The Remote Field Technique (RFT) is extremely suitable for the inspection of ferrite pipes. RFT is particularly suitable for the detection and quantification of wall thickness reductions. Internal and external defects can be detected but not distinguished. The filling factor is much less important for the sensitivity and reliability of the inspection than the other applicable techniques. RFT testing is not suitable for testing finned pipes.
The measurement probe is positioned in the pipe. The probe contains a transmission and reception coil. The transmission coil generates a magnetic field that penetrates through the pipe wall to the outside. This field then penetrates back in through the pipe wall at the height of the reception coil. Because the magnetic field penetrates the pipe wall twice, it is slowed and weakened to a certain extent. The slowing and weakening that occur depend on the local pipe thickness and are measured.
Contrary to eddy current testing (ECT), remote field testing (RFT) is suitable for ferromagnetic materials such as pipes and tubes found in boilers and heat exchangers. Inspecting the full thickness of a tube wall using conventional ECT is difficult due to the strong skin depth effect in ferromagnetic materials; with RFT, wall thicknesses of up to 12 mm can be inspected and this, at a pulling speed of 0.1 m/s to 0.3 m/s. About technique sensitivity, you get nearly the same detection sensitivity for both internal and external flaws. Moreover, this method is highly sensitive to variations in wall thicknesses and tends to be less sensitive to probe lift-off, centering, wobbling, and fill-factor.
That “indirect field” spreads through the tube wall, propagating along the tube axis, and emerges back inside the tube. When the probe encounters a decrease in wall material, the field intensity, picked up by the receivers, varies. Therefore, the measurement corresponds to the total material thickness that the magnetic field has gone through.
Basically, an RFT probe is constituted from one exciter and two receiver coils set apart at a distance equal to twice the tube outer diameter or more. The zone where the receivers are located is called the remote field zone. Then, receiver coils are going to feel the indirect field from the exciter rather than the direct one, typically used with ECT.
Because the RFT technique is equally sensitive to ID and OD defects, it cannot be used to determine if wall loss occurs inside or outside the tube wall. RFT is a through-wall transmission technique where the magnetic field passes through the tube wall twice: once at the exciter location and once at the receiver location. Whether a defect is aligned with the exciter or the receivers, an effect will be felt by the receivers. The measurement is relative to the total material thickness crossed by the indirect magnetic field.