Ultrasonic flow meter with asymmetric transducer window

Abstract

Systems and methods provided herein relate to an ultrasonic flow meter comprising: a transmitter connected to a first acoustic window in contact with a transmission medium; and a receiver connected to a second acoustic window in contact with the transmission medium wherein the geometry and / or material properties of the first acoustic window and the second acoustic window are asymmetrical.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of and priority to Provisional Application No. 63 / 336,704, filed on April 29, 2022, the entire disclosure of which is hereby incorporated by reference herein. Background Art

[0003] The present disclosure relates to flow meters. Flow meters can be used to measure fluids (i.e., production fluids and injection fluids) passing through conduits. In some cases, operators use ultrasonic flow meters to make such measurements. Ultrasonic flow meters include ultrasonic transducers for transmitting and detecting ultrasonic waves (i.e., ultrasonic signals) in the fluid passing through the flow meter. The flowing fluid interacts with the ultrasonic signals transmitted through the fluid. This allows the received ultrasonic signals to be used to infer the characteristics of the fluid, such as velocity and volume flow rate.

[0004] A transit time ultrasonic flow meter quantifies fluid flow based on the difference in transit time of an upstream ultrasonic signal and a downstream ultrasonic signal. For example, U.S. Patent No. 4,300,400, entitled "Acoustic flowmeter with Reynolds number compensation," discloses one such transit time ultrasonic flow meter. In some cases, reflections of the ultrasonic signal may result in undesirable signal characteristics. These undesirable signal characteristics may cause errors in applications such as transit time ultrasonic flow measurement. Summary of the invention

[0005] One embodiment of the present disclosure relates to an ultrasonic transmission link, which includes: a transmitting ultrasonic transducer connected to a first acoustic window in contact with a transmission medium; and a receiving ultrasonic transducer connected to a second acoustic window in contact with the transmission medium, wherein the geometric shapes and / or material properties of the first acoustic window and the second acoustic window are asymmetric.

[0006] Some embodiments relate to an ultrasound system comprising a transmitter connected to a first acoustic window and a receiver connected to a second acoustic window, the first acoustic window being in contact with a transmission medium, the second acoustic window also being in contact with the transmission medium, the geometry and / or material properties of the first acoustic window and the second acoustic window being asymmetric.

[0007] In some embodiments, the parameters of the first acoustic window and the second acoustic window are selected to suppress constructive interference of secondary reflections. In some embodiments, asymmetry is achieved by having the first acoustic window and the second acoustic window with different thicknesses. In some embodiments, the material properties of the first acoustic window and the second acoustic window are changed to achieve the desired asymmetry. In some embodiments, the thickness properties of the first acoustic window and the second acoustic window are changed to achieve the desired asymmetry. In some embodiments, both the material properties and the thickness are changed to achieve the desired asymmetry.

[0008] In some embodiments, the difference in transit time through the first acoustic window and the second acoustic window is given by (n+0.5)T, where n is an integer and T is the period of the primary signal. In some embodiments, the asymmetry in the parameters is large enough that the difference between the round-trip transit time in the first acoustic window and the second acoustic window is greater than the duration of the primary signal. In some embodiments, the acoustic windows are made of the same material and the difference in thickness of the acoustic windows is given by 0.5(n+0.5)λ, where n is an integer and λ is the wavelength of the primary signal.

[0009] Some embodiments relate to a method used in an ultrasonic system for measuring flow in a medium, the ultrasonic system comprising a transmitter and a receiver. The method comprises providing an ultrasonic signal through a first acoustic window of the transmitter, the first acoustic window being in contact with a transmission medium, and the method comprises receiving the ultrasonic signal through a second acoustic window of the receiver. The second acoustic window is in contact with the transmission medium, and the geometry and / or material properties of the first acoustic window and the second acoustic window are asymmetric.

[0010] In some embodiments, the parameters of the first acoustic window and the second acoustic window are selected to suppress constructive interference of secondary reflections. In some embodiments, asymmetry is achieved by having a first acoustic window and a second acoustic window of different thicknesses. In some embodiments, the material properties of the respective acoustic windows are changed to achieve the desired asymmetry. In some embodiments, the thickness properties of the first acoustic window and the second acoustic window are changed to achieve the desired asymmetry. In some embodiments, wherein both the material properties and the thickness are changed to achieve the desired asymmetry.

[0011] In some embodiments, the difference in transit time through the first acoustic window and the second acoustic window is given by (n+0.5)T, where n is an integer and T is the period of the primary signal. In some embodiments, the asymmetry in the parameters is large enough that the difference between the round-trip transit time in the first acoustic window and the second acoustic window is greater than the duration of the primary signal. In some embodiments, the first acoustic window and the second acoustic window are made of the same material, and the difference in thickness of the first acoustic window and the second acoustic window is given by 0.5(n+0.5)λ, where n is an integer and λ is the wavelength of the primary signal.

[0012] Some embodiments relate to an ultrasonic flow meter. The flow meter includes: a pair of ultrasonic devices; a meter body configured to be coupled to a conduit for a medium; and a meter electronics device that communicates with the pair of ultrasonic devices, the meter electronics device being configured to determine flow in response to ultrasonic signals. The pair of ultrasonic devices are relatively disposed in the meter body. A first device in the pair of ultrasonic devices has a first acoustic window in contact with the medium, and a second device in the pair of ultrasonic devices has a second acoustic window that is also in contact with the transmission medium. The geometry and / or material properties of the first acoustic window and the second acoustic window are asymmetric.

[0013] In some embodiments, the difference in transit time through the first and second acoustic windows is given by (n+0.5)T, where n is an integer and T is the period of the primary signal.

[0014] This summary is illustrative only and is not intended to be limiting in any way.Other aspects, inventive features, and advantages of the apparatus or processes described herein will become apparent in the detailed description set forth herein in conjunction with the accompanying drawings, wherein like reference numerals refer to like elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a block diagram of a flow measurement system according to some implementations.

[0016] Figure 2 is a perspective cross-sectional view of a flow meter according to some embodiments, the flow meter may be Figure 1 It is implemented in the flow measurement system.

[0017] Figure 3 is a cross-sectional view of a flow meter according to some embodiments.

[0018] Figure 4 is a cross-sectional view of a flow meter body according to some embodiments.

[0019] Figure 5A According to some embodiments, Figure 2 A cross-sectional view of a transducer assembly implemented in a flow meter.

[0020] Figure 5B According to some embodiments Figure 5A Exploded view of the transducer assembly.

