Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

(e) Non-ferromagnetic support or baffle plates and conducting deposits at
the outside surface.
A non-ferromagnetic tube support of baffle plate gives the combined effect of an
increase in thickness of the tube, and, if the support has a different conductivity
from that of the tube, a change of conductivity, rotated clockwise because of
phase shift. At f 90 the operating point is relatively low on the thickness curve,
similar to that shown in FIG. 5.25 for surface thickness testing. The large
increase in thickness (greater than the effective depth of penetration) caused by
a support plate moves the spot to the Effective Depth of Penetration (EDP)
location on the conductivity curve, and so gives a signal in the lower left
quadrant, as shown in FIG. 5.36.
Charlie Chong/ Fion Zhang