Abstrict A Karman vortex flow meter for measuring the flow rate of a fluid
under test in a pipeline having a base to be secured to the pipeline,
and a vortex detector having an axis intended to intersect the pipeline.
The vortex detector includes a post, a detector flange at least
partially surrounding the post having a free surface, a columnar
force receiving part extending from the post into the pipeline for
producing a Karman vortex, and a strain detecting element coupled
to the free surface for detecting the Karman vortex and producing
an output signal corresponding to the detected Karman vortex.
Claims What is claimed is:
1. A Karman vortex flow meter for measuring the flow rate of a
fluid under test, in a pipeline containing the fluid and having
a longitudinal axis along which the fluid flows in a selected direction,
comprising:
a base secured to said pipeline abut an opening therein; and
a vortex detector having a detector axis generally perpendicular
to the longitudinal axis of the pipeline, said detector including:
a post supported by the base and aligned with the detector axis,
a balance weight aligned with and at least partially surrounding
the post, said balance weight being movable along the axis of the
post,
a detector flange supported by the base and substantially coaxial
with and at least partially surrounding said post, said detector
flange having a flexible surface orthogonal to the post,
a columnar force receiving part axially aligned with the post and
extending from the bottom of said post into the pipeline, said receiving
part shaped for producing a Karman vortex when inserted into the
pipeline, wherein the Karman vortex applies a force to the force
receiving part, and
a strain detecting element coupled to said flexible surface of
the detector flange for detecting the Karman vortex force applied
to the force receiving part and for producing an output signal corresponding
to the detected Karman vortex.
2. The Karman vortex flow meter of claim 1 further comprising
an upstream columnar body adjacent and substantially parallel with
said force receiving part, and extending from the base into the
pipeline.
3. The Karman vortex flow meter of claim 2 further comprising
a downstream columnar body extending substantially parallel with
said upstream columnar body and supported by the base, said downstream
columnar body having a substantial hollow portion along its longitudinal
axis to receive said force receiving part into said hollow portion.
4. The Karman vortex flow meter of claim 3 wherein said downstream
columnar body further includes a pair of side openings facing one
another, said side openings extending substantially parallel with
and partially surrounding said hollow portion of the downstream
columnar body, wherein said side openings communicate to one another
and to said hollow portion.
5. The Karman vortex flow meter of claim 2 further comprising
means for securing said force receiving part and said upstream columnar
body to the pipeline opposite to the opening therein.
6. The Karman vortex flow meter of claim 2 wherein said upstream
columnar body has a triangular cross-section having a base and two
sides, and wherein said columnar force receiving part has a trapezoidal
cross-section having a largest side opposing the base of the triangular
upstream columnar body.
7. The Karman vortex flow meter of claim 6 wherein the base of
the triangular upstream columnar body is substantially parallel
with the largest side of the trapezoidal columnar force receiving
part.
8. The Karman vortex flow meter of claim 1 wherein said strain
detecting element of the vortex detector has first and second electrodes
on opposite ends, said first electrode has a pair of split electrode
portions having a splitting axis substantially parallel to the axis
of the pipeline.
9. The Karman vortex flow meter of claim 8 wherein said strain
detecting element of the vortex detector has a piezoelectric material
element between said first and second electrode.
