Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

5.2.1.2 Selection of test frequency
The impedance diagram for testing tubes with internal probes is of the same
general shape as the impedance diagram for surface testing of materials. Since
detection of flaws at both the inner and outer surfaces is required, the test
frequency is chosen so that the tube wall thickness is less than the effective
depth of penetration. That is, the operating point is located on a thickness curve
rather than on the conductivity curve. The location where each of the thickness
curves meets the conductivity curve occurs when the tube wall thickness equals
the effective depth of penetration. The location of this point on the conductivity
curve depends on the test frequency, the conductivity of the curve and the tube
diameter - as any of these increases, this point moves further down the
conductivity curve.
Keywords:
Since detection of flaws at both the inner and outer surfaces is required, the test
frequency is chosen so that the tube wall thickness is less than the effective
depth of penetration.
Charlie Chong/ Fion Zhang