VDI Numbers

[Ks Coyote]

I know this is a basic question but on whites detectors that use VDI numbers how are they calibrated, by size or by composition? I've noticed that on coins like half dimes and three cent pieces the numbers seem to be more erratic then on dimes and quarters.

 

[widebody]

[size=large]if you can get your hands on jeff foster's book "the mxt edge", whites vdi numbers are explained in it. can't say i've learned it yet but i'm trying.

HH[/size]

 

[Larry (IL)]

I believe they are based on the targets conductance. Lower the conductance the lower the VDI.

 

[Tom_in_CA]

Yes, as Larry says, TID readings are based on conductivity. However, it's based on conductivity PER SIZE of the target too. So sort of like calibrated for conductivity, for items in coin-sized categories. So for example: a snippet of aluminum (or simply the tab or beavertail portion) from an aluminum can, will read low, right? Yet what happens if you wave AN ENTIRE CAN in front of the coil? It's read up at quarter or whatever (high TID), right? But notice that in each case, the composition (aluminum in this case) never changed, right?

Therefore size plays into it. So it's correct to say it's based on conductivity, yet the targets have to be commensurately sized.

And be aware, that a lot of different metal can share the same conductivities (gold and aluminum for instance). And there is inifinate variables of alloys out there.