Ultrasonic flow measurement system

An ultrasonic flow measurement system having a first ultrasonic sensor with a first ultrasonic buffer and a second ultrasonic sensor with a second ultrasonic buffer is disclosed. The first and second ultrasonic buffers have different cross sections in order to reduce distortion of the ultrasonic signals.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to an ultrasonic flow measurement system.

Flow meters, including ultrasonic flow sensors, are used to determine the characteristics (e.g., flow rate, pressure, temperature, etc.) of liquids, gases, etc. flowing in pipes of different sizes and shapes. Knowledge of these characteristics of the fluid can enable other physical properties or qualities of the fluid to be determined. For example, in some fluid custody-transfer applications, the flow rate of the fluid can be used to determine the volume of a fluid (e.g., oil or gas) being transferred from a seller to a buyer through a pipe over a period of time to determine the costs for the transaction/The volume is equal to the measured fluid flow rate multiplied by the cross sectional area of the pipe multiplied by the period of time over which the fluid flow is measured.

In one type of ultrasonic flow sensor employing transit time flow metering, one or more pairs of ultrasonic flow sensors can be installed along a portion of a pipe, referred to as a flow cell. Each pair of ultrasonic flow sensors contain an ultrasonic transducer and an ultrasonic buffer that are located upstream and downstream from each other, forming an ultrasonic path between these ultrasonic flow sensors at particular chordal locations across the pipe.

Each transducer, when energized, transmits an ultrasonic signal (e.g., a sound wave) along an ultrasonic path through the flowing fluid that is received by and detected by the other transducer. The path velocity of the fluid averaged along the ultrasonic path at a particular chordal location can be determined as a function of the differential between (1) the transit time of an ultrasonic signal traveling along the ultrasonic path from the downstream transducer upstream to the upstream transducer against the fluid flow direction, and (2) the transit time of an ultrasonic signal traveling along the ultrasonic path from the upstream transducer downstream to the downstream transducer with the fluid flow direction. Ultrasonic flow meters use signal processing techniques to identify the ultrasonic signals received by the transducers and the time that those ultrasonic signals were received in order to determine the transit times used to determine the flow rate of the fluid.

In some ultrasonic flow measurement systems (e.g., ultrasonic signals of a few megahertz or less where the wavelength of the ultrasonic signal is not significantly less than the diameter of the ultrasonic buffers), the spreading of the ultrasonic signal beam as it propagates within the ultrasonic buffer can result in distortion in the form of multiple ring-down signals and peaks in the received ultrasonic signal. This distortion is a result of angular portions of the ultrasonic signal reflecting off of the walls of the ultrasonic buffer. When these ring-down signals and peaks resulting from the distortion have amplitudes that are comparable or greater than the amplitudes of the main (non-angular) portion of the ultrasonic signal, the signal processing of the ultrasonic flow meter may not be able to accurately identify the main portion of the ultrasonic signal from which the transit time is determined.

BRIEF DESCRIPTION OF THE INVENTION

An ultrasonic flow measurement system having a first ultrasonic sensor with a first ultrasonic buffer and a second ultrasonic sensor with a second ultrasonic buffer is disclosed. The first and second ultrasonic buffers have different cross sections in order to reduce distortion of the ultrasonic signals. An advantage that may be realized by the practice of some of the disclosed embodiments of the ultrasonic flow measurement system is improving the accuracy of the measured transit times.

In one embodiment, an ultrasonic flow measurement system is disclosed. The system comprises a first ultrasonic sensor comprising a first ultrasonic transducer and a first ultrasonic buffer, and a second ultrasonic sensor comprising a second ultrasonic transducer and a second ultrasonic buffer, the second ultrasonic sensor is aligned with the first ultrasonic sensor along an axis, wherein the first ultrasonic buffer has a first cross section that is perpendicular to the axis and the second ultrasonic buffer has a second cross section that is perpendicular to the axis, and wherein the first cross section of the first ultrasonic buffer is different from the second cross section of the second ultrasonic buffer.

