Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

The basis for the selection of test frequency is the same as that for the selection
of frequency for surface testing for subsurface flaws or thickness measurement.
The frequency should be low enough so that the eddy current intensity at the
outside surface is relatively high, but low enough to give a relatively high degree
of phase separation between signals from different thicknesses. Therefore, the
normal operating frequency should be the frequency which gives a f 90 separation
between the signals from a shallow inside surface flaw and a shallow outside
surface flaw. However, because of the difference in configuration between
surface testing and tube testing, this frequency is that at which the tube wall
thickness equals approximately 1.1 standard depths of penetration.
f 90 =530/(t 2 σ) (5.4)
Where:
f 90 = the operating frequency (kHz),
t = the tube wall thickness (mm) , and
σ = the conductivity of the test material (% IACS)
Charlie Chong/ Fion Zhang