10. A Karman vortex flow meter for measuring the flow rate of a
fluid under test, in a pipeline containing the fluid and having
a longitudinal axis along which the fluid flows, comprising:
a base secured to the pipeline about an opening therein; and
a vortex detector having a detector axis generally perpendicular
to the longitudinal axis of the pipeline, said detector including:
a post supported by the base and aligned with the detector axis,
a detector flange supported by the base and substantially coaxial
with and at least partially surrounding said post, said detector
flange having a flexible surface orthogonal to the post,
a columnar force receiving part axially aligned with the post and
extending from the bottom of said post into the pipeline, said receiving
part having a length of approximately one half of the inside diameter
of the pipeline, wherein the Karman vortex applies a force to the
force receiving part,
an upstream columnar body adjacent to and substantially parallel
with said force receiving part, and extending from the base into
the pipeline,
a downstream columnar body extending from said base into the pipeline,
said downstream columnar body substantially parallel with said upstream
columnar body and said force receiving part, wherein the downstream
columnar body includes a hollow portion along its longitudinal axis
for receiving said force receiving part and a plurality of side
openings which communicate with each other and said hollow portion,
and
a strain detecting element coupled to said flexible surface of
the detector flange for detecting the Karman vortex force applied
to the force receiving part and for producing an output signal corresponding
to the detected Karman vortex force.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a Karman vortex flow meter which measures
the flow speed, or rate of a fluid under test, by detecting the
frequency of Karman vortex streets formed on both sides of a vortex
generating element placed in the stream of the fluid under test
and its manufacturing method.
2. Discussion of the Related Art
In a conventional Karman vortex flow meter, for example, as disclosed
by Japanese Patent Application Publication No. 4967/1983 a stress
detecting unit having a piezo-electric element is sealed (e.g.,
with glass) in a vortex generating element to detect variations
in pressure caused by alternating forces of Karman vortexes in the
vortex generating element. The Karman vortex flow meter has an advantage
in that the stress detecting unit is not in contact with the fluid
under test which flows in the pipeline of the flow meter, and thus
is not contaminated by the fluid. However, in the conventional Karman
vortex flow meter, when external vibrations are generated in the
pipeline, vibrations having a mode the same as that of the vortex
pressure are introduced as noise in the vortex generating element
of the flow meter, resulting in low S/N ratio, i.e., low measurement
accuracy.
Several attempts have been made to solve these problems in the
past. For example, the Karman vortex flow meter having a vibration
compensating device has been proposed by Japanese Patent Application
Publication No. 32127/1988. The vibration compensating device is
outside the pipeline and the vortex generating element is partially
inside the side wall of the pipeline. One vibration sensor is provided
in the portion of the vortex generating element inside the side
wall of the pipeline to produce an output signal corresponding to
the frequency of Karman vortexes produced in the vortex generating
element. Another vibration sensor is provided in the vibration compensating
unit outside the pipeline to produce an output signal to cancel
the portions of the output signal of the first vibration sensor
associated with the external vibration.
In the above type Karman vortex flow meter, to eliminate noises
from the external vibration, a plurality of vibration sensors are
required, and the measurement accuracy of the flow meter substantially
depends on the quality of the output signals of these vibration
sensors. Therefore, to insure the measurement accuracy, the output
signals of these sensors are adjusted (called the "actual flow
adjustment" hereinafter).
Since the vortex generating element must be vibrated during the
adjustment, in the actual flow adjustment, the output signals of
the vibration sensors are adjusted while the fluid under test is
allowed to flow in the pipeline. This makes the actual flow adjustment
very difficult a special skill and thus expensive.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a Karman vortex
flow meter which can accurately measure the flow rate of the fluid
under test, without being affected by external vibrations or requiring
the actual flow adjustment.
To achieve the object and in accordance with the purpose of the
invention, as embodied and broadly described herein, the Karman
vortex flow meter of the present invention includes a pipeline for
containing a fluid under test flowing in a lateral direction with
respect to the pipeline, a base fixedly secured to the pipeline,
and a vortex detector having an axis substantially perpendicular
with the lateral direction. The vortex detector includes a post
coaxial with the vortex detector, a detector flange substantially
coaxial with and at least partially surrounding the post, which
detector flange has a free surface on one side and a peripheral
end portion fixedly secured to the base on the other side, a columnar
force receiving part coaxial with the post and extending from the
post into the pipeline for producing a Karman vortex, and a strain
detecting element coupled to the free surface for detecting the
Karman vortex and producing an output signal corresponding to the
detected Karman vortex.
