Abstrict A vortex flow meter includes a measuring tube in which a fluid
is carried, a vortex generator provided in the measuring tube for
developing a Karman vortex in the fluid, a magnetic field generator
generating a magnetic field across the measuring tube downstream
of the vortex generator, a pair of electromotive force measuring
electrodes provided downstream of the vortex generator for measuring
an electromotive force generated by the Karman vortex passing across
the magnetic field, a pair of reference electrodes provided at locations
upstream and downstream of the electromotive force measuring electrodes,
respectively, for measuring a potential at each location, and a
detector circuit electrically connected to the electromotive force
measuring electrodes and the reference electrodes for calculating
the flow of the fluid from the electromotive force and the potential
measured by the reference electrodes. The flow is calculated by
subtracting the potential difference of the reference potential
measuring electrodes from the electromotive force measured by the
electromotive force measuring electrodes. This allows the flow meter
to have an increased measuring range, meet a wide range of flow
conditions, have a simple construction, and have measurement accuracy.
Claims What is claimed is:
1. A vortex flow meter comprising: a measuring tube in which a
fluid is carried; a vortex generator provided in the measuring tube
for developing a Karman vortex in the fluid; a magnetic field generator
for generating a magnetic field applied downstream of the vortex
generator across the measuring tube; a pair of electromotive force
measuring electrodes provided downstream of the vortex generator
for measuring an electromotive force generated when the Karman vortex
passes across the magnetic field; a first reference electrode provided
upstream of said vortex generator and a second reference electrode
provided downstream of the electromotive force measuring electrodes,
for measuring a potential at each location; and a detector circuit
electrically connected to the electromotive force measuring electrodes
and the first and second reference electrodes for calculating a
flow of the fluid from the electromotive force and the potential
measured by the first and second reference electrodes.
2. The vortex flow meter according to claim 1 wherein the detector
circuit comprises a differential amplifier for eliminating common-mode
disturbing noise components induced at the electromotive force measuring
electrodes and the signal between the paired first and second reference
electrodes.
3. The vortex flow meter according to claim 2 wherein the detector
circuit further comprises: a first positive amplifier for amplifying
the electromotive force generated between the electromotive force
measuring electrodes and inputting the amplified electromotive force
to a first input of the differential amplifier; and a second positive
amplifier for amplifying the electromotive force generated between
the first and second reference electrodes and inputting the amplified
electromotive force to a second input of the differential amplifier.
4. The vortex flow meter according to claim 1 wherein the detector
circuit further comprises: a coupling capacitor for coupling the
electromotive force measuring electrodes to the differential amplifier;
and a voltage follower circuit for generating a reference potential
from the potential measured at the first and second reference electrodes.
5. The vortex flow meter according to claim 4 wherein the voltage
follower circuit generates the reference potential in which the
potential measured by the first and second reference electrodes
is superimposed with a certain potential.
6. The vortex flow meter according to claim 4 wherein the detector
circuit further comprises a high impedance circuit for determining
the reference potential.
7. The vortex flow meter according to claim 1 wherein the detector
circuit further comprises an amplifier for amplifying an output
of the differential amplifier.
8. The vortex flow meter according to claim 1 wherein each of
the first and second reference electrodes has a diameter not greater
than 1/2 of a width of the vortex generator.
9. The vortex flow meter according to claim 1 wherein each of
the first and second reference electrodes has the same shapes.
10. The vortex flow meter according to claim 1 wherein each of
the electromotive force measuring electrodes has a diameter not
greater than 1/2 of a width of the vortex generator.
11. The vortex flow meter according to claim 1 wherein the electromotive
force measuring electrodes have the same diameters.
12. The vortex flow meter according to claim 1 wherein each of
the electromotive force measuring electrodes has, in the measuring
tube, a length ranging 2 to 2.5 times as large as a width of the
vortex generator.
13. The vortex flow meter according to claim 1 wherein the electromotive
force measuring electrodes have the same length in the measuring
tube.
