Patent Description:
Ultrasonic meters (USMs) are also known as Ultrasonic flow meters (UFMs) are becoming popular for fluid flow metering because of their capability to measure a wide range of different flow rates, cause minimal pressure drops, and have no-moving parts thus providing less mechanical maintenance and better reliability. A key hardware component in the USM is a piezoelectric element that comprises a piezoelectric crystal or a piezoelectric ceramic. As known in physics the Piezoelectric Effect is the ability of certain materials to generate an electric charge responsive to an applied mechanical stress.

For the application of flow metering two or more piezoelectric element-based transducers transmit and receive ultrasound signals through either a completely or partially encapsulated enclosure to and from fluids flowing in the pipe. There are the challenges of obtaining high ultrasonic signal quality and reliability which remain in industries such oil and gas. High pressure (gauge pressure) flow can reach <NUM> bar or more for industrial applications, while low and medium pressure is usually between <NUM> and <NUM> bar for commercial applications, such as being <NUM> to <NUM> bar.

Commercial and/or city gas distribution networks often face the issues of performance degradation when using conventional USMs at low and medium pressure, since the lower pressure is, the lower the density of the gas, and the higher the attenuation of ultrasonic signals. As a result, when USMs are operated at relatively low pressure there is a lower signal to noise ratio (SNR) and reduced detectability, reliability and/or stability of the USM. Currently turbine meters are the next best alternative (NBA) to USM's, but mechanical movements and a narrower turn-down ratio or measuring range make turbine meters difficult to serve commercial gas distribution markets predominantly because there may be a large difference between the high peak and the low valley of gas usage for businesses such as restaurants and hotels in cities where the velocity of gas (VoG) generally changes between <NUM>/s and <NUM>/s. Therefore, there is a high demand for USMs for city or commercial gas distribution networks. <CIT> discloses an electroacoustic transducer comprising a housing structure having an opening and a piezoelectric disc located within said opening.

This Summary is provided to introduce a brief selection of disclosed concepts in a simplified form that are further described below in the Detailed Description including the drawings provided. This Summary is not intended to limit the claimed subject matter's scope.

Disclosed aspects recognize that in general, USM signal quality can be degraded by a plurality of different factors. These USM signal degrading factors include operational modes of the piezoelectric element, the surrounding housing (or enclosure) creating large amplitude and long-lasting ringing effects that can undermine useful signal detectability and system sensitivity. Ringing effects that last a long time increase the minimum measuring range (blind zone), which is recognized to be disadvantageous to short range measurements, especially in small size USMs, while matching and mismatching handling mechanisms cause signal loss and multiple reflections. Improper filling and backing materials can cause back coupling that increases the noise level, which reduces the SNR. Low or medium pressure of the fluid can cause severe attenuation effects of the signal amplitude, dust and dirt contamination on the piezoelectric transducer can cause a reduction in the SNR due to an increase in the noise level, and strong unwanted signals can saturate the receiver's low-noise amplifier (LNA) used to amplify the generally weak received ultrasonic signals.

Disclosed USMs have features that can generally address all these USM signal quality degrading factors described above in one design with an emphasis on the being configured for the relatively more difficult application to low and medium pressure fluid flow sensing which as noted above is generally between <NUM> and <NUM> bar, since high-pressure applications can or less difficult because of the enhancement in the receive signal quality due to a lower attenuation/damping effect. As a result, conventional high-pressure transducer designs in general will not fit for such low and medium pressure applications in terms of signal quality, especially for compressible fluids such as natural gas that can cause considerable attenuation/damping on signals travelling through it, so that the transducer paths will not work properly, resulting in low signal quality or the failure of the USM's flow measurements.

