An magnetic flow meter is provided which includes a reference electrode
configured to electrically couple process fluid flowing within a
flowtube of the flow meter. The reference electrode is adapted to
measure potential of the process fluid. A current limiter is configured
to limit current flow through the reference electrode and thereby
reduce corrosion of the reference electrode.
What is claimed is:
1. A magnetic flow meter comprising: measurement circuitry; a flowtube;
at least first and second electrodes disposed within the flowtube
and coupled to the measurement circuitry; at least a reference electrode
operably coupled to the measurement circuitry and disposed to electrically
couple to process fluid within the flowtube; and a current limiter
coupled to the reference electrode and adapted to couple to the
measurement circuitry, the current limiter configured to reduce
corrosion of the reference electrode.
2. The flow meter of claim 1 wherein the reference electrode comprises
3. The flow meter of claim 1 and further comprising another reference
electrode operably coupled to the measurement circuitry and disposed
to contact process fluid.
4. The flow meter of claim 1 wherein the current limiter comprises
5. The flow meter of claim 4 wherein the resistor has a resistance
of between about 10 ohm and about 150 kohm.
6. The flow meter of claim 1 and further comprising a conductive
flow meter case containing the transmitter circuitry and being coupled
to the flowtube, wherein the current limiter is coupled to the case
and the case is adapted to coupled to ground.
7. The flow meter of claim 1 wherein the reference electrode is
coupled to the flowtube via a non-conductive coupler to electrically
isolate the reference electrode from the tube.
8. The flow meter of claim 1 wherein the measurement circuitry
includes an amplifier coupled to the first electrode and wherein
the amplifier is referenced to a potential of the process fluid
through the reference electrode and current limiter.
9. The flowmeter of claim 1 wherein the reference electrode comprises
a ground ring.
10. A flowtube for a magnetic flow meter, the flowtube comprising:
a conductive tube having a non-conductive inner surface; first and
second electrodes disposed on an inner surface and being adapted
to contact process fluid; a reference electrode mounted to the conductive
tube, and electrically isolated therefrom, the reference electrode
being disposed to electrically couple to process fluid; and a current
limiter configured electrically coupled to the reference electrode
and being adapted to couple in series to a measurement circuitry.
11. The flowtube of claim 10 wherein the current limiter is a resistor.
12. The flow meter of claim 10 wherein the resistor has a resistance
of between about 10 ohm and about 150 kohm.
13. The flowtube of claim 10 wherein the reference electrode comprises
14. The flowtube of claim 10 wherein the reference electrode is
mounted to the flowtube via a non-conductive coupler.
15. The flowtube of claim 10 wherein the reference electrode comprises
a ground ring.
16. A method of reducing corrosion of a reference electrode configured
to sense a potential of process fluid in a magnetic flow meter,
comprising: disposing at least first and second electrodes within
a flowtube and coupled to flow measurement circuitry; obtaining
a current limiter; and placing the current limiter electrically
in series with the reference electrode and flow measurement circuitry,
the current limiter configured to reduce corrosion of the reference
17. The method of claim 16 wherein the current limiter comprises
18. The method of claim 16 wherein the resistor has a resistance
of between about 10 ohm and about 150 kohm.
19. The method of claim 16 wherein the reference comprises a ground
FIELD OF THE INVENTION
The present invention is related to the process measurement and
control industry. More specifically, the present invention is related
to magnetic flow meters.
BACKGROUND OF THE INVENTION
Magnetic flow meters are used to measure flow of a conductive process
fluid through a flowtube. The conductive fluid flows past an electromagnet
and electrodes. In accordance with Faraday's law of electromagnetic
induction, an electromotive force (EMF) is induced in the fluid
due to an applied magnetic field. The EMF is proportional to the
flow rate of the fluid. The electrodes are positioned in the flowtube
to make electrical contact with the flowing fluid. The electrodes
sense the EMF that is magnetically induced in the fluid which can
then be used to determine flow rate. The EMF is measured by the
flow meter using a differential front end amplifier connected across
the electrodes. The potential of the process fluid is used as a
reference for the differential amplifier. Note that this reference
may not necessarily be Earth ground.