In another embodiment, the ultrasonic flow measurement system comprises a first ultrasonic sensor comprising a first ultrasonic transducer and a first ultrasonic buffer, and a second ultrasonic sensor comprising a second ultrasonic transducer and a second ultrasonic buffer, the second ultrasonic sensor is aligned with the first ultrasonic sensor along an axis, wherein the first ultrasonic buffer has a first cross section that is perpendicular to the axis and the second ultrasonic buffer has a second cross section that is perpendicular to the axis, and wherein the first cross section of the first ultrasonic buffer has a first shape and the second cross section of the second ultrasonic buffer has a second shape, and wherein the first shape is different from the second shape.

In yet another embodiment, the ultrasonic flow measurement system comprises a first ultrasonic sensor comprising a first ultrasonic transducer and a first ultrasonic buffer, and a second ultrasonic sensor comprising a second ultrasonic transducer and a second ultrasonic buffer, the second ultrasonic sensor is aligned with the first ultrasonic sensor along an axis, wherein the first ultrasonic buffer has a first cross section that is perpendicular to the axis and the second ultrasonic buffer has a second cross section that is perpendicular to the axis, and wherein the first cross section of the first ultrasonic buffer has a first diameter and the second cross section of the second ultrasonic buffer has a second diameter, and wherein the first diameter is different from the second diameter.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a diagram of an exemplary ultrasonic flow measurement system100. As shown, this system100includes a first ultrasonic flow sensor170and a second ultrasonic flow sensor180. The first ultrasonic flow sensor170includes a first ultrasonic buffer172and a first transducer174. The second ultrasonic flow sensor180includes a second ultrasonic buffer182and a second transducer184. Each ultrasonic flow sensor170,180is designed to transmit and receive ultrasonic signals. The pipe190is designed to transport a fluid192that is in a liquid state, a gas state, or a combination of liquid and gas. The first and second ultrasonic flow sensors170,180are each installed into the pipe190and each have ultrasonic buffers172,182aligned along a common axis150. The common axis150can be at an angle which is perpendicular to the axis of the pipe190where no flow is detected or at an angle other than ninety degrees with respect to the axis of the pipe190to detect flow.

FIG. 2illustrates a cross sectional view of the exemplary first ultrasonic buffer172, whileFIG. 3illustrates a cross sectional view of the exemplary second ultrasonic buffer182ofFIG. 1. As shown in these figures, the first ultrasonic buffer172has a cross section (i.e., circular in shape) with a first diameter179, while the second ultrasonic buffer182has a cross section (i.e., circular in shape) with a second diameter189. Both cross sections and other cross sections described within this document are defined along a plane that is perpendicular to the axis150(FIG. 1). ComparingFIGS. 2 and 3shows that the cross section of the first ultrasonic buffer172is the same shape and size as the cross section of the second ultrasonic buffer182. In one embodiment, the diameters179,189are 0.5 inches (12.7 millimeters). As will be explained, since the cross section of the first ultrasonic buffer172is the same as the cross section of the second ultrasonic buffer182, this symmetry will produce significant distortions (ring-down signals, peaks) in the received ultrasonic signal.

FIG. 4illustrates a plurality of ultrasonic signal paths110,120,130traveling from the first ultrasonic buffer172to the second ultrasonic buffer182ofFIGS. 1-3. The ultrasonic signal is transmitted from the first transducer174via the first ultrasonic buffer172to the second ultrasonic flow sensor180via the second ultrasonic buffer182. A portion of the ultrasonic signal is transmitted along a first (main) ultrasonic signal path110, which is co-axial with axis150ofFIG. 1. This first ultrasonic signal path110is a straight and direct path from the first ultrasonic buffer172to the second ultrasonic buffer182. As a result of beam spread of the ultrasonic signal, angular components of the ultrasonic signal travel along many other ultrasonic signal paths in addition to the first ultrasonic signal path110. For example, portions of the ultrasonic signal also travel along a second ultrasonic signal path120and a third ultrasonic signal path130.