14. The vortex flow meter according to claim 1 wherein a distance
between the electromotive force measuring electrodes is 2 to 2.5
times as large as a width of the vortex generator.
15. The vortex flow meter according to claim 1 wherein a width
between the magnetic field generators in a radial direction of the
measuring tube is 1.5 to 2 times as large as a width of the vortex
generator.
16. The vortex flow meter according to claim 1 wherein the measuring
tube has an undulated portion over an inner wall surface thereof.
17. The vortex flow meter according to claim 16 further comprising
a separate undulated member provided on an inner wall of the measuring
tube.
18. The vortex flow meter according to claim 17 wherein the separate
undulated member is electrically conductive and contacts directly
with a portion of one of the first and second reference electrodes.
19. The vortex flow meter according to claim 17 wherein the separate
undulated member is a coil spring.
20. The vortex flow meter according to claim 19 wherein the coil
spring has a portion having a pitch not greater than a diameter
of the reference electrodes, the portion accepting one of the first
and second reference electrodes.
21. The vortex flow meter according to claim 1 wherein a cross
section of the measuring tube has a track shape having linear portions
orthogonal to the magnetic field and arcuate portions bridging over
the linear portions, the arcuate portions arranged symmetrical about
a direction orthogonal to the magnetic field, a distance between
the arcuate portions being greater than a width between the linear
portions.
22. The vortex flow meter according to claim 21 wherein each of
the arcuate portions has a semi-circular shape.
Description FIELD OF THE INVENTION
The present invention relates to a flow meter for measuring the
flow of a fluid, such as air or liquid running in a measuring tube,
accurately throughout a wide range.
BACKGROUND OF THE INVENTION
Among flow meters for measuring the flow of a fluid which runs
in a measuring tube, a Karman-vortex flow meter is known. A conventional
Karman-vortex flow meter disclosed in Japanese Patent Laid-open
No. 60-40914 develops a Karman vortex in the flow of a fluid, and
a generating frequency (referred to as a frequency hereinafter)
of the vortex is measured for calculating the rate of the flow.
The calculation is based on the fact that the Karman-vortex generating
frequency is proportional to the flow. For measuring the Karman-vortex
frequency, the meter disclosed in the Japanese Patent Laid-open
No. 60-40914 employs an ultrasonic or oscillation technique. It
is known that when ultrasonic or oscillating waves directed to a
Karman vortex have a frequency or phase change, the change in the
ultrasonic or oscillating waves may be measured with only a large,
complex, expensive meter even if the measuring tube is relatively
small. Because the measuring accuracy of such an expensive meter
depends primarily on the generating mechanism of the Karman vortex,
the accuracy is easily reduced by a condition such as an ambient
temperature or disturbing turbulence which affects the generation
of a Karman vortex.
Japanese Patent No. 3113946 discloses that a Karman-vortex frequency
is measured with a magnetic field. The modified flow meter will
be explained. FIG. 7 is a cross sectional view of a conventional
flow meter. The meter includes a measuring tube 1 in which an electrically
conductive fluid flows and a vortex generator 2 provided in the
measuring tube 1. The vortex generator 2 generates a Karman vortex
3. The meter also includes a pair of electromotive force measuring
electrodes 4a and 4b, a detector circuit 5 electrically connected
to the electromotive force measuring electrodes 4a and 4b for measuring
a voltage between the electrodes 4a and 4b to calculate a flow rate
of the fluid running in the measuring tube 1 and a pair of magnetic
field generators 7a and 7b mounted around the measuring tube 1.
The magnetic field generators 7a and 7b are two magnets mounted
to both sides of the measuring tube 1 respectively, so that the
two, N and S, poles are opposite to each other. More specifically,
the magnetic field generators 7a and 7b are arranged so that the
orientation of the magnetic field from the N pole to the S pole
is perpendicular to the axis of the vortex generator 2 and to the
electromotive force measuring electrodes 4a and 4b. Downstream of
the vortex generator 7 in the flow direction, a pair of lines of
Karman vortices are generated in which alternate vortices of opposite
rotation are developed at a frequency proportional to the representative
dimension of the vortex generator 2. The electromotive force measuring
electrode 4b is located downstream of the vortex generator 2. The
electromotive force measuring electrode 4a opposite to the electromotive
force measuring electrode 4b is located downstream of the vortex
generator 2 and upstream of the electromotive force measuring electrode
4b. FIG. 7 illustrates the electromotive force measuring electrode
4a arranged unitarily with the vortex generator 2 for simplicity.
