Post subject: Measuring Specific Gravity [SG] - would this idea work?
Posted: Sun Jan 13, 2008 1:28 pm
New to the Forum or The Quiet Type
Joined: Sat Jan 05, 2008 12:44 pm Posts: 10 Location: san diego, ca
Heres my idea:
1. I take any balance beam scale, like a Ohaus triple beam scale which is $60 or soon ebay.
2. weigh the stone in air.
3. weigh a glass of water.
4. drop the stone in the glass of water, then weigh glass of water with the stone in it.
5. SG = Weight of Mineral in Air /
( Weight of Mineral in Air - Weight of Mineral in Water)
6. Plan B, same as above, BUT with one difference: The glass is water is filled to the very top and weighed. The gem is then dropped in the glass of water, and of course water overflows. Now, weigh the glass of water with the gem in it.
Joined: Sun Oct 07, 2007 2:04 pm Posts: 623 Location: Southern OK/North TX
Doos, you're so funny!
====================
Capybara, Let's go thru that...
2. weigh the stone in air. OK - LET'S SAY 4.05ct
3. weigh a glass of water. LET'S CALL IT 35.00ct for consistancy
4. drop the stone in the glass of water, then weigh glass of water with the stone in it. SCALE NOW READS 39.05ct
5. SG = Weight of Mineral in Air /
( Weight of Mineral in Air - Weight of Mineral in Water)
OK - THAT WOULD BE 4.05ct./4.05ct MINUS....HUH?? 39.05ct???
Now why in the world did THAT experiment fail? You weighed the stone then you weighed the stone plus the glass of water, you never found the weight of the stone IN water....
You're not the first, second or even thousandth to think of this....and even you learned something too!
Read the link in the posting above for a complete explanation of this!
Joined: Sun Jul 22, 2007 5:58 pm Posts: 294 Location: California desert
Plan "B" would work if a final step of then removing the gem and weighing the container and the remaining water before the math.
full container of water -(minus) end result of partial filled container = volume of stone.
dry weight of stone divided by the volume of stone = sp.gr. ( density)
BUT!!
the common Ohaus has a resolution of about 1/10 gram or 0.5ct, so significant figures of a determined SG would be incomparable with those listed in reference guides for gems. Also the loss of water volume using common containers and loss of water adhering to the gemstone could be as much or more than the volume of the gemstone itself.
In another thread, G4LAB mentioned specific gravity bottles which contained a very accurate amount of water and the modified process "B" could be done. I tried a 25 ml sp. gr. bottle from the chem lab and only the smallest gemstone could enter the neck, and I had no way of fishing the gem out to measure the partial water without picking up a significant amount of water which would give faulty results.
So, failure arising from two reasons. Reason one is incorrect understanding of the measurement...weighing stone and water together isn't the same thing as weighing the stone in water. Reason two is underestimating the precision required for measurement... you ain't weighing a bag of grapes at the grocery store.
Good try capybara, and good explanations squeaky and desertgem. Doos, hope your exams went ok.
P.S. isn't it interesting: drying off the stone significantly change its weight reading. I tried it for myself, right after a shower I stepped on the scales and then again after I dried off. No change.
Joined: Sat Jan 05, 2008 12:44 pm Posts: 10 Location: san diego, ca
desertgem wrote:
Plan "B" would work if a final step of then removing the gem and weighing the container and the remaining water before the math.
full container of water -(minus) end result of partial filled container = volume of stone.
dry weight of stone divided by the volume of stone = sp.gr. ( density)
BUT!! the common Ohaus has a resolution of about 1/10 gram or 0.5ct, so significant figures of a determined SG would be incomparable with those listed in reference guides for gems. Also the loss of water volume using common containers and loss of water adhering to the gemstone could be as much or more than the volume of the gemstone itself. Jim
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Im going to use it on larger samples - usually 0.5 oz = 70 ct = 14 gr
or more. i think as the sample size increases, the relative size of the error of an Ohaus scale [0.1 gr] becomes smaller. For a sample size of
14 grams, and error of 0.1 grams is < 1%.
But i do see how my idea is inaccurate for smaller gems
Proposed test: im going to try it on 0.5 oz or larger samples of:
quartz
chacedony / jasper
topaz
nephrite
lapis
and i'll post the results, accurate or not, ok?
