Remote Field
Remote Field
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... is an electromagnetic test that makes use of an AC excitation source without any attempt at tube magnetization or
saturation.  The varying magnetic field is affected by the abnormalities of the material. These changes in the field are
measured a few tube diameters away from the source.
the primary magnetic field strength that exists at a given distance from the exciter
The probe is passed through the tube which shall be inspected.  The exciter coil is fed with a low frequency alternating current
(normally 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 secondary field is much weaker than the primary field directly
conventional internal probes, very low frequencies (e.g. 30 Hz for a steel pipe 10 mm thick) are necessary to achieve the
through-penetration of the eddy currents.  This situation produces a very low sensitivity of flaw detection.  The degree of
penetration can in principle be increased by the application of a saturation magnetic field.   However, because of the large
saturating field.  The difficulties encountered in the internal testing of ferromagnetic tubes can be greatly alleviated with the use
of the remote field eddy current method, which allows measurable through penetration of the walls at three times the
maximum frequency possible with the conventional direct field method. This technique was introduced by Schmidt in 1958.  
Although it has been used by the petroleum industry for detecting corrosion in their installations since the early 1960s, it has
only recently evoked general interest.  This interest is largely because the method highly sensitive to variations in wall
thickness, but relative insensitive to fill-factor changes. The method has the added advantage of allowing equal sensitivities of
detection at both inner and outer surfaces of a ferromagnetic tube.  With technique manipulation it can differentiate between
signals from these respective surfaces.  In its basic form, the probe arrangement consists of an exciting coil and a receiver coil
kept at a rigidly fixed separation along the axial direction.
should be at least twice the inner diameter of the tube,
The exciting coil induces magnetic field in the normal 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 reverse
field weakens that part of the field remaining within the air space, which decreases to zero before
reaching receiver coil.  The region that is active where the field induces directly by the exciting The
exciting coil induces magnetic field in the normal 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 concentric
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 remaining within the air space, which decreases to zero before
reaching receiver coil.  The region that is active where the field induces directly by the exciting coil is
called the direct field zone.  This field can produce a current in any coil suitably placed within the place
between the exciting coil and any receiver coil inside it.  Small variations in the incident magnetic field
can produce large changes in the resultant field, thus increasing the sensitivity of defect detection.  
With a careful choice of frequency it is possible to resolve signals indicating variations of magnetic
permeability from signals indicating the presence and size of defects.
REMOTE FIELD TESTING (RTF) IS an
ELECTROMAGNETIC TEST that ...
The RFT system actually measures
coil.  That field strength determines the included voltage across the pickup coil.  That voltage, expressed as a phase and
amplitude vector with respect to the exciter coil, is represented on an X-Y screen in the standard format that most Eddy
Current Technology technicians can recognize.  Through-transmission is sometimes used to describe the RFT process.  It
implies that there is a source of energy that transmits "through" a medium.
An RFEC probe consists basically of
an exciter coil and a detector coil in a certain arrangement between each other.
The separation between exciting coil and receiver coil
preferably two and a half times, for the reasons explained as follows:
An RFT system contains 4 major components
Principle of Remote Field Sensing Effect-Additional View
Discretization of the Tube and Exciter Coil
Experimental Determination of the Permeability
Iteration Procedure
Defect Signal Simulation Procedure
Detector Coil Voltage as Function of Exciter Current
Calculated Defect Signals of Absolute Probe
Real Part of Calculated Field Distribution around Exciter Coil
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1159 East North St.,
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