The second ultrasonic signal path120is initially directed at a 10 degree angle122from a center location121along the back wall175of the first ultrasonic buffer172. This second ultrasonic signal path120changes direction at the face176of the first ultrasonic buffer172, where it refracts into the fluid192. The second ultrasonic signal path120changes direction at the face186of the second ultrasonic buffer182where it refracts from the fluid192into the second ultrasonic buffer182. The second ultrasonic signal path120changes direction again when it reflects off the lower wall187of the second ultrasonic buffer182. The second ultrasonic signal path120changes direction again when it reflects off the upper wall188of the second ultrasonic buffer182. The second ultrasonic signal path120exits the second ultrasonic buffer182along the back wall185of the second ultrasonic buffer182and is received by the second ultrasonic transducer184. As shown, the second ultrasonic signal path120is longer than the first ultrasonic signal path110.

The third ultrasonic signal path130is initially directed at a 10 degree angle132from a non-center location131along the back wall175of the first ultrasonic buffer172. The third ultrasonic signal path130changes direction when it reflects off the lower wall177of the first ultrasonic buffer172. The third ultrasonic signal path130changes direction and reflects off an upper wall178of the first ultrasonic buffer172. Next, the third ultrasonic signal path130changes direction at the face176of the first ultrasonic buffer172, where it refracts into the fluid192. The third ultrasonic signal path130changes direction at the face186of the second ultrasonic buffer182where it refracts from the fluid192into the second ultrasonic buffer182. The third ultrasonic signal path130exits the second ultrasonic buffer182along the back wall185of the second ultrasonic buffer182and is received by the second ultrasonic transducer184. As shown, the third ultrasonic signal path130is longer than the first ultrasonic signal path110, but is the same length as the second ultrasonic signal path120.

FIG. 5illustrates a plurality of received ultrasonic signal waveforms210,220,230corresponding to the ultrasonic signal as it is received by the second ultrasonic transducer184via each of the ultrasonic signal paths110,120,130ofFIG. 4. It should be noted that the graph ofFIG. 5(and all other graphs herein) show the ultrasonic signal waveforms on the same voltage (Y) axis to illustrate the relative amplitudes of each ultrasonic waveform. Each ultrasonic signal waveform210,220,230is a representative portion of the ultrasonic signal. As shown, the first ultrasonic signal waveform210is received via the first ultrasonic signal path110, the second ultrasonic signal waveform220is received via the second ultrasonic signal path120, and the third ultrasonic signal waveform230is received via the third ultrasonic signal path130. The fourth (combined) ultrasonic signal waveform240is the ultrasonic signal waveform received from the combination of the ultrasonic signal paths110,120,130.

Each of the ultrasonic signal waveforms210,220,230includes a leading edge212,222,232, respectively, that arrives at a particular time. In one exemplary ultrasonic flow measurement system (1 MHz ultrasonic signal, the first and second ultrasonic buffers172,182are made from SS316 stainless steel, each having a 0.50 in (12.70 mm) diameter by 0.75 in. (19.05 mm) length, with 1.0 in. (25.4 mm) water separation between the ultrasonic buffers172,182), the leading edge212of the first ultrasonic signal waveform210is received by the second ultrasonic transducer184via the first ultrasonic signal path110at a time of 23.53 microseconds. The leading edge222of the second ultrasonic signal waveform220is received via the second ultrasonic signal path120at a time of 27.11 microseconds. The leading edge232of the third ultrasonic signal waveform230is received via the third ultrasonic signal path130at a time of 27.11 microseconds, the same time as the leading edge222of the second ultrasonic signal waveform220.

