National Physical Laboratory

Which weighs the least, a vessel containing air, helium or a vacuum? (FAQ - Mass & Density)

The evacuated box.

If the box, of mass X kg, had an internal volume of about 1 m3 its mass would be as shