Patent Description:
This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.

Ultrasound transducers are used to transmit and receive ultrasonic signals in a flowmeter system. Several problems were solved by using the present invention, they are: The ability to remove or repair transducers from existing ultrasonic flow meters required either depressurizing the pipe gas line or the use of special tools for transducer replacement; transducers not in direct contact with the gas contained in the pipe usually have poor transducer performance resulting in poor signal detection for gas meters. Current ultrasonic gas meters use gas "wetted" transducers, that is, the transducer itself is in direct contact with the gas and cannot be removed without special tools or the elimination of the pressure in the pipe. Traditional transducer housings that put the transducer outside of the pipe pressures, such as those are used in liquid meters, have not been used since the acoustic losses through the housings combined with the noise produced by the mounting of the transducer housings make the acoustic signals unusable. The traditional transducer housings do not work in a gas environment for two reasons. First, conventional transducer housings had thick metal windows that poorly match the acoustic impedance of gas and therefore would not transmit sound into the gas. Second, the traditional transducer housings are rigidly attached to the meter body. These rigid attachments provide pathways for sound to be transmitted through the meter body and not through the gas resulting in poor signal to noise ratios.

The current ultrasonic gas meters have their wetted transducers exposed to the gas which can contain hydrogen sulfide or other contaminants. Hydrogen sulfide can over time deteriorate transducers made of conventional solder joints and epoxies for electrical and mechanical connections within the transducer. In current ultrasonic gas meters, a metal seal is placed behind the transducer in order to maintain the pipe pressure and to prevent gas from leaking out. So if a transducer fails, the transducer has to be replaced using special tools that prevent the transducer from bursting out of the flowmeter at high speeds due to pipe gas pressure. This may be deadly to the replacer if the tools are improperly handled because the transducer becomes a projectile under pipe gas pressure. In addition, the escaping gas is usually highly flammable, presenting an extreme hazard to the replacer and others nearby. If the gas pipeline/process is depressurized to replace transducers for safety reasons the pipeline flow ceases causing lost revenue.

Current gas meter transducers use either a monolithic PZT ceramic transducer or a Tonpilz transducer. These transducers suffer from poor bandwidth, poor signal to noise ratio and radial modes in the <NUM> to <NUM> frequency range, the preferred ultrasonic operating frequency range for gas meters. As a result, the received signals can be very distorted. This results in poor transit time measurements and poor accuracy of a gas flowmeter Current gas meters also have metal to metal contact between parts even with wetted transducers when cases enclosing the transducers are made of metal so the flow meter can suffer from poor signal to noise ratio due to acoustic noise of the system.

<CIT> discusses transducers mounted in a housing or vessel to propagate signals along a fluid measurement path, and a plurality of massive elements placed between transmitting and receiving transducers in the acoustic propagation path through the solid body of the housing or vessel to remove crosstalk. <CIT>discusses a device for measurement of entrained and dissolved gas which has a first module arranged in relation to a process line for providing a first signal containing information about a sensed entrained air/gas in a fluid or process mixture flowing in the process line at a process line pressure.

The present invention pertains to a flowmeter according to claim <NUM> and a method according to claim <NUM> for measurement of gas where accuracy and reliability are essential. The flowmeter measures gas flow rates in a pipe having a channel disposed in the pipe through which gas in the pipe flows and plane waves generated by multiple upstream ultrasonic transducers and multiple downstream ultrasonic transducers propagate. The arrangement of the transducers defines two crossing planes but may have more or fewer. An important feature of this meter is that the transducer elements can be safely checked or replaced without special tools and without depressurizing the line because of housings which hold the transducer and contain the pipe pressure.

In the accompanying drawings, the preferred embodiment of the invention and preferred methods of practicing the invention are illustrated in which:.

