Use and Accuracy of a Clamp-on
Direct Current Ammeter
Unabridged Version
Introduction
This paper is about solving cathodic protection
problems with a clamp-on direct current ammeter.
Clamp-ons 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.
The question is - how much lost current can you
find? What is the resolution? Accuracy? Zero offset?
The answer depends on:
- The equipment. This paper reports on results
using the best DC Amp Clips.™ [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. Good use of the
Floating Zero Procedure can reduce the error by
ten to one.
The Equipment
The results obtained using clamp-on DC
ammeters and sensors which are our best DC Amp Clips
[MER™ Meter™ specification]. Production includes 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" and are secured with brass finger nuts on
captive studs. This is shown in Fig. 1.
Figure 1. The sensor of a clamp-on DC
ammeter is secured around a pipe carrying 1.83 Amp
CP current. The sensor 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 IM read 0.0 when all current in the pipe was cut
off. The half percent high reading (IM+ = 1.84 A) is
quite likely when the pipe current IC is switched
back on.
When the pipe is a bit bigger than the aperture
of their clamp, a few persons have asked if clamps
"stretch", or if they need to fully close around the
pipe. Really, no and yes. For any respectable
accuracy, the clamp must close fully around the pipe
as shown in Fig. 1.
However, there is no known limit on how big the
clamp can be and how small the conductor carrying
the direct current to be measured can be.
Experiments with a #10 cable more or less centered
in the aperture of a 3-foot clamp showed no
important change in the precision of the meter. It
is best to avoid a small cable held against the side
of a big clamp, especially in the lips. Otherwise
the error may approach 5%.
There are reasons for having a clamp or clip
(sensor) no larger than twice the conductor's
outside diameter, all covering and insulation
included. Among these:
- Very large sensors cost more and are more
easily damaged than smaller ones.
- Very large sensors can be difficult to get
on one pipe in the midst of a group of closely
spaced pipes.
- The Earth Field Effect (He) of sensors
increases with size. This means that the zero
offset of the meter will change more as the
clamp is rotated in a vertical North - South
plane. The equivalent input current change in
zero offset is 0 ± 3 mA for a ¾" clip; 0 ± 100
mA for a 13" clamp, and 0 ± .75 Amp (750 mA) for
a 48" diameter aperture clamp.
The sensitivity to nearby magnets may also be
greater if the clamp is too big.
Resolution
Accuracy is a function of resolution. CP current in
a 13" pipe or rail of an electric train can be
measured up to 2 Amperes with 1 mA resolution; to 20
Amp with 10 mA resolution, or 200 Amp with 100 mA
resolution. From 14 to 82 inch diameter aperture,
the resolution is 10 mA on the 20 Amp range, or 100
mA on the 200 Amp range. The 2 Amp and 20 Amp
current ranges having 1 and 10 mA resolution may be
extended by as much as 5 times using the recorder
output connector provided for logging data.
Current Reading - Precision and Linearity
The panel meter reading and the output of the
recorder connector are proportional to (a linear
function of) 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 1.8 Ampere
interrupted current is good to ± 1% of 1.8 Amp (±
.018 Amp or 18 mA), except that the last digit may
be off as much as 3. On the 2 Amp range, this is ± 3
mA, but on the 20 Amp range this is ± 30 mA.
So 1.8 Amp interrupted current will read:
1.8 A ± .018 A ± .003A; i.e.,
1.8 A ± 21 mA, or
1.8 A ± 1.2%.
Measuring by Changing the Current with an
Interrupter
When the CP direct current to be measured is
interrupted, the accuracy obtainable is better, and
quicker than otherwise. For example, if the 1.83 Amp
CP current (IC) flowing in the pipe in Figure 1 is
entirely due to a rectifier and this rectifier is
disconnected, the current changes -1.83 Amp, and IC
= 0. The zero control (Z) on the indicator can be
set so that the indicator meter reads zero.