_________________ dont put the horse before the cart
What is specific gravity? Archimedes discovered that the weight of body in air minus its weight in water is equivalent to the weight of the water displaced by the body. When a body is placed in water, the volume of water displaced is equal to the volume of the body. When the body is placed in water that it undergoes an apparent loss of weight. This loss of weight is equal to the weight of the water displaced.
We'll substitute the word a "body" to a "gem stone" to explain specific gravity although you can really determine the specific gravity of any material.
When a gem stone is weighed in air and then weighed in water the loss of weight is equal to the weight of its volume in water displaced. The weight of the gem in air divided by the loss of weight in water gives the specific gravity of the stone.
If the displaced water can be measured, it gives the volume of the stone, and the dry weight divided by the volume = sp. gr. ( density).
Alternatively, if the actual amount of water( ml, grams, cts) displaced can not be determined by weighing or volumetric measurement, it can be found by subtracting the wt of the stone in the water from the wt of the stone in air, as the difference is equal to the amount of the water displaced ( bouyancy).
If one can measure the amount of water displaced, the weighing of the stone in water doesn't have to be made ( in theory) as the amount of water displaced is equal to the difference in weight of the stone in air and in water.
Joined: Sun Jul 22, 2007 5:58 pm Posts: 294 Location: California desert
Hi Barbra,
Actually, I use the "weigh in air, weigh suspended in water", method as I find it is more accurate with the equipment I have, than the displaced water method, which as I mentioned have several measurement deficiencies. I have used the displaced water method on gemstones, but found the accuracy was less, due to the problems I mentioned in earlier post here, and also sensitive weight measuring balances are more common and available than sensitive volume measuring devices.
I was commenting on the theory of the measurement rather than the resolution of the accuracy. Both "weigh in air/water" and "water displacement" methods will give the sp. gr. of a gemstone, the accuracy may vary.
Many use inexpensive digital gem balances that have a rounding resolution ( always end in 0 or 5, never another number) so that using 2 weights ( one dry, one wet) that have already been rounded, perhaps in same or different direction(s) can give inaccuracies that are not often considered.
Joined: Sun Jul 22, 2007 5:58 pm Posts: 294 Location: California desert
Just a side note on density/sp. gravity, I read in "DISCOVER" that the density of Ceres ( asteroid/dwarf planet) is slightly over 2, compared to Vesta ( another asteroid which is iron-silicates) 3.5 and Earth's 5.5. This indicates that Ceres has a "Lot" of water in its makeup. the Dawn project is head there and should reach Ceres in 2015.
Joined: Sat Sep 22, 2007 1:21 am Posts: 99 Location: Salt Lake City, Utah
hello all;
i have struggled with the 'let's see if i can make things a little harder' syndrome most of my life. Of course i am seriously ADD also, so i have grown to expect this sort of thing from myself. It's everyone else that suffers for my behavior. HOWEVER, in the last few years the prospect of reaching 60 yrs of age has made me say "G__ dAMMIT BOB, YOU'VE GOT TO QUIT DOING THIS" at least 3 times before proceeding. Now if you think my words here are a waste of our time you're right. After reading some of the ideas out there about how to get an accurate sg reading i'd say read my words of self-examination and then to yourself say "G__DAMMIT (your name) i've got to get some help with this first"
I will admit i haven't did an sg since high school chemistry. I like the idea of using wire stone holder that can be suspended from your scale. Subtract the wt of the wire, or zero your scale before adding the sample stone. then do the exact same thing with the stone suspended in a cup of water. Be sure the cup is wide enough and the sample is suspended in the center and not close to the surface or the cup. Also, if you cannot zero the scale the weight of the wire will be different in air and in the water so you need to get a wire only weight in and out of the water. I'm just trying to decide the best scale to buy as far as accuracy. Maybe those of you who have already done this can suggest a good scale for the money? Most of my work will be with stones in the 1-20 caret range.
Well, my cat blackjack is bawling and has his toy at my feet so gotta go..
take care all, bob
Joined: Mon Dec 10, 2007 9:00 pm Posts: 13 Location: South Carolina
Please note that while most people use density and specific gravity interchangeably, they are actually two different measurements. Density is measured in mass/volume (eg. grams/cc), while specific gravity has no units, it is a ratio number comparing the weight of the substance in air to the weight of the substance while suspended in water.
SG can be measured fairly easily using simple digital gem scale. In fact there is a SG device for any gem scale that costs about $80. I made my own from light wire and use a small glass of water. This works fairly well as long as the gem is above 1 carat of so. Does not work well with very small stones.
email me and I will send picture and instructions for SG testing; too much to post in forum.
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