Theory of Operation
A probe consisting of an exciter coil and one or more detectors
is pulled through the tube. The exciter coil and the detector
coil(s) are rigidly fixed at an axial distance of two tube
diameters or more between them. The exciter coil is driven with a relatively
low frequency sinusoidal current to produce a magnetic field.
This changing magnetic field induces strong circumferential eddy
currents which extend axially, as well as radially in the tube
These eddy currents, in turn, produce their own magnetic field,
which opposes the magnetic field from the exciter coil. Due to
resistance in the tube wall and imperfect inductive coupling,
the magnetic field from the eddy currents does not fully counterbalance
the magnetic exciting field. However, since the eddy current field
is more spread out than the exciter field, the magnetic field
from the eddy currents extends farther along the tube axis. The
interaction between the two fields is fairly complex but the simple
fact is that the exciter field is dominant near the exciter coil and
the eddy current field becomes dominant at some distance away
from the exciter coil.
The receiving coils are positioned at a distance where the magnetic
field from the eddy currents is dominant. In other words, they are placed at a
distance where they are unaffected by the magnetic field from
the exciter coil but can still adequately measure the field strength
from the secondary magnetic field. Electromagnetic induction occurs
as the changing magnetic field cuts across the pick-up coil array.
By monitoring the consistency of the voltage induced in the pick-up
coils one can monitor changes in the test specimen. The strength
of the magnetic field at this distance from the excitation coil
is fairly weak but it is sensitive to changes in the pipe wall
from the I.D. to the O.D.