After a half minute the rectifier is reconnected,
and IC = 1.83 Amp flows. The indicator's meter may
show that IM = 1.84 Amp.
This is a current change method of measurement.
It is not necessary to set the panel meter to
read zero when the rectifier is disconnected.
Instead, when the CP current IC = 0, we can note the
meter reading and call it ID. For example, ID may be
-0.4 Amp. Then, after 2/3 minute, when the rectifier
is reconnected for 1/3 minute we can again note the
meter reading and call it IR. For example, IR may be
+1.43 Amp. The measured CP current IMC is the
algebraic difference. Restated:
IMC = IR - ID
= +1.43 - (-0.4) Amp
= +1.83 Amp
The greatest anticipated uncertainty is still ± 0.21
Amp. Hence, the real CP current is:
IC = IMC ± .021
= +1.83 ± .021 Amp
= +1.83 Amp ± 1.2%
Put another way, the likely CP current is:
+1.809 < IC < 1.851 Amp
Unmeasured Current
A disadvantage of the current change or interrupted
current method is that an important part of the CP
current IC flowing in the pipe may be missed. For
example, a serious interference current will not be
noticed if it is steady. The current change method
ignores constant current.
Constant Current
Suppose that the 1.83 Amp current IC flowing in the
pipe in Figure 1 is practically constant for at
least 20 minutes - perhaps a day, and it cannot be
interrupted. This could be because it is
interference current, and the source is out of our
control. Or perhaps it is at least partly from
sacrificial anodes hard wired to the pipe. Here the
indicator's panel meter and recorder output voltage
show the algebraic sum of the pipe current IC plus
the input current equivalent of the zero offset
called IZ. Restated:
IM = IC + IZ
For example, if the unknown IZ were -0.4 A; and the
true IC were 1.83 A, then the meter would read:
IM = 1.83 - 0.4 A
= 1.43 A
This reading is considerably off the mark. We need
to do better.
The cause of zero offset IZ can be a magnet Hn
near the sensor (magnetized pipe, rebar, etc.), or
the Earth's magnetic field He. Also, if the
indicator's zero control is well off-center, there
can be a considerable zero offset.
If we knew IZ, the rest would be easy, but
generally we do not. Unless, for example, a bond is
opened at a known good insulating flange, and the
clamp is close by. Likely we need to cancel IZ out.
Canceling IZ by Changing the Sensor
To delete IZ when the current must be 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 true CP current
IC with reasonable accuracy is to cancel out IZ. This process
can be thought of as "changing the sensor"
instead of changing the pipe current. It is not
quite as quick or quite as accurate, but it gets the
job done.
The usual way to "change the sensor" is to move
it. 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+ = 1.84 Amp. 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- = -1.84 Amp.
However the zero offset may have changed somewhat
when the clamp was moved. Magnetic effects - He and
Hn - are the likely causes. Smaller clips are
better. A good 4" clip has less than He = 0 ± 30 mA
peak. A good 24" clamp has less than He = 0 ± 300 mA
peak.
This "change the sensor" method is called the 2
Step Floating Zero (FZ) Procedure. It is usually
quite accurate, and easy to do.
Interpretation is convenient. The most likely
magnitude of IC is:
For example, in Figure 1, IM+ = 1.84 A. In Figure
2 IM- = 1.79A. Then the most likely magnitude of IC
is:
In this example, the error, i.e., the deviation
from the true IC = 1.83 Amp is -.015 Amp (-15 mA).
This 2 step FZ can work just fine. However, there
is an element of chance. It can be a lot less
accurate than the 8 or 16 step FZ procedure,
especially if the pipe or manhole is strongly
magnetized, or if the sensor is damaged.
2 Step with 4" Clip on Copper
A 4 inch diameter aperture MER™ Meter2 was used to
measure the current in a #16 wire strung
north-south. 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.
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 (average) is:
The true current was 0.500 Amp.
