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Author Topic: I measured the density of my silver/gold coins...  (Read 16031 times)
johnniewalker
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March 25, 2013, 01:08:03 AM
 #21

This is very interesting and I am impressed by your research. But I am sure that such prestigious mints have a very high standard of quality control; I find it hard to believe those margins of error are accurate.
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March 25, 2013, 05:36:34 AM
 #22

This is very interesting and I am impressed by your research. But I am sure that such prestigious mints have a very high standard of quality control; I find it hard to believe those margins of error are accurate.

again: I meant error of my measurement. I'm sure these coins are both legit and 99.9+% pure silver.

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March 26, 2013, 01:09:01 AM
 #23

Ok, I'll explain the "frame" a bit more. You can make it out of a coathanger, but ideally something a bit flimsier is lighter and so better. It does not have to  be totally rigid, if it moves out of shape, it doesnt matter. I use copper wire because its easy to bend and holds its shape.

Here is a diagram:                                                  .------------.
                                                                           |              |
                                                                     ________        |
     Scales                                                         :::::::::         |
     Table edge     ===========================       |
                                                                                          |
                                                                                          |
     Bent Wire Frame                                                 .-----------.
                                                                            .
     Cotton Thread                                                    .
                                                                            .
                                                                            .
                                                                   |        .        |
     Object                                                      |      |||||     |
                                                                   |                 |
                                                                   |                 |
     Beaker of water                                        |                 |
                                                                   |                 |
                                                                    .__________.

                                                                       
The frame can be C shaped, it doesnt have to be rectangular, the important point is that the object is directly under the place where it rests on the scales so that it balences.  In fact it will pretty much hang that way anyway.

I have drawn a table in here, and you do want the scales to be horizontal and not moving, so this is quite a good way to do it. You take two mass measurements as |I explained, first with no beaker or water, then you raise the beaker and note the loss of weight due to boyancy with the object fully submerged.

Now one problem here is that any airbubbles that stick to the object represent an inaccuracy. Really you would like to use a bit of soap to prevent this, but that might alter the purity of the water. If you use a very small amount this would probably be a good idea. There is an alternative however. You can use any liquid provided that you take its density into account, so if you decided to use say parafin, because you will not get air bubbles in parafin because of the low density. But you probably do not know the density of parifin given thatit may vary quite a bit from sample to sample. So, use a known good coin or other object as a "calibration", that gives you the density acurately, which you can then use in your calculation of the 2nd object. The known good coin in fact can be anything you know the volume of, a freshly minted coin is convienient since you know the mass and the density of the material its made to high accuracy, so you know the volume!! But A 1cc cube or other known volume  of anything that sinks (in parafin) would be just as good.

Yeah, this is probably overkill, the smidgeon of soap method would be much simpler and probably work just fine.


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The Casascius 1oz 10BTC Silver Round (w/ Gold B)


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March 26, 2013, 02:50:26 AM
 #24

I'm curious to know how a Casascius silver coin would stack up, but the sticker and private key paper would probably throw things off by quite a bit.

Companies claiming they got hacked and lost your coins sounds like fraud so perfect it could be called fashionable.  I never believe them.  If I ever experience the misfortune of a real intrusion, I declare I have been honest about the way I have managed the keys in Casascius Coins.  I maintain no ability to recover or reproduce the keys, not even under limitless duress or total intrusion.  Remember that trusting strangers with your coins without any recourse is, as a matter of principle, not a best practice.  Don't keep coins online. Use paper or hardware wallets instead.
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March 26, 2013, 04:00:29 AM
 #25

Watch for air bubbles when suspending the object into water. Gently tap the thread to get rid of them.

Clean the object, handle with gloves (non-powdered).

Use distilled water, and read the density from the table for the particular temperature.

Repeat several times. What is the variation between measurements? Is it random, or is there a consistent up or down trend?

Make sure the balance is level.


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molecular
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March 26, 2013, 05:57:52 AM
 #26

Ok, I'll explain the "frame" a bit more. You can make it out of a coathanger, but ideally something a bit flimsier is lighter and so better. It does not have to  be totally rigid, if it moves out of shape, it doesnt matter. I use copper wire because its easy to bend and holds its shape.

Here is a diagram:                                                  .------------.
                                                                           |              |
                                                                     ________        |
     Scales                                                         :::::::::         |
     Table edge     ===========================       |
                                                                                          |
                                                                                          |
     Bent Wire Frame                                                 .-----------.
                                                                            .
     Cotton Thread                                                    .
                                                                            .
                                                                            .
                                                                   |        .        |
     Object                                                      |      |||||     |
                                                                   |                 |
                                                                   |                 |
     Beaker of water                                        |                 |
                                                                   |                 |
                                                                    .__________.

                                                                       
The frame can be C shaped, it doesnt have to be rectangular, the important point is that the object is directly under the place where it rests on the scales so that it balences.  In fact it will pretty much hang that way anyway.

holy f..k! That is cool! I will try this and see if I can get better accuracy from my scale this way.

Thanks for taking the time to explain (and make that great drawing) for me Wink

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March 26, 2013, 06:01:31 AM
 #27

Most likely the difference in calculations are caused by impurity or different density of water due to temperature conditions. The error of +/- 2% might be caused by other factors and measurement tolerances. The scale is not laboratory grade but for most purposes even the cheapest 8$ scales are extremely accurate compared to what was available in labs 15 years ago.
Um, scales in labs 15 years ago could measure the weight of fingerprints.

