Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

(b) Test part thickness
The conductivity reading will be affected if the test part thickness is less than
the ‘effective depth of penetration, which, for the coil diameter normally used
for conductivity testing (approximately 10 mm), can be taken as 3 times the
standard depth of penetration. If there is doubt as to whether the test part is
thick enough, it should be tested, if possible, with and without a block of
copper held against the far surface. Any difference in the readings indicates
that the thickness is less than the effective depth of penetration. Most eddy
current conductivity meters operate at a fixed, common frequency of 60 kHz.
This is low enough to give sufficient penetration to avoid any contamination of
the test surface having a significant effect, and high enough to allow testing of
reasonably thin materials.
Charlie Chong/ Fion Zhang