Abstrict An electromagnetic flow meter includes a fluid-carrying conduit.
A signal detecting element detects an electromotive force induced
in the fluid in the conduit to produce a first detecting signal.
A conduit voltage detecting element detects a voltage of the conduit
to produce a second detecting signal. An electrical energy applying
device is responsive to the first and second detecting signals for
applying a selected amount of voltage to the conduit so that the
voltage of the first detecting signal and the detected voltage of
the conduit become equal to each other, whereby a profile of the
voltage of the conduit is substantially similar to a profile of
the electromotive force at least at a position of the signal detecting
element.
Claims What is claimed is:
1. An electromagnetic flow meter comprising:
(a) a conductive conduit through which a fluid flows;
(b) means for applying a magnetic field to said conduit to induce
an electromotive force in said fluid;
(c) signal detecting means for detecting the electromotive force
induced in said fluid to produce a first detecting signal;
(d) means disposed in the vicinity of said signal detecting means
for detecting a voltage of said conduit to produce a second detecting
signal;
(e) electrical energy applying means responsive to said first and
second detecting signals for applying a selected amount of voltage
to said conduit so that the voltage of said first detecting signal
and the said detected voltage of said conduit become equal to each
other, whereby a profile of the voltage of said conduit is substantially
similar to a profile of said electromotive force at least at a position
of said signal detecting means.
2. An electromagnetic flow meter according to claim 1 in which
said conduit has a lining of an electrical insulating material mounted
on the internal surface thereof.
3. An electromagnetic flow meter according to claim 2 in which
said signal detecting means comprises a pair of diametrically opposed
electrodes mounted within said lining in capacitive coupling relation
to said fluid.
4. An electromagnetic flow meter according to claim 1 in which
an apparent conductivity of said conduit varies circumferentially
thereof.
5. An electromagnetic flow meter according to claim 1 in which
said signal detecting means comprises a pair of diametrically opposed
electrodes mounted on said conduit through electrical insulating
means and disposed in contact with said fluid.
6. An electromagnetic flow meter according to claim 1 in which
said electrical energy applying means comprises a pair of elongated
auxiliary electrodes mounted on said conduit for electrical connection
thereto and extend along the axis of said conduit and an amplifier
connected to said auxiliary electrodes for applying its output voltage
thereto, said signal detecting means and said conduit voltage detecting
means lying between said pair of auxiliary electrodes.
7. An electromagnetic flow meter according to claim 6 in which
said auxiliary electrodes are provided in a plurality of pairs.
8. An electromagnetic flow meter according to claim 6 in which
two points of said auxiliary electrodes to which the output voltage
of said amplifier is applied are disposed in a plane which is disposed
perpendicular to the longitudinal axis of said conduit and contains
said signal detecting means.
9. An electromagnetic flow meter according to claim 1 in which
said conduit voltage detecting means comprises an elongated electrode
mounted on said conduit for electrical connection thereto and extending
along the longitudinal axis thereof.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electromagnetic flow meters.
2. Prior Art
One conventional flow meter 10 of the electromagnetic type shown
in FIG. 1 comprises a fluid-carrying conduit 11 including a pipe
12 of a conductive material and a lining 13 of an insulating material
such as polytetrafluoroethylene mounted on the internal surface
of the pipe 12. A magnetic field B produced by a magnetic field
generating means (shown by a coil) is exerted on the conduit 11
so that an electromotive force E is induced in the fluid 14 flowing
through the conduit 11. A pair of diametrically opposed measuring
electrodes or probes 15a and 15b are mounted on the conduit 11
and the voltage between the electrodes 15a and 15b is detected to
measure the flow rate of the fluid through the conduit 11.
