gold nugget simulation?

[Skippy]

Thanks for the maths reminder, Eric ..... but we see you've made a basic error in interpreting your results.

The basic exponential decay formula for a voltage decay is:

(V / V₀) = e^(-t/tau)

..notice the time-constant, tau is on the bottom of the exponent.

Your charts in post #167 have a 'best-fit' exponential formula showing, for example y = 5287.5 e^-0.176x

The 0.176 figure is the reciprocal of the tau. So tau = 1/0.176 = 5.68 , allowing for the fact x is microsecs, not secs, means this is actually tau = 5.68 microsecs. Which is what Green and myself have calculated, in different ways, directly from your graphs.

And putting the dimensions of the nugget into the mathematical model:

Assume nugget is 6.5mm diameter circle - assuming that the 11.1mm length contributes little to the TC, only to the amplitude.
Assume thickness = 1.2mm (presumably 1.4mm is the thickest point)
And TC = 5.6 usec
This gives nugget %IACS = 5.6 / (6.5 x 1.2 x 0.055) = 13%, a figure that agrees well with other tests.

[Ferric Toes]

Hi Green and Skippy,
Yes, it was under my nose and I didn't spot it, particularly in my last post (179) where both x axes start at 0. Although the delay is 10uS, this is actually where the tau zero
measurement starts. If the axis is renumbered then, as you both say, the nugget, bronze ball, and washer comes out at about 5uS.

Some IACS measurements that Skippy might find useful -
Euros.
10C.....16.3
20C.....16.2
50C.....16.3

UK shilling
1953....5.53
1954....5.50
2004 10p.....5.53

UK Halfcrown
1948.....5.71
1956.....5.44

UK two shilling
1956.....5.54

Eric.

[Skippy]

As far as I've been able to work out, ALL UK CuNi coins have been made from the CuNi25 alloy, with the sole exception of the 20 pence, which is stated as being a 16% Nickel alloy.
CuNi25 is widely used around the world for CuNi coinage, but good reliable conductivity data is a bit hard to find online, most sources say the %IACS figure is from 5.1% to 5.3%.
The 16% Ni 20p alloy has an %IACS value about 8%; 7.7 - 8.1% is possible.
The Eurocent alloy is seemingly unique to coinage, there's little data available, 14 - 15% was what I was originally anticipating.

I think it's likely your meter has its readings skewed by the surface being struck. Firstly there's the obvious fact that it's no longer flat, so surface currents have slightly longer paths to travel. Then there's the issue of the striking process modifying the metals characteristics, by work-hardening etc. And I'm sure there's been some micro-investigation of US 5c 'nickel' coins surface behaviour. There's a intriguing method of revealing 'hidden' features from very worn nickels. It involves etching the surface with Ferric Chloride (PCB etchant) which selectively removed copper, and leaves a stain like a 'ghost' of long worn-away features, including inscriptions and date. It's sold commercially as 'Nic-a-Date' . This all works because the coins surface is not consistent.

https://www.coincommunity.com/forum/...OPIC_ID=272838

https://www.wizardcoinsupply.com/pro...ic-a-date.html

[Ferric Toes]

I had two 20p coins handy and they read 7.8 and 8.0%. One of the copper calibration samples has a plastic film over it to give a lift-off of 0.25mm. I get an equal reading of 100.10 on both samples. When measuring a coin, I sometimes find that one surface has a slightly different reading to the other. I alway take the higher one.

Eric.

[green]

Tried a couple targets. Used A Popsicle stick(2mm+thick). Drilled a hole 9/64(3.7mm) and 13/64(5.16mm). Melted some SN60 PB40 solder in holes. Ended up with a piece 3.2x2.1,TC=2.3usec and 4.3x2.32(small width), TC=4.1usec. Calculates a multiplier close to .04. Will try again tomorrow with some lead and solder. The procedure looked good, need to use more care.

[Skippy]

I'm making progress with my suite of dummies, the 99C, lead, and 60/40 ones are the right thickness, they just need the edges filing/sanding to get the profile correct. The CuNi is going to take more effort, as it's harder to work.

