MD Physics

[Skippy]

OK, Carl, I did wonder if that was what you meant, a few hours after I typed it .... one of those quirks of MD terminology that's bound to cause confusion.
I would say that thick (and usually large diameter ) coins of high conductivity metal WOULD be affected by skin effects, because of their thickness, which could be greater than 'two skin-depths' thick. Thinner coins, like US 10c dime and 25c quarter would be less affected, unless perhaps you were using a higher-frequency machine, say 25kHz+. But a US silver dollar (Morgan/Peace) appears to show skin-effect. It seems based on others work, that it has a corner frequency of about 800Hz. I have measured one ( a Peace dollar) with a 13kHz machine as having a corner freq = 1200 to 1300Hz. I think this discrepancy is caused by skin-effect, the coin appearing thinner than it is. It's an experiment I want to try some time, actually measure the dollar at near it's f_c. Sadly, Mr. Green doesn't have a US 1 dollar coin, so (on another thread) we weren't able to get his PI assessment of it.

And just for info: Typical silver hammered coins aren't low down the MD 'conductivity scale' because they're thin. The main reason is because the metal is low-conductivity. They were (usually) Sterling 92.5% when made, but centuries of corrosion leaches out the copper, leaving a porous sponge-like silver coin behind, which conducts electricity quite poorly, compared to it's original state.
Some observations:
* I have a Scottish hammered coin that reads way 'higher up' than any other comparable one, I believe it is high-fineness silver, so it's been little-changed by corrosion. Coin fineness has been tinkered with by various monarch, usually to the low-fineness direction (eg. Henry 8th, Edward 6th) but others have used high purity ( in gold as well as silver) at times.

*I have a medieval penny that is physically identical to its peers, but reads much lower on the 'conductivity scale', I haven't weighed it (yet) but I think it's likely a low fineness example ( produced by unscrupulous mint managers).

*We have small Saxon silver coins called Sceat. These are small diameter (10mm) but thick (1mm), and known to be low fineness, 50 - 70% silver. If they were 'freshly-made' Sterling silver, they would read pretty high up the ID range, probably near 'Zinc 1 cent'. But dug up examples read way down in the 'foil' range. This is entirely due to the metal change caused by corrosion.

And @ Melbeta:
See the wikipedia article in my earlier post.
Basically, AC currents don't circulate deep inside a conductor, due to magnetic effects. If there's no current in part of the conductor, then the electrical resistance of the sample is higher, as only part of it is being used. Example: a 2mm diameter wire might behave like a 2mm diameter tube, with a 1mm diameter bore.

[pustareka]

https://www.easycalculation.com/physics/electromagnetism/electron-debye-length.php

metal=~ 10e-7 cm

[Melbeta]

Well, I no longer normally use a BFO, unless I am out searching for metal ores, and I primarily use a VLF, not a PI detector. So what I do is use all metal mode, and move the coil around slowly, to give the detector time to really soak the target with its AC field. Swinging the coil too fast does not normally make the target hot enough. The coil is over it for a fraction of a second and in my thinking that is not sufficient long enough to make the target very hot.

I am not interested in searching for poor alloy newer coins, nor do I search for small pieces of aluminum. I am not after the surface trash. I search for the smaller older deeper targets, which are usually vintage coins, vintage jewelry, vintage tokens, and vintage relics. In other words, if it is not valuable, why search for it?

Now I can tell a small target from a larger target, and while I used to dig up pieces of old vintage iron coal cook stoves, not really into those anymore. I have enough vintage mining picks, vintage old tools, vintage drilling bits, and other vintage relics, that finding them is no longer a great thrill. I do find lots of older car license plates deeper in the ground. My oldest is 1912 and it is a dealer license plate. And I have a large carton of vintage horse shoes, vintage mule shoes, and even some vintage oxen shoes.

