Does a tank of helium weigh more or less when empty?

March 24, 2007 6:43 PM

Does a helium tank weigh more or less after it is empty?

I've been turning this over in my head all day and short of buying a helium tank to test this out, haven't really found any satisfactory answers online either. Would a tank of helium weigh more or less when it is empty?

posted by perpetualstroll to science & nature (13 comments total)

Empty = full of air? Then it would weigh more. If empty means a vacuum, it would weigh less.
posted by TonyRobots at 6:50 PM on March 24, 2007

It would weigh less as typically the contents would be under pressure.

http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/rcs/press.html
posted by jkaczor at 6:51 PM on March 24, 2007

Clearly it will be lightest when it has nothing in it. It's heavier when it contains a nitrogen/oxygen mix at one atmosphere than it is when it contains helium at the same pressure. It's heaviest still when it contains helium above some pressure, but what that pressure is I couldn't tell you.
posted by Mr. President Dr. Steve Elvis America at 6:51 PM on March 24, 2007

weighs less. see the prez. Elvis above
posted by defcom1 at 6:52 PM on March 24, 2007

Mr. President Dr. Steve Elvis America (who also has a great nick from a fantastic flash cartoon) has it right:

The weight will be highest when it contains He at high pressure. As the pressure is reduce, the weight will decrease. If you have a one-way valve, then the tank will never truly be empty, but the tank pressure will approach the pressure on the other side of the valve. At this point, it will be at its lightest.

If you allow air to start flowing into it (e.g. no one-way valve), it will start getting (very slightly) heavier again.
posted by JMOZ at 7:03 PM on March 24, 2007

The threshold pressure must be N atm, where N = (mass of 1 liter air)/(mass of 1 liter of He)
posted by DU at 7:14 PM on March 24, 2007

avg. atomic weight of air = 28
atomic weight of helium = 4
atmospheric pressure is 14.7 psi

define empty. a quick search shows helium for balloons in 2,000 psi tanks. despite the high pressure the helium weighs almost nothing by comparison with the metal required to retain that pressure. nevertheless it weighs about 136 time more than the volume of air at atmospheric pressure. as the pressure drops the tank will get lighter until the pressure reaches atmospheric when it will stop flowing. if you have no one way valve and leave the tank open eventually air will diffuse in and helium will diffuse out, especially if the valve is at the top of the tank it will rise out due to its buoyancy. the tank will then get heavier. when it is all air it will be as heavy as it was when the pressure was seven times atmospheric, which is 102.9 psi, or 88.2 psig, or something like that.
posted by caddis at 9:38 PM on March 24, 2007

I think people are missing the point. A compressed-gas tank doesn't automatically get filled with air as you release gas from it. It just has less of the gas, at a lower pressure. (When the pressure inside and outside the tank are equal, no more flows out easily, so you think of it as empty.)

When a helium tank is spent, it can be assumed to contain mostly helium, at atmospheric pressure. (Okay, if you leave the valve open for long enough and it doesn't have a check valve in it, some air will migrate in, but we'll ignore that.)

So when the tank is full, it's charged with something like 2000psi of helium.

When it's empty, it's 1atm, or 14.7psi, of helium.

You can do the ideal gas law if you want (PV=nRT), but the full tank is going to be heavier. It has to; it's conservation of mass.
posted by Kadin2048 at 11:58 PM on March 24, 2007

Kadin2048: I think I addressed that in my post. It really depends on whether there's any sort of one-way valve and how you empty it.

If you simply open the valve and leave it open, it won't stop at 1atm of He. Instead, when the He level gets to 1atm, you'll start having diffusive exchange of He and N2/O2/misc. In essentially no time, you'll have 1 atm of air. But, if you want to ignore that, you can.

I think for the poster's purposes, the answer everyone has given is correct: it will be heavier when full than empty.

Oh, and b1tr0t: nah. You can not believe in atoms and believe that the weight of the tank magically changes. I'll call that the "Art history major approximation." [NOT COLLEGE-MAJORIST]
posted by JMOZ at 12:09 AM on March 25, 2007

I think people are missing the point.

What people? I thought JMOZ was pretty clear above too.
posted by vacapinta at 1:15 AM on March 26, 2007

He was.
posted by Kirth Gerson at 8:36 AM on March 26, 2007

From actually having to move a helium tank, it's lighter when it's empty. They're pretty darn heavy when they are full.
posted by debit at 1:12 PM on March 26, 2007

From actually having had to move dewars of liquid Helium, they are much, much heavier when full. I don't think that's really what the question was about, though.
posted by Kirth Gerson at 3:32 PM on March 26, 2007