In Eddy current testing, the analysis of impedance diagrams is simplified by use of normalized frequency. That is, by using a ratio of the actual frequency used and the "limiting frequency" fg. In the ASNT handbook, I found the following formulas for fg (in non-ferromagnetic material): 1. for cylinder in an encircling coil: fg=a/(sigma*D^2) 2. for tube in an encircling coil: fg=a/(sigma*D*W)

Where a is a constant whose value depends on the units used, sigma is the conductivity, D is the diameter of the cylinder or the outer diameter of the tube and W is the tube wall thickness.

Strangely, these two expressions do not coincide when W=D/2, that is when the tube becomes a full cylinder. Have I missed something here / Does formula 2 have a limited range of application so that it does apply when W=D/2 ?