[0021] Figure 6 According to some embodiments Figure 5A and Figure 5B A cross-sectional view of the transducer housing.

[0022] Figure 7According to some embodiments Figure 5A and Figure 5B An exploded view of a transducer interacting with a second transducer.

[0023] Fig. 8A and Figure 8B According to some embodiments Figure 7 Diagram of the interaction between transducers and the ultrasonic transmission path.

[0024] Fig. 9 is a waveform graph depicting a signal having characteristics corresponding to constructive interference between reflected ultrasonic signals that arrive later than a primary ultrasonic signal, according to some embodiments.

[0025] Fig.10 is a waveform graph depicting a signal having characteristics corresponding to destructive interference between reflected ultrasonic signals that arrive later than a primary ultrasonic signal, according to some embodiments.

[0026] Fig.11A and Fig. 11B is a waveform graph depicting destructive interference between reflected ultrasonic signals that arrive later than a primary ultrasonic signal, according to some embodiments.

[0027] Fig.12 is a set of waveform graphs depicting the varying amplitude of the reflected signal depending on the relative delay between the primary ultrasonic signal and the reflected signal according to some embodiments.

[0028] Fig.13A and Fig. 13B is a waveform graph depicting separation of reflected ultrasonic signals based on specific relative delays after a primary ultrasonic transducer according to some embodiments. DETAILED DESCRIPTION

[0029] summary

[0030] Before turning to the accompanying drawings that illustrate certain exemplary embodiments in detail, it should be understood that the present disclosure is not limited to the details or methods set forth in the specification or illustrated in the accompanying drawings. It should also be understood that the terminology used herein is for descriptive purposes only and should not be considered limiting.

[0031] The present disclosure relates to ultrasonic measurement systems, including but not limited to ultrasonic flow meters. Some ultrasonic flow meters use ultrasonic transducers arranged to determine the flow rate of a fluid through a conduit. When a fluid flows in a conduit, an acoustic pulse (i.e., an ultrasonic signal) transmitted through the conduit in the direction of the flow from one transducer (i.e., a downstream ultrasonic signal) will propagate faster than a pulse transmitted from another transducer in the opposite direction of the flow (i.e., an upstream ultrasonic signal). Transit-time ultrasonic flow meters quantify fluid flow based on measured signal transit times and other computational inputs. The difference in transit time of a downstream signal and an upstream signal on a given path is proportional to the velocity of the fluid on that path.

[0032] A transit time flow meter may arrange the transducers in an emit-capture configuration (i.e., using two transducers per signal path). In this case, the first transducer may be considered a transmitting transducer (i.e., a "transmitting transducer"), and the second transducer may be considered a receiving transducer (i.e., a "capturing transducer"). The transmitting transducer generates a pressure wave (i.e., an ultrasonic signal) that is received by the receiving transducer. Thus, the excitation of the transmitting transducer may be used to determine the "departure time" of the ultrasonic signal, and the signal received at the capturing transducer may be used to record the "arrival time" of the ultrasonic signal. One such method uses detection of peaks and associated zero crossings within the "leading edge" of the received ultrasonic signal. Therefore, repeatable and reliable detection of the arrival time of the ultrasonic signal is essential to the measurement of transit time. Therefore, a transit time flow meter may rely on the detection of specific peaks within the leading edge of the received signal waveform to calculate the transit time for the ultrasonic signal to travel from the transmitting transducer to the receiving transducer. For accurate transit time measurements, it is necessary to be able to consistently identify peaks and zero crossings within the ultrasonic signal.

[0033] The transducers used in ultrasonic flow meters typically include a housing that may include an acoustic window (sometimes called a "buffer"). An acoustic window is a thickness of material placed between the transducer and the medium through which the signal is to be transmitted, typically to protect the transducer or prevent the transducer from being exposed to process conditions. These windows are typically required to withstand relatively high process pressures, and these windows are typically equal in thickness to multiple ultrasonic wavelengths. Each material that forms the ultrasonic signal transmission path, including the transducer housing and the medium through which the signal is to be sent, exhibits an acoustic impedance. Due to the acoustic impedance mismatch between the materials that form the ultrasonic transmission path, a portion of the ultrasonic signal is reflected at different interfaces instead of being transmitted through these different interfaces.

[0034] At interfaces with high acoustic impedance mismatch, the reflected portion of the signal may be significant compared to the transmitted portion of the signal. The reflected signal in the transmitting transducer housing and transducer assembly may eventually be transmitted to the receiving transducer housing through the fluid. The signal is delayed relative to the primary signal by a transit time, which corresponds to the "round trip" of the reflection of the signal from the window / fluid interface back through the window material and any other materials, to the second reflection point and then forward again through any other materials and the window material. When the primary signal reaches the receiving transducer, a portion of it will also be reflected within the receiving transducer and transducer housing assembly (including the acoustic window). These reflections may result in undesirable signal characteristics, which may cause errors in applications such as transit time ultrasonic flow measurement.

[0035] In some cases, the housings of the transmitting and receiving transducers are formed with the same (i.e., "symmetrical") size and / or geometry. In the case of symmetric transducer geometry and materials, the aforementioned reflections at the receiving transducer will occur with the same delay time as the reflections occurring at the transmitting transducer, and in phase with the reflections occurring at the transmitting transducer. These "in-phase" reflections can eventually constructively combine at the receiving transducer, thereby producing a series of secondary peaks that are delayed relative to the leading edge. Under certain conditions, the amplitude of these secondary peaks can be greater than the amplitude of the peak at the leading edge of the waveform.

[0036] The presence of secondary peaks may have undesirable effects on signal processing strategies, including automatic gain control (AGC) and applications based on threshold peak detection. The purpose of AGC is to amplify the received signal until it reaches the desired maximum peak voltage. The intention is that the measured signal peak that drives the AGC process is located at the "leading edge" of the signal. However, due to the presence of a strong secondary reflection peak that exceeds the size of the peak in the leading edge, the secondary peak may be unintentionally used to set the AGC level. Similarly, the threshold-based peak detection algorithm used in ultrasonic signal transit time measurement, which is intended to consistently identify the peak in the leading edge of the signal, may suffer from inaccuracy when there are large secondary peaks. More specifically, when the size of the secondary signal peak is greater than the size of the primary signal peak, the primary signal peak may drop below the amplitude threshold defined by the threshold-based peak detection algorithm. This may cause the threshold-based peak detection algorithm to mistakenly identify the secondary peak as a leading ultrasonic signal. Therefore, there is a need for a transducer housing design for a transit time ultrasonic flow meter that solves the aforementioned problems associated with symmetrical transducer geometry and materials.

