JPH0921665A - Ultrasonic flow meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve measurement accuracy of an ultrasonic flow meter. SOLUTION: A pair of ultrasonic vibrators 11, 12 covered by streamlined packages 13, 14 are provided oppositely to each other and symmetrically with respect to an axis upstream and downstream in a flow path 8. This constitution allows fluid flowing in the flow path 8 to flow along the packages 13, 14 and prevents an irregular eddy or stagnation from being formed. Therefore, a highly precise ultrasonic flow meter 6 can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic flowmeter for measuring the flow rate of fluids such as gas and water using ultrasonic waves.

[0002]

2. Description of the Related Art As shown in FIG. 7, a conventional ultrasonic flowmeter of this type has a pair of ultrasonic transducers provided with bent portions 2 and 3 which are bent at right angles upstream and downstream of a flow path 1. Nos. 4 and 5 are fixed to the outside of the wall surface of the curved portion, ultrasonic waves are propagated in parallel to the flow direction of the fluid, and they are operated as an ultrasonic flowmeter. The arrows in the figure indicate the flow direction of the fluid (see Japanese Patent Laid-Open No. 60-115810).

With such a configuration, an ultrasonic wave is transmitted from the vibrator 4 on the upstream side in the flow direction, and this ultrasonic wave is received by the vibrator 5 on the downstream side. The sound wave propagation time, Tdn, is measured. On the contrary, the oscillator 5 on the downstream side
To transmit an ultrasonic wave against the flow of the ultrasonic wave, the ultrasonic wave is received by the upstream transducer 4, and the propagation time of the ultrasonic wave from the transducers 5 to 4, Tup, is measured. The average flow velocity of the fluid flowing through the flow channel 1 is calculated from these two propagation times, and the flow rate of the fluid is measured from the cross-sectional area of the flow channel 1 which is known in advance.

[0004]

However, in the above-mentioned conventional ultrasonic flowmeter, vortices are irregularly generated in the fluid at the bent portions 2 and 3 which are bent at right angles, or irregular in the flow path. A stagnation point etc. occurred, which was an error factor in flow rate measurement. Further, the ultrasonic waves transmitted in the flow path are repeatedly reflected by the wall of the pipe and the like, and remain as reverberation waves forever, and the reverberation waves are received by the ultrasonic transducers, which also causes an error.

The present invention solves the above problems, and provides an ultrasonic flowmeter capable of reducing reverberation waves and measuring flow with high accuracy.

[0006]

In order to achieve the above object, the ultrasonic flowmeter of the present invention has the following constitution.

That is, a tubular flow path through which a fluid flows and a pair of ultrasonic transducers covered by an exterior part are provided axially symmetrically in the upstream and downstream of the flow path.

In addition, a tubular flow path through which a fluid flows, and parabolic ultrasonic reflectors on the inner surface are provided upstream and downstream of the flow path, and the outer surface is covered with an exterior part. The ultrasonic transducers are arranged so as to face each other in axial symmetry.

Further, a tubular flow path through which a fluid flows, and parabolic ultrasonic reflectors on the inner surface are provided upstream and downstream of the flow path, and the outer surface is covered with an exterior part and the front surface is provided. In addition, a pair of ultrasonic transducers having a streamlined ultrasonic transmission film are axially symmetrically opposed to each other.

In addition, a tubular flow path through which a fluid flows, and parabolic ultrasonic reflectors on the inner surface are provided upstream and downstream of the flow path, and the outer surface is a streamlined conductive exterior portion. A pair of ultrasonic transducers, which are covered and have a conductive ultrasonic transmission film on the front surface, are provided so as to face each other in axial symmetry.

Further, a tubular flow path through which the fluid flows, and parabolic ultrasonic reflectors on the inner surface are provided upstream and downstream in the flow path, and the outer surface is covered with an exterior part and the front surface. And a pair of ultrasonic transducers each having a directional horn, which are opposed to each other in axial symmetry.

[0012] Further, a tubular flow channel in which a fluid flows, the inner wall surface of which is rough, and an inner cylinder made of an ultrasonic wave permeable film are provided in the tubular flow channel. A pair of ultrasonic transducers having parabolic ultrasonic reflectors on the inner surface, an outer surface covered with an exterior part, and an ultrasonic transmission film on the front surface are axially symmetrical in the upstream and the downstream of the inside. It was configured to face each other.

