Abstrict A wedge unit according to the present invention is used for an
ultrasonic Doppler flow meter, being mounted on the outer wall of
a pipe in which a fluid flows, supplying an ultrasonic wave to the
fluid, receives the reflected wave and supplies the reflected wave
to a flow rate calculation unit, comprises a wedge with one surface
thereof being mounted on a part of the outer circumference of the
pipe and on another surface thereof being equipped with an ultrasonic
oscillator that generates the ultrasonic wave in response to an
electric signal and receives the reflected wave; and an ultrasonic
wave attenuation unit being mounted on the outer circumference of
the pipe so as to include a position where an ultrasonic wave injected
from the ultrasonic oscillator into the pipe by way of the wedge
first reaches the outer wall of the pipe after being reflected by
the inner wall thereof.
Claims What is claimed is:
1. A wedge unit used for an ultrasonic Doppler flow meter, being
mounted on the outer wall of a pipe in which a fluid flows, supplying
an ultrasonic wave to the fluid, receives the reflected wave and
supplies the reflected wave to a flow rate calculation unit, comprising:
a wedge with one surface thereof being mounted on a part of the
outer circumference of the pipe and on another surface thereof being
equipped with an ultrasonic oscillator that generates the ultrasonic
wave in response to an electric signal and receives the reflected
wave; and an ultrasonic wave attenuation unit being mounted on the
outer circumference of the pipe; wherein the ultrasonic wave attenuation
unit is configured to be in contact with a position where an ultrasonic
wave injected by the ultrasonic oscillator into the pipe by way
of the wedge first reaches the outer wall of the pipe after being
reflected by the inner wall of the pipe.
2. The wedge unit according to claim 1 wherein said ultrasonic
wave attenuation unit is composed of a material having a smaller
acoustic impedance compared to the material of said pipe.
3. The wedge unit according to claim 1 wherein said ultrasonic
wave attenuation unit is composed of tungsten elastomer.
4. The wedge unit according to claim 1 wherein a part of said
ultrasonic wave attenuation unit contacting with said pipe is featured
with the same radius as said pipe.
5. A wedge unit used for an ultrasonic Doppler flow meter, being
mounted on the outer wall of a pipe in which a fluid flows, supplying
an ultrasonic wave to the fluid, receives the reflected wave and
supplies the reflected wave to a flow rate calculation unit, comprising:
a wedge with one surface thereof being mounted on a part of the
outer circumference of the pipe and on another surface thereof being
equipped with an ultrasonic oscillator that generates the ultrasonic
wave in response to an electric signal and receives the reflected
wave; and an ultrasonic wave transmission unit having an acoustic
impedance approximately the same as the pipe and being mounted on
the outer circumference of the pipe; wherein the ultrasonic wave
transmission unit is configured to be in contact with a position
where an ultrasonic wave injected from the ultrasonic oscillator
into the pipe by way of the wedge first reaches the outer wall of
the pipe after being reflected by the inner wall of the pipe.
6. The wedge unit according to claim 5 wherein a part of said
ultrasonic transmission unit contacting with said pipe is featured
with the same radius as said pipe.
7. The wedge unit according to claim 5 wherein said ultrasonic
wave transmission unit is configured by a feature on the outer surfaces
thereof for causing an ultrasonic wave to be diffused.
8. A wedge used for an ultrasonic Doppler flow meter, being mounted
on the outer wall of a pipe in which a fluid flows, supplying an
ultrasonic wave to the fluid, receives the reflected wave and supplies
the reflected wave to a flow rate calculation unit, wherein one
surface is mounted on a part of the outer circumference of the pipe
and another surface is equipped with an ultrasonic oscillator that
generates the ultrasonic wave in response to an electric signal
and receives the reflected wave, and the diameter of the ultrasonic
oscillator is defined so that the projected size of the ultrasonic
beam emitted by the ultrasonic oscillator impressed on the outer
wall of the pipe, which is dependent on the inclination angle of
another surface of the wedge being mounted by the ultrasonic oscillator,
does not exceed the difference between a position where the ultrasonic
wave is injected from the outer wall of the pipe and a position
where the ultrasonic wave first reaches the outer wall of the pipe
after being reflected by the inner wall thereof.
9. The wedge according to claim 8 wherein the diameter of said
ultrasonic oscillator is defined so that said projected size is
equal, or approximately equal, to said difference.
10. The wedge according to claim 8 wherein the diameter, D, of
said ultrasonic oscillator is defined so as to satisfy the following
conditional equation, where t is the thickness of said pipe, .theta.p
is a propagation angle of ultrasonic wave in a pipe and .theta.w
is an inclination angle of wedge: D.ltoreq.2t*tan .theta.p*cos .theta.w
11. The wedge according to claim 8 wherein the diameter, D, of
said ultrasonic oscillator is defined so as to satisfy, or satisfy
approximately, the following equation, where t is the thickness
of said pipe, .theta.p is a propagation angle of ultrasonic wave
in a pipe and .theta.w is an inclination angle of wedge: D=2t*tan
.theta.p*cos .theta.w
12. A wedge unit used for an ultrasonic Doppler flow meter, being
mounted on the outer wall of a pipe in which a fluid flows, supplying
an ultrasonic wave to the fluid, receives the reflected wave and
supplies the reflected wave to a flow rate calculation unit, comprising:
a wedge with one surface thereof being mounted on a part of the
outer circumference of the pipe and on another surface thereof being
equipped with an ultrasonic oscillator that generates the ultrasonic
wave by using an electric signal and receives the reflected wave;
and an ultrasonic wave attenuation member for attenuating an ultrasonic
wave component adding a noise to an ultrasonic echo signal, wherein
the diameter of the ultrasonic oscillator is defined so that the
projected size of the ultrasonic beam emitted by the ultrasonic
oscillator impressed on the outer wall of the pipe, depending on
the inclination angle of another surface of the wedge being equipped
by the ultrasonic oscillator does not exceed the difference between
a position where the ultrasonic wave is injected from the outer
wall of the pipe and a position where the ultrasonic wave first
reaches at the outer wall of the pipe after being reflected by the
inner wall thereof, and an ultrasonic wave attenuation member is
mounted on the outer circumference of the pipe so as to avoid the
projection of an ultrasonic beam by the ultrasonic oscillator.
13. The wedge unit according to claim 12 wherein said ultrasonic
wave attenuation member is mounted on the outer circumference of
said pipe so as to further include a position where an ultrasonic
wave from the ultrasonic oscillator first reaches the outer wall
of the pipe after being reflected by the inner wall thereof.
14. The wedge unit according to claim 12 wherein the diameter
of said ultrasonic oscillator is defined so that said projected
size is equal, or approximately equal, to said difference.
15. The wedge unit according to claim 12 wherein the acoustic
impedance of said ultrasonic wave attenuation member is smaller
than that of said pipe.
16. The wedge unit according to claim 12 wherein said ultrasonic
wave attenuation member is made of tungsten elastomer.
17. A wedge unit used for an ultrasonic Doppler flow meter, being
mounted on the outer wall of a pipe in which a fluid flows, supplying
an ultrasonic wave to the fluid, receives the reflected wave and
supplies the reflected wave to a flow rate calculation unit, comprising:
a wedge with one surface thereof being mounted on a part of the
outer circumference of the pipe and on another surface thereof being
equipped with an ultrasonic oscillator that generates the ultrasonic
wave in response to an electric signal and receives the reflected
wave; and first and second beam diameter limitation units for limiting
an ultrasonic beam diameter emitted by the ultrasonic oscillator
and being mounted on the bottom surface of the wedge, wherein at
least one of the first and second beam diameter limitation units
doubles as an ultrasonic wave attenuation member for attenuating
an ultrasonic wave component adding noise to an ultrasonic echo
signal.
18. The wedge unit according to claim 17 wherein said beam diameter
limitation unit or said ultrasonic wave attenuation member is mounted
for limiting a beam diameter so that the projected size of the beam
impressed on the outer wall of the pipe does not exceed the difference
between a position where any of the beam element (the ultrasonic
wave) is injected from the outer wall of pipe and a position where
the beam element first reaches the outer wall of the pipe after
being reflected by the inner wall thereof.
19. The wedge according to claim 18 wherein said beam diameter
limitation unit or said ultrasonic wave attenuation member is mounted
for limiting the diameter, D, of said ultrasonic oscillator so as
to satisfy the following conditional equation, where t is the thickness
of a pipe, .theta.p is a propagation angle of ultrasonic wave in
the pipe and .theta.w is an inclination angle of wedge: D.ltoreq.2t*tan
.theta.p*cos .theta.w
20. The wedge according to claim 17 wherein said beam diameter
limitation unit has a smaller acoustic impedance than the wedge
material and is a slit which is composed of a gaseous body such
as air.
21. The wedge according to claim 17 wherein said beam diameter
limitation unit is a slit being composed of a material for absorbing
and/or attenuating an ultrasonic wave.
22. The wedge according to claim 21 wherein said material for
absorbing and/or attenuating an ultrasonic wave is tungsten elastomer.
23. The wedge according to claim 17 wherein said beam diameter
limitation unit is an ultrasonic wave reflection member being composed
of a material having larger acoustic impedance than the wedge material.
24. The wedge according to claim 23 wherein said ultrasonic wave
reflection member is made of a metallic material such as stainless
steel or aluminum.
25. A wedge used for an ultrasonic Doppler flow meter, being mounted
on the outer wall of a pipe in which a fluid flows, supplying an
ultrasonic wave to the fluid, receives the reflected wave and supplies
the reflected wave to a flow rate calculation unit, wherein the
wedge on one surface thereof is mounted on a part of the outer circumference
of the pipe and on another surface thereof is equipped with an ultrasonic
oscillator for generating the ultrasonic wave in response to an
electric signal and receiving the reflected signal, and is equipped
by a beam diameter limitation unit for limiting the ultrasonic beam
diameter emitted from the ultrasonic oscillator inside the wedge.