The first portion214of the combined ultrasonic signal waveform240is the portion of the received ultrasonic signal contributed by the first ultrasonic signal waveform210received via the first ultrasonic signal path110from which the transit time should be determined. But since the second and third ultrasonic signal waveforms220,230arrive at the same time (27.11 microseconds), the constructive combination of these ultrasonic signal waveforms220,230forms a second portion244of the combined ultrasonic signal waveform240that has a greater amplitude than the first portion214of the combined ultrasonic signal waveform240. This second portion244of the combined waveform can produce unwanted ring-down signals and peaks in the combined ultrasonic signal waveform240that can result in the signal processing electronics of the ultrasonic flow measurement system being unable to accurately identify the first portion214of the combined ultrasonic waveform240from which the transit time of the ultrasonic signal is determined.

To address this problem, embodiments of the invention reduce the constructive combination of ultrasonic signal waveforms that are received from the angular ultrasonic signal paths so that the first (main) ultrasonic signal waveform of the first (main) ultrasonic signal path is more easily identifiable by the signal processing electronics. This reduction is accomplished using ultrasonic buffers having different cross sections to reduce the symmetry of the ultrasonic signal paths in the ultrasonic flow measurement system, creating different ultrasonic signal path lengths for the angular components of the ultrasonic signal.

FIG. 6illustrates a cross sectional view of an exemplary third ultrasonic buffer382. ComparingFIG. 2withFIG. 6shows that the cross section of the first ultrasonic buffer172is different than the cross section of the third ultrasonic buffer382. As shown, the third ultrasonic buffer382has a cross section having a first section383that has a circular shape and a second section384forming a straight line edge398. This cross section is referred to herein as having a letter “D” shape. The “D” shape shown inFIG. 6is different than the circular “O” cross section shape of first ultrasonic buffer172shown inFIG. 2. This “D” shape cross section has a first dimension397and a second dimension399. In one embodiment, the first dimension397is 0.427 inches (10.85 millimeters) and the second dimension399is 0.5 inches (12.7 millimeters).

FIG. 7illustrates a plurality of ultrasonic signal paths310,320traveling from the first ultrasonic buffer172(FIG. 2) to the third ultrasonic buffer382ofFIG. 6, as seen from a first perspective. The third ultrasonic buffer382is shown where the first dimension397is the distance between the lower wall387and the straight line edge398, which constitutes an upper wall of the third ultrasonic buffer382. The ultrasonic signal is transmitted from the first transducer174via the first ultrasonic buffer172to the second transducer184via the third ultrasonic buffer382. A portion of the ultrasonic signal is transmitted along a first (main) ultrasonic signal path310, which is co-axial with axis150ofFIG. 1. This first ultrasonic signal path310is a straight and direct path from the first ultrasonic buffer172to the third ultrasonic buffer382. As a result of beam spread of the ultrasonic signal, angular components of the ultrasonic signal travel along many other ultrasonic signal paths in addition to the first ultrasonic signal path310. For example, portions of the ultrasonic signal also travel along a second ultrasonic signal path320(FIG. 7) and a third ultrasonic signal path330(FIG. 8).

The second ultrasonic signal path320is initially directed at a 10 degree angle122from a center location121along the back wall175of the first ultrasonic buffer172. This second ultrasonic signal path320changes direction at the face176of the first ultrasonic buffer172, where it refracts into the fluid192. This second ultrasonic signal path320changes direction at the face386of the third ultrasonic buffer382where it refracts from the fluid192into the third ultrasonic buffer382. The second ultrasonic signal path320changes direction again when it reflects off the lower wall387of the third ultrasonic buffer382. The second ultrasonic signal path320changes direction again when it reflects off the straight line edge398or upper wall of the third ultrasonic buffer382. The second ultrasonic signal path320exits the second ultrasonic buffer382along the back wall385of the third ultrasonic buffer382and is received by the second ultrasonic transducer184. As shown, the second ultrasonic signal path320is longer than the first ultrasonic signal path310.

