Relative Density
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Author:  Bill Hanneman [ Sun Sep 20, 2015 11:04 pm ]
Post subject:  Relative Density

I believe any gemologist who really understands the use of SG should be able to devise an instrument to determine the relative density of any sample from 1 to 1000 carats without weighing anything.

I also believe that no gemologists here can do that, :| :| and I am willing to GIVE portable instruments for characterizing OTL GEMS to the first two people to describe how to make such a SG instrument in less than 100 words the using the following list of materials.

A base or stand (your choice)
Nylon thread
meter or yard stick
a couple of pebbles and/or rocks
glass of water
piece of paper, pencil
a dab of Blue-Tack or Stick-Tack
If you want to measure very small stones, you might employ mending tape and/or Super glue.

Author:  Gabroo [ Mon Sep 21, 2015 6:53 am ]
Post subject:  Re: Relative Density

Hello Dr. Hanneman, the inventor of gemological instruments S.O.G. If i am not wrong, i am new here though but i always tried my best to acquire your balance scale but could not find anywhere,

its really very difficult to devise such an S.G. instrument as your described material, but will try though :)

I will use a round rough stone at least 6" high as a stand, then stick "Stick-Tack" on the far edges of both sides of the yard stick to make it level, one side of the edge of yard will be tied to nylon string over a glass filled water and now 2 equally weighted stone or pebble will be used one tied to nylon string and will be submerged in water and the other stone will be put on the tack other side of yard stick,

that's may be an instrument and i could be wrong though!
but how will i calculate S.G may be i'm missing something


Author:  dchallener [ Mon Sep 21, 2015 8:08 pm ]
Post subject:  Re: Relative Density

hang the specimen on the left end of the yardstick. Hang the pebbles from the other end (or set them on the other end). Place your fingers lightly underneath the yardstick and move them toward one another till they meet. The place that they stop is X from the left and Y from the right.
Lower the yardstick until the specimen is under the water in the beaker and do it again. At this point, the place where your fingers meet is A from the left and B from the right.

From the first you know that the specimen is Y/X times the weight of the pebbles in air. From the second you know the specimen is B/A times the weight of the specimen in water.

B/A over Y/X is the ratio of the weight specimen in air over the weight specimen in water.

Now 1/Specific gravity = (Weight in air -weight in water) / (Weight in air) = 1 - B/A over Y/X

So specific gravity = 1 over (1-BY/AX)

How did i do?

Author:  wilsonintexas [ Mon Sep 21, 2015 11:35 pm ]
Post subject:  Re: Relative Density

try 4. lost several. keep editing trying to clean it up....
Spend the day learning how Microsoft Word deals with equations, only to say daaaaa.
- you can not paste them in here.
so. I made a pdf.
sorry forgot how to align things..... best I can do for now. down load the PDF for a better drawing.

...|....b length in air...........^ ............a length in air........|
...|...y in water..................|..............x length in water...|

Anyhow the directions:
Glue gem and rock to strings (or hang in thin wire baskets)
Blue tack so gem is on right and rock is on left hanging off of the end of the yard stick
place pencil on the corner of table
Balance yardstick on pencil (record lengths a- right side, b-left side)
using glass of water raise it up so the gem is in the water and rebalance (record lengths x-right side y-left side)
Plug a, b, x, and y into this formula

sg = 1/(1-ay/bx)

(word count = 94)

I did not have a yard stick, but I did have a 31 inch piece of 1/4 brass rod..... (really long dop)

Here are some actual measurements from 2 pieces of CZ hung using thin wire:

KNOW that the decimals are to long... but I left them in so if someone put it into excel they would see the same numbers.....

overall length = 30.75
a air = 14.5
b air = 16.25

x water = 15.9 (just over 7/8)
y water = 14.85
shift in balance = 1.4

a *y = 215.325
b*x = 258.375

ay/bx = 0.833381713

1 - ay/bx = 0.166618287

sg (1/ 1-ay/bx) 6.00174216

Now let's see how sensitive it is...... what if the shift was 2 inches..... (and the length changes, while playing with the numbers....)

how sensative
overall length 31.25
a air 14.5
b air 16.75

x water 16.5
y water 14.75
shift in balance 2

a *y 213.875
b*x 276.375

ay/bx 0.773857983

1 - ay/bx 0.226142017

sg (1 / above) 4.422

The explanation of the formula is in the attached pdf.

Bill Hanneman wrote:

I also believe that no gemologists here can do that, :| :|

It is the Engineer in me that did it. I do not qualify as a gemologist.

It look me all day to dust off the cob webs... I have not done algebra for a while.....

I had to use excel to help me verify that I had things correct. the first few attempts I had something wrong.

explination yard stick sg.pdf [247.18 KiB]
Downloaded 222 times

Author:  wilsonintexas [ Wed Nov 09, 2016 7:05 pm ]
Post subject:  Re: Relative Density

Dr Hanneman.

It has been a while since I have seen a post from you I hope that everything is ok

I recently created a problem for myself. I came up to Alaska to help my daughter and brought lot of rough and my faceting machine, but not much else. Over the past year, I have managed to get all of my rough mixed up. I do have your book, back in Dallas, which is not doing me much good here.

I may give myself a Christmas present and buying some of your portable tools, and wondered which ones you would recommend to help me sort a bag of very mixed up rough?

I know that this is a strange place to post this, but I remembered this challenge that you made, and wanted to see if there was enough here to get me started.

I hope that you are doing well.

Author:  Bill Hanneman [ Fri Nov 11, 2016 4:00 am ]
Post subject:  Re: Relative Density

I am still alive and going fishing next week.
Send me your mailing adress and I will loan you a prototype of what you are looking for. When you finish with it, just send it back and also give me an idea about what is missing and what I should charge for the kit. Remember, this will be a limited edition and should become a collector's item. :D

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