[0037] Various embodiments disclosed herein relate to an ultrasonic transmission link having a transducer in a launch-capture configuration, the transducer having an asymmetric acoustic window of varying thickness and / or geometry such that constructive interference of secondary reflections is prevented or avoided. The use of asymmetric window thicknesses can promote destructive interference of the reflections described above, thereby reducing undesirable signal characteristics that might otherwise occur as a result. The use of appropriately designed asymmetric windows can also avoid constructive interference of reflections that would otherwise result in undesirable signal characteristics.

[0038] In various embodiments disclosed herein, the round trip time of reflection through the transmitter window and the round trip time of reflection through the receiving window are controlled to ensure that constructive interference does not occur. In some embodiments, this is achieved by ensuring that the round trip time of reflection through the transmitter window differs from the round trip time of reflection through the receiving window by half (or an odd multiple of half) of the expected signal period, or by ensuring that the round trip time of reflection through the receiving window differs from the round trip time of reflection through the transmitter window by half (or an odd multiple of half) of the expected signal period. If the asymmetry of the acoustic window is achieved only by thickness, the difference in thickness expected between the two windows is given by 0.5(n+0.5)λ, where n is an integer and λ is the wavelength of the signal in the window material. For example, if the transmitter window is 5 wavelengths thick, the receiver window thickness will be 5 plus or minus 0.25, 0.75, 1.25, 1.75, etc. wavelengths. The desired time difference can also be achieved via an asymmetry of the material properties (in particular the speed of sound) of the respective windows or by a combination of thickness properties and material properties.

[0039] Although the present disclosure is generally described herein with respect to flow meters, it will be appreciated that the present disclosure may be applied to other ultrasonic-based measurement devices that utilize transducers in a launch-and-catch arrangement where it is desirable to suppress secondary reflections originating from the transducer housing.

[0040] Ultrasonic measurement system

[0041] Now refer to Figure 1, a block diagram of an ultrasonic measurement system 100 according to some embodiments is shown. In some embodiments, the ultrasonic measurement system 100 includes a meter body 101, a flow conduit 102, and a meter electronic device 103. The meter electronic device 103 may include a display 104, an input / output circuit 105, and a processor 106. The processor 106 can be configured to control the transmission and reception of ultrasonic signals, as described in more detail herein. The processor 106 can also be configured to perform signal and data processing and to perform flow and output calculations. In some embodiments, the meter electronic device 103 can be included in the meter body 101 (i.e., assembled as a unit). In other embodiments, the meter electronic device 103 can be separately mounted on the meter body 101 or the flow conduit 102. The input / output circuit 105 can include various analog and / or digital inputs and outputs to perform the systems and methods described herein.

[0042] In some embodiments, and as described in more detail below, the ultrasonic measurement system 100 includes at least one pair of transducers (i.e., transducer assemblies) that exhibit a transmit-capture operation, wherein a signal is transmitted from one transducer to another transducer. The transducers may be located in a housing that, in some cases, has the asymmetric properties of the present disclosure. The processor 106 controls the application of electronic signals to the transmitting transducer and the reception of electronic signals from the receiving transducer. In addition to controlling the excitation of the transmitting transducer and the acquisition of the received signal by the receiving transducer, the processor 106 may also perform other functions, such as signal processing and calculation of the obtained results. The processor 106 may be connected to an output device that may be included in the input / output circuit 105. The processor 106 may be configured to perform flow measurement based on the determination of the transit time of the ultrasonic signal. Such flow measurement results may be recorded and stored or output in analog or digital form.

[0043] Flow Meter

[0044] Now refer to Figure 2 , showing a perspective cross-sectional view of a flow meter 200 according to some embodiments. The flow meter 200 can be similar to the flow meter 200 described above. Figure 1 The ultrasonic measurement system 100 described is the same or substantially similar. In some embodiments, the flow meter 200 includes a flow meter body 201, a flow meter electronics 203, and a fluid inlet port 205 / outlet port 206. Although the systems and methods disclosed herein generally relate to ultrasonic flow meters, such as the flow meter 200, this is merely exemplary and should not be considered limiting, as other types of ultrasonic systems are contemplated. Ports 205, 206 can be used as inlet ports or outlet ports, respectively, and / or ports 205, 206 can be used as outlet ports or inlet ports, respectively.

[0045] In some embodiments, the flow meter electronics 203 includes processing circuitry including a memory (not shown) and one or more processors. The processor may be implemented as a general purpose processor, an application specific integrated circuit (ASIC), one or more field programmable gate arrays (FPGA), a group of processing components, or other suitable electronic processing components. The flow meter electronics 203 may be substantially similar to Figure 1 The measuring instrument electronics 103 depicted in FIG. 1 functions.

[0046] In some embodiments, the flow meter 200 includes transducer assemblies 204 arranged upstream and downstream of each other in a launch-capture relationship to transmit acoustic energy along an acoustic path through a fluid flowing in a conduit, as described below with reference to Figure 3 and Figure 4 The transducer assembly 204 can be disposed within an orifice formed by the flow meter body 201. In some embodiments, the transducer can be slightly recessed within the orifice so that the transducer assembly 204 does not directly contact the inner surface 210 of the flow meter body 201. The flow meter electronics 203 determines the transit times for the upstream and downstream signal transmissions and uses those measured upstream and downstream transit times in conjunction with other inputs to calculate the velocity in each measurement plane and infer the flow rate of the fluid.

[0047] A non-limiting example of this type of meter is the CALDON TM (Carlton TM )Ultrasonic flowmeter. CALDON TM The meter uses a compact transmitter housing that can be integrally mounted to the meter body or remotely pipe mounted. Within the meter body are multiple pairs of fully integrated piezoelectric ultrasonic transducers that form acoustic measurement paths. These paths typically cross the flow stream at angles between 45 and 65 degrees, resulting in differences in the transit time of the ultrasonic signal, depending on whether the acoustic pulse is traveling in the direction of the flow or against the flow. The difference in transit time is measured along each path. The meter electronics infer the velocity on each path and perform an integration of the axial velocity to calculate the output of volume flow rate.

