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DIY Specific Gravity Kit (tutorial) - (19)
  
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cascaillou




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PostPosted: Apr 08, 2014 05:53    Post subject: DIY Specific Gravity Kit (tutorial) - (19)  

There are two ways to build a Specific Gravity kit to fit a digital scale. It's important to note that the formula for SG calculation will be different depending on the method used:

1)Method number one: the gallow that holds the basket sits on the scale weighing tray, but the glass of water doesn't makes any contact with the tray. Commercially available SG kits are based on this method (and the kit I have built myself is also based on this method). Let's note that with this method, stability and good balance of the stand (gallow+basket) are important factors so to get accurate readings, so you really need to build things properly.
When using this setting SG = weight of stone in air / (weight of stone in air - weight of stone in water)

2)Method number two: the glass of water sits on the scale weighing tray, but the gallow that holds the basket doesn't make any contact with the tray (also make sure the gallow and basket are not making any contact with the glass). No commercial kits are based on this method, but you can easily build one yourself. However, this method requires a high capacity scale, and yields less accurate SG measurements for smaller stones (which is why method.1 is best). On the other hand, when using this method, accuracy shouldn't suffer much from a somewhat approximative shaping of the stand.
When using this setting SG = weight of stone in air / weight read with stone in water

NOTE: in both settings (method.1 and method.2), for more accurate SG results, the basket part has to be made of very thin wire (1mm thick). Also, in method.1 you will get more accurate SG results if not using a narrow diameter glass (better use a large diameter glass), while in method.2 you would want to save some scale capacity by using a light glass made of plastic.



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PostPosted: Apr 08, 2014 05:57    Post subject: Re: DIY Specific Gravity Kit (tutorial) - (19)  

As said previously, Method number one is best, so the following tutorial will explain how to build an SG kit based on that method.
I own both a Tanita 1230 scale (20x0.002g) and a Tanita 1579D scale (200x0.01g), so I've been working on a small specific gravity kit that would fit both models, however I think this SG kit design should actually fit most existing scales.

STEP BY STEP TUTORIAL:

First I built a base out of plexiglass (pmma) that can fit both models. The circular base is 5cm diameter, and weighs only 2.6g.
To make the base I needed a 1mm thick plexiglass sheet, and a 8mm thick plexiglass sheet. For carving the plexiglass I used a Dremel tool. Some strong glue is used for assembly of the three parts making up the base. The three parts making up the base are seen into details in the next two pictures:

click on images to see full size (so you can read the indications):



2-base underside.jpg
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2-base underside.jpg



3-base top.jpg
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PostPosted: Apr 08, 2014 05:58    Post subject: Re: DIY Specific Gravity Kit (tutorial) - (19)  

Indeed, this base will fit any large flat weighing tray (such as the 1579D) but it will also fit perfectly the circular Tanita 1230 weighing tray (as shown in this last pic):


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cascaillou




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PostPosted: Apr 08, 2014 05:59    Post subject: Re: DIY Specific Gravity Kit (tutorial) - (19)  

Once the base has been made, one has to build the weighing stand. This part is made up of two separate elements: a gallow shaped stand, and a basket (hanging to the gallow). One will need a vise and small pointed pliers to build these.
I've actually been building two different weighing stands (both fit the single plexiglass base that was made previously):
->the first is intended for the milligram scale (thus it is made very lightweight): the gallow is made of 2mm thick duraluminium wire, and the basket is made of 1mm thick non-magnetic stainless steel wire.
->the second is intended for 0.01g scale (it is bigger and more sturdy, but also heavier): the gallow is made of 2mm thick non-magnetic stainless steel wire, and the basket is made of 1mm thick non-magnetic stainless steel wire.

Please note that duraluminium is a light weight and non-magnetic aluminium alloy which is more rigid than common aluminium.
Also, concerning stainless steel wire, one must check it with a magnet to make sure it is of the non-magnetic kind.

