MD Physics

[green]

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.

I read reducing the target size by a half reduces the sensitivity to one eight. Did a test this morning, appears to be true if I double the thickness to keep the TC the same. The 8x8mm 1 layer projects a higher amplitude than the 8x8mm 2 layer at 3usec. Test is with a PI, would a VLF show different? What am I missing or doing wrong?
My testing didn't show a 1/8 drop in sensitivity. Wondered if it was because I was using a PI instead of VLF or I was doing something wrong. Got some strange readings with the ferrite beads awhile back, tester was overloading causing error readings. Trying to explain why what I read or test isn't the same or how to explain something that doesn't make sense. What doesn't make sense to me might make perfect sense to someone else and they could explain what I'm missing. Can't explain results, MTammet coin test. My testing shows little difference in signal amplitude clad or no clad with a PI. Wondered if the gold platting on the ring could make a difference. Seems there has to be an explanation why the smaller coin detects a larger distance, maybe physics maybe something else. Don't know his coil size but I would guess he might need 4 times the signal with the smaller ring. Wondering if I should have named thread PI Physics instead of MD Physics.

Copper coin chart: Tx off, large change in decay slope, do to skin effect? Tx on, constant slope, VLF not effected by skin effect? Just questions.

Saw some change in slope when I stacked the US quarters. Does a VLF ID the same for a single quarter or stacked quarters?

[Skippy]

My good VLF that I do my testing on is out of action, and I'm too busy to find time to sort it. So I never got round to trying out a multitude of experiments, including signal strength vs target size, for varying target time-constant.

Re: the high initial decay slope on your 'coin' targets, I thought this had come up one one of the other threads on the topic .... I answered that I thought it was because the PI pulse excited circulating currents in all directions within the coin/target, but over time, they decayed leaving only the 'dominant' ones still active. In the case of a flat coin, these are the ones that circulate in the plane of the coin, rather like those in a ring. These decay in a more well-defined way, giving your constant slope decay. [ I suggested some experiment with a second search coil at 90 degrees to the flat target ... probably hard to do ]
VLF's don't do anything odd like this, the target behaviour is the same all the way through, as far as I can tell.

And regarding Mikhal's rings: A gold plating will slow down the effects of corrosion on a copper item. It's not the plating itself that is responsible for the different resistivity, it's how it has stopped the good copper turning into rough-surfaced, porous high-resistivity copper.

[green]

My good VLF that I do my testing on is out of action, and I'm too busy to find time to sort it. So I never got round to trying out a multitude of experiments, including signal strength vs target size, for varying target time-constant.

Re: the high initial decay slope on your 'coin' targets, I thought this had come up one one of the other threads on the topic .... I answered that I thought it was because the PI pulse excited circulating currents in all directions within the coin/target, but over time, they decayed leaving only the 'dominant' ones still active. In the case of a flat coin, these are the ones that circulate in the plane of the coin, rather like those in a ring. These decay in a more well-defined way, giving your constant slope decay. [ I suggested some experiment with a second search coil at 90 degrees to the flat target ... probably hard to do ]
VLF's don't do anything odd like this, the target behaviour is the same all the way through, as far as I can tell.

And regarding Mikhal's rings: A gold plating will slow down the effects of corrosion on a copper item. It's not the plating itself that is responsible for the different resistivity, it's how it has stopped the good copper turning into rough-surfaced, porous high-resistivity copper.

You might be correct about the high initial decay of the coin targets. The PI current pulse is about 1usec long, so I was thinking skin effect might be the cause. Don't know how to prove which is correct. Would rotating the coin be different than a second coil at 90 degrees to the flat target? I've been thinking TC is proportional to width*thickness. Rotating the target 90 degrees would have the same TC. I know it would decay a lot faster but skin effect would be a lot higher so I still don't know how to prove TC proportional to width*thickness is correct or not.

[MTammet]

I can make test with Vallon and factory coil. Test will include following: small mediaeval coin silver plated 0,3g, mediaeval coin copper 0,5 g, good silver coin 0.25 g. All coins are less than 1mm thick. I will do the same with rings adding roman bronze ring. measuring amplitude change, if I get Tesla meter i gan record chaneges in magnetic moment. I have to figure out how to do that. Some other ideas ?

[Skippy]

Mikhal: The problem with your proposed tests is they use non-standard targets. Sure, it may be useful and interesting to you, but it's impossible for anyone else to reproduce, because we don't have corroded medieval Estonian coins in my country, and Mr. Green certainly doesn't find them in the US of A.

Green: I think the idea of the second pickup coil at 90 degrees was so the transmit coil was unchanged. You would just have two different RX signals for each test, one from the main TX coil, one from the secondary. It's an exotic experiment, and whether you could learn anything relevant I don't know. Practical uses? Well you would have a clumsy detector that had coils tilted at 45 degrees ...

[MTammet]

I found an article which explained the phenomenon. Highly recommended. Thing is you have to replace in article are body fluid and tissue with soil and you will get the idea how things are in the metal detecting world. To a certain extend it follows you ideas. file:///C:/Users/mihkel.tammet/Downloads/SkinEffectOnRoughenedRod_SeshadriScott_ENZCon2017_ paper_16_small.pdf

BR
Mihkel

[green]

Many PI designs are more sensitive to low conductors than high conductors; they almost always detect nickels deeper than quarters, even though quarters are bigger. From Carl's reply#8

Some data for a nickel and quarter. Charted data I did awhile back, scope pictures tonight. Easy to see the nickel is higher amplitude.