[0048] Now refer to Figure 3 , according to some embodiments, a cross-sectional view is shown that is orthogonal to one of the transducer planes and parallel to the other transducer plane. As shown, the transducer assembly 204 may include a transducer module 302 located (i.e., mounted) inside a transducer housing 303. Figure 5A and Figure 5BThe transducer housing 303 is described in further detail. The transducer housing 303 may be as follows: Figure 3 , such that the transducer module 302 within the transducer housing 303 can provide an ultrasonic signal through the following fluid: the fluid flows through the channel 304, and the fluid flow rate will be measured through the channel 304. Therefore, the fluid flow rate can be measured at least in part by using the transducer module 302 that at least partially forms the transducer assembly 204 within the transducer housing 303. As shown, and described in more detail below, the transducer assembly 204 is arranged in an emit-capture configuration. In some embodiments, the transducer housing 303 is attached to the flow meter body 201, thereby forming a seal that prevents the fluid from flowing out. Of course, the attachment method of the transducer housing 303 and the seal can vary (i.e., a flange on the outside of the measuring instrument body, the use of an O-ring, the use of a retaining ring, the use of a threaded seal, etc.), and should not be considered limiting. The transducer housing 303 and the corresponding window (see below) Figure 5A and Figure 5B The measuring device (described in more detail) can also be formed as a single piece with the measuring device body itself, rather than as separate components.

[0049] Now refer to Figure 4 , shows another cross-sectional view of the interior of the flow meter 200 according to some embodiments. Figure 4 A view of the transducer housing 303 mounted through the flow meter body 201 is shown. The arrows show the flow path for the channel 304. As described above, the transducer module 302 for the transducer assembly 204 that performs the ultrasonic pulses is located within the transducer housing 303. As shown, and described in more detail below, the transducer assembly 204 is arranged in a launch-capture configuration.

[0050] Transducer Overview

[0051] Now refer to Figure 5A , Figure 5B and Figure 6 , according to some embodiments, transducer assembly 204 is shown in greater detail. Figure 5A A side cross-sectional view of transducer assembly 204 is shown. Figure 5B 2 shows an exploded perspective view of the transducer assembly 204 according to some embodiments. Generally, the transducer assembly 204 includes a transducer module 302 and a transducer housing 303. However, Figure 6 A side cross-sectional view of the transducer assembly 204 without the transducer module 302 is shown. The transducer housing 303 includes an acoustic window 502 (ie, a buffer, etc.). Figure 5A, the transducer housing 303 can form a cavity 503, and the transducer module 302 can be located within the cavity 503. In some embodiments, the transducer assembly 204 can also include a cap (i.e., a "compression screw") for sealing the transducer module 302 within the transducer housing 303. As described above, the transducer assembly 204 can be installed in the flow meter body 201. The transducer module 302 can provide an ultrasonic signal (i.e., an ultrasonic pulse, waveform, etc.) along a signal path. The acoustic window 502 is the thickness of the material disposed between the transducer module 302 and the medium through which the ultrasonic signal will be transmitted, such as a flow through Figure 3 and Figure 4 In particular, as shown in FIG. Figure 5A As shown in , the transducer housing 303 can form an acoustic window / process fluid interface 504 and a transducer module / acoustic window interface 505, through which ultrasonic signals are transmitted accordingly. The ultrasonic signal generated by the transducer module 302 travels into and through the acoustic window 502, and ultimately travels into the fluid flowing through the channel 304. In some embodiments, the acoustic window 502 is used to protect the transducer assembly 204 or prevent the transducer assembly 204 from being exposed to process conditions occurring in the channel 304, including the fluid flowing through the channel 304. In some embodiments, the thickness of the acoustic window 502 is multiple wavelengths. The acoustic window 502 can be made of any suitable material for ultrasonic signal transmission, including but not limited to stainless steel, carbon steel, aluminum, epoxy resin, plastic, and polyurethane. In some embodiments, some or all of the transducer housing 303, the acoustic window 502, and the flow meter body 201 are mounted together as separate components. In other embodiments, some or all of the transducer housing 303, acoustic window 502, and flow meter body 201 are machined from or manufactured as a single piece of material rather than being mounted together as separate components.

[0052] Asymmetric transducer window

[0053] Now refer to Figure 7 According to some embodiments, the measurement path (or emit-capture transmission link) 700 includes Figure 2 , Figure 3 , Figure 4 , Figure 5A , Figure 5B and Figure 6 The transducer assembly 204 depicted in Figure 2 , Figure 3 and Figure 4The transducer assembly 214 may be similar to the transducer assembly 214, but positioned so that the transducer assemblies 204 and 214 form an emission-capture relationship, as described above with reference to Figure 3 and Figure 4 As depicted. As shown, transducer assembly 204 and transducer assembly 214 form a transmission link along signal path 701. Transducer assembly 204 transmits ultrasonic pulses (via transducer module 302) along signal path 701 to transducer assembly 214 (i.e., the ultrasonic pulses are received by transducer module 312). Therefore, acoustic window 502 can be referred to as a "transmitter" window, and acoustic window 512 included in transducer assembly 214 can be referred to as a "receiver" window. However, transducer assemblies 204 and 214 can achieve interchangeable functions (i.e., transmitting and receiving) by alternating between transmitting ultrasonic signals and receiving ultrasonic signals along signal path 701. The emission-capture configuration does not depend on which of the transducer assemblies 204 or 214 is the transmitter and which is the receiver, but rather a pair of transducer assemblies 204 and 214 form a transmission link (i.e., signal path 701) for transmitting and receiving ultrasonic signals. Thus, in various embodiments, either of transducer assembly 204 and transducer assembly 214 may be described as "transmitting" or "receiving". The ultrasonic signal itself is a pressure wave with various characteristics, one of which is wavelength. Acoustic windows 502 and 512 have thicknesses 702 and 712, respectively. In some embodiments, and as discussed in more detail below, thicknesses 702 and 712 may be different (i.e., asymmetric) and may be multiple wavelengths in size.

[0054] In some embodiments, and as described below with respect to Fig. 8A and Figure 8B As discussed in more detail, the ultrasonic signals generated by the transducer module 302 are at least partially reflected within the transducer housings 303 and 313 and the acoustic windows 502 and 512. As described in more detail below, by implementing the acoustic windows 502 and 512 with asymmetric geometries (including but not limited to variations in thickness 702 and 712) and / or asymmetric material properties, one or the other of the reflected signal generated in the transmitting transducer housing and the reflected signal generated in the receiving transducer housing can be delayed relative to the other such that they are out of phase with each other and thus interfere destructively rather than constructively.

[0055] In some embodiments, and as described in more detail below, if the aforementioned asymmetry is appropriately chosen (i.e., resulting in a half-cycle delay), the peak associated with the transmit transducer reflection and the peak associated with the receive transducer reflection will at some point in time exhibit opposite polarity, resulting in a signal having a significantly reduced amplitude. In this way, the trailing secondary peak can be suppressed.

