The following technical papers describe our
instruments and how to use them:
How to Get Better Accuracy with a Clamp-on Direct
Current Ammeter
Introduction
Clamp-on direct current ammeters are tools for
solving cathodic protection problems. They can measure interference
in a gas pipe or support column, measure anode current on a subsea
structure, or find lost CP current due to bad pipeline insulation,
defective anode leads, or poor structural electrical contact.
This abbreviated paper discusses how much lost
current can you find? What is the resolution? Accuracy? The answer
depends on:
- The equipment. This paper reports on results
using our technology. [Patent 3,768,011 and others pending
describe the concepts we use to build our clamp-on ammeters.]
- The application. Accuracy is different with a
¾" cable or a 48" oil pipe.
- The method. A "pig" may leave spots having
considerable magnetic intensity. Proper use of the Floating Zero
Procedure can reduce the error by ten to one.
The unabridged
version of this paper is available upon request.
The Equipment
Sensors
MER2™ Meters come with ¾" to 82 inch diameter aperture
sensors. The 6 inch and smaller are clips for one hand use. The 8"
to 82" are clamps having two "C" sections which mate at the "lips"
to form a closed ring.
These are secured with brass finger nuts on captive studs. A clamp
is shown in Fig. 1.
Figure 1
The sensor of a clamp-on DC ammeter is secured around a pipe
carrying 0.500 A CP current. This is the signal input to the
ammeter.
The clamp is said to be placed in the positive polarity sense
because the bridle is nearer the source of positive return
current from the earth.
The zero control (Z) on the indicator was set so I read 0.0
when all current in the pipe was temporarily cut off. When the
current came back on, the meter reading was 0.503 A. This
is acceptably close to the true current of 0.500 A.
It is better to use a clamp which is big enough to
fit comfortably around the pipe, but not more than twice as big.
This is because the zero offset due to the Earth's magnetic field He
increases with the size of the sensor, and so also does the
sensitivity to nearby magnets. He is measured by rotating the clamp
in a vertical North - South plane. The equivalent input current
change in zero offset, He, is less than 0 ± 3 mA for a ¾" clip; 0 ±
100 mA for a 13" clamp, and 0 ± .75 A (750 mA) for a 48" diameter
aperture clamp.
Resolution
One mA resolution is available for clamps up
to 13" diameter on the 2 A range. A 1.9 A current in a 13" pipe,
or in the rail of an electric train can be measured with 1 mA
resolution. From 14 to 82 inch diameter aperture, the resolution is
10 mA on the 20 A range, or 100 mA on the 200 A range. The
recorder jack overrange capability on the 2 A and 20 A range is
about 3 times full scale, i.e., 60 A on the 20 A range.
Current Reading -- Precision and Linearity
The panel meter reading and the output of the
recorder connector are proportional to the direct current flowing
through the aperture of the sensor to within 0 ± 1% of reading, ± 3
least significant decimal counts.
In other words, a meter reading of 0.500 A
interrupted current is good to ± .005 A, except that the last
digit may be off as much as .003 A.
So 0.500 A interrupted current will read 0.500 A
± .005 A ± .003A; or 0.500 A within ± .008 A.
Measuring by Changing the Current with an
Interrupter
The simplest way to get 1% accuracy is to interrupt
the current. This cancels out the zero offset. In Fig. 1 for
example, if the 0.500 A current flowing in the pipe is entirely
due to a rectifier and this rectifier is disconnected, the current
changes -0.500 A.
When the rectifier is reconnected, 0.500 A
flows. The indicator's meter shows IM = 0.503 A. So the indicated
change in current magnitude was +0.503 A.
This is an example of the change in current method
of measurement.
It is not necessary to set the panel meter to read
zero when the rectifier is disconnected. Instead, when the current
is zero, we note the meter reading. Then, when the rectifier is
reconnected again, note the meter reading. The measured current IM
is the algebraic difference.
For example, if the zero offset is -0.2 A, this
will be the meter reading when the current is off. When the 0.500
A current comes back on, the meter still reads 0.2 A less than
true, so it reads +0.3 A. The true current is the difference:
IC = 0.3 A - (-.2A)
= 0.5 A
Unmeasured Current
A disadvantage of the change in current method is
that an important current in the pipe may be missed. For example, a
serious interference current will not be noticed if it is steady.
The change in current method ignores zero offset, but it also
ignores a constant signal input current.