So the error was -.005 Amp, or error = -1% of
reading. Since the He spec for the 4" clip is .03 A,
the error = -0.2 He.
0.2 He accuracy is good with the 2 step FZ.
Figure 2. Here the "sensor has been changed". The
sensor in Fig. 1 has been turned over ("changed") so
that now the bridle is farther away from the source
of positive pipe current IC = 1.83 Amp. 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 IM- is reversed.
Instead of positive, IM- reads -1.79 Amp. The ideal
reading is IM- = -1.84 A, exactly the reverse of
Fig. 1, but in the field it is likely that the
Earth's magnetic field He and nearby magnet Hn will
cause the reading IM- to deviate from the ideal. A
reasonable reading is 2.2% low, i.e., IM- = -1.79 A.
8 Step
For comparison, a full 8 step FZ run gave average IC
= 0.498 Amp; error = -.002 Amp; -0.4% of reading;
.07 He.
2 Step with 4" Clip on Steel Pipe
Copper, aluminum, lead, etc., conductors do not have
internal magnetization, but steel pipe may. The 2
step, was repeated, this time on a 3" diameter, .2"
wall steel pipe with some internal magnetism. In a
relatively "cool" sector (centerline + 1"), the east
pointing 2 step FZ gave:
IM+ = .502 A
IM- = -.492 A
Average IC = .497 Amp
Since the true pipe current was still 0.5 Amp, error
= -.003 A; -0.6% of reading; -.1 He. Very good!
Chance
However, when a full 8 step FZ was done, the up pointing 2 step gave average IC = .471 A.
This worst orientation could have been chosenin the
first place and been -.029 Amp in error; i.e., 1 He
off the mark.
8 Step
The Full 8 step FZ is more reliable. The result was:
average IC = .486 Amp; error = -.014 A; -3% of
reading; -.5 He.
This is acceptable at a "cool" sector on
steel pipe having mild local magnetism Hn.
General Floating Zero Procedure (FZ)
When the CP current IC in the pipe is constant and
cannot readily be changed, the method of measurement
is to change the position of the sensor. Meter
readings IM corresponding to several sensor clamp
positions are averaged. The result is likely closer
to the true pipe current IC.
The next examples are taken from data measured*
on a lab pipe having a calibrated continuous IC =
0.50 Amp current. This made it possible to find out
what accuracy was achieved.
* 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. 4 orientations, each read in
both the positive and negative sense, will do for
most pipes in the field. But the data in Figs. 3 and
4 and also Table 1 shows that 1" left of #4 is a
"HOT" sector requiring 16 steps.
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.
Figure 3. In this end-on view, a 4 inch diameter
aperture clamp is shown mounted on a 3 inch pipe
having 0.2 inch wall thickness. Four shims (s), or
preferably a 9/16" foam belt position the clamp so
that it is more or less centered about the pipe. The
clamp is shown in the nose down orientation. The
clamp is in the positive sense, i.e., the bridle is
positioned closer to the source of a known pipe
current ICtrue = 0.500 Amp. The meter on the
indicator reads IM+ = .479 Amp.
In Figures 3 and 4 the 4 inch diameter aperture
MER™ Clip is represented as a clamp. In this end-on
view the clamp is oriented nose down on the pipe.
The pipe current IC is 0.5 Amp.
Figure 3 shows the clamp in the positive sense,
i.e., the bridle is in front of the clamp, out above
the page. The current ICtrue = 0.5 A is shown
flowing into the page, so the meter reads IM+ =
+0.479 A. This was the meter reading in Amperes in
my experiment.
The 4 shims were really a 9/16" foam belt, loose
on the pipe. Wood or plastic shims have been used,
but a foam belt works better.
Figure 4 again shows the clamp on the pipe in the
nose down orientation, but this time the clamp is in
the negative sense; i.e., the bridle is down into
the page. So when the ICtrue = 0.5 Amp pipe current
flows into the page, the current IC is flowing
through the clamp's aperture and out into the bridle
- the reverse of the positive sense. So the meter on
the indicator shows IM- = -0.461 Amp. This too is
the current measured in lab setup.
