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Question About Specific Gravity Testing

 
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Author Previous TopicReplies: 5 / Views: 422Next Topic  
Valued Member
United States
288 Posts
 Posted 03/28/2021  6:57 pm Show Profile   Bookmark this topic Add MisterT to your friends list Get a Link to this Message
Greetings fellow collectors. For those of you who conduct your own specific gravity tests, I have a question. While conducting this test recently I had observed erroneous results. Then I noticed that the waxy cord I was using to suspend my coins in water actually had a weight of 0.11 grams just by itself. Once I started deducting that 0.11 grams from my reading, the results came out as expected. So my question is what type of rig do you use to suspend your coins in solution and should I use something other than this waxed cord?
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United States
1128 Posts
 Posted 04/05/2021  9:37 pm  Show Profile   Bookmark this reply Add Ballyhoo to your friends list Get a Link to this Reply
While I fully understand the process I have yet to do so. But the wax string would have been my choice as well followed by the necessary deduction.
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Valued Member
United States
288 Posts
 Posted 04/05/2021  10:43 pm  Show Profile   Bookmark this reply Add MisterT to your friends list Get a Link to this Reply
Ballyhoo, the test is not difficult but it does require attention to accuracy. You begin by weighing your raw coin and record that weight on paper. Then you fill a container with preferably distilled water and place it on the scale, then zero out the scale again. Then you attach a string to your coin and submerge in in the water making sure it doesn't touch the sides or bottom of the container. Then you record the wet weight of the coin and write it down on paper. The final calculation is in dividing the dry weight by the wet weight. 90% silver should be about 10.33 while copper is around 8.9. Gold is in the 17.2 range or higher. A good test to determine the composition of any suspect coin. As mentioned, my erroneous readings were coming from a miscalculation (I didn't factor in the weight of the string). So if you perform this test, check to see if your string has any weight in the water BEFORE you attach a coin to it.
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United States
1054 Posts
 Posted 04/06/2021  4:56 pm  Show Profile   Bookmark this reply Add machine20 to your friends list Get a Link to this Reply
only the fraction of string that displaces any water should matter. the submerged part of the test is a measure of volume
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Canada
4142 Posts
 Posted 04/06/2021  5:22 pm  Show Profile   Bookmark this reply Add oriole to your friends list Get a Link to this Reply
You might consider using a very fine thread rather than a string-it will reduce a source of inaccuracy. Don't forget to factor in the error in the scale into your calculation. You will always have uncertainty. A very sensitive scale will also be affected by by breathing or other air currents, even aside from the intrinsic inaccuracy (only measures to a certain decimal point).
Valued Member
United States
288 Posts
 Posted 04/06/2021  11:48 pm  Show Profile   Bookmark this reply Add MisterT to your friends list Get a Link to this Reply
Very smart people on here! You are all correct. The test must be performed with lab like accuracy. I always check my scale with a calibration weight before using (scale reads to 2 decimal points) and I make sure it is on a stable surface free of vibration and air current. The string I am using is a flat waxed cord normally used in craft work or for making the web of mandelas. Tied with a slip knot to adjust for different coins. I only used distilled water in a plastic container and an alligator clip mounted above the container gives me something secure to suspend from. I try to suspend the coin consistently only up to the knot just being submerged. When I discovered errors in my readings I submerged the string only up to the knot to determine its displacement was 0.11 grams. Once I started deducting the 0.11 grams from my "wet weight", then my division calculations were accurate. I tested a Trade dollar that kept reading 8.74 which is consistent with copper. Confident in my test, I took a file to the reeded edge and sure enough I exposed a copper core.
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