The Karman vortex 3 generated by the vortex generator 2 changes
the velocity of the flow thus causing a change in the magnetic flux
of the magnetic field developed between the magnetic field generators
7a and 7b. The change in the magnetic flux then generates an inductive
electromotive force between the electromotive force measuring electrodes
4a and 4b. The number of voltage changes is proportional to the
number of vortices and is measured by the detector circuit 5 for
calculating the flow.
However, as the electrodes in the conventional Karman-vortex flow
meter are directly provided downstream of the vortex generator,
they detect vortices in an area where the vortices do not depart
completely from the vortex generator and before the vortices grow
up to a measurable size. Therefore, the meter receives an influence
of fluctuations of the Karman vortices.
Also, as the electrodes are arranged at a point and thus has a
small sensing area, the accuracy of measurements may stay low. Particularly,
when the electric conductivity is low, a small flow is hardly measured.
The meter is susceptible to disturbing noises and thus requires
a scheme for diminishing the affect of noises. Having to include
a sophisticated filter circuit makes the overall arrangement of
the meter intricate and expensive, thus creating a secondary drawback.
Moreover, when the flow of the fluid is small where Reynolds number
is less than 3000 calculated from Re=UL/v (where U is the average
flow velocity in the cross section, L is the representative length,
and v is the kinetic viscosity coefficient), the velocity distribution
significantly varies by the resistance of the inner wall of the
measuring tube. This makes Karman vortices generate unstable, thus
reducing the accuracy and repeatability of measurements and reducing
the accuracy of the calculation of the flow.
SUMMARY OF THE INVENTION
A vortex flow meter follows: a measuring tube in which a fluid
is carried; a vortex generator provided in the measuring tube for
developing a Karman vortex in the fluid; a magnetic field generator
for generating a magnetic field to be applied downstream of the
vortex generator across the measuring tube; a pair of electromotive
force measuring electrodes provided downstream of the vortex generator
for measuring an electromotive force which is generated when the
Karman vortex passes across the magnetic field; a pair of reference
electrodes provided at locations upstream and downstream, respectively,
of the electromotive force measuring electrodes for measuring potentials
at the locations, respectively; and a detector circuit electrically
connected to the electromotive force measuring electrodes and reference
electrodes for calculating the flow of the fluid from the electromotive
force and the potential measured by the reference electrodes.
The flow meter measures the flow while offsetting a change of the
flow caused by a change of the measuring environments and conditions,
hence increasing the measurement range, decreasing the cost with
no use of extra components for a noise reduction, and improving
accuracy of measurement.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view of a flow meter according to Embodiment
1 of the present invention;
FIG. 2 is a block diagram of a detector circuit in the flow meter
of Embodiment 1;
FIG. 3 is a block diagram of a detector circuit in a flow meter
according to Embodiment 2 of the present invention;
FIG. 4 is a cross sectional view of a flow meter according to Embodiment
3 of the present invention;
FIG. 5 is a cross sectional view of an arrangement with a flow
meter according to Embodiment 4 of the present invention;
FIG. 6A is a cross sectional view of a flow input section of the
flow meter of Embodiment 4;
FIG. 6B is a cross sectional view of the flow meter of Embodiment
4; and
FIG. 7 is a cross sectional view of a conventional flow meter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(Embodiment 1)
A flow meter according to Embodiment 1 of the present invention
will be described referring to FIG. 1 and FIG. 2. Like components
are denoted by like numerals as those of the conventional flow meter
and will be explained in no more detail. FIG. 1 is a cross sectional
view of the flow meter of Embodiment 1 and FIG. 2 is a block diagram
of a detector circuit in the flow meter of Embodiment 1.