26. The wedge according to claim 25 wherein said beam diameter
limitation unit is mounted inside the wedge so that the projected
size of said limited beam diameter impressed on the outer wall of
the pipe does not exceed the difference between a position where
any of the beam element (the ultrasonic wave) is injected from the
outer wall of pipe and a position where the beam element first reaches
the outer wall of the pipe after being reflected by the inner wall
thereof.
27. A wedge unit used for an ultrasonic Doppler flow meter, being
mounted on the outer wall of a pipe in which a fluid flows, supplying
an ultrasonic wave to the fluid, receives the reflected wave and
supplies the reflected wave to a flow rate calculation unit, comprising:
a wedge with one surface thereof being mounted on a part of the
outer circumference of the pipe and on another surface thereof being
equipped with an ultrasonic oscillator that generates the ultrasonic
wave in response to an electric signal and receives the reflected
wave, and additionally inside thereof being equipped by a beam diameter
limitation unit for limiting an ultrasonic wave beam diameter emitted
by the ultrasonic oscillator; and an ultrasonic wave attenuation
member for attenuating an ultrasonic wave component adding noise
to an ultrasonic echo signal.
28. The wedge unit according to claim 27 wherein said beam diameter
limitation unit is mounted inside the wedge so that the projected
size of said limited beam diameter impressed on the outer wall of
a pipe does not exceed the difference between a position where any
of the beam is injected from the outer wall of the pipe and a position
where the beam first reaches at the outer wall of the pipe after
being reflected by the inner wall thereof.
29. The wedge unit according to claim 27 wherein said ultrasonic
wave attenuation member is mounted on the outer circumference of
said pipe so as to avoid a position where an ultrasonic wave emitted
from said ultrasonic oscillator first reaches the outer wall of
the pipe.
30. The wedge unit according to claim 27 wherein said ultrasonic
wave attenuation member is further mounted on the outer circumference
of said pipe so as to include a position where an ultrasonic beam
reaches the outer wall of the pipe after being reflected by the
inner wall thereof.
31. The wedge unit according to claim 27 wherein said beam diameter
limitation unit is mounted inside the wedge for limiting the diameter,
D, of said ultrasonic beam so as to satisfy the following conditional
equation, where t is a thickness of a pipe, .theta.p is a propagation
angle of ultrasonic wave in the pipe and .theta.w is an inclination
angle of the wedge: D.ltoreq.2t*tan .theta.p*cos .theta.w
32. The wedge unit according to claim 27 wherein said beam diameter
limitation unit has a smaller acoustic impedance than the wedge
material and is a slit which is configured by a gaseous body such
as air.
33. The wedge unit according to claim 27 wherein said beam diameter
limitation unit is a slit being configured by a material for absorbing
and/or attenuating an ultrasonic wave.
34. The wedge unit according to claim 33 wherein said material
for absorbing and/or attenuating an ultrasonic wave is tungsten
elastomer.
35. The wedge unit according to claim 27 wherein said beam diameter
limitation unit is an ultrasonic wave reflection member being configured
by a material having larger acoustic impedance than the wedge material.
36. The wedge unit according to claim 35 wherein said ultrasonic
wave reflection member is made of a metallic material such as stainless
steel or aluminum.
37. A wedge unit used for an ultrasonic Doppler flow meter, being
mounted on the outer wall of a pipe in which a fluid flows, supplying
an ultrasonic wave to the fluid, receives the reflected wave and
supplies the reflected wave to a flow rate calculation unit, comprising:
a wedge with one surface thereof being mounted on a part of the
outer circumference of the pipe and on another surface thereof being
equipped with an ultrasonic oscillator that generates the ultrasonic
wave in response to an electric signal and receives the reflected
wave; and a spacer being installed between the wedge and the pipe.
38. The wedge unit according to claim 37 wherein the thickness
of said spacer is defined so that a size of the projection of the
ultrasonic beam, which is dependent on the inclination angle of
another surface of said wedge being equipped by said ultrasonic
oscillator, impressed on a contact surface of the spacer, does not
exceed the difference between a position where the ultrasonic wave
enters from the contact surface and a position where the ultrasonic
wave first reaches the contact surface after being reflected by
the inner wall of said pipe.
39. The wedge unit according to claim 38 wherein the thickness
of said spacer is defined so that said projected size is equal,
or approximately equal, to said difference.
40. The wedge unit according to claim 37 wherein said spacer is
constituted of a material having the same, or approximately the
same, acoustic impedance as the pipe material.
41. The wedge unit according to claim 40 wherein said spacer thickness
is defined so as to satisfy the following conditional equation,
where tp is the thickness of said pipe, ts is the thickness of said
spacer, .theta.p is the propagation angle of the ultrasonic wave
in the pipe or spacer, .theta.w is the inclination angle of wedge
and D is the diameter of the ultrasonic oscillator: D/(2*tan .theta.p*cos
.theta.w)-tp.ltoreq.ts
42. The wedge unit according to claim 40 wherein said spacer thickness
is defined so as to satisfy, or approximately satisfy, the following
conditional equation, where tp is the thickness of said pipe, ts
is the thickness of said spacer, .theta.p is the propagation angle
of ultrasonic wave in the pipe or spacer, .theta.w is the inclination
angle of wedge and D is the diameter of the ultrasonic oscillator:
D/(2*tan .theta.p*cos .theta.w)-tp=ts
43. The wedge unit according to claim 37 wherein said spacer is
extended in the propagating direction of said ultrasonic wave, an
ultrasonic wave attenuation member is further comprised for attenuating
an ultrasonic wave component adding noise to an ultrasonic echo
signal, and the ultrasonic wave attenuation member is mounted onto
the spacer.
44. The wedge unit according to claim 43 wherein said ultrasonic
wave attenuation member is mounted on said spacer so as to avoid
a projection, which is dependent on the inclination angle of another
surface of the wedge being equipped by said ultrasonic oscillator,
impressed on a contact surface of the spacer with the wedge by an
ultrasonic wave emitted from the ultrasonic oscillator.
45. The wedge unit according to claim 44 wherein an ultrasonic
wave attenuation member is mounted on said spacer so as to include
a position where an ultrasonic wave first reaches the contact surface
of said spacer after being reflected by the inner wall of pipe.
46. The wedge unit according to claim 43 wherein said ultrasonic
wave attenuation member has a smaller acoustic impedance than the
pipe material.
47. The wedge unit in claim 43 wherein said ultrasonic wave attenuation
member is tungsten elastomer.
48. The wedge unit in claim 43 wherein said ultrasonic wave attenuation
member is composed of a material having the same or approximately
the same impedance as the pipe material and also features corrugated
surfaces where being exposed to the air.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wedge and a wedge unit for use
in an ultrasonic Doppler flow meter which is mounted (i.e., clamped)
on the outer wall of a pipe having a fluid flowing inside thereof,
supplying an ultrasonic wave (ultrasound) to the fluid, receives
the reflected wave and supplies the reflected wave to a flow rate
calculation unit.
2. Description of the Related Art
One of a conventional ultrasonic Doppler flow velocity profile
meter is a clamp-on ultrasonic flow meter. The clamp-on ultrasonic
flow meter is for measuring a flow rate of a flowing fluid inside
a pipe by mounting (i.e., clamping) a material for propagating a
wave into the pipe, i.e., a wedge, on a part of the outer circumference
of the pipe and emitting the wave into the pipe by way of the wedge.
Let it be assumed herein that a fluid is flowing horizontally in
the pipe unless otherwise noted.
Clamp-on type ultrasonic flow meters include a propagation time
difference and a Doppler method types.
In a propagation time difference-method clamp-on type ultrasonic
flow meter, the ultrasonic wave is diagonally injected to the flowing
fluid and returned therefrom, thereby measuring the flow rate by
the difference in propagation time between the outward and homeward
propagations.
While in a Doppler-method clamp-on type ultrasonic flow meter,
the velocity of the fluid is measured (i.e., calculated) by that
of suspended particles, et cetera, based on the assumption that
suspended particles and air bubbles contained in a fluid flow at
the same velocity as the fluid. In the Doppler method, an attention
is focused on the fact that the frequency of an ultrasonic wave
injected into a fluid is changed by the Doppler Effect as a result
of being reflected by a suspended particle, and therefore the velocity
of the particle is measured by detecting the frequency of the reflected
ultrasonic wave.
FIG. 1 shows a configuration of a conventional Doppler-method clamp-on
type ultrasonic flow meter.
In FIG. 1 the Doppler-method clamp-on type ultrasonic flow meter
for example comprises a wedge 14 on one surface thereof being mounted
on a part of the outer circumference of a pipe 31 and on another
surface thereof being equipped with an ultrasonic oscillator 13
for generating an ultrasonic wave in response to an electric signal
and receiving the reflecting ultrasonic wave back from a fluid within
the pipe 31 a transmitter/receiver circuit 12 for generating a
pulsed electric signal and supplying the signal to the ultrasonic
oscillator 13 for driving it and a flow rate calculation unit (including
an amplifier 21 A/D converter 22 velocity profile measurement
unit 23 computer 24 and display unit 25).
The transmitter/receiver circuit 12 is, for example, comprised
of an oscillator and a pulse generation circuit. The oscillator
generates an electric signal having a basic frequency of f0 and
the pulse generation circuit outputs a pulsed electric signal at
a prescribed interval (i.e., 1/F rpf). The ultrasonic oscillator
13 generates an ultrasonic pulse by application of the pulsed electric
signal thereto. The ultrasonic pulse is then transmitted to the
pipe 31 by way of the wedge 14.
Note that the basic frequency f0 is essentially a required frequency
defined in inverse proportion to the inner diameter of the pipe
31. Also, the ultrasonic pulse is a beam of translatory movement
having a pulse width of approximately 5 mm for example.
Meanwhile, the surface of wedge 14 on which the ultrasonic oscillator
13 is mounted is inclined by a certain angle so that the normal
line to the surface crosses with the direction of the normal line
to the transverse section surface of the pipe 31 at an angle smaller
than 90 degrees.