Another object of the invention is to provide a method of making
a Karman vortex flow meter including a base having a base flange,
and a vortex detector having a detector flange with a peripheral
end portion adjacent the base flange. The method of the present
invention, as embodied and broadly described herein, includes forming
a protrusion on the peripheral end portion of the detector flange,
positioning the protrusion in contact with the base flange to form
a joint, and applying pressure and electric current simultaneously
to the joint to thermally fuse the protrusion of the detector flange
and base flange.
Additional objects and advantages of the invention will be set
forth in part in the description which follows, and in part will
be obvious from the description, or may be learned by practice of
the invention. The objects and advantages of the invention will
be realized and attained by means of the elements and combinations
particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated in and constitute
a part of this specification, illustrate several embodiments of
the invention and together with the description, serve to explain
the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of the Karman vortex flow meter
according to the first embodiment of the present invention.
FIG. 2 is a cross-sectional view of the upstream columnar body
and the force receiving part of the vortex detector of FIG. 1 taken
along line A--A.
FIG. 3 illustrates the principle of operation of the vortex detector
of FIG. 1 under one condition.
FIG. 4 illustrates the principle of operation of the vortex detector
of FIG. 1 under another condition.
FIG. 5 is a cross-sectional view of an upper portion of the stain
detecting element of the vortex detector of FIG. 1.
FIG. 6 is a cross-sectional view of a lower portion of the strain
detecting element of the vortex detector of FIG. 1.
FIG. 7 is a cross-sectional view of the Karman vortex flow meter
according to the second embodiment of the present invention.
FIG. 8 is a cross-sectional view of FIG. 7 taken along line B--B
from a different perspective.
FIG. 9 is a cross-sectional view of the upstream columnar body
and the force receiving part of the vortex detector of FIG. 7 taken
along line A--A.
FIG. 10 is a detailed cross-sectional view of a portion of the
detector flange of the vortex detector of the Karman vortex flow
of the present invention.
FIG. 11 is another cross-sectional view of the Karman vortex flow
meter of the present invention to illustrate the manufacturing method
of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will now be made in detail to the present preferred embodiments
of the invention, examples of which are illustrated in the accompanying
drawings. Wherever possible, the same reference numbers will be
used throughout the drawings to refer to the same or like parts.
The Karman vortex flow meter according to a first embodiment of
the present invention will be described in reference to FIGS. 1-6.
In FIG. 1 the Karman vortex flow meter of the first embodiment
of the invention includes a pipeline 1 a base 2 and a vortex detector
3. The fluid under test flows in pipeline 1 in a lateral direction
with respect to pipeline 1.
Base 2 essentially includes a base flange 21 for securing base
2 to the side wall of pipeline 1 preferably with screws; a support
22 formed at the center of base flange 21 and shaped to be inserted
into the side wall of an upper portion la of pipeline 1; an upstream
columnar body 23 extending from support 22 to the side wall of a
lower portion 1b of pipeline 1 for accelerating the generation of
Karman vortexes; and a support 24 formed at an end portion of upstream
columnar body 23 as means for securing the elongated detector elements
of the side wall of lower portion 1bof pipeline 1. Supports 22 and
24 are inserted into the side wall of respective portions of pipeline
1 through O-rings 4 and 5 respectively, to prevent the vibration
of upstream columnar body 23.
The vortex detector of the Karman vortex flow meter detects Karman
vortex streets produced downstream of a vortex generating element
inserted into a pipeline through which the fluid under test flows.
Vortex detector 3 as embodied and broadly described herein, essentially
includes a detector flange 31 secured to an upper surface of base
flange 21 of base 2; a post 32 having a lower portion extending
from detector flange 31 into support 22 and an upper portion extending
outwardly from detector flange 31; a force receiving part 33 extending
from the lower portion of post 32 parallel to the detector axis;
a support 34 formed at an end portion of force receiving part 33
and inserted, through an O-ring 6 as a means for securing it to
the central portion of support 24 of base 2; and a balance weight
35 axially aligned with post 32 and movable upwardly and downwardly
along the upper portion of post 32. Balance weight 35 has a weight
substantially equal to that of force receiving part 33.