As shown in FIG. 1 the flow meter includes a measuring tube 1
a vortex generator 2 provided in the measuring tube 1 for developing
Karman vortices, a pair of electromotive force measuring electrodes
4a and 4b, a pair of reference electrodes 6a and 6b, a detector
circuit 5 for measuring an inducted electromotive force to calculate
a rate of the flow and offsetting the influence of disturbing noises,
and a pair of magnetic field generators 7a and 7b. A Karman vortex
3 is generated while alternate vortices of opposite rotation are
developed at a frequency being proportional to the representative
dimension of the vortex generator 2. The small inner diameter of
the measuring tube 1 has an inner wall affecting the Karman vortex
3 significantly. Too large of diameter makes the flow velocity slow
down and may hardly produce a Karman vortex. At an appropriate flow
velocity, the Reynolds number preferably ranges from 3000 to 100000.
The vortex generator 2 of this embodiment has a triangular column
shape, and may have any possible shape that will develop a Karman
vortex. The vortex generator 2 of this embodiment is mounted to
the inner wall of the measuring tube 1 so that one of the sides
of the generator is perpendicular to the direction of the flow.
In the flow meter of this embodiment, the measuring tube 1 has an
inner diameter of 7 mm for measuring a range of the flow rate from
1 L/min to 10 L/min. Under that condition, it is most desirable
that the triangular column body of the vortex generator 2 is an
isosceles triangle in the cross section having a width of 2 mm and
a height of 3 mm.
The electromotive force measuring electrodes 4a and 4b are arranged
in parallel with each other downstream of the vortex generator 2
so that their axes may extend at a right angle to the axis of the
vortex generator 2 and the direction of the flow. Each line of flow
passes the electromotive force measuring electrode 4a and passes
the electromotive force measuring electrode 4b. The magnetic field
generators 7a and 7b are permanent magnets mounted on both sides
of the measuring tube 1 to sandwich the electromotive force measuring
electrodes 4a and 4b with two, N and S, poles facing opposite to
each other. When the range of the flow measurement is set from 1
L/min to 10 L/min, the density of magnetic flux in the measuring
tube 1 has to be increased by the magnetic field generators 7a and
7b. Therefore, a rare-earth group permanent magnet having a width
1.5 times greater than that of the vortex generator 2 is used as
the magnetic field generators 7a and 7b.
The reference electrodes 6a and 6b measure a potential difference
between locations upstream and downstream of the electromotive force
measuring electrodes 4a and 4b. The reference electrodes 6a and
6b are also arranged in parallel with the electromotive force measuring
electrodes 4a and 4b. This allows a flow line passing the reference
electrodes 6a and 6b to pass the electromotive force measuring electrodes
4a and 4b. More specifically, the four electrodes are aligned in
a row along the direction of the flow from upstream to downstream.
The Karman-vortex flow meter has to be carefully sized for the
representative dimension of each component and the Reynolds number
of a fluid for steadily developing a Karman vortex 3 and for not
generating a noise with a disturbance in the measurement tube 1.
FIG. 1 shows a width D which is the representative dimension of
the vortex generator 2 diameters da and db of the electromotive
force measuring electrodes 4a and 4b, respectively, a diameter dc
of each of the reference electrodes 6a and 6b, a length h of each
of the electromotive force measuring electrodes 4a and 4b in the
measuring tube 2 a distance L between the electromotive force measuring
electrodes 4a and 4b, and a width Dm along the tube diameter of
each of the magnetic field generators 7a and 7b. Those dimensions
are determined in precise balance so as to steadily develop the
Karman vortex 3 but not any disturbing noise, as explained below.
For example, the diameter dc of each of the reference electrodes
6a and 6b is not greater than 1/2 of the width D of the vortex generator
2. This holds 1/2 or smaller of the Reynolds number. Accordingly,
as laminar flows appear about the reference electrodes 6a and 6b,
the Karman vortex 3 is rarely interrupted. Also, the reference electrodes
6a and 6b develop few vertices. Such a change in the flow velocity
does not disturb the magnetic flux between the electromotive force
measuring electrodes 4a and 4b and does not adversely affect measurement
of the flow by reducing the accuracy.