In addition, a tubular flow channel having a rough inner wall surface, in which a fluid flows, and an inner cylinder made of an ultrasonic wave permeable film are provided in the tubular flow channel, and the tubular flow channel inner surface has the flow channel. A pair of ultrasonic transducers having parabolic ultrasonic reflectors on the inner surface, an outer surface covered with an exterior part, and an ultrasonic transmission film on the front surface are axially symmetrical in the upstream and the downstream of the inside. The pair of ultrasonic transducers are provided so as to face each other, and a fluid permeable membrane having a finer mesh than the ultrasonic permeable membrane is provided upstream and downstream of the pair of ultrasonic transducers.

[0014]

The present invention has a structure in which a tubular flow path through which a fluid flows and a pair of ultrasonic transducers covered by an exterior part are axially symmetrically opposed to each other upstream and downstream of the flow path. Therefore, generation of vortices and stagnation points in the fluid are eliminated.

Further, a tubular flow path through which the fluid flows, and parabolic ultrasonic reflectors on the inner surface are provided upstream and downstream of the flow path, and the outer surface is covered with an exterior part. Since the acoustic wave oscillators are arranged so as to face each other in axial symmetry, the generation of vortices and stagnation points in the fluid is eliminated, and the ultrasonic waves passing near the oscillators are parabolic. Because it reflects on the reflective surface,
The reflected ultrasonic waves converge on one point, can be easily absorbed and attenuated, and reverberation waves are reduced.

In addition, a tubular flow path through which a fluid flows, and parabolic ultrasonic reflectors on the inner surface are provided upstream and downstream of the flow path, and the outer surface is covered with an exterior part and the front surface is provided. Since a pair of ultrasonic transducers having ultrasonic transmission membranes are provided so as to be opposed to each other in axial symmetry, the fluid flowing in the flow path flows along the ultrasonic transmission membrane on the front surface of the transducer. Generation of vortices and stagnation points are eliminated, and ultrasonic waves are efficiently transmitted through the ultrasonic transmission film.

Further, the ultrasonic waves that pass through the ultrasonic wave transmitting film and pass through the vicinity of the vibrator are reflected by the parabolic reflection surface on the inner surface of the exterior part and converge at one point, so that they are easily absorbed and attenuated. be able to.

In addition, a tubular flow path through which the fluid flows, and parabolic ultrasonic reflectors on the inner surface are provided upstream and downstream of the flow path, and the outer surface is covered with a conductive exterior part. And, since it has a configuration in which a pair of ultrasonic transducers having a conductive ultrasonic transmission film on the front surface are provided axially symmetrically facing each other, a conductive exterior portion,
By grounding the conductive ultrasonic wave transmitting film, it is resistant to electromagnetic noise.

Further, a tubular flow path through which the fluid flows, and parabolic ultrasonic reflectors on the inner surface are provided upstream and downstream of the flow path, and the outer surface is covered with the exterior part and the front surface is provided. Since the ultrasonic wave transmission film is provided and a pair of ultrasonic transducers having a directional horn are provided in the transmission film so as to face each other in axial symmetry, the ultrasonic wave transmitted in the flow channel is generated by the horn. The directivity is improved, and the reflection on the pipe wall of the flow channel is significantly reduced. Further, since the horn is formed inside the ultrasonic wave transmitting film on the front surface of the vibrator, it does not disturb the fluid flow. Therefore, it is possible to provide an ultrasonic flowmeter that can measure the flow rate with higher accuracy.

Further, a tubular flow channel having an inner wall surface in which a fluid flows and an inner cylinder made of an ultrasonic wave permeable film are provided in the tubular flow channel, and the tubular flow channel inner surface is provided in the flow channel. A pair of ultrasonic transducers having parabolic ultrasonic reflectors on the inner surface, an outer surface covered with an exterior, and an ultrasonic transmission film on the front surface are axially opposed to each other upstream and downstream of As a result, the ultrasonic waves transmitted from the vibrator penetrate the inner cylinder made of the ultrasonic wave transmission film provided on the inner surface of the tubular flow path and reach the tube wall of the rough tubular tube directly. , Is reflected, and thus is largely scattered or absorbed. Therefore, reverberation waves are reduced.

