We Have Gone Backwards On Detection Depth Since 1973
21 posts in this topic
By Steve Herschbach
A common misperception among those new to metal detecting is that metal detectors can identify one metal from another. How much we wish that were true. The reality is that for all practical purposes the common metal detector target id scale is based on a combination of the conductive or ferrous properties of the item multiplied by the size and shape of the item.
There are two common terms in use for this scale. The Target ID or TID scale is the most generic. White's also popularized the use of Visual Discrimination Indicator or VDI numbers. You will see references to both TID and VDI numbers and both refer to the same thing. The problem when you use Google is that TID also refers to Terminal ID number, which is for credit card machines. VDI gets far better results as the preferred term and so is what I will use from now on.
The VDI scale is almost always arranged the same way by common convention although in theory it can be rearranged any way you want. The common scale has ferrous items on the low end and non-ferrous items on the high end. Ferrous items are like mirror images of non-ferrous items and so the most common arrangement of the VDI scale is with small items in the middle with ferrous getting larger in one direction and non-ferrous getting larger in the other direction. The ferrous and non-ferrous ranges actually overlap in the middle.
Tiny Ferrous/Non-Ferrous Overlap
We can assign a numeric range to this basic VDI scale any way we want. Many early machines went with a 0 - 100 scale, with the ferrous compressed into the low end of the scale:
100 Large Non-Ferrous
50 Medium Non-Ferrous
20 Small Non-Ferrous
5 Tiny Ferrous/Non-Ferrous Overlap
3 Small Ferrous
1 Medium Ferrous
0 Large Ferrous
The idea of ferrous as negative numbers made sense due to the mirror imaging in size between ferrous and non-ferrous. A very common White's scale runs from -95 to 0 to +95
95 Large Non-Ferrous
50 Medium Non-Ferrous
15 Small Non-Ferrous
0 Tiny Ferrous/Non-Ferrous Overlap
-15 Small Ferrous
- 20 Medium Ferrous
- 40 Large Ferrous
The "positive only" 0 - 100 VDI scale seems most popular these days with other manufacturers, but the scheme varies. Two very common setups are 0-40 ferrous and 41-99 non-ferrous OR 0-10 ferrous and 11-99 non-ferrous. But as I noted you can set this up any way you want and so other scales do exist.
When we look at just the non-ferrous part of the scale, what is important is how the detector "sees" the target. In very simple terms conductive targets are either very weak or very strong or somewhere in between. Small items are weak targets. Low conductive metals are weak targets. Large items are strong targets. High conductive metals are strong targets. The shape matters. Irregular shapes or thin items are weak targets. Rounded and thick items are strong targets.
On a conductive scale of 0 to 100:
0 = very small targets 100 = very large targets
0 = very thin targets 100 = very thick targets
0 = very low conductive metals 100 = very high conductive metals
0 = very irregular shaped targets 100 = very rounded targets, especially is a hole in the middle
Add this all up and small gold items are low on the VDI scale and large gold items high on the scale. Silver being a better conductor than gold, a silver item will read higher on the scale than the identical size and shape gold item. In general silver will read higher than gold. However, a very large gold item can read higher than a very small silver item. Chasing thin hammered silver coins in the U.K., especially the cut varieties, is not that different than hunting gold nuggets.
What you rapidly figure out is the metal detector VDI scale can only get repeatable results on certain man made items that are the same every time, like a U.S. nickel or a U.S. dime. And even these signals degrade when deep in the ground or in proximity to other items under the search coil at the same time. Given all the limitations, it is a wonder we get any degree of accuracy at all with detector discrimination systems.
With that, I give you a standardized White's VDI scale taken directly from the control box of my White's DFX. This -95 to 0 to +95 scale is common on many modern White's detectors. Nearly all other detectors have the same relative positioning of items just with different numeric scales, an exception of note being the Fisher CZ detectors, which use a rearranged scale. This DFX scale is helpful because it includes gold coins.
The main thing I want you to focus on here is the relative positioning of items on the scale. As a detectorist operating in the United States, I always pay attention to just three things 1. where do the ferrous numbers start? 2. where does a U.S. nickel read? and 3. where does a U.S. dime read? If I know those three things, I can adjust almost instantly to any detector scale in existence, because I know how everything else reads in relation to those three points on the scale.
Looking at the scale you can use gold coins as a rough guide to where large gold nuggets will read, although coins being pure gold and round will read much better than gold nuggets of the same size. It might take a one pound gold nugget to read the same as a one ounce $20 gold coin, which in turn reads very close to the U.S. silver quarter reading.
On the other end, tiny gold, tiny ferrous, and salt water, being a low conductive target, all overlap. This is why if you tune out salt water on the beach, you also tune out single post gold ear rings and thin gold chains, which read like small gold nuggets. If a prospector tunes out salt alkali readings on a salt lake, there go the small gold readings. And the chart shows that if you get too aggressive in rejecting all ferrous items, good items can be lost also.
When I say small it is important to note what we are really talking about is small/weak readings. A large gold item buried very deep in mineralized ground will have a very weak reading and appear as a small target to the detector. This means a very deep large items can appear just like a very small gold item and be lost for the very same reasons as those small items. Again, think weak targets and strong targets to get a better feel for how things react in the field.
To sum up, gold and platinum are low conductive metals, and when also small in size read very low on the VDI scale, even dipping into the ferrous range. The foil range is the sweet spot for ear rings, thin gold chains, small womens rings, and platinum items. In general women's gold rings will read below a U.S. nickel and men's gold rings will fall above a U.S. nickel on the VDI scale. Nearly all gold nuggets found by most people are going to read nickel and lower just because nearly all gold nuggets are small. However, as this photo I made using my DFX and some gold nuggets shows, gold nuggets can read all over the place due to their shape and purity. Surprisingly, if you add silver to gold the conductivity drops as alloys are less conductive than pure metals. This makes many gold jewelry items and gold nuggets far harder to detect than would be the case were they pure gold. See this article for details on this nugget photo Some Gold Nugget VDI Numbers
You can get some great spreadsheets for jewelry VDI numbers for White's and Minelab detectors here.
There are no doubt many people who have read this who are just shaking their head and thinking "this is why I just dig everything". I absolutely agree, when at all possible, that is the best solution. Unfortunately it simply is not possible in some locations where trash targets outnumber the good by thousands to one. This is where knowing the VDI scale and how it works can pay off.
The best book ever written on the subject of discrimination is "Taking A Closer Look At Metal Detector Discrimination" by Robert C. Brockett. It is out of print but if you find a copy grab it, assuming the topic interests you.
By Steve Herschbach
Note: thread was split from this previous thread Tone By TID Selection Option?
Thanks for posting that reminder on the F44 Mike. I had forgot about it, and added the chart page to your post. To my mind for coin and jewelry detecting I simply have no interest in owning machines that do not allow me to customize tone ranges and tones. My current stable of coin/jewelry machines are the White's DFX, Minelab CTX, Nokta Impact, and XP DEUS, and all four offer this capability (the ability to cusomize tone ranges and tones) in one form or another. It really is a killer feature on the F44 at such a low price, only $349 these days. If all I could have is one detector and had to buy a new one under $400 I have no doubt the F44 is what I would end up with. Funny that it gets so little interest on the forums but I guess that reflects the fact most of us tend to be using higher end product. This is a case where Fisher may have sold more by pricing it higher! People may snicker at that but there are sound sales reasons for why that may be true. Look at what you get in a Gold Bug Pro and you would think it should be $349 and the F44 should be $649.
By Steve Herschbach
For background on electrical interference in VLF detectors here is a great essay on electrical interference by Dave Johnson of First Texas Products. You can also find a more detailed discussion that includes PI detectors in section 2.1 of this Minelab document by Bruce Candy.
