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Click Here
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Magnetism
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It is based on the induction of an electromagnetic field in
the component undergoing inspection. It can be applied
to electrically conducive but non-ferromagnetic materials,
and can detect various forms of internal and external dam-
age to the object being tested.
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It uses a probe with a built-in bobbin coil that induces eddy cur-
rents in adjacent material. Should the probe encounter flaws as
it is pulled through the tube, the eddy currents are interrupted,
causing an impedance change in the coil. The impedance
change is measured and displayed on a monitor.
Eddy Current testing is very sensitive to most defects that occur
in tubes. It is critical to use correct eddy current techniques and
procedures for conclusive results.
The three primary techniques are differential, absolute, and multi-
frequency.
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ADVANCED TECHNIQUES USE
COMPUTER CONTROLLED
INSTRUMENTS
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These digital multi-frequency, multiple-
channel units have the ability to simul-
taneously conduct multiple differential
and absolute tests using multiple fre-
quency mix channels.
Combining the defect detection abilities
of these 3 methods in a single examina-
tion, provides the most accurate, and
rapid examination of heat exchanger
tubes.
THE MAIN DISADVANTAGE
It is that it can be used only for the in-
spection of non-ferromagnetic materials.
THE MAIN ADVANTAGE
Speed is the main advantage of this tech-
nique. It is the fastest of all the techni-
ques.
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Differential Method:
Differential Method:
Absolute Method:
Absolute Method:
The Absolute Method uses two separate probes, one positioned in a known reference
tube, the second positioned in the tube under examination. The instrument compares
the signals from the two probes to detect and evaluate the defects. This method is
sensitive to gradual volumetric changes in the tube wall.
Multi-frequency Method:
Multi-frequency Method:
The Multi-frequency Method simultaneously conducts multiple differential and
absolute tests with frequency mix techniques. This powerful examination combines
the defect detection capabilities of the previous methods into a single, accurate test.
It also provides enhanced defect detection for each type of test through the use of
multiple frequencies sensitive to different types of defects.
The differential method uses two coils in one probe to compare adjacent sections of
the tube. When both coils are positioned on sound areas, the signals from both are
identical. When one coil passes over a defect, the instrument compares the differences
in the two signals to evaluate the defect. This method is very sensitive to defects that
cause an abrupt wall section change.
Magnetec
THE EDDY CURRENT
TECHNIQUE