The operation of this conventional electromagnetic flow meter will
now be described with reference to an equivalent circuit of the
flow meter (FIG. 2). In FIG. 2 E denotes an electromotive force
proportional to the flow rate of the fluid, and R.sub.I denotes
an internal impedance of the fluid 14 and R.sub.W denotes an impedance
representing the total of an impedance at the boundary between the
fluid 14 and the conduit 11 and an impedance of the conduit 11.
Also, V.sub.M denotes a potential difference between the measuring
electrodes 15a and 15b, that is to say, an output voltage of the
electromagnetic flow meter. Using this equivalent circuit, the output
voltage V.sub.M is represented by the following formula: ##EQU1##
In this case, since the lining 13 is made of an insulating material,
the following formula is obtained:
Then, the following formula is obtained from the formulas (1) and
(2):
Thus, the output voltage V.sub.M of the electromagnetic flow meter
is substantially equal to the electromotive force E proportional
to the flow rate of the fluid.
This conventional flow meter 10 of the electromagnetic type have
the following disadvantages because of the use of the lining 13:
(a) Since the lining 13 of a uniform thickness has to be mounted
on the internal surface of the pipe 12 the manufacture of the flow
meter requires much labor and cost.
(b) It is necessary that the lining 13 should have an adequate
mechanical strength as well as a corrosion resistance, a wear resistance
and a thermal resistance. Therefore, generally, the lining 13 is
made of a polymeric material such as polytetrafluoroethylene and
rubber. However, the lining made of such a material has failed to
meet the above-mentioned requirements satisfactorily.
(c) When a fluid of a conductive nature is deposited on the internal
surface of the lining 3 the impedance R.sub.W is lowered so that
the formula (2) (R.sub.W >>R.sub.I) is not established. As
a result, the formula (3) is also not established so that the output
voltage V.sub.M is not proportional to the flow rate of the fluid.
Therefore, the flow rate can not be measured accurately.
In order to overcome such disadvantages arising from the use of
the lining, there have been proposed electromagnetic flow meters
having no lining. One such flow meter 10a is shown in FIGS. 3 and
4 and FIG. 5 shows an equivalent circuit of the flow meter 10a.
The electromagnetic flow meter 10a comprises an electrical energy
applying means which includes a pair of annular auxiliary electrodes
17a and 17b of copper and a pair of amplifiers 18a and 18b each
having a voltage gain of 1. The pair of auxiliary electrodes 17a
and 17b are welded to a pipe 12 and surround measuring electrodes
or probes 15a and 15b, respectively. The measuring electrode 15a,
15b is connected to one input terminal of the amplifier 18a, 18b
while the auxiliary electrode 17a, 17b is connected to the other
input terminal of the amplifier 18a, 18b and to the output terminal
thereof. With this arrangement, the voltage of the measuring electrode
15a, 15b is rendered equal to the voltage of the auxiliary electrode
17a, 17b. Therefore, as shown in FIG. 5 no current flows through
the impedance R.sub.1 between the measuring electrode 15a and the
auxiliary electrode 17a and also through the impedance R.sub.2 between
the measuring electrode 15b and the auxiliary electrode 17b. Therefore,
no current flows through a path constituted by the impedance R.sub.1
the impedance R.sub.W of the pipe 12 and the impedance R.sub.2.
As a result, the output voltage V.sub.M of the electromagnetic flow
meter 10a is equal to the electromotive force E which is proportional
to the flow rate of the fluid 14. In other words, the output voltage
V.sub.M of the flow meter is proportional to the flow rate of the
fluid 14 so that the flow rate can be measured. The amplifiers
18a and 18b function to cause an output current J to flow through
a path extending therebetween and containing the impedance R.sub.W,
the output voltage V.sub.M is not affected by the output current
J.
Coordinates shown in FIG. 6A indicates the relation between the
pipe 12 and a detecting point on the internal surface of the pipe
12 when a magnetic field is applied in a plane perpendicular to
the longitudinal axis of the pipe, with the pipe having a uniform
thickness. FIG. 6B is a diagrammatic illustration showing the relation
between the detecting point and the voltage at the detecting point.