Figure 4. This end-on view is similar to Figure 3
except that the 4" clamp is in the negative sense,
meaning that the bridle is into the page, farther
away from the source of a known 3" pipe current
ICtrue = 0.500 Amp. This causes the meter on the
indicator to show a negative number. Here IM- =
-.461 Amp. The negative sign on the meter shows that
the current is flowing from above the page, through
the aperture of the clamp, and out past the bridle.
This is the reverse of the positive polarity symbol
marked on the clamp.
The zero control (Z) in Figs. 3 and 4 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 (IM = 0) 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.
Two Step FZ
Combining Figures 3 and 4 yields a two step FZ run.
In Fig. 3, IM+ = .479 A.
In Fig. 4, IM- = -.461 A.
The scalar magnitude average is IC = .470 A.
The error, i.e., deviation from ICtrue = 0.5 A,
is E = -.03 A, or E = -1.0 He equivalent.
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, but it is better than no FZ. In
the single reading IM- = -.461 , E = -.039 A, or
-1.3 equivalent He.
Eight Step FZ
To help organize the data for a FZ run, we can use a
chart such as Table 1. Clamp orientations can be
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,
even if they are negative when read from the meter
on the indicator, and they are written as negative.
The currents shown are what was measured on the lab
pipe with 4" MER™Clip #563.
Table 1
Summary of
Eight "Square" Current Readings
"Hot" Pipe Location: {1" left of #4} |
Current
Reading
IM-
Negative
Clamp Sense |
Clamp
Orientation |
Current
Reading
IM+
Positive
Clamp Sense |
Orientation
Scalar
Average |
| -.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 |
The four negative clamp sense readings IM- are
presented in the left hand column, even if they are
positive, and in such a case they are written as
positive.
The result shown in Table 1 is a disappointment.
This {1" left of #4} sector on our lab pipe is
"Hot". The "square" 8 step FZ is no better than the
2 step. Sixteen steps are needed at this sector.
The question is - How to know that a particular
pipe sector requires a 16 step FZ?
"Hot" Sector Recognition
Table 1 is for a "hot" steel pipe. Other FZ runs may
be on a lead cable or copper pipe - not magnetic in
themselves, but positioned in a "hot" manhole or
building adjacent to magnetized material. In any
case, we want to know if the readings are "hot."
Table 1A highlights the deviation of individual
current readings from the scalar average of all -
called the "Reference Average." The largest are -.05
A and +.049 A. These are -1.7 He and +1.6 He. In
other words, the "square" orientation data is spread
about a lot more than the Earth Field specification
He for the 4" clip used. This is the warning to do
what we can to get a more reliable result.
Table 1A
Deviation
Annotated Summary of Eight "Square" Current
Readings
Hot Pipe Location: {1" left of #4} |
Scalar
Deviation
From
Average
IC = .461 A |
Current
Reading
IM-
Negative
Clamp
Sense |
Clamp
Orientation |
Current
Reading
IM+
Positive
Clamp
Sense |
Scalar
Deviation
From
Average
IC = .461 A |
Orientation
Scalar
Average |
| -.014 A |
-.447 A |
East
|
+.465 A |
+.004 A |
.456 A |
| -.05
A |
-.411 A |
West
|
+.450 A |
-.011 A |
.431 A |
| +.005 A |
-.466 A |
Up
|
+.510 A |
+.049 A |
.488 A |
| 0 A |
-.461 A |
Down
|
+.479 A |
+.018 A |
.470 A |
| Reference
Average: IC = |
.461
A |
Table 2 is a summary of the 8 "square" current
readings of Table 1 with 8 more "angle readings
added. The "angle readings are taken with the clamp
orientated more or less half way between the
adjacent "square" orientation. Both are measurements
made with 4" MER™clip #563 on the 3 ¼" lab pipe.