All of the above components of vortex detector 3 are axially aligned
with one another. O-ring 6 is provided to prevent the vibration
of the end portion of force receiving part 33. Detector flange 31
includes a free surface 31a which serves as a diaphragm. The peripheral
portions of detector flange 31 are welded gas tight to the upper
surface of base flange 21. A strain detecting element 12 (i.e.,
a sensor) is secured to free surface 31a and is prevented from contacting
the fluid under test in pipeline 1 so as not to be contaminated
by the fluid.
FIG. 2 is a cross-sectional view of upstream columnar body 23 at
base 2 and force receiving part 33 of vortex detector 3 taken along
line A--A in FIG. 1. The sectional view of upstream columnar body
23 has an isosceles triangular shape having two sides of equal length
and a base, and that of force receiving part 33 of vortex detector
3 has a trapezoid shape having a base in parallel with a top side.
The base of the triangle of upstream columnar body 23 is adjacent
the base (the longest side) of the trapezoid of force receiving
part 33 and these bases are substantially equal to one another
in length. An arrow Q in FIG. 2 indicates the direction of flow
of the fluid under test in the pipeline.
Upstream columnar body 23 and force receiving part 33 essentially
form the Karman vortex generating element in a broad sense: force
receiving part 33 being a main part for forming vortexes, and upstream
columnar body 23 being auxiliary thereto to accelerate the formation
of vortexes. Karman vortex streets are produced downstream of the
Karman vortex generating element when the Karman vortex generating
element is inserted in the pipeline to detect the Karman vortex
streets to measure the flow speed of the fluid under test in the
pipeline.
The Karman vortex flow meter, as embodied herein, is designed such
that when vortex detector 3 is externally vibrated or shocked (referred
to as "external vibration" hereinafter), the amount of
the bending moments externally applied to balance weight 35 and
to force receiving part 33 with respect to free surface 31a by an
inertial force (referred to as "an inertial bending moment"
hereinafter) is substantially equal in magnitude and opposite in
their applied directions to eliminate noise resulting from the external
vibration. The positioning along the axis of post 32 dimension
and weight of balance weight 35 is carefully controlled to maintain
the above balance. In other words, when vortex detector 3 is externally
vibrated, the inertial bending moment (with respect to free surface
31a) is canceled.
More specifically, in FIG. 3 an inertial bending moment M1 with
respect to free surface 31a is caused by inertial forces (indicated
by arrows) applied to the lower portion of post 32 and force receiving
part 33. An inertial bending moment M2 is caused by inertial forces
applied to the upper portion of post 32 and balance weight 35. Inertial
bending moments M1 and M2 are substantially equal in magnitude,
and opposite in directions, so that they are canceled out. Under
this balanced condition, as shown in FIG. 4 when a bending moment
M with respect to free surface 31a is caused by Karman vortex forces
(indicated by arrows) applied to force receiving part 33 strain
detecting element 12 (shown in FIG. 1), which is secured to free
surface 31a, produces an output signal corresponding only to the
vibration frequency of the Karman vortex forces, free from the external
vibration.
Strain detecting element 12 may be an annular piezo-electric element
bondable to free surface 31a. FIG. 5 is a plan view of a top layer
of strain detecting element 12 and FIG. 6 is that of a bottom layer
thereof. As shown in these figures, the top layer of strain detecting
element 12 includes electrodes 12a, 12b, and 12c and the bottom
layer includes an electrode 12d. The piezo-electric material is
sandwiched between the top and bottom layers.