FIG. 8illustrates a plurality of ultrasonic signal paths310,330traveling from the first ultrasonic buffer172(FIG. 2) to the third ultrasonic buffer382ofFIG. 6, as seen from a second perspective. The third ultrasonic buffer382is shown where the second dimension399is the distance between the lower wall387and the upper wall388of the third ultrasonic buffer382. Relative toFIG. 7, the third ultrasonic buffer382has been rotated 90 degrees.

The third ultrasonic signal path330is initially directed at a 10 degree angle122from a center location121along the back wall175of the first ultrasonic buffer172. This third ultrasonic signal path330changes direction at the face176of the first ultrasonic buffer172, where it refracts into the fluid192. This third ultrasonic signal path330changes direction at the face386of the third ultrasonic buffer382where it refracts from the fluid192into the third ultrasonic buffer382. The third ultrasonic signal path330changes direction again when it reflects off the lower wall387of the third ultrasonic buffer382. The third ultrasonic signal path330changes direction again when it reflects off the or upper wall388of the third ultrasonic buffer382. The third ultrasonic signal path330exits the third ultrasonic buffer382along the back wall385of the third ultrasonic buffer382and is received by the second ultrasonic transducer184. As shown, the third ultrasonic signal path330is longer than the first ultrasonic signal path310, and is slightly longer than the second ultrasonic signal path320.

FIG. 9illustrates a plurality of received ultrasonic signal waveforms410,420,430corresponding to the ultrasonic signal as it is received by the second ultrasonic transducer184via each of the ultrasonic signal paths310,320,330ofFIGS. 7-8. Each ultrasonic signal waveform410,420,430is a representative portion of the ultrasonic signal. As shown, the first ultrasonic signal waveform410is received via the first ultrasonic signal path310, the second ultrasonic signal waveform420is received via the second ultrasonic signal path320, and the third ultrasonic signal waveform430is received via the third ultrasonic signal path330. The fourth (combined) ultrasonic signal waveform440is the combination of the ultrasonic signal paths310,320,330.

Each of the ultrasonic signal waveforms410,420,430, respectively includes a leading edge412,422,432that arrives at a particular time. In one exemplary ultrasonic flow measurement system (1 MHz ultrasonic signal, the first ultrasonic buffer172and the third ultrasonic buffer382are made from SS316 stainless steel, each 0.75 in. (19.05 mm) length, with 1.0 in. (25.4 mm) water separation between the ultrasonic buffers172,382), the leading edge412of the first ultrasonic signal waveform410is received by the second ultrasonic transducer184via the first ultrasonic signal path310at a time of 23.53 microseconds. The leading edge422of the second ultrasonic signal waveform420is received via the second ultrasonic signal path320at a time of 27.12 microseconds. The leading edge432of the third ultrasonic signal waveform430is received via the third ultrasonic signal path330at a time of 27.61 microseconds, different than the time of the leading edge422of the second ultrasonic signal waveform420because of the different cross-sections of the ultrasonic buffers172,382.

The first portion414of the combined ultrasonic signal waveform440is the portion of the received ultrasonic signal contributed by the first ultrasonic signal waveform410received via the first ultrasonic signal path410from which the transit time should be determined. Since the second and third ultrasonic signal waveforms420,430arrive at different times that are approximately one half cycle/period apart (0.5 microseconds for a 1 MHz signal), the destructive combination of these ultrasonic signal waveforms420,430forms a second portion444of the combined ultrasonic signal waveform440that has a smaller amplitude than the first portion414of the combined ultrasonic signal waveform440. Since the amplitude of the second portion444is smaller than the amplitude of the first portion414of the combined ultrasonic signal waveform440, the signal processing electronics of the ultrasonic flow measurement system can more easily identify the first portion414of the combined ultrasonic signal waveform440from which the transit time of the ultrasonic signal is determined.

