It is with great sadness I'd like to announce Dr. Hanneman passed away on December 12, 2020. His legacy will live on forever!
Welcome to the GemologyOnline.com Forum
A non-profit Forum for the exchange of gemological ideas
It is currently Sun Jan 17, 2021 9:36 pm

All times are UTC - 4 hours




Post new topic Reply to topic  [ 30 posts ]  Go to page Previous  1, 2
Author Message
 Post subject:
PostPosted: Fri Mar 21, 2008 12:13 pm 
Offline
Gold Member

Joined: Mon May 14, 2007 11:00 am
Posts: 1133
Location: Monterey, CA
While density and specific gravity are indeed two different measurements, the numerical values are the same if one reports results to only two decimal places. Otherwise, one must specify the temperature at which the measurements are made.

Insofar as few gems are "pure" and isomorphic substitution is rampant, the third decimal place is useless for identification purposes.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Tue Mar 25, 2008 9:07 am 
Offline
Established Member

Joined: Tue Mar 25, 2008 8:37 am
Posts: 15
Location: various
Hi everybody,

IMHO capybara's idea works.
Desertgems said
Quote:
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.

yes, this apparatus is called a 'pycnometer'
see for exemple http://www.uic.edu/classes/cemm/cemmlab ... ravity.pdf
It is usually used for crushed materials but some bottles are fitted with a wide opening so that large pieces can fit in, see e.g.
http://www.educnet.education.fr/rnchimi ... _ts1_2.pdf
very helpful particularly for those who can read french, or for a picture:
http://www.uni.edu/~rachor/Pycnometre1.gif

Yep, it works!


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Tue Mar 25, 2008 2:10 pm 
Offline
Gemology Online Übergod
User avatar

Joined: Mon Oct 17, 2005 2:39 pm
Posts: 3530
Location: San Francisco Bay Area, CA
Wikipedia on the pyknometer.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Tue Mar 25, 2008 2:19 pm 
Offline
Platinum Member

Joined: Sun Oct 16, 2005 12:42 pm
Posts: 2591
Hi Gravillon and welcome,

Since you are so sure it works, could you give us an empirical example related to gemstones?

_________________
Proud to be a DSN and JTV shopper, just love the guys!


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Tue Mar 25, 2008 4:05 pm 
Offline
Valued Contributor
User avatar

Joined: Sun Jul 22, 2007 5:58 pm
Posts: 295
Location: California desert
Hi Gravillon,

I had never seen this tall large pycnometer ( pyknometer) container before. Although the photo angle might distort its size, I would guess it would weigh empty several hundred grams, and of course filled with water and a gem , much much more. Thus it would be usable on a large sample mass, but it would be an unusual balance to be able to do significant figures on a sample only a few carats in size, IMO. Of course, this is hypothetical, as I do not know the actual size and weight of the container. I would be interested in knowing the specifications of the large container. A short squat 15 ml container with ground glass-capillary tube type stopper would be my ideal :)

Oh, and Welcome !

Jim :)


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Mon Apr 14, 2008 8:22 am 
Offline
Platinum Member
User avatar

Joined: Tue Jan 02, 2007 8:20 pm
Posts: 2658
Desertgems version of Capybara's plan B will give the correct result.

It is a correct rendition of the pycnometric method.

It will work correctly if the overflowing steps are not done on the platform of the scale and the outside of the beaker is kept dry.

They have determined the weight of the water in the full beaker and the weight of the water in the beaker after overflowing it by the volume of the stone. Since the density of water is close to one this is the volume of the stone.
The stone volume determination can be made either by overflowing the water drying the outside and reweighing the beaker which has now lost a volume of water equal to the volume of the stone. Or the volume of the stone can be determined by the weight of the water in the beaker with the stone in it minus the weight of the stone. (The weight of the empty beaker is always tared off)

The stone was weighed separately. Weight divided by volume equals density.
If reasonably clean water is used the density and the SG are close enough for government work.