Electrodes 12a and 12b are split electrode portions which are symmetrically
spaced from one another with respect to an axis X--X which axis
is intended to be parallel with the direction of flow of the fluid
under test in pipeline 1. Electrodes 12a and 12b are fixedly secured
to one side of strain detecting element 12. Electrode 12d is electrically
coupled to the slit electrodes 12a and 12b, and fixedly secured
to the opposite side. Electrode 12c is disposed intermediate electrodes
12a and 12b, and spaced therefrom. Electrode 12c is secured to the
opposite side and integral with common electrode 12d. Since it is
difficult to provide an output signal from the inner electrode 12d,
the output signal is provided from outer electrode 12c since it
is electrically coupled to electrode 12d.
As is apparent from the above description, strain detecting element
12 for example, of piezo-electric type, detects alternating forces
acting on force receiving part 33 only in directions along an axis
Y--Y (i.e., the direction of dynamic lift), independent of the forces
acting in axis X--X (i.e., the direction of the external vibration)
or direction Q of the stream of the fluid under test. Therefore,
the output signal of strain detecting element 12 corresponds only
to the flow speed or rate of the fluid under test, and is not affected
by the external vibration. As a result, the output signal is doubled
in strength compared to the conventional electronic circuit, thus
greatly improving the sensitivity and accuracy of measurement.
The Karman vortex flow meter according to a second embodiment of
the invention will be described with reference to FIGS. 7-9. The
flow meter of the second embodiment is particularly suitable for
a pipeline having a relatively large diameter.
The Karman vortex flow meter of the first embodiment of the present
invention is effective for removing the external vibration noise,
particularly for a pipeline having a relatively small diameter,
but yet it has a simple construction. However, for a pipeline with
a relatively large diameter, the length of force receiving part
33 of vortex detector 3 of the flow meter of the first embodiment
has to be substantially extended, thereby reducing its natural frequency.
For example, in the Karman vortex flow meter of the first embodiment,
force receiving part 33 functions as a main part of the vortex generating
elements. When the extraneous (not flow inducing) vibration frequency
in the vortex generating element is high, force receiving part 33
tends to resonate with the extraneous vibration frequency, causing
vortex detector 3 to operate erroneously.
In view of the foregoing, in the Karman vortex flow meter of the
second embodiment of the present invention, force receiving part
33 of vortex detector 3 is made independent of the vortex generating
element to not be affected by its effect such that vortex detector
3 operates free of error even for a pipeline with a relatively large
diameter.
In FIGS. 7-9 those components which have been previously described
in reference to the Karman vortex flow meter of the first embodiment
are designated by the same reference numerals.
In FIGS. 7 and 8 base 7 includes a base flange 71 secured to pipeline
1 with screws; a support 72 inserted in the side wall of upper portion
1a of pipeline 1; a support 74 inserted and secured with screws
in the side wall of lower portion 1b of pipeline 1; an upstream
columnar body 73 having a cross-section of an isosceles triangular
shape having a base and two sides of equal length, which extends
between supports 72 and 74; a downstream columnar body 75 having
a cross-section of an isosceles trapezoidal shape having a top and
bottom in parallel to one another and two sides, which extends into
lower portion 1b of pipeline 1; and a circuit box fixing member
9 secured to pipeline 1.
Upstream and downstream columnar bodies 73 and 75 essentially form
the vortex generating element. The peripheral end portions of detector
flange 31 are welded to the upper surface of base flange 71 of base
7. A circuit box 76 is disposed on base flange 71 and covers vortex
detector 3. A circuit board 8 on the circuit box 76 is electrically
coupled to and detects the output signal of strain detecting element
12 of vortex detector 3. An O-ring 10 between the side wall of circuit
box 76 and the side wall of circuit box fixing member 9 secures
circuit box 76 gas-tight to circuit box fixing member 9.
As shown in FIG. 9 downstream columnar body 75 which functions
as part of the vortex generating element, has side openings 75b
and 75c on each respective side of the trapezoid, and an inserting
hole 75a being hollow for a substantial portion of columnar body
75 and extending along the axis of downstream columnar body 75.
Side openings 75b and 75c communicate to one another, and hole 75a
communicates to side openings 75b and 75c.
