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accurate&precise measurement of volume w/o mass/weight

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Hon1nbo

Wed Dec 10 2008, 03:35AM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

the title says it: what is the most accurate/precise method, available to the hobbyist, of measuring volume (a sphere for example), without using any measurements of mass/weight or contaminating the object (and it must be pretty dang accurate, to say for a sphere to +/- 0.00001 meter for radius or diameter...
I thought about a laser setup that uses a high resolution sensor, or film, to make the edges outlined that can then be measured by other means...

...

Wed Dec 10 2008, 03:46AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

We need a lot more information... If you know it is a sphere you can try to measure the diameter, but chances are that if you are trying to measure down to the nearest um that sphere isn't so round anymore.

If you need an accurate volume the only thing that I can think of that a hobbyist can get a hold of is displacement, use a liquid that doesn't evaporate quickly (water would probably be fine) and measure the volume that is displaced. You could use a container that is overflowing and measure the mass of the overflow which would get you to a decent number of sig figs at least. Getting past surface tension would be tricky, but I bet you can come up with some way to deal with it...

But in short, there isn't an easy way to make um tolerance measurements. Even a light setup would be tricky, since the wavelength of even UV light is about .5um

Proud Mary

Wed Dec 10 2008, 04:20AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The displacement of water in a measuring cylinder is probably the easiest way if water is not "contamination."

If the solid is known to be a regular solid, such as a sphere, you could measure it with a micrometer and then use simple geometry to calculate its volume.

No doubt there are more complex, costly, modern, methods involving lasers as has been suggested, but is the "hobbyist" of your question likely to have them?

Hon1nbo

Wed Dec 10 2008, 04:29AM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

sorry, typo
it was meant to be 0.00001 meter (fixed)
the thing about displacement is that I need to measure the mass if the liquid, since using anything graduated would be a problem
I did just realize typing tv last line though, if I used a gas at known temperature and volume (which I can get a precisely sized container), I might be able to acurratly find V displaced by using two chambers, one with N2 at STP and the other, a vacuum. Knowing the gas is at STP, I could deduce when the gas is pumped into chamber two (so now I can open chamber one and not change my amountof N2). I then place the object inside and form a vacuum. Then I use the vacuum pump to resinsert the gas into chamber one and, since the temperature is constant, find the change in pressure by a pressure sensor, or some moving object whose position varies with force applied by gas and measuredby light pulses...

Proud Mary

Wed Dec 10 2008, 06:40AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Surely the "hobbyist" is unlikely to have ready access to such means, and that the question is just a way of trying to see if you understand the fundamental formula that density = weight/volume.

Bored Chemist

Wed Dec 10 2008, 06:49AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

How big is the thing?

Hon1nbo

Wed Dec 10 2008, 01:32PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

Bored Chemist wrote ...

How big is the thing?

small, within an inch

and the main issue is that I cannot use weight or mass in my measurements, as the point is to use Avogadro number (and with a chemically near pure crystal given to us) to find mass

Steve Conner

Wed Dec 10 2008, 02:59PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

This sounds like one of these "What are you really trying to do" threads, where the actual problem needing solved is completely different to the question being asked, because of some unquestioned wrong assumption made earlier.

Bored Chemist

Wed Dec 10 2008, 06:53PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

If it's a fairly rigid ball then measuring it lots of times with a micrometer then calculating the volume from the diameter would give a pretty good answer (albeit a rather dull one)

Hon1nbo

Wed Dec 10 2008, 10:58PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

I guess a micrometer would have to do... do they come in digital or do I need a magnifying glass to read the scale lol
the point of this project is to be given n ACS/reagent grade crystal, which will already be shaped (a cube seems more proper to order, or a sphere if i really take my time with letting the machine run on its own or order one), then (knowing it's unit cell size) to find it's volume, and there for the rough (in field of study terms) number of unit cells present: I expect to be way off, but in the end since I am using this number to find mass, it wouldn't be off by THAT much in the final, desired value...

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