Disclosed aspects include an ultrasonic flow meter that includes a housing including an upper housing portion and a lower housing portion for attaching to a fluid pipe. A piezoelectric element coupled to a transmitter and receiver is configured to emit ultrasonic waves in an axial direction perpendicular to a horizontal plane defined by the piezoelectric element. A lens combination is in a lower housing portion that includes a refocusing lens positioned radially outside the first piezoelectric element that is ring-shaped which is configured for redirecting received radial ultrasonic waves to travel in the axial direction, and has a thickness profile configured to act as a matching layer for reducing multiple reflections within the lower housing portion. A second lens that is flat disc-shaped is below the refocusing lens that includes an outer portion that is radially outside the first piezoelectric element which is configured for redirecting the radial ultrasonic waves to travel in the axial direction The ultrasonic flow meter according to the invention is defined in claim <NUM>. The method of ultrasonic fluid flow sensing according to the invention is defined in claim <NUM>.

Disclosed embodiments are described with reference to the attached figures, wherein like reference numerals, are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate aspects disclosed herein. Several disclosed aspects are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the embodiments disclosed herein.

One having ordinary skill in the relevant art, however, will readily recognize that the disclosed embodiments can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring aspects disclosed herein. Disclosed embodiments are not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with this Disclosure.

Also, the terms "coupled to" or "couples with" (and the like) as used herein without further qualification are intended to describe either an indirect or direct electrical connection. Thus, if a first device "couples" to a second device, that connection can be through a direct electrical connection where there are only parasitics in the pathway, or through an indirect electrical connection via intervening items including other devices and connections. For indirect coupling, the intervening item generally does not modify the information of a signal but may adjust its current level, voltage level, and/or power level.

<FIG> is block diagram depiction of an USM <NUM> that includes at least the disclosed lens combination comprising the refocusing lens <NUM> and the second lens <NUM>, and generally also includes a plurality of the other above-described features for overcoming USM signal quality degrading factors, which is installed within a meter body for ultrasonic-based flow measurements of a fluid flowing in a section of a pipe <NUM>. The USM <NUM> is shown including an outer housing <NUM> also known as an enclosure providing a meter body with a piezoelectric element pair shown therein comprising a first piezoelectric element shown as T<NUM> and a second piezoelectric element Tz on the same side of the pipe <NUM> that are spaced apart from one another by a distance shown as d, that implements a reflective path (indirect path) sensing configuration. Disclosed USM's may also have only a single piezoelectric element, such as shown in <FIG> described below. The housing <NUM> can comprise brass, titanium, or an aluminum alloy, such as aluminum <NUM> which includes magnesium and silicon as its major alloying elements.

Although the USM <NUM> is shown implementing a reflective path sensing configuration, a direct path configuration shown in <FIG> can also be implemented where there is a first USM 100a having a first piezoelectric element and a second USM 100b having a second piezoelectric element, where the respective USMs are instead located on the opposite sides of the pipe <NUM>, with a face of the respective piezoelectric elements <NUM> pointing toward one another. As known in the art, the piezoelectric elements <NUM> generally have a disc-shape, that being flat, thin circular objects, that can be considered because of their thin nature providing a horizontal plane which in the USM is in the radial direction, that is perpendicular to the actual direction in which the ultrasonic wave is emitted by the USM. The reflective (indirect) path configuration in <FIG> is shown implemented with a first USM 100c having a first piezoelectric element <NUM> and a second USM 100d having a second piezoelectric element <NUM>, where the respective USMs are now located on the same side of the pipe <NUM>.

The USM <NUM> is attached to the pipe <NUM>, generally either being inserted into the pipe <NUM> using a gas tight and high-pressure resistant mechanism, or configured as a clamp-on device installed on the outside of the pipe <NUM>. Although not shown in <FIG>, the piezoelectric elements T<NUM> and T<NUM> may each have their own separate encapsulation chamber (see the inner encapsulated (isolated) chamber <NUM> shown in <FIG> described below that encapsulates the piezoelectric element, and is shown also encapsulating the refocusing lens <NUM>.

The piezoelectric elements T<NUM> and T<NUM> employ piezoelectric crystals or piezoelectric ceramics that are set into vibration when a pulsed voltage signal (receipt from a transmitter) is applied to their piezoelectric element, thereby generating ultrasonic waves. In operation, ultrasonic pulses are alternately transmitted by one of the piezoelectric elements of the pair and are received by the other piezoelectric element of the pair needed for the flow measurement.

