# How to weigh gasses that are less dense than air?

I know that everything that has less of a density than air produces lift. So if you have a closed box filled (half full) with scales and the other half is filled with air. Together it ways 25 kg if you put it on a scale, then you replace the air with helium won't the scale give a lower weight because of the lift produced by the helium?

And what if you pump out all of the air and make it vacuum? What will the scale say then, that the box filled with scales/weights weighs? Will it give the exact weight of the box and the scales together or will it give a lower weight? (does the vacuum inside the box produce lift?)

I hope it is a bit clear, what I am trying to ask.

### 3 Answers

- Andrew SmithLv 76 years agoBest Answer
Lets try it a different way.

The AIR outside the box produces lift.

That is Archimedes' principle.

It does so no matter what is inside the box.

The amount of lift is equal to the weight of the air that the box displaces.

Now the box itself has weight and, if it is filled with air, that air also has weight.

Hence air inside the box adds weight and the air outside the box provides lift. The two approximately cancel so you observe the weight of the box.

By removing the air, or replacing it with something lighter then the total weight of the box + contents is reduced. But it still has the lift from the air outside so the apparent weight is less than the true weight of the box alone.

A vacuum is the lightest contents possible. Lighter than hydrogen or helium.

So the box with the vacuum will have the least weight.

Now back to one part of your question. How to find the weight of a gas that is less dense than air.

Take your box and fill it with the gas. Weigh the result.

Now double the pressure of that gas. So we now have EXTRA gas.

Weigh the box again. The increase in weight is the weight of that extra gas.

Which, at STP, would have a volume equal to the volume of the box itself.

- Steve4PhysicsLv 76 years ago
That is too difficult to understand. You seem to have different scales inside and outside the box - but you are not clear which ones you are referring to.

You could call the ones inside the box scale-I and the one outside the box scale-O for example. Then specifically state one you are referring to.

Is there anything on scale-I?

What are the 'weights' you are referring to?

I'd guess you need to post a new question if you don't get any answers.

- 6 years ago
Fill an air tight room/container that is secured to the ground with the gas you are trying to weigh. Then put scales in there with a ballon or something filled with the same gas.

Subtract the weight of the ballon/contain

Will it work? If not then in the air tight room, use a lighter gas or a vacuum instead of the same gas