[Seany (OP)]

How do I weigh a balloon filled with Helium, to measure the mass of the rubber skin and the gas inside?

Put the scales upside-down?

[science_guy]

basicly, a helium filled balloon has no weight.  The only way you can find the weight is by knowing the mass beforehand, and you would end up with a negative weight for the balloon.  Unless, of course, a passing helium balloon expert would come and correct me and my lighter-than-air cluelessness.

[another_someone]

The problem you have is knowing the pressure the helium is under, and so knowing how much helium is within the balloon.  With any gas, if you know how much gas there is, it is a very simple calculation to know the mass of that gas - the relative masses of similar volumes of two gasses is always in proportion to their molecular weight.

Ofcourse, this is talking about mass rather than weight, and measured weight  depends on the environment you are weighing the thing in.  Weigh a helium balloon in air, and (assuming it has sufficient helium within the balloon, and not at too high a pressure or too low a temperature) it will have negative weight in air, but weight that same balloon in a hydrogen atmosphere and it will have positive weight.

If you weight the balloon in a helium atmosphere, and the pressure of the helium without is the same as the pressure of the helium within (which may not be easy to achieve, since the balloon itself, if it is a simply rubber type balloon, is likely to be put the helium under additional pressure no matter what the external pressure) then the only weight of the helium filled balloon would be that of the rubber balloon, since the helium within would balance the density of gas of the helium without.

[Bored chemist]

Weight isn't the same as mass.
Hop on board the space shuttle and ask them to put it in orbit, unless you have a license to drive it yourself (strictly you don't need to do this but it does show that you can "weigh" things in zero gravity too). Tie the baloon to the end of a spring (remember to take one with you ; its a long way to nip back for one) strech the spring an let go of it. The spring will now bounce back and to with a frequency that you can measure. This frequency can be related to the mass of the baloon.
OK That would be an utterly absurd way to weigh anything wouldn't it? Nobody would ever think of doing that would they? Except they do.
http://en.wikipedia.org/wiki/Quartz_crystal_microbalance

Less amusingly you could weigh the baloon on different days with different atmospheric pressures. Draw a line of "weight" (which would be negative) versus pressure and extrapolate to zero pressure.
Easier still would be to weigh the thing in a vacuum- but the baloon would burst (maybe some of the small mylar ones wouldn't).

[lightarrow]

How do I weigh a balloon filled with Helium, to measure the mass of the rubber skin and the gas inside?
Put the scales upside-down?

1. Measure of volume V. You could do it putting the ballon in a container filled with water and weigh the water it remains.
2. Measure of its up push S. You attach the ballon to a spring and see how much the balloon is pushed up; you call this force "S".
3. Computing weigth
be:
Balloon weigth = m*g  (m = mass, g = acc. of gravity)
Archimede's push = A = V*ρ_air*g
ρ_air = air density
you have:
S = A - m*g -->
--> m*g = A - S = V*ρ_air*g - S.

P.S. As you can see, the balloon's weigth m*g is not negative

[nizam]

In the case of a balloon, the helium inside the balloon weighs less than the same volume of air the balloon displaces. This air outside pushes on the balloon, and up it goes!

So a helium balloon has two opposing forces acting on it: gravity pulling it down, and buoyancy pushing it up. Buoyancy wins, so you can’t really weigh a balloon, even though it does have weight.

[Bored chemist]

This air outside pushes on the balloon, and up it goes!

That's not really how it works; the air pushes down on the top of  the balloon almost exactly as much as it pushed up on the bottom of the balloon.

[syhprum]

When weighing anything you have to consider the effects of buoyancy if you are weighing slabs of lead its no big deal but would be a highly distorting factor when you tried to weigh your Helium filled balloon.
The only way to weigh it would be in a vacuum chamber.

[Rodin1880]

Tie known weights to the string until the balloon can barely lift them...

[syhprum]

This would only give you the effective negative weight of the gas filled balloon when immersed in the buoyant atmosphere you would have no information about actual weight .
 

[Halc]

When weighing anything you have to consider the effects of buoyancy if you are weighing slabs of lead its no big deal but would be a highly distorting factor when you tried to weigh your Helium filled balloon.
The only way to weigh it would be in a vacuum chamber.

