The standard depth of penetration describes the depth (dependent on the electrical conductivity and magnetic permeability of the workpiece, as well as the excitation frequency used) at which the density of the eddy current has dropped to ca. 37%. Penetration performance depends on the nature of the eddy currents. Principally the eddy current field penetrates very deeply into the workpiece at low excitation frequencies, and at high frequencies it only forms near the periphery. Technically, tests at up to 3 to 5 times the standard penetration depth are possible. The following figure depicts standard penetration depths of various materials, based on the excitation frequency. Put simply, with the excitation frequency one can adjust the depth resolution of the eddy current test.