Meanwhile, the ultrasonic oscillator 13 functions as receiver for
receiving the echo ultrasonic wave generated by an ultrasonic wave
emitted by the ultrasonic oscillator 13 being reflected from a reflecting
body suspended in a fluid 32 flowing in the pipe 31 in addition
to the function of transmitter.
Such reflecting bodies in the fluid 32 include an air bubble consistently
contained in a fluid, a particle such as aluminum particulate, a
foreign material having a different acoustic impedance from the
fluid subjected to measurement, et cetera.
An operation of the Doppler-method clamp-on type ultrasonic flow
meter shown in FIG. 1 is then described as follows.
First, an ultrasonic pulse is injected into the fluid 32 in the
pipe 31 from the ultrasonic oscillator 13 by application of a pulsed
electric signal thereto, the ultrasonic pulse is reflected by a
reflecting body suspended in the fluid 32 is received by the ultrasonic
oscillator 13 as an ultrasonic echo, and then converted into an
echo electric signal.
The echo electric signal is amplified by the amplifier 21 and
the amplified echo electric signal is digitized by the A/D converter
22.
The digitized echo electric signal is then input to the velocity
profile measurement unit 23. While FIG. 1 does not delineate clearly,
the velocity profile measurement unit 23 receives an electric signal
having the basic frequency of f0 from the oscillator of the transmitter/receiver
circuit 12 measures velocity changes based on a Doppler shift according
to the frequency difference between an echo electric signal and
the electric signal having the basic frequency, calculates a velocity
profile along the line of measurement in the respective area; and
accordingly calculates a flow velocity profile across the transverse
section of the pipe 31 by modifying the flow velocity profile calculated
for the measurement area with the angle of the above described inclination.
The configuration of the flow rate calculation unit and the transmitter/receiver
circuit shown in FIG. 1 is an example, and other types (of transmitter/receiver
and calculation unit) are applicable.
DISCLOSURE OF THE INVENTION
The problem in relation to flow velocity profile measurement technique
by a conventional Doppler-method clamp-on type ultrasonic flow profile
meter is then described below. Before going into a detail, the problem
is summarized as follows. That is, the ultrasonic echo used for
measuring flow rate or a velocity profile is accompanied by acoustic
noise caused by multiple reflections.
Since the identification of the problem plays a major role in the
approach to the present invention, the above described problem will
be further described below.
The operating principle of an ultrasonic Doppler flow meter is
described in reference to FIG. 2.
As shown in the top of FIG. 2 an ultrasonic pulse emitted by the
ultrasonic oscillator 13 is injected into the pipe 31 in a manner
merging with the flow direction of the fluid 32 being subjected
to measurement with an angle of .alpha. relative to the vertical.
The ultrasonic pulse is met by a reflecting body consistently suspended
in the fluid for instance and is reflected thereby, transforms itself
to an ultrasonic echo, comes back the opposite way, and is received
by the ultrasonic oscillator 13 and is then converted to an echo
electric signal.
The second part of FIG. 2 shows the output, for example, of the
A/D converter 22 shown in FIG. 1. In the second part of FIG. 2
the part indicated by the sign "a" corresponds to an ultrasonic
echo being reflected by a reflecting body, "b" corresponds
to a multiple reflection echo reflected by the pipe wall on the
side where the ultrasonic pulse is emitted, and, "c" corresponds
to multiple reflection echoes being reflected by the pipe wall on
the side opposite the side where the ultrasonic pulse is injected
from. Those signal parts indicated by the signs "a," "b"
and "c" will be repeated in response to the ultrasonic
pulse emitted from the ultrasonic oscillator in the prescribed interval
(1/F rpf).
An A/D conversion process and a filtering process are further performed
to the echo electric signal shown in the second part of FIG. 2
and then a flow velocity profile along the measuring line is calculated
according to the Doppler shift method by the flow velocity profile
measurement unit. The Doppler shift method is a velocity measurement
method using the principle that the frequency of the above described
ultrasonic echo shifts in proportion to the flow velocity.
The third part of FIG. 2 (at the bottom) shows the output of the
flow velocity profile measurement unit, in which the horizontal
axis indicates the positions along the line of measurement, while
the vertical axis indicates the flow velocities corresponding to
the respective positions.
Following the flow velocity acquisition, a flow rate is calculated
using the following procedure. Such a method is disclosed in various
documents such as the Patent Document 1 noted below.
First, a flow rate, m, of a fluid at a time, t, is given by the
Equation (1) below, where .rho. is the viscosity of a fluid subjected
to measurement, and v(x, t) is a velocity component at the time,
t. m(t)=.rho..intg.v(x,t)dA (1)
From the equation (1), a flow rate, m, of a fluid flowing in the
pipe at a time, t, can be converted to the equation (2) below. m(t)=.rho..intg..intg.vx(r,.theta.,t)rdrd.theta.
(2)
Here, vx(r, .theta., t) indicates a velocity component along the
axis of the pipe (i.e., pipe axis direction) at a time, t, at a
distance, r, from the center of the transverse section of the pipe,
and in a direction of the angle, .theta., viewed from a certain
direction going through the center of the transverse section.
[Patent Document 1] Japanese patent laid-open application publication
2000-97742 "Ultrasonic Doppler flow meter".
The above described procedure makes it necessary to detect a velocity
profile accurately to calculate the flow rate with accuracy. This
necessity is independent of the fluid subjected to measurement being
in a normal or abnormal condition.
Furthermore, since the flow velocity profole is obtained by signal
processing of an ultrasonic echo returning from a reflecting body,
ideally the ultrasonic echo should contain the required (preferable)
acoustic signal alone.
In such a Doppler-method clamp-on type ultrasonic flow profile
meter, however, because the acoustic impedance of a pipe (i.e.,
the material thereof) is larger than that of the fluid in the pipe,
an ultrasonic wave injected from the ultrasonic oscillator into
the pipe by way of the wedge is in large part reflected into the
pipe wall at the border between the pipe and the fluid, followed
by a multiple reflections within the pipe wall (i.e., between the
outer and inner walls of the pipe). The fact that the multiple reflections
are larger than the emission from the inner wall to the inside of
the pipe, results in the required ultrasonic echo being coupled
with a large amount of acoustic noise, thus causing error in the
flow rate determination.
The above described phenomenon is then elaborated in reference
to FIG. 3.
In FIG. 3 an ultrasonic wave emitted from the ultrasonic oscillator
13 is injected to the wedge 14 along the line of incidence 201
then into the pipe 31 along the incident line 202a, as far as the
inside wall of the pipe 31.
On the inside wall of the pipe 31 the ultrasonic wave branches
into one component ultrasonic wave penetrating the inside wall and
penetrating into the fluid along the incident line 202b, and another
component reflecting against the inside wall of the pipe towards
the outside wall of the pipe along a sidetrack 203.
The ultrasonic wave reaching the outer wall is reflected thereby
in almost its entirety and once again is directed toward the inside
wall along the sidetrack 204a, followed by similar branching into
one component ultrasonic wave penetrating into the fluid 32 along
the sidetrack 204b and another component ultrasonic wave being reflected
by the inside wall and directed toward the outside wall.
Each component ultrasonic wave, while going back and forth along
these lines (paths) as described above, will be received by the
ultrasonic oscillator 13 as an ultrasonic echo, and thereby a flow
velocity profile and the resultant flow rate are obtained.
That is, the ultrasonic echo going along the incident paths 202b,
202a, 201 back to the ultrasonic oscillator 13 and the echo going
along the sidetracks 204b, 204a, 203 202a, 201 back to the ultrasonic
oscillator 13. Among these, the ultrasonic echo going along the
incident lines 202b, 202a, 201 back to the ultrasonic oscillator
13 is called the "preferable ultrasonic echo."
The problem associated with FIG. 3 is that the preferable ultrasonic
echo is accompanied by the ultrasonic echoes going back to the ultrasonic
oscillator 13 along the sidetracks 204b, 204a, 203 202a, 201 for
example, as acoustic noise.
First, well known equations will be derived in order to describe
the above problem.
FIG. 4 describes the way a sound wave, in heading from a medium
1 to medium 2 either is reflected or penetrates at the interface
between the two media 1 and 2.
In FIG. 4 when a sound wave enters from the media 1 to 2 at an
incident angle of .theta.in from the vertical direction of the interface,
the relationship between the incident, reflected and transmitted
(penetrating) waves is given by the following equation (3) (based
on Snell's law).
.times..times..theta..times..times..times..times..theta..times..times..the-
ta. ##EQU00001##
Where in equation (3), c1 is the sound velocity in medium 1 c2
is the sound velocity in medium 2 .theta.in is the angle of incidence
in medium 1 .theta.out is the angle of incidence in medium 2 and
.theta.ref is the reflected angle in medium 1.
Meanwhile, the acoustic impedance Z1 and Z2 of the media 1 and
2 respectively, each is given by the equations (4) and (5). z.sub.1=.rho..sub.1c.sub.1
(4) z.sub.2=.rho..sub.2c.sub.2 (5)
In the equations (4) and (5), c1 is the sound velocity in medium
1 c2 is the sound velocity in medium 2 .rho.1 is the density of
medium 1 and .rho.2 is the density of medium 2.
In this case, the penetration and the reflection ratios of a sound
pressure wave are given by the equations (6) and (7), respectively.
.times..times..times..times..theta..times..times..times..times..times..tim-
es..theta..times..times..times..times..times..theta..times..times..times..-
theta..times..times..times..times..times..theta..times..times..times..time-
s..theta..times..times..times..times..times..theta. ##EQU00002##
The reflection ratio and the penetration ratio at the interface
of the pipe and the fluid in the pipe are obtained by applying these
equations to the pipe (material thereof) and the fluid.
FIG. 5 shows an example calculation in the case of using stainless
steel for the pipe material and water as the fluid flowing therein.
