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specific gravity

Posted by Gord Howe  
avatar Re: specific gravity
February 03, 2011 05:26PM
Hello all,

For Mindat'rs yourselfers, I propose a simple installation of hydrostatic balance. Good luck.
First the ingredients: - a support arm with weighted (eg, old desk lamp)
- A precision balance (around 30 euros)
- A container door set at the end of the arm
- Support weighing up a ring of PVC, with one side a pedestal to be placed on the balance pan, and, unlike a stem for receiving the sample. (I used a welding rod is brass and an old electric socket, which are welded to tin) because the device must not be magnetic ... logical.
We put the bracket on the weighing scale platform, we tare to zero, then we put the sample in the old electric socket. We note the outcome of the measure.
Identical operation, sleeve immersed in the container - do not touch the sides or bottom-. We rebuild a tare to zero. Sample is deposited gently into the socket carrier is allowed to stand so that the system is stabilized and there has been the result of the measure. We have a friendly thought for good old Archimedes helps us well for a successful conclusion to these measures.
We have the dry weight of the sample and its mass in water, then applying the following formula:
Density = Weight in air
Weight in air - weight in water
we get a number that is enough to compare to those of many tables that are found in the literature or the internet, and we have an idea of the nature of the sample. The ideal is a small specimen consists only of the unknown material. Of course this is only an index, further testing will be needed, hardness, color and mineral line, form, place of origin, etc. ...

Bests regard's. Michel
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avatar Re: specific gravity
February 04, 2011 03:16PM
A;ysson, your fishing-line springs sound great! I have to try that. How would a long threaded bolt work for a winding mandrell? (I think it would provide even spacing for the turns in the spring}
avatar Re: specific gravity
February 04, 2011 10:31PM
A bolt works well, but it takes a long time to get it up to temperature so that the spring doesn't simply unravel when released. The other thing is that you may find your fingers rather close to a source of live steam! My one attempt with this method was a heavy-weight line that was heated for an hour in a thermostatically controlled oven (120 Celsius is about right)

The other option is to wrap a bare copper wire around a wooden mandrel and then wrap the fishing line over that - it would heat much quicker than a bolt.

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.
Re: specific gravity
February 05, 2011 12:14AM
I was going to wrap the fishing line around a length of about 7/8"(~22mm) ready rod, secure both ends with hose clamps and bake it in an oven for awhile at about 150C or maybe use an electric heat gun to hot form it. I have also considered using a high e guitar string and wrapping it under tension around a mandrel say 3/8 to 1/2" (~10-12mm)in diameter. Its diamter will expand quite a bit when the tension is released but it should hold it shape. I am unclear about the length, assuming the longer the better, and the larger the diameter the better. Also, I'm not sure how to terminate the ends so the load stays as close to the centreline of the spring as possible. Hopefully, I'm way over thinking this.
avatar Re: specific gravity
February 05, 2011 04:57PM
150C will do the job, but keep an eye on the filament. A hot-air gun will fuse the filament rapidly and unevenly unless care is exercised. Monofilament, though, is cheap enough for experimentation.

Centering the load is relatively unimportant for a longer spring.

The actual length of the monofilament of the spring is important in that the longer the filament, the larger the displacement under load - however, you must use a rigid enough spring to ensure that it doesn't 'bottom-out' under tension.

You need to experiment, but ideally, you will have a good few milimetres change in extension between dry-weight and wet-weight of your specimen.

The physics of spring extension states that while the change in displacement under load doesn't significantly alter the geometry of the spring, then the displacement is proportional to the load.

What this means in practical terms, is that the spring should open only slightly (less than, say, 45 degrees) between neutral load (just the balance pan) and under maximum load (balance pan and specimen). Under no circumstances should readings be taken when the spring is over-extended, as the larger the extension, the lower the accuracy of measurement.

This limitation is reduced in impact in a longer spring by making the changes in extension larger - but at the expense of using a more rigid spring for larger specimens - a 5lbs monofilament spring will only be useful for a few grammes of specimen at most.

As an adjunct, it is possible to make a simple torsion balance using monofilament that will operate in exactly the same way as a Jolly balance. Whilst it is has a less linear callibration, it is a more robust piece of equipment that requires less head room.

Tonight's homework is to work out why the callibration is not linear.

