Topic: I measured the density of my silver/gold coins... (Read 16260 times)

This thread is the second result for me when i google "gold measure density"

I just used the improved density measuring method by minitmark to measure the density of 8 silver coins. Took roughly 20 minutes including setup (found the old C-shaped metal thingy with the thread) and cleanup.

Well I have a method that I think will work and avoid all these problems. It uses magnetically induced eddy currents, and uses no test leads. It measures energy loss in the coin or bar. It tells you thickness - if the conductivity is wrong, the thickness will be out by the same factor. (Typically a factor of 1.5 or 2)  BUT I could really do with a transformer like the one I described to test this. Someone must have one??

You'd also be dealing with measuring a coin who's conductivity is ten times higher than even the multimeter test leads.

On page 10 is a subject called "milliohmmeter" discussing circuits and methodology to measure such low resistances:
http://cds.linear.com/docs/en/application-note/an98f.pdf

Since silver is the best conductor in the world at temperatures ~293K, what you'd be looking for is any resistance higher than a known good reference coin of the same minting. To establish what an expected resistance is, I'm not aware of what software can model electrical resistance @ current vs skin effects etc on various shapes, such a project would likely be PhD-earning material.

I had been wondering about that and indeed couldn't think of a way around that, except for actually having a numerical simulation program in which you fill in the details, and which then simulates the resistance. Then you still have to account for contact point resistance, which you might do by measuring different points on the coin (edge to opposite side edge, middle of face to middle of flipside face, edge to face, etc) and then correcting for the constant factor.

Please note that the shape of an object can affect the conductivity, so a conductivity test isn't a good way to test for purity. This is one of the reasons why Shire Silver has never implemented a method of directly accessing the metal through the plastic. We considered it when we were creating our prototypes, putting holes at each end of the embedded wire to allow probes to be used. For more on this, you might start with http://en.wikipedia.org/wiki/Electrical_conductor and particularly note sections like
and
This last one indicates that a conductivity test would likely be more accurate for Shire Silver than traditional bullion, but it would still not be accurate enough and adding the probe access points would add significant cost to the card production.

And yes, we get our silver and gold from a well known jewelry supply company who has been in the business for a long time. We also try to make our cards contain at least as much metal as they say they have plus a small amount to help account for our production variance. For example, a half gram silver card will have 0.52 +/- 0.02 grams of .999 fine silver.

I suppose I should get around to making a Shire Silver specific thread so I can post stuff like the news that our recent site upgrade has temporarily cut off the ability to accept bitcoins in the shopping cart, which we're trying to fix.

And people following this thread might find this page interesting: https://shiresilver.com/counterfeiting

The (molar) heat capacity might also be useful.  For background, read up on Trouton's rule.  The data you can collect at home is to heat your coin in boiling water, measure the temperature, put the coin into a thermos with a known amount of water, and measure the temperature rise.  You might want to arrange the volume of water so that the start and end temperature are on scale with a digital fever thermometer.

Then make a 2D plot, with density and heat capacity as the two axis. 

Any method that you use will easily detect some problems, and be blind to others.  Your own personal crooks will specialize in the blind spots that you have.  It is a locksmith - thief race.

Very nice testing, molecular I love it!

That's a method that shows eddy currents, but the problem is, eddy currents happen in every metal. Sure, much more in gold or silver (less resistance), but it's hard to make this a quantitative method, suitable for testing if your coin is genuin.

One of the most amazing demos is to hang a gold coin by two threads, so it will swing but not spin.  Place a super magnet just past the place where the coin will rest when it stops.  Lift the coin back, release it, and watch as it swings into the magnet and comes to a dead stop.

This is a variation of the "induced eddy current method" and it's totally cool.

There is a factory in China that makes fake gold coins, so far as I know it has been making them for many years. They are made of tungsten, and gold plated. The density of Gold and Tungsten is nearly exactly the same, far to close to call.

The electrical conductivity is very different, and so too I imagine is the thermal conductivity. Goldmoney.com use a sonar device to check all their bars for suspicious inconsistencies, and I am sure you would see a core of different material, but I am not sure if you would be able to tell if the whole bar was made of Tungsten and gold plated, as seems certain some are.

So far as silver is concerned, there are (I suppose) many alloys that would have the right density, but the right conductivity would be impossible to fake - silver is the best metalic conductor known.

I believe an induced eddy current method could work very well.

Honestly, no clue. Impure silver would conduct both heat and electricity differently, and I'm pretty sure the the lead core/lighter alloy wrapping would throw them off, too... but I'm uncertain in exactly what way.


All my silver is genuine. Right, ShireSilver?

That's very interesting. Gives one a feel for how expensive such accuracy is and what factors have to be taken into account in cases where such accuracy is necessary.

I think for the "detect fake silver coin" application using the described method (and ignoring pressure, temperature, water purity, coin uncleanliness, etc) and just getting a scale that does 0.001 g instead of 0.01 gaccuracy will go a long enough ways to be good enough.

Probably makes more sense to measure other properties in addition to density before scaling accuracy of density measurement, no?

The conductivity ideas (both of them) sound good. Still looking for something simple, though. I have a voltmeter but no thermometer... well, maybe I could build one using an arduino and some semiconductor...

I teach analytical chemistry.  Undergrad analytical labs have balances that read 0.000 1 g.  Well respected manufacturers include Mettler and Sartorius.  Troemmler also comes to mind, though I don't remember if they make balances, or only calibration weights.  I have my students do a very similar lab, and at that sensitivity level we can watch evaporation as it happens.  We need to know the barometric pressure, to account for the buoyancy of the objects in air.

The next level of sensitivity, 0.000 01 g, escalates the cost to about $25,000.

I wish all bitcointalk threads were of this kind.

which of these 2 is harder to fake (by using impure silver or using core of different material(s)).

I'd love to have some fake silver coins. Anyone have any?

Hmm, I have an idea for that too.  But I need something and wondered if anyone has it.  You used to be able to buy a transformer where the core is retangular, and the primary and secondary are on seperate bobins.

So I want to remove the secondary and then cut a 1cm air gap in the core that I can easily fit a coin or even a gold bar into.

This doesnt really work well with the normal "B" shaped transformer core. So if anyone has one of these in their junk box, I would really appreciate it...

Electrical. You probably could do it with thermal, though.

thermal or electric conductivity?

Probable source: https://www.kitcomm.com/showpost.php?p=1619194&postcount=30

Analogous argument:

I don't know how much more accuracy I could expect than the above results, they are very good but probably within the range of what determined counterfeiters might be able to produce with an alloy. This test of pocket scales shows $60 0.01g generic scales are more like +/- 0.09g: http://www.digitalscale.com/Precision.htm. Here's that scale on eBay. www.ebay.com/itm/170625095817

Next the conductivity tests!

Haha! Exactly what I had in mind!!

I am glad it actually worked well :-)