Remote Field Testing is an effective non-destructive testing method for the inspection of ferromagnetic as well as slightly ferrtic alloy tubes, based on the transmission of an electromagnetic field through the tube material. This through-transmission defines remote field testing. Typically used on heat exchangers, boilers and air heaters.
These probes come in various configurations, but essentially consist of a transmitter coil positioned about two and a half tube diameters apart from the receiver coils. This spacing is necessary as it positions the receiver coils in the indirect/remote field zone, this is the zone where the magnetic field from the eddy currents is dominant, and unaffected by the magnetic field from the transmitter coil.
The exciter coil is driven with a relatively low alternating current to produce a magnetic field. This changing magnetic field induces strong circumferential eddy currents into the tube wall, these eddy currents in turn produce their own magnetic field, which opposes the magnetic field from the exciter coil.
The magnetic field that originates from the transmitter coil, is rapidly attenuated by the circumferential eddy currents induced into the tube’s wall. Since the eddy current field is more spread out than the field produced by the exciter, it extends farther along the tube axis. At a distance of approximately two and a half tube diameters from the exciter the primary field is no longer effective, so from this distance, the magnetic field produced by the eddy currents is dominant.