The results in Table 2 are more encouraging. The
Error (difference between the calculated average IC
- .490 A and the known true IC = .500 A) is -.01
Amp. This is a lot (4 times) better accuracy than
the 2 step or 8 step and it may be about as well as
can be done. The -.01 A error is only -.33 He, i.e.,
1/3 of the 4" clip's Earth Field effect rating.
Table 2
Summary of 16
Step FZ Procedure
"Hot" Pipe Location 1" Left of #4 |
Current
Reading
IM-
Negative
Clamp Sense |
Clamp
Orientation |
Current
Reading
IM+
Positive
Clamp Sense |
Scalar
Average
of Current Reading
at Orientation |
| -.447 A |
East
|
+.465 A |
.456 A |
| -.609 A |
East
 |
+.643 A |
.626 A |
| -.411 A |
West
|
+.450 A |
.431 A |
| -.612 A |
West
 |
+.650 A |
.631 A |
| -.466 A |
Up
|
+.510 A |
.488 A |
| -.386 A |
Up
 |
+.421 A |
.404 A |
| -.461 A |
Down
|
+.479 A |
.470 A |
| -.393 A |
Down
 |
+.433 A |
.413 A |
| Average: IC = |
.490 A |
In the field where the true current IC is known
only by our measurements, how do we show that this
is accurate? Suggestions:
- Move to another location on the pipe. Be
close enough so that the true current is likely
the same, and measure there. If the results
agree within 1 He, their average is the best
bet.
- If the diameter of the pipe exceeds 5
inches, increase the number of clamp
orientations. Likely 16 orientations will work
better on a 10" to 14" pipe.
- Change the method of calculating the average
IC. Throw out the "hot" data and use only the
"cool" data. This method is illustrated in Table
2A. In Table 2A, called "hot", any reading having
a deviation from the average IC = .490 A greater
than 2 He. The 4" clip #563 has He = .030 Amp,
so any deviation over .060 Amp is highlighted as
"hot".
In Table 2A, shown is the orientation scalar average
for only the 3 "cool" rows having deviation less
than 2 He.
Table 2A
The "use of this cool, delete the hot" method is
a help. It gives IC = .471 A, less than true by one
He. It serves as a check on other methods, and adds
confidence to their result.
In Table 2A, the average of the three remaining
"cool" rows is IC = .471 Amp. The error with respect
to the known true IC = .500 Amp is -.029 Amp, i.e.,
-1 He. So this worked well enough in the lab.
Summary of Several FZ Runs
Confidence in the practical accuracy - say within ±
1 He - is gained when the result of several FZ runs
pretty well agree.
"Hot" lab pipe location {1" left of #4} current
IC has been measured four ways:
- Two step FZ, Fig. 3 and 4: Result: IC = .470
Amp
- Eight step FZ, Table 1: Result IC = .461 Amp
- Sixteen step FZ, Table 2: Result: IC = .490
Amp
- Deviation less than 2 He, Table 2A: Result:
IC = .471 Amp
All four agree within 1 He, so we can have
confidence that IC is likely between .461 and .490
Amp. Is there a basis for selecting one of the four
results? Perhaps yes.Selection Criteria
Preferred is the use of the result obtained by averaging all
the current readings available. This is c) above;
i.e., Table 2, where average IC = .490 A. However,
some of the readings are "hot". Is it reasonably
safe to rely on Table 2?Perhaps yes, on condition
its result is within one He of the rest.
To see if this condition is satisfied, compare
the average IC = .490 Amp obtained from the 16 step
FZ of Table 2 with the other three.
In a), IC = .470 Amp. The deviation is -.02 A;
i.e., -.7 He; This is acceptable.
In b), IC = .461 Amp. 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
Table 2, the 16 step FZ called c), and say: "quite
likely IC = .490 Amp, within ± He."
Conclusion
If in doubt, or especially when the pipe is "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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