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Author Topic: I measured the density of my silver/gold coins...  (Read 16102 times)
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March 24, 2013, 11:16:04 AM
 #1

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

EDIT: a better method was suggested by mintymark and yielded better results.



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?

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March 24, 2013, 12:32:03 PM
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There's definitely a good bit of error there, which I really doubt since the coins at minted by respected sources. Those coins are probably either .999 or .9999 and that thousandth or ten thousandth of an percent does not result in a 1-2% error.
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March 24, 2013, 03:07:54 PM
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There's definitely a good bit of error there, which I really doubt since the coins at minted by respected sources. Those coins are probably either .999 or .9999 and that thousandth or ten thousandth of an percent does not result in a 1-2% error.

Still would catch most real fakes as the density would be off by more then 5%.

 A bigger water container could help, and maybe a more accurate scale.  Finally having a consistent amount of string in the water and taking that into the calculation would help.   

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March 24, 2013, 03:22:23 PM
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I dont think your method is correct. This is the method I used to satisfy myself that a oplatinum ring was truly pure.

a) Weigh Ring.   
b) Weigh ring under water.
    This can be done by suspending the ring on a frame such that it hangs from a thread, and is imersed.
c) As an alternative to b), weigh the difference in weight between b) and a) , many scales can do this automatically.

Either way, you have the in air weight , and the loss of weight from imersion in water, c.

c is also the of mass of water displaced, and so gives the volume of the coin or ring.
If you do this in grams, the volume is in cubic centimeteres.

The density (or specific gravity) is found as normal by dividing the weight in air in grammes by the volume in cubic centimetres.

I found this method pretty accurate.

Main sources of inacuracy, I guess:

 Water not pure - use deionised water.
 Density varies with temperature, both of water and of the metals, corrections could be applied for this.
 Make sure you weight the item before you wet it, you do not want to weigh a wet item.
 Displacement of anything used to hold item, minimal if it is a thread.

I believe that a modern 'cheap' pair of digital scales is more accuracy than you will need. For their price these items give fantastic value. Just bear in mind that the weight of the frame, which can be a bent wire to suspend the item in a beaker of water under the scales should be as low as possible. Although the frame is zeroed out of the calculations, the errors in these types of scales go up when you do this type of thing.

I think you can get better accuracy than you did by using this method.







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March 24, 2013, 03:22:35 PM
 #5

There's definitely a good bit of error there, which I really doubt since the coins at minted by respected sources. Those coins are probably either .999 or .9999 and that thousandth or ten thousandth of an percent does not result in a 1-2% error.

with error I meant error of my measurement.

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March 24, 2013, 03:24:35 PM
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There's definitely a good bit of error there, which I really doubt since the coins at minted by respected sources. Those coins are probably either .999 or .9999 and that thousandth or ten thousandth of an percent does not result in a 1-2% error.

Still would catch most real fakes as the density would be off by more then 5%.

What density could a fake have? What kind of materials/techniques might be used?

A bigger water container could help, and maybe a more accurate scale.  Finally having a consistent amount of string in the water and taking that into the calculation would help.   

I thought about that. I think lowest hanging fruit would be a better scale. I can't use more water, scale can only do 200g.

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March 24, 2013, 03:30:12 PM
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I dont think your method is correct. This is the method I used to satisfy myself that a oplatinum ring was truly pure.

a) Weigh Ring.   
b) Weigh ring under water.
    This can be done by suspending the ring on a frame such that it hangs from a thread, and is imersed.
c) As an alternative to b), weigh the difference in weight between b) and a) , many scales can do this automatically.

Either way, you have the in air weight , and the loss of weight from imersion in water, c.

Hmm. I think this is what I did, no? What's the difference?

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March 24, 2013, 05:03:36 PM
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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.

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March 24, 2013, 05:12:04 PM
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There's definitely a good bit of error there, which I really doubt since the coins at minted by respected sources. Those coins are probably either .999 or .9999 and that thousandth or ten thousandth of an percent does not result in a 1-2% error.

Still would catch most real fakes as the density would be off by more then 5%.

What density could a fake have? What kind of materials/techniques might be used?

Common metals used:
Tungsten: 19.25 g/cm^3
Lead: 11.34 g/cm^3
Iron: 7.874 g/cm^3
Steel: 7.859 g/cm^3
They might also use Uranium: 19.05 g/cm^3

Some people are sneaky, and use a lead core, and lighter alloy on the outside, mimicking the density and weight of silver. There's almost always a plating of real metal in any case, so the surest non-destructive way to tell is ultrasound, or a carefully calibrated electrical test.

Failing that, there's always the ear test. "Ring true," as it were.

