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

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

perpetualstroll — Sat, 24 Mar 2007 18:43:17 -0800

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?

By: TonyRobots

TonyRobots — Sat, 24 Mar 2007 18:50:12 -0800

Empty = full of air? Then it would weigh more. If empty means a vacuum, it would weigh less.

By: jkaczor

jkaczor — Sat, 24 Mar 2007 18:51:11 -0800

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

http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/rcs/press.html

By: Mr. President Dr. Steve Elvis America

Mr. President Dr. Steve Elvis America — Sat, 24 Mar 2007 18:51:16 -0800

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.

By: defcom1

defcom1 — Sat, 24 Mar 2007 18:52:54 -0800

weighs less. see the prez. Elvis above

By: JMOZ

JMOZ — Sat, 24 Mar 2007 19:03:04 -0800

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.

By: DU

DU — Sat, 24 Mar 2007 19:14:35 -0800

The threshold pressure must be N atm, where N = (mass of 1 liter air)/(mass of 1 liter of He)

By: caddis

caddis — Sat, 24 Mar 2007 21:38:40 -0800

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.

By: Kadin2048

Kadin2048 — Sat, 24 Mar 2007 23:58:56 -0800

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.

By: JMOZ

JMOZ — Sun, 25 Mar 2007 00:09:24 -0800

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]

By: vacapinta

vacapinta — Mon, 26 Mar 2007 01:15:50 -0800

I think people are missing the point.

What people? I thought JMOZ was pretty clear above too.

By: Kirth Gerson

Kirth Gerson — Mon, 26 Mar 2007 08:36:26 -0800

He was.

By: debit

debit — Mon, 26 Mar 2007 13:12:32 -0800

From actually having to move a helium tank, it's lighter when it's empty. They're pretty darn heavy when they are full.

By: Kirth Gerson

Kirth Gerson — Mon, 26 Mar 2007 15:32:16 -0800

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.