Further, a tubular flow channel having an inner wall surface through which the fluid flows and an inner cylinder made of an ultrasonic wave permeable film are provided in the tubular flow channel, and the inner surface of the tubular flow channel is upstream of the flow channel. A pair of ultrasonic transducers having a parabolic ultrasonic reflector on the inner surface, an outer surface covered with an exterior portion, and an ultrasonic transmission film on the front surface are axially opposed to each other and downstream. Since a fluid permeable membrane with a finer mesh than the ultrasonic permeable membrane is provided on the upstream side and the downstream side of the pair of ultrasonic transducers, ultrasonic transmission with a coarser mesh provided in the flow path is performed. Dust, water droplets, etc. do not adhere to the film, and the durability and dust resistance of the ultrasonic wave transmissive film are greatly improved and the reliability is improved.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an ultrasonic flowmeter 6 according to an embodiment of the present invention, in which a flow path 7 through which a fluid flows has a straight pipe portion 8 with a diameter of 20 mm and a length of 80 mm for measuring the flow rate, and upstream and downstream. And the narrowed portions 9 and 10. These flow paths were made of acrylic resin so that the inside could be seen. The arrow in the figure indicates the direction of fluid flow. The pair of ultrasonic transducers 11 and 12 facing upstream and downstream of the flow path have a diameter of 10 mm.
And Streamlined exterior parts 13 and 14 covering the ultrasonic transducer
The outer diameter was 15 mm. The insides of the exterior parts 13 and 14 were filled with rubber, silicone resin, or the like so as to attenuate ultrasonic waves. Further, the narrowed portions 9 and 10 are wide enough to prevent the flow disturbance due to the exterior portion from occurring. Reference numerals 15 and 16 denote signal lines for driving the ultrasonic transducer, which are arranged so as to be coaxial so that the fluid flow is not disturbed. The drive frequency of the ultrasonic transducer used was 100 to 500 kHz.

When smoke was introduced into the fluid with this configuration and the flow behavior was visually evaluated, no vortex or stagnation point was found over a wide flow rate range. Therefore, an ultrasonic flowmeter capable of measuring the flow rate with good reproducibility was realized.

FIG. 2 shows an ultrasonic flowmeter 17 according to the second embodiment of the present invention. Although it has the same configuration as the ultrasonic flowmeter shown in FIG. 1, the insides 17 and 18 of the exterior parts 13 and 14 of the ultrasonic transducer are formed in a parabolic shape so that the transmitted ultrasonic waves are reflected. . The curvature was adjusted so that the reflected ultrasonic waves converge on the back surface portions 19 and 20 of the transducer.

The vibrators 11 and 1 inside the exterior parts 13 and 14
The back surface of No. 2 was filled with rubber, silicone resin, glass wool, etc. so that the reflected and converged ultrasonic waves were well attenuated. With this configuration, the ultrasonic waves once reaching the back surface of the vibrator are strongly absorbed. Therefore, an ultrasonic flowmeter with few reverberation waves was realized.

FIG. 3 shows an ultrasonic flowmeter 21 according to the third embodiment of the present invention. It has the same configuration as the ultrasonic flowmeter shown in FIG. 2, but the streamlined ultrasonic transmission films 22 and 23 are provided on the front surface of the ultrasonic transducer, and the exterior portions 13 and 14 are further disturbed with respect to the flow. It is configured so that it does not occur. That is, the ultrasonic transducers 11 and 12 were surrounded by a streamlined cover. The ultrasonic transmission films 22 and 23 are composed of screen mesh used for thick film printing. The mesh numbers of the screen mesh used are # 8.6 to # 35.
It was 0. The driving frequency of the ultrasonic transducer used is 10
It was 0 to 500 kHz.

