Abstrict A magnetic flow meter has a diagnostic circuit that indicates leakage
from flowtube electrodes. The diagnostic circuit couples to the
electrodes and to ground. The diagnostic circuit samples diagnostic
potentials between each electrode and ground, and then generates
a sum of the diagnostic potentials between a first and a second
electrode that indicates the leakage. The flowtube includes an insulated
tube and an electromagnet. A transmitter circuit couples to the
electromagnet and electrodes, and then generates a transmitter output
representing a flow rate, which can be corrected for the leakage.
Claims What is claimed is:
1. A magnetic flow meter, comprising: a flowtube having an insulated
tube adapted to carry a flowing liquid that is coupled to a ground,
the flowtube having an electromagnet, and an electrode circuit including
first and second electrodes; a transmitter circuit coupled to the
electromagnet, to the electrode circuit and to the ground, the transmitter
circuit generating a transmitter output representing a flow rate
of the liquid as a function of a differential potential on the electrode
circuit; and a diagnostic circuit coupled to the electrode circuit
and to the ground, the diagnostic circuit sensing a first diagnostic
potential between the first electrode and ground, and sensing a
second diagnostic potential between the second electrode and ground,
and generating a diagnostic output indicating leakage from the electrode
circuit as a function of a sum of the first and second diagnostic
potentials.
2. The magnetic flow meter of claim 1 wherein the leakage is from
the electrode circuit to ground.
3. The magnetic flow meter of claim 1 wherein the leakage is from
the electrode circuit to the electromagnet.
4. The magnetic flow meter of claim 1 wherein the diagnostic output
indicates electrode-to-ground leakage when the sum of diagnostic
potentials is in a first range, and indicates electrode-to-electromagnet
leakage when the sum of diagnostic potentials is in a second range
larger than the first range.
5. The magnetic flow meter of claim 1 wherein transmitter circuit
couples a drive output to the electromagnet, and the diagnostic
circuit calculates a sampled sum that is sampled in synchronization
with the drive output.
6. The magnetic flow meter of claim 5 wherein the diagnostic circuit
calculates an absolute value of the sampled sum.
7. The magnetic flow meter of claim 1 wherein the transmitter circuit
includes a correction circuit generating a corrected transmitter
output as a function of the transmitter output and the diagnostic
output.
8. The magnetic flow meter of claim 7 wherein the correction circuit
scales the corrected transmitter output as a function of a ratio
of the diagnostic output to the transmitter output when the diagnostic
output is in the first range.
9. The magnetic flow meter of claim 8 wherein the transmitter output
is corrected according to the equation:
where CM is one half of the sum, and DM is the differential potential.
10. A magnetic flow meter, comprising: a flowtube having an insulated
tube adapted to carry a flowing liquid that is coupled to ground,
the flowtube having an electromagnet, and first and second electrodes;
a transmitter circuit coupled to the electromagnet, the first and
second electrodes and the ground, the transmitter circuit generating
a transmitter output representing a flow rate of the liquid as a
function of a differential potential between the first and second
electrodes; and diagnostic means coupled to the electrodes and the
ground for sensing a first diagnostic potential between the first
electrode and ground, and for sensing a second diagnostic potential
between the second electrode and ground, and for generating a diagnostic
output indicating electrode leakage as a function of a sum of the
first and second diagnostic potentials.
11. A computer readable medium having stored thereon a plurality
of sequences of instructions, the plurality of sequences of instructions
including sequences which, when executed by a processor in a magnetic
flow meter, cause the processor to perform the sequence: receiving
a first diagnostic potential between a first electrode of a magnetic
flow meter and ground; receiving a second diagnostic potential between
a second electrode of a magnetic flow meter and ground; and generating
a diagnostic output indicating a presence of electrode leakage as
a function of a sum of the first and second diagnostic potentials.
12. The computer readable medium of claim 11 further having sequences
of instructions that perform the following sequence: summing the
first and second diagnostic potentials; sampling the diagnostic
potentials; calculating an absolute value of the diagnostic potentials;
comparing the absolute value of diagnostic potentials to a stored
reference; indicating electrode leakage as a function of the comparing;
and correcting the transmitter output for the indicated leakage.
13. A process for operating a magnetic flow transmitter, comprising:
summing first and second diagnostic potentials received from corresponding
first and second electrodes of a flowtube; sampling the diagnostic
potentials; calculating an absolute value of the diagnostic potentials;
comparing the absolute value of diagnostic potentials to a stored
reference; indicating electrode leakage as a function of the comparing;
and correcting the transmitter output for the indicated leakage.