Using an Ohaus triple beam and beaker will of course limit the size on the small end, but the method is correct. The beaker is not as easy to fill unifromly as a pycnometer. But pycnometers have necks that limit the diameter of stones to be put in them. This kind of measurement can also be done with a graduated cylinder, especially one with a squat aspect ratio.

When I first got on fleabay a guy whose handle was "retiredAZgemologist"
was selling off all his equipment. He had a pycnometer that solved both problems. Unfortunately I did not win it. I am going to build one of those.

The one in Gravillon's last link is the one that is needed.
I have a couple as in the earlier links. They are limited by neck size.
The one in the picture does not weigh several hundred grams. I would say about 30grams. I have some similar weighing bottles that dont have the tubulation for filling to an exact volume. With a good analytical balance you could do SGs by both the hydrostatic and the pycnometric method and thus have two datapoints which would be handy since uncertainty goes up as stone size goes down.(As Dr. Bill Hanneman pointed out decades ago!)


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Fri May 29, 2009 10:12 am 
Offline
Active Member
User avatar

Joined: Tue Apr 07, 2009 12:57 pm
Posts: 96
Location: Mumbai, India
Hey...I have racked my brain a lot over this...the best and most suitable method is to take a long glass tube with one open end and one closed with diameter just of the size of the stone..now take a red/colored tape and wind it round the glass tube so that you get a marking to see the level of water.Then do the following steps..
1) weight the stone in air eg. 1.00 cts
2) weight the tube with the lower meniscus of water just touching the marking.(you can use a syringe to adjust the water level) eg. 35.00 cts
3) slowly put the stone in the glass tube with water without spilling any water out.NOW AGAIN WITH THE SYRINGE REMOVE THE EXCESS OF WATER WHICH HAS OVERFLOWED DUE TO IMMERSION OF THE GEM...IE. BRING THE WATER LEVEL BACK TO THE MARKING BBY REMOVING SOME WATER.
4) Now weight this setup it will be less than weight from step one and two added together...ie.
35+1=36
but the actual weight will be 35.5 etc.
5) now subtract 35.5 from 36...this is the loss in weight in water ie. 0.5
6) now use this formula for s.g
S.G.= WEIGHT OF STONE IN AIR/LOSS IN WEIGHT IN WATER
=1/36-35.5
=1/0.5
=2
This is a reliable and accurate enough method to find the s.g.

_________________
Exotic Gemstones: All Natural & Untreated
The Star Ruby Shop


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Fri May 29, 2009 1:44 pm 
Offline
Moderator
User avatar

Joined: Thu Dec 13, 2007 3:42 pm
Posts: 4086
Location: the Netherlands
The things people come up with... :wink:

I like my good old hydrostatic balance.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sat Mar 13, 2010 8:48 am 
Offline
Gold Member

Joined: Sun Jun 28, 2009 9:56 am
Posts: 1256
Quote:
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.


I think that's because the term for the thing differs from one language to another.
The french term 'densité' (which my english dictionnary will erroneously translate into 'density') actually is the equivalent to the english term 'specific gravity'.
The english term 'density' would have to be translated into the french term 'masse volumique' (mass per volume)

That might be a source of confusion for the forum members which aren't aware of this translation trick. I guess the problem might be the same in many other languages.


Personally, I'm struggling to find the english equivalents for stone inclusions french descriptive terms. How the heck do you say a 'givre' (literally frost) in english?! :lol:


Top
 Profile  
Reply with quote  
 Post subject: Re: Measuring Specific Gravity [SG] - would this idea work?
PostPosted: Fri Oct 04, 2013 5:16 am 
Offline
New to the Forum or The Quiet Type

Joined: Mon Sep 02, 2013 11:46 am
Posts: 5
two questions

1. how to correctly make your own SG attachment
2. when the stone is suspended in water does the suspended stone holder supposed to immerse in water or floats in water . if immerced will it make any difference to SG or weight is adjusted before weighing.

thanks
Pal


Top
 Profile  
Reply with quote  
 Post subject: Re: Measuring Specific Gravity [SG] - would this idea work?
PostPosted: Fri Oct 04, 2013 10:53 pm 
Offline
Site Admin
User avatar

Joined: Sun Oct 16, 2005 12:22 pm
Posts: 20554
Location: San Francisco
Why don't you read through this thread:
viewtopic.php?f=37&t=15506

Might help.