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March 24, 2013, 05:14:43 PM
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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.
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March 24, 2013, 05:18:04 PM
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Um, scales in labs 15 years ago could measure the weight of fingerprints.
In ordinary chemistry labs - not true. Mechanical scales for 1/10 of gram with some error.

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March 24, 2013, 05:43:48 PM
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Um, scales in labs 15 years ago could measure the weight of fingerprints.
In ordinary chemistry labs - not true. Mechanical scales for 1/10 of gram with some error.
An E1 1mg test weight has a tolerance of ±0.003mg. The scale in the OP likely would not change from 0.00 to 0.01 with several of these weights on it. A slight fingerprint weighs about 0.1mg and an ungloved hand on a beaker can leave behind 1mg. The university chemlab where I work-studied had several scales this accurate just for undergraduate chemistry. Put sample in, close door, wait. (I should add that these weren't generally used for student experiments, they were in the lab where chemicals and glassware were stored).

If you are serious about weighing the coins, washing them first may be in order. You're probably not going to have an ultrasonic cleaner around, but dawn detergent followed by lots of rinsing and gloved handling will remove the contaminants that may contribute to the differences in measurement between coins. A troy ounce is 31.1034768g; all the coins weighed more than this.

While the accuracy of measurements can't be verified without a calibration weight that has not been mishandled, the precision can, by determining repeatability - does one kronwitter always measure 1% more than another?
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March 24, 2013, 05:58:31 PM
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I dont think your method is correct. This is the method I used to satisfy myself that a oplatinum ring was truly pure.

a) Weigh Ring.   
b) Weigh ring under water.
    This can be done by suspending the ring on a frame such that it hangs from a thread, and is imersed.
c) As an alternative to b), weigh the difference in weight between b) and a) , many scales can do this automatically.

Either way, you have the in air weight , and the loss of weight from imersion in water, c.

Hmm. I think this is what I did, no? What's the difference?

His test relies on more implied concepts of physics. Mainly Archimedes's bouyancy principles. This helps to get rid of errors in weighing or measuring the volume of the displaced water.
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March 24, 2013, 06:38:49 PM
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Failing that, there's always the ear test. "Ring true," as it were.

You telling me that with all the high-tech doodads on the market for musicians, there's not one reasonably priced device that can detect the purity of precious metals by sound alone?

If that's the case, then there's money to made by the first person who invents/builds one.

A true Krugerrand has a distinct tone, whereas a fake would have a definite different tone.

How hard could making such a device be?
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March 24, 2013, 06:46:08 PM
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Failing that, there's always the ear test. "Ring true," as it were.

You telling me that with all the high-tech doodads on the market for musicians, there's not one reasonably priced device that can detect the purity of precious metals by sound alone?

If that's the case, then there's money to made by the first person who invents/builds one.

A true Krugerrand has a distinct tone, whereas a fake would have a definite different tone.

How hard could making such a device be?

The programming, I think, would be the hardest. Though one could always just use a frequency analyzer and compare each coin against a known good example.

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March 24, 2013, 06:56:49 PM
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Here's more fun science - remove silver tarnish: http://scifun.chem.wisc.edu/homeexpts/tarnish.html

Both silver sulfide and silver oxide have a lower density than pure silver.
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March 24, 2013, 07:22:03 PM
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Failing that, there's always the ear test. "Ring true," as it were.

You telling me that with all the high-tech doodads on the market for musicians, there's not one reasonably priced device that can detect the purity of precious metals by sound alone?

If that's the case, then there's money to made by the first person who invents/builds one.

A true Krugerrand has a distinct tone, whereas a fake would have a definite different tone.

How hard could making such a device be?

The programming, I think, would be the hardest. Though one could always just use a frequency analyzer and compare each coin against a known good example.

We're getting closer!: http://en.wikipedia.org/wiki/Spectrum_analyzer

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March 24, 2013, 07:46:24 PM
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I dont think your method is correct. This is the method I used to satisfy myself that a oplatinum ring was truly pure.

a) Weigh Ring.   
b) Weigh ring under water.
    This can be done by suspending the ring on a frame such that it hangs from a thread, and is imersed.
c) As an alternative to b), weigh the difference in weight between b) and a) , many scales can do this automatically.

Either way, you have the in air weight , and the loss of weight from imersion in water, c.

Hmm. I think this is what I did, no? What's the difference?

His test relies on more implied concepts of physics. Mainly Archimedes's bouyancy principles. This helps to get rid of errors in weighing or measuring the volume of the displaced water.

