How a detector reads coin conductivity?

Look at these 2 coins, both silver and of the same thickness, can anyone explain to me how they can ID 13 points different on a Deus?
Does roundness and texture come into play as well?

They are not technically the same size are they ?? so its not a true like for like test,also the silver content can change by a massive amount,so my way of think is that this is probably the reason why they are not giving the same reading not only on the Deus but possibly on other detectors as well.Just my theory of course

If you found say 2 identical coins from the same hole and same size and weight i reckon that they would be much closer on the i.d !!



Edited 1 time(s). Last edit at 07/31/2017 10:15AM by Junk and Disorderly.

I'm taking it that there both of the same silver purity Rick, would that effect the reading?

ghound Wrote:
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> I'm taking it that there both of the same silver p
> urity Rick, would that effect the reading?

That can effect the reading alot,the silver content varied by a massive amount it basically it depended how much silver was around at that time,the same can also go for 'gold hammered coins' as well until milled coinage came of age and then it was basically all uniformal weight as well as content wise.

Weight for hammered silver coins like for like was pretty consistent but the quality of the silver could/can vary,so yes this will show different readings on your screen,i do use a Deus infact i have 2 of them as one is used a backup,but for the most part just use it as a audio only machine,always keep the controller with me for altering the settings but certainly never make a 'dig/no dig decision on what the screen tells me,audio is far more reliable and also at much greater depth as well.



Edited 1 time(s). Last edit at 07/31/2017 11:01AM by Junk and Disorderly.

Thanks that's what was throwing me off, i expected the larger heavy coin to read the highest but it was the other way round.
I'm still learning the Deus, so i'm using the screen to help me avoid digging coke, i'm digging everthing else though.

You're not really arguing your case very well, here. If you told us which coin read higher/lower it would help. Then tell us what operating frequency you had your Deus running at, too. And were you ID-normalised or not?

The coins are clearly different, one is 60% heavier than the other, and while their diameters are clearly different, I suspect one is also a little thinner, too...a bit of mathematics would tell you the theoretical thicknesses.
And thickness is one of the key things that causes hammered coins to have their low 'conductivity' reading.

Then there's the metal itself. Ideally, they both started off as 92.5% Sterling Silver -- but you don't definitely know that, and the Medieval long-cross penny would be the most likely to be sub-standard, and a lower silver content will ultimately lead to a lower conductivity reading.

Then the big unknown, and a very important one, too: CORROSION. Over the centuries, that 7.5% copper can corrode out (forming the familiar green copper hydroxide/verdigris product), leaving the silver only lightly affected. The remaining copper-less coin is like a sponge on the microscopic level. Electrical currents cannot flow nearly as easily as they would on a freshly-struck coin. This increase in electrical resistance drops the 'conductivity' reading down quite significantly. How corroded each of these coins is is hard to define, it depends on the ground they've been buried in, and how acidic/over-fertilised/undisturbed etc it is.
If you were to get some Sterling Silver sheet, and make a 'fake hammy', you would find it was markedly different to most dug-up ones, especially the long-buried ones.



Edited 2 time(s). Last edit at 07/31/2017 11:22AM by Pimento.

I'm not trying to argue any case, i'm trying to understand. I did say the smaller coin read higher, i was running 18khz
Thanks for explaining it though.


Pimento Wrote:
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> You're not really arguing your case very well, her
> e. If you told us which coin read higher/lower it
> would help.

You didn't say which read higher in the original post, I was typing my reply when you added this info in your later post, so never saw it.

Your diameter measurements are also odd. 28mm for what looks like a regular long-cross penny? All mine are about 18/19mm unclipped.
Likewise the other coin, 14mm might be more correct for a James I penny? . If that's a regular metric ruler on the bottom, then I'm correct.



Edited 1 time(s). Last edit at 07/31/2017 11:48AM by Pimento.

Yes, sorry i got the long cross wrong, it's roughly 18mm depending where you measure, the other i think is a half groat and probably closer to 17mm.
The slightly small lighter coin i thought would have read lower, so i was scratching my head, but your reply explains it well thanks.



Edited 2 time(s). Last edit at 07/31/2017 12:05PM by ghound.

I can't say whether there is much difference in thickness, if one is 0.35mm and the other 0.40mm, for example, it can affect the ID reading.

However, I think corrosion of the older coin is the main reason for your observation. This is compounded by the possibility of the silver content being debased. If it was 85% silver, for example, there would be twice as much copper to leach away, leaving what's left even more porous.

I do have a few 'outlier' hammered pennies myself. I've a voided long-cross penny that reads way below what it's contemporaries do. It looks fine, it's not clipped badly, or cracked, I think it's a debased example. And I have a Scottish Alexander 3rd penny, also in good shape, that reads much higher than expected. I did wonder if it was perhaps better silver, possibly 100%. But as far as I can find out, the Scots used Sterling silver too at that time, hence why their coins were freely-traded between England and Scotland. It was found in land that's been unploughed for about 400 years, so it's not had fertiliser abuse, (though animal 'waste' and acid rain will no doubt have affected it). I guess it's just less corroded that they normally are from cultivated land.

