In this video I discuss the principle of a VLF type induction balanced metal detector (Garrett AT pro), in particular how the discrimination between different metals works. I do this by looking at the emitted and received signals of a test coil, so not by opening up the detector.

00:00 General Intro
00:34 Need for discrimination / ID in metal detection
03:19 Test setup
03:55 Principle of double D coil explained
08:29 ID and phase shift for different metals (para and diamagnetism)

It turns out that the detector measures the phase delay between the emission and reception coil. The reception coil shows this delay because of changes in the inductance which are introduced by the conductivity and intrinsic magnetic properties of metal objects in the vicinity.