Agree.  Weight being m*g only works in a vacuum.  In air or water or anything else, one has to subtract buoyancy to get weight.  Hence a helium balloon has negative weight, which is why it can be used to lift an equivalent weight.
I weigh nothing in water, but a rock still does weigh a significant fraction of what it does on land.

This air outside pushes on the balloon, and up it goes!

That's not really how it works; the air pushes down on the top of  the balloon almost exactly as much as it pushed up on the bottom of the balloon.

That's exactly how it works.
You say 'almost', and the difference in pressure on the bottom vs the top is what pushes the balloon up.

[Bored chemist]

That's interesting.
I can get a conical balloon.
Obviously, that has much more area at the bottom than the (pointy) top so it will go up.
But if I turn it upside down there will be much more area at the top than at the bottom so it will go down.
Like this ^ it should go up, but like this v  it should go down.
Unless what drives it is something else.

[jeffreyH]

Helium is less dense than air by volume and therefore lighter. This means that gravity has a greater effect on the air than the helium and therefore the helium rises.

[Halc]

That's interesting.
I can get a conical balloon.
Obviously, that has much more area at the bottom than the (pointy) top so it will go up.
But if I turn it upside down there will be much more area at the top than at the bottom so it will go down.
Like this ^ it should go up, but like this v  it should go down.
Unless what drives it is something else.

If that was sound reasoning, you have your perpetual motion machine.

[Bored chemist]

That's interesting.
I can get a conical balloon.
Obviously, that has much more area at the bottom than the (pointy) top so it will go up.
But if I turn it upside down there will be much more area at the top than at the bottom so it will go down.
Like this ^ it should go up, but like this v  it should go down.
Unless what drives it is something else.

If that was sound reasoning, you have your perpetual motion machine.

Yes, and it's based on the idea that the pressure presses harder on the bottom than on the top.

The words "top" and "bottom" need defining here.

[Halc]

Yes, and it's based on the idea that the pressure presses harder on the bottom than on the top.

The words "top" and "bottom" need defining here.

Shape has nowt to do with it. Cut up the object into thin vertical prism slices and thus integrate the thickness difference over the entire profile of the object.  It buoys up the most where it is thickest (there is a greater difference between pressure on top vs the bottom of that thin slice.  No matter what shape the object, two objects with the same volume will yield the same answer when integrated that way.  It is in fact how the volume of any irregular object can be computed.

Any two objects with the same volume have the same buoyancy in the same environment (such as at sea level).  On the ISS there is none since there is no significant pressure difference.  A balloon goes nowhere in the ISS, but it does move to the windshield of my car when I accelerate due to the horizontal buoyancy it gets from the pressure gradient inside my car.  It certainly isn't inertia that makes it accelerate harder than the car does.

[Rasull]

Helium gas is lighter than air, so there will not be any weight in the helium balloon. According to the statement provided by the experts, we can able to find the mass of the helium balloon.

[David Cooper]

Cool it down until the gas becomes liquid, then weigh it.

[Hayseed]

Have you ever studied chemistry?  One of the first things you learn is how to count atoms.  And it(chemistry) is super neat on the ways that this is done.    Chemist are this best bean counters in the world.  If you need to check inventory or the numbers of it, hired a chemist.

Anyhow, I would make sure that there is no air in balloon and weight it.  Then simply count the atoms put in balloon.  The result will be the same whether you are at sea level, 100,000 ft., or under water.

[evan_au]

It certainly isn't inertia that makes [a helium balloon] accelerate harder than the car does.

I beg to differ.

It is the inertia of the air in your car that makes air rush towards the back of the car when the car accelerates...(Newton said objects tend to stay in uniform motion unless acted on by an external force - like the rear window of your accelerating car).

It is the (smaller) inertia of a helium balloon that causes the air rushing to the rear of the car to exert a force on the helium balloon to push it to the front of the accelerating car.

So I argue that it is inertia that causes this behavior - but perhaps the inertia of the (invisible) air has a bigger influence than the (smaller) inertia of the (visible) balloon.