14. A magnetic flow meter, comprising: a flowtube having an insulated
tube adapted to carry a flowing liquid that is coupled to ground,
the flowtube having an electromagnet, and first and second electrodes;
a processor system coupled to the electromagnet, and the first and
second electrodes, the processor system generating a transmitter
output representing a flow rate of the liquid as a function of a
differential potential between the first and second electrodes,
and the processor system sensing a first diagnostic potential between
the first electrode and ground, and sensing a second diagnostic
potential between the second electrode and ground, and generating
a diagnostic output indicating a presence of electrode leakage as
a function of a sum of the first and second diagnostic potentials.
Description BACKGROUND OF THE INVENTION
The present invention relates to magnetic flow meters that sense
liquids flowing in industrial process plants. In particular, the
present invention relates to electrode circuits in such magnetic
flow meters.
Magnetic flow meters utilize an insulated flowtube that carries
liquid flowing past an electromagnet and electrodes. The electrodes
are sealed in the flowtube to make contact with the flowing liquid.
The electrodes sense an electromotive force (EMF) magnetically induced
in the liquid, and proportional to flow rate according to Faraday's
law of electromagnetic induction.
Electrical leakage from the electrodes or electrode wiring can
give rise to measurement errors in the transmitter output that can
go undiagnosed by the operator of the process plant for long periods
of time. One technique to address the problem of electrical leakage
is to attempt to limit errors due to the electrical leakage. For
example, a transmitter circuit with an extremely high input impedance
is used to sense the EMF. The wiring between the electrodes and
the transmitter is also carefully insulated to avoid leakage or
extraneous noise. However, these techniques do not attempt to diagnose
or quantify the electrical leakage.
SUMMARY OF THE INVENTION
A magnetic flow meter is disclosed that includes a diagnostic circuit
indicating a presence of electrical leakage in an electrode circuit
in the magnetic flow meter. The diagnostic circuit couples to first
and second electrodes in the flowtube and to the flowtube ground.
The diagnostic circuit senses a first diagnostic potential between
the first electrode and ground, and senses a second diagnostic potential
between the second electrode and ground.
The diagnostic circuit generates a diagnostic output as a function
of a sum of the first and second diagnostic potentials. The sum
of the potentials indicates whether there is electrical leakage.
The flowtube includes an insulated tube adapted to carry a flowing
liquid that is coupled to the ground. The flowtube also includes
an electromagnet.
A transmitter circuit couples to the electromagnet, the first and
second electrodes and the ground. The transmitter circuit generates
a transmitter output representing a flow rate of the liquid as a
function of a differential potential between the first and second
electrodes.
The diagnostic output indicates whether the accuracy of the transmitter
output is affected by leakage so that corrective action can be taken.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a magnetic flow meter;
FIG. 2 illustrates leakage between an electrode circuit and an
electromagnet in an magnetic flow meter;
FIG. 3 illustrates leakage between an electrode and ground in a
magnetic flow meter;
FIG. 4 illustrates a first embodiment of a magnetic flow meter
with a diagnostic circuit;
FIG. 5 illustrates a second embodiment of a magnetic flow meter
with a diagnostic circuit;
FIG. 6 illustrates a sampled waveform of a differential electrode
signal under normal and leakage conditions;
FIG. 7 illustrates a sampled waveform of a summed (common mode)
electrode signal under normal and leakage conditions;
FIG. 8 illustrates a transmitter output (flow) signal and a summed
electrode signal during a transition from normal to leaking conditions;
FIG. 9 illustrates correction of a transmitter output (flow) signal
during a transition from normal to leaking conditions in the flowtube;
FIG. 10 illustrates a third embodiment of a magnetic flow meter
with a diagnostic circuit; and
FIG. 11 is a flow chart of a diagnostic process.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
A magnetic flow transmitter is disclosed in which a diagnostic
circuit detects undesired excessive electrical leakage in an electrode
circuit of a magnetic flow tube. The electrical leakage is often
the result of process liquid leaking past a damaged seal around
one of the magnetic flow meter electrodes. The electrical leakage
can reduce the accuracy of the transmitter output. The diagnostic
circuit senses electrode-to-ground diagnostic potentials at each
of two electrodes and forms a sum of the two diagnostic potentials.