Preferably, the diameters da and db of the respective electromotive
force measuring electrodes 4a and 4b may not be greater than 1/2
the width D of the vortex generator 2 preferably. This holds 1/2
or smaller of the Reynolds number and can create laminar flows about
the electrodes 4a and 4b. As a result, the Karman vortex 3 is hardly
disturbed or fractured. The flow disturbed by the electromotive
force measuring electrode 4a is spread and runs to the downstream
side before reaching the electromotive force measuring electrode
4b. In a case in which the diameters da and db of the electromotive
force measuring electrodes 4a and 4b are equal, any noise developed
at the electromotive force measuring electrodes 4a and 4b can be
minimized. More specifically, the diameters da and db being different
from each other may vary the resistance to fluid and the friction
between the two electrodes 4a and 4b. Accordingly, the flow is generated
irregularly, thus causing noises. The electrodes 4a and 4b having
the same diameters minimizes generation of noises. In this embodiment,
the diameters da and db being equal to each other allows a substantially
uniform noise to be generated. The detector circuit 5 includes a
differential amplifier for removing, from the detection signal,
a disturbing noise induced on the electromotive force measuring
electrodes 4a and 4b and reduces the noise in the signal processing
process.
When the length h in the tube of the electromotive force measuring
electrodes 4a and 4b is 2 to 2.5 times as large as the width D of
the vortex generator 2 the inner diameter of the measuring tube
1 is preferably 3 to 4 times as large as the width D for taking
an appropriate balance between the flow and the size. Those dimensions
permit the electromotive force measuring electrodes 4a and 4b to
sandwich two strings of the Karman vortex 3 at their distant ends
with a minimum height and thus measure a change of the electromotive
force favorably at the center of the string of the Karman vortex
3 thus mesuring the flow accurately. The detector circuit 5 offsets
eddies generated in the wake of the distal ends of the electromotive
force measuring electrodes 4a and 4b or disturbing noises developed
by turbulence of the flow excluding the Karman vortex 3. Thus, an
overall noise is reduced, and the flow measurement can be improved
in the accuracy.
In a case in which the distance L between the electromotive force
measuring electrodes 4a and 4b is 2 to 2.5 times as large as the
width D of the vortex generator 2 the distance L can be smaller
than the distance between two strings of the Karman vortex 3. Therefore,
as there is one or no vortex passing between the electromotive force
measuring electrodes 4a and 4b, the level of noise can be reduced.
And thus, one Karman vortex 3 in the string corresponds to one of
the detection signal (one pulse), the frequency can be counted very
easily, and the flow measurement can further be improved in the
accuracy.
In a case in which the width Dm along the tube diameter of the
magnetic field generators 7a and 7b is 1.5 to 2 times as large as
the width D of the vortex generator, the magnetic field developed
in the measuring tube 1 is targeted to the area in the measuring
tube 1 where the Karman vortex 3 is developed. Accordingly, a noise
generated from undesired eddies developed on the inner wall of the
measuring tube 1 can be reduced. In the flow meter of this embodiment,
the width Dm is 1.5 times as large as the width D.
In the flow meter sized as described above, when the Karman vortex
3 runs across the magnetic field between the electromotive force
measuring electrodes 4a and 4b, the vortex changes the magnetic
field and generates pulses of the electromotive force at the electromotive
force measuring electrodes 4a and 4b. Simultaneously, the reference
electrodes 6a and 6b measure a potential difference around this
area, i.e., a reference potential difference between the electromotive
force measuring electrodes 4a and 4b. To the potential difference,
the measuring conditions (direct current factors and disturbing
noises excluding the Karman vortex) are reflected. As a result,
the detector circuit 5 offsets the reference potential difference
in the electromotive force and thus removes undesired noise or signal
components in the detection signal.
