Menu
What's new
Find's Treasure Forums

Welcome to Find's Treasure Forums, Guests!

You are viewing this forums as a guest which limits you to read only status.

Only registered members may post stories, questions, classifieds, reply to other posts, contact other members using built in messaging and use many other features found on these forums.

Why not register and join us today? It's free! (We don't share your email addresses with anyone.) We keep email addresses of our users to protect them and others from bad people posting things they shouldn't.

Click here to register!

Need Support Help?

Cannot log in?, click here to have new password emailed to you

Changed email? Forgot to update your account with new email address? Need assistance with something else?, click here to go to Find's Support Form and fill out the form.

What's Going On Here ?

R

Ralph Bryant

New member
Can anyone offer up any ideas about the results of this little test ?

[attachment 13072 Untitled-Scanned-01.jpg]

We're all familiar with the theory of "target averaging", but this one has me stumped. If you take a nickel with an I.D. of 12 by itself on the Xterra 30, and place it inside a bent over beavertail pull-tab as shown which I.D.s at 24 by itself, the combination actually rises to a higher I.D. (combined conductivity) than either of the parts, in this case giving an I.D. of 28. This same effect carries over to every VDI machine I've ever tried it on.

Any thoughts ?

Ralph
 
Ralph,

What do you think the results would be with thin insulation between the two items so that there was no electrical contact between the two items?

I am thinking that target averaging will be considerable different if the targets are sufficiently separated so that there is no interaction between how each target distorts the magnetic field of the transmitter.

HH,
Glenn
 
Merry Christmas!
MM
 
Ralph,

there are two types of discrimination/ID technologies I am aware of.

1) averaging which most tectors use

2) additive. This is what it sounds like is happening here.


1266x comes to mind when I think of additive.

I havent noticed this on mine yet, probably because I been having too much fun in the field using the x-30

Tony
 
This time, I tried the same two targets from different orientations.

First, I placed the nickel and tab side by side about 1/4 inch apart, and from each sweep direction I got a 24 reading, same as the tab alone. But when I placed the tab and nickel to where they were oriented one atop the other (but not touching), they always read 28, whether the nickel was on top or visa versa.

At first I thought that the two targets being placed together and touching was increasing the I.D. response due to the greater mass of the two combined, but this obviously wasn't the case. It seems it has more to do with the effect of one on the other, but still strange that two lower conductivity targets can combine their signals to result in a higher apparent conductivity than either of the constituents alone.

Ralph
 
Ralph, I don't think we actually measure conductivity but conductance. The two targets have more conductance just as a larger piece of metal low conductive can be identified as a small piece of high conductive metal.

There are most likely many targets with different shapes and allows that will give us these odd reading that we try to equate to more or less conductivity. Throw in a little iron and it is even more interesting. Cut the circular tab so it could be straightened out into a kind long bar shape but keep it in original shape and see what happens.

This is not the same thing as bonding two metals in an alloy to change the conductivity as you know.
 
Hi Cody,

I think it is actually a combination of both conductivity AND conductance, or wouldn't all larger items read higher on the VDI than all smaller items of a similar material ? This isn't always the case. Target shape or cross-sectional mass and how the eddy currents are allowed to flow (or are stiffled) is another factor, just as you mention in cutting a ring shank to change the VDI value. In that case, the mass of the item hasn't changed (actual conductance), nor has the conductivity of the material. The difference is in the current "flow" within the item. In the case of the cut ring, the eddy current ceases to flow in a smooth circular pattern around the full circle of the ring, but actually reverses back upon itself at the location of the cut. At least that is my understanding. It's strange that the experiment with the nickel and the tab could never be duplicated with any other standard U.S. coins other than the nickel. In the case of other types of coins, including copper cents, silver or clad dimes and quarters, the coin and tab never read any higher than the highest value of either item. Might be due to the ferrous nature of the actual nickel metal contained in the nickel, not sure, but the other clads have some nickel content also only separated in the center by pure copper. But on the X-30 the same tab with a copper cent or clad or silver dime averaged the otherwise "36" VDI coins down to "32", and averaged the otherwise "40" VDI clad or silver quarter down to "36". But like you, I thought there might be other combinations of targets that could show similar results, but in all of the bench testing I've done through the years, I've never seemed to find another target combination that would do that.

Ralph
 
I've always looked at detector function like this. Conductance is what allows the metal to be detected "in the raw", but conductivity is what allows metals to be differentiated or discriminated. It's kind of like trying to separate the runner from his feet. He can have feet, and not necessarily run (ability or conductivity), but he still has to have his feet to do so (current or conductance).

(....or something like that.) :lol:

Ralph
 
Ralph,

This is interesting and something I have seen before.
My "unofficial" explanation would be that with ring pulls, and in particular the folded over example in your photo, you are not dealing with just one eddy current flow but in fact there are at least three different currents being generated. The tear drop is one, the outside of the round part is another and the inside of the round part is the third. Introduce your medium condutive nickel and the machine then sees 2 types of metal instead of 3 + another separate current = 4.
Are you suggesting that the tab + nickel combo "should" be 36.
I've repeated your experiment here. I don't have a ring pull that generates a 24. The samples I have produce 18. If I introduce the nickel the combination then produces 21 - a difference of just 3 which is less than your difference by 1.
As a sidenote, our two and one Euro coins have two metals in their physical make-up being, "Outer part copper nickel/inner part three layer nickel-brass,nickel,nickel brass" which in the case of the two Euro coin produces an ID reading of 27 on X-Terra which is not too far away from your "2-metal combo" of 28.
I've sought the advice of the gurus from Australia and would hope to perhaps be able to provide the physics of your conundrum very soon!

Good Hunting!

Des Dunne
Minelab
 
I think you nailed it. It is those darn eddy currents all confused about who and what they are. I think the believe they are protons and and some may even think they are neutrons.

HAHA

Great explanation, I always look at those questions in those terms but it is "politically correct" to say conductance and conductive. Would eddy current in the two targets induce eddy currents in the other or should I say attempt to do so. I have seen this several times and it depends on the tab and nickel as to what reading we get. Our nickels suck as do our clad coins!
 
You must log in or register to reply here.
Top