Remote field transition and probe spacing

Remote field testing exists because conventional eddy currents cannot get through a ferromagnetic tube wall. A relative permeability of a hundred divides the standard depth of penetration by ten, so a bobbin coil in a carbon steel tube sees essentially only the inside surface. Remote field testing sidesteps the problem by not trying to read the field that stayed inside the tube at all.

Two fields leave the exciter. The direct field travels along the bore and is attenuated very rapidly by the tube wall acting as a shield. The indirect field passes outward through the wall, travels along the outside of the tube where attenuation is low, and re-enters through the wall further along. The direct field falls off far faster with axial distance, so beyond roughly two tube diameters the indirect field dominates. That region is the remote field zone; the span around 1.5 to 2 diameters where the two are comparable is the transition zone, where they interfere and the phase is unstable. Detector coils are therefore placed two to three tube diameters from the exciter, and never in the transition zone.

Because the energy crosses the wall twice, the phase lag is about double the one-way wall lag, roughly 2 x 57.3 x t / delta degrees, and the amplitude loss is doubled too. This double transit is what gives remote field testing its defining property: approximately equal sensitivity to inside and outside surface flaws, since both lie in the same through-transmission path. It is also why the technique is slow and low frequency - tens to a few hundred hertz - and why signals are 30 dB or more below what a conventional bobbin would give on non-ferrous tube.

Use this when laying out or checking a remote field probe, when a procedure specifies a spacing you want to verify against the tube size, and when deciding a test frequency: too high and the double-transit attenuation buries the signal, too low and the phase lag becomes too small to resolve depth.

remote field zone begins at about 2 x tube outside diameter from the exciter
exciter-to-detector spacing L = k x OD, with k typically 2 to 3
delta = 1 / sqrt(pi f mu0 mur sigma)
double-transit phase lag = 2 x 57.3 x t / delta  degrees
double-transit loss = 20 x log10( e^(2t/delta) )  dB