As shown in FIG. 7 force receiving part 33 of vortex detector
3 extends from post 32 of vortex detector 3 at least to the center
or a midpoint between upper and lower portions 1a and 1b of pipeline
1 and is inserted into inserting hole 75a of downstream columnar
body 75 fixedly secured by an O-ring 5 therein. Therefore, force
receiving part 33 of vortex detector 3 of the second embodiment,
is made independent of the vortex generating element.
In the Karman vortex flow meter constructed as such, Karman vortexes
are detected as follows. Referring to FIG. 9 when Karman vortexes
are produced by upstream and downstream columnar bodies 73 and 75
the pressure at side opening 75b of downstream columnar body 75
is decreased relative to the pressure at side opening 75c thereof,
causing force receiving part 33 of vortex detector 3 in inserting
hole 75a deformed towards side opening 75b. The Karman vortexes
are produced on each side (of the trapezoid) of downstream columnar
body 75 alternately, causing force receiving part 33 to vibrate
in directions along an axis Y--Y.
The vibration of force receiving part 33 deforms free surface 73a
of vortex detector 3. Strain detecting element 12 which is bonded
to free surface 31a produces an output voltage signal corresponding
to the deforming of free surface 73a and associated with the frequency
of the Karman vortexes. Noise from the external vibration is eliminated
by the bending moment associated with the inertial force acting
on the part of vortex detector 3 above free surface 31a in a manner
similar to the Karman vortex flow meter of the first embodiment.
In the Karman vortex flow meter of the second embodiment of the
invention, since force receiving part 33 of vortex detector 3 is
inserted into inserting hole 75a of downstream columnar body 75
independent thereof, variations in pressure associated with the
Karman vortexes are induced to openings 75a, 75b, and 75c of downstream
columnar body 75 making vortex detector 3 independent of the vortex
generating element. Therefore, even for a pipeline with a large
diameter, force receiving part 33 of vortex detector 3 does not
need to be extended beyond the center point of the pipeline and,
thus, can be made smaller than otherwise required to set its natural
frequency to a high value.
The Karman vortex flow meter of the second embodiment does not
operate erroneously even when the frequency of the external vibration
applied to the flow meter is high. Furthermore, when the flow rate
of the fluid under test is measured for more than one pipeline having
a large diameter, only one common vortex detector 3 can be used
for pipelines having different diameters. Therefore, a different
vortex detector for each pipeline having a different diameter is
not required, reducing manufacturing costs.
Now, the method of manufacturing the Karman vortex flow meter of
the invention will be described in reference to FIGS. 1 10 and
11. FIG. 10 is a cross-sectional view of a portion of detector flange
31 of vortex detector 3 of FIG. 1 to illustrate how detector flange
31 is welded to base flange 21. FIG. 11 is a cross-sectional view
of an upper portion of the Karman vortex flow meter to illustrate
how the vortex detector is coupled to the base of the flow meter.
As discussed above and shown in FIG. 1 the Karman vortex flow
meter includes base 2 and vortex detector 3. Base 2 includes upstream
columnar body 23 and base flange 21. Vortex detector 3 includes
force receiving part 33 which functions as a vortex generating
element with upstream columnar body 23. Vortex detector 3 is fixedly
secured by welding detector flange 31 to base flange 21.
A full-circled arc welding can be used to weld detector flange
31 to base flange 21. However, in the full-circled arc welding,
peripheral end portion 31b of detector flange 31 is welded continuously,
locally thermally straining the weld of detector flange 31 and base
flange 21. As a result, upstream columnar body 23 is axially out
of alignment with respect to force receiving part 33.
The misalignment adversely affects not only the generation of Karman
vortexes, but also the proportional relationship between the vortex
frequency and the flow speed of the fluid under test. Furthermore,
since the full-circled arc welding normally takes relatively a long
time, and thus produces residual stresses in free surface 31a of
detector flange 31 and strain detecting element 12 is bonded to
free surface 31a, variations in the output signal from strain detecting
element 12 occur.
