RFT 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 wall.
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.