[0056] Now refer to Fig. 8A and Figure 8B , shows a schematic diagram of a measurement path 800 according to some embodiments. According to some embodiments, the measurement path 800 includes the transducer assembly 204 and the transducer assembly 214. As shown, the ultrasonic signal emitted from the transducer assembly 204 is depicted as being reflected on various surfaces between the transducer assembly 204 and the transducer assembly 214. As described above with respect to Figure 5A and Figure 5B As depicted, the transducer assembly 204 includes a transducer module 302 and a transducer housing 303. The transducer module 302 includes a back wall 506, a component interface (i.e., a piezoelectric disc to backing mass interface) 507, and a window interface (i.e., an acoustic window interface, a front interface, etc.) 505. The transducer housing 303 includes an acoustic window 502 and an acoustic window process fluid interface (i.e., a fluid interface) 504. Similarly, the transducer assembly 214 includes a transducer module 312, which includes a back wall 516, a component interface 517 within the transducer module 312, a window interface 515, and a transducer housing 313, which forms the acoustic window 512 and the fluid interface 514.

[0057] As shown, the ultrasonic signal is transmitted via transducer assembly 204 to travel along a path (i.e., signal path, measurement path, etc.) (along direction 801) to transducer assembly 214. Therefore, in this example, transducer assembly 204 can be interpreted as a transmitting transducer and transducer assembly 214 can be interpreted as a receiving transducer. However, as discussed above, this only illustrates a single transmission, and either transducer assembly 204 and 214 can be used as a transmitting transducer or a receiving transducer. The ultrasonic signal generated by transducer assembly 204 must pass through some or all of the components (i.e., materials) described above that form transducer assemblies 204 and 214 on the signal path to reach transducer assembly 214. As described above, the ultrasonic signal can be reflected on various surfaces between transducer assembly 204 and transducer assembly 214. The various reflections described herein can be reflected by Fig. 8A The example paths 803, 804 and 805 depicted in FIG. Figure 8B, and 807 are illustrated in the example paths 803, 806, and 807 depicted in FIG. However, it should be understood that paths 803, 804, 805, 806, and 807 are depicted for reference only. For example, paths 803, 804, and 805 and paths 803, 806, and 807 are shown offset from each other. Ultrasonic signal paths typically have a finite width and may therefore overlap. Only for clarity, paths 803, 804, 805, 806, and 807 are depicted offset from each other. Ultimately, paths 803, 804, 805, 806, and 807 are depicted herein only to indicate where various reflections may occur and the number of times a signal may traverse acoustic windows 502 and 512, and are not depicted to illustrate specific locations relative to each other.

[0058] In some embodiments, the ratio of the transmitted ultrasonic signal to the portion of the reflected ultrasonic signal depends on the acoustic impedance of the adjacent material. Each material exhibits an acoustic impedance that depends on its density and its acoustic velocity. The greater the difference in the acoustic impedance of two adjacent materials, the greater the ratio of the reflected ultrasonic signal to the ultrasonic signal transmitted to the adjacent material.

[0059] As shown, an ultrasonic signal (ie, a primary ultrasonic signal) is transmitted from the transducer module 302 along direction 801 into the acoustic window 502 (described above with respect to Figure 5A , Figure 5B , Figure 6 and Figure 7 The primary ultrasonic signal (see path 803 for an exemplary illustration) travels through acoustic window 502 and into the process fluid between transducer assemblies 204 and 214, such as into the process fluid flowing through Figure 3 and Figure 4 304. When the primary ultrasonic signal reaches the fluid interface 504, a portion of the ultrasonic signal will be reflected back toward the transducer module 302 due to the acoustic impedance mismatch of the acoustic window 502 and the process fluid (see paths 805 and 807 for exemplary illustration). The reflected portion of the primary ultrasonic signal will travel back toward the transducer module 302 along direction 802. At the window interface 505, the internal component interface 507, or the back wall 506, a portion of the reflected ultrasonic signal will again be reflected back toward the acoustic window 502 and the fluid interface 504 along direction 801. Which interface of the window interface 505, the component interface 507, or the back wall 506 contributes to the strongest reflection will depend on the acoustic impedance mismatch between the adjacent media forming the acoustic window 502 and the transducer module 302 itself. At the fluid interface 504, a portion of the reflected ultrasonic signal is then transmitted through the fluid interface 504 along direction 801 and transmitted through the process fluid. The ultrasonic signal will be delayed from the primary signal in direction 801 by an amount of time that includes at least the time of traversing the thickness of the acoustic window (e.g., Figure 7 The thickness 702 depicted in FIG. 7 is the transit time required twice.

[0060] As shown, the primary signal first transmitted to the process fluid traveling in direction 801 will travel to the transducer assembly 214. A portion of the primary signal will be transmitted through the fluid interface 514, where this portion of the signal will travel through the acoustic window 512 to the window interface 515. A portion of the signal will be transmitted to the transducer module 312, thereby generating a measured primary signal, such as Fig. 9 . However, a portion of the primary signal will be reflected at the window interface 515, the component interface 517, or the back wall 516 (see paths 804 and 806 for exemplary illustration). Which interface of the window interface 515, the component interface 517, or the back wall 516 contributes the strongest reflection will depend on the acoustic impedance mismatch between the adjacent media including the acoustic window 512 and the transducer module 312 itself. In order to achieve the desired characteristics in the primary signal, the transducer modules 302 and 312 are manufactured to be nominally identical, and therefore the interface (window interface, component interface, or back wall) for the maximum reflection is usually the same on both sides of the ultrasonic signal path. The reflected signal will then return to the fluid interface 514 through the acoustic window 512 along the direction 802. If the acoustic window 502 and the acoustic window 512 have the same geometry and thickness (i.e., symmetry), the reflected signal in the receiving transducer acoustic window 512 will arrive at the fluid interface 514 at the same time as the aforementioned reflected signal from the transducer assembly 204. In other words, the reflected signal for reference illustrated by paths 804 and / or 806 will arrive at fluid interface 514 simultaneously with the reflected signal for reference illustrated by paths 805 and / or 807. The portion of the reflected signal from transducer assembly 204 that is transmitted into acoustic window 512 will advantageously combine with the reflected signal and will arrive in phase at window interface 515. For commonly used transducer materials and ultrasonic flow meter materials, the constructive interference of these two reflected signals will produce a secondary ultrasonic signal, such as Fig. 9 , the secondary ultrasonic signal 905 depicted in , which exhibits a peak value that may be larger in amplitude than the primary ultrasonic signal 903 .