It really does not make too much sense for me to dwell on what kind of AC force field eddy current is on the target, or even inside the target itself, just that the targets in the ground are lit up very well so it responds very good. I do not use discrimination at all except to help identify the target. Discrimination rejects targets and that is not my business. I want to search with 100% of detector power, not lesser detector power! And I use the sensitivity turned wide open! I do not care if it buzzes or does not buzz. I am at 100% sensitivity. I have even found platinum engraved wedding rings from the Victorian period. My best one was almost 12" down and the sound was so faint I almost gave up trying to find that elusive target.

I am not into newer computerized machines, I am into the older machines that are not as much filtering out unwanted targets. I like to hear every doggone target, and then I decide to dig or not dig. I always dig a solid target though as I found those can really fool the detector user. And it is very interesting to see what the old guys from years ago used to make and lose. I have also found a lot of meteorites. I used to discard them as hot rocks, until I visited a museum in Denver, then I realized I was discarding meteorites. So I again have three meteorites safely stored away again.

No matter how you try, the metal detector is merely a machine, and it does not know what the target is at all. It can sometimes give you an indication of what it thinks the PROBABLE target is, but it still is a GUESS, although you can flip a coin heads and tails and be as accurate as the detector was in guessing the probable target and be as accurate as the TID machine. Now one can use different frequency detectors when detecting and one WILL ENHANCE one's ability to find more of a desire alloy then another unwanted alloy target. For example lower frequencies do find more silver coins and silver jewelry over higher frequency detectors...
Melbeta

[Dave J.]

Quote:"Is there a best VLF Tx frequency for a 1usec TC target?"

A 1 microsec TC target would be equivalent to a 159kHz corner frequency. So in principle, a detector operating in this sort of range should be best. However, I assume a 1 usec target is the smallest target you're realistically trying to find, you would rather hope to find ones with 5 or 10 usec TC ? So a typical VLF 'gold' machine operating at 35 - 70 kHz would probably be a realistic choice.

Dang, Skippy, it's like you've hacked my in-house file on that very subject.
---Dave J.

[Carl-NC]

I would say that thick (and usually large diameter ) coins of high conductivity metal WOULD be affected by skin effects, because of their thickness, which could be greater than 'two skin-depths' thick.

I think my explanation was not very good. Big thick targets are limited by skin effect so that their phase response is pretty much the same regardless of what their skin depth actually is, and also regardless of frequency and thickness. That is, once you are past 1 or 2 skin depths, higher frequency and thicker metal don't affect the response much. Thin targets operate well below the skin depth, so skin effect is very dominant in how they respond. Change the thickness, change the size, or change the frequency, and the response changes because of skin effect. That's the point I was trying to make to Green, with thin targets he may have to account for skin effect, but with big thick coins not so much.

Probably you are right about hammered silver coins, I don't have any and should get a few. But I do have some very thin modern silver coins and they ring up fairly low.

[pustareka]

1uS no posibile got target. 1/r^4 rul.

[green]

More charts.
Some observations
Top left: Tx off, not straight line decay(skin effect?), less than 2 to 1 difference in Rx amplitude. Tx on, straight line decay(no skin effect?), less than 2 to 1 difference in Rx amplitude at Tx start, slope difference between single and stacked(should I.D. different with VLF?)
Top right: silver coin TC a little higher than clad, Tx on or off.
Bottom left: Tx off, not straight line decay(skin effect?), Tx on time has small effect on Rx amplitude with low Tx off delay times. Tx on, straight line decay(no skin effect?), Rx signal proportional to Tx current rate.

Bottom right: 10x10x.89mm Rx signal, 1/8 of 20x20x.89 Rx signal at start of Tx on( same as ITMD, maybe VLF different than PI?), not 8 to 1 at start of Tx off.

[MTammet]

Hello!