The voltage at the detecting point determined by an angle .theta.
is indicated by .phi.. A profile of the voltage induced in the fluid
14 at the internal surface of the pipe of the flow meter 10 is shown
in a solid line a in the diagrammatic illustration on FIG. 6A. Also,
a profile of the voltage produced at the internal surface of the
pipe of the flow meter 10a having no lining is shown in a broken
line b in the diagrammatic illustration of FIG. 6B. In FIGS. 6A
and 6B, the positions of the measuring electrodes 15a and 15b are
indicated by .theta.a and .theta.b, respectively. The positions
of the auxiliary electrodes 17a and 17b are indicated by .theta.c
and .theta.d, respectively. Thus, in the prior art, the flow meter
having no lining can properly measure the flow rate of the fluid
by causing the cosine wave-shaped voltage profile indicated by the
solid line a to approximate the angularly-bent voltage profile indicated
by the broken line b. In this case, it is important that the voltage
at those portions of the pipe internal surface adjacent to the measuring
electrodes 15a and 15b should be equal to the voltage of those portions
of the fluids adjacent to said those portions of the pipe internal
surface. In other words, in FIG. 6B, the voltage profile a must
coincide with the voltage profile b when .theta. is 0.degree. and
180.degree..
In the electromagnetic flow meter 10a, the output current J of
a large magnitude is caused to flow through the auxiliary electrodes
17a and 17b welded to the pipe 12 so that voltage drops develop
at the welding portions, thereby rendering the measurement of the
flow rate inaccurate. This will now be described with reference
to FIG. 7 in which V.sub.M denotes the output voltage of the measuring
electrode 15a, V.sub.2 denotes the output voltage of the amplifier
18a, V.sub.3 denotes a voltage applied to the auxiliary electrode,
and V.sub.4 denotes a voltage of the portion of the pipe 12 adjacent
to the measuring electrode 15a. Since the amplifier 18a has a voltage
gain of 1 the following formula is obtained:
Thus, the output voltage V.sub.M is equal to the voltage V.sub.3
applied to the auxiliary electrodes 17a. In this condition, output
current J of the amplifier 18a flows through the auxiliary electrode
17a into the pipe 12 so that a voltage drop develops at the welded
portion between the auxiliary electrode 17a. and the pipe 12. As
a result, the voltage V.sub.4 becomes smaller than the voltage V.sub.3
by an amount equal to this voltage drop. Therefore, the following
formula is obtained:
Further, the following formula is obtained from the formulas (4)
and (5):
Thus, the voltage V.sub.4 of the pipe 12 becomes smaller than the
output voltage V.sub.M of the detecting electrode 15a, thereby giving
rise to an error in the measurement of the flow rate. As is clear
from the foregoing, an accurate measurement of the flow rate can
be made when the output voltage V.sub.M of the measuring electrode
15a is equal to the voltage V.sub.4 of the pipe 12.
This phenomenon also occurs at the side of the measuring electrode
15b. In conclusion, in the electromagnetic flow meter 10a in FIG.
3 it is necessary that the output voltage V.sub.M should be equal
to the voltage V.sub.4 of the pipe 12 in order to achieve an accurate
measurement of the flow rate. Actually, however, by making the output
voltage V.sub.M equal to the voltage V.sub.3 it is assumed that
the voltage V.sub.M is equal to the voltage V.sub.4. As mentioned
above, the voltage profile a coincides with the voltage profile
b when .theta. is 0.degree. and 180.degree. (FIG. 6B). However,
this can be achieved only when the voltage drops at the welding
portions of the auxiliary electrodes 17a and 17b are ignored. With
the conventional electromagnetic flow meter, it is impossible that
the voltage profile a coincides with the voltage profile b accurately.