Referring now to the drawings wherein like reference numerals refer to similar or identical parts throughout the several views, and more specifically to <FIG> thereof, there is shown a flowmeter <NUM> for detecting gas flow rates in a pipe <NUM>. The flowmeter <NUM> comprises a container <NUM> configured to be attached to the pipe <NUM> having a channel <NUM> through which the gas flows, and a plurality of recesses <NUM> that extend through the container <NUM> and a plurality of housings <NUM>. Each recess <NUM> has a housing <NUM> which contains pressure in the channel <NUM>. Each housing <NUM> has a window <NUM> that is in acoustic communication with the channel <NUM>. The flowmeter <NUM> comprises a plurality of transducers <NUM>, with one transducer <NUM> of the plurality of transducers <NUM> disposed in each recess <NUM>. The transducers <NUM> transmit ultrasonic signals into and receive ultrasonic signals from the channel <NUM> through the window <NUM> in the housing <NUM> in which a transducer <NUM> is disposed. The flowmeter <NUM> comprises a controller <NUM> in electrical communication with the plurality of transducers <NUM> which determines the gas flow rate through the channel <NUM> by measuring transit times of signals transmitted by and received by the transducers <NUM>.

The window <NUM> may have a thickness less than <NUM>/<NUM> wavelength of ultrasound in the window's material. The window <NUM> thickness may be about <NUM>/<NUM> wavelength of ultrasound in the window's material. One transducer <NUM> of the plurality of transducers <NUM> may be disposed in each housing <NUM> disposed in each recess <NUM>. The housing <NUM> is a pressure boundary which contains the pressure in the channel <NUM> and essentially prevents gas in the channel <NUM> from escaping into the housing <NUM>. The housing <NUM> forms a gas tight seal with the channel <NUM>.

The transducers <NUM> are arranged to be removed from the recesses <NUM> without having to depressurize the pipe <NUM> or having to use an extraction tool that removes the transducers <NUM> through a pressure containing component that would contain the pressure in the channel <NUM>. The ultrasonic signals transmitted and received by the transducers <NUM> may define a first path in a first plane and a second path in a second plane which paths cross in the channel <NUM>.

The flowmeter <NUM> may include acoustic isolators <NUM> which acoustically isolate the transducer housings <NUM> from the container <NUM>. The transducer <NUM> may couple to the window <NUM>. The window <NUM> is made of metal or plastic. The window <NUM> may be made of titanium, PEEK or PPS.

Each transducer <NUM> may be a broad band piezoelectric composite transducer <NUM> with a coupling coefficient greater than. <NUM> out of <NUM> and an acoustic impedance of less than <NUM> Mrayls.

The present invention pertains to a housing <NUM>, as shown in <FIG>, for an ultrasonic transducer <NUM> for a flowmeter <NUM> which is inserted into a recess <NUM> of a container <NUM> through which gas flows. The housing <NUM> comprises a shell <NUM> in which the transducer <NUM> is disposed. The shell <NUM> has an outer surface and a flange <NUM> that extends from the outer surface of the shell <NUM> and a plurality of acoustic ribs <NUM> disposed in proximity to an end of the shell <NUM> from which signals are emitted by the transducer <NUM>. The shell improves the signal-to-noise ratio to greater than <NUM>:<NUM>, the ribs attenuate all non-gas paths of sound by at least <NUM>%.

The housing <NUM> may include an acoustic isolator <NUM> disposed about the housing <NUM>, which isolator contacts the container <NUM> and the housing <NUM> when the transducer <NUM> is disposed in the recess <NUM> of the container <NUM>, with the acoustic isolator <NUM> disposed between the container <NUM> and the housing <NUM> so the housing <NUM> does not contact the container <NUM>. The acoustic isolator <NUM> may include discs disposed about the flange <NUM> of the housing <NUM>. The discs may be made of plastic, syntactic foam or rubber. Impedance matching material may not be used on the exterior surface to improve transmission of signals from the transducer <NUM> into the gas, but uses a window <NUM> thickness less than ¼ wave length, which makes the window <NUM> acoustically transparent. There may be no metal to metal contact between the housing <NUM> and the container <NUM>.