GEB was adjusted to cancel ground when enabled.

Maybe MTammet scope pictures will explain the difference in detection for his rings.

[Qiaozhi]

Many PI designs are more sensitive to low conductors than high conductors; they almost always detect nickels deeper than quarters, even though quarters are bigger. From Carl's reply#8

Some data for a nickel and quarter. Charted data I did awhile back, scope pictures tonight. Easy to see the nickel is higher amplitude.

GEB was adjusted to cancel ground when enabled.

Maybe MTammet scope pictures will explain the difference in detection for his rings.

A PI detector design I'm working on at the moment has a better response to quarters than it has to nickels. In this particular case, a nickel has 64.3% the depth of a quarter.
The relationship between the depths of these two coins depends on the TX pulse width and the main sample delay. If the detector is optimized for small low conductivity targets, then it tends to be desensitized to high conductivity items, and vice-versa. As usual, it's horses for courses.

[Skippy]

That's why I didn't go into detail about which of Mikhal's 2 rings would be lower/higher resistivity metal, simply stating they are almost certainly different, which combined with the different cross-sectional area, makes them have a different loop resistance. Add in differences in detector response to different targets, and you have different detection depths as a result.

[green]

A PI detector design I'm working on at the moment has a better response to quarters than it has to nickels. In this particular case, a nickel has 64.3% the depth of a quarter.
The relationship between the depths of these two coins depends on the TX pulse width and the main sample delay. If the detector is optimized for small low conductivity targets, then it tends to be desensitized to high conductivity items, and vice-versa. As usual, it's horses for courses.

Pictures reply #37. Signal for nickel higher than quarter before 25usec, lower after. Are you just sampling latter or have you found away to get signal for quarter higher than nickel for early delays?

[green]

That's why I didn't go into detail about which of Mikhal's 2 rings would be lower/higher resistivity metal, simply stating they are almost certainly different, which combined with the different cross-sectional area, makes them have a different loop resistance. Add in differences in detector response to different targets, and you have different detection depths as a result.

If rings were made of different materials, smaller copper, larger stainless I could see a big difference. Wondering why the smaller would detect 7cm more distance if both made of copper.

[MTammet]

One thing I forgot to mention copper ring has rough vertical cuts as ornament. This increases the area of ring surface. If the current flows on surface as in case of skin effect there will be skin effect loss. Made scope tests and will post pictures soon. Added to test one bronze ring with nice patina and one ring with thick copper oxide. Amplitude was measured with different PI (goldscan with 15 in commander coil) and from 6 cm distance). There is nothing interesting in these readings. I At least at close proximity. One of my fiends is involved in eddy current research. They are working with the algorithms to distinguish forged euro coins using eddy current analyser. I will ask him to give a scientific explanation on how eddy currents, skn effect, material composition, shape, magnetic field orientation are related. They are doing multi parameter research.

[Qiaozhi]

Pictures reply #37. Signal for nickel higher than quarter before 25usec, lower after. Are you just sampling latter or have you found away to get signal for quarter higher than nickel for early delays?

I'm sampling later.

[Skippy]

Quote:"Wondering why the smaller would detect 7cm more distance if both made of copper."


Not wanting to be repetitive, but CORROSION is the reason.

"I suggest that corrosion is one reason for the difference, especially if they are old and long-buried."

"Regarding Mikhal's rings: A gold plating will slow down the effects of corrosion on a copper item. It's not the plating itself that is responsible for the different resistivity, it's how it has stopped the good copper turning into rough-surfaced, porous high-resistivity copper."

Even if they were quite good quality copper, they would corrode a bit, and it's likely they will have some natural impurity in them, even if it's just 1%. Some of our British copper coins from the 1700's were made from natural unrefined copper, and they are usually very corroded, little detail remaining, often having lost 20% of their weight.
We find plenty of gilded buttons here, typically from the mid-1800's. It's pretty apparent when you've seen a few, that if the gilding is still well attached, the underlying metal ( brass or bronze in this case) must be uncorroded. Anywhere the gilding is absent has a rough surface, obviously heavily corroded.

Unless Mikhal mails his rings to us for testing, we're not going to make any progress. He could, however, post up some photographs of them.

Perhaps it's worth starting a new thread specifically on finger ring detection ? Tests on real rings, test rings made from known metals, mathematical analysis/modelling ? I'm not that interested myself, as I'm not a jewellery-hunter ... I've never done beach detecting.

[MTammet]

I is a good idea to start object size and material oriented thread. Why not to start from rings. I agree with Skippi that it is probably surface roughness , corrosion(Cu2O is as a matter of fact slightly ferromagnetic and has magnetic sustainability ca +400 compared to copper 40. We have same type of coins here in Estonia and I fully understand your point. I am trying to understand physics behind it. I can post pictures with scope results, but we have to model what happens in a object with certain shape, composition ect having multiparameter approach. If we do that we can compare stuff we detect with that model and may be we can improve our technology.