passage of each portion of passage is located at the right of each slot in said wall. 24. - The fluid meter, according to claim 22 or 23, characterized in that the measuring conduit is a tube. 25- Fluid meter, according to claim 24, characterized in that the grooves are annular and arranged along the tube. 26- Fluid meter, according to claim 24, characterized in that a helical groove is formed on the external surface of the measuring screw. SUMMARY OF THE INVENTION The invention provides a fluid meter (10) comprising ultrasonic transducers (26, 28), attenuation means for the attenuation of parasitic ultrasound waves of wavelength?, And constituted by at least one passage (36, 38, 40) in which said waves propagate along a principal direction corresponding to a longitudinal dimension a of said passage, said passage having a transverse dimension b, perpendicular to a, and much smaller than the wavelength. of the parasitic waves in the propagation medium, said passage comprising a plurality of consecutive passage portions (46, 48) each having a part (46) that presents a reduction in the propagation cross section along the dimension b of the passage, the longitudinal dimension of each passage portion being substantially equal to? . 1/11 2/11 Fg.3 (A-A) Fig. 4 Fig.4b Fig.4c Fig.2 4/11

3. 2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 F (x1 O4) Fig.4d Fig.5 Fg.6 15 81 148 Flg.d 176 Fig.16b Fig.16a 11/11 Fig.22a