How much does air weigh?

[Jules]

I am sure someone can tell me how to work this out. Just curious becuase Noel and Iwere discussing whether my dumpty 12s are heavier than his or not. His weigh 17 kg empty. Mine weigh 19.5 kg and have about 80 Bar in. When you are calculating weight how much does air weigh at certain pressures?
Can some one explain how you would work it out.

I am curious because my twin 10s weigh about 28 kg (again with about 100 bar in) and i was wondering what the weight with a full fill would be. I could always get them filled and find out empiracally then i would know but that qould be cheating.

jules

[beanie]

<font color='#8D38C9'>Air has a density of 0.0012 kg/litre.
The air in a full cylinder weighs 0.0012 x 80 bar x 12.2 litres = 1.2 kg.

calc

[turbanator]

IIRC, your cylinders should have the weight empty stamped on the neck, however this wil not include the valves.

If I can remember about20-25 years back.
The molecular weight of O2 is 16g/mole and N2 is 14g/mole.
1 mole of free gas occupies 24 liters
Air is 20% O2 80% N2
Cylinders are 12 litres pumped to 232 bar.

There, that should get you started

The thing I can never remember is:

is the 16g/mole for O or O2, I think it's O,
so 24 l of O2 weighs 32g and N2 28g.

Oh well, may as well carry on now.

12x232 = 2,784l
556.8L/24 of O2 x 32 g = 742.4g of O2
2227.2L/24 of N2 x 28g = 2598.4 of N2

=3340.4 g or 3.34kg.
I think the BS-AC SD manula says about 4-6 pounds,
so that *seems* right.

[Jules]

so the 80 bar in the tank weighs 1.2kg which means that the tank empty would be about 18.5kg which is really heavy compared to those ones on the link beanie posted.
Serves me right for buying cheap tanks!

[beanie]

<font color='#8D38C9'>yeah my 10s are 11.1kg without the valve (stamped weight)

[Finless]

I have a spreadsheet (MS XLS) thingy to work this kind of stuff out (sent to me by Rob YBOD Evans, I believe) which I have never actually used.

I would be happy to forward it on to anyone who is interested. To potentially save me time later if anyone wants a copy pls note - I think you need to run the spreadsheet with &quot;macros enabled&quot; (question is asked as the spreadsheet loads).

I also remember Dominic posting a link to somewhere that listed weights empty/full although that might have only been for Fabers (that helps doesn't it   ).

[Conor]

Originally Posted by turbanator

IIRC, your cylinders should have the weight empty stamped on the neck, however this wil not include the valves.

If I can remember about20-25 years back.
The molecular weight of O2 is 16g/mole and N2 is 14g/mole.
1 mole of free gas occupies 24 liters
Air is 20% O2 80% N2
Cylinders are 12 litres pumped to 232 bar.

There, that should get you started

The thing I can never remember is:

is the 16g/mole for O or O2, I think it's O,
so 24 l of O2 weighs 32g and N2 28g.

Oh well, may as well carry on now.

12x232 = 2,784l
556.8L/24 of O2 x 32 g = 742.4g of O2
2227.2L/24 of N2 x 28g = 2598.4 of N2

=3340.4 g or 3.34kg.
I think the BS-AC SD manula says about 4-6 pounds,
so that *seems* right.

(corrected cos I was a muppet with atomic weights)

Sorry to bump an old post, but am interested in the content and not sure if and answer was reached.

The above is nearly spot on.

Vol of 1 mol of gas = 22.4 dm3
O2 = 2x16 gmol-1 = 32
N2 = 2x14 gmol-1 = 28
Air = (0.79x32)+(0.21x28)g (per 22.4dm3) = 31.16 per 22.4dm3 = 1.391gdm-3 = 1.391 gl-1

weight of cylinder at 1 ATM = mass of metal
weight of cylinder at 2 ATM = mass of metal + mass of cylinders volume of air

weight of air in cylinder= volume of cylinder x [(pressure of air -1)x1.391]

So for the original example, a 12L at 80bar;

12x(79x1.391) = 1318.668g

For the total mass of air in a 230 bar fill for a 12L;

12x(229x1.391) = 3822.468g

So a full 12L should way ca 4kg more than an empty one (Does this seem low?)

By extension, the weight change between start and end of a dive, from 230bar to 50 bar = mass of 12L of air x 180
12x(180x1.391) = 3004.56g

Around 3kg should be added if a buoyancy check was conducted at 230 bar.

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Feel free to correct my maths or science, I was doing this out of idle curiousity

Conor