An angled axial signal path is shown in <FIG> by the dashed line inside the pipe <NUM>. As known in the art, the USM can include more than the <NUM> piezoelectric elements T<NUM> and T<NUM> shown in <FIG>, typically from two up to <NUM> piezoelectric elements. USM <NUM> includes a transmitter (Tx) <NUM> and a receiver (Rx) <NUM>, or as an alternative to the separate Tx and Rx as shown there can be a single transceiver, coupled to T<NUM> and T<NUM> respectively by a digitally controlled multiplexer (MUX) <NUM> that enables the piezoelectric elements T<NUM> and T<NUM> to swap (alternate) transmit and receive roles so that in one moment, Tx <NUM> is on and the Rx <NUM> is on another moment. The pipe <NUM> in operation has a fluid therein, being a liquid or a gas, such as natural gas. The fluid can be at a low pressure or a medium pressure that is usually between <NUM> and <NUM> bar, such as <NUM> to <NUM> bar.

The USM <NUM> generally also includes a controller <NUM>, generally comprising a microprocessor, digital signal processor (DSP), or a microcontroller unit (MCU), that has an associated memory shown as 'MEM' <NUM> that can store code for algorithms including algorithms for implementing disclosed fluid velocity measurement methods. The controller <NUM> is coupled to the Tx <NUM> and the Rx <NUM>, and is also coupled to control the MUX <NUM>. The controller <NUM> also is configured to switch the Tx and Rx roles in designated timing intervals.

However, as known in the art, algorithms run by the controller <NUM> may be implemented by hardware and/or be implemented by software. Regarding hardware-based implementations, algorithm equations can be converted into a digital logic gate pattern, such as using VHDL (a Hardware Description Language) that can then be realized using a programmable device such as a field-programmable gate array (FPGA) or complex programmable logic device (CPLD), or a dedicated application-specific integrated circuit (ASIC) to implement the logic gate pattern. Regarding software-based implementations, code for the algorithm is generally stored in a memory such as memory <NUM> that can be implemented by the controller <NUM>.

There is also a human-machine interface (HMI) <NUM> shown in <FIG> coupled to the controller <NUM> that may include a keyboard and a display if deemed needed. An operator can use the HMI <NUM> to adjust operating parameters of the USM.

The USM <NUM> also includes a disclosed lens arrangement that can be a better seen in <FIG> described below, where the piezoelectric element is now shown as <NUM>. <FIG> is a cut-away view that shows an example arrangement in the lower housing portion 108b showing a disclosed refocusing lens <NUM> and a disclosed second lens <NUM> that generally comprises a bottom section of the lower housing portion 108b, and how they are positioned relative to one another and relative to the piezoelectric element <NUM>. An axial direction and a radial direction are both shown. The radial direction is the same direction as the horizontal plane defined by the piezoelectric element <NUM>, where the piezoelectric element <NUM> as noted above is generally disc-shaped.

<FIG> also shows back filling and damping materials <NUM> on top of the damping and canceling layer <NUM>, a matching and temperature insulation layer <NUM> below and on the sides of the piezoelectric element <NUM>, and side damping <NUM> including between the lower housing portion and the matching and temperature insulation layer <NUM> and damping and canceling layer <NUM>.

The lens arrangement comprises a refocusing lens <NUM> and a second lens <NUM>, that are both positioned in the lower housing portion 108b, where the refocusing lens <NUM> is radially outside the first piezoelectric element <NUM>, and an outer portion of the second lens <NUM> is radially outside the first piezoelectric element <NUM>. The second lens <NUM> is generally disc-shaped, generally being a solid disk, and is positioned below the refocusing lens <NUM>, where the ultrasonic signal is transmitted (e.g., emitted and optionally also received) through the inner portion the second lens <NUM>.