FIG. 10illustrates a cross sectional view of an exemplary fourth ultrasonic buffer582. ComparingFIG. 2withFIG. 10shows that the cross section of the first ultrasonic buffer172is different than the cross section of the fourth ultrasonic buffer582. As shown, the fourth ultrasonic buffer582has a diameter599of 0.427 inches (10.5 millimeters), which is smaller than the diameter179of the first ultrasonic buffer172, which is 0.5 inches (12.7 millimeters).

FIG. 11illustrates a plurality of ultrasonic signal paths510,520,530traveling from the first ultrasonic buffer172(FIG. 2) to the fourth ultrasonic buffer582ofFIG. 10. The ultrasonic signal is transmitted from the first transducer174via the first ultrasonic buffer172to the second transducer184via the fourth ultrasonic buffer582. A portion of the ultrasonic signal is transmitted along a first (main) ultrasonic signal path510, which is co-axial with axis150ofFIG. 1. This first ultrasonic signal path510is a straight and direct path from the first ultrasonic buffer172to the fourth ultrasonic buffer582. As a result of beam spread of the ultrasonic signal, angular components of the ultrasonic signal travel along many other ultrasonic signal paths in addition to the first ultrasonic signal path510. For example, a portion of the ultrasonic signal also travels along a second ultrasonic signal path520and a third ultrasonic signal path530.

The second ultrasonic signal path520is initially directed at a 10 degree angle122from a center location121along the back wall175of the first ultrasonic buffer172. This second ultrasonic signal path520changes direction at the face176of the first ultrasonic buffer172, where it refracts into the fluid192. This second ultrasonic signal path520changes direction at the face586of the fourth ultrasonic buffer582where it refracts from the fluid192into the fourth ultrasonic buffer582. The second ultrasonic signal path520changes direction again when it reflects off the lower wall587of the fourth ultrasonic buffer582. The second ultrasonic signal path520changes direction again when it reflects off the upper wall588of the third ultrasonic buffer582. The second ultrasonic signal path520exits the fourth ultrasonic buffer582along the back wall585of the fourth ultrasonic buffer582and is received by the second ultrasonic transducer184. As shown, the second ultrasonic signal path520is longer than the first ultrasonic signal path510.

The third ultrasonic signal path530is initially directed at a 10 degree angle132from a non-center location131along the back wall175of the first ultrasonic buffer172. The second ultrasonic signal path530changes direction when it reflects off the lower wall177of the first ultrasonic buffer172. The third ultrasonic signal path530changes direction and reflects off an upper wall178of the first ultrasonic buffer172. Next, the third ultrasonic signal path530changes direction at the face176of the first ultrasonic buffer172, where it refracts into the fluid192. The third ultrasonic signal path530changes direction at the face586of the fourth ultrasonic buffer582where it refracts from the fluid192into the fourth ultrasonic buffer582. The third ultrasonic signal path530exits the fourth ultrasonic buffer582along the back wall585of the fourth ultrasonic buffer582and is received by the second ultrasonic transducer184. As shown, the ultrasonic signal path530is longer than the first ultrasonic signal path510, and is slightly longer than the second ultrasonic signal path520.

FIG. 12illustrates a plurality of received ultrasonic signal waveforms610,620,630corresponding to the ultrasonic signal as it is received by the second ultrasonic transducer184via the ultrasonic signal paths510,520,530ofFIG. 11. Each ultrasonic signal waveform610,620,630is a representative portion of the ultrasonic signal. As shown, the first ultrasonic signal waveform610is received via the first ultrasonic signal path510, the second ultrasonic signal waveform620is received via the second ultrasonic signal path520, and the third ultrasonic signal waveform630is received via the third ultrasonic signal path530. The fourth (combined) ultrasonic signal waveform640is the combination of the ultrasonic signal paths510,520,530.

