VDI (Visual Discrimination Indicator) Numbers Used by Metal Detector Manufacturers.

[ToddB66]

Hi !

I assume that VDI scale numbers will usually vary from one metal detector brand to another, due to the different kHz frequencies and circuit designs employed ?

I further assume that the development of the VDI numbers is not based on the test results specified in
Siemens/m Conductivity of Metals Sorted by Resistivity charts ?

Are my assumptions correct ? If not, please explain.

ToddB66

[Dave J.]

All that is correct. Furthermore:
VDI scale numbers can vary even on the same or similar platform due to different market emphasis.
Beepers do not measure material properties, they measure the effective properties of the object in question, and therefore scientific resistivity charts are irrelevant.

In other words, the VDI scales are arbitrary according to what the engineer or company feels will be the most useful for that particular product.

[Carl-NC]

Within a brand, VDI numbers are usually adjusted to some reference scale, so that detectors with different frequencies produce the same VDIs. Some companies (like White's) have a single reference scale that was established with the 6.592kHz designs, some (like FTP) have more than one reference scale for different purposes. Regardless of the brand or the circuit design, everyone could use the same VDI scale, they just choose not to.

VDI has less to do with conductivity than it does with size, shape, and thickness. Ferinstance, the US 10c, 25c, 50c, and 1$ silver coins all have exactly the same alloy, but always different VDIs.

[ToddB66]

All that is correct. Furthermore:
VDI scale numbers can vary even on the same or similar platform due to different market emphasis.
Beepers do not measure material properties, they measure the effective properties of the object in question, and therefore scientific resistivity charts are irrelevant.

In other words, the VDI scales are arbitrary according to what the engineer or company feels will be the most useful for that particular product.

Dave J.,
Thanks for the information in your reply !

ToddB66

[ToddB66]

Within a brand, VDI numbers are usually adjusted to some reference scale, so that detectors with different frequencies produce the same VDIs. Some companies (like White's) have a single reference scale that was established with the 6.592kHz designs, some (like FTP) have more than one reference scale for different purposes. Regardless of the brand or the circuit design, everyone could use the same VDI scale, they just choose not to.

VDI has less to do with conductivity than it does with size, shape, and thickness. Ferinstance, the US 10c, 25c, 50c, and 1$ silver coins all have exactly the same alloy, but always different VDIs.

Carl-NC,

In your first paragraph, is FTP a metal detector manufacturer ? What is their full name ? I don't recognize the acronym.


Thanks for the very interesting information in your above reply, especially the second paragraph !


ToddB66

[Qiaozhi]

In your first paragraph, is FTP a metal detector manufacturer ? What is their full name ? I don't recognize the acronym.

I believe the acronym means "First Texas Products".

[ToddB66]

I believe the acronym means "First Texas Products".

Thanks Qiaozhi ! I'm sure you are correct. At age 75½ my recall isn't real sharp anymore.

ToddB66