When the ultrasonic wave propagation characteristics of the ultrasonic flowmeter 21 having such a structure were evaluated, the wire diameter of the screen meshes forming the ultrasonic wave transmission films 22 and 23 was found to be as follows.
If the wavelength of the ultrasonic wave to be used is large, the propagation loss was a little large and the propagation loss was a little large, but if the wire diameter is (1/3) or less of the wavelength of the ultrasonic wave to be used, the transmittance of the ultrasonic wave is 80% or more. Became. The porosity of the screen mesh (light transmission area per unit area) showed a substantially constant value regardless of the mesh number and was about 40%. The transmittance of ultrasonic waves increased to 80% or more because the thickness of the screen mesh used was very thin compared to the wavelength of ultrasonic waves. However, it is thought that it became larger than that of light. Also,
Smoke was introduced into the flow path 8 and the effect on the flow path was visually evaluated to find that the wire diameter used for the screen mesh was the flow path 8
If the wire diameter is less than (1/200), that is, the wire diameter is 0.1
If it is less than mm, almost no influence on the flow such as generation of vortex or stagnation point in the fluid is recognized. In addition,
Nylon and SUS were used as the material of the screen mesh, but no difference was observed. It is considered that SUS is more suitable for this kind of purpose from the viewpoint of moisture resistance and durability.

As described above, the ultrasonic wave permeable film 22 made of a screen mesh having a wire diameter of about 1/200 or less of the flow path diameter,
In 23, by covering the front surfaces of the ultrasonic transducers 11 and 12, the fluid flowing in the flow path 8 was able to realize a smooth flow without generation of irregular vortices or stagnation points. Further, by using a screen mesh having a wire diameter of ⅓ or less of the wavelength of ultrasonic waves, there was little reflection and good ultrasonic transmission characteristics were obtained. Therefore, a highly accurate ultrasonic flowmeter can be realized in a wide range of flow rate.

Further, the ultrasonic flowmeter 21 described above is used.
In, the screen meshes forming the ultrasonic transmission films 22 and 23 are made of SUS, and the exterior parts 13 and 14
Was made of a conductive material such as metal, and the ultrasonic transmission film and the exterior part were electrically connected. With this configuration, the ultrasonic transducers 11 and 12 are electrically shielded,
The configuration is strong against electromagnetic noise. As a result, the electrical noise was greatly reduced and the ultrasonic flowmeter was highly stabilized. In the normal case, the transmitting side of the ultrasonic transducer is driven by high voltage, high frequency pulses. Therefore, it is difficult to measure a minute flow rate with high accuracy such that electric noise as an electromagnetic wave easily enters the vibrator on the receiving side and the measurement circuit becomes complicated. However, as in the present invention, low noise can be realized and a highly accurate ultrasonic flowmeter can be realized by adopting a configuration in which the ultrasonic transducer that transmits and receives is electrically shielded.

FIG. 4 shows an ultrasonic flowmeter 24 according to a fourth embodiment of the present invention. Although having the same configuration as the ultrasonic flowmeter shown in FIG. 3, a cylindrical shape for improving the directivity of ultrasonic waves is provided inside the ultrasonic transmission films 22 and 23 integrated with the streamlined exterior parts 13 and 14. Or a conical horn 25,
26 was provided on the front surface of the ultrasonic transducer. With this configuration, the ultrasonic waves transmitted from the ultrasonic transducers 11 and 12 are
The propagation direction is restricted by the horns 25 and 26, and the horn 25 travels straight, so that the propagation is performed without spreading to the pipe wall of the flow path 8.

Therefore, since the ultrasonic waves reflected by the tube wall of the flow channel 8 and delayed by the ultrasonic waves do not reach the vibrator for receiving the ultrasonic waves, the propagation time of the ultrasonic waves can be measured with high accuracy. Also,
The ultrasonic waves that have passed the vicinity of the ultrasonic transducers 11 and 12 are reflected by the paraboloids inside the exterior portions 13 and 14 of the ultrasonic transducers 11 and 12 and converge on the back surfaces of the ultrasonic transducers 11 and 12. Then
It is absorbed by absorbent materials such as rubber, silicone resin and glass wool. Therefore, a highly accurate ultrasonic flowmeter with few reverberation waves can be realized.

FIG. 5 shows an ultrasonic flowmeter 27 according to the fifth embodiment of the present invention. It has the same configuration as the ultrasonic flowmeter shown in FIG. 3, but a cylindrical ultrasonic transmission film 28 is provided inside the channel 8 and the surface roughness of the wavelength of ultrasonic waves using the inner wall surface of the channel 8 is set. It was coarse so that

That is, the driving frequency of ultrasonic waves used in this type of ultrasonic flowmeter is 100 to 500 kHz, and the wavelength thereof is about 0.7 to 3.4 mm. Therefore, the amplitude of the surface roughness was adjusted to about 0.5 to 2 mm.

