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 Post subject: SG reading without an hydrostatic kit
PostPosted: Mon Oct 11, 2010 11:16 am 
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Here's a crude field technique for SG of rough stones and minerals:

Image
(click on the image once, and then click again to make it big enough so to read instructions)

This technique is somewhat approximative but is fine for use with a pocket sized 0.01g scale, which would be accurate enough for stones weighing 3g or more (when using this technique with a 0.01g scale, theoretically maximum error in the SG reading for a 3g stone which has an actual SG of 5.00 would be 2%)

All you need is a scale, a plastic cup, a thin string (or fishing line), and some thin non-magnetic metallic wire (either stainless steel or copper wire).

-First weight the stone in air (this is Ma)
-Then shape the metal wire into a crude basket
-Tie the string to the basket
-Make a red mark on the basket metal wire (with a pen)
-Fill the plastic glass with distilled water and place it on the scale
-Immerse the basket in water (simply holding it with your fingers) so the red mark is exactly at the surface of water, and tare the scale.
-Then, place the stone in the basket and immerse it so the red mark is exactly at the surface of water again, and read the weight on the scale at this point (this is V)
-Then SG=Ma/V

Both when you tare the scale and when you weight the immersed stone, it is important that the basket and your fingers are not making any contact with the glass.

During the taring of the scale and during the weighing, stabilise your forearm so to avoid shaking (comfortably lean on your elbows over the table, and support one hand with the other hand)


Last edited by cascaillou on Mon Apr 21, 2014 1:09 pm, edited 64 times in total.

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 Post subject: Re: SG reading without an hydrostatic kit
PostPosted: Mon Oct 11, 2010 11:44 am 
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Hi,

I would consider what you just described to be a hydrostatic SG method. (and the line tied to the stone could be considered a very simple hydrostatic kit)

No5 should read SG = m1 / m1-m2


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 Post subject: Re: SG reading without an hydrostatic kit
PostPosted: Mon Oct 11, 2010 11:48 am 
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If you substitute a little basket on a wire for the string, it IS a hydrostatic kit. :wink:


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 Post subject: Re: SG reading without an hydrostatic kit
PostPosted: Mon Oct 11, 2010 11:54 am 
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Quote:
If you substitute a little basket on a wire for the string, it IS a hydrostatic kit.


Nop, it's not the same:
Note that the basket is hold with your fingers (or hanged to a stand that does NOT sit on the scale weighing tray), and that the glass of water sits directly onto the scale weighing tray. Which means that the basket is NOT applying any force on the scale weighing tray, neither is the weight of the immersed stone sensed by the scale weighing tray considering that the basket that hold the stone doesn't sit on it (unlike the classic hydrostatic method were the basket is supported by a stand that do sit on the scale tray)

Quote:
No5 should read SG = m1 / m1-m2


not in this method.

It is different as you are not weighing the stone in water, but rather direct reading the volume of the stone. At the end the result is the same, but there's a real difference in the procedure.
Therefore, the calculation is different than when using a classic hydrostatic kit:
with classic hydrostatic kit you will have SG= weigh in air / (weigh in air - weigh in water),
while in the method I decsribed SG is: weigh of the stone in air (Ma) / volume of the stone (V).
With this setting, what the scale reads when the stone is immersed in water is indeed V in cube centimeters.


Last edited by cascaillou on Wed Oct 23, 2013 10:48 am, edited 7 times in total.

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 Post subject: Re: SG reading without an hydrostatic kit
PostPosted: Mon Oct 11, 2010 1:58 pm 
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In the illustration of "M2", this can not measure the volume of the stone. The stone being held up by the string will displace it's volume in water, but the water will just rise in the container. The weight of the water will not change. To measure volume, the amount of water displaced will have to be measured. There are special volumetric containers that the overflow ( representing the volume of the stone) is collected and then weighed or measured in cc, to determine the volume of the the object.
Jim


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 Post subject: Re: SG reading without an hydrostatic kit
PostPosted: Mon Oct 11, 2010 3:50 pm 
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Quote:
In the illustration of "M2", this can not measure the volume of the stone. The stone being held up by the string will displace it's volume in water, but the water will just rise in the container. The weight of the water will not change


Yes it can :-)

the total weight of water will not change (and anyway, it's not taken into account as after placing the cup of water on the scale weighing tray tare has been done), but the scale will sense the displaced amount of water due to archimede principle which is stating that 'when immersing an object into water, the object will be subjected to a vertical force oriented up, equal and opposed to the weigh of the water volume dispalced by the object'. That vertical force is what the scale tray will sense, and considering that 1g of water is 1cc, then what you are reading is equivalent to the exact volume of the stone in cube centimeters.

This method has been used for a while by mineralogists and works fine.


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