My method also used the bouyancy principle and find the volume of the coin by "weighing" the displaced water. I also suspend the coin from a thread. I don't see how his method differs at all from what I did, practically. Am I overlooking something?


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March 24, 2013, 08:28:12 PM
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I dont think your method is correct. This is the method I used to satisfy myself that a oplatinum ring was truly pure.

a) Weigh Ring.   
b) Weigh ring under water.
    This can be done by suspending the ring on a frame such that it hangs from a thread, and is imersed.
c) As an alternative to b), weigh the difference in weight between b) and a) , many scales can do this automatically.

Either way, you have the in air weight , and the loss of weight from imersion in water, c.

Hmm. I think this is what I did, no? What's the difference?

His test relies on more implied concepts of physics. Mainly Archimedes's bouyancy principles. This helps to get rid of errors in weighing or measuring the volume of the displaced water.

My method also used the bouyancy principle and find the volume of the coin by "weighing" the displaced water. I also suspend the coin from a thread. I don't see how his method differs at all from what I did, practically. Am I overlooking something?



Well, for a start at no point does the weight of the beaker of water go on the scales. Your photograph seems to show the beaker of water on the scales. The point about these small scales is they are very accurate for small masses, less so for larger masses. Especially so as regards repeat readings which of course is what counts. Partly its a percentage thing. So its much better to not weigh large weights as far as you can. My method does involve weighing a wire frame so that you can weigh under the scales (and submerse the dangling object in water, which of itself is never weighed. You could use a 100 ton bucket of watter when weighing a 10g coin, and it would make no difference.

I think you are displacing the water out of the contain. There is an acuracy problem with this, and that is that surface tension may result in inconsistent results. Its inherrently sticky, and a few drips more or less may splut out as you do it. Any amount of grease or detergent on the object may also change the surface tension and that is worse. You may say its a trivial amount, but the point is, its multiplied by the comparatively large area of the top of the beaker. So it ends up as quite a bit.


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March 24, 2013, 09:01:06 PM
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thanks for the clarifications...

I dont think your method is correct. This is the method I used to satisfy myself that a oplatinum ring was truly pure.

a) Weigh Ring.   
b) Weigh ring under water.
    This can be done by suspending the ring on a frame such that it hangs from a thread, and is imersed.
c) As an alternative to b), weigh the difference in weight between b) and a) , many scales can do this automatically.

Either way, you have the in air weight , and the loss of weight from imersion in water, c.

Hmm. I think this is what I did, no? What's the difference?

His test relies on more implied concepts of physics. Mainly Archimedes's bouyancy principles. This helps to get rid of errors in weighing or measuring the volume of the displaced water.

My method also used the bouyancy principle and find the volume of the coin by "weighing" the displaced water. I also suspend the coin from a thread. I don't see how his method differs at all from what I did, practically. Am I overlooking something?



Well, for a start at no point does the weight of the beaker of water go on the scales. Your photograph seems to show the beaker of water on the scales. The point about these small scales is they are very accurate for small masses, less so for larger masses. Especially so as regards repeat readings which of course is what counts. Partly its a percentage thing. So its much better to not weigh large weights as far as you can. My method does involve weighing a wire frame so that you can weigh under the scales (and submerse the dangling object in water, which of itself is never weighed. You could use a 100 ton bucket of watter when weighing a 10g coin, and it would make no difference.

Sounds great, I'd love to be able to do that. I understand your point about the accuracy deteriorating with higher weight and I'd like to avoid that.

Can you maybe find or produce a drawing of that frame and the other stuff? I'm still having problems. The setup I come up with in my mind has other problems... I'm probably thinking too complicatedly.

I think you are displacing the water out of the contain. There is an acuracy problem with this, and that is that surface tension may result in inconsistent results. Its inherrently sticky, and a few drips more or less may splut out as you do it. Any amount of grease or detergent on the object may also change the surface tension and that is worse. You may say its a trivial amount, but the point is, its multiplied by the comparatively large area of the top of the beaker. So it ends up as quite a bit.

I'm not sure what you mean by "you're displacing water out of the cointainer". No water leaves the container (except after the measurement is complete and I remove the coin some drops stick to the coin of course). I'm weighing the container with the water first (resetting scale to 0), then I'm submersing the coin hanging it from the string (so it doesn't touch either the floor of the container of the top of the water). The scale then shows the weight of the displaced water. (Actually I did it a little differently: For resetting the scale I put the coin (attached to the string but not hanging from it) into the container (so it's laying on the ground of the container). Scale shows zero at that point. Then I lift it up while keeping it submerged to it doesn't touch the ground or surface. That's why the scale show a negative value in my shot (the weight of the displaced water minus the weight of the coin)).

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