As known in the art, during USM <NUM> operation when the piezoelectric element <NUM> vibrates, both p-wave and s-wave are produced with an inflection slope including in the radial direction. The refocusing lens <NUM> is configured for redirecting the radial ultrasonic waves from the first piezoelectric element <NUM> to provide a diffraction or inflection to redirect these radial ultrasonic waves in the axial direction for reducing a signal loss.

As known in acoustics, the thickness of a matching layer is usually equal to one quarter of the sound wavelength (λ/<NUM>), to minimize the reflection at the front side of the matching layer, in favor of transmitted waves. Having found an optimum impedance value, the next determination is for finding a material having that particular impedance. One can use the known Mason Model which is a theoretical matching layer modeling algorithm. It is also possible to find the most suitable thickness by using a computer numerical simulation or comparative tests.

The second lens <NUM> is also for redirecting the radial ultrasonic waves it receives to travel more directionally in the axial direction. The nominal center diameter of the second lens <NUM> is generally λ/<NUM>, that can generally be ± <NUM>%, and the nominal thickness for the second lens <NUM> is as also with the refocusing lens <NUM> generally determined by acoustic impedance matching considerations to provide a matching layer. The second lens <NUM> may have a nominal thickness of λ/<NUM> that can generally be ± <NUM>% to provide an impedance matching layer which was determined to be the best acoustic matching thickness according to the results of a comparative test performed by the Inventors for a particular set of operating conditions.

With the diameter of the second lens <NUM> being disc surface that utilizes the bottommost part of the housing specified to generally be about λ/<NUM>, as well as its thickness to be about λ/<NUM>, with these two defined geometrical parameters, the bottommost part of the housing acts as a lens to redirect acoustical waves that are received in the USM, such as where the same piezoelectric elements are alternating as transmitter or receiver in a pair. The bottommost flat surface of the housing thus provides an additional new function as an acoustic lens.

As known in the art, during operation when the piezoelectric element <NUM> is controlled to vibrate, both p-wave (also known as compressional waves) and s-wave (also known as secondary waves) are produced with an inflection slope including in the radial direction, where resonances can occur when the radial spacing (the outer diameter of the piezoelectric element <NUM> to the inner wall of the lower housing portion 108b) is too tight, so that standing waves can be created.

The refocusing lens <NUM> is generally ring-shaped with a right-angled triangle (cross-section (see <FIG> described below) and is configured to redirect received ultrasonic waves away in the axial direction which as described above is perpendicular to horizontal plane of the piezoelectric element <NUM>, so that standing acoustic waves are not created. Specifically, an inflection slope and diffraction/inflection of the received radial ultrasonic waves provided by the refocusing lens <NUM> is for overcoming acoustic impedance mismatching with the housing which generally comprises a metal or metal alloy to a gas such as air, natural gas, or nitrogen, which has a large difference in impedance as compared to the temperature insulation layer <NUM> which is shown positioned between the piezoelectric element <NUM> and the refocusing lens <NUM> and the second lens <NUM>.

Regarding flow velocity measurement, acoustic pulses from the piezoelectric elements T<NUM> and T<NUM> are crossing the pipe <NUM> like a ferryman crossing a river. Without fluid flowing in the pipe <NUM>, acoustic pulses propagate with the same speed in both directions. If the fluid in the pipe <NUM> has a flow velocity different from zero, acoustic pulses travelling downstream (from T<NUM> to T<NUM>) with the fluid flow will move faster, while those travelling upstream (from T<NUM> to T<NUM>) against the fluid flow will move slower. Thus, the downstream travel times "tAB" will be shorter, while the upstream travel times ones "tBA" will be longer as compared when the fluid is not moving. Time of flight (TOF) which herein refers to directly measuring the travel time of the signal, or indirect measurement methods such as Tx signal-based system cross-correlation or post-processing based cross-correlation can also be used to determine the travel time. The equations below illustrate the computation principle, representing the travel time, and velocity of the fluid and velocity of sound in the fluid as a function of the path length and angle of the path relative to the pipe <NUM>. <MAT> <MAT> <MAT> <MAT>.