Stainless steel has a sound velocity of 3250 m/sec and a density
of 7.91*10.sup.3 kg/m.sup.3 while water has a sound velocity of
1490 m/sec and a density of 1.00*10.sup.3 kg/m.sup.3.
As shown in FIG. 5 given that the angle of incidence of the ultrasonic
wave from the pipe is 47.degree. (degree), a penetration ratio of
6% and a reflectance ratio of 94% are obtained by using equations
(6) and (7), making it apparent that most of the ultrasonic wave
is reflected within the pipe wall, leaving only a small fraction
thereof penetrating the water.
The penetration and reflectance ratios of the ultrasonic wave reflected
by the inner wall of the pipe can be likewise calculated.
Since the stainless steel contacts with air on the outer wall of
the pipe, which has a sound velocity of 344 m/sec and a density
of 1.293*10.sup.3 kg/m.sup.3 using equations (6) and (7), a penetration
ratio of 0.001% and a reflectance ratio of 99.999% are obtained.
That is, most of the ultrasonic wave is reflected inside the wall,
instead of being emitted into the air.
Again the same calculation for the ultrasonic wave reaching the
interface between the pipe (made of stainless steel) and a fluid
(water in this case) gives a ratio of sound pressure penetrating
into water of 5.4%, where the ratio is relative to the sound pressure
initially penetrating the pipe, which is considered to be 100%.
In order to show how the ultrasonic echo responding to an initial
penetrating wave, that is, the preferable ultrasonic wave, is actually
accompanied by an ultrasonic wave resulting from a reflected wave,
the wall thickness and inner diameter of a pipe have to be specified.
A thickness of 6 mm and inner diameter of 102 mm are assumed for
the pipe here.
The incident path (length) is calculated from the incident angle
(i.e., 47.degree. in this case), and a time of travel in a medium
is calculated by dividing by the sound velocity of the respective
medium (i.e., stainless steel or water in this case).
Comparing the corresponding positions along the sidetracks 204
band 202b of the inner wall, the ultrasonic echo wave occurring
in a certain position along sidetrack 204b is received by the ultrasonic
oscillator 13 later by a time corresponding to traveling (back and
forth) along sidetracks 203 and 204a as compared with the ultrasonic
echo wave occurring in the corresponding position along sidetrack
202b.
Therefore, a period of time in which the ultrasonic echoes occurring
in random positions along the sidetrack 204b are received by the
ultrasonic oscillator 13 continuously in terms of time is overlapped
by the delay of time which the ultrasonic wave or the ultrasonic
echo wave travels back and forth along sidetracks 203 and 204a,
with a period of time in which the ultrasonic echoes occurring in
random positions along the incident line 202b are received by the
ultrasonic oscillator 13 continuously in terms of time.
FIG. 6 shows how the ultrasonic echoes are overlapped and received
by the ultrasonic oscillator.
In FIG. 6 from the above described thickness and inner diameter
of the pipe and the angle of incidence, the distance of a return
trip along the sidetracks 203 and 204a is 12.2 mm*4=48.8 mm so that
the delay time due to the return trip is 15 micro sec, taking the
transverse wave velocity as 3250 m/sec for the stainless steel pipe.
The time for the ultrasonic wave to take a return trip in water
along the sidetracks such as 202b and 204b is 137 micro sec by taking
the sound velocity in water as 1490 m/sec. Therefore the overlapped
ultrasonic echo signals from the sidetracks 202b and 204b are overlapped
and received by the ultrasonic oscillator 13 for the duration X
shown by FIG. 6.
FIG. 7 describes how noise is generated as a result of echo signals
being overlapped.
In FIG. 7 the label I shows the flow velocity profile based on
the ultrasonic echo along the incident path 202b; the label II shows
the flow velocity profile based on the ultrasonic echo along the
sidetrack 204b; and the label III shows the flow velocity profile
as a result of overlapping the flow velocity profiles based on the
ultrasonic echoes along the incident and sidetrack paths. FIG. 7
makes it apparent that the flow velocity profile (i.e., as indicated
by III) as a measurement result is shifted from a preferable (required,
desired, actual, needed) flow velocity profile.
FIG. 8 is a cross section view of a wedge equipped conventional
Doppler-method clamp-on type ultrasonic flow meter together with
part of the pipe it is clamped to. This figure also shows a second
problem associated with the conventional technique.
In FIG. 8 a wedge 52 equipped with an ultrasonic oscillator 51
is clamped to a part of the outer wall of a pipe 53.
FIG. 8 corresponds to a case in which the thickness of the pipe
is small as compared to the diameter of the ultrasonic oscillator
(i.e., the ratio of the former to the latter is less than a prescribed
value). In this case, multiple reflections occur within the width
of the ultrasonic beam as shown in the figure. That is, an ultrasonic
beam entering from the outer wall of the pipe at the position P11
for instance reaches the position P12 where it gets overlapped with
another ultrasonic beam entering from the outer wall, thereby causing
multiple reflections.
The number of the incident lines (paths) used for measuring (calculating)
a flow velocity in a pipe is proliferated in accordance with the
number of multiple reflections. The ultrasonic echo signals traveling
along the resultant sidetracks being overlapped with the required
ultrasonic echo signal causes the problem of an error in calculating
the flow velocity profile or flow rate.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a wedge and a
wedge unit for use in an ultrasonic Doppler flow meter capable of
reducing acoustic noise.
A first wedge unit of the present invention is a wedge unit used
for an ultrasonic Doppler flow meter, being mounted on the outer
wall of a pipe in which a fluid flows, supplying an ultrasonic wave
to the fluid, receives the reflected wave and supplies the reflected
wave to a flow rate calculation unit, comprises a wedge with one
surface thereof being mounted on a part of the outer circumference
of the pipe and on another surface thereof being equipped with an
ultrasonic oscillator that generates the ultrasonic wave in response
to an electric signal and receives the reflected wave; and an ultrasonic
wave attenuation unit being mounted on the outer circumference of
the pipe so as to include a position where an ultrasonic wave injected
from the ultrasonic oscillator into the pipe by way of the wedge
first reaches the outer wall of pipe after being reflected by the
inner wall of the pipe.
Meanwhile, a wedge of the present invention is a wedge used for
an ultrasonic Doppler flow meter, being mounted on an outer wall
of a pipe in which a fluid flows, supplying an ultrasonic signal
to the fluid, receives the reflected wave and supplies the reflected
wave to a flow rate calculation unit, wherein one surface of the
wedge is mounted on a part of the outer circumference of the pipe
and another surface thereof is equipped with an ultrasonic oscillator
that generates the ultrasonic wave in response to an electric signal
and receives the reflected wave, and the diameter of the ultrasonic
oscillator is defined so that the projected size of the ultrasonic
wave emitted by the ultrasonic oscillator impressed against the
outer wall of the pipe, depending on the inclination angle of another
surface of the wedge being equipped with the ultrasonic oscillator,
does not exceed the difference between a position where the ultrasonic
wave is injected from the outer wall of the pipe and a position
where the ultrasonic wave first reaches the outer wall of the pipe
after being reflected by the inner wall thereof.
Meanwhile, a second wedge unit of the present invention is a wedge
unit used for an ultrasonic Doppler flow meter, being mounted on
the outer wall of a pipe in which a fluid flows, supplying an ultrasonic
wave to the fluid, receives the reflected wave and supplies the
reflected wave to a flow rate calculation unit, comprises a wedge
with one surface thereof being mounted on a part of the outer circumference
of the pipe and on another surface thereof being equipped by an
ultrasonic oscillator that generates the ultrasonic wave in response
to an electric signal and receives the reflected wave, and a spacer
being installed between the wedge and the pipe.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the configuration of a conventional Doppler-method
clamp-on type ultrasonic flow meter;
FIG. 2 shows the principle of operation of an ultrasonic Doppler
flow meter in which the first part shows how an ultrasonic pulse
is emitted into the pipe; the second part shows the output from
the A/D converter shown in FIG. 1; and the bottom part shows the
output of the flow velocity profile measurement unit shown in FIG.
1;
FIG. 3 describes an ultrasonic echo accompanied by noise in a conventional
example;
FIG. 4 describes the way a sound wave, in traveling from a medium
1 to medium 2 is either reflected or penetrates at the interface
between the two media 1 and 2;
FIG. 5 shows an example calculation in the case of a stainless
steel pipe and water flowing therein;
FIG. 6 shows how the ultrasonic echoes along various sidetracks
are overlapped and received by the ultrasonic oscillator in a conventional
example;
FIG. 7 shows how noise is generated as a result of the echo signals
being overlapped in a conventional example;
FIG. 8 is a cross section view of a conventional wedge equipped
Doppler-method clamp-on type ultrasonic flow meter together with
part of the pipe it is clamped to, and also explains a second problem
associated with the conventional technique;
FIG. 9 shows the configuration of a wedge unit for use in an ultrasonic
Doppler flow meter of the first embodiment according to the present
invention;
FIG. 10 is a transverse cross section view (No 1) from the right
of FIG. 9;
FIG. 11 is a transverse cross section view (No 2) from the right
of FIG. 9;
FIG. 12 shows a variation of the outer surface of a material in
the case of using the material transmitting an ultrasonic wave;
FIG. 13 shows a cross sectional view of a wedge unit for use in
an ultrasonic flow meter of the second embodiment according to the
present invention;
FIG. 14 shows how the diameter of an ultrasonic oscillator is determined;
FIG. 15 shows a cross sectional view of a wedge unit for use in
an ultrasonic flow meter of the third embodiment according to the
present invention;
FIG. 16 shows a cross sectional view of a wedge for use in an ultrasonic
flow meter of the fourth embodiment according to the present invention;
FIG. 17 shows a cross sectional view of a wedge unit for use in
an ultrasonic flow meter of the fifth embodiment according to the
present invention;
FIG. 18 shows how the thickness of a spacer is determined.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred embodiment for achieving the present invention is
described in detail while referring to the accompanying drawings
as follows.