The present invention pertains to a method for detecting gas flow rates in a pipe <NUM>. The method comprises the steps of transmitting ultrasonic signals from a plurality of transducers <NUM> disposed in recesses <NUM> in a container <NUM> attached to the pipe <NUM> through a window <NUM> made of metal of each recess <NUM> into a channel <NUM> of the container <NUM> in which the gas flows, with one transducer <NUM> of the plurality of transducers <NUM> disposed in each recess <NUM> and the window <NUM> in acoustic communication with the channel <NUM>. There is the step of receiving ultrasonic signals from the channel <NUM> through the windows <NUM> by the transducers <NUM> in the recesses <NUM>. There is the step of determining the gas flow rate through the channel <NUM> by measuring transit times of the signals transmitted by and received by the transducers <NUM> with a controller <NUM> in electrical communication with the plurality of transducers <NUM>.

The window <NUM> is a pressure boundary which contains the pressure in the channel <NUM> and prevents gas in the channel <NUM> from escaping into the housing <NUM>, the window <NUM> forming a gas tight seal with the housing <NUM>, so that a transducer <NUM> can be replaced without depressurizing the channel <NUM> or without using an extraction tool that removes a transducer <NUM> through a pressure containing component that would contain the pressure in the channel <NUM>.

The transmitting step may include the step of transmitting ultrasonic signals by the transducers <NUM> along a first path in a first plane and a second path in a second plane which cross in the channel <NUM> and the receiving step may include the step of receiving ultrasonic signals by the transducers <NUM> from the first path and from the second path. There may be the step of acoustically isolating housings <NUM> in which the transducers <NUM> are disposed in the recesses <NUM> from the container <NUM> with acoustic isolators <NUM>. There may be the step of applying a force against the transducer <NUM> to hold the transducer <NUM> in contact with the window <NUM>.

The transmitting step may include the steps of generating with an upstream ultrasonic transducer <NUM> of the first path plane waves that propagate through the channel <NUM> and are received by a downstream ultrasonic transducer <NUM> of the first path; producing a downstream transducer <NUM> signal with the downstream transducer <NUM> from the plane waves the downstream transducer <NUM> receives; generating with the downstream ultrasonic transducer of the first path plane waves that propagate through the channel <NUM> and are received by the upstream ultrasonic transducer of the first path; producing an upstream transducer <NUM> signal with the upstream transducer <NUM> from the plane waves the upstream transducer <NUM> receives; and determining with the controller <NUM> the gas flow rate from transit times of the signals generated and received by the upstream transducer <NUM> and downstream transducer <NUM>.

The replacing step includes the step of replacing the transducer <NUM> without using an extraction tool that removes the transducer <NUM> through a pressure containing component that provides a gas tight, pressure-bearing enclosure, within which the transducer <NUM> can be unfastened from the container <NUM> and allowing the gas in the channel <NUM> to fill the enclosure without leaking into an external environment about the container <NUM>. The replacing step includes the step of replacing the transducer <NUM> without using a valve to seal the recess <NUM> from which the transducer <NUM> is removed thereby allowing the enclosure within the extraction tool to be vented.

Unlike the present invention, in a design in which the transducer itself is immersed in the gas, the extraction tool performs the following functions:.

The present invention pertains to a transducer <NUM>, as shown in <FIG>, for an ultrasonic flowmeter <NUM>. The transducer <NUM> comprises a case <NUM>. The transducer <NUM> comprises a broad band piezoelectric composite <NUM> disposed in the case <NUM> with a coupling coefficient greater than. <NUM> out of <NUM> and an acoustic impedance of less than <NUM> Mrayls.

The transducer <NUM> may include a transformer <NUM> which matches the transducer's electrical impedance.

In another embodiment, the housing <NUM> and transducer <NUM> described herein, as well as the overall technique described herein, may be applied directly to a pipe <NUM> where the recesses <NUM> are formed in the pipe <NUM> and the housings <NUM> with transducers <NUM> are inserted into the recesses <NUM> in the pipe <NUM> itself.