Suppose it's a perfect example of not pre judging a target on it's number ID or low-mid tone, so many differing coins and their make up in the UK.

Ghound,
Try the following experiement.
Get 2 just alike modern coins.
Find 2 that weigh the same using your scales.
Then sweep with Deus.

Take one and grind to make thinner.
Take the other and grind to make out of round leaving thickness the same generally.

After grinding weigh each.
Place on flat surface (clean) and sweep.

You can then grind each some more and weigh, and check.

This experiement will maybe answer at least some part of your question here. (As far as the mass/shape thingy)

Btw, this what you are asking about, is one reason the folks who run Minelab FBS tech units, had better not be too cute when it comes to trying to cherry pick coins.
Worn US silver dime can mimmick clad dime even a wheathead penny with a little wear.

It is not necessary to know the why behind your original question in order to be a good to great detectorist.
Just be able to realize the affects.

This gent Allain, behind the Deus, smart fella.
This explains the "why" behind the way Deus is engineered.

I could make a case, if Deus did infact ID deeper nonferrous objects, the Deus likely wouldn't be as popular and respected as it is currently.
If Deus did ID deeper, allbeit, with additional depth would come more error, wonder how many good to great finds would still be living in the ground that have been swept with Deus coil previously?

Certainly my friend who is handy with his etrac says he finds this on some silver hammered.
But what did throw me was the smaller (and probably thinner if measured with a caliper) was reading much higher.
Just for reference, the smaller coin reads 70, same as a pull tab and a gold 1/2 sovereign coin, what a lottery lol



Edited 2 time(s). Last edit at 07/31/2017 01:15PM by ghound.

The simple answer is the size, shape, weight, surface features, materials it is made of, depth, proximity to other targets, ground phase, moisture content, sweep speed, sensitivity, operating frequency, and about a dozen other things determines TID of a target. I dont know what algorithms the Deus uses, so I can't say exactly why one target in air differs from another when both targets are, for the most part, identical.

On the CTX, I air tested 5 silver nickels and they are read the same with-in a point or two. Then I ran those same nickels on the f75 and the were all over the board with a 20 point(crazy) difference. I could only assume that the CTX was using an averaging algorithm and the f75 was using something different.

When my Dad found the .50 cent gold coin, the tone was very pronounced and loud. I can assume that the Deus may be more sensitive or hard hitting on smaller object.

I would say since there Medieval coins (LUCKY DOG) there was not a whole lot of standardization going on ...the larger if reading lower may well of been of lower purity silver..

BUT you say on the DEUS...does this imply on other detectors it reads the same on both coins??

Keith

“I don't care that they stole my idea . . I care that they don't have any of their own”
-Nikola Tesla

A bit about Mint quality control, and Henry 1's method of improving it...
[books.google.co.uk]

No messing about back then, you could lose your balls !!!!


Pimento Wrote:
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> A bit about Mint quality control, and Henry 1's me
> thod of improving it...
> [books.google.co.uk]

I just repeated the experiment using 'same coins' assuming the smaller one is a Charles 1, 1625 - 1649, 20pence John Falconer Issue - a Scottish coin
Using a variety of English hammered from Edward 1 to Henry IV etc .

Deus Fast
Disc 6.8
Sens 65 (inside)
Freq 11.8
Iron Vol 3
React 3

Thicker full penny - 63/64

John Falconer 20pence - 63/64

Thinner/narrower penny - 65/66/67

Thick penny - 59/60

Smaller thick penny - 59/60

Small thick penny - 67

* Density
* Purity

Both the above are the deciding factors as detectors "read" density of objects and react accordingly.
The silver content can vary from coin to coin, mint to mint, period in history, silver content, silver mix etc.

The 'John Falconer' is a nice find. Well done as they are 'smaller, not as common as others and quite interesting.

There are also small 'Soldino' coins that are about the same size and are also 'rarer' to surface.

I guess the moral of the story is don't rely on meters...use your ears, the best discriminator!"

There could have been something else in the ground that may have made one of the coins either up-average or down-average. No way to know this now. To me, this is nothing to be concerned with, and the Deus is surly working properly, but it is a learning experience none the less. Hey, you dug it up! That is the plus side to this. I have only had my Deus for a short time but I am impressed with it. It is a sparky machine! Keep on diggin!!!!!

So many factors involved. The higher op freq of the Deus at 18Khz would produce current flow in the coin that is shallower (surface) than say the CTX (very low primary op freq). It could be that the surface flow encounters a rougher pitted coin outer layer or the raised relief of the coin is more aggressive, which slightly lowers the conductive report.

The CTX would, in theory, drive current a bit deeper giving (perhaps) a more stable reading than say the F75 at 13 kHz. But also less post-processing in the F75 could tighten up the report which may be another factor making the F75 more sparky with quicker updates (vs. the CTX which seems much slower. If the F75 and the CTX were headed to school, the CTX would be taking the short bus).

john