RFT Theory of Operation (cont.)
Direct Couple Zone
The region where the magnetic field from the exciter coil is interacting with the tube wall to produce a concentrated field of eddy currents is called the direct field or direct coupled zone. This zone does not contribute a great deal of useful data to the RFT inspection due to problems with rather high noise levels due to the intense varying magnetic field from the excitation coil.
The region just outside the direct couple zone is known as the transition zone. In this zone there is a great deal of interaction between the magnet flux from the exciter coil and the flux induced by the eddy currents. As can be seen in the graph, the interaction of the two opposing fields is strongest near the ID of the tube and fairly subtle at the OD of the tube. The "resultant" field strength (the magnetic field at the sum of the two fields) in this region tends to change abruptly on the ID due to the interaction of the fields with differing directional characteristics of the two fields.
The receiver coil's signal phase, with respect to the exciter coil, as a function of distance between the two coils is also shown in the graph. When the two coils are directly coupled and there is no interference from a secondary field, their currents are in phase as seen at location zero. In the transition zone, it can be seen that the phase swiftly shifts, indicating the location where the magnetic field from the eddy currents becomes dominate and the start of the remote field.
Remote Field Zone
The remote field zone is the region in which direct coupling between the exciter coil and the receiver coil(s) is negligible. Coupling takes place indirectly through the generation of eddy currents and their resulting magnetic field. The remote field zone starts to occur at approximately two tube diameters away from the exciter coil. The amplitude of the field strength on the OD actually exceeds that of the ID after an axial distance of approximately 1.65 tube diameters. Therefore, RFT is sensitive to changes in material that occur at the outside diameter as well as the inside diameter of the tube.