The sum of the diagnostic potentials indicates whether there is
excessive leakage in the flow meter electrode circuit. When leakage
is detected with the diagnostic circuit, corrective action can be
taken by the process plant operator or by a correction circuit in
the transmitter.
Use of the diagnostic circuit avoids a situation where the magnetic
flow transmitter output appears to be indicating flow accurately,
but actually is inaccurate due to undetected leakage in the electrode
circuit.
In FIG. 1 a partially cutaway view of an embodiment of a magnetic
flow meter 20 is illustrated. Magnetic flow meter 20 includes a
flowtube 22 formed of low magnetic permeability material with an
electrically insulating liner 23 an electromagnet 24 with coils
26 a ferromagnetic core or shield 28 and electrodes 30 32. The
electromagnet 24 and the electrodes 30 32 are wired to a transmitter
circuit 34. In operation, the transmitter circuit 34 drives the
electromagnet 24 with an electrical current, and the electromagnet
24 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 flow tube 22.
The EMF from electrodes 30 32 is carried to the transmitter circuit
34 by leads 38 that are insulated to avoid leakage. The transmitter
circuit 34 has an electrode input circuit with high input impedance
to limit leakage as well.
The electrodes 30 32 are sealed to the insulating liner 23 however,
with aging, wear or corrosion damage, the seal between the electrodes
30 32 and the insulating liner 23 can be broken. Process liquid
21 can seep past the broken seal and can form electrical leakage
paths from the electrode circuit to the flowtube 22 which is grounded.
Liquid leakage can also form electrical leakage paths from the electrode
leads 28 to the electromagnet 24. In most instances, the flowtube
20 or the transmitter 34 includes terminal blocks (not shown in
FIG. 1) for connecting electrode leads 38. These terminal blocks
can become contaminated with liquid that also forms leakage paths
from the electrode wiring to ground or to the drive circuit for
the electromagnet 24.
In FIG. 2 a partial cross-sectional view of an embodiment of a
flowtube 50 is illustrated. Flowtube 50 includes electromagnet coils
52 54 that are wired by leads 56 58 60 to a terminal block 62.
A flowtube 64 lined with an insulating liner 66 is filled with a
flowing process liquid 68. Electrodes 70 72 contact the process
liquid 68 and are sealed to the liner 66. Electrodes 70 72 are
insulated from the flowtube 64 to prevent electrical leakage. Electrode
leads 74 76 are insulated and shielded and connect the electrodes
70 72 to the terminal block 62. A cable (not shown) connects the
leads at terminal block 62 to electronic transmitter circuitry which
is explained in more detail below. When the seal between electrode
72 and liner 66 is damaged or broken, process liquid 68 can leak
past the seal as illustrated by dots 80 and run or condense in various
locations on the electrode 72 the electrode leads 7476 or the
electromagnet coils 52 54. The leaked process liquid forms undesired
electrical leakage paths from the electrode 72 electrode lead 76
(i.e., the electrode circuit) to the grounded flowtube 64 or to
the electromagnet coils 52 54.
In FIG. 3 an enlarged partial cross-sectional view of an electrode
90 illustrates the liquid leakage in more detail. Electrode 90 is
mounted in a flowtube 92 that has an insulating liner 94. Electrode
90 has a shaft with a threaded portion 96 that engages a nut 98.
Nut 98 is advanced on the threaded portion to compress a spring
washer 100 ("Belleville spring") against a metal thrust
washer 102. Thrust washer 102 in turn, presses against insulating
bushing 104 which presses against the flowtube 92. The force from
the compression of the spring washer 100 causes the sharp outer
rim 106 of the electrode 90 to sink into the insulating liner 94
and form a liquid seal. The liquid seal thus formed is generally
reliable, however, with aging, misuse, corrosion, etc. the seal
can eventually fail, allowing process liquid 108 represented by
dots, to seep past the failed seal and complete an electrical leakage
path 110 from the electrode 90 to the grounded flowtube 92. This
leakage path 110 loads the flow-induced EMF and causes a flow measurement
error, however, this error is often not noticeable by an operator
of a process plant for a long time.
In FIG. 4 an embodiment of a magnetic flow meter 120 is illustrated.