OK, so I am bench testing my new Teknetics G2 at home recently. The 19 kHz models are renowned for being immune to EMI (ElectroMagnetic Interference). No electrical interference ever (well, almost never) even at highest gain levels.
As a rule, the lower the frequency, the more issues you have with EMI. It is especially bad under 10 kHz. DEUS owners may see significant EMI at 4 kHz, only a little at 8 kHz, and none at 12 and 18 kHz. Another reason why manufacturers favor mid frequency over low frequency detectors these days.
I have been bench testing detectors in my house for years so think I know the EMI levels. No issues with previous 19 kHz units like the Gold Bug Pro and F19. Yet this new G2 chatters like crazy! I fire off an email to First Texas asking if some change I was unaware of. Nope. And could not be a bad coil because both coils I have did it.
Then on another go I noticed the EMI was bad on one end of house but not the other. I walk around and the detector leads me to a new LED bulb I installed recently. This thing is pumping out 19 kHz EMI like crazy! Not long ago I installed a number of these cheap LED bulbs in my house. https://www.amazon.com/Feit-Electric-Replacement-CEOM60-927/dp/B01BJ0Y1MC Looks like my mission to upgade my house to all LED just ran into a snag!
More on LED bulb interference - https://www.google.com/search?num=30&q=led+bulb+electrical+interference&oq=led+bulb+electrical+interference
Just shows how more and more we are surrounded by new forms of electrical interference to make life harder for detector engineers.
By Mark Gillespie
For over a decade I've ask for one small feature to a good detector.
Say for instance on the Fisher F75 detector or another machine of comparable abilities.
Add a two tone function in the motion all metal mode.
Nothing fancy, just one tone for ferrous and another for non-ferrous.
Being a computer programmer for several years I can't imagine this would be very difficult.
Now on the F75, while in motion all metal mode the machine gives better depth and also gives an ID for detected metal objects.
So since nothing is really needed except assigning a tone to the ID number scheme why is it so hard to acquire a unit with that feature.
I know the V3i has a feature similar, but it lacks the depth capability of the F75 in my ground.
Now I've ask again.
I'll check back in another decade.
By Steve Herschbach
From Interview with Dave Johnson at http://www.fisherlab.com/hobby/davejohnson/Interview%20with%20David%20Johnson.pdf
Many of the people reading this interview unknowingly own products of your design. Would you mind listing them?
My first metal detector (in 1971) was a portable experimental vehicle detector for use on roadway loops. It discriminated between cars and trucks, but to become a practical product would have required a lot of development and nobody was interested in investing in it.
Fisher in California: 1260, 1220, 1210, 1235, 1225, 1212, 1265, 1266, CZ6, CZ5, CZ20, original Gold Bug, Gold Bug II, Gemini, and industrial instruments including TW6, FX3, XLT-16, PF-18, and circuitry of the TW-770.
Tesoro: Diablo MicroMax, Lobo SuperTraq.
White's: GMT, MXT, analog circuitry of DFX, Beach Hunter and PCL-600 line tracer.
Troy: X-5 and X-3.
FTP Bounty Hunter: major revisions to existing platforms most of which originated with George Payne. The BH Junior, Platinum, Gold and security wand (sold under various trademarks) were new designs.
FTP Teknetics: T2, Alpha, Delta, Gamma, Omega, G2.
FTP Fisher: F2, F4, F5, F75, F70, new Gold Bug, circuitry of the TW-82 industrial line tracer.
In the case of microprocessor-driven FTP products, the software was coded by John Gardiner and Jorge Anton Saad. Mechanical designs were done mostly by other people, but I engineered the ergonomics of the T2 mechanical design, which is also used on the F75.
See also Detector Stuff Interviews FT-Fisher Engineers, David Johnson and John Gardiner at http://detectorstuff.com/detector-stuff-interviews-ft-fisher-engineers-david-johnson-and-john-gardiner/
Want people to know you own one of the Dave Johnson detectors listed above? Just download and print the attached logo and apply to your detector.