The transmitter must be referenced to the process to provide a
stable reading. This process connection is established by insuring
electrical connection between the flowtube and the process. This
can be done with ground rings which strap to flowtube, a ground
electrode which is connected directly to the flowtube, or a strap
between the flowtube and the adjacent conductive pipe. Earth ground
can provide a low noise reference and often is required by electrical
safety code. However, earth ground is not necessarily required for
proper operation. Some installations due to the electrical nature
of the process or the corrosiveness of the process fluid use either
plastic or non-conductive pipe or a lining in the metal pipe. In
these cases, the process may be at a different electrical potential
than earth ground. The connection between the ground electrode and
flowtube through bolts or some other means can provide a path for
electrical current to ground which may lead to corrosion of the
ground ring or ground electrode.
In many process installations, process piping carrying the process
fluid is conductive and is in contact with the process fluid. Accordingly,
simply connecting a strap from the flowtube to the process piping
will ensure that the conductive fluid is at the same potential as
the flowtube. However, in some applications, the process piping
itself may be non-conductive, or may have a non-conductive inner
lining. Thus, electrical contact to the process piping itself will
not establish a reference to the process fluid. In these situations,
an alternative technique must be used to electrically couple to
the process fluid. For example, a process reference can be accomplished
by using either ground rings or a ground electrode within or adjacent
to the flow meter.
One of the problems that has occurred in magnetic flow meters in
accordance with the prior art is significant corrosion of ground
electrodes. The connection between the ground electrode and flowtube
through bolts or some other means can provide a path for electrical
connection to ground which may lead to corrosion of the ground ring
or ground electrode. In installations where ground electrodes tend
to corrode, the flowtube can be electrically isolated from earth
ground to remove the electrical path to ground. This will generally
prevent any electrical current from flowing through the process
fluid and the ground electrode to earth ground. While this approach
has generally resolved many problems, it has not addressed all situations.
Some situations continue to exist where it is not feasible to isolate
the flowtube from ground due to the particular application. One
example of such a case is where the bolts themselves used to install
the flowtube provide an electrical path between the flowtube and
the adjacent process piping. Another example is the use of metal
lined pipe which prevents isolation of the flowtube from adjacent
piping. However, this will likely provide some path to earth resulting
in stray current corrosion of the ground electrode or ground ring.
In such environments, grounding rings can be used. Grounding rings
provide a greater surface area in comparison to a ground electrode
and the corrosion is much less problematic. However, in some situations,
ground rings are impractical. For example, the failure of a ground
ring can result in leaking of the process fluid. Further, the use
of an inert metal such as platinum is expensive. Accordingly, providing
a magnetic flow meter with a ground electrode that can resist corrosion
and is less expensive than ground rings would be particularly useful
in some installations.
SUMMARY OF THE INVENTION
A magnetic flow meter includes circuitry that is adapted to be
electrically coupled to a process fluid. A reference contact is
configured to contact the process fluid flowing within a flowtube.
An electrical component is provided in series between the reference
contact and the circuitry to reduce the flow of electrical current
through the reference contact.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partially cut away view of a magnetic flow meter in
which embodiments of the present invention are particularly useful.
FIG. 2 is a diagrammatic view of a magnetic flow meter in which
embodiments of the present invention are particularly useful.
FIG. 3 is a diagrammatic view of a portion of the flowtube for
use within a magnetic flow meter in accordance with an embodiment
of the present invention.
FIG. 4 is a diagrammatic view of a magnetic flow meter in accordance
with an embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A magnetic flow meter is disclosed that provides increased ground
electrode corrosion resistance in response to stray currents present
in the process. In particular, embodiments of the present invention
act to limit, or other inhibit, stray currents present in some process
installations from flowing through the ground electrode to ground.
FIG. 1 is a partially cut away view of an embodiment of a magnetic
flow meter in which embodiments of the present invention are particularly
useful. Magnetic flow meter 20 includes a flowtube 22 formed of
low magnetic permeability material with an electrically insulating
liner 23 an electromagnet 26 is formed by a coil, a ferromagnetic
core or shield 28 and electrodes 30 32. The electromagnet 26 and
the electrodes 30 32 are wired to a transmitter circuit 34 as is
ground electrode 35. In operation, the transmitter circuit 34 drives
the electromagnet 26 with an electrical current, and the electromagnet
26 produces a magnetic field 36 indicated by arrows inside the flowtube
22. Process liquid 21 flows through the magnetic field in the flowtube
22 and the flow induces an electromotive force (EMF, voltage) in
the liquid 21. The insulating liner 23 prevents leakage of the EMF
from the liquid 21 to the metal flowtube 22. The electrodes 30
32 contact the liquid 21 and pick up or sense the EMF which, according
to Faraday's law, is proportional to the flow rate of the liquid
21 in the flowtube 22.
