# Naked Science Forum

## General Science => General Science => Topic started by: Seany on 29/05/2007 14:48:54

Title: How do I weigh a Helium-Filled Balloon?
Post by: Seany on 29/05/2007 14:48:54
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?
Title: How do I weigh a Helium-Filled Balloon?
Post by: science_guy on 29/05/2007 15:51:28
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.
Title: How do I weigh a Helium-Filled Balloon?
Post by: another_someone on 29/05/2007 16:17:16
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.
Title: How do I weigh a Helium-Filled Balloon?
Post by: Bored chemist on 29/05/2007 18:25:09
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).
Title: How do I weigh a Helium-Filled Balloon?
Post by: lightarrow on 31/05/2007 21:03:04
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*ρair*g
ρair = air density
you have:
S = A - m*g -->
--> m*g = A - S = V*ρair*g - S.

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