Yes.  And the evaporation rate of the water.
Those balances cost $5,000.

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March 26, 2013, 07:22:50 AM
 #28

This is very interesting and I am impressed by your research. But I am sure that such prestigious mints have a very high standard of quality control; I find it hard to believe those margins of error are accurate.

A fake would not be from the claimed mint, though Wink

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March 26, 2013, 07:25:49 AM
 #29

I'm curious to know how a Casascius silver coin would stack up, but the sticker and private key paper would probably throw things off by quite a bit.

I'll do the (modified) process on my 10 BTC silver round when I have some time.

The sticker plus the volume (and paper) below it we could probably estimate pretty well. How thick is the hologram and how deep the "inlet" in the coin for the paper with the key? They weight of sticker+paper we can probably neglegt or guesstimate also.

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molecular
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March 26, 2013, 07:28:55 AM
 #30

Watch for air bubbles when suspending the object into water. Gently tap the thread to get rid of them.

Clean the object, handle with gloves (non-powdered).

Use distilled water, and read the density from the table for the particular temperature.

Repeat several times. What is the variation between measurements? Is it random, or is there a consistent up or down trend?

Make sure the balance is level.

thanks for these tips, I'll try to do my best.

The variation between measurements was (if I recall correctly) up to 0.03 g. There have even been some outlieres with something like 0.08 if I recall correctly. I tried this on a couple of measurements with 2 of the coins. The variation was present only on the measurements using the container. The simple weighing of the coin showed max of 0.01 g variation.

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March 26, 2013, 12:44:59 PM
 #31

I measured the density of my silver/gold coins unsing this setup



these are the results:



Here's how I did it:

  • Weigh the coin itself
  • Put container with water and the coin attached to string on scale and reset scale to 0
  • pull on string, effectively measuring weight of coin minus weight of displaced water
  • calculate density (assuming water density is 1g/cm³) by dividing weight of coin by volume of coin (equals volume of displaced water)

I used a cheap scale I ordered from Hong Kong for around €5. Yet the results seem to be quite reassuring (error margin (EDIT: of my measurements) seems to be around 1%-2%)

Comments?


If you would put the spreadsheet on Google docs I might look at it more.

I notice that one measurement for each coin is in disagreement with the later measurements on the coin.

I would add some other calculations.  From the average density of the coin, calculate the weight of water that you expect for the measured weight of the coin, also, calculate the expected weight of the coin given the measured weight of water.



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molecular
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March 26, 2013, 01:17:47 PM
 #32


If you would put the spreadsheet on Google docs I might look at it more.

I notice that one measurement for each coin is in disagreement with the later measurements on the coin.

I would add some other calculations.  From the average density of the coin, calculate the weight of water that you expect for the measured weight of the coin, also, calculate the expected weight of the coin given the measured weight of water.


https://docs.google.com/spreadsheet/ccc?key=0Au-mVSBh0PA4dFRMMTV5RmM5S0ppZDVJSVR6RUROb2c#gid=0

But please note: each row is a seperate coin, so we're talking about 15 distinct coins. I did take multiple measurements for 2 of the coins, but only recorded one.

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March 26, 2013, 07:04:46 PM
 #33

much better results using mintymarks method:


(click image above for google doc, these are single measurements of 7 individual coins)



As expected, the Casascius 10 BTC round has lower density than silver because of the sticker and space (filled with paper and maybe some air) behind it.

The scale zeroed in much quicker than with my first setup. I conclude that the higher weight of the water cointainer in my first setup (about 150g, scale max is 200g) as opposed to the weight of the frame used here (< 3g) really did add a lot of inaccuracy in the scale.

Repeated measurements had a variation of +/- 0.01g max (resulting already in a ~0.24% jump in resulting "error").

Thanks mintymarks, now I have a much better method at hand.

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March 26, 2013, 07:06:18 PM
 #34

I'm curious to know how a Casascius silver coin would stack up, but the sticker and private key paper would probably throw things off by quite a bit.

I measured a densitiy of 10.40 g/cm³. Slightly less heavy than silver, as expected.

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March 26, 2013, 07:51:48 PM
 #35

Haha! Exactly what I had in mind!!

I am glad it actually worked well :-)

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March 26, 2013, 08:56:10 PM
 #36

Most likely the difference in calculations are caused by impurity or different density of water due to temperature conditions. The error of +/- 2% might be caused by other factors and measurement tolerances. The scale is not laboratory grade but for most purposes even the cheapest 8$ scales are extremely accurate compared to what was available in labs 15 years ago.
Um, scales in labs 15 years ago could measure the weight of fingerprints.

Yes.  And the evaporation rate of the water.
Those balances cost $5,000.
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Analogous argument:
Quote
Quote
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A's Assertion: "Even the cheapest tablet computers are extremely fast compared to what was available in supercomputer centers 15 years ago"
B's Evidence disproving assertion: "iPad4 - 76 GFLOPS; 1996 Sandia Laboratories ASCI Red - 1100 GFLOPS"
C's Statement not disproving evidence: "That computer cost $55 Million"

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!


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March 26, 2013, 09:33:44 PM
 #37

Next the conductivity tests!

thermal or electric conductivity?

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March 26, 2013, 10:00:34 PM
 #38

Next the conductivity tests!

thermal or electric conductivity?

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

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March 26, 2013, 10:07:45 PM
 #39

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...

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March 26, 2013, 10:14:54 PM
 #40

Next the conductivity tests!

thermal or electric conductivity?

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

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?

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