Magnetic flow meter 120 includes a diagnostic circuit 122 which
can sense electrical leakage and provide an indication 164 to the
operator when leakage occurs. The electrical leakage is usually
caused by liquid leakage as illustrated in FIGS. 2-3. The magnetic
flow meter 120 includes a flowtube 124 that has an insulated tube
or liner 126 adapted to carry a flowing liquid 128 that is coupled
to a ground 130. The coupling of the liquid 128 to ground is usually
completed by way of contact between the liquid 128 and metal piping
mating with the flowmeter. The flowtube 124 has an electromagnet
132 mounted on it. Electromagnet 132 includes coils 134 and a magnetic
return path or core, illustrated schematically at 136. First and
second electrodes 138 140 together with electrode leads 142 144
form an electrode circuit 146.
The electrode circuit 146 can also include amplifiers 148 150.
Amplifiers 148 150 are typically unity gain buffers (also called
impedance converters) that have extremely high impedance, low leakage
inputs, but low impedance outputs. The amplifiers 148 150 simply
replicate each electrode voltage at the corresponding amplifier
output, but isolate the electrodes from the loads connected to the
outputs of amplifiers 148 150. The amplifiers 148 150 may be mounted
on the flowtube 124 or mounted in the transmitter housing, depending
on the needs of the application. The amplifiers provide a low leakage
sensing input for the electrode circuit 146 and may be seen as
part of the electrode circuit. The electrode circuit 146 may also
be shielded with driven shields (not illustrated) that are driven
by the outputs of the amplifiers 148 150.
A transmitter circuit 152 which can be of conventional design,
couples to the electromagnet 132 to the electrode circuit 146 (by
way of buffers or amplifiers 148 150) and to the ground 130. The
transmitter circuit 152 generates a transmitter output 154 representing
a flow rate of the liquid 128 as a function of a differential potential
on the electrode circuit 146. In transmitter circuit 152 the outputs
of amplifiers 148 150 are subtracted to provide an indication of
flow. This subtraction can be done in transmitter circuit 152 using
an analog differential amplifier or various known types of digital
signal processing circuits that compute a difference or subtraction.
The diagnostic circuit 122 is also coupled to the electrode circuit
146 (via buffer amplifiers 148 150) and to the ground 130. The
diagnostic circuit 122 senses a first diagnostic potential 160 between
the first electrode 138 and ground 130. The diagnostic circuit 122
also senses a second diagnostic potential 162 between the second
electrode 140 and ground 130. The diagnostic circuit 122 generates
a diagnostic output 164 that indicates leakage from the electrode
circuit 146 as a function of a sum of the first and second diagnostic
potentials 160 162. The diagnostic potentials 160 162 require
a liquid ground reference for measurement of each diagnostic potential,
whereas the flow or differential potential can be measured without
reference to the ground 130. Comparison of the diagnostic potentials
160 162 provides an indication as to whether the ground 130 is
centered or balanced relative to the electrode potentials. If the
ground is not centered or balanced, then electrode leakage can be
inferred.
When the electrode circuit 146 is free of leakage, it is found
that the flow-induced EMF on each electrode relative to ground (diagnostic
potentials) tend to be balanced or equal, but of opposite polarity.
When these two diagnostic potentials 160 162 are summed or added
together, the result tends to be near zero under normal operating
conditions without leakage.
When there is leakage, however, it is found that the diagnostic
potentials 160 162 tend to be imbalanced or unequal. When these
two diagnostic potentials are summed or added together, the result
tends to be a substantial non-zero value that indicates that leakage
is present.
When the sum of the diagnostic potentials is imbalanced but approximately
in the range of the normal differential flow-induced EMF, then the
leakage can be inferred to be a leakage from some part of the electrode
circuit to ground.
When the sum of the diagnostic potentials is imbalanced and much
larger than the normal range of differential flow induced EMF, then
the leakage can be inferred to be leakage from some part of the
electrode circuit to some part of the much higher voltage electromagnet
and its associated wiring.
The diagnostic output 164 can be arranged to indicate electrode-to-ground
leakage when the sum of diagnostic potentials is in a first, lower
range, and indicates electrode-to-electromagnet leakage when the
sum of diagnostic potentials is in a second, higher range, that
is larger than the first range. This is explained in more detail
below in connection with FIG. 11.
Typically, the transmitter output 154 will be a 4-20 mA analog
signal, and the diagnostic output 164 will be a HART protocol signal
superimposed on the 4-20 mA analog loop signal.