A wedge unit of a first aspect of the present invention, is used
for an ultrasonic Doppler flow meter, being mounted on the outer
wall of a pipe in which a fluid flows, supplying an ultrasonic wave
to the fluid, receives the reflected wave and supplies the reflected
wave to a flow rate calculation unit, comprises a wedge with one
surface thereof being mounted on a part of the outer circumference
of the pipe and on another surface thereof being equipped with an
ultrasonic oscillator that generates the ultrasonic wave in response
to an electric signal and receives the reflected wave; and an ultrasonic
wave attenuation unit being mounted on the outer circumference of
the pipe so as to include a position where an ultrasonic wave emitted
by the ultrasonic oscillator into the pipe by way of the wedge first
reaches the outer wall of the pipe after being reflected by the
inner wall of the pipe.
Here, equipping an ultrasonic wave attenuation unit being mounted
on the outer circumference of the pipe so as to include a position
where the reflected ultrasonic wave first reaches the outer wall
of the pipe and thereby absorbs a part of the ultrasonic wave reaching
the outer wall of the pipe enables attenuation of echo signals causing
noise coming back along sidetracks caused by further reflections
from the outer wall of the pipe and being received by the ultrasonic
oscillator and reduction of acoustic noise.
A wedge unit of a second aspect of the present invention, is used
for an ultrasonic Doppler flow meter, being mounted on the outer
wall of a pipe in which a fluid flows, supplying an ultrasonic wave
to the fluid, receives the reflected wave and supplies the reflected
wave to a flow rate calculation unit, comprises a wedge with one
surface thereof being mounted on part of the outer circumference
of the pipe and on another surface thereof being equipped with an
ultrasonic oscillator that generates the ultrasonic wave in response
to an electric signal and receives the reflected wave; and an ultrasonic
wave transmission unit having an acoustic impedance approximately
the same as the pipe and being mounted on the outer circumference
of the pipe so as to include a position where an ultrasonic wave
emitted by the ultrasonic oscillator into the pipe by way of the
wedge first reaches the outer wall of the pipe after being reflected
by the inner wall thereof.
Here, mounting an ultrasonic wave transmission unit on the outer
circumference of the pipe so as to include a position where the
reflected ultrasonic wave first reaches the outer wall of the pipe
and thereby transmits the ultrasonic wave reaching the outer wall
of the pipe enables a reduction of echo signals returning to the
ultrasonic oscillator by diffusing ultrasonic waves transmitted
from the outer wall of the pipe as a result of further reflections
against surfaces of the ultrasonic wave transmission unit and a
reduction of acoustic noise.
A wedge of a third aspect of the present invention, is used for
an ultrasonic Doppler flow meter, being mounted on the outer wall
of a pipe in which a fluid flows, supplying an ultrasonic wave to
the fluid, receives the reflected wave, and supplies the reflected
wave to a flow rate calculation unit, wherein one surface is mounted
on a part of the outer circumference of the pipe and another surface
is equipped with an ultrasonic oscillator that generates the ultrasonic
wave by using an electric signal and receives the reflected wave,
and the diameter of the ultrasonic oscillator is defined so that
the projected size of the ultrasonic wave emitted by the ultrasonic
oscillator impressed against the outer wall of the pipe determined
by the inclination angle of another surface of the wedge being equipped
with the ultrasonic oscillator, does not exceed the difference between
a position where the ultrasonic wave is injected from the outer
wall of the pipe and a position where the ultrasonic wave first
reaches the outer wall of the pipe after being reflected by the
inner wall thereof.
Here, it is possible to avoid proliferation of sidetracks by ultrasonic
waves overlapping with one another within the projected diameter
of the ultrasonic oscillator and eliminate deviation from the preferable
ultrasonic echo as a result of it being accompanied by ultrasonic
echo signals returning along the multiplied sidetracks.
Meanwhile, a wedge unit of a fourth aspect of the present invention,
is used for an ultrasonic Doppler flow meter, being mounted on an
outer wall of a pipe in which a fluid flows, supplying an ultrasonic
wave to the fluid, receives the reflected wave and supplies the
reflected wave to a flow rate calculation unit, comprises a wedge
with one surface thereof being mounted on a part of the outer circumference
of the pipe and on another surface thereof being equipped with an
ultrasonic oscillator that generates the ultrasonic wave in response
to an electric signal and receives the reflected wave; and an ultrasonic
wave attenuation member for attenuating an ultrasonic wave component
adding a noise to an ultrasonic echo signal, wherein the diameter
of the ultrasonic oscillator is defined so that the projected size
of the ultrasonic beam emitted by the ultrasonic oscillator impressed
on the outer wall of the pipe determined by the inclination angle
of another surface of the wedge being mounted by the ultrasonic
oscillator, does not exceed the difference between a position where
the ultrasonic wave is injected from the outer wall of the pipe
and a position where the ultrasonic wave first reaches the outer
wall of the pipe after being reflected by the inner wall thereof,
and an ultrasonic wave attenuation member is mounted on the outer
circumference of the pipe so as to avoid the projection of the ultrasonic
beam by the ultrasonic oscillator.
Here, it is possible to prevent the ultrasonic wave from entering
the outer wall of the pipe and being reflected thereby as a result
of entering the ultrasonic attenuation material first in the fourth
aspect, in addition to the third aspect.
Meanwhile, a wedge unit of a fifth aspect of the present invention,
is used for an ultrasonic Doppler flow meter, being mounted on the
outer wall of a pipe in which a fluid flows, supplying an ultrasonic
wave to the fluid, receives the reflected wave and supplies the
reflected wave to a flow rate calculation unit, comprises a wedge
with one surface thereof being mounted on a part of the outer circumference
of the pipe and on another surface thereof being equipped with an
ultrasonic oscillator that generates the ultrasonic wave in response
to an electric signal and receives the reflected wave; first and
second beam diameter limitation units for limiting an ultrasonic
beam diameter emitted by the ultrasonic oscillator and being mounted
on the bottom surface of the wedge, wherein at least one of the
first and second beam diameter limitation units doubles as an ultrasonic
wave attenuation member for attenuating an ultrasonic wave component
adding noise to an ultrasonic echo signal.
Here, it is possible to reduce the rate of sidetrack multiplication
due to overlapping ultrasonic waves with one another within the
limited beam diameter, responding to a combination of a slit and
an ultrasonic wave attenuation member, and the error caused by the
required ultrasonic echo signal being overlapped by an ultrasonic
echo signal received by way of the multiplied sidetracks.
In the above described fifth aspect, the beam diameter limitation
unit or the ultrasonic wave attenuation member may be mounted so
that the projected size of the beam incident on the outer wall of
the pipe does not exceed the difference between a position where
any of the beam gets injected from the outer wall of the pipe and
a position where the beam first reaches the outer wall of the pipe
after being reflected by the inner wall of the pipe.
Meanwhile, a wedge of a sixth aspect of the present invention,
is used for an ultrasonic Doppler flow meter, being mounted on the
outer wall of a pipe in which a fluid flows, supplying an ultrasonic
wave to the fluid, receives the reflected wave and supplies the
reflected wave to a flow rate calculation unit, wherein the wedge
on one surface thereof is mounted on a part of the outer circumference
of the pipe and on another surface thereof is equipped with an ultrasonic
oscillator for generating the ultrasonic wave in response to an
electric signal and receiving the reflected signal, and is equipped
by a beam diameter limitation unit for limiting the ultrasonic beam
diameter emitted by the ultrasonic oscillator inside the wedge.
Here, it is possible to reduce the rate of sidetrack multiplication
due to overlapping ultrasonic waves with one another, responding
to an extension of the slit limiting the beam diameter, and an error
caused by the required (preferable) ultrasonic echo signal being
overlapped with an ultrasonic echo signal received by way of the
multiplied sidetracks.
Meanwhile, a wedge unit of a seventh aspect of the present invention,
is used for an ultrasonic Doppler flow meter, being mounted on the
outer wall of a pipe in which a fluid flows, supplying an ultrasonic
wave to the fluid, receives the reflected wave and supplies the
reflected wave to a flow rate calculation unit, comprises a wedge
with one surface thereof being mounted on part of the outer circumference
of the pipe and on another surface thereof being equipped with an
ultrasonic oscillator that generates the ultrasonic wave by using
an electric signal and receives the reflected wave, and additionally
inside thereof being equipped by a beam diameter limitation unit
for limiting the ultrasonic beam diameter emitted by the ultrasonic
oscillator; and an ultrasonic wave attenuation member for attenuating
an ultrasonic wave component adding noise to an ultrasonic echo
signal.
In the above described seventh aspect, the beam diameter limitation
unit may be mounted inside the wedge so that the projected size
of the limited beam diameter incident on the outer wall of the pipe
does not exceed the difference between a position where any of the
beam enters from the outer wall of the pipe and a position where
the beam first reaches the outer wall of the pipe after being reflected
by the inner wall thereof.
Also in the above described seventh aspect, the ultrasonic wave
attenuation member may be mounted on the outer circumference of
the pipe so as to avoid a position where an ultrasonic wave emitted
from the ultrasonic oscillator first reaches the outer wall of the
pipe. Also in the above described seventh aspect, the ultrasonic
wave attenuation member may be mounted on the outer circumference
of the pipe so as to include a position where an ultrasonic beam
reaches the outer wall of the pipe after being reflected by the
inner wall thereof.
Meanwhile, a wedge unit of an eighth aspect of the present invention,
is used for an ultrasonic Doppler flow meter, being mounted on the
outer wall of a pipe in which a fluid flows, supplying an ultrasonic
wave to the fluid, receives the reflected wave and supplies the
reflected wave to a flow rate calculation unit, comprises a wedge
with one surface thereof being mounted on a part of the outer circumference
of the pipe and on another surface thereof being equipped with an
ultrasonic oscillator that generates the ultrasonic wave by using
an electric signal and receives the reflected wave, and a spacer
being installed between the wedge and the pipe.
