In the choice of test frequency (aka excitation frequency) the requirements of the application and the probe to be used must be taken into account. The frequency range recommended for the probe can be found in the manufacturer’s data sheet for the probe.
The test frequency has a decisive effect on penetration capability; i.e. the distribution of the eddy current strength in the direction of depth:
With increasing distance away from the surface the eddy current strength drops considerably. The eddy currents concentrate themselves largely on the surface; they are shielded to some extent in the direction of depth. This is known as the “skin effect”. One measure of the drop in eddy current strength with depth is the standard depth of penetration.
The higher the test frequency, the larger the strength of the eddy currents induced on the surface of the test object (according to the law of induction). But on the other hand, the higher the test frequency, the more the eddy current strength drops in the direction of depth. (Note: The electrical conductivity and the relative permeability of the test object have the same influence on the distribution of eddy current strength as does the excitation frequency.)
For that reason, with the choice of test frequency it is possible to exercise specific control on the measurement sensitivity as well as the zone of interaction (volume penetrated by eddy currents):
High test frequencies generate strong eddy currents at the surface of the test object and so they offer excellent sensitivity for surface defects.
Because of their better penetration capability, low test frequencies offer good sensitivity (detection capability) for defects lying under the surface (hidden defects).
The choice of test frequency also has an influence on the phase separation angle of defects of different depths. This can be exploited for example in the examination of pipes using an internal coaxial probe (aka “bobbin”) with phase analysis:
At low test frequencies the directions of signal drift (phase angles of signals) of internal defects of different depths or external defects of different depths hardly differ from one another.
At increasing test frequency the separation angle of defects of different depths grows. This corresponds to better resolution capacity in the direction of depth. (Note: The electrical conductivity and the relative permeability of the test object have the same influence on the phase separation angle as does the excitation frequency.)