[0061] Now refer to Fig. 9, according to some embodiments, the primary ultrasonic signal 903 and the secondary ultrasonic signal 905 described above are shown on a waveform graph 900. The presence of a peak of the secondary ultrasonic signal 905 having a larger amplitude than the peak of the primary ultrasonic signal 903 may cause control and measurement problems for the algorithms used in ultrasonic flow meters or other ultrasonic measurement devices that determine the transit time of ultrasonic signals. One such algorithm that the presence of a larger secondary signal peak may have an undesirable effect on is an automatic gain control (AGC) algorithm. The purpose of the AGC is to amplify the signal until it reaches the desired maximum peak voltage. The intention is that the signal peak in the measured signal that drives the AGC algorithm is located at the leading edge of the signal also referred to as the primary signal (i.e., the primary ultrasonic signal 903). However, in the presence of a larger secondary signal peak, shown herein as the secondary ultrasonic signal 905, the secondary signal peak may be incorrectly used to set the AGC level. Similarly, the threshold-based peak detection algorithm used in time measurement - which is intended to consistently identify the peak in the leading edge of the signal, such as the peak in the leading edge 904 - may suffer from inaccuracy when a larger secondary peak is present. This may occur when the relative amplitude of the primary signal peaks decreases due to the AGC operating on the secondary peaks. The lower relative amplitude of the primary signal peaks may then cause the peaks in the primary ultrasonic signal 903 to fall below the amplitude threshold 901 which is intended to be used to identify specific peaks and zero crossings to be used for transit time measurements.

[0062] The present disclosure relates to a method for Fig. 9 The solution to the problem in question. Again, refer to Figure 7 , the measurement path 700 in the emission-capture configuration includes the transducer assembly 204 and the transducer assembly 214 having acoustic windows 502 and 512, respectively. The acoustic windows 502 and 512 can be asymmetric acoustic windows of different thicknesses / geometries and / or materials. For example, the acoustic window 502 can form the thickness 702 to have a different thickness and / or geometry than the thickness 712 of the acoustic window 512. Although not explicitly shown, alternatively or in addition to forming different thicknesses, the acoustic window 502 and the acoustic window 512 can be formed of different materials. Therefore, the desired time difference can also be achieved via the asymmetry of the material properties (particularly the speed of sound) of the corresponding windows or by a combination of thickness properties and material properties.

[0063] Now refer to Fig.10 (and additionally with reference to FIG. 8 ), according to some embodiments, waveform graph 1000 depicts a primary ultrasonic signal 1001 and a secondary ultrasonic signal 1002. The purpose of the different acoustic window thicknesses / geometries and / or materials is to prevent constructive interference of reflected signals that would otherwise produce a large secondary received signal, such as Fig. 9, wherein the peak in the secondary received signal is greater in amplitude than the peak in the primary signal, such as greater than the peak in the primary ultrasonic signal 903. For asymmetries that result in a time difference between reflected signals that is shorter than the duration of the primary signal, appropriate design of the thickness / geometry and / or material of the acoustic window will promote destructive interference of the reflected signals, thereby reducing secondary peaks, such as the secondary ultrasonic signal 1002, in the ultrasonic signal forming the waveform graph 1000. In some embodiments, the round-trip time of the reflection through the transmitter window is designed to differ from the round-trip time of the reflection through the receiving window by half (or an odd multiple of half) of the expected signal period, or the round-trip time of the reflection through the receiving window is designed to differ from the round-trip time of the reflection through the transmitter window by half (or an odd multiple of half) of the expected signal period. Therefore, when the two signals meet at the window interface, the reflected signal will be 180 degrees out of phase with the other reflected signal, as described above with reference to Fig. 8A and Figure 8B As described. The out-of-phase signals will destructively combine resulting in a secondary peak of greatly reduced amplitude, as shown by the secondary ultrasonic signal 1002 compared to the primary ultrasonic signal 1001. To achieve this phasing of the reflected signal, if the asymmetry is achieved only by thickness, the desired thickness difference between the two windows is given by 0.5(n+0.5)λ, where n is an integer and λ is the wavelength of the signal in the window material. λ can be calculated as the period multiplied by the speed of sound of the medium. The period is the time required for the signal to complete a complete cycle (i.e., the round-trip transit time). For example, if the transmitter window is 5 wavelengths thick, the receiving acoustic window thickness will be 5 plus or minus 0.25, 0.75, 1.25, 1.75, etc. wavelengths.

[0064] In some embodiments, the transit time of a signal experiencing reflection in the transmit transducer window may be given by Equation 1 below:

[0065] t ref_T =3t w_T +t w_R +t other

[0066] Equation 1

[0067] Among them, t ref_T is the round-trip transit time of the signal experiencing reflection in the transmit transducer window, t w_T is the transit time required to cross the transmitter window, t w_R is the transit time required to cross the receiver window, and t other denotes any other contribution to the transit time. Similarly, for a signal that experiences reflection in the receive transducer window, the transit time can be given by Equation 2 below:

[0068] t ref_R =3t w_R +t w_T +t other

[0069] Equation 2

[0070] Among them, t ref_R is the round trip transit time of the signal experiencing reflection in the transmit transducer window. Therefore, the difference in the total transit time between the signal experiencing reflection in the transmit transducer window and the signal experiencing reflection in the receiver window can be given by combining equations 1 and 2 by equation 3 below:

[0071]

[0072]

[0073] As described above, the round trip time of the reflection through the transmitter window and the round trip time of the reflection through the receiving window can be designed to differ by half (or an odd multiple of half) of the expected signal period, or the round trip time of the reflection through the receiving window and the round trip time of the reflection through the transmitter window can be designed to differ by half (or an odd multiple of half) of the expected signal period, so as to promote destructive interference of the reflected signals. Therefore, the expected difference in round trip time can be applied to Equation 3 as given by Equation 4 below:

[0074] 2(t w_T -t w_R )=(n+0.5)T

[0075] Equation 4

[0076] Where n is an integer and T is the expected signal period. In general, and as described above, the travel time of a reflection in a window can be described as the ratio between the thickness of the transducer window and the speed of sound. Therefore, the difference in round trip time between two transducer windows can be described as the difference of these ratios. Therefore, the expected difference in round trip time as defined by Equation 4 can alternatively be given by Equation 5 below:

[0077]