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.
Re: specific gravity
February 06, 2011 12:36AM
I built the one in Sinkankas's book. It has been an invaluable tool. It's extremely accurate, relatively easy to build and inexpensive. I used a pica points rule instead of the graph paper suggested by Sinkankas, this makes it incredibly precise! Paper clips make good carriers and weights which can be selected and cut to different sizes according to specimen weight.
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Re: specific gravity
February 06, 2011 07:44PM
Nice job Rudy! Alysson's article also has plans to build a walking beam balance not unlike yours. I'm the kind that likes to build things perhaps even more than owning them. My toolbox and workshop drawers will attest to that. I decided to "Go Jolly" because I want to learn a bit more about springs and load deflection etc and I'm already wondering how I can use this info for another project. Once I nail down the basics, I will go my own way. Its all about keeping the mind ticking over, and, more importantly, having fun doing it! Having a useful instrument at the end of it all is a great bonus.
avatar Re: specific gravity
February 06, 2011 07:55PM
Rudy Bolona Wrote:
I built the one in Sinkankas's book. It has been an invaluable tool. It's extremely accurate, relatively easy to build and inexpensive. I used a pica points rule instead of the graph paper suggested by Sinkankas, this makes it incredibly precise! Paper clips make good carriers and weights which can be selected and cut to different sizes according to specimen weight.

That is a really good looking piece of equipment, Rudy. Sadly, I have neither the skill nor the patience to actually build a beam balance of that quality (which is why I use a chemical balance or a Jolly balance).

The beam balance, when well made, is incredibly accurate and consistent in use, and, by use of suitable adjustments to the size of the riders, is capable of a broad range of measurement. The limitations to the design are the friction of the pivot and the rigidity of the beam.

There is no limit to the choice of scale used on this type of balance provided that it is linear and is aligned properly with the pivot (the pivot should be at zero units). Indeed, a scrap metal merchant I once visited had a beam balance weighbridge scaled in hundredweights - that is 4" per Ton, and was capable of handling 20 Tons of scrap. (a beam of 80 inches length that is 6ft 8" or 2m).

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.
Re: specific gravity
February 06, 2011 08:59PM
Thank you for the kind words! I'm a wood floor installer by trade, so working with such materials is not much of a problem for me. Once built, this balance does require a learning curve. I have learned several tricks to getting good, consistent results. It must be calibrated and made sure it's level before each measurement. The balsam wood of the beam itself is quite sensitive to changing humidities. I use a simple compass to record balanced beam distance in air and match that setting in the submerged in water reading. Very accurate this way.
Re: specific gravity
February 07, 2011 01:27AM
I found an old analytical balance in an old photo studio years ago. The photographer had no further use for it so I built a double pan assembly for the left side, balanced it, added a beaker for DI water and it has served me well for 20 years! Quite repeatable and since D is essentially a "dimensionless" number, weights for the right pan can be anything - using the beam rider and chain assembly gives a good four-place reading, so far repeatable to three places. The agate fulcrum & pivots (knife-edges) make friction negligable.

Don S.
Re: specific gravity
February 07, 2011 05:45PM
I am sorry for machine translation.

I am taken by definition of density of minerals many years.
My councils:
1. For the big crystal almost any physical method on the basis of Archimedes law allows to define easily density +/-0.1 gram/sm3. All methods simple. Choose a method under scales available for you. The scale of 0,01 gram is sufficient.
2. Small and smallest crystals demand a special technique. Here careful, laborious work and other scale of scales is necessary.
3. For primary diagnostics accuracy +/-0.1 gram/sm3 is quite sufficient.
4. Accuracy above +/-0.01 gram/sm3 for mineral definition is superfluous. (You will see, than one crystal of the same mineral differs from another. Even if crystals seem faultless. This influence of micropores and inclusion other minerals.)

Re: specific gravity
February 07, 2011 07:48PM
Alexander, you are, of course, correct. +/- .1 is sufficient. +/- .01 better, if repeatable. Since I do three seperate determinations over two days (wet sample!) and average results, I'm looking for repeatability affected by initial moisture and or solubles in the sample. I go to +/- .0001 simply because it's easy and is procedural in other aspects of my work (assay). I guess you could say it has become habit!! I'm addicted!!

Don S.
avatar Re: specific gravity
February 09, 2011 07:55PM
For those tiny specimens, there is nothing better than a pycnometer, though for this method a consistent analytical technique is the deciding factor on the accuracy and reproducibility. A single drop of teepol or similar wetting agent per litre of distilled water is a must for this technique, since it relies on near-perfect wetting of small samples.

My article does explain the method and, more particularly, how to make a servicable pycnometer bottle.

I have seen this method used to find the specific gravity of a single, small sand grain using a tiny (0.5 ml) pycnometer bottle.

You will require a precision dry-pan balance (Jolly or beam balances are perfectly acceptable) for this method, and the mass of the bottle is irrelevant as it is weighed dry to start with.

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.

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