<FIG> shows a cut-away view of an example USM <NUM> having a disclosed refocusing lens <NUM> and a second lens <NUM>, according to an example embodiment. The operating wavelength for the USM is generally in a range from <NUM> to <NUM>. The outer housing <NUM> is as in <FIG> shown in <FIG> having an upper housing portion 108a attached to a lower housing portion 108b. Chamber connecting threads <NUM> are shown in <FIG> coupling the upper housing portion 108a to the lower housing portion 108b. The housing <NUM> being configured as separate upper housing portion 108a and lower housing portion 108b reduces the amount of bubbles introduced when filling materials generally added in liquid form such as the damping layer <NUM>, damping and canceling layer <NUM>, and backfilling and damping material <NUM> are added to the inside of the housing <NUM> using a conventional long tube, difficulty in filling the inside of the housing <NUM> in production, and further to be able to adjust the joint in the air space (air interface) between the upper housing portion 108a and lower housing portion 108b to mitigate undesirable back coupling of signals from the front to the back of the USM <NUM> and then to the front-again.

The housing <NUM> can comprise a metal material or a non-metallic material. The upper housing portion 108a has an attachment feature shown as mounting connecting threads <NUM> for attaching to a pipe having a fluid herein. Regarding terminology, the top of the USM <NUM> is the side of the USM with the threads <NUM> as shown in <FIG> with the O-ring <NUM> under the threads <NUM> that is on an opposite side relative to the piezoelectric element <NUM>, while the bottom of the USM <NUM> is the end with the piezoelectric element <NUM> that is the front-acoustic active end for the wanted acoustics transmitting and receiving direction, where the bottom of the USM <NUM> (front-end) is designed to be positioned down inside the pipe of the meter body as shown in <FIG> and <FIG>. The piezoelectric element <NUM> is shown in the lower housing portion 108b. However, the piezoelectric element <NUM> can also be in border region between the upper housing portion 108a and the lower housing portion 108b. The piezoelectric element <NUM> is coupled to a transceiver (Tx <NUM>/Rx <NUM>) as shown in <FIG> that may be within an electronics housing (not shown) together with a controller <NUM>.

Above the piezoelectric element <NUM> is a damping layer <NUM> that generally comprises a material with low density such as a porous foam or a polymer with a damping and cancelling layer <NUM> that also generally comprises a material with a low density such as porous foam or a polymer. The layer thickness of the damping layer <NUM> is generally about λ/<NUM> in an axial direction backward direction towards upper housing portion 108a. The piezoelectric element <NUM> is surrounded by a matching and temperature insulation layer <NUM> that generally comprises a composite material having a low thermal conductivity, such as an epoxy glass, or a thermoplastic compound foam. There is also a side damping layer <NUM> outside the damping and the cancelling layer <NUM> and over the matching and temperature insulation layer <NUM> portion over the piezoelectric element <NUM> and the damping layer <NUM>.

The refocusing lens <NUM> is configured according to piezoelectric element's <NUM> vibrational mode as well as the arrangement of the piezoelectric element <NUM> in the housing (the lower housing portion 108b). The refocusing lens <NUM> is shown in the triangular space between the matching and temperature insulation layer <NUM> and the lower housing portion 108b, and can comprise an adhesive epoxy resin mixed with hardener for filling the gap between metal lower housing 108b and matching and temperature insulation layer <NUM>. The filling material using an adhesive compound not only bonds two different material without voids, but also seals the gap with a designated shape so that the refocusing lens <NUM> can be formed in the gap.

The refocusing lens <NUM> which as noted above can be shaped as a right-angled triangle, can provide an angle of the hypotenuse which is generally greater than or equal to <NUM> degrees, defined by the gap present in the structure, and the center width (thickness) is about <NUM>/<NUM> wavelength, which is a relatively small thickness, typically less than <NUM>. As a result, most of the ultrasonic signals leaked towards the side of the USM <NUM> between refocusing lens <NUM> and λ/<NUM> wall of the lower housing portion 108b will be reflected by the refocusing lens <NUM>, and a λ/<NUM> thick housing wall to not interfere with the signal going forward at the bottom of the USM <NUM> that is its acoustically active front-end.