In view of the foregoing and as shown in FIG. 10 in the manufacturing
method according to the present invention, an annular protrusion
31c is formed on the lower surface of peripheral end portion 31b
of detector flange 31 adjacent base flange 21 and resistance welding
is done on annular protrusion 31c. Annular protrusion 31c may have
a cross-section of a trapezoid shape.
Referring to FIG. 11 during the resistance welding, a lower member
15a of a lower electrode 15 is placed inside a guide 14 of the welding
device, and positioning pins 16 with springs 17 are inserted into
an upper member 15b of lower electrode 15. Upper member 15b is disposed
on lower member 15a of lower electrode 15. Base flange 21 of upstream
vortex body 23 is mounted on the lower member 15b of lower electrode
15 with positioning pins 16 as a guide. Annular protrusion 31c of
detector flange 31 of vortex detector 3 is placed to be in direct
contact with base flange 21. Then, upper and lower members 18a and
18b of upper electrode 18 are mounted over and surrounding vortex
detector 3.
According to the method of the present invention, since detector
flange 31 of vortex detector 3 and base flange 21 of upstream columnar
body 23 are laterally positioned in parallel to one another during
the resistance welding, the axial misalignment of upstream columnar
body 23 with respect to force receiving part 33 does not occur.
Under this condition, a 12.5 kA current is applied to lower and
upper electrodes 15 and 18 for twenty seconds while a pressure (of
300 kgf, for example) is applied coaxially to electrodes 15 and
18. Since the current and pressure is concentrated on annular protrusion
31c of detector flange 31 of vortex detector 3 and base flange 21
of upstream columnar body 23 thermal fusion takes place by contact
resistance, thus permitting quick welding (e.g., two seconds).
In the above example, annular protrusion 31c of detector flange
31 has a cross-section of a trapezoid shape. The invention is not
limited thereto. For instance, the cross-section of annular protrusion
31c may have a semicircular, triangular, or any other shape which
may facilitate concentrating the current and pressure on annular
protrusion 31c.
According to the method of the present invention, since detector
flange 31 of vortex detector 3 is not strained during welding, and
the axial misalignment is minimized, variations in the production
of Karman vortexes and in the proportional relationship between
the vortex frequency and the flow speed of the fluid under test
can be controlled. Further, since no residual stress or strain exists
in free surface 31a of detector flange 31 strain detecting element
12 can detect a horizontally symmetrical strain produced by vortex
pressure, i.e., little variations in the output from strain detecting
element 12. Further, since the welding is done in a relatively short
time, little heat is generated during the welding, allowing the
welding to be performed even when strain detecting element 12 (e.g.,
of a piezo-electric element) is connected to free surface 31a of
detector flange 31. Thus, according to the manufacturing method
of the present invention, the welding to be done with ease and high
yield.
In summary, the Karman vortex flow meter of the present invention
has several advantages. For example,
(1) A highly accurate measurement of the flow speed or rate of
the fluid under test is obtained and the measurement is not affected
by the external or extraneous vibration or shock.
(2) Since the actual flow adjustment is not required, the operability
of the flow meter is improved, and the manufacturing cost is reduced.
(3) Since the output of the strain detecting element of the flow
meter responds only to the flow speed or rate of the fluid under
test, and thus an additional vibration compensating electronic circuit
is not required. The flow meter of the present invention is simple
in construction, highly reliable and inexpensive.
Furthermore, since the manufacturing method of the present invention
does not produce residual stresses in the free surface of the vortex
detector, the Karman vortex flow meter made by the inventive method
further improves the measurement accuracy.
Other embodiments of the invention will be apparent to those skilled
in the art from consideration of the specification and practice
of the invention disclosed herein. It is intended that the specification
and examples be considered as exemplary only, with a true scope
and spirit of the invention being indicated by the following claims.
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