Top
 Profile  
Reply with quote  
 Post subject: Re: Measuring Specific Gravity [SG] - would this idea work?
PostPosted: Wed Oct 16, 2013 2:19 pm 
Offline
Gold Member

Joined: Sun Jun 28, 2009 9:56 am
Posts: 1256
When using a pycnometer, one still need to weigh the stone with a scale so to calculate SG, therefore I think methods that require nothing but a scale are more practical than using a pycnometer.

There are actually two easy methods for specific gravity measurements using nothing but a scale. It's important to note that the formula for SG calculation will be different depending on the method used:

Image

Method 1:
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 (also make sure that the gallow and basket are not making any contact with the glass). Commercially available SG kits are based on this method, but you can also build one yourself.
With method.1 SG = weight of the stone in air / (weight of the stone in air - weight of the stone in water)

Method 2:
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).
With method.2 SG = weight of the stone in air / weight read when the stone is immersed in water

Let's note that 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), and it's also best not to use a small diameter glass.


Last edited by cascaillou on Wed Oct 23, 2013 11:08 am, edited 4 times in total.

Top
 Profile  
Reply with quote  
 Post subject: Re: Measuring Specific Gravity [SG] - would this idea work?
PostPosted: Thu Oct 17, 2013 3:33 pm 
Offline
Gold Member

Joined: Mon May 14, 2007 11:00 am
Posts: 1133
Location: Monterey, CA
cascaillou wrote:
There are actually two easy methods for specific gravity measurements using nothing but a scale. It's important to note that the formula for SG calculation will be different depending on the method used (although I'm not so comfortable with physics, I'll try to explain why):

Why don't you simply use a Hanneman Balance. Then, you need NO weights, NO scales, and NO calculations.
It might even be cheaper. One can even build one youself to measure any sample weighing from ten pounds down to 10 points. :D :D


Top
 Profile  
Reply with quote  
 Post subject: Re: Measuring Specific Gravity [SG] - would this idea work?
PostPosted: Thu Oct 17, 2013 7:44 pm 
Offline
Gold Member

Joined: Sun Jun 28, 2009 9:56 am
Posts: 1256
your balance is a clever device but it's a large beam scale, and I like to keep my equipment portable.

Here is my smaller SG kit:
Image
Once dismantled, the scale and all the parts of the kit will fit inside the cigarettes packet.

Of course, such a tiny 20$ scale (0.01g accuracy) is low quality and poorly reliable, thus as it comes to SG measurements, it can't be trusted for stones under 15ct (3grams).
Indeed, from experimentations with this setting, maximum error for 15ct stones with sg of 2.6 was a bit less than 1% while maximum error for 15ct stones with sg of 4.9 was a bit less than 3%.


Last edited by cascaillou on Wed Oct 23, 2013 11:01 am, edited 2 times in total.

Top
 Profile  
Reply with quote  
 Post subject: Re: Measuring Specific Gravity [SG] - would this idea work?
PostPosted: Fri Oct 18, 2013 10:04 pm 
Offline
Gold Member

Joined: Mon May 14, 2007 11:00 am
Posts: 1133
Location: Monterey, CA
cascaillou wrote:
your balance is a clever device but it's a large beam scale, and I like to keep my equipment portable.
Once dismantled, the scale and all the parts of the kit will fit inside the cigarettes packet.

Of course, such a tiny 20$ scale has poor accuracy and as it comes to SG measurements, it can't be trusted for stones under 15ct (3grams).

Your kit is also a clever device-- for mineral specimens.
However, I really don't believe it has much to offer for gem identification. It is two orders of magnitude too coarse. :(


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 30 posts ]  Go to page Previous  1, 2

All times are UTC - 4 hours


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
Gemology Style ported to phpBB3 by Christian Bullock