[Skippy]

I did consider using something like Green's method for making the correct profile. I was intending to drill the correct size hole ( or a tiny bit oversize) in a steel plate 2 - 3mm thick. Insert the under-diameter/over-thick solder/lead blob in the hole, then crush it in a vice so it fills out the drilled hole. Then hand file/abrasive paper the flat surface(s) to get the desired thickness. The 12 x 4.5 dummy could also be made this way, with the correct profile cut in the steel sheet.
What I actually did was just use vice-crushing to get the correct thickness, and hand-finish the edge profile. The 12 x 4.5 solder blobs were 'pre-shaped' with some GRP sheet to contain the blob to 'long and narrow'. After crushing, the 99C and 60/40 blanks look pretty good, not too much trimming needed. The lead blobs were made in a stainless-steel spoon on a gas stove. They were all made over-size, to allow for filing off the surface dross, then crushed. So the 12 x 4.5 blank was more like 15mm diameter x 2mm, so needs a lot more work to get the correct profile.

[Skippy]

Green: I don't know if you were intending to make any of the 7mm x 2mm round targets, as it seemed unlikely that it would sucessfully mimic the 12x4.5 nugget. But if you are, can you please make it 8mm diameter instead ? This will make it a more useful size, not so close to the 5mm x 2 dummy. I think my maths was a bit sloppy when I worked out that '7mm diameter' figure, a value of 7.5 - 8 is a more accurate representation of the area of your long nugget.

[green]

Green: I don't know if you were intending to make any of the 7mm x 2mm round targets, as it seemed unlikely that it would sucessfully mimic the 12x4.5 nugget. But if you are, can you please make it 8mm diameter instead ? This will make it a more useful size, not so close to the 5mm x 2 dummy. I think my maths was a bit sloppy when I worked out that '7mm diameter' figure, a value of 7.5 - 8 is a more accurate representation of the area of your long nugget.

I can do that. Right now I'm trying to make sense of the smaller targets, the formula multiplier needs to be lower for the calculated TC's to match target TC's. Maybe something I'm doing wrong or method of making them. Tried lead and SN60 PB40 targets.

[Skippy]

The multiplier is low as it is, I'm sure the true figure is nearer to 0.060, I only stated a 'first attempt' figure of 0.055 so we could get on with the experiment.
I intend (soon) preparing some test targets to try to pin down the multipliers for square and circular targets, that will be for the 'test targets' thread.
I think lack of precision is an obvious difficulty with making such small items, I may experiment in the New Year with different ideas. I have access to a lathe, but even that has limited uses - you can't turn lead easily.

[Skippy]

I see what you're meaning about the 'low multiplier', there will be some point where thickness increase doesn't have a proportional effect on TC. Like your copper wire experiments, increasing the length had no effect on TC, ie. if the wire was mathematically considered a disc, with the thickness being the length of the wire.

[Ferric Toes]

Nugget IACS




I managed to get a reasonably flat surface on a nugget from SE Australia such that it would read the %IACS on the conductivity meter. Pure gold would read 70% relative to copper according to my conductivity chart.

Eric.

[green]

[ATTACH]44764[/ATTACH]

I managed to get a reasonably flat surface on a nugget from SE Australia such that it would read the %IACS on the conductivity meter. Pure gold would read 70% relative to copper according to my conductivity chart.

Eric.

What does your meter read for a 1inch square and a 1/2inch square of regular strength aluminum foil? Would or does heavy duty read the same?

T-C = 0.072 x D x T x %IACS Skippy's formula reply#165. If TC is proportional to D and T and multiplier doesn't change, they should read the same?

[green]

[ATTACH]44764[/ATTACH]

I managed to get a reasonably flat surface on a nugget from SE Australia such that it would read the %IACS on the conductivity meter. Pure gold would read 70% relative to copper according to my conductivity chart.

Eric.

What does your meter read for a 1inch square and a 1/2inch square of regular strength aluminum foil? Would or does heavy duty read the same?

T-C = 0.072 x D x T x %IACS Skippy's formula reply#165. If TC is proportional to D and T and multiplier doesn't change, they should read the same?

[green]

Would your nugget read the same %IACS if it were thin? Was trying to Edit reply above, reason for same replies. Not computer smart.