U.S. Pat. No. 2733604 discloses another electromagnetic flow
meter which comprises a pair of probes mounted on a fluid-carrying
conduit having no lining, one of the probes being connected to an
amplifier in the form of a pentode constituting a cathode follower
while the other probe is grounded. The one probe detects a voltage
of the fluid in the conduit to send a detecting signal to the amplifier
which in turn sends an output voltage through an auto-transformer,
which serves to correct the output voltage, to plate-like auxiliary
electrodes mounted on the internal surface of the conduit. With
this construction, a profile of the voltage of the fluid in the
vicinity of the one probe coincides with a profile of the voltage
of the auxiliary electrodes. This flow meter is disadvantageous
in that the voltage of the one probe and the voltage of the auxiliary
electrodes do not coincide with each other when external conditions
vary, because no feed-back loop is established. Another disadvantage
is that the coincidence of the probe voltage with the auxiliary
electrode voltage must be made by manipulating a slider of a variable
resistor of the auto-transformer, which is rather troublesome.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide an electromagnetic
flow meter which can measure the flow rate of a fluid accurately.
According to the present invention, there is provided an electromagnetic
flow meter comprising a conductive conduit through which a fluid
flows; means for applying a magnetic field to the conduit to induce
an electromotive force in the fluid; signal detecting means for
detecting the electromotive force induced in the fluid to produce
a first detecting signal; means disposed in the vicinity of the
signal detecting means for detecting a voltage of the conduit to
produce a second detecting signal; electrical energy applying means
responsive to the first and second detecting signals for applying
a selected amount of voltage to the conduit so that the voltage
of the first detecting signal and the detected voltage of the conduit
become equal to each other, whereby a profile of the voltage of
the conduit is substantially similar to a profile of the electromotive
force at least at a position of the signal detecting means.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a conventional electromagnetic
flow meter having a lining;
FIG. 2 is a circuit diagram of an equivalent circuit of the flow
meter of FIG. 1;
FIG. 3 is another conventional electromagnetic flow meter having
no lining;
FIG. 4 is a schematic front elevational of the flow meter of FIG.
3;
FIG. 5 is a circuit diagram of the flow meter of FIG. 3
FIG. 6A is a diagrammatic illustration showing the relation between
a pipe and a detecting point on an internal surface of the pipe;
FIG. 6B is a diagrammatic illustration showing the relation between
the detecting point and the voltage at the detecting point;
FIG. 7 is a fragmentary view of the flow meter of FIG. 3;
FIG. 8 is a cross-sectional view of an electromagnetic flow meter
provided in accordance with the present invention;
FIG. 9 is a front elevational view of the flow meter of FIG. 8;
FIG. 10 is an illustration similar to FIG. 6A but representing
the present invention; and
FIGS. 11 to 18 are views showing modified electromagnetic flow
meters.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
An electromagnetic flow meter 10b shown in FIGS. 8 and 9 comprise
a fluid-carrying conduit 11 in the form of a conductive pipe and
a pair of diametrically-opposed measuring electrodes 15a and 15b
mounted on the conduit 11 through insulating members 16a and 16b.
