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[whaleswiththumbs]

That's a tricky one. Let me make a totally ignorant hypothesis:

Although the air has the same weight whether it's compressed or uncompressed, in its compressed form there is less upward pressure as it takes up less space. The inflatable life raft has say 4m^2 for the water to push against, compared to only say 20cm^2 for the canister. Since the weight of the water is the same at any point, objects with a larger surface area are more buoyant since they are pushed up by more water; conversely, when the total weight of the air is concentrated in one place, its weight is increased relative to the amount of upward pressure from the water, or more accurately there is less upward pressure of water lifting the air.

It's like diving into a pool: if you pin-drop, you'll sink very far, but if you land flat (belly flop), the water keeps you up, because you have a greater horizontal surface area.

That's my guess. Can anyone confirm or deny?

(I'm not that good at being a science nerd)
How do you weigh air?


I'd say that the person holding the tank was enough weight to hold the air down, and the weight of the canister helped some. I'm not sure what a zodiac is made of but that might not be the most float-able thing when uninflated.

 

[Johnmw]

A few people here have mentioned density. That's the crux of the matter.
When people say something is "lighter than air" they mean less dense than air. As the gas is compressed, it becomes quite dense, thus causing it to sink. Once the gas is allowed to expand, it becomes less dense. This makes its effective buoyancy greater, causing it to float.

Says my sleep deprived brain, at least.

This guy got it.

 

[ontherisess]

(I'm not that good at being a science nerd)
How do you weigh air?

By the atomic weight of the elemnents in it.

 

[TheMatt]

How do you weigh air?

You measure it's pressure (p), temperature (T) and volume (V) and with the Clapeyron's equation, you know the amount of O, N and other particles, which come in particular proportions and have a fixed weight (M), in mols .

pV=nRT
n=pV/RT

nM=m

ummmm..... I feel lost and alone after reading that. Wayyy over my head.

Quick, someone ask me something obscure about Star Wars so I can feel smart again!

 

[Lukeje]

How do you weigh air?

You measure it's pressure (p), temperature (T) and volume (V) and with the Clapeyron's equation, you know the amount of O, N and other particles, which come in particular proportions and have a fixed weight (M), in mols .

pV=nRT
n=pV/RT

nM=m

How is that related to the Clapeyron eq'n? That's
(dp/dT)[small]eq'm[/small] = (Enthalpy)/(T * Change in volume)

...you're thinking of the perfect gas eq'n.

 

[Zacharine]

How do you weigh air?

You measure it's pressure (p), temperature (T) and volume (V) and with the Clapeyron's equation, you know the amount of O, N and other particles, which come in particular proportions and have a fixed weight (M), in mols .

pV=nRT
n=pV/RT

nM=m

Let me dumb it down a bit.

The ideal function of gases states that pV=nRT, where p=pressure, V=volume n=the amount of particles, R=Rydberg's constant (a constant like the value of pi) and T=temperature.

We wish to calculate the mass of the air, or m.

We know that nM=m or the amount of particles times their atomic weight (M)= mass (m).

We know the precise mix of gases atmosphere or air is composed of and therefore can simply check the total atomic mass (or M) from the values given in the periodic table of elements. Therefore mass = a constant (the atomic weight of air) times the amount of particles.

We can calculate the amount of particles with the equation for the Ideal gas. pV=nRT, therefore n=pV/RT.

Pressure is nothing more than mass divided by an area. Industrial containers and the bottles used by divers have built-in pressure indicators. There are also hand-held pressure measuring tools.

The volume (or V) of the gas (air in this case) is simply the internal volume of the container (usually engraved to the side of an industrial container bottle or easily calculated by measuring the container).

R, as stated earlier, is a constant, with a value of approx. 8,31451 Joules/mol*Kelvin or 0,0831451 bar*cubic desimeter/mol*Kelvin.

Temperature is simply the temperature the gas is in (measured in Kelvins in this case).

Now we have it. mass = atomic weight of air * the result of pV/RT. Input values to your calculator and you have an answer.

 

[whaleswiththumbs]

How do you weigh air?

You measure it's pressure (p), temperature (T) and volume (V) and with the Clapeyron's equation, you know the amount of O, N and other particles, which come in particular proportions and have a fixed weight (M), in mols .

pV=nRT
n=pV/RT

nM=m

You are the true science nerd here.

 

[Spacelord]

How come "compressed" air does not float? I mean, does compressing it actually also minimize its buoyancy? If so, how? Even though it occupies less space, is it not the same volume of air with the same properties?

Compressed air tanks sink because they do not displace water mass equal or greater than its own, which is how you make stuff float. Eureka!

Edit: mass stays the same, but the volume decreases (hence, 'compressing'), increasing its density. When it is denser than water it sinks. When it is not, it floats. I just thought I'd reiterate because in what I posted earlier I kinda explained it awkwardly, I think.

 

[DrunkenKitty]

Things float based on a ratio of weight to surface area.

 

[jasoncyrus]

It's not much of a trick, its simple math.

The canisters wieght a crap load>< You ever try lifting one? those things are HEAVY. Heavy duty steel made of 2 pieces, the canister and the valve. Thats it, no other seems. just one big giant big of steel.

 

[quiet_samurai]

Swamp Gas....