Here, it is possible to reduce the rate of sidetrack multiplication
due to overlapping ultrasonic waves with one another within the
diameter of an ultrasonic oscillator, by the spacer installed between
the wedge and the outer wall of the pipe, and an error caused by
the required (preferable) ultrasonic echo signal being overlapped
with ultrasonic echo signals received by way of the multiplied sidetracks.
In the above described eighth aspect, the thickness of the spacer
may be adjusted so that the projected size of the ultrasonic beam
emitted by the ultrasonic oscillator, which is dependent on the
inclination angle of another surface of the wedge being equipped
by the ultrasonic oscillator, impressed on the contact surface of
the spacer with the wedge does not exceed the difference between
a position where the ultrasonic wave enters from the contact surface
and a position where the ultrasonic wave first reaches the contact
surface after being reflected by the inner wall of the pipe.
By this configuration, it is possible to avoid a multiplication
of sidetracks within the diameter of an ultrasonic oscillator, and
an error caused by the required (preferable) ultrasonic echo signal
being overlapped with ultrasonic echo signals received by way of
the multiplied sidetracks.
According to a wedge unit for use in an ultrasonic Doppler flow
meter of the first aspect of the present invention, since an ultrasonic
wave attenuation unit is mounted so as to include a position where
an ultrasonic wave first reaches the outer wall of a pipe, thereby
absorbing a part of the ultrasonic wave reaching the outer wall
of the pipe, it is possible to attenuate noise-adding ultrasonic
echo signals received by the ultrasonic oscillator by way of the
sidetracks caused by further reflections at the outer wall of the
pipe, and reduce acoustic noise. Also a reduction of acoustic noise
can improve the accuracy of measurement of velocity profile and
flow rate.
According to a wedge unit for use in an ultrasonic Doppler flow
meter of the second aspect of the present invention, since an ultrasonic
wave transmission unit is mounted so as to include a position where
an ultrasonic wave first reaches the outer wall of a pipe, thereby
transmitting an ultrasonic wave first reaches the outer wall of
the pipe, it is possible to reduce noise-adding ultrasonic echo
signals received by the ultrasonic oscillator by diffusing transmitted
wave from the outer wall of the pipe by further reflections at the
surface of the ultrasonic wave transmission unit, and acoustic noise.
Also a reduction of acoustic noise can improve the accuracy of measurement
of velocity profile and flow rate.
According to a wedge for use in an ultrasonic Doppler flow meter
of the third aspect of the present invention, it is possible to
avoid a multiplication of sidetracks by ultrasonic waves overlapping
with one another within the diameter of the ultrasonic oscillator
and eliminate error in a required ultrasonic echo as a result of
it being accompanied by ultrasonic echo signals returning along
the multiplied sidetracks. Therefore a reduction of acoustic noise
is enabled.
According to a wedge unit for use in an ultrasonic Doppler flow
meter of the fifth aspect of the present invention, it is possible
to reduce the rate of sidetrack multiplication due to overlapping
ultrasonic waves with one another within the limited beam diameter,
responding to a combination of a slit and an ultrasonic wave attenuation
member, and error caused by the required (preferable) ultrasonic
echo signal being overlapped with an ultrasonic echo signal received
by way of the multiplied sidetracks. This then enables a reduction
of acoustic noise.
According to a wedge for use in an ultrasonic Doppler flow meter
of the sixth aspect of the present invention, it is possible to
reduce the rate of sidetrack multiplication due to overlapping ultrasonic
waves with one another within the beam diameter, corresponding to
an extension of the slit limiting the beam diameter, and error caused
by the required ultrasonic echo signal being overlapped with ultrasonic
echo signals received by way of the multiplied sidetracks. Therefore
a reduction of acoustic noise is enabled.
According to a wedge unit for use in an ultrasonic Doppler flow
meter of the eighth aspect of the present invention, it is possible
to reduce or eliminate the rate of sidetrack multiplication due
to ultrasonic waves overlapping with one another within the diameter
of the ultrasonic oscillator, by the spacer installed between the
wedge and the outer wall of the pipe, and error caused by the required
ultrasonic echo signal overlapping with ultrasonic echo signals
received by way of the multiplied sidetracks. Therefore a reduction
of acoustic noise is enabled.
FIG. 9 shows a configuration of wedge unit for use in an ultrasonic
Doppler flow meter of a first embodiment according to the present
invention.
In FIG. 9 the wedge unit for use in an ultrasonic Doppler flow
meter is configured by a wedge 62 with one surface thereof being
mounted on a part of the outer circumference of a pipe 71 and on
another surface thereof being equipped with an ultrasonic oscillator
61 that generates an ultrasonic wave in response to an electric
signal and receives the reflected (ultrasonic) wave from a fluid
72 in the pipe 71; and an ultrasonic wave attenuation unit 63 being
mounted on the outer circumference of the pipe 71 so as to include
a position where an ultrasonic wave injected from the ultrasonic
oscillator 61 into the pipe 71 by way of the wedge 62 first reaches
the outer wall of the pipe 71 after being reflected by the inner
wall of the pipe 71.
Let it be known that the ultrasonic pulse is a beam of translatory
movement having a pulse width of approximately 5 mm for example.
Also, the wedge 62 as a medium conveying an ultrasonic wave generated
by an ultrasonic oscillator 61 to the pipe 71 is configured by a
plastic material such as acrylic, polyvinyl chloride, et cetera,
while the ultrasonic oscillator 61 is configured by a piezoelectric
material such as PZT (lead zirconate titanate) and fixed onto the
wedge 62 by using an epoxy resin adhesive for instance.
The surface of the wedge 62 which the ultrasonic oscillator 61
is mounted on is inclined by a prescribed angle so that the line
normal to the surface crosses the normal to the transverse section
surface (i.e., the longitudinal direction) of the pipe 31 at an
angle smaller than 90.degree. (90.degree.-.theta.w).
Meanwhile, the ultrasonic oscillator 61 functions, in addition
to a transmitter, as receiver for receiving echo ultrasonic waves
borne by an ultrasonic wave emitted from the ultrasonic oscillator
61 colliding with and being reflected by a reflecting body suspended
in the fluid 72 flowing in the pipe 71.
In FIG. 9 an ultrasonic wave emitted from the ultrasonic oscillator
61 is injected into the wedge 62 along the line of incidence 301
and into the pipe 71 along the line of incidence 302a, and then
reaches the inner wall of the pipe 71 along the line of incidence
302a.
At the inner wall of the pipe 71 the ultrasonic wave splits into
an ultrasonic wave component penetrating the inner wall of the pipe
and penetrating the fluid along the line of incidence 302b, and
another ultrasonic wave component at the inner wall of the pipe
71 being reflected by the inner wall of the pipe and going toward
the outer wall of the pipe along a sidetrack 303.
A certain portion of the ultrasonic wave component reaching the
outer wall gets injected into an ultrasonic wave attenuation member
63 which is mounted onto the outer wall so as to include the relevant
position, and the rest of the ultrasonic wave component gets reflected
by the outer wall, again going toward the inner wall along the sidetrack
304a.
By thus letting the ultrasonic wave attenuation member absorb a
portion of the ultrasonic wave reaching the interface with the pipe,
the ultrasonic wave component going toward the inner wall along
the sidetrack 304a is weakened and the noise added to the required
ultrasonic echo (i.e., the ultrasonic wave echo corresponding to
an ultrasonic wave penetrating into the fluid along the incident
line 302b) by the ultrasonic wave component penetrating into the
fluid along the sidetrack 304b is thereby reduced to a level that
causes no error in the measurement data.
As such, since a certain portion of ultrasonic waves reaching the
outer wall of pipe are absorbed by the ultrasonic wave attenuation
member 63 installed so as to cover the position where the ultrasonic
wave component reflected on the inner wall of the pipe first reaches
the outer wall of the pipe (along the sidetrack 303), it is possible
to attenuate noise-adding echo signals received by the ultrasonic
oscillator 61 by way of the sidetracks proliferating as a result
of further reflections at the outer wall of the pipe, and therefore
reduce the acoustic noise.
In the meantime, an ultrasonic wave reaching the inner wall likewise
splits into an ultrasonic wave component being injected into the
fluid 72 in the pipe 71 along the sidetrack 304b and the other ultrasonic
wave component getting reflected by the inner wall and going toward
the outer wall.
Each of the ultrasonic wave components is again received by the
ultrasonic oscillator 61 as an ultrasonic echo after traveling back
and forth along the sidetracks, and a flow velocity profile and
a flow rate are calculated by a flow rate calculation unit (not
shown) based on the ultrasonic wave echo.
Shown in FIG. 9 for example are the ultrasonic echo going back
to the ultrasonic oscillator 61 along the incident paths 302b, 302a
and 301 and another ultrasonic echo going back to the ultrasonic
oscillator 61 along the sidetracks 304b, 304a, 303 302a and 301.
FIG. 10 is a cross section viewed from the right of FIG. 9.
As shown by FIG. 10 the wedge 62 and the ultrasonic wave attenuation
member 63 are mounted contacting on the pipe 71.
Due to the nature of a clamp-on type, the above described mounting
is detachable afterwards in that the wedge 62 and ultrasonic wave
attenuation member 63 are generally mounted onto the pipe 71 by
being wrapped around using a steel belt, et cetera. The mounting
can be done by fixing onto the pipe 71 with an adhesive for instance
if no consideration is required for a removal later. Also, the ultrasonic
wave attenuation member 63 can be fixed onto the wedge 62 with an
adhesive.
The above described ultrasonic wave attenuation member 63 can be
fabricated from a material, such as tungsten elastomer, having an
acoustic impedance lower than the above described pipe 71.
Meanwhile, even if the wedge 62 is mounted onto the pipe 71 contacting
it as indicated by FIG. 10 an ultrasonic wave emitted from the
ultrasonic oscillator 61 actually keeps reflecting in the gap between
the outer wall and inner wall of the pipe in a two dimensional spread.