In the operation of the invention, the flowmeter <NUM> is capable of measuring gas flow rates with ultrasonic transit time technology. The application is specifically applied to natural gas metering. A novel aspect of the flowmeter <NUM> is the transducer housing <NUM> which has a pressure containing window <NUM> less than <NUM>/<NUM> of a wavelength of the ultrasound of the window <NUM> material in thickness but nevertheless complies with the strength and tightness requirements imposed by the full pressure of the gas on its exterior. The preferred window <NUM> is a titanium window <NUM>. The titanium window <NUM> is thin enough such that it is nearly acoustically transparent, it serves as a hermetic seal from the natural gas which may contain hydrogen sulfide or other contaminants, and it acts as a pressure barrier to the gas in the pipe <NUM>. A wavelength (λ) of titanium at <NUM> is <NUM> when the speed of sound in the material is <NUM>/s. The titanium window <NUM> is laser welded to the titanium transducer housing <NUM>. Laser welding is a low heat process compared to torch welding therefore there is no damage to the mechanical integrity of the λ/<NUM> thick (<NUM>) titanium window <NUM> to the housing <NUM>. The window <NUM> serves to maintain the mechanical integrity of the transducer housing <NUM> by being a pressure barrier from the pressurized gas. Typical operation pressure ratings range from below <NUM> psi (ANSI <NUM>) to <NUM>,<NUM> psi (ANSI <NUM>). The operating temperatures range from -<NUM> C to <NUM> C. The gas flow meter has replaceable transducers <NUM> without having to depressurize the line. All transducer housings <NUM> have acoustic ribs <NUM> which are spaced λ/<NUM> to λ/<NUM> apart to isolate and cancel the acoustic noise of the transducer housing <NUM> and breakup the transducer housing <NUM> resonance. In addition, all transducer housings <NUM> are acoustically isolated from the meter body by acoustic isolators <NUM>.

Special transducers <NUM> have been fabricated using composite piezoelectric technology. The composite piezoelectric material has a kt of. <NUM> and low Z of <NUM> MRayls. This enables broad band performance for an easily detectable leading edge for precise time measurements. Each transducer <NUM> uses a <NUM>:<NUM> impedance matching transformer <NUM> which improves signal strength by <NUM> dB. This transducer arrangement coupled to a titanium window <NUM>, λ/<NUM> is desired for performance of an ultrasonic gas meter that uses a window <NUM> as a pressure barrier greater than <NUM>,<NUM> psi.

A flowmeter <NUM> for detecting gas flow rates in a pipe <NUM> preferably includes multiple paths disposed in the pipe <NUM> through which gas in the pipe <NUM> flows. The upstream transducer <NUM> is in contact with the titanium window <NUM> within the transducer housing <NUM> and positioned such that plane waves generated by the upstream transducer <NUM> propagates through the channel <NUM>. The downstream transducer <NUM> is acoustically isolated from the pipe <NUM> and positioned such that plane waves generated by the downstream transducer <NUM> propagate through the channel <NUM> and are received by the upstream transducer <NUM>, which produces an upstream transducer <NUM> signal that is provided to a controller <NUM>. The downstream transducer <NUM> receives the plane waves from the upstream transducer <NUM>, and provides a downstream transducer <NUM> signal that is provided to the controller <NUM>. The flowmeter <NUM> includes a signal processor, otherwise known as the controller <NUM>, in communication with the upstream and downstream transducers <NUM>, <NUM> which determines the gas flow rate through the channel <NUM> by measuring transit times of signals transmitted by and received by the transducers.

Referring to <FIG>, the flowmeter <NUM> comprises a channel <NUM> disposed in the pipe <NUM> through which gas in the pipe <NUM> flows. The flowmeter <NUM> comprises multiple paths. Multiple upstream ultrasonic transducers <NUM> within transducer housings <NUM> acoustically isolated from the pipe <NUM> and positioned so plane waves generated by an upstream transducer <NUM> propagate through the channel <NUM>. Each path comprises a downstream ultrasonic transducer <NUM>, within a transducer housing <NUM> acoustically isolated from the pipe <NUM> and positioned so plane waves generated by the downstream transducer <NUM> propagate through the channel <NUM> and are received by the upstream transducer <NUM> which produces an upstream transducer <NUM> signal. The downstream transducer <NUM> receives the plane waves from the upstream transducer <NUM> and provides a downstream transducer <NUM> signal. The flowmeter <NUM> comprises a controller <NUM> in communication with the upstream and downstream transducers <NUM>, <NUM> which determines the gas flow rate through the channel <NUM> by measuring transit times of signals transmitted by and received by the transducers.