FIG. 2 is a diagrammatic view of circuitry of a prior art magnetic
flow meter. The magnetic flow meter 120 includes a flowtube 124
that has an insulated liner 126 adapted to carry a flowing liquid
128 that is electrically coupled to the flowtube 124 and is generally
connected to earth ground 130. When the process piping is electrically
coupled to the process fluid, an electrical connection between the
piping and the flowtube provides the required electrical coupling
of process fluid 128 to the flowtube. Coils 134 are positioned to
apply a magnetic field to the process fluid in response to a drive
signal from drive circuitry 152. Electrodes 138 and 140 couple to
measurement circuitry 154 through amplifiers 150 and 148 respectively.
Measurement circuitry 154 provides an output related to flow in
accordance with known techniques.
As illustrated in FIG. 2 components within magnetic flow meter
120 are typically coupled to a common reference. For example, amplifiers
148 and 150 are referenced to a common reference which is connected
to flowtube. This allows the transmitter to eliminate noise common
to each electrode with reference to the process.
The configuration illustrated in FIG. 2 works particularly well
where the process piping itself is metallic and thus can be connected
directly to flowtube providing a strong electrical reference to
the process fluid. There are however some situations where the process
piping does not provide an electrical reference to the process.
Specifically, some process installations use non-conductive piping
or use conductive piping with non-conductive inner linings. In these
cases, it is still important for the front end amplifier to be reference
to the potential of the process fluid. This is because while the
potential of the process fluid may vary significantly depending
on stray currents, and/or interference, the potential measured across
the electrodes 138 140 is typically on the order of one or more
millivolts. In these cases, a third grounding electrode is used
with the magnetic flow meter. This grounding electrode is used to
electrically contact the process fluid. However, in some installations,
corrosion of the grounding electrode occurred unacceptably rapidly.
The invention includes the recognition that excessive corrosion
of the ground electrode can be caused by stray currents present
in the process fluid which are shunted to ground through the electrode.
For example, some processes require application of large potentials
or electrical currents to the process fluid which may leak through
the ground electrode.
FIG. 3 is a diagrammatic view of a portion of a flowtube for use
within magnetic flow meter in accordance with an embodiment of the
present invention. Portion 200 of flowtube includes a pair of electrodes
138 140 extending through conductive casing 202 via non-conductive
couplers 204 206 respectively. Electrodes 138 140 further extend
through non-conductive lining 208 such that each of the electrodes
138 140 electrically contact the fluid flowing within portion 200.
Electrodes 138 and 140 couple to circuitry 198 (shown in FIG. 4)
through connectors 222 and 224 respectively. In FIG. 3 ground
electrode 212 passes through case 202 via a non-conductive coupler
214 which is preferably of a similar type of couplers 204 and 206.
However, any manner of passing an electrically conductive electrode
through conductive casing 202 in a non-conductive manner, or otherwise
providing electrical access to the interior of case 202 while isolating
electrode 212 therefrom can be used. Ground electrode 212 is coupled
to circuitry 198 (shown in FIG. 4) through a current limiter 216
and connection 225. In one embodiment, current limiter 216 is simply
a resistor. However, any device, or circuit which can function to
limit or reduce the current component passing therethrough can be
used to practice embodiments of the present invention. Preferably,
current limiter 216 allows the potential of the process fluid to
be coupled to measurement circuitry 198. Accordingly, current limiter
216 can include a filter or other electrical component or circuit.
Additionally, while FIG. 3 illustrates simply one ground electrode
212 any number or configuration of such electrodes can be used
in order to spread the corrosion over a plurality of such electrodes.
In some embodiments, the ground electrode 212 can comprise a ground
FIG. 4 illustrates a magnetic flow meter 300 in accordance with
an embodiment of the present invention. Components which are similar
to components shown in FIG. 2 are numbered the same. The flowtube
includes a ground electrode 212 that is operably coupled to amplifiers
148 150 through current limiter 216. Accordingly, the output of
amplifiers 148 150 are referenced to the potential of the process
Although the present invention has been described with reference
to preferred embodiments, workers skilled in the art will recognize
that changes may be made in form and detail without departing from
the spirit and scope of the invention. Typically, when a resistor
is employed for the current limiter, its resistance will be between
10 ohm and 50 kohm, however, any appropriate value can be used,
for example 100 kohm, 150 kohm or more. The ground electrode can
be of any appropriate material such as platinum. The current limiter
can be an integral component of the ground electrode, for example
by adding impurities to the electrode or fabricating the limiter
with the electrode.