Surf Pi, Nickel has similar detecting distance to other coins. Why?

[Polymer]

I am using a 1946 Nickel to experiment with my Surf PI.
Solid detecting distance with threshhold absolutely quiet: 24cm (9.5")
That's quite a good result from what I have read and understood on this forum.

What has me puzzled is why Euro coins can only be detected around ~25cm (10")

Adjusting the delay is what I would call "finicky" and it's hard to hit the max distance.
I do that by measuring the voltage at PIN 7 of the 358 which is 1.7v in this case.

I don't have an Oscilloscope yet, so am doing everything by trial and error.

Here are the specs:

Coil: 20 Windings Teflon, Oval, 12.6"x5", R=1.4Ω
Rdamp= ~750Ω (found thru pot adjust))

Power: 85mA
Frequency: 2560 Hz, Txon=32us, Txoff=358us
(calculated with values of 555 C & R's)

My question: why is the detecting distance of other coins only marginally better than that of a nickel?

Have I been messing around too much in the circuit and gotten lost ... Any pointers much appreciated!

[Davor]

You'll find occasional reference to the "6th power law" of signal waning with distance. It is a dominating factor. When you compare the coins' conductivity, you'll find it grouped well within an order of a magnitude, and hence the small difference. If conductivities were scattered over a few orders of magnitude, you'd see more difference in depth.
Now about Euro coins - they are rubbish. They are minted in sandwich made of cheap multilayer metal core covered with a thin shiny crust. All of them are literally junk money. Bulk alloy coins are usually much more expensive to make than their denomination, hence they became cheap rubbish.
It is the same with small change in Croatian kuna - they all stick to magnet because of a steel core. Rubbish. However, 1, 2 and 5 kuna coins are solid alloy coins, but one kuna costs over 2 kuna to make.

[6666]

Coil: 20 Windings Teflon, Oval, 12.6"x5", R=1.4Ω

Make your coil a full circle not an Oval and test.

[greylourie]

Thats good distance.... 24-25cm, for a coil that is 127mm wide.

The good news about lost old coins is that they dont corrode as rapidly. The modern sandwich variety does not even need much moisture or any salt to rot to a lump of rust. Bit like some Soviet AK rounds with the steel core. The galvanic action just needs a touch of moisture.

[scrungy_doolittle]

Thats good distance.... 24-25cm, for a coil that is 127mm wide.

The good news about lost old coins is that they dont corrode as rapidly. The modern sandwich variety does not even need much moisture or any salt to rot to a lump of rust. Bit like some Soviet AK rounds with the steel core. The galvanic action just needs a touch of moisture.

Our use pennies are worse.. Copper flashing over zinc. First flaw in the coin and it eats it's self. And banks wont accept them the are considered mutilated currency

[Polymer]

Thank You All!

Davor: Sooo very true ... especially near salt water.

Here's a list of Euro coin specs including alloys for all who are interested.
I researched what the Ni5 or Ni12 (inner layer) of the 3 layer core means. Apparently it refers to weight % of complete coin and not just the inner core.
Perhaps someone else has better information on this, I couldn't find much info about it.

6666:
Noted. You want me going round eh

greylourie:
The old coins I find are usually in much better condition than the modern junk. I was wondering about that.
So modern multi-layer coins are basically relatively "bad" eddy current conductors given how big they are. Correct?

Thanks for the postive feedback of coil depth ... am not looking for super duper deep for where I am searching now.

scrungy_doolittle:
Guess what? I have found quite a few us cents here in Europe by the sea and was wondering why some look good & have a great signal
and others are quite crappy and have white oxide crusts on them like cheap jewellery I also find.

I just realized this a few weeks ago when looking up what americans mean with "zincolns" or so.
I did not know there were two versions of the us 1cent coin!

fruitloops, nearly forgot the pic