Which weighs more, an empty gas cylinder or the same cylinder filled with Helium?
It depends on three things:
- How heavy the gas cylinder itself is,
- What volume of helium is contained, and
- What pressure the helium is at.
Note, of course, that the cylinder with helium will always have more mass…
And the answer is: the filled cylinder, assuming you mean a cylinder filled with pressurized helium.
The “empty” cylinder actually is filled with air at standard temperature and pressure (STP). But the helium at pressure is denser. So the filled cylinder has more mass, and thus weighs more.
The buoyancy of both cylinders is the same, that is, they both displace the same amount of outside air, so they get the same amount of buoyancy lift.
If the gas cylinder is truly EMPTY, that means it’s full of nothing. It’s a sealed vacuum. So of course the cylinder with helium is heavier. 
Allow me to clarify, i.e, try desperately to recover from asking such a moronic question. Explain why a Helium cylinder at 2500 psi weighs more than the same cylinder after it has bled down to 200 psi. The cylinder weighs approx. 100 lbs. and its approx. volume is 30 gals.
I think that this
http://boards.straightdope.com/sdmb/showthread.php?threadid=62824
is proper answer for the OP.
Ah, this is a perfect time to offer one of my all-time favorite riddles.
Which weighs more, an ounce of lead, or an ounce of feathers?
Think about that for a second or two before coming to the obvious conclusion.
Think again. I’ts a trick question.
The answer: an ounce of lead. But why?
Feathers are weighed in conventional avoirdupois, while metals are weighed in Troy ounces. One Troy ounce is equal to about 1.01 avoirdupois ounce.
A given volume of gas (any gas) will weigh more at a high pressure than a low pressure simply because there are more gas molecules. More mass = more weight. There is nothing magical about helium that gives it “negative gravity”, such that more of it will be lighter.
Or to do the math:
For the 2500 psi tank, there will be:
2500 psi 3.78 l 4 g. 1 mole
-------- x 30 gal. x ------ x ---- x ------ =
14.7 psi gal mole 22.4 l
3443.9 g = 3.4439 kg.
of helium in it.
For the 200 psi tank, there will be:
200 psi 3.78 l 4 g. 1 mole
-------- x 30 gal. x ------ x ---- x ------ =
14.7 psi gal mole 22.4 l
275.5 g = 0.2755 kg
So the 2500 psi tank will “weigh” 3.1684 kg. (7 lbs) more.
[quote]
More mass = more weight. There is nothing magical about helium that gives it “negative gravity”, such that more of it will be lighter.
[quote]
Its probably worth mentioning that even a ballon full of helium weighs more then an empty one though its apparent weight will be lower.
[QUOTE]
*Originally posted by Ned *
**
Its mass will be greater, but its weight is less in an STP atmosphere. In a pure vacuum, though, yes the helium-filled balloon will weigh more.
Well, since weight is the gravitational force of attraction on an object its boyancy in air just makes it tough to measure.
i notice this is being argued on the front page today regarding the weight of clouds.
In a vacuum, a helium-filled balloon will pop.
Oooh, here’s an interesting question…say you take your everyday party balloon and put just a little bit of air in it – not even enough to stretch it tight (say you just found it lying there on the table, picked it up, and tied it off). Now you put it in a vacuum. Obviously, it will inflate, but will it be enough to pop the balloon? Presumably, the balloon pops when the pressure differential reaches some limit*, but how is that behavior affected by the outside pressure being zero?
*the function for when the balloon breaks might be more complex – it could, for instance, be not only a function of the pressure difference, but the necessary differential could vary as a function of the absolute pressures, as well. I’m not up on balloon physics, and didn’t want someone to pick this nit and distract from the point.