A pair of electrodes 20a and 20b are mounted on the conduit 11 adjacent
to respective measuring electrodes 15a and 15b and spaced therefrom
in the axial direction of the conduit 11. The detecting electrodes
20a and 20b serve to detect the voltages at the portions of the
conduits where they are mounted, that is, the voltages V.sub.5 at
the portions of the conduit 11 adjacent to the measuring electrodes
15a and 15b. A pair of elongated auxiliary electrodes 17a and 17a
are mounted on the conduit 11 and extend in the axial direction
of the conduit 11 the electrodes 17a and 17a being circumferentially
spaced equidistantly from the measuring electrode 15a. Another pair
of elongated auxiliary electrodes 17b and 17b are mounted on the
conduit 11 and extend in the axial direction of the conduit, the
electrodes 17b and 17b being circumferentially spaced equidistantly
from the measuring electrode 15b. The measuring electrode 15a, 15b
is connected to one input terminal of amplifier 18a, 18b, and the
detecting electrode 20a, 20b is connected to the other input terminal
of the amplifier 18a, 18b. The output terminal of the amplifier
18a, 18b is connected to the auxiliary electrodes 17a, 17b. With
this arrangement, the amplifier 18a, 18b applies an output voltage
V.sub.6 to the auxiliary electrodes 17a, 17b so that the voltage
V.sub.5 is equal to the voltage V.sub.M. The auxiliary electrodes
17a, 17b and the amplifier 18a, 18b constitute an electrical energy
applying means. A pair of diametrically opposed grounding electrodes
(not shown) are mounted on the conduit 11 and disposed in a plane
perpendicular to the plane in which the measuring electrodes 15a
and 15b lie, the grounding electrodes being disposed respectively
between one of the electrodes 15a and one of the electrodes 15b
and between the other electrode 15a and the other electrode 15b.
With this construction, no current flows through the measuring
electrode 15a, 15b and the conduit voltage detecting electrode 20a,
20b, so that they are not subjected to a voltage drop. Therefore,
although current flows through the auxiliary electrode 17a, 17b
so that it is subjected to a voltage drop, the voltage V.sub.M of
each of the measuring electrodes 15a and 15b accurately coincides
with the voltage V.sub.5 of each of the detecting electrodes 20a
and 20b. In other words, the voltage V.sub.5 of the detecting electrode
20a, 20b accurately represents the voltage at that portion of the
conduit 11 adjacent to the measuring electrode 15a, 15b, and the
voltage at said that portion of the conduit 11 is equal to the voltage
V.sub.M of the measuring electrode 15a, 15b. The requirement that
the voltage V.sub.M is equal to the voltage at the portion of the
conduit 11 adjacent to the measuring electrode 15a, 15b is essential
for the electromagnetic flow meter having no lining to carry out
an accurate measurement of the flow rate of the fluid 14 through
the conduit 11. Thus, the electromagnetic flow meter 10b in FIGS.
8 and 9 can effect an accurate measurement of the flow rate.
Preferably, points A and B of the auxiliary electrodes 17a to which
the amplifying voltage V.sub.6 is supplied from the amplifier 18a
are disposed in a plane which is disposed perpendicular to the longitudinal
axis of the conduit 11 and contains the measuring electrode 15a.
Referring to FIG. 9 in this respect, the point A, the measuring
electrode 15a and the point B are disposed in alignment with one
another. Also, the points of the auxiliary electrodes 17b to which
the amplifying voltage is supplied from the amplifier 18b are disposed
in the same manner described for the points A and B. With this arrangement,
a conduit voltage profile is produced symmetrically with respect
to the plane in which the measuring electrode 15a, 15b lie. The
voltage profile of the electromagnetic flow meter 10b is shown in
an diagrammatic illustration of FIG. 10 in which a broken line c
represents a profile of voltage produced in the conduit 11. A solid
line a represents a profile of the voltage induced in the fluid
at the internal surface of the pipe 12 of the electromagnetic flow
meter 10 of FIG. 1 having the lining 13. As is seen from FIG. 10
the voltage profile c and the voltage profile a coincide with each
other accurately when .theta. is 0.degree. and 180.degree..
The conduit voltage detecting electrode 20a can be disposed at
any position between the auxiliary electrodes 17a and 17a, and similarly
the other detecting electrode 20b can be disposed at any position
between the auxiliary electrodes 17b and 17b.
FIG. 11 shows a modified electromagnetic flow meter 10c which differs
from the electromagnetic flow meter 10b in that each of the conduit
voltage detecting electrodes 20a and 20b is replaced by a pair of
longitudinally spaced electrodes 20c and 20c between which the measuring
electrode 15a, 15b is disposed, the spaced electrodes 20c and 20c
being connected together by an electrical lead 23.
