Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

Higher frequencies than 2.2f 90 may give greater accuracy, but the signal from a
shallow flaw at the outside surface will then approach the horizontal (180 °) and
so could be confused with a dent indication or probe wobble signals. Also,
higher frequencies reduce the strength of signals from flaws at the outside
surface. It should be noted that operating at 2f 90 rotates signals from conducting
non-ferromagnetic deposits at the outside surface to over 360° – that is, to the
region occupied by flaws at the inside surface. Care must be taken therefore to
correctly identify signals from such deposits at the frequency used for detection
of signals (f 90 ) before changing to the evaluation frequency. The accuracy of
flaw depth estimation increases with increasing flaw depth. This is
advantageous because the greater flaw depths are of course more critical.
Charlie Chong/ Fion Zhang