Remote field testing (RFT) probes are being used to successfully inspect ferromagnetic tubing such as carbon steel or ferritic stainless steel. They are very sensitive for detecting and measuring volumetric defects resulting from erosion, corrosion, wear, and baffle cuts. Sensitivity to pitting has been further improved with the new design of the remote field probe.
The remote field probe is a low-frequency variant of the exciter- (driver) pickup eddy current probe, characterized by a exciter-pickup distance of at least 2.5 to 3 times the tube OD. This distance is necessary and critical in order to have the pickup coils sense the “re- mote” magnetic field rather than the “direct” field.
Olympus remote field probes and equipment are used successfully around the world to inspect heat exchangers, feedwater heaters, and boiler tubes. RFT is a through-wall transmission technique. The basic probe is made of one exciter coil and two pickup coils. Two magnetic fields are present: the direct field, in the vicinity of the exciter coil, is rapidly attenuated with distance, while the indirect field is diffused outward through the tube wall. The near field then propagates along the tube axis and is then rediffused back through the tube wall.
The zone where the indirect field is dominant is called the remote field. This zone is present at a distance greater than 2 tube diameters. All remote field probes have their pickup coils set at 2.5 to 3 times the tube OD to ensure that only the indirect field is picked up. All
Olympus RFT probes have a set of circumferential pickup coils that can be operated simultaneously in absolute and differential mode