Each of the ultrasonic signal waveforms610,620,630, respectively includes a leading edge612,622,632that arrives at a particular time. In one exemplary ultrasonic flow measurement system (1 MHz ultrasonic signal, the first ultrasonic buffer172and the fourth ultrasonic buffer582are made from SS316 stainless steel, each 0.75 in. (19.05 mm) length, with 1.0 in. (25.4 mm) water separation between the ultrasonic buffers172,582), the leading edge612of the first ultrasonic signal waveform610is received by the second ultrasonic transducer184via the first ultrasonic signal path510at a time of 23.53 microseconds. The leading edge622of the second ultrasonic signal waveform620is received via the second ultrasonic signal path520at a time of 26.61 microseconds. The leading edge632of the third ultrasonic signal waveform630is received via the third ultrasonic signal path530at a time of 27.12 microseconds, different than the time of the leading edge622of the second ultrasonic signal waveform620because of the different cross-sections of the ultrasonic buffers172,582.

The first portion614of the combined ultrasonic signal waveform640is the portion of the received ultrasonic signal contributed by the first ultrasonic signal waveform610received via the first ultrasonic signal path510from which the transit time should be determined. Since the second and third ultrasonic signal waveforms620,630arrive at different times that are approximately one half cycle/period apart (0.5 microseconds for a 1 MHz signal), the destructive combination of these ultrasonic signal waveforms620,630forms a second portion644of the combined ultrasonic signal waveform640that has a smaller amplitude than the first portion614of the combined ultrasonic signal waveform640. Since the amplitude of the second portion644is smaller than the amplitude of the first portion614of the combined ultrasonic signal waveform640, the signal processing electronics of the ultrasonic flow measurement system can more easily identify the first portion614of the combined ultrasonic signal waveform640from which the transit time of the ultrasonic signal is determined.

FIG. 13illustrates a cross sectional view of an exemplary fifth ultrasonic buffer782. ComparingFIG. 2withFIG. 13shows that the cross section of the first ultrasonic buffer172is slightly different than the cross section of the fifth ultrasonic buffer782. As shown, the fifth ultrasonic buffer782has a diameter799of 0.51 inches (12.95 millimeters), which is slightly larger than the diameter179of the first ultrasonic buffer172, which is 0.50 inches (12.7 millimeters). Further, in this embodiment, the first ultrasonic buffer172is here made from Molybdenum and the fifth ultrasonic buffer782is made from SS316 stainless steel. It will be understood that other materials can be used for the ultrasonic buffers, which may result in the use of different diameters for those buffers.

FIG. 14illustrates a plurality of ultrasonic signal paths710,720,730traveling from the first ultrasonic buffer172(FIG. 2) to the fifth ultrasonic buffer782ofFIG. 13. The ultrasonic signal is transmitted from the first transducer174via the first ultrasonic buffer172to the second transducer184via the fifth ultrasonic buffer782. A portion of the ultrasonic signal is transmitted along a first (main) ultrasonic signal path710, which is co-axial with axis150ofFIG. 1. This first ultrasonic signal path710is a straight and direct path from the first ultrasonic buffer172(FIG. 2) to the fifth ultrasonic buffer782. As a result of beam spread of the ultrasonic signal, other angular components of the ultrasonic signal travel along many other ultrasonic signal paths in addition to the first ultrasonic signal path710. For example, a portion of the ultrasonic signal also travels along a second ultrasonic signal path720and a third ultrasonic path730.

The second ultrasonic signal path720is initially directed at a 10 degree angle122from a center location121along the back wall175of the first ultrasonic buffer172. The second ultrasonic signal path720changes direction at the face176of the first ultrasonic buffer172, where it refracts into the fluid192. The second ultrasonic signal path720changes direction at the face786of the fifth ultrasonic buffer782where it refracts from the fluid192into the fifth ultrasonic buffer782. The second ultrasonic signal path720changes direction again when it reflects off the lower wall787of the fifth ultrasonic buffer782. The second ultrasonic signal path720changes direction again when it reflects off the upper wall788of the fifth ultrasonic buffer782. The second ultrasonic signal path720exits the fifth ultrasonic buffer782along the back wall785of the fifth ultrasonic buffer782and is received by the second ultrasonic transducer184. As shown, the second ultrasonic signal path720is longer than the first ultrasonic signal path710.

