Remote field testing (RFT) bobin probes are being used to successfully inspect ferromagnetic tubing such as carbon steel or ferritic stainless steel. They are very sensitive in detecting and measuring volumetric defects resulting from erosion, corrosion, wear, and baffle cuts. Sensitivity to pitting has been further enhanced with the remote field probe's new design.

The remote field probe is a low-frequency variant of the exciter (driver)-pickup eddy current probe, which is characterized by an exciter-

pickup distance of at least 2.5 to 3 times the tube OD. This distance is essential and critical for the pickup coils to be able to sense the “remote” magnetic field rather than the “direct” field.

AVN 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. There are two magnetic fields 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, before being rediffused back through the tube wall. The zone in which the indirect field is dominant is called the remote field. This zone is present at a distance greater than two tube diameters.