[0078] Among them, x w_T is the thickness of the transmitting transducer window, x w_R is the thickness of the receiving transducer window, c w_T is the speed of sound through the transmitting transducer window (which may be based on the material properties of the transducer window), and c w_Ris the speed of sound through the receiving transducer window. In some embodiments, the transmitting transducer window and the receiving transducer window are made of the same material, so c in Equation 5 is w_T and c w_R can each be defined as c, and thus the expected difference in round trip time as defined in Equation 5 can be simplified to the relationship between the window thicknesses given by Equation 6 below:

[0079]

[0080] Assuming that the wavelength (λ) is equal to the speed of sound divided by the frequency (c / f), and that the expected signal period T is equal to 1 / f, then cT in Equation 6 is equal to λ, and therefore the relationship between the window thicknesses as defined in Equation 6 (yielding the expected difference in round-trip travel time) can be further simplified to be given by Equation 7 below:

[0081] x w_T -x w_R =0.5(n+0.5)λ

[0082] Equation 7

[0083] Thus, in some embodiments, a desired difference in round trip time can be achieved by a relative difference in window thickness, as summarized by Equation 7.

[0084] In other embodiments, and as described above, the desired difference in round trip time may be achieved by relative changes in the material properties of the windows rather than differences in window thickness. As described above, the material properties of the windows may produce relative differences in the speed of sound through the windows, and thus at equal window thicknesses, i.e., x w_T =x w_R = x, the expected difference in transit time defined in equation 5 can be interpreted as c wT and c wR is a function of , as given by Equation 8 below:

[0085]

[0086] To achieve the desired difference in round trip time, it may be useful to vary the material properties of one window relative to the other, and so the relationship defined by Equation 8 may be adjusted to be given by Equation 9 below, and simplified to be given by Equation 10 below:

[0087]

[0088] As summarized by Equation 10, the speed of sound through the receive transducer window may be determined based on the speed of sound measured through the transmit transducer window (or the speed of sound through the transmit transducer window may be determined based on the speed of sound measured through the receive transducer window) in order to achieve a desired difference in round-trip travel time.

[0089] In still other embodiments, where it may be difficult to achieve the desired difference in sound speed and / or window geometry, it will be clear to those skilled in the art that the frequency, and therefore the period T, may also be varied to achieve the desired difference in round trip time.

[0090] It should be noted that, in reality, the ultrasonic signal is a transient signal comprising a plurality of quasi-sinusoidal half cycles. When n in the above equation is equal to 0 (i.e., when the difference in arrival time is equal to half of the period T), the reduction of the reflected peak in the received signal waveform will generally be most effective. However, if this is difficult to achieve for practical reasons, a difference of (n+0.5)T may still be effective.

[0091] The above description and associated equations are provided to provide insight into how an asymmetric window may be designed and are not intended to be limiting. As described above, and as will be appreciated by those skilled in the art, there are many ways to vary the geometry and material properties of the transmitting and receiving transducers to achieve desired results.

[0092] Now refer to Fig.11A and Fig. 11B , according to some embodiments, an example waveform graph 1100 depicting destructive interference is shown. Fig.11A In FIG. 1 , the waveform graph 1100 includes a primary ultrasonic signal 1101, a first reflected ultrasonic signal 1102, and a second reflected ultrasonic signal 1103. Fig.11A As shown in , the first reflected ultrasonic signal 1102 and the second reflected ultrasonic signal 1103 are delayed by 5 signal cycles and 5.5 signal cycles respectively relative to the primary ultrasonic signal 1101. Fig. 11B As shown in , a result waveform 1104 is depicted, which shows that in the result waveform 1104, Fig.11A How does the difference of 0.5 signal cycles between the first reflected ultrasonic signal 1102 and the second reflected ultrasonic signal 1103 cause them to be Fig.11A The primary ultrasonic signal 1101 depicted in FIG. 1 decreases.

[0093] Now refer to Fig.12 , according to some embodiments, showing Fig.11A and Fig. 11B1200 of resultant waveforms produced by the same process as the process of . Group 1200 includes waveforms 1201 to 1208, in which the delay between the first reflected signal and the second reflected signal varies from 0 to 3.5 in small amounts of 0.5 signal cycles (as marked below each waveform). It can be observed from the 1201 waveform that the amplitude of the secondary peak is the largest (and therefore the least ideal) when the delay is zero. Similarly, it can be observed from the 1202 waveform that the amplitude of the secondary peak is the lowest (and therefore the most ideal) when the delay between the reflections is 0.5 signal cycles. It can also be observed in the remaining cases depicted in waveforms 1203 to 1208 that, in general, the amplitude of the secondary peak is lower in the remaining cases where the delay is an odd multiple of 0.5 signal cycles as shown in waveforms 1204, 1206 and 1208 compared to waveforms 1201, 1203, 1205 and 1207. It can also be observed that when the delay between reflections is longer than the duration of the primary signal, the reflections can become completely separated in time (as shown in waveforms 1206, 1207, and 1208), resulting in another way that the present disclosure can reduce secondary peaks, as described below with respect to Fig.13A and Fig. 13B as described.

[0094] Now refer to Fig.13A and Fig. 13B , according to some embodiments, waveform graphs 1300 and 1310 are shown. The waveform graph 1300 includes a primary ultrasonic signal 1301, a first reflected ultrasonic signal 1302, and a second reflected ultrasonic signal 1303. As shown, the delay between the first reflected ultrasonic signal 1302 and the second reflected ultrasonic signal 1303 has a length in time similar to the duration of the primary ultrasonic signal 1301. In this case, the performance of the acoustic window does not need to be a specific thickness, as long as the time delay introduced by the asymmetry between the acoustic windows is longer than the total duration of the primary ultrasonic signal 1301. Combining this concept with Equation 4 above, the expected difference in round-trip time can be given by Equation 11 below:

[0095] 2(t w_T -t w_R )>Y

[0096] Equation 11

[0097] Where Y is the effective duration of the primary signal. In practice, duration Y may be the time taken from the start of the signal to the point where its amplitude has decreased to such a point that the sum of the amplitudes of the first and last peaks within duration Y is less than the amplitude of the maximum peak in the primary signal.

[0098] In some embodiments, the acoustic windows need not be parallel or have a constant thickness. For example, if the windows have stepped or curved surfaces, a similar process can be applied by one skilled in the art to achieve asymmetry between the transmit and receive windows to avoid constructive interference of secondary reflections.

