THREE COINS IN THE P.I. FOUNTAIN

[mickstv]

Hi Mick,

The phenomenon of "Electronic Holes" has been known for some time, but you have demonstrated it in a very elegant way. This could be duplicated with coins of other sizes and even with a coin and a nugget. Perhaps even with two nuggets?

Are you by any chance related to the Prince of Serendip? I guess you didn't set out to find a hole, so this was a lucky accident...

Best Regards,

Allan

Hi Allan, the big problem is trying to overcome these holes in detection. I have experimented with a short and long TX pulse, all this seems to do is move the electronic hole.

Hi Mick,

I found one 50c coin and it measures 5.5% of the conductivity of the copper standard; exactly the same as our old half crown, and about the same size, so the two coins are identical electrically. I found that at 100uS delay, the initial reading stars to increase from 25 when the second coin is about an inch above the first, until the reading is 161 when sitting on top of the first. This all indicates a substantial increase in TC. With subtractive GB in operation, you would be fine tuning the TC to the point where the GB ignored the two coins as they have dropped into the notch. The same thing happened with all of the cupro-nickel coins I have. Coins with higher conductivity behave a bit differently. The TC goes up initially but reaches a point when the second coin is close to the first when the TC goes down again. US quarters gave really strange results. Maybe there are different layers to the core and the plating on the surface has a reaction too. Look into that further when I have more time.

Eric.

Hi Eric, thanks for the reply. I'll try the side by side with some coins tomorrow, I also have some sinkers of the same size/weight I'll do some testing with them also.


Mick

[Prospector_Al]

Hi Allan, the big problem is trying to overcome these holes in detection. I have experimented with a short and long TX pulse, all this seems to do is move the electronic hole.



Hi Eric, thanks for the reply. I'll try the side by side with some coins tomorrow, I also have some sinkers of the same size/weight I'll do some testing with them also.


Mick

Hi Mick,

You're doing important work. I'm looking forward to seeing your results with the lead sinkers.

There's some mystery here--at least, I can't understand how the interaction between the targets works. If you think in terms of the magnetic coupiing, it's easy to understand that the eddy current path is partially duplicated in the second, third, etc. targets. The total resistance in the combined eddy current paths is thus lower. Since Tc = L/R, a lower resistance yields a longer Tc. BUT, what about the inductance? When you parallel inductances, the combined inductance is lowered. Aren't the inductances parallelled? If they were, the combined inductances and resistances should be affected to the same extent and the TC would remain unchanged...

It's understood that according to Lenz's Law, the field from each coin weakens the field reaching the successive coins, but that should not affect the inductance or resisstance of the eddy current paths.

I believe only experimentation can sort this out!

Good Luck,

Allan

[Prospector_Al]

Some years ago a PI detector user here dug a signal which at first sight looked like an iron concretion. He wacked it with his spade and out shot a coin. So he took the brown lump home and cleaned it up and what it turned out to be was a Victorian leather purse, inside which were some silver coins together with a gold sovereign. A good example of coins being in very close proximity.

Eric

[ATTACH]25188[/ATTACH]

A fantastic find! If the coins are spread out and only partially overlapping, the increase in Tc is minimised and the cache is detected more easily. (The area intercepting the flux is bigger also...)

I'm going to try something... When you get coins from the bank, they come neatly wrapped in a paper tube. I'm curious to find out if a single coin yields a higher target signal that the whole roll. Any guesses?

[WM6]

I'm going to try something... When you get coins from the bank, they come neatly wrapped in a paper tube. I'm curious to find out if a single coin yields a higher target signal that the whole roll. Any guesses?

Unshielded (magnetically) "coin tube" is no more stacked coins, but rod.

[Ferric Toes]

A fantastic find! If the coins are spread out and only partially overlapping, the increase in Tc is minimised and the cache is detected more easily. (The area intercepting the flux is bigger also...)

I'm going to try something... When you get coins from the bank, they come neatly wrapped in a paper tube. I'm curious to find out if a single coin yields a higher target signal that the whole roll. Any guesses?

You can see a "shadow" on some of the silver coins where another coin overlapped, and the degree of overlap was considerable on some of them. However, as they are silver coins and high conductivity, the rules are different. I suspect that the TC may well have been reduced over that of a single coin - the larger ones anyway.

The results you get from a tube of coins will depend very much on the coins. Low conductivity cupro-nickel alloy will give the results I have found. Higher conductivity copper alloy, or silver, will be quite different. How about getting a tube of gold coins, say 90% gold and 10% copper or silver alloy (approx 22 carat). Even a 90% gold alloy will show a relatively fast decay and when such coins are stacked, you should see the TC increase. Unfortunately, I haven't been able confirm this in a test.

Eric.

[Ferric Toes]

Hi Mick,

You're doing important work. I'm looking forward to seeing your results with the lead sinkers.

There's some mystery here--at least, I can't understand how the interaction between the targets works. If you think in terms of the magnetic coupiing, it's easy to understand that the eddy current path is partially duplicated in the second, third, etc. targets. The total resistance in the combined eddy current paths is thus lower. Since Tc = L/R, a lower resistance yields a longer Tc. BUT, what about the inductance? When you parallel inductances, the combined inductance is lowered. Aren't the inductances parallelled? If they were, the combined inductances and resistances should be affected to the same extent and the TC would remain unchanged...

It's understood that according to Lenz's Law, the field from each coin weakens the field reaching the successive coins, but that should not affect the inductance or resisstance of the eddy current paths.

I believe only experimentation can sort this out!

Good Luck,

Allan

Hi Allan and Mick,

Paralleling inductors that are not coupled is like resistors in parallel. However, for coupled inductances it is different and you can end up with a value that is little different to that of a single element. Stacked coins would classify as coupled inductors. I had to Google information on "coupled inductors in parallel" to sort this out as I initially thought the TC would remain the same and I was puzzled that my test results showed otherwise in quite a dramatic way.

For horizontally space coins, the coin-coin coupling is greatly reduced and there is little change in the TC over one coin. However, the amplitude goes up for each coin added due to the increasing surface area of radiation. All the individual eddy currents will be circulating in the same direction and although there will be "cancellation" at the edges of two adjacent coins the net field is now that of two conductors in parallel. Much like the adjacent turns on your TX coil. If you looked at the field between turns it would be zero due to cancellation, but the external net field increases the more turns you add.

I just found 6 off 20c Oz coins so I set my measurement at 100uS delay as most of the signal will have gone by then as cu/ni alloy. Any increase in amplitude at that late delay will relate to a TC increase i.e. if the TC for two coins was the same as for one, then the reading should just double. Starting at one coin and then stacking one at a time we get 10, 90, 188, 262, 312, 345.

Eric.

[Qiaozhi]

Star = 1 coin, Circle = 2 coin, Triangle = 3 coin.

Here is a further plot for my coins where you see the initial amplitude reduced for the next coin in line, but the enhancement at late times. I am hoping that some brave person will try a Spice simulation - if it is possible.

Eric.

Does this look familiar?