In one preferred embodiment, the transmitter circuit 152 provides
an approximately square wave drive or excitation current to electromagnet
132 and the corresponding electrode potentials are also approximately
square waves, including "flat" time intervals when the
flow induced EMF is flat or stable. In this preferred embodiment,
the diagnostic potentials are sampled during the time intervals
when the flow-induced EMF is flat or stable. The diagnostic circuit
122 calculates a sampled sum that is sampled in synchronization
with the drive to the electromagnet 132 ensuring that sampling
is done during a stable interval. The sampled sum alternates along
with the drive, and the diagnostic circuit also preferably calculates
an absolute value of the sampled sum to remove this alternation.
In FIG. 5 a second embodiment of a magnetic flow meter 180 with
a diagnostic circuit 182 is illustrated. The magnetic flow meter
180 shown in FIG. 5 is similar to the magnetic flow meter 120 shown
in FIG. 4 and the same or similar parts in FIGS. 4 and 5 are identified
using the same reference numerals. Diagnostic circuit 182 includes
an adder 186 a sampling circuit 188 and an absolute value calculating
circuit 190. The sampling circuit 188 is synchronized by synchronization
line 192 so that diagnostic potentials are obtained during a flat
or stable portion of the electromagnet pulsed or square wave drive.
Magnetic flow meter 180 also includes a correction circuit 184.
The correction circuit 184 generates a corrected transmitter output
194 as a function of a transmitter output 196 (that is not corrected
for leakage) and the diagnostic output 198. The correction circuit
184 scales the corrected transmitter output 194 as a function of
a ratio of the diagnostic output 198 to the uncorrected transmitter
output 196) when the diagnostic output is in a first or lower range.
In this first or lower range, the sum of the diagnostic potentials
is low enough to indicate that the leakage detected is leakage to
ground, which can be estimated and corrected. Preferably, the transmitter
output is corrected according to the equation:
The diagnostic output 198 can also be coupled outside the transmitter
180 for use by a technician or operator.
FIG. 6 is a display image of digitally sampled waveforms of differential
electrode signal under normal and leakage-to-ground conditions.
The waveforms of normal and leakage conditions are superimposed
on the same display to provide convenient comparison of the two
waveforms. The vertical axis 200 represents differential flow signal
amplitude expressed in normalized counts of an A/D converter in
a digital sampling oscilloscope. The horizontal axis 202 represents
elapsed time expressed as sample numbers. A first waveform 204 illustrates
a normal differential electrode signal waveform under test conditions
of approximately 10 foot per second liquid flow rate and an approximately
square wave electromagnet drive at a frequency of about 6 Hertz.
The peak-to-peak amplitude between level or stable portions of this
normal waveform 204 is approximately 40000 counts peak-to-peak.
Next, one of the electrodes is sprayed with water to create a leakage
to ground condition, and a second waveform 206 is sampled under
this leakage to ground condition. The second waveform has a peak-to-peak
amplitude between level portions of about 24000 counts. In other
words, when one electrode has a leakage to ground, the amplitude
of the differential electrode has a error of approximately 15%.
The differential waveform 206 however, appears normal in other
respects and gives no hint to the operator that the flow meter is
malfunctioning due to leakage.
FIG. 7 is a display image of superimposed, digitally sampled waveforms
of summed (common mode) diagnostic potentials under normal and leakage-to-ground
conditions. In FIG. 7 the vertical and horizontal axes are as explained
in connection with FIG. 6 above. Under normal operating conditions,
the summed diagnostic potential 210 ranges between plus and minus
5000 counts due to power line noise, but has approximately a zero
count value when the power line (60 Hz) noise is averaged or filtered
out. Under conditions of leakage, however, the average summed diagnostic
potential 212 shifts back and forth between -3000 and +3000 counts
each time the polarity of the electromagnet drive changes. The summed
diagnostic potential gives a detectable indication of electrode
leakage.
FIG. 8 illustrates a differential (flow) signal and a summed (common
mode) electrode signal during a transition from normal to leaking
conditions. The vertical axis 200 represents electrode signal amplitudes
expressed in normalized counts of an A/D converter in a digital
sampling oscilloscope. The horizontal axis 202 represents elapsed
time expressed as sample numbers. A leakage-to-ground condition
is simulated by pouring water over a portion of one the electrodes
that is external to the flow tube as shown at time 218.
In FIG. 8 a digitally sampled waveform of differential electrode
signal under normal conditions is shown at 220 and under leakage-to-ground
conditions is shown at 222. The change in this differential electrode
signal, which represents flow, after the leak is about -21.62%.
This amount of change is within the normal range of expected flow
signals and thus cannot be distinguished from an actual change in
flow rate, and can go undetected for a long period of time.
