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Target ID / Vdi Numbers For Gold Nuggets And Gold Jewelry


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Great thread here Steve..... Kudos..:smile: 

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  • 1 month later...

This is a very informative thread.  Thanks Steve.  Does anyone have a metallurgical explanation why some some steel indicates non-ferrous when it rusts?   I find this most often on some forms of steel wire or cable that has rusted to the core, often disintegrating when you dig it.

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Metal detectors can sense both conductive and magnetic effects. Non-ferrous items have only conductive properties, whereas ferrous items have both conductive and magnetic properties. Many iron and steel items can unfortunately read non-ferrous. This thread goes into the details.

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  • 7 months later...

Setting up your own VLF VDI reference chart.  

Found this thread very interesting Steve, thanks.  

I have a query for yourself and anyone else willing to chime in.

Obviously there are charts showing where certain coins/objects show on the VDI scale - as you have posted.  Due to obvious differences with coins in different countries a different chart would be required for Australia, England, China, etc.  And I'm sure that many of you take your own readings and make your own reference charts.  

My question is:  What is your particular methodology in working out what VDI number correlates to any particular object?  

Do you simply move a dime past the coil, it reads 26 (or whatever depending on the machine) and you record that number?

Or, do you want the dime read at a distance of 6 inches from the coil and record that number?

Or, do you want an 'at coil' reading, a 6" from the coil reading and a 12" from the coil reading.  And in your experience do the VDI numbers change significantly at those 3 different depths?  

Does anyone ever bury these targets in the type of ground they are likely to hunt and then document those VDI numbers rather than the readings of air tests.

The same procedure would obviously need to be done with rings, chains, etc, etc.  

The reason I ask is that I am seriously considering purchasing an Equinox and would need to do this for Australian coins and make my own reference chart.  Simply wondering how others go about this.  

Im not sure if there is going to be a simple or complex answer to this - happy for it to be a thread of its own somewhere more appropriate Steve if you think it warrants it. 

Thanks in advance :wink:

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Charts are usually constructed using simple air tests. The big caveat on all discrimination systems is that ground effects and nearby targets can cause target id numbers to read both lower and higher, so all this really only works on relatively isolated targets in mild ground. It all goes out the window for anything more complex, and in places like Europe with thousands of years of history with different types of finds to be made, you simply dig all non-ferrous targets.

Therefore constructing charts they are only for reference under perfect conditions, and they must be able to be duplicated by others. Air test with moderate sensitivity a few inches from the coil. You want clean, solid repeatable numbers. Trying to go farther will just give you erratic numbers that bounce all over the place.

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Charting is useful when you first start out.  Chart everything.  Treasure and trash.   Chart every type of target identification feature you have available to you.  Chart close, chart far away.   But when you get through charting everything, you'll find that all you really need to chart are a couple of reference items.   

For me,  I only chart nickels and dimes.   Oh..I'll do depth testing with a quarter and a half and a dollar but in reality, if its a high conductor it doesn't really matter what the number is, it gets dug anyway, right?  So the dime is the number I look at, and I want to know stable and fringe depth audio and number response.

For low conductors I really only need the nickel number for a known reference point and then knowledge on how well it holds a nickel id at depth.   Below the nickel is small rings, above the nickel is large rings.

The only other thing I look at is the ferrous / non-ferrous break point.   How soft is this boundary? Is it sharp or is it 6 numbers soft?   

But starting out is always best to bench test the crap out of a detector.  

And then the ground throws it all out the window, of course.   

HH
Mike

 

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2 hours ago, Mike Hillis said:

Charting is useful when you first start out.  Chart everything.  Treasure and trash.   Chart every type of target identification feature you have available to you.  Chart close, chart far away.   But when you get through charting everything, you'll find that all you really need to chart are a couple of reference items.   

For me,  I only chart nickels and dimes.   Oh..I'll do depth testing with a quarter and a half and a dollar but in reality, if its a high conductor it doesn't really matter what the number is, it gets dug anyway, right?  So the dime is the number I look at, and I want to know stable and fringe depth audio and number response.

For low conductors I really only need the nickel number for a known reference point and then knowledge on how well it holds a nickel id at depth.   Below the nickel is small rings, above the nickel is large rings.

The only other thing I look at is the ferrous / non-ferrous break point.   How soft is this boundary? Is it sharp or is it 6 numbers soft?   

But starting out is always best to bench test the crap out of a detector.  

And then the ground throws it all out the window, of course.   

HH
Mike

 

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Thanks Steve, Mike and MineBlab.

Always good to know what the experienced among us do when learning a new machine. 

Thanks again :wink:

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  • 2 weeks later...
On ‎4‎/‎25‎/‎2017 at 11:05 PM, Steve Herschbach said:

Here is the final thought for the night. Back when Fisher did the CZ series of detectors they did lots of ring testing. They tested 255 rings for the CZ-70 and came up with some interesting percentages. I have seen these percentages reflected in the field.

2% of the rings were in the copper penny, dime, quarter range.

4% of the rings were in the zinc penny/screw cap range

49% of the rings were in the "pull tab" range above U.S. nickel

10% of the rings were in the nickel range

36% of the rings were in the foil range below nickel

0% of the rings were in the iron range

Now look at my simplified X-Terra VDI chart in the post immediately above. What we are seeing is the difference between women's rings and men's rings. Women's rings tend to be small high quality rings, and cluster in the foil range. They are the rings most likely to have stones and be of high value. Men's rings are significantly larger and heavier, and fall into the area above U.S. nickel. There is a weak spot or gap between women's and men's rings in the nickel range. This is of course a gross generality but I have found it to be true in my own detecting. You can use this to good effect when looking at an area and deciding what might be found there. Is the area more likely to hold men's rings and women's rings? The football field will more likely have men's rings. The shallow childs wading area or tot lot will lean women's rings. I do a lot of heavy surf detecting, and nearly all my finds are men's rings.

I have mentioned the Fisher CZ detectors. Rumor has it that CZ stood for "Coin Zapper". The CZ detector are unique in having a shuffled discrimination scale that puts nickels up high with the other coins. Little attention is paid to the fact they also lumped the bulk of the ring zones together. In addition to visual target identification, the CZ  has 3-tone, audio target ID. A low tone is for iron, a medium tone Is for pull tabs and foil and a high tone is for coins. A fourth tone, which sounds like a telephone, alerts you to large, shallow targets that are usually (but not always!) trash.

Basically with a CZ, low tone is iron, medium tone the ring range, and high tone the coin range. The newest model, the CZ-3D adds a fourth tone to the scheme designed to capture old coins in the zinc penny range, but for jewelry detecting the original CZ scheme is pretty simple and ingenious.

The good news is with newer detectors that have custom tone id ranges you can duplicate this setup yourself. Fisher also offers this modified tone scheme in the F75 models. The main thing is to be aware, no matter what detector you are using, or where the possible ring "hot spots" are on the VDI scale depending on where you are hunting.

From Fisher CZ-70 Pro Owners Manual, page 24 (tones added):

fisher-cz-7a-pro-ring-percentages-chart.jpg

 

This is fascinating stuff.  Interestingly enough, I dug 6 gold rings last year.  A solitaire, 3 bands, and 2 child's rings.  The solitaire fell in the nickel range, the 3 bands were all in the zinc penny range, and the two child's rings were down in foil range (both tot lot finds).   Based on the percentages above...aside from the "dig it all" tot lot finds.... I walked over 25 gold rings.  And I believe those statistics.  

It is a lot of work to commit to the mid tones... but it is obvious that is where the yellow is.  

Tim.

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