The separation between exciting coil and receiver
coil should be at least twice the inner diameter of
the tube, preferably two and a half times, for the
reasons explained below.
The exciting coil induces magnetic field in the nor-
mal manner; some of the field penetrates the wall
of the tube and the rest remains within the tube's
air space. Eddy currents follow circular paths con-
centric with the axis of the tube flow within the
tube wall and set up a reverse magnetic field. The
reverse field weakens that part of the field remain-
ing within the air space, which decreases to zero
before reaching receiver coil. The region that is
active where the field induces directly by the exci-
ting coil, is called the direct field zone. This field
can produce a current in any coil suitably placed
within the zone. The remote field zone is the re-
gion in which no direct coupling,or joining togther,
can take place between the exciting coil and any
receiver coil inside it. Small variations in the inci-
dent magnetic field can produce large changes in
the resultant field, thus increasing the sensitivity
of defect detection. With a careful choice of fre-
quency it is possible to resolve signals indicating
variations of magnetic permeability from signals
indicating the presence and size of defects.
coil should be at least twice the inner diameter of
the tube, preferably two and a half times, for the
reasons explained below.
The exciting coil induces magnetic field in the nor-
mal manner; some of the field penetrates the wall
of the tube and the rest remains within the tube's
air space. Eddy currents follow circular paths con-
centric with the axis of the tube flow within the
tube wall and set up a reverse magnetic field. The
reverse field weakens that part of the field remain-
ing within the air space, which decreases to zero
before reaching receiver coil. The region that is
active where the field induces directly by the exci-
ting coil, is called the direct field zone. This field
can produce a current in any coil suitably placed
within the zone. The remote field zone is the re-
gion in which no direct coupling,or joining togther,
can take place between the exciting coil and any
receiver coil inside it. Small variations in the inci-
dent magnetic field can produce large changes in
the resultant field, thus increasing the sensitivity
of defect detection. With a careful choice of fre-
quency it is possible to resolve signals indicating
variations of magnetic permeability from signals
indicating the presence and size of defects.
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Magnetec
| REMOTE FIELD |
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REMOTE FIELD
EDDY CURRENT
EDDY CURRENT
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Remote Field Testing (RFT) is an electromag-
netic test that makes use of an AC excitation
source without any attempt at tube magneti-
zation or saturation. The varying magnetic
field is affected by the abnormalities of the
material. These changes in the field are mea-
sured a few tube diameters away from the
source.
The RFT system actually measures the pri-
mary magnetic field strength that exists at
a given distance from the exiter coil. That
field strength determines the included vol-
tage across the pickup coil. That voltage,
expressed as a phase and amplitude vector
with respect to the excitor coil, is represen-
ted on an X-Y screen in the standard format
that most Eddy Current Technology techni-
cians can recognize. Through-transmission
is sometimes used to describe the RFT pro-
cess. It implies that there is a source of en-
ergy that transmits "through" a medium.
netic test that makes use of an AC excitation
source without any attempt at tube magneti-
zation or saturation. The varying magnetic
field is affected by the abnormalities of the
material. These changes in the field are mea-
sured a few tube diameters away from the
source.
The RFT system actually measures the pri-
mary magnetic field strength that exists at
a given distance from the exiter coil. That
field strength determines the included vol-
tage across the pickup coil. That voltage,
expressed as a phase and amplitude vector
with respect to the excitor coil, is represen-
ted on an X-Y screen in the standard format
that most Eddy Current Technology techni-
cians can recognize. Through-transmission
is sometimes used to describe the RFT pro-
cess. It implies that there is a source of en-
ergy that transmits "through" a medium.
An RFEC probe consists basically of a exciter coil
and a detector coil in a certain arrangement be-
tween each other. The probe is passed through the
tube which shall be inspected. The exciter coil is
fed with a low frequency alternating current (nor-
mally sinusoidal) which causes an electromagnetic
field around the exciter coil. The energy of the field
spreads out in axial direction inside the tube as well
as into the tube wall. The eddy currents which are
induced in the tube wall generate a secondary field
which can be measured outside the tube. This se-
condary field is much weaker than the primary field
directly at the exciter coil inside the tube and has a
significant phase shift. When testing thick-walled
ferromagnetic metal pipes with conventional inter-
nal probes, very low frequencies (e.g. 30 Hz for a
teel pipe 10 mm thick) are necessary to achieve
the through-penetration of the eddy currents. This
situation produces a very low sensitivity of flaw de-
tection. The degree of penetration can in principle,
be increased by the application of a saturation mag-
netic field. However, because of the large volume
of metal present, a large saturation unit carrying a
heavy direct current may be required to produce
an adequate saturating field. The difficulties encoun-
tered in the internal testing of ferromagnetic tubes
can be greatly alleviated with the use of the remote
field eddy current method, which allows measur-
able through penetration of the walls at three times
the maximum frequency possible with the conven-
tional direct field method. This technique was intro-
duced by Schmidt in 1958. Although it has been
used by the petroleum industry for detecting corro-
sion in their installations since the early 1960s, it
has only recently evoked general interest. This in-
terest is largely because the method highly sensi-
tive to variations in wall thickness, but relative in-
sensitive to fill-factor changes. The method has the
added advantage of allowing equal sensitivities of
detection at both inner and outer surfaces of a ferro-
magnetic tube. With technique manipulation it can
differentiate between signals from these respec-
tive surfaces. In its basic form, the probe arrange-
ment consists of an exciting coil and a receiver coil
kept at a rigidly fixed separation along the axial
direction.
and a detector coil in a certain arrangement be-
tween each other. The probe is passed through the
tube which shall be inspected. The exciter coil is
fed with a low frequency alternating current (nor-
mally sinusoidal) which causes an electromagnetic
field around the exciter coil. The energy of the field
spreads out in axial direction inside the tube as well
as into the tube wall. The eddy currents which are
induced in the tube wall generate a secondary field
which can be measured outside the tube. This se-
condary field is much weaker than the primary field
directly at the exciter coil inside the tube and has a
significant phase shift. When testing thick-walled
ferromagnetic metal pipes with conventional inter-
nal probes, very low frequencies (e.g. 30 Hz for a
teel pipe 10 mm thick) are necessary to achieve
the through-penetration of the eddy currents. This
situation produces a very low sensitivity of flaw de-
tection. The degree of penetration can in principle,
be increased by the application of a saturation mag-
netic field. However, because of the large volume
of metal present, a large saturation unit carrying a
heavy direct current may be required to produce
an adequate saturating field. The difficulties encoun-
tered in the internal testing of ferromagnetic tubes
can be greatly alleviated with the use of the remote
field eddy current method, which allows measur-
able through penetration of the walls at three times
the maximum frequency possible with the conven-
tional direct field method. This technique was intro-
duced by Schmidt in 1958. Although it has been
used by the petroleum industry for detecting corro-
sion in their installations since the early 1960s, it
has only recently evoked general interest. This in-
terest is largely because the method highly sensi-
tive to variations in wall thickness, but relative in-
sensitive to fill-factor changes. The method has the
added advantage of allowing equal sensitivities of
detection at both inner and outer surfaces of a ferro-
magnetic tube. With technique manipulation it can
differentiate between signals from these respec-
tive surfaces. In its basic form, the probe arrange-
ment consists of an exciting coil and a receiver coil
kept at a rigidly fixed separation along the axial
direction.