Also shown in <FIG> is an inner encapsulating chamber <NUM> that encapsulates the piezoelectric element <NUM>, that is also shown encapsulating the refocusing lens <NUM>. The second lens <NUM> generally a portion (the bottom part) of the housing is outside of the encapsulating chamber <NUM>.

The refocusing lens <NUM> orientation is generally ring-shaped and positioned radially outside the piezoelectric element <NUM>, but lower in the lower housing portion 108b relative to the piezoelectric element <NUM>, while the second lens <NUM> is generally below the refocusing lens <NUM>. The second lens <NUM> as noted above is generally an area of the lower housing portion 108b and is not a separate component. This housing area for providing second lens <NUM> can be disc-shaped with a nominal center diameter of λ/<NUM> and a thickness of about λ/<NUM>. The refocusing lenses <NUM> and the second lens <NUM> are thus configured so that reflected and refracted ultrasonic waves from its own acoustics are directed and/or redirected in the axial direction toward the fluid medium in the pipe, where the so-called mismatching of acoustic impedance of different media is known to cause undesirable reflection back to piezoelectric element <NUM>.

The angle of <NUM> degrees (set by the housing wall above the second lens <NUM> as shown in <FIG>) thus does not interfere with the signal transmitted by the piezoelectric element <NUM> from the lower housing portion 108b being the front-acoustic active end of the USM <NUM> by redirecting lateral waves to the front-acoustic active end to avoid multiple reflections within the relatively small space and for canceling standing resonance waves. The refocusing lens <NUM> is configured for cancelling leakage of ultrasound waves reaching to the wall of the housing was generally comprises a metal where the impedance discontinuity is more severe at the outer surface of the lower housing. This mismatching can further create multiple reflections (a standing wave) between the region of the piezoelectric element <NUM> and the refocusing lens <NUM>. Accordingly, the λ/<NUM> housing wall thickness is meant to suppress standing waves in that space to enhance the axial ultrasonic waves from the piezoelectric element <NUM>.

As noted above the second lens <NUM> can be λ/<NUM> ± <NUM>% in center diameter of its disk shape, where as noted above λ is the wavelength of the ultrasound signal used in the sensing application, such as <NUM> to <NUM>. The material for second lens <NUM> generally being a portion of the lower housing portion can comprise a metal or a non-metal, but will generally be a metal such as stainless steel or titanium that is recognized to be a housing material that is well adapted for harsh environment conditions. The outer area of the second lens <NUM> shown in <FIG> reduces the radial ultrasonic wave propagation (i.e., cancels them with a phase reversal) perpendicular to the axial direction and suppresses sidelobes to make the ultrasonic wave travel more directionally, in the axial direction.

The refocusing lens <NUM> and second lens <NUM> are generally partial concentric with one another with the second lens <NUM> extending radially outside the refocusing lens, and is at least partially below the refocusing lens <NUM> generally shown completely below the refocusing lens <NUM>. As noted above, the piezoelectric element <NUM> is typically disc-shaped, and the refocusing lens <NUM> is a generally a right triangle shaped ring radially outside of the piezoelectric element <NUM>, and as described above second lens <NUM> is generally a section of the lower housing portion 108b that is typically disc-shaped and is positioned below the piezoelectric element <NUM> relative to the piezoelectric element's <NUM> front face 128a.

Lens materials for the refocusing lens <NUM> and the second lens <NUM> are generally selected that are applicable to low and medium pressure conditions which as noted above is up to <NUM> bar, which should not be too soft to enable withstanding pressure conditions. For example, titanium, SS304, <NUM>, aluminum alloys, or non-metallic material such as plastics may be used, depending on the requirements of fluid measurement conditions, e.g., corrosive or dust. That is the reason the ¼ λ matching layer shown as matching and temperature insulation layer <NUM>, and the refocusing lens <NUM> are configured to redirect the sidelobe ultrasonic waves to the front of the USM <NUM> and suppress multiple reflections that can take place within the generally metallic rigid housing 108a, 108b.