Constant Current
When the signal input current is not interrupted, the
zero offset IZ adds to the input current IC so that the meter
reading IM can be in error. It represents an error because it is
unknown. The cause of zero offset can be a magnet near the sensor
(magnetized pipe, rebar, etc.), or the Earth's magnetic field. And
it can be due to the indicator's zero control. If this is well
off-center, the zero offset IZ can be big.
To see the effect of zero offset, suppose that the
0.500 A current IC flowing in the pipe in Figure 1 is constant.
This could be interference current. Or sacrificial anodes hard wired
to the pipe. Then the indicator's panel meter IM and recorder output
voltage show the algebraic sum of the pipe current IC
plus the input
current equivalent of the unknown zero offset IZ. Restated:
IM = IC + IZ.
If, at the same time IZ were -0.2 A; and the true IC
were 0.50 A, then the meter would read:
IM = 0.5 - 0.2 A
= 0.3 A.
This reading is not good. Zero offset IZ needs to be
canceled.
Canceling IZ by "Changing the Sensor"
To delete zero offset IZ when the input current is
constant, we change the sensor instead of changing the current.
Changing the clip or clamp's position on the pipe can pretty well
cancel out IZ.
Canceling Zero Offset Using a 2 Step Floating
Zero Procedure.
Generally the simplest way to find the true value of
an unchanging input current, IC in Fig. 1, is to cancel out zero
offset IZ. I think of this process as "changing the sensor."
The usual way to "change the sensor" is to move
it. First read the current with the sensor placed on the pipe in a
positive sense, as shown in Fig. 1. In this example, the meter reads IM
= 0.503 A. Then turn it over to a negative sense as shown in
Fig. 2. Ideally, the effect is to exactly reverse the current
reading to IM
= -0.503 A.
However the zero offset may have changed somewhat
when the clamp was moved. Magnetic effect is the likely cause.
Figure 2
Here the sensor is placed in the negative polarity sense.
This illustrates "changing the sensor". The sensor in Fig. 1
has been turned over so that now the bridle is farther away from
the source of positive pipe current IC = 0.500 A. This current
flows through the aperture of the clamp in the opposite
direction from that in Fig. 1, so the polarity of the meter
reading is reversed. Instead of positive, IM reads -0.490 A.
The ideal reading is IM = -.503 A, exactly the reverse of Fig.
1, but it is likely that the Earth's magnetic field and nearby
magnet will cause the reading IM to deviate from the ideal. A
reasonable reading is IM = -.490 A.
The 2 Step Floating Zero (FZ) Procedure is an
example of the "change the sensor" method for canceling zero offset.
It is usually quite accurate, and easy to do. The most likely
magnitude of IC
is:
IM+ - IM-
IC = ---------
2
For example, in Figure 1, IM+
= 0.503 A. In Figure 2, IM- = -0.490 A.
Then the most likely magnitude of IC is:
0.503 - (-0.490)
IC = ----------------
2
= 0.497 A.
The error, i.e., the deviation from the true IC =
0.500 A is -.003 A.
The 2 step FZ can work just fine. However, there
is an element of chance. If the pipe or manhole is strongly
magnetized, it can be a lot less accurate than the 8 or 16 step FZ
procedure.
Lab Trial of the 2 Step using a 4" Clip on Copper
I used a 4 inch diameter aperture MER™ Meter to
measure the current in a #16 wire strung north-south. The actual
(true) current was 0.500 A from a lab current source. The zero
knob on the indicator was set so that IM (Fig. 1) read zero when the
clip pointed due east. There was no significant magnetism except the
Earth Field. The real He of this clip was 0 ± 13 mA peak. The
specification requires that the He of a 4" MER™Clip be less than ±
30 mA peak equivalent input current. I use this as the normalizer in
checking to see if a result is reasonable.
With the clip around the wire in the positive
sense (Fig. 1), still pointing east, the meter read IM+ = +.498 A.
With the clip turned over in the negative sense
(Fig. 2), but still pointing east the meter read IM- = -.491 A.
Then the most likely value of IC
is:
IM+ - IM-
IC = ---------
2
0.498 - (-0.491)
= ----------------
2
= 0.495 A
The error was -.005 A. The error is -0.2 He. A good
result.