FIG. 12 shows another modified electromagnetic flow meter 10d which
differs from the electromagnetic flow meter 10b of FIG. 8 in that
a conduit 11 is cut away at diametrically opposed portions, so that
the conduit wall varies in thickness at these portions. With this
arrangement, an apparent conductivity of the conduit 11 varies circumferentially
of the conduit. Therefore, the voltage profile of the conduit 11
and the voltage profile of the fluid 14 resemble each other both
in the vicinity of the measuring electrodes 15a and 15b and at positions
spaced from the electrodes 15a and 15b. In other words, referring
to FIG. 10 the voltage profile c approaches the voltage profile
a even when .theta. is not 0.degree. and 180.degree.. These two
voltage profiles of course coincide with each other when .theta.
is 0.degree. and 180.degree.. Therefore, this flow meter 10d can
measure the flow rate more accurately.
FIG. 13 shows a further modified electromagnetic flow meter 10e
which achieves the same effect as the electromagnetic flow meter
10d of FIG. 12. In this embodiment, each pair of auxiliary electrodes
17a and 17b have a half crescent shape. Thus, since the thickness
of the auxiliary electrodes varies circumferentially of the conduit
11 the same effect that the conductivity of the conduit 11 is equivalently
varied is achieved.
FIG. 14 shows a still further modified electromagnetic flow meter
10f which differs from the electromagnetic flow meter 10b of FIG.
8 in that a pair of additional amplifiers 25a and 25b and two pairs
of additional auxiliary electrodes 26a and 26b are provided. An
output voltage of the amplifier 25a is applied to the electrodes
26a and 26a so that the voltage of the conduit 11 and the voltage
of the fluid 14 at the auxiliary electrodes 26a and 26a become equal
to each other. Also, an output voltage of the amplifier 25b is applied
to the electrodes 26b and 26b for the same purposes. Therefore,
the voltage profile of the conduit 11 resembles the voltage profile
of the fluid 14 even at positions spaced from the measuring electrodes
15a and 15b, so that the flow rate can be measured more accurately.
FIG. 15 shows a further modified electromagnetic flow meter 10g
which differs from the electromagnetic flow meter 10b of FIG. 8
in that a lining 13 is mounted on the internal surface of conduit
12. Also, the electromagnetic flow meters in the preceding embodiments
may have a lining mounted internally of conduit 12. Such electromagnetic
flow meters having the conduit lining can properly measure the flow
rate of the fluid even when the lining is subjected to damage.
FIGS. 16 and 17 show a further modified electromagnetic flow meter
10h which differs from the electromagnetic flow meter 10b of FIG.
8 in that a pair of generally rectangular measuring electrodes 15c
and 15d are mounted in a lining 13 in insulating relation to conduit
12 and that a pair of guard electrodes 28a and 28b are mounted in
the lining 13 in surrounding relation to the respective measuring
electrodes. The guard electrodes 28a and 28b are connected to conduit
voltage detecting electrodes 20a and 20b, respectively. Thus, in
this embodiment, the measuring electrodes 15c and 15d are not in
contact with the fluid 14 in the conduit 12 and are disposed in
capacitive coupling relation to the fluid to detect the flow rate
thereof.
As described above, according to the present invention, a selected
amount of voltage is applied to the conduit so that the voltage
detected by the measuring electrodes and the voltage detected by
the conduit voltage detecting electrodes become equal to each other.
Therefore, the profile of the conduit voltage resembles the profile
of the fluid voltage in the vicinity of the measuring electrodes.
As a result, the flow rate of the fluid through the conduit can
be measured accurately.
While the electromagnetic flow meters according to the present
invention have been specifically shown and described herein, the
invention itself is not restricted by the exact showing of the drawings
or the description thereof. For example, each of the measuring electrodes
15a and 15b may be mounted on a recess 30 in the conduit, as shown
in FIG. 18. |