The transducer housing <NUM> is preferably made of titanium which is corrosion resistant to natural gas contaminants such as hydrogen sulfide. The upstream transducer <NUM> and the downstream transducer <NUM> are coupled to a window <NUM> within the transducer housing <NUM>. The transducer housings <NUM> are acoustically isolated by acoustic isolators <NUM> which contact the pipe <NUM>. The window <NUM> forms a seal with the transducer housing <NUM> preventing gas in the pipe <NUM> leaking into the transducer housing <NUM>. The window <NUM> which is inclusive of the transducer housing <NUM> is sealed via laser welding such that hydrogen sulfide cannot leak inside. The transducer <NUM> couples to the acoustic window <NUM> using a couplant, preferably a silicone grease. A spring assembly <NUM> applies pressure to the transducer <NUM> in order to couple the ultrasonic signal more effectively to the window <NUM>. A spacer <NUM> and compression nut <NUM> compress the spring assembly <NUM> such that at least <NUM> psi is applied to the transducer <NUM>. O-rings <NUM> are placed around the transducer housing <NUM> and a bushing <NUM> in order to create another gas tight seal between the transducer housing <NUM> and the pipe <NUM>. The bushing <NUM> provides mechanical support for the transducer housing <NUM> and positions the transducer housing <NUM> within the container <NUM>. The load nut <NUM> and lock nut <NUM> secure the transducer housing <NUM> to the container <NUM> by applying pressure to the acoustic isolators <NUM> and prevents the transducer housing <NUM> from being forced out of the container <NUM> under pipe pressure. There is a gas gap radially disposed between the transducer housing <NUM> and load nut <NUM> and lock nut <NUM> so there is no metal to metal contact between the container <NUM> and the transducer housings <NUM>. It should be noted that basically, the container <NUM> with the channel <NUM> is an extension of the pipe <NUM> and for all intent and purpose in regard to this invention is considered a part of the pipe <NUM>.

Shown in <FIG> is a transducer <NUM> consisting of a wearface <NUM>, piezocomposite <NUM> and delay line <NUM> that is potted in a brass case <NUM> with a load cylinder <NUM> and a cap <NUM>. The wearface <NUM> is made of impedance matching material preferably of high purity alumina (Al<NUM>O<NUM>) greater than <NUM>% with a density of around <NUM> gm/cm<NUM>, a hardness greater than <NUM> Knoops and the thickness << λ. The piezocomposite <NUM> is a <NUM>-<NUM> composite material preferably made from PZT-<NUM> and epoxy (Smart Material Inc. , Sarasota, FL). The volume fraction of PZT-<NUM> is around <NUM>% this produces a high coupling coefficient, kt of. <NUM> out of <NUM>. Having a high coupling coefficient is necessary because it improves the signal to noise ratio of the ultrasonic signal. The acoustic impedance known as Z = ρv, where density ρ (kg/m<NUM>) and velocity v (m/s), is around <NUM> MRayls. The delay line <NUM> serves two purposes: as a reflection delay line <NUM> and as an acoustic attenuator. The delay line <NUM> is made of metal filings filled with epoxy of at a low volume fraction, typically <NUM>%. The delay line <NUM> is long enough such that the reflection is more than several wavelengths away from the piezoelectric element. A typical speed of sound for the delay line <NUM> is <NUM>/s. A wave length in a delay line <NUM> at <NUM> is <NUM>. A typical length of a delay line <NUM> for this application is thirty two mm. Therefore, once a signal is received by the transducer <NUM> it takes a reflection twice the time of a delay line <NUM> before the reflection is received as reverberation by the transducer <NUM>. In this case the reflection occurs fifty microseconds later than the detectable signal. Attenuation in the delay line <NUM> is typically, <NUM> dB/cm, so round trip attenuation of a reflected signal would be <NUM> dB. In order to make electrical connections to piezocomposite <NUM>, a (+) wire <NUM> and a (-) wire <NUM> are soldered to silver foil which is bonded to the piezocomposite <NUM> using silver epoxy. The entire transducer <NUM> is potted with non conductive epoxy in order to encapsulate and insulate all components.