The cylindrical ultrasonic transmission film 28 is
The screen mesh is made of nylon, SUS or the like described in the above embodiment. With this configuration, the ultrasonic waves transmitted from the ultrasonic transducer pass through the cylindrical ultrasonic transmission film 28, reach the inner wall surface of the flow path 8, and are scattered or attenuated by the inner wall surface. Therefore, in the transducer that receives ultrasonic waves,
Since ultrasonic waves reflected from the tube wall of the flow path 8 and delayed in time do not arrive, the propagation time of ultrasonic waves can be measured with high accuracy.
Further, the fluid flowing in the flow path 8 flows along the tubular ultrasonic transmission film 28 provided in the flow path 8. Therefore, the surface roughness of the inner wall surface of the flow path 8 did not affect the flow of the fluid, and neither irregular vortex nor stagnation point was observed. Therefore, a highly accurate ultrasonic flowmeter with few reflected waves can be realized.

FIG. 6 shows an ultrasonic flowmeter 29 according to the sixth embodiment of the present invention. 5 has the same configuration as the ultrasonic flowmeter shown in FIG. 5, but the ultrasonic transmission used on the front surface of the ultrasonic transducers 11 and 12 is upstream and downstream of the ultrasonic transducers 11 and 12 in the flow path 8. Fluid permeable membranes 29 and 30 having finer mesh than the cylindrical ultrasonic permeable membrane 28 used on the inner surfaces of the membranes 22 and 23 and the channel 8 were provided. For example, in the ultrasonic wave transmissive films 22 and 23 and the cylindrical ultrasonic wave transmissive film 28,
When the screen mesh of mesh number # 200 is used, the fluid permeable membranes 29 and 30 have mesh number # 250.
The above screen mesh will be used. With this configuration, finer screen meshes provided on the upstream and downstream sides of the ultrasonic transducer perform dustproof and drip-proof functions.
Moreover, dust and water drops do not adhere to the cylindrical ultrasonic wave transmission film 28. Therefore, a highly reliable ultrasonic flowmeter can be realized. For this type of purpose, moisture resistance,
In terms of durability, SUS screen mesh is more suitable than nylon screen mesh.

[0037]

As is apparent from the above description, the ultrasonic flowmeter of the present invention has the following effects.

(1) It is possible to provide a highly accurate ultrasonic flowmeter capable of stably measuring the flow rate by eliminating generation of irregular vortices and stagnation points in the fluid.

(2) Generation of irregular vortices and stagnation points in the fluid is eliminated, and ultrasonic waves passing near the oscillator are reflected by the parabolic reflection surface and are therefore reflected. It is possible to provide an ultrasonic flowmeter that can converge an ultrasonic wave to one point, easily absorb and attenuate it, reduce reverberation waves, and perform highly accurate flow rate measurement.

(3) Since the fluid flowing through the flow path flows along the streamlined ultrasonic wave permeable membrane on the front surface of the oscillator, the occurrence of irregular vortices and stagnation points in the fluid is eliminated. In addition, the ultrasonic waves are efficiently transmitted through the ultrasonic transmission film composed of the screen mesh. In addition, ultrasonic waves that pass through the ultrasonic transmission film and pass through the vicinity of the vibrator are reflected by the parabolic reflection surface on the inner surface of the exterior part, converge to a single point, and can be easily absorbed and attenuated. Reverberation waves can be reduced. Therefore, it is possible to provide an ultrasonic flowmeter that can measure the flow rate with higher accuracy.

(4) By grounding the conductive exterior part and the conductive ultrasonic wave permeable film, it is possible to provide an ultrasonic flowmeter that is resistant to electromagnetic noise and can measure flow rate with high accuracy.

(5) The directivity of the ultrasonic waves transmitted in the channel is improved by the horn, and the reflection on the tube wall of the channel is greatly reduced. Further, since the horn is formed inside the ultrasonic wave transmitting film on the front surface of the vibrator, it does not disturb the fluid flow. Therefore, it is possible to provide an ultrasonic flowmeter that can measure the flow rate with higher accuracy.