[0099] As described herein, the use of an asymmetric window to avoid constructive interference or to promote destructive interference of reflections can avoid undesired secondary signal peaks having amplitudes greater than the amplitude of the primary signal peak. The absence of these larger secondary signals can allow the AGC and threshold-based peak detection algorithms to operate precisely and accurately identify leading edge signals.

[0100] Configuration of the exemplary embodiment

[0101] As used herein, the terms "approximately," "about," "substantially," and similar terms are intended to have a broad meaning consistent with the common and accepted usage by those of ordinary skill in the art to which the subject matter of the present disclosure belongs. It should be understood by those skilled in the art who review the present disclosure that these terms are intended to allow description of specific features described and claimed without limiting the scope of these features to the precise numerical ranges provided. Therefore, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or changes to the subject matter described and claimed are considered to be within the scope of the present disclosure as recited in the appended claims.

[0102] It should be noted that the term "exemplary" and variations thereof used herein to describe various embodiments are intended to indicate that such embodiments are possible examples, representations, or illustrations of possible embodiments (and such terms are not intended to mean that such embodiments are necessarily unusual or best examples).

[0103] As used herein, the term "coupled" and its variants mean that two members are directly or indirectly connected to each other. Such a connection can be constant (e.g., permanent or fixed) or movable (e.g., removable or releasable). Such a connection can be achieved by directly connecting the two members to each other, by using a separate intervening member and any additional intermediate members that are connected to each other to connect the two members to each other, or by using an intervening member that is integrally formed with one of the two members to form a single monolithic body to connect the two members to each other. If "coupled" or its variants are modified by additional terms (i.e., directly connected), the general definition of "coupled" provided above is modified by the colloquial language meaning of the additional terms (i.e., "directly connected" means that the two members are connected without any separate intervening members), resulting in a narrower definition than the general definition of "coupled" provided above. Such a connection can be mechanical, electrical, or fluid.

[0104] As used herein, the term "or" is used in its inclusive sense (and not in its exclusive sense), so when used to connect a series of elements, the term "or" means one, some, or all of the elements in the series. Unless otherwise specifically stated, connective language such as the phrase "at least one of X, Y, and Z" is understood to convey that the element can be any one of X, Y, Z; X and Y; X and Z; Y and Z; or X, Y and Z (i.e., any combination of X, Y, and Z). Therefore, unless otherwise indicated, such connective language is generally not intended to imply that certain embodiments require at least one of X, at least one of Y, and at least one of Z to each be present.

[0105] References herein to the position of elements (i.e., "top," "bottom," "above," "below") are only used to describe the orientation of the various elements in the drawings. It should be noted that the orientation of the various elements may be different according to other exemplary embodiments, and the present disclosure is intended to cover such variations.

[0106] Although the drawings and description may illustrate a particular order of method steps, the order of these steps may be different from that depicted and described, unless otherwise noted above. In addition, two or more steps may be performed simultaneously or partially simultaneously, unless otherwise noted above. Such variations may depend, for example, on the software and hardware systems selected and on the designer's choice. All such variations are within the scope of the present disclosure.

[0107] It is important to note that the construction and arrangement of the devices shown in the various exemplary embodiments are illustrative only. Additionally, any element disclosed in one embodiment may be combined or used with any other embodiment disclosed herein. Although only one example of an element from one embodiment that may be combined or used in another embodiment is described above, it should be understood that other elements of the various embodiments may be combined or used with any other embodiment disclosed herein.

Claims

1. An ultrasonic flow meter, comprising: a pair of ultrasonic devices; a meter body configured to be coupled to a conduit for a medium; and a meter electronics device in communication with the pair of ultrasonic devices, the meter electronics device being configured to determine flow in response to ultrasonic signals, further characterized in that the pair of ultrasonic devices are relatively disposed in the meter body, wherein a first device of the pair of ultrasonic devices has a first acoustic window in contact with the medium, and a second device of the pair of ultrasonic devices has a second acoustic window also in contact with the medium, wherein the geometric shapes and / or material properties of the first acoustic window and the second acoustic window are asymmetric.

2. The ultrasonic flow meter according to claim 1, further characterized in that: Parameters of the first acoustic window and the second acoustic window are selected to suppress constructive interference of secondary reflections.

3. The ultrasonic flow meter according to claim 1, further characterized in that: The asymmetry is achieved by having the first and second acoustic windows with different thicknesses.

4. The ultrasonic flow meter according to claim 1, further characterized in that: The material properties of the respective first and second acoustic windows are made different to achieve the desired asymmetry.

5. The ultrasonic flow meter according to claim 1, further characterized in that: The thickness of the respective first and second acoustic windows are made different to achieve the desired asymmetry.

6. The ultrasonic flow meter according to claim 1, further characterized in that: Both material properties and thickness are varied to achieve the desired asymmetry.

7. The ultrasonic flow meter according to any one of claims 2 to 6, further characterized in that: The difference in transit time through the first window and the second window is given by (n+0.5)T, where n is an integer and T is the period of the signal provided through the medium.

8. The ultrasonic flow meter according to any one of claims 2 to 6, further characterized in that: The asymmetry of the window parameters is large enough so that the difference between the round trip transit times in the first window and the second window is greater than the duration of the primary signal.

9. The ultrasonic flow meter according to claim 3, further characterized in that: The first acoustic window and the second acoustic window are made of the same material and the difference in thickness of the windows is given by 0.5(n+0.5)λ, where n is an integer and λ is the wavelength of a signal provided through the medium.

10. A method for use in an ultrasonic system for measuring flow in a medium, the ultrasonic system comprising a transmitter and a receiver, the method being characterized by: providing an ultrasonic signal through a first acoustic window of the transmitter, the first acoustic window being in contact with the medium; and The ultrasonic signal is received through a second acoustic window of the receiver, the second acoustic window being in contact with the medium, wherein: The geometry and / or material properties of the first acoustic window and the second acoustic window are asymmetric.

11. The method according to claim 10, further characterized in that: Parameters of the first acoustic window and the second acoustic window are selected to suppress constructive interference of secondary reflections.

12. The method according to claim 10, further characterized in that: The asymmetry is achieved by having the first and second acoustic windows with different thicknesses.

13. The method according to claim 10, further characterized in that: The material properties of the respective acoustic windows are made different to achieve the desired asymmetry.

14. The method according to claim 10, further characterized in that: The thickness of the first and second acoustic windows are made different to achieve the desired asymmetry.

15. The method according to any one of claims 10 to 14, further characterized in that: Both material properties and thickness are varied to achieve the desired asymmetry.

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