Depending on the specific operating conditions of the meter, a range of frequencies can be used, from <NUM> to <NUM>. A broad band composite transducer <NUM> is preferred for metering gas. Unique to gas applications, when flow rate > <NUM> feet/second is present the acoustic wave is distorted by the flow and the wave front is received by a transducer <NUM>. If a broad band composite transducer <NUM> is used instead of a typical narrow band monolithic PZT transducer or Tonpilz transducer there is less distortion of the received acoustic wave. The upstream and downstream transit time signals are detected in order to make a flow measurement by the controller <NUM>. The composite transducer <NUM> has a rising edge of the received ultrasonic pulse that can be more accurately detected by the controller <NUM> thus calculating a more accurate transit time measurement.

An electrical transformer <NUM> is put in series with each transducer <NUM> to match the electrical impedance of the transducer <NUM> to that of the electronics and cables, <FIG>. The electronics are represented by an AC voltage V in series with a transformer, L<NUM>, the first winding of the transformer and the impedance Z of a transducer is in series with the secondary winding of the transformer, L<NUM>. The equivalent circuit is shown when the electronics, V, are directly in series with the impedance Z of the transducer and the impedance is reduced by the number of turns squared, N<NUM>. The electrical impedance of each transducer <NUM> (<NUM>Ω) is reduced N<NUM> by <NUM> to <NUM>Ω in order to match to the <NUM>Ω transmit and receive electronics. Each transducer <NUM> is spring loaded by the spring assembly <NUM> inside the transducer housing <NUM> in order for the wearface <NUM> to make sufficient mechanical contact to the window <NUM>. Grease is used for acoustic coupling of the transducer <NUM> to the window <NUM>.

The transducer housing <NUM> has acoustic ribs <NUM>, acoustic isolators <NUM>, and a window <NUM>. The acoustic ribs <NUM> are spaced λ/<NUM> to λ/<NUM> apart in order to cancel acoustic noise. During the excitation of an ultrasonic transducer <NUM> which is coupled to the window <NUM>, a low amplitude acoustic wave either longitudinal or shear propagates along the transducer housing <NUM>. The wave propagates according to the wave equation:
<MAT>.

When a longitudinal or shear wave is reflected from a rib interface its phase changes ω = π or <NUM> degrees ~ t = T/<NUM> or x = λ/<NUM>, it then interferes with an incoming wave to destructively cancel it. Therefore,
<MAT>.

Since the ambient noise generated in the transducer housing <NUM> can be at many frequencies and amplitudes, cancelation is not complete. The noise cancellation improves the signal to noise ratio of the received ultrasound signal up <NUM> dB.

The acoustic isolators <NUM> are preferably made of Polyether Ether Ketone (PEEK) disks (thickness > λ) that fit around the transducer housing <NUM> between a flange <NUM>. Other low acoustic impedance materials may be used, for example syntactic foam. The acoustic isolators <NUM> are in contact with the pipe <NUM> but the flange <NUM> is not. The total noise loss from noise attenuation can be calculated using transmission equations using the following formula:
<MAT>.

Where Po is pressure output, Pin pressure input, Zpeek, the acoustic impedance of PEEK is <NUM> Mrayls, Zsteel, the acoustic impedance of Steel is <NUM> Mrayls, and ZTi, the acoustic impedance of Titanium is <NUM> Mrayls. The loss calculation is <NUM> dB but more loss is possible since the PEEK, steel and titanium parts are pressed together and not mechanically bonded.

The window <NUM> is less than wavelength thick such that it becomes virtually acoustically transparent at operating frequencies. According to transmission line theory the acoustic impedance of material becomes closer to that acoustic impedance the thinner it becomes. When the thickness of the titanium window l is λ/<NUM> of titanium, the acoustic energy transmits through the window <NUM> into gas without signal distortion. The equation for transmission line theory is:
<MAT>.