The detector circuit and an operation of the circuit in the flow
meter of Embodiment 1 will be described in more detail. As shown
in FIG. 2 the detector circuit 5 includes operational amplifiers
10a, 10b, 10c, 10d, and 10e, a comparator 11 a band-pass filter
12 resistors, and coupling capacitors.
The operational amplifier 10a has a positive input terminal connected
to a coupling capacitor 14a and has a negative input terminal to
which an output is fed back via a resistor 13c, thus forming a first
positive amplifier. The electromotive force measuring electrode
4a is connected to the positive input terminal via the coupling
capacitor 14a, and the electromotive force measuring electrode 4b
is connected to the negative input terminal via a resistor 13b which
determines an input potential. Similarly, the operational amplifier
10b has a positive input terminal connected to a coupling capacitor
14b and has a negative input terminal to which an output is fed
back via a resistor 13f, thus forming a second positive amplifier.
The reference electrode 6a is connected to the positive input terminal
via the coupling capacitor 14b, and the reference electrode 6b is
connected to the negative input terminal via a resistor 13e which
determines an input potential. The gain of each of the first and
second positive amplifiers is determined by the two resistors connected
to the negative input terminal. The electromotive force signals
developed on the electromotive force measuring electrodes 4a and
4b and the reference electrodes 6a and 6b is amplified to a desired
level by the gain.
The operational amplifier 10c is a buffer amplifier. The operational
amplifier 10d operates as a differential amplifier for positive-amplifying
an output of the first positive amplifier and for subtracting, from
the amplified output, an output of the second positive amplifier.
That is, the differential amplifier removes a common-mode signal
caused by the disturbing noise from the detection signal and amplifies
the detection signal to a desired level easily detected by the comparator
11. An output signal of the operational amplifier 10d passes through
the band-pass filter 12 and has the waveform shaped by the comparator
11 before being released out from the detector circuit 5. In brief,
the electromotive force signal induced by the Karman vortex 3 passing
between the electromotive force measuring electrodes 4a and 4b is
subjected to the amplification and the noise elimination of the
first and second positive amplifiers and differential amplifier.
And then, the electromotive force is shaped to a square wave by
the comparator 11 and then released as a string of pulses having
a frequency proportional to the rate of the flow. As described previously,
one pulse corresponds one Karman vortex 3 and is counted to measure
the flow accurately.
(Embodiment 2)
A flow meter according to Embodiment 2 of the present invention
will be described referring to FIG. 3. Like components are denoted
by like numerals as those of Embodiment 1 and will be explained
in no more detail. FIG. 3 is a block diagram of a detector circuit
according to Embodiment 2.
An operational amplifier 10a positive-amplifies a signal received
via a coupling capacitor from the electromotive force measuring
electrode 4a with a negative feedback. The amplified signal and
a signal received from the electromotive force measuring electrode
4b are differentially-amplified by an operational amplifier 10b.
The amplifier 10b removes a common-mode signal caused by disturbing
noises from the amplified output and amplifies the detection signal
to a desired level. The detection signal then passes through a band-pass
filter 12 and is amplified by a positive amplifier, which includes
mainly an operational amplifier 10c, to a desired level easily detected
by a comparator 11. In brief, the electromotive force induced by
the Karman vortex 3 passing between the electromotive force measuring
electrodes 4a and 4b is amplified by the differential amplifier
and the positive amplifiers. The electromotive force has the waveform
shaped by the comparator 11 and is released as a string of pulses
having a frequency proportional to the rate of the flow.
An operational amplifier 10e operates as a voltage follower circuit.
This circuit has an extremely high input impedance and a low output
impedance. The reference electrodes 6a and 6b are connected to the
grounding side of voltage dividing resistors 13k and 13l loaded
with a source voltage Vcc.
Therefore, an electrode potential actually measured between the
reference electrodes 6a and 6b is superimposed on a desired partial
determined from the source voltage Vcc divided by resistance, and
the impendance of the desired potential becomes lower. This allows
the output of the operational amplifier 10d (the reference potential
applied to the detector circuit 5) to remain constant even if the
reference electrode potential actually measured varies. As a result,
an influence of external potential fluctuations is eliminated. A
noise can be canceled by the differential amplifier including mainly
the operational amplifiers 10a and 10b even when the electromotive
force developed at the electromotive force measuring electrodes
4a and 4b is small. Consequently, a change in the magnetic field
derived from the Karman vortex 3 can favorably be measured.