The refocusing lens <NUM> and the second lens <NUM> are thus for redirecting received radial ultrasonic waves and to prevent multiple reflections within the walls of the lower housing portion 108b. The refocusing lens <NUM> and second lens <NUM> will redirect the received radial ultrasonic waves in the axial direction towards its counterpart piezoelectric element in the piezoelectric element pair. <FIG> shows a cut-away view of an example USM <NUM> with an outer housing having a refocusing lens <NUM> and a second lens <NUM>. The region shown as <NUM> is a fastener, such as comprising a heady-duty rubber ring.

<FIG> shows a cut-away view of an example USM <NUM> having a refocusing lens <NUM> and a second lens <NUM>, with an outer housing variant, according to an example embodiment. The variation in the outer housing shown eliminates the external step such as shown in <FIG> and <FIG>, now in <FIG> being a straight cylinder, and the internal rear structure comprising back filling and damping materials <NUM> and <NUM> can comprise epoxy resin, or room temperature vulcanizing (RTV) rubber. The back filling and damping material <NUM> between the back filling and damping material <NUM> and damping and cancelling layer <NUM> is used to better fill the back filling and damping material <NUM> and reduce the back-signal coupling. The back filling and damping materials region <NUM> may be pre-formed, such as comprising rubber.

In summary, the main features for disclosed USMs for addressing USM signal quality degrading factors are:.

An example assembly sequence to fabricate a disclosed USM is now provided. In a first step the piezoelectric element <NUM> can be bonded to a temperature insulation layer <NUM> which lines a lower housing portion 108b, thus being above the second lens <NUM>, which is one of the core assembly steps, generally requiring special attention to operating conditions such as the temperature, preload, dust, and static. In a second step, the damping layer <NUM> while in is liquid form can be poured on top of the piezoelectric element <NUM>. In a third step, the above-described components can be assembled with the side damping layer <NUM>, and the damping and cancelling layer <NUM> while liquid form can be then poured in.

In a fourth step, the above components can be assembled within the lower housing portion 108b, which is one of the core assembly steps, and requires special attention to the bottom and side coupling, such as paying attention to the joint completely fitting, leaving no air gaps, such as using the matching and temperature insulation layer <NUM> to fill the gap. A fifth step can comprise assembling the upper housing 108a to the lower housing portion 108b, shown above as chamber connecting threads <NUM>. In a sixth step, the back filling and damping material <NUM> while liquid form can be poured into the housing, paying attention to the cable in the middle position and the bonding well. In the sixth step, the liquid material will be converted to solid, as described above, while avoiding bubbles.

While various disclosed embodiments have been described above, it should be understood that they are presented by way of example only, and not as a limitation. Thus, the breadth and scope of this Disclosure should not be limited by any of the above-described embodiments. Rather, the scope of this Disclosure is defined in accordance with the following claims.

Claim 1:
An ultrasonic flow meter, comprising:
an outer housing having an upper housing portion attached to a lower housing portion, with the upper housing portion having an attachment feature for attaching to a pipe that is adapted for having a fluid flowing therein;
at least a first piezoelectric element having a planar surface that defines a horizontal plane coupled to a transmitter and to a receiver, wherein the first piezoelectric element is configured to emit ultrasonic waves at an operating wavelength (λ) in an axial direction which is perpendicular to the horizontal plane;
a lens combination in the lower housing portion, including:
a refocusing lens positioned radially outside the first piezoelectric element that is ring-shaped configured for redirecting received radial ones of the ultrasonic waves to travel in the axial direction for reducing a signal loss, and
a second lens that is flat disc-shaped positioned below the refocusing lens that includes an outer portion which is radially outside the first piezoelectric element configured for redirecting the radial ones of the ultrasonic waves to travel in the axial direction, the second lens has a thickness configured to act as a matching layer for reducing multiple reflections within the lower housing portion.