This question has bugged me for a long time.  From practical
observation, if you get a tank of helium, and then use up all the
helium filling baloons, the empty tank weighs more than the tank did
when it was full of helium.  This doesn't seem to make sense.  More of
something should weigh more, not less.  Why does the empty tank weigh
more than the tank full of helium?

---

Clarification of Question by nathan_brunner-ga on 04 Nov 2005 16:51 PST

I needed the balloons for a party.  I rented a tank full of helium (or
whatever the standard helium/air mixture is that goes in to helium
baloons).  The tank was a standard tank of compressed air, similar to
what a diver would use, and had an attachment for filling the
balloons.  After filling the baloons, there was obviously less
pressurized gas in the tanks, but the tank appeared noticeably
heavier.  Nothing replaced the helium in the tank, as the tak was
still sealed, with lower internal pressure.

Your "practical observation" must be mistaken. The weight on a scale
is the weight of the container, plus the weight of the gas inside it,
minus the weight of the air displaced. If the tank does not expand in
size when there is more helium in it (like a balloon), then the volume
of air it displaces is fixed and the more helium in the tank, the more
it weighs.

hm... let me think about this.  this is just a brainstorm, so it's
probably wrong, but:

the density of atmospheric air (containing O2, N2, CO2, all that good
stuff - heavy molecules compared to helium!) is about 1.3 g/L.  Let's
assume that's at STP (standard temperature and pressure.  it's not,
because it's not at 0 degrees C, but whatever.  this is just an
approximation).

Now, if helium gas is approx. 4 g/mol, then at STP, given 22.4 L/mol,
the density is about 0.18 g/L.  In order for helium gas to have the
same density as air, its molar volume must be 3.1 L/mol, which is a
pressure of about 7.9 atm... which I think is about 116 PSI.  So if
the pressure of the compressed helium in the tank is less than that, I
could see how what you're saying might be true...

Heh, it's 1:15 in the morning, I bet there's a HUGE flaw in that
reasoning somewhere :-)  Oh well, it's just a thought.

Again, if the container is always the same size, it's bouyancy is a
constant, the weight of the displaced air. The more helium I put into
the container, the more it weighs.

Where's harryfshaw-ga when you need him?

Uh huh

- so you unscrew the valve and replace the helium with air ?

If the tank came with a pressure of under 7 atmospheres then that would happen.

Air is mostly Nitrogen - atomic weight 14 
- the atomic weight of Helium is 2

Rig the tank upside down on the scales, it should act like a balloon.

If not, cut the tank open and inspect the mechanism, it might be that
there is a scavenging mechanism - like the pipe and divider inside a
disposable cigarette lighter.
- somehow the Helium is getting replaced with air.

Quite neat - that way you get about 3 more balloons from a smallish cannister.
- also it would not explode in an incinerator (if empty).

Frde - Good thought, I took the word empty too literally. I did not
consider that the Helium might be replaced with air. I was picturing a
standard gas cylinder, which will end up with Helium at atmospheric
pressure, which is no big deal since the starting pressure is many
atmospheres. Perhaps you are right that the vessel is effectively
changing in size internally, which of course violates my assumption
that the vessel is fixed in size. Your factor of 7 is good, but the
atomic mass of Helium is 4 and the molecular weight of air is closer
to 28 since Nitrogen and Oxygen are diatomic molecules.

@kottekoe-ga 

Yes, I decided not to go in for molecules, moles and Avagadro

I too started thinking of something like a diver's gas tank, but then
the 'balloons' bit got me thinking of a small plastic consumers 'party
kit'.

The mechanism is intriguing, one does not get much benefit from
entirely filling the balloons with Helium unless one is going in for
some sort of intercontinental balloon race, and even then it would be
sub optimal.

My hunch is that the balloons are filled with a mixture of air and
Helium, which would make one small hand held cannister go a lot
further.

The (postulated) replacement of Helium with air at atmospheric
pressure could well be an intentional side effect.  It would be very
interesting to cut one open and look at the mechanism.

Yes easiast way to measure the Mass of object is to create the
pressures equal in side and outside of container. The computation
effort of air replaced will be saved.

@nathan_brunner-ga 

Thanks for the clarification, now I'm really baffled.

Helium is a light gas.It is found at the top of the atmosphere.it has
a UPWARD BOYENCY.Due to which it the bellon fly.Similarly when this
gas in the cylinder,it put a pressur upward.So that the weight is
less, when the gas is full.When we release the gas through the ballon,
the CYLINDER weight becomes more.Because the upward pressure of helium
becomes less due to decrease in quantity.So the weight of the cylinder
is more in compairision to the the cylinder full of helium gas.

lucky, that is perfectly wrong.  Helium is not antimatter.  It it
lighter than air, but it is still heavier than nothing.  More helium
weighs more, not less.

I think that nathan_brunner is mistaken.  He uses words like "appeared
noticeably heavier," suggesting he did not actually weigh the tank on
an accurate scale.  If indeed no weighing was done, I think the answer
is obvious: an error of perception.  Perhaps things feel heavier after
all the toil of filling all those balloons.

If you drain the tank rapidly, it will get cold.  This can cause
condensation of water on the tank.  However, then you would notice the
tank was wet (or icy, depending on how cold).  Also, I haven't done
the calculation, but I'm pretty sure that regardless of the humidity,
the weight lost with the helium will be more than the weight gained
from condensed water.

the answer is that the helium is less denced than the air outside as
helium is less denced than the air outside, so u can say that the
helium tries to push the tank upwards but it is not able to escape the
earth's escape vilocity(the escape vilocity of the earth is
11.19km/sec)but it does opposes the gravity so, when the helium is
finished then it does not tries to oppose the gravitational force of
the earth and it weights normal as it was before filling it with
helium.So we conclude that the helium makes the tank to oppose the
gravity but due to its mass it opposes the gravity and wieghts lighter
but does not escapes the gravity.

yours truely
golu

No, Golu. I agree that helium is less dense than air at the same
pressure, but if it is contained in a rigid tank then the amount of
air it displaces is constant, so the weight of the tank gets smaller
as the helium in it is released.