In this context, installation of an ultrasonic wave attenuation
member 64 being featured with a radius in contour of the outer wall
of the pipe so as to include a position where such reflected wave
having a two-dimensional spread first reaches the outer wall of
the pipe 71 as indicated by FIG. 11 will make it possible to further
attenuate the above described noise-adding echo signals, thereby
greatly reducing acoustic noise.
Meanwhile, referring to FIGS. 9 through 11 use of ultrasonic wave
transmission material having approximately the same acoustic impedance
as the pipe material in place of an ultrasonic wave attenuation
member, that is, a stainless steel member in a designed form being
mounted on a stainless steel pipe for example, most of the ultrasonic
wave gets transmitted through the aforementioned member at a position
where an ultrasonic wave first reaches the outer wall of the pipe
after being reflected from the inner wall thereof, although a little
reflection occurs at the interface with the member. As a result
of this, the wave transmitted through the outer wall is diffused
by further reflections at the surface, et cetera, of the stainless
steel member, thus enabling reduction of the noise-adding echo signals
returning to the ultrasonic oscillator 61 and the resultant acoustic
noise.
Also in this case, an additional structure may be mounted on the
outer surface of the ultrasonic wave transmission material for further
diffusing the reflections so as to attenuate substantially the ultrasonic
waves entering the ultrasonic wave transmission material by diffusion
(i.e., a random reflection). Such a structure is exemplified in
FIG. 12 in which a consideration may be given to the features of
the surface of an ultrasonic wave transmission material 65 having
a triangular shape with the same pitch or nearly the same pitch
as the wave length of the injected ultrasonic wave.
FIG. 13 shows a cross sectional view of a wedge unit for use in
an ultrasonic flow meter of a second embodiment according to the
present invention. The wedge unit comprises a wedge being equipped
with an ultrasonic oscillator, and an ultrasonic wave attenuation
member.
In FIG. 13 a wedge 82 and an ultrasonic wave attenuation member
88 are mounted on the outer wall of a pipe 83 in which a fluid 84
flows. One surface of the wedge 82 is mounted on a part of the outer
circumference of the pipe 83. Another surface of the wedge 82 is
equipped with an ultrasonic oscillator 81 which generates an ultrasonic
wave in response to an electric signal supplied by a drive circuit
(not shown), injects the ultrasonic wave into the fluid 84 and receives
the reflected signal thereof. The received reflected signal is then
supplied to a flow rate calculation unit (not shown) as an ultrasonic
echo signal.
The wedge 82 is preferably constituted of a plastic resin material
such as acrylic, polyvinyl chloride, et cetera, while the ultrasonic
oscillator 81 is preferably constituted of a piezoelectric material
such as PZT (lead zirconate titanate). The ultrasonic oscillator
81 is fixed onto the wedge 82 by an adhesive such as epoxy resin
adhesive. Note that the surface of the wedge 82 on which the ultrasonic
oscillator 81 is equipped (i.e., fixed) is inclined by .theta.w
degrees in reference to the vertical viewed from the longitudinal
direction of the pipe 81 as shown by FIG. 13.
In the present embodiment, the diameter of the ultrasonic oscillator
81 is defined so that the projected size of the ultrasonic beam
emitted by the ultrasonic oscillator 81 impressed on the outer wall
of the pipe 83 dependent on an inclination angle of another surface
of the wedge 82 being equipped by the ultrasonic oscillator 81 does
not exceed the difference between a position where the ultrasonic
wave is injected from the outer wall of the pipe and a position
where the ultrasonic wave first reaches the outer wall of the pipe
after being reflected by the inner wall thereof.
By the above described configuration, it is possible to avoid the
multiplication of sidetracks as a result of ultrasonic waves overlapping
with one another within the diameter of the ultrasonic oscillator
81 and eliminate error in the preferable ultrasonic echo as a result
of it being accompanied by ultrasonic echo signals returning along
the multiplied sidetracks.
Meanwhile, FIG. 13 shows a configuration further comprising an
ultrasonic wave attenuation member 88 in which the ultrasonic wave
attenuation member 88 is mounted on the outer circumference of the
pipe 83 so as to avoid the above described projection incident on
the outer wall of the pipe by the ultrasonic wave emitted from the
ultrasonic oscillator 81 that is, the position where the ultrasonic
wave first reaches the outer wall of the pipe. By this configuration,
the ultrasonic wave enters the ultrasonic wave attenuation member
88 before reaching the outer wall of the pipe, thereby preventing
further reflection.
Furthermore, an installation of the ultrasonic wave attenuation
member 88 so as to include the position where the ultrasonic wave
first reaches the outer wall of the pipe after being reflected by
the inner wall of the pipe effectively reduces the amplitude of
the initial reflected wave which would otherwise cause subsequent
reflections, and thus is capable of further reducing the acoustic
noise.
Note here that the ultrasonic wave attenuation member 88 is preferably
of a size large enough to intercept more than one time of multiple
reflections of an ultrasonic wave in consideration of the propagating
direction of the ultrasonic wave. The ultrasonic wave attenuation
member 88 is preferably constructed of a material having a smaller
acoustic impedance than the pipe 83 such as tungsten elastomer.
Meanwhile, the ultrasonic wave attenuation member 88 may be fixed
onto the wedge 82 by using an adhesive for example, or directly
fixed to the pipe by using a fixing unit such as a steel belt.
FIG. 14 shows how the diameter of an ultrasonic oscillator is determined.
In FIG. 14 the diameter D of the ultrasonic oscillator is defined
so that the projected size (i.e., the distance between the points
P1 and P2 that is, L') of the ultrasonic beam emitted by the ultrasonic
oscillator incident on the outer wall of the pipe, which depends
on the inclination angle of another surface of the wedge being equipped
by the ultrasonic oscillator, does not exceed the difference, L,
between a position (i.e., the point P1) where the ultrasonic wave
is injected from the outer wall of the pipe and another position
(i.e., the point P3) where the ultrasonic wave first reaches the
outer wall of the pipe after being reflected by the inner wall thereof.
That is, the diameter D is determined in accordance with the following
equation (A1): L'.ltoreq.L (A1)
Meanwhile, the following equation (A2) is derived, where .theta.w
is the angle of inclination for the surface of the wedge on which
the ultrasonic oscillator is equipped: D=L'*cos .theta.w (A2)
Meanwhile, the following equation (A3) is derived, where t is the
thickness of the pipe wall, and .theta.p is the angle showing the
direction of propagation of the ultrasonic wave within the pipe:
L=2t*tan .theta.p (A3)
Then the following equation (A4) is derived by substituting the
equations (A2) and (A3) into (A1), replacing L and L': (D/cos .theta.w).ltoreq.2t*tan
.theta.p (A4)
Because .theta.w.ltoreq..pi./2 rearranging the equation (A4) obtains
the equation (A5): D.ltoreq.2t*tan .theta.p*cos .theta.w (A5)
Determining the diameter D of an ultrasonic oscillator so that
the projection size L' is equal to the difference L between the
above described positions and the ultrasonic oscillator is realized
by the maximum transmission power with an acceptable level of noise
cut, thus deriving the following equation (A6): D=2t*tan .theta.p*cos
.theta.w (A6)
FIG. 15 shows a cross sectional view of a wedge unit for use in
an ultrasonic flow meter of the third embodiment according to the
present invention. The wedge unit comprises a wedge being equipped
by an ultrasonic oscillator, and an ultrasonic wave attenuation
member. Descriptions will be omitted from the description of FIG.
15 where there is duplication with FIG. 13.
In FIG. 15 mounted on the bottom of a wedge 92 are a slit 89 for
limiting the diameter of the ultrasonic beam emitted by an ultrasonic
oscillator 91 and an ultrasonic wave attenuation member 88 for attenuating
an ultrasonic wave component adding noise to an ultrasonic echo
signal. Note that in the case an emitted ultrasonic wave is to be
injected into the ultrasonic wave attenuation member 88 before reaching
the outer wall of pipe, the ultrasonic wave attenuation member 88
doubles as a slit for limiting the beam diameter of the ultrasonic
wave.
The slit 89 is constituted of a material having a smaller acoustic
impedance than the wedge material, such as air or some other gaseous
body, or a material absorbing or attenuating ultrasonic waves (such
as tungsten elastomer), or an ultrasonic wave reflection member
(e.g., a metallic material such as stainless steel or aluminum)
made of a material having a larger acoustic impedance compared to
the wedge material.
By the above described method, it is possible to reduce the rate
of sidetrack multiplication due to overlap between ultrasonic waves
within the limited beam diameter, corresponding to a combination
of the slit 89 and the ultrasonic wave attenuation member 88 and
error caused by the preferable ultrasonic echo signal being overlapped
with ultrasonic echo signals received by way of the multiplied sidetracks.
Meanwhile, the slit 89 or the ultrasonic wave attenuation member
88 is preferably mounted so that the projected size of the beam
incident on the outer wall of the pipe 83 does not exceed the difference
between a position where any of the beam is injected from the outer
wall of the pipe and a point where the beam first reaches the outer
wall of the pipe after being reflected by the inner wall of the
pipe.
This prevents overlapping between the ultrasonic beams within the
above described beam diameter, adding further effectiveness.
In the meantime, the slit 89 or the ultrasonic wave attenuation
member 88 is preferably mounted on the bottom of the wedge in the
third embodiment so as to limit the beam diameter D of the ultrasonic
beam emitted by the ultrasonic oscillator 91 by satisfying the conditional
equation (i.e., D.ltoreq.2t*tan .theta.p*cos .theta.w), where t
is the thickness of the pipe 83 and .theta.p is the angle of propagation
of the ultrasonic wave within the pipe and .theta.w is the inclination
angle of the wedge.
FIG. 16 shows a cross sectional view of a wedge for use in an ultrasonic
flow meter of a fourth embodiment according to the present invention.
The wedge is equipped with an ultrasonic oscillator and featured
with a slit therein. In describing FIG. 16 where common with FIG.
13 descriptions are omitted.
In FIG. 16 inside a wedge 122 there is a slit 110 for limiting
the beam diameter of the ultrasonic beam emitted from an ultrasonic
oscillator 121.