Here are diagrams of both settings (the 5x5mm squares on the paper should give you a good view of the angles and proportions):
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PostPosted: Apr 08, 2014 06:00    Post subject: Re: DIY Specific Gravity Kit (tutorial) - (19)  

1230 SG kit: duraluminium gallow shaped stand (2mm thick wire) with non-magnetic stainless steel basket (1mm thick wire):

it is 9,5cm high and as you can see maximum sample size would be about 3x3x3cm. Total weight of the SG kit: 5.9g (considering that the Tanita 1230 has 20g capacity, that leaves you with about 14g capacity once the kit is set on the scale)



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PostPosted: Apr 08, 2014 06:00    Post subject: Re: DIY Specific Gravity Kit (tutorial) - (19)  

1579D SG kit: non-magnetic stainless steel gallow shaped stand (2mm thick wire) and non-magnetic stainless steel basket (1mm thick wire):

it is 12cm high and as you can see maximum sample size would be about 4x4x4cm. Total weight of the SG kit: 10.4g.
Optional suggestion: let's note that 1mm thick wire is best for the basket (indeed thinner wire means more accurate SG results), but if you plan to be measuring SG of heavy stones (80g or more) with the 1579D kit, then I'd suggest building an extra basket out of 2mm thick non-magnetic stainless steel wire (if so total weight of the kit rises to 17g)



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PostPosted: Apr 08, 2014 06:01    Post subject: Re: DIY Specific Gravity Kit (tutorial) - (19)  

Please note that in both diagrams, the gray hatched thing under the glass of water is a duraluminium support (it is supporting the glass, considering that the glass should not make any contact with the scale weighing tray). This duraluminium support was made made out of a 1,5mm thick duraluminium sheet (mostly because it non-magnetic, lightweight and sturdy).
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PostPosted: Apr 08, 2014 06:03    Post subject: Re: DIY Specific Gravity Kit (tutorial) - (19)  

Pictures of the Tanita 1230 SG kit:

First picture = Gallow (2mm thick duraluminium wire)
Second picture = Basket (1mm thick non-magnetic stainless steel wire)
Third picture = Complete SG kit (base+gallow+basket) set on the scale



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7-1230 dural gallow.jpg



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8-1230 stainless basket.jpg



9-1230 complete kit on scale.jpg
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9-1230 complete kit on scale.jpg


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PostPosted: Apr 08, 2014 06:04    Post subject: Re: DIY Specific Gravity Kit (tutorial) - (19)  

Pictures of the Tanita 1579D SG kit:

First picture: Gallow (2mm thick non-magnetic stainless steel wire)
Second picture: Basket (1mm thick non-magnetic stainless steel wire)
Third picture: Complete SG kit (base+gallow+basket) set on the scale
Fourth picture: Optional additional basket for use with heavier stones (2mm thick non-magnetic stainless steel wire, for higher stiffness)



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10-1579D stainless gallow.jpg



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11-1579D stainless basket.jpg



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12-1579D complete kit on scale.jpg



13-1579D optional thick stainless basket.jpg
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13-1579D optional thick stainless basket.jpg


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PostPosted: Apr 08, 2014 06:05    Post subject: Re: DIY Specific Gravity Kit (tutorial) - (19)  

How to use these SG kits:

Place the scale on a stable flat horizontal surface, away from vibrations and away from air drafts (milligram scales are sensitive to air drafts).
If the scale had been unused for a long time, or if you've been moving the scale around (which is likely for a pocket carry scale), then you first need to calibrate it (with proper calibration weight).
At that point, weight the stone on the scale (that measurement is the weight in air).
Then you need a colorless transparent glass filled with distilled water. With this SG setting (based on method.1), for more accurate SG measurements, do not use a narrow diameter glass (better use a large diameter glass).
The glass of water will be supported by a duraluminium support (which is made out of a 1,5mm thick duraluminium sheet).
You have to position the SG kit on the scale making sure that the kit is not making any contact with the glass nor with supporting duraluminium tray, nor with the sides of the scale body.
Once the SG kit is set on the scale (with basket immersed in water), one must tare the scale.
To place the stones on the sg kit, it's best to use non-magnetic tweezers (I use titanium tweezers).
Make sure the stone is completely immersed in water, and make sure there are no air bubbles stuck to the basket or stuck to the stone. At that point read the weight of the stone immersed in water.