As described above, the detector circuit 5 of Embodiment 2 eliminates
a fluctuation in the actual reference electrode potential with the
voltage follower circuit and the reference electrodes 6a and 6b.
And the circuit 5 removes the common- mode disturbing noise from
the electromotive force received from the coupling capacitor with
the differential amplifier circuit, thus detecting the electromotive
force induced by a change of the magnetic field caused by the Karman
vortex. Accordingly, the flow meter according to this embodiment
has a simple construction, is less expensive, and measures the flow
accurately.
(Embodiment 3)
A flow meter according to Embodiment 3 of the present invention
will be described referring the cross sectional view of FIG. 4.
Also, like components are denoted by like numerals as those of Embodiment
1 and will be explained in no more detail.
Undulation members 8a and 8b extending uniformly along the inner
wall of the measuring tube 1 are provided. The members are particularly
implemented in this embodiment by coil springs which are less expensive.
As described previously, a Karman vortex 3 is steadily developed
and has the frequency proportional to the rate of flow when a fluid
to be measured is carried in turbulent flows. This condition may
be expressed with the Reynolds number. In a proper range of flow
velocities, the Reynolds number ranges from 3000 to 100000. The
Reynolds number may however be varied depending on the shape of
the inner wall of the measuring tube 1. Particularly, the Reynolds
number ranges from 2320 to 3000 at the transition between a laminar
flow and a turbulent flow. For measuring a flow in the range of
the Reynolds number, the inner wall of the measuring tube 1 has
to be roughed uniformly on the surface upstream of the vortex generator
2 in order to develop turbulent flows steadily.
For that reason, Embodiment 3 employs the two coil springs 8a and
8b provided upstream and downstream of the vortex generator 2 along
the inner wall of the measuring tube 1 respectively. The coil springs
8a and 8b, as each having a uniform pitch, are most favorable materials
for implementing uniform undulation over the inner wall. Also, the
coil springs 8a and 8b are easily assembled and has the effect precisely
predicted. The coil springs 8a and 8b prevent the measuring tube
1 from being delicately machined in the inner wall, hence contributing
to the easy fabrication of the flow meter with the measuring tube
1 undulated on the inner wall without costly processes.
The coil springs 8a and 8b are made of electrically conductive
material and directly connected with the reference electrodes 6a
and 6b, respectively. The reference electrodes 6a and 6b, which
are directly connected with the coil springs 8a and 8b, have the
performance for measuring the reference potential improved. Upon
having the pitch between two adjacent windings partially greater
than the diameter of the reference electrodes 6a and 6b, the coil
springs 8a and 8b hold the reference electrode 6a and 6b securely
and tight with a yielding force, respectively. The coil springs
8a and 8b, when formed of close-coiled helical type springs, have
increased areas contacting with the fluid thus improving the effectiveness.
For inhibiting the reference electrodes 6a and 6b from being deformed
by the yielding force of the coil springs 8a and 8b, the coil springs
8a and 8b have to be made from the same material and to have the
same diameter as the reference electrodes 6a and 6b.
An operation of the flow meter according to Embodiment 3 will be
explained. A fluid to be measured running in the measuring tube
1 is disturbed by the coil spring 8a to cause a turbulent flow.
Accordingly, a Karman vortex 3 is developed with the vortex generator
2 as proportional to the velocity of the flow. As the Karman vortex
3 crosses the magnetic field between the electromotive force measuring
electrodes 4a and 4b, the vortex creates regularly a string of pulses
of electromotive force. Simultaneously, a potential difference between
the electromotive force measuring electrodes 4a and 4b to which
the other factors (a direct current and disturbing noises other
than the Karman vortex) is reflected is measured with the reference
electrodes 6a and 6b. As a result, an undesired noise or signal
component in the detection signal can readily be removed by the
detector circuit 5 offsetting the component in the reference potential
signal.