The third ultrasonic signal path730is initially directed at a 10 degree angle132from a non-center location131along the back wall175of the first ultrasonic buffer172. The third ultrasonic signal path730changes direction when it reflects off the lower wall177of the first ultrasonic buffer172. The third ultrasonic signal path730changes direction and reflects off an upper wall178of the first ultrasonic buffer172. Next, the third ultrasonic signal path730changes direction at the face176of the first ultrasonic buffer172, where it refracts into the fluid192. The third ultrasonic signal path730changes direction at the face786of the fifth ultrasonic buffer782where it refracts from the fluid192into the fifth ultrasonic buffer782. The third ultrasonic signal path730exits the fifth ultrasonic buffer782along the back wall785of the fifth ultrasonic buffer782and is received by the second ultrasonic transducer184. As shown, the third ultrasonic signal path730is longer than the first ultrasonic signal path710, and is slightly shorter than the second ultrasonic signal path720.

FIG. 15illustrates a plurality of received ultrasonic signal waveforms810,820,830corresponding to the ultrasonic signal as it is received by the second transducer184via each of the ultrasonic signal paths710,720,730ofFIG. 14. Each waveform810,820,830is a representative portion of the ultrasonic signal. As shown, the first ultrasonic signal waveform810is received via the first ultrasonic signal path710, the second ultrasonic signal waveform820is received via the second ultrasonic signal path720, and the third ultrasonic signal waveform830is received via the third ultrasonic signal path730. The fourth (combined) ultrasonic signal waveform840is the combination of the ultrasonic signal paths710,720,730.

Each of the ultrasonic signal waveforms810,820,830, respectively includes a leading edge812,822,832that arrives at a particular time. In one exemplary ultrasonic flow measurement system (1 MHz ultrasonic signal, the first ultrasonic buffer172is made from Molybdenum and the fifth ultrasonic buffer782is made from SS316 stainless steel, each are 0.75 in. (19.05 mm) length, with 1.0 in. (25.4 mm) water separation between the ultrasonic buffers172,782), the leading edge812of the first ultrasonic signal waveform710is received by the second ultrasonic transducer184via the first ultrasonic signal path710at a time of 23.17 microseconds. The leading edge822of the second ultrasonic signal waveform820is received via the second ultrasonic signal path720at a time of 26.82 microseconds. The leading edge832of the third ultrasonic signal waveform830is received via the third ultrasonic signal path730at a time of 26.30 microseconds, different than the time of the leading edge822of the second ultrasonic signal waveform820because of the different cross-sections and materials of the ultrasonic buffers172,782.

The first portion814of the combined ultrasonic signal waveform840is the portion of the received ultrasonic signal contributed by the first ultrasonic signal waveform810received via the first ultrasonic signal path710from which the transit time should be determined. Since the second and third ultrasonic signal waveforms820,830arrive at different times that are approximately one half cycle/period apart (0.5 microseconds for a 1 MHz signal), the destructive combination of these ultrasonic signal waveforms820,830forms a second portion844of the combined ultrasonic signal waveform840that has a smaller amplitude than the first portion814of the combined ultrasonic signal waveform840. Since the amplitude of the second portion844is smaller than the amplitude of the first portion814of the combined ultrasonic signal waveform840, the signal processing electronics of the ultrasonic flow measurement system can more easily identify the first portion814of the combined ultrasonic signal waveform840from which the transit time of the ultrasonic signal is determined.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. For example, while the first section and second section of the cap are shown disposed substantially planar to the first planar surface of the substrate in the disclosed embodiments, it will be understood that the sections can be disposed at a different orientation (e.g., at a slope relative to the first planar surface of the substrate).