Since titanium has lower acoustic impedance than steel (typical material of gas pipes), it is the best metal to use acoustically that is resistant to hydrogen sulfide and other contaminants.

The ultrasonic flow meter arrangement <NUM> uses multiple transducers <NUM> each within a transducer housing <NUM>, there are multiple chordal paths distributed in spacing according to numerical integration rules in order to accurately sample the velocity profile in a pipe <NUM>. A path consists of one transducer <NUM> upstream from the gas flow and another is downstream from the gas flow, both transducers <NUM> transmit and receive signals. The difference in transit times between the upstream and downstream signal is used to calculate a velocity per path. The path velocities are integrated by the controller <NUM> to calculate a flow rate. These equations can be modified for Mach number.

In order directly to measure volumetric flow, one must integrate the axial fluid velocity over a cross section normal to the pipe <NUM> axis. In order to solve for the speed of sound in gas and gas velocity, the upstream and downstream transit times need to be measured via a controller <NUM>. The controller <NUM> computes the transit time differences between the upstream and downstream transit times per path length. The VaxialLchord product is exactly the line integral of Vaxial dy at a chord location. The VaxialLchord product is calculated for each location x<NUM>, x<NUM>, x<NUM>, x<NUM> in <FIG>, effectively dividing the pipe <NUM> cross-section into four segments per plane. The effective width of each segment is a fraction of the internal diameter, D, measured along the x axis. Either Legendre or Jacobian/Chebychev spacing or weighting are used for chordal flowmeters, the path locations y, and weighting factors w were not chosen arbitrarily but comply with numerical integration rules. The spacing is measured from the center of the pipe <NUM>, the spacing y<NUM>, y<NUM>, - y<NUM>, -y<NUM> is shown in Table <NUM> along with the weighting factors. The length of each chord is known either by calculation or measurement.

The flow Q can now be calculated by the following equation: <MAT>
where w<NUM> = w<NUM> and w<NUM> = w<NUM>; Lchord<NUM> = Lchord<NUM>, Lchord<NUM> = Lchord<NUM>.

A <NUM> inch diameter (<NUM>" ID) ultrasonic gas flowmeter <NUM> with eight paths was fabricated as described in this invention and calibrated to a known standard at the CEESI gas calibration facility in Iowa. Results show (<FIG>) the percent error as function of a range of velocities from <NUM> ft/s to <NUM> ft/s. Native linearity (that is; the <NUM> inch meter linearity without correction) was determined to be only +/- <NUM>%. The pipe <NUM> is fabricated from carbon steel in accordance ASME B31. <NUM> Process Piping Code. The controller <NUM> is designed in compliance to UL/cUL Class <NUM>, Division <NUM>, Groups C & D.

The meter sizes, flowrates and velocities are described in Table <NUM>. Flowrates shown are based on schedule <NUM> pipe ID. Over-range flowrates are at <NUM> fps.

Claim 1:
A flowmeter (<NUM>) for detecting a gas flow rate in a pipe (<NUM>), the flowmeter comprising:
a container (<NUM>) configured to be attached to the pipe (<NUM>), the container (<NUM>) having a channel (<NUM>) through which a gas flows and a plurality of apertures (<NUM>) that extend through the container (<NUM>); and
a plurality of transducer devices disposed in the apertures (<NUM>) such that a respective transducer device is disposed in a respective aperture of the apertures, the transducer devices each comprising a transducer element (<NUM>) and a housing (<NUM>) configured to contain a pressure in the channel (<NUM>) and to prevent gas in the channel (<NUM>) from escaping into the housing (<NUM>), each housing (<NUM>) having a window (<NUM>) that is in acoustic communication with the channel (<NUM>);
wherein each transducer element is configured to transmit an ultrasonic signal into the channel and receive an ultrasonic signal from the channel (<NUM>) through the window (<NUM>), wherein each of the plurality of transducer elements (<NUM>) are arranged to be removable from the housing (<NUM>) without depressurization of the pipe (<NUM>); and
a controller (<NUM>) in electrical communication with the plurality of transducer devices which determines the gas flow rate through the channel (<NUM>) using ultrasonic signals transmitted by and received by the transducer devices.