The slit 110 is constituted either of a material having a smaller
acoustic impedance than the wedge material, such as air or some
other gaseous body, a material absorbing or attenuating ultrasonic
waves (such as tungsten elastomer), or an ultrasonic wave reflection
member (e.g., a metallic material such as stainless steel or aluminum)
made of a material having a larger acoustic impedance than the wedge
material.
By the above described configuration, it is possible to reduce
the rate of sidetrack multiplication due to overlapping ultrasonic
waves with one another, responding to an extension of the slit 110
limiting the beam diameter, and error caused by the required ultrasonic
echo signal being overlapped by ultrasonic echo signals received
by way of multiplied sidetracks.
The ultrasonic wave attenuation member 88 is preferably mounted
on the outer circumference of the pipe 83 so as to avoid a position
where an ultrasonic wave emitted by the ultrasonic oscillator 121
first reaches the outer wall of the pipe 83.
Meanwhile, the slit 110 limits the beam diameter of the ultrasonic
oscillator 121 so that size of the projected beam diameter incident
on the outer wall of the pipe does not exceed the difference between
a position where any of the beam enters the outer wall of the pipe
and a position where the beam first reaches the outer wall of the
pipe after being reflected by the inner wall thereof.
This prevents overlapping between the ultrasonic waves within the
above described beam diameter, adding further effectiveness.
Further, the ultrasonic wave enters the ultrasonic wave attenuation
member 88 before reaching the outer wall of the pipe, thereby preventing
further reflection.
And furthermore, an installation of the ultrasonic wave attenuation
member 88 so as to include the position where the ultrasonic wave
first reaches the outer wall of the pipe 83 after being reflected
by the inner wall of the pipe effectively reduces the strength of
the initial reflection wave which would otherwise cause subsequent
reflections.
In the meantime, the slit 110 is preferably mounted inside the
wedge in the fourth embodiment so as to limit the beam diameter
D of ultrasonic wave emitted from the ultrasonic oscillator 121
by satisfying the conditional equation (i.e., D.ltoreq.2t*tan .theta.p*cos
.theta.w), where t is the thickness of the pipe 83 and .theta.p
is the angle of propagation of the ultrasonic wave within the pipe
and .theta.w is the inclination angle of the wedge.
Meanwhile, in the above description, while the ultrasonic wave
attenuation member 88 is installed in the propagating direction
of the ultrasonic wave as shown by FIGS. 13 and 16 the ultrasonic
wave attenuation member 88 may be replaced by an ultrasonic wave
transmission member having the same or approximately the same acoustic
impedance as the pipe material. In such case, the interface between
the ultrasonic wave transmission member and the air will preferably
be rugged so as to diffuse the ultrasonic wave reaching thereto.
FIG. 17 shows a cross sectional view of a wedge unit for use in
an ultrasonic flow meter of a fifth embodiment according to the
present invention. The wedge unit comprises a wedge 132 being equipped
with an ultrasonic oscillator 131 and an ultrasonic attenuation
member 138.
In FIG. 17 a spacer 139 is mounted between the wedge 132 and the
pipe 133 in which a fluid 134 flows, and the wedge 132 is mounted
on a part of the outer circumference of the pipe 133 by way of the
spacer 139 which is extended in the propagating direction of the
ultrasonic wave. The extended part of the spacer 139 is mounted
by an ultrasonic wave attenuation member 138 for attenuating an
ultrasonic wave component adding noise to the preferable ultrasonic
echo signal.
Meanwhile, another surface of the wedge 132 is equipped by an ultrasonic
oscillator 131 which generates an ultrasonic wave in response to
an electric signal from a drive circuit (not shown), injects the
ultrasonic wave into a fluid 134 in a pipe 133 and receives the
reflected wave. The received reflected wave is then supplied to
a flow rate calculation unit (not shown) as an ultrasonic echo signal.
The wedge 132 is preferably composed of a plastic resin material
such as acrylic, polyvinyl chloride, et cetera, while the ultrasonic
oscillator 131 is preferably composed of a piezoelectric material
such as PZT (lead zirconate titanate). The ultrasonic oscillator
131 is fixed to the wedge 132 by an adhesive such as epoxy resin
adhesive. Note that the surface of the wedge 132 on which the ultrasonic
oscillator 131 is equipped (i.e., fixed) is inclined by .theta.w
degrees in reference to the vertical viewed from the longitudinal
direction of the pipe 133 as shown by FIG. 13.
Here, it is possible to reduce the rate of sidetrack multiplication
due to ultrasonic waves overlapping with one another within the
diameter of the ultrasonic oscillator 131 by the spacer 139 installed
between the wedge 132 and the outer wall of the pipe, and an error
caused by the preferable ultrasonic echo signal being overlapped
with ultrasonic echo signals received by way of the multiple sidetracks.
Furthermore, in the present embodiment, the thickness of the spacer
139 is adjusted so that the size of the projection of the ultrasonic
beam emitted by the ultrasonic oscillator 131 which is defined
by the inclination angle of the surface of the wedge 132 to which
the ultrasonic oscillator 131 is attached, impressed on the contact
surface of the spacer 139 with the wedge 132 does not exceed the
difference between a position where the ultrasonic wave enters from
the contact surface and a position where the ultrasonic wave first
reaches the contact surface after being reflected by the inner wall
of the pipe.
By this configuration, it is possible to avoid a multiplication
of sidetracks within the diameter of the ultrasonic oscillator 131
and error caused by the preferable ultrasonic echo signal being
overlapped with ultrasonic echo signals received by way of the multiple
sidetracks.
Meanwhile, the ultrasonic wave attenuation member 138 is mounted
on the outer wall of the pipe in FIG. 17 making it possible to
reduce the influence of multiple reflections between the inner and
outer walls of the pipe.
Installing the ultrasonic wave attenuation member 138 on the outer
circumference of the pipe 133 so as to avoid a projection of an
ultrasonic wave emitted by the ultrasonic oscillator 131 impressed
on the contact surface of the spacer 139 with the wedge 132 that
is, the position where the ultrasonic wave first reaches the contact
surface of the spacer 139 will prevent a reflection because the
ultrasonic wave enters the ultrasonic wave attenuation member 138
before reaching the outer wall of the pipe.
Furthermore, installation of the ultrasonic wave attenuation member
138 on the spacer 139 so as to include the position where the ultrasonic
wave first reaches the contact surface of the spacer 138 (also including
an extended position contacting the wedge 132) after being reflected
by the inner wall of the pipe will be capable of effectively reducing
the strength of the initial reflected wave which would otherwise
cause subsequent reflections, and thus reduces the acoustic noise
substantially.
Meanwhile, the ultrasonic wave attenuation member 138 is preferably
large enough to intercept multiple reflections of the ultrasonic
wave in the pipe at least once, considering the propagating direction
of ultrasonic wave in the pipe. Also, the ultrasonic wave attenuation
member 138 is preferably configured by a material having a smaller
acoustic impedance compared to the pipe 133 such as tungsten elastomer.
Meanwhile, the ultrasonic wave attenuation member 138 may be fixed
onto the wedge 132 by using an adhesive for example, or directly
fixed to the pipe by using a fixing unit such as a steel belt.
FIG. 18 shows how the thickness of the spacer is determined.
In FIG. 18 the thickness of the spacer 139 is adjusted so that
the size of the projection (the distance between points P1 and P2
that is L') of an ultrasonic beam emitted by the ultrasonic oscillator
131 which is dependent on the inclination angle of the surface
of the wedge 132 being equipped by the ultrasonic oscillator 131
impressed on the contact surface of the spacer 139 with the wedge
132 does not exceed the difference, L, between a position (the point
P1) where the ultrasonic wave enters from the contact surface and
another position (the point P3) where the ultrasonic wave first
reaches the contact surface after being reflected by the inner wall
of the pipe. That is, the thickness of the spacer 139 is determined
in accordance with the following equation (B1): L'.ltoreq.L (B1)
Meanwhile, the following equation (B2) is derived, where .theta.w
is the angle of inclination of the surface of the wedge on which
the ultrasonic oscillator is equipped: D=L'*cos .theta.w (B2).
Note that the spacer is composed of a material which is assumed
to have the same or approximately the same acoustic impedance as
the pipe wall, to simplify the equations, et cetera, in the following.
However, a similar argument applies if such a limitation is not
imposed.
Meanwhile, the following equation (B3) is derived, where tp is
the thickness of pipe, ts is the thickness of spacer and .theta.p
is the direction of the ultrasonic wave propagation (B3): L=2*(tp+ts)*tan
.theta.p (B3)
Then the following equation (B4) is obtained when replacing L and
L' by substituting the equations (B2) and (B3) for (B1): (D/cos
.theta.w).ltoreq.2*(tp+ts)*tan .theta.p (B4)
Since 0.ltoreq..theta.p and .theta.w.ltoreq..pi./2 rearranging
equation (B4) will obtain (B5): D/(2*tan .theta.p*cos .theta.w)-tp.ltoreq.ts
(B5)
The thickness ts of the spacer which makes the projected width
L equals to the difference L' between the above described positions,
gives the minimum thickness of the spacer capable of efficiently
cutting noise. In this case, the following equation (B6) holds:
D/(2*tan .theta.p*cos .theta.w)-tp=ts (B6)
Note that, while the ultrasonic wave attenuation member 138 is
mounted in the direction of ultrasonic wave propagation as shown
by FIG. 17 in the above description, an ultrasonic wave transmission
member having the same or approximately the same acoustic impedance
as the pipe material may be substituted for the ultrasonic wave
attenuation member 138. In such a case, the interface of the ultrasonic
wave transmission member and the air is preferably be rugged so
as to diffuse the ultrasonic waves reaching thereto.
APPLICABILITY TO INDUSTRIES
The wedge and the wedge unit according to the present invention
are applicable to a Doppler-method clamp-on type ultrasonic flow
meter for use by mounting on (i.e., clamping on) a part of the outer
circumference of a pipe. |