Then SG = weight of stone in air / (weight of stone in air - weight of stone in water)

Please note that a 0.01g scale (such as Tanita 1579D) isn't accurate enough for measuring specific gravity of stones weighing less than 3g (theoretically, when using a 0.01g scale with the described SG kit setting, maximum error for 3g stones with sg of 2 should be 1%, while maximum error for 3g stones with sg of 5 should be 3%)
Actually, for optimal accuracy I'd suggest to stick to the milligram scale as long as the scale capacity is enough for the stone.
When using a 0.002g scale (such as Tanita 1230) with the described SG kit setting, theoretically maximum error for a 0.6g stone with sg of 2 should be 1%, while maximum error for a 0.6g stone with sg of 5 should be 3%.
When using a 0.001g scale with the described SG kit setting, theoretically maximum error for a 0.3g stone with sg of 2 should be 1%, while maximum error for a 0.3g stone with sg of 5 should be 3%.
For instance, with a 0.002g scale, for a 0.6g pyrite which would have an actual SG of 5.00 the maximum error would be 3% from actual SG which means +/-0.15 (that is to say that the obtained SG measurement could range anywhere from 4.85 to 5.15, while the actual SG is exactly 5.00)

Here are pictures of the scales, with the duraluminium support for the glass:



14-1230 kit with dural support.jpg
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14-1230 kit with dural support.jpg



15-1579D kit with dural support.jpg
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15-1579D kit with dural support.jpg


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PostPosted: Apr 08, 2014 06:06    Post subject: Re: DIY Specific Gravity Kit (tutorial) - (19)  

Advantages of my kit design: simple multipurpose design, easily made yourself for cheap, small sized, light weight (for higher capacity), and can be dismantled for easy carry. This SG kit design should fit most existing scales. This setting yields accurate SG results even for smaller stones (when using the milligram scale).

Drawbacks: the kit I built uses a single-legged gallow, however if you don't care about portability, then a three-legged design is an even better way to go. Indeed, a single-legged gallow might create a turning moment in the base, so that the scale might think that the weight is placed on one side of the pan, which may introduce some error in the readings. That is why professional SG kits usually rely on a three-legged design for the stand, which is a bit more bulky but allows one to achieve uniform repartition of weight on the scale weighing tray (thus ensuring better accuracy of the readings). Let's note that you can still build such a three-legged kit yourself (using 1mm thick non-magnetic stainless steel wire for building the three-legged gallow as well as for building the basket).
This also means that, whatever you build (single-legged or three-legged stand), it is very important to apply yourself in building it properly so it is as stable and as well-balanced as possible.

For comparison, here you can see a commercial three-legged universal SG kit:
http://www.mineralab.com/SGK-C/
(link normalized by FMF)
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PostPosted: Apr 08, 2014 06:17    Post subject: Re: DIY Specific Gravity Kit (tutorial) - (19)  

SUSPENSION TECHNIQUE:

Now, if you don't yet have a specific gravity kit, or if your specimen is too big to fit your kit, then you can use the suspension technique, which is a simplified version of method.2 that doesn't require building anything, but it's not appropriate for smaller stones. With this technique, beware that using very very thin line is essential to get accurate SG measurements.

Using a milligram scale is irrelevant here. But with a 0.01g scale, the suspension technique can be used for measuring specific gravity of any stone which is at least 3g, provided that the fishing line is 0.1mm thick or less (the thinner the better!). Alternatively, one could use a 0.1g scale if the stone is at least 30g (but for more accurate results I'd suggest to stick to a 0.01g scale).
In both cases, the maximum error in the obtained SG for a stone which has an actual SG of 5 would be 2%

-First weight the stone on the scale
-Then, fill a plastic glass with distilled water, place it on the scale and tare the scale.
-Tie the stone with some fishing line. As the immersed line volume will introduce an error, you want to minimize this error by using very thin line, but also by managing to tie the stone with as little line length as possible.
-Holding the line with your fingers, suspend the stone in water so the stone is completely immersed but not making any contact with the glass itself (also, don't immerse the stone deeper than 2cm under water). Also make sure there are no air bubbles stuck to the fishing line or stuck to the stone. At this point read the weight on the scale.
Note: during the weighing, try to stabilize your forearm so to avoid shaking (comfortably lean on your elbows over the table, and support one hand with the other hand)
-Then SG = weight of the stone / weight read with stone in water



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