A waveform of summed, also called common mode, electrode signal
is displayed under normal conditions at 224 and under leakage to
ground conditions at 226. The change in the common mode electrode
signal when the leak is introduced is approximately 1000% which
is easily distinguishable from normal operating conditions, and
provides a good indication of leakage. These waveforms are obtained
under test conditions of approximately 10 foot per second liquid
flow rate and an approximately square wave electromagnet drive at
a frequency of about 6 Hertz.
FIG. 9 is a display image of digitally sampled waveforms of an
uncorrected transmitter flow output signal under normal conditions
at 230 and under leakage-to-ground conditions at 232. The uncorrected
change or error in the flow output, after the leak is about -21.62%.
The transmitter flow output shown at 230 232 has not been automatically
corrected based on the common mode signal.
A waveform of summed, also called common mode, electrode signal
is displayed under normal conditions at 234 and under leakage to
ground conditions at 236. The change in the common mode electrode
signal when the leak is introduced is approximately 1000% which
is easily distinguishable from normal operating conditions, and
provides a good indication of leakage.
These waveforms are obtained under test conditions of approximately
10 foot per second liquid flow rate and an approximately square
wave electromagnet drive at a frequency of about 6 Hertz.
When automatic correction based on the common mode signal is used,
the corrected flow output has an error of 0.12% before the leak
is introduced as shown at 238 and the corrected flow output has
an error of -1.77% after the leak is introduced. The automatic correction
reduces the flow output error from -21.62% to only -1.77% in this
particular test. Results will vary depending on the test conditions,
however, generally a more accurate indication of flow is obtained
under leakage conditions when the correction is made.
FIG. 10 illustrates a magnetic flow meter 250 utilizing a processor
system 252 that combines the functions of the transmitter circuit
and the diagnostic circuit. The flow meter 250 is similar to the
flow meters 120 180 shown in FIG. 10 and features that are identical
or similar to features in FIGS. 4 5A flowtube have the same reference
numbers.
Processor system 252 includes a processor 254 and memory 256. A
diagnostic algorithm 258 is stored in memory 256. The processor
system 252 is coupled to a coil driver 152 and to first and second
electrodes via amplifiers 148 150 and an analog-to-digital converter
260. The processor system generates a transmitter output 154 representing
a flow rate of liquid as a function of a differential potential
between the first and second electrodes. The processor system senses
a first diagnostic potential between the first electrode and ground,
and also senses a second diagnostic potential between the second
electrode and ground. The processor system generates a diagnostic
output 164 indicating a presence of electrode leakage as a function
of a sum of the first and second diagnostic potentials. The processor
system, if desired, can correct the transmitter output as a function
of the correction output using the diagnostic algorithm 258.
FIG. 11 illustrates the diagnostic process 270 performed in the
processor system 252 shown in FIG. 10. The process steps can be
stored as a diagnostic algorithm 258 in the processor memory 256.
The diagnostic algorithm can be stored in ROM, or if desired, the
diagnostic algorithm can be stored in alterable memory such as EEPROM.
The algorithm can be loaded in memory from a computer readable medium
having stored thereon a plurality of sequences of instructions,
the plurality of sequences of instructions including sequences which,
when executed by a processor in a magnetic flow meter, cause the
processor to perform the diagnostic sequence.
In FIG. 11 the diagnostic algorithm 270 starts at 272. The sum
of electrode voltages is calculated at 274. The resulting sum is
then sampled at 276 preferably during a time interval when the
magnetic field and electrode voltage are flat or stable. Next, an
absolute value of the sampled sum is calculated at 278 to remove
alternations in polarity. The absolute value is then compared at
280 to a reference 282 to classify the leakage conditions. If the
absolute value is low, then no leakage or malfunction is indicated
as shown at 284. If the absolute value is approximately in the range
of the normal flow signal, then leakage to ground is indicated at
286. If the absolute value is much large than normal flow signals,
then leakage to an electromagnet coil is indicated at 288.
The leakage conditions including leakage or malfunction are output
as shown at 290 and the transmitter output can be automatically
corrected, if desired, as shown at 292. After completion of a diagnosis,
the algorithm returns at 294 to the start to repeat the algorithm.
Use of the diagnostic algorithm 270 avoids a situation where the
magnetic flow transmitter output appears to be indicating flow accurately,
but actually is inaccurate due to undetected leakage in the electrode
circuit.
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. |