Specific gravity test for specimens and minerals

[Hawkear]

Just working through the Betts Specific Gravity Test Greenhornet linked to in his post on “OLD LEAD PIECE"
At first I didn’t quite believe that it would work then afterwards just felt so dumb that I had been doing the SG test the hard way for all my prospecting life.
The difficulty I had with the method, was how could the weight of a bowl of water sitting on a set of scales increase just by dangling or holding something submerged in it. Even a ping pong ball for that matter, It just didn’t seem logical.
A quick trip to the kitchen, filled a small bowl with water, set it on some scales, stuck my finger in and lo and behold the weight increased even though I did not touch the bottom or sides of the bowl.
Googling this phenomenon, it seems that the explanation is that the scales are not measuring the weight of the water directly but the force caused by the pressure of the water exerted on the bottom of the container. Whenever something is put in the water which causes its level to be raised, additional hydrostatic pressure is created at the bottom and it is this increase in force that is transferred to the scales via the container walls.
So even if you stick a finger, ping pong ball or a gold speci in the water to increase the water level an increase in weight will be seen on the scales
Furthermore this increase in pressure will logically be proportional to the volume of water raised ie the volume of the object submerged.
So no more tricky and fiddly devices to suspend a specimen from a set of digital scales in the air while a cup of water is brought up from below.
Just my morning cup of coffee sitting on a set of scales, a thin bit of stiff wire to dangle my speci in it and I can have all the info I need to calculate the SG.

 

[grubstake]

As referred to above:
http://www.johnbetts-fineminerals.com/jhbnyc/articles/specific_gravity.htm

 

[Nightjar]

Just working through the Betts Specific Gravity Test Greenhornet linked to in his post on “OLD LEAD PIECE"
At first I didn’t quite believe that it would work then afterwards just felt so dumb that I had been doing the SG test the hard way for all my prospecting life.
The difficulty I had with the method, was how could the weight of a bowl of water sitting on a set of scales increase just by dangling or holding something submerged in it. Even a ping pong ball for that matter, It just didn’t seem logical.
A quick trip to the kitchen, filled a small bowl with water, set it on some scales, stuck my finger in and lo and behold the weight increased even though I did not touch the bottom or sides of the bowl.
Googling this phenomenon, it seems that the explanation is that the scales are not measuring the weight of the water directly but the force caused by the pressure of the water exerted on the bottom of the container. Whenever something is put in the water which causes its level to be raised, additional hydrostatic pressure is created at the bottom and it is this increase in force that is transferred to the scales via the container walls.
So even if you stick a finger, ping pong ball or a gold speci in the water to increase the water level an increase in weight will be seen on the scales
Furthermore this increase in pressure will logically be proportional to the volume of water raised ie the volume of the object submerged.
So no more tricky and fiddly devices to suspend a specimen from a set of digital scales in the air while a cup of water is brought up from below.
Just my morning cup of coffee sitting on a set of scales, a thin bit of stiff wire to dangle my speci in it and I can have all the info I need to calculate the SG.

Bill & Udo, WA, put this video together.
https://www.gold-prospecting-wa.com/gold-in-quartz.html

 

[Hawkear]

I think the essence of the Betts method is that instead of weighing a specimen while it is suspended in a container of water, you simply weigh the container of water it is suspended in. Much simpler.

 

[user 4386]

I think the essence of the Betts method is that instead of weighing a specimen while it is suspended in a container of water, you simply weigh the container of water it is suspended in. Much simpler.

Yes, that is the method I outlined nearly five years ago (Apr 13 2018) in :

Series on identifying minerals - part 8 SPECIFIC GRAVITY

Except that on my digital scale I simply zero out the container full of water before weighing the specimen in it. So I directly get a figure for the specimen in air, then one in water.

Otherwise it is just Archimedes Principle "the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object".

i.e. pressure at depth in a fluid =rho (Greek symbol) x g x h where rho =1 for water (a constant, its density), g is the gravitational constant (so does not vary) and h the depth (a variable). The last is therefore the only thing varying, and the pressure at the bottom of the specimen is greater than the pressure at the top of the specimen, because pressure increases with increased depth. Pressure is force per unit area, so this difference is the bouyancy force (per unit area) which makes it appear to weigh less in water than in air because it will be greater at the bottom of the specimen than at the top (an upward force).

I think.....

 

[Greenhornet_au]

Just working through the Betts Specific Gravity Test Greenhornet linked to in his post on “OLD LEAD PIECE"
At first I didn’t quite believe that it would work then afterwards just felt so dumb that I had been doing the SG test the hard way for all my prospecting life.
The difficulty I had with the method, was how could the weight of a bowl of water sitting on a set of scales increase just by dangling or holding something submerged in it. Even a ping pong ball for that matter, It just didn’t seem logical.
A quick trip to the kitchen, filled a small bowl with water, set it on some scales, stuck my finger in and lo and behold the weight increased even though I did not touch the bottom or sides of the bowl.
Googling this phenomenon, it seems that the explanation is that the scales are not measuring the weight of the water directly but the force caused by the pressure of the water exerted on the bottom of the container. Whenever something is put in the water which causes its level to be raised, additional hydrostatic pressure is created at the bottom and it is this increase in force that is transferred to the scales via the container walls.
So even if you stick a finger, ping pong ball or a gold speci in the water to increase the water level an increase in weight will be seen on the scales
Furthermore this increase in pressure will logically be proportional to the volume of water raised ie the volume of the object submerged.
So no more tricky and fiddly devices to suspend a specimen from a set of digital scales in the air while a cup of water is brought up from below.
Just my morning cup of coffee sitting on a set of scales, a thin bit of stiff wire to dangle my speci in it and I can have all the info I need to calculate the SG.

Being a stand up guy, I will expose that the links I provided for SG testing, did not work properly, so mbasko went to the trouble to
swap my link out for the "Betts Specific Gravity Test", and PM'd me to advise.

So cudo's to him, and I am in total agreement with that action, the Betts is clear and simple.

 

[mbasko]

Betts Method has been around for awhile.
First came to my attention in 2016 after Heatho posted about it: https://www.prospectingaustralia.com/threads/gemstone-and-mineral-identification.17698/
Not that it's a competition on who put it up first - the more informative type posts/links to good info like this around the forum, the easier it is for members to find it.
John Betts deserves all the cudos for putting together an easy to follow method with video.

 

[Hawkear]

Unfortunately sometimes good information gets buried in the depths of time and other threads.
Accurate measurement of SG is such an important thing for mineral identification and determining gold content in specis, that however it comes about, it is worthwhile to put out this information again every now and again for the benefit of newbies and people like me who just didn’t pick up on those earlier threads.
Just wish I’d picked up on Goldierocks, Heatho and probably others members’ threads about it years ago.