I have two rings, normal simple bands. One weights 2.4 and the other one weights 1.6 gram. First one is made of copper. Probably from copper coin. The other is made of copper but plated with gold. They are both pretty old made somewhere at the end of 19 hundreds. Found with the detector. When tested with Vallon (I have done the same test years ago with different detectors and the result has been always the same), frist ring is detected at 23 cm and the second one at 30 cm in normal mode sensitivity 13 , EMI ch 5/11. It is due to the skin effect. Eddy current and magnetic moment of EMF in induced ring behaves same in both rings due to their shape. Please explain the physics of metal detecting behind this phenomenon. I will have one small abstract explaining skin effect in plated PCB tracks:
https://www.polarinstruments.com/sup...its/AP171.html

BR

Mihkel

[Skippy]

Mikhel:
I suggest that corrosion is one reason for the difference, especially if they are old and long-buried. But the alloy they are made from can also be important. You say they are copper, I think this is unlikely, they will be some alloy like brass or bronze. These alloys are very different in their electrical conductivity compared to pure copper. Often 25 - 50% of the pure copper conductivity.
These two factors can easily make one ring have an electrical resistance 4 times greater than the other. This affects detection depth of PI machines.

[MTammet]

I agree with you Skippy, but Russian copper coins are copper coins in the very sense of it and most of the peasant rings were made of them. corrosion is light, but gold plated ring shows copper gold diffusion. They are not made of brass or bronze.

BR

Mihkel

[green]

Does a thin coating(clad)have much of an effect on target response, VLF? PI?

[MTammet]

I tried with XTerra 70 with 18 mhz 6x10 coil and result is same as with Vallon. I have seen same phenomenon with silvered hammered coins (poor quality coins basically copper but with thin silver on outside are very common in 16th/17 centuries). same with gilded bronze Roman jewellery. Same with silver plated Viking age sword hilts )pommels ect). Silver and gold will reduce skin effect of iron on certain frequencies reducing overall resistance in iron conductors several times. Magnetical field induced by eddy currents has also role to play. Not all coloured metals have same magnetic properties. Al is paramagnetic versus gold being mostly diamagnetic. With the strong skin effect conductor becomes less conductive when measured at cross section. Plating reduces skin effect making object better conductor. But skin effect helps in certain frequencies to detect better small items. Eddy currents induced in skin have magnetic moment towards inductor. Better than without skin effect.

[green]

Hello!

I have two rings, normal simple bands. One weights 2.4 and the other one weights 1.6 gram. First one is made of copper. Probably from copper coin. The other is made of copper but plated with gold. They are both pretty old made somewhere at the end of 19 hundreds. Found with the detector. When tested with Vallon (I have done the same test years ago with different detectors and the result has been always the same), frist ring is detected at 23 cm and the second one at 30 cm in normal mode sensitivity 13 , EMI ch 5/11. It is due to the skin effect. Eddy current and magnetic moment of EMF in induced ring behaves same in both rings due to their shape. Please explain the physics of metal detecting behind this phenomenon. I will have one small abstract explaining skin effect in plated PCB tracks:
https://www.polarinstruments.com/sup...its/AP171.html

BR

Mihkel

Have you looked at amplifier out with a scope to see what the difference is with each ring placed the same location on the coil ?

[MTammet]

I will do that with test coil attached to scope as receiver. As i have no scope at home you have to wait. I have not done that with PI. I can try it with winscope, but...

BR

Mihkel

[Skippy]

Mr. Green , a question:
Where are we going with this new thread you've started? It was about nuggets and their behaviour, their simulation. Then you post up response curves for 1 ounce copper coins and threw us all. Now you're asking about clad coins ...... ?

Though I have some US clad coins, I've not tested them in any way that may show up unusual/different/worth-knowing-about characteristics.

We do have copper-plated steel coins here in the U.K, the 1 pence and 2 pence. Fresh ones read much like their solid bronze (97% Cu ) predecessors, in fact they read slightly higher up the 'conductivity scale', presumably due to the pure copper plating. However in real life, the buried ones soon start falling apart, the steel rusts, and they start giving jumpy readings as the 'iron' starts to have an effect.

Your pressure-clad coins don't have a continuous covering - your quarters have copper edges, for example.

This is why I avoid using clad/plated/bi-metallic coins for testing.