Understanding VDI in Depth
WARNING, not for the faint of heart.
For those that want a more complete understanding of the underlying principle behind VDI, read on.
Everyone else can skip this post.
Detector discrimination and consequentially VDI numbers as well, depend on the lag between the peak amplitude of the transmit waveform and the peak amplitude of the Eddy current induced in the target.
We normally, but incorrectly, refer to this as being based on the target's conductance. Thus you hear about targets being high or low conductors.
If you wanted to be picky about it, it is actually a measure of the target's admittance.
Admittance is a vector quantity (ie. it has a magnitude and a direction) made of two components: Conductance and Susceptance.
You can think of them as an east-west axis for the conductance part of the number and a north-south axis for the susceptance component.
Conductance is associated with the electrical resistance of the target to pass a DC current.
Susceptance is associated with the ability of the target to temporarily store and later release energy from the transmitter signal.
Both of these components make up the Admittance vector. It is the phase angle between the applied transmitter signal and the target's admittance vector that forms the basis of discrimination and VDI number.
Different manufacturers map this phase angle into an arbitrary number scale. On the White's machines this arbitrary scale goes from -95 to +95.
Silver and other "high conductors" have an admittance vector mostly determined by the conductance part, while in iron and other largely ferrous materials, the susceptance part dominates the admittance vector.