The flow meter according to Embodiment 3 has the coil springs 8a
and 8b provided for implementing uniform undulation over the inner
wall of the measuring tube 1 allowing a Karman vortex 3 to be developed
steadily. This can extend the measuring range and improve the resistance
to noises. Upon having the uniform undulation implemented by separate
members, the measuring tube 1 can be less expensive.
(Embodiment 4)
A flow meter according to Embodiment 4 of the present invention
will be described referring to FIGS. 5 and 6. Like components are
denoted by like numerals as those of Embodiment 1 and will be explained
in no more detail. FIG. 5 is a cross sectional view of an arrangement
of the flow-meter of Embodiment 4. FIG. 6A is a cross sectional
view of a flow input section of the flow meter and FIG. 6B is a
cross sectional view of the flow meter of Embodiment 4.
As shown in FIG. 5 the flow meter has a metal yoke 9 for reducing
a leakage of magnetic fluxes generated by magnetic field generators
7a and 7b.
As shown in FIG. 6B, the cross section of a measuring tube 1b of
Embodiment 4 consists of a track shape having an arcuate portion.
The shape has linear portions orthogonal to the magnetic field generated
by the magnetic field generators 7a and 7b and arcuate portions
bridging over the linear portions. This permits the distance dg
between the magnetic field generators 7a and 7b to be minimized
without disturbing a Karman vortex 3 generated by a vortex generator
2. The arcuate portions are curved outward and symmetrical about
a direction perpendicular to the magnetic field. The symmetry about
the direction perpendicular to the magnetic field is equivalent
to the symmetry about a line extending between and in parallel with
the linear portions. The arcuate portions are also symmetrical about
the magnetic field in Embodiment 4. For reducing the distance dg
in the cross section of the measuring tube 1b, the distance X between
the inner sides of arcuate portions (referred to as an arcuate distance)
is not smaller than the length Y between the linear portions (referred
to as a linear distance). More particularly, the arcuate distance
X is a long axis length of the track shape while the linear distance
Y is a short axis length.
The arcuate portion may preferably have a semi-circular shape of
which the diameter is equal to the linear distance Y. This allows
the measuring tube 1b to be fabricated more easily and less expensively.
The measuring tube having this shape is smoothly joined with the
linear edges in the direction of a tangent, thus rarely producing
secondary eddies which disturb the measurement and can have an increased
measuring range and improved measuring accuracy.
The smaller the arcuate distance X, the higher the velocity of
the flow will increase to provide a higher Reynolds number, hence
making the measurement easy. If the arcuate distance X is too small,
the inner wall surface disturbs the flow and prevents a Karman vortex
3 from being generated. Therefore, the arcuate distance X and the
width W of the vortex generator 2 preferably have the ratio of W/X
ranging 0.2 to 0.4. The measuring tube 1b of Embodiment 4 for having
a measuring range from 1 L/min to 10 L/min, has the linear distance
of 3.8 mm, the diameter of the arcuate portion of 3.8 mm, and the
arcuate distance of 6.8 mm. The vortex generator 2 in the measuring
tube 1b may preferably be a triangular column having an isosceles
triangle in the cross section, a width of 2 mm, and a height of
3 mm.
The magnetic field generators 7a and 7b are accompanied with a
metal yoke 9 and spaced by a small distance dg from each other for
reducing the leakage and thus increasing the magnetic flux. In case
that the measuring tube 1b of this embodiment has the linear distance
of 3.8 mm, the diameter of the arcuate portion of 3.8 mm, and the
arcuate distance of 6.8 mm for having a measuring range from 1 L/min
to 10 L/min, the magnetic field generators 7a and 7b may be implemented
by a ferrite magnet. More preferably, the magnetic material is a
rare-earth permanent magnet, such as neodymium (Ne), samarium (Sm),
or cerium (Ce), which has a higher magnetic flux density and a favorable
thermal property for further improving the accuracy of measurement. |