(6) The ultrasonic waves transmitted from the vibrator pass through the inner cylinder made of an ultrasonic wave transmitting film provided on the inner surface of the tubular flow path and directly reach the tube wall of the rough tubular tube flow path. , It is largely scattered or absorbed because it reflects. Therefore, it is possible to provide an ultrasonic flowmeter capable of highly accurate flow rate measurement with less time-delayed reverberation waves.

(7) Dust, water droplets, etc. will not adhere to the coarser ultrasonic wave transmission film provided in the flow path, and the durability and dust resistance of the ultrasonic wave transmission film will be greatly improved and reliability will be improved. It is highly likely. It is possible to provide an ultrasonic flowmeter that is highly durable and capable of highly accurate flow rate measurement.

[Brief description of drawings]

FIG. 1 is a sectional view of an ultrasonic flowmeter according to a first embodiment of the present invention.

FIG. 2 is a sectional view of an ultrasonic flowmeter according to a second embodiment of the present invention.

FIG. 3 is a sectional view of an ultrasonic flowmeter according to a third embodiment of the present invention.

FIG. 4 is a sectional view of an ultrasonic flowmeter according to a fourth embodiment of the present invention.

FIG. 5 is a sectional view of an ultrasonic flowmeter according to a fifth embodiment of the present invention.

FIG. 6 is a sectional view of an ultrasonic flowmeter according to a sixth embodiment of the present invention.

FIG. 7 is a sectional view of a conventional ultrasonic flowmeter.

[Explanation of symbols]

 6 Ultrasonic flowmeter 8 Flow path 11, 12 Ultrasonic transducer 13, 14 Exterior part

Claims (7)

[Claims] 1. An ultrasonic flowmeter comprising a tubular flow path through which a fluid flows, and a pair of ultrasonic transducers covered by an exterior part, which are axially symmetrically opposed to each other upstream and downstream of the flow path. . 2. A tubular flow path through which a fluid flows, and parabolic ultrasonic reflectors on the inner surface upstream and downstream of the flow path. An ultrasonic flowmeter equipped with ultrasonic transducers facing each other in axial symmetry. 3. A tubular flow path through which a fluid flows, and parabolic ultrasonic reflectors on the inner surface upstream and downstream of the flow path, the outer surface being covered with an exterior part, and An ultrasonic flowmeter equipped with a pair of ultrasonic transducers having an ultrasonic transmitting film on the front surface, which are axially symmetrical to each other. 4. A tubular flow path through which a fluid flows, and parabolic ultrasonic reflectors on the inner surface upstream and downstream of the flow path, the outer surface of which is covered with a conductive exterior part. And an ultrasonic flowmeter, which is provided with a pair of ultrasonic transducers having a conductive ultrasonic transmission film on the front surface thereof so as to be axially symmetrical to each other. 5. A tubular flow path through which a fluid flows, and parabolic ultrasonic reflectors on its inner surface upstream and downstream of the flow path, and its outer surface covered with an exterior part, and An ultrasonic flowmeter having an ultrasonic wave transmissive film on its front surface, and a pair of ultrasonic wave oscillators having directional horns facing each other axially symmetrically in the transmissive film. 6. A tubular flow channel in which a fluid flows, the inner wall surface of which is rough, and an inner cylinder made of an ultrasonic wave permeable membrane are provided in the tubular flow channel. A pair of ultrasonic transducers having a parabolic ultrasonic reflector on the inner surface, an outer surface covered with an exterior portion, and an ultrasonic transmission film on the front surface are provided upstream and downstream in the passage. An ultrasonic flow meter equipped symmetrically opposite. 7. A tubular flow channel in which a fluid flows, the inner wall surface of which is rough, and an inner cylinder made of an ultrasonic wave permeable film are provided in the tubular flow channel. A pair of ultrasonic transducers having a parabolic ultrasonic reflector on the inner surface, an outer surface covered with an exterior portion, and an ultrasonic transmission film on the front surface are provided upstream and downstream in the passage. An ultrasonic flowmeter, which is provided so as to be symmetrically opposed to each other, and includes a fluid permeable film having a finer mesh than the ultrasonic permeable film upstream and downstream of the pair of ultrasonic transducers.