Lab Trial of the 2 Step using the same 4" Clip on
a Steel Pipe
Copper, aluminum, lead, etc., conductors do not have
internal magnetization, but steel pipe may. I repeated the 2 step,
this time on a 3" diameter, .2" wall steel pipe having some internal
magnetism. In a relatively "cool" sector, the East pointing 2 step
FZ gave:
IM+ = .502 A
IM- = -.492 A
Average: IC = .497 A
Since the true pipe current was still 0.5 A, the
error was -.003. A good result.
The Element of Chance
When I did a full 8 step FZ using the same setup in
the same location, I found that the up pointing 2 step gave IC
=
.471 A. I could have chosen this worst orientation in the first
place and been -.029 A in error. Not good. This was 1 He off the
mark.
The 8 Step is more reliably accurate than the 2
Step FZ procedure.
Lab Trial of the 8 Step in the same "Cool" Sector
The result was twice as accurate as the up pointing 2
step. To be sure, it was less accurate than the East pointing 2
Step, but it is more dependable.
General Floating Zero Procedure (FZ)
This section describes the 8 Step Floating Zero
Procedure for measuring a constant current. It describes how to
largely cancel zero offset error, even in a magnetized pipe or near
other magnets. It is an outline for the 16 step procedure which is
basically the same, but more reliably accurate in a tough spot.
When the CP current IC in the pipe in Fig. 1 is
constant, the method of measurement is to change the position of the
sensor. This largely cancels zero offset. Meter readings
corresponding to several sensor clamp positions are averaged. The
result is closer to the true pipe current.
The next examples are taken from data measured on
a lab pipe having a calibrated continuous IC = 0.50 A true
current. [AutoMER™™ SN 2517 and 4" MER™Clip #563. The measured He is
0 ± 14 mA peak to peak. The specified maximum He is 0 ± 30 mA peak.
The lab pipe is 3.3" OD; has 0.22 wall and is 45" long. It was spot
magnetized at various times in connection with the design of
Magnetic Error Reduction clips.]
The lab pipe is locally magnetized -- some places
more than others. In the "cool" sectors a two step Floating Zero (FZ)
Procedure worked fine. In the very "hot" sectors having a lot of
local magnetism, it was necessary to use a 16 step FZ. This utilized
8 different orientations of the clip on the pipe -- first in the
positive sense and then again in the negative sense. I think 4
orientations, each read in both the positive and negative sense,
will do for most pipes in the field. However, the "Hot" sector shown
in Table 1 was more accurately measured with the 16 step FZ.
The 4 inch diameter aperture MER™ Clip is
represented as a clamp in Figures 1 and 2. The clamp is oriented
nose up and to the left on the pipe. The actual pipe current IC is
0.5 A.
Figure 1 shows the clamp in the positive sense,
i.e., the bridle is in front of the clamp. The 0.5 A true current is
shown flowing into the bridle, so the meter reads positive. In my
experiment the meter reading was IM+ = +0.479 A when the nose
pointed down.
To position the clamp symmetrically around the
pipe I used a 9/16" foam belt, loose on the pipe. Wood or plastic
shims have been used, but a foam belt works better.
Like Figure 1, Figure 2 shows the clamp on the
pipe in the nose up and to the left orientation. However, in Fig. 2
the clamp is in the negative sense; i.e., the bridle is behind the
clamp. The 0.5 A pipe current flows through the clamp's aperture
and out into the bridle. This is the reverse of the positive sense,
so the meter on the indicator reads negative. I measured IM-
=
-0.461 A when the nose pointed down.
The zero control (Z) in Figs. 1 and 2 should be
set when the FZ procedure is started, and then not touched. Straight
up is a good guess. In my experiment the meter read zero when the
clip was held in a magnet free sector (except for the Earth's
magnetism), in a vertical plane, pointing east. Since the lab pipe
was horizontal, running north-south, this was a good start. But it
is not essential. The FZ cancels out zero offset error due to
indicator miss-adjustment as well as zero offset error due to
magnets -- both local and Earth.
What is essential is that the zero
adjustment (Z) not be touched once a FZ run is started. Changing the
zero control (Z) on the indicator during a FZ run invalidates the
run. The data is likely bad.
Measurements Made During a Two Step FZ run
Combining Figures 1 and 2 using the data in my lab
tests yields a two step FZ run.
In the setup of Fig. 1, IM+
= .479 A.
In the setup of Fig. 2, IM-
= -.461 A.
The scalar magnitude average is IC
= .470 A.
The error at this "Hot" sector, i.e., deviation
from 0.5 A, is -.03 A.
This -30 mA error is equivalent to 1.0 times the
He specification of the 4" clip used. It is more than we want to
see, even in a "Hot" sector, but it is better than no FZ.
Measurements Made During an 8 Step FZ run in a
Hot Magnetic Location
Table 1 is a chart of the type I use to organize the
data for a FZ run. Clamp orientations are represented by arrows for
nose up, down, east, and west. The four positive clamp sense
readings IM+ are presented in the column to the right of
orientation. This is done even if they are negative when read from
the meter on the indicator. In such an event they are written as
negative. The currents shown are what I measured at a magnetically
speaking "Hot" location on the 3" lab pipe.
This can be seen in the data. For example, in the
left hand column of Table 1, the west current reading was -.411 A.
Its scalar magnitude is .05 A less than the average IC = .461 A on
Table 1. Likewise the up reading is +.510 A; high by .049 A. These
"standout" readings are 50 mA and 49 mA off the average IC = .461 A;
i.e., off by 1.7 He. Too much, if you need to know the current
within a fraction of one He.
For a "Hot" pipe such as this, it is best to
double the number of orientations, making a 16 step FZ run. Or
better, if feasible move to a "cooler" spot on the pipe, at least
several clamp diameters up or down the length of the pipe.
Assurance that the measurement is accurate enough
can be had by deleting all lines of data wherein a measurement is
more than 2 He from the average. This is best done on a 16 step FZ.
In the lab I made a 16 step FZ run in the same
"Hot" sector. I got average IC = .490 A. This is He/3 shy of the
true 0.5 A current. A good result. The added 8 steps increased the
accuracy and confidence considerably.
Table 1
Summary of Eight Current Readings made at a "Hot"
Pipe Location
Current
Reading
IM-
Clamp in the
Negative Sense |
Clamp
Orientation |
Current
Reading
IM+
Clamp in the
Positive Sense |
Scalar
Average
for the
Clamp
Orientation |
| -.447 A |
East
 |
+.465 A |
.456 A |
| -.411 A |
West
 |
+.450 A |
.431 A |
| -.466 A |
Up
 |
+.510 A |
.488 A |
| -.461 A |
Down
 |
+.479 A |
.470 A |
| Average: IC = |
.461 A |
| Known True IC = |
.500 A |
| Error = |
-.039 A |
| Error = |
-1.3 He |
In Table 1, the four readings IM- taken with the
clamp in the negative sense, are presented in the left hand column,
even if they are positive, and in such a case they are written as
positive.
The error shown in Table 1 is more than is usually
allowed. This is because the lab pipe was "Hot" in the sector
measured, i.e., strongly magnetized in a spotty fashion.
Compare Several FZ Runs
Confidence in the practical accuracy -- say within ±
1 He
-- is gained when the result of several FZ runs pretty well
agree.
The 0.5 A true current IC has been measured four
ways at the "Hot" lab pipe sector:
- Two step FZ, Fig. 1 and 2: Result: IC = .470
A
- Eight step FZ, Table 1: Result IC = .461 A
- Sixteen step FZ: Result: IC = .490 A
- Delete data rows having a deviation from
average greater than 2 He; Result: IC = .471 A.
All four agree within 1 He , so we can have confidence
that IC is likely between .461 and .490 A, or at least close
thereto.
Is there a basis for selecting one of the four
results? I think yes.
Criteria for Selecting the Result from One
Process
I prefer to use the result obtained by averaging all
the current readings available. This is the 16 step in c) above. The
process is similar to that shown in Table 1. The average IC is .490
A, which is only He/3 less than the true 0.5 A.
However, some of the readings are "hot". Is it
reasonably safe to rely on Table 1 or the 16 step? I think yes, on
condition its result is within one He of the rest.
To see if this condition is satisfied, check the
other three against the average IC = .490 A obtained from c)
above, using the 16 step FZ.
In a), IC = .470 A. The deviation is -.02 A;
i.e., -.7 He; This is acceptable.
In b), IC = .461 A. The deviation is -.029 A;
i.e., -1 He. This is acceptable.
In d), IC = .471 A. The deviation is -.019 A;
i.e., -0.6 He. This is acceptable.
Conclusion
We estimate we can rely on the 16 step FZ called c),
and say:
"quite likely IC = .490 A, within ± He ."
If in doubt, or especially when the pipe is
magnetically "hot":
- Use all 4 methods (a, b, c, and d, in the
Summary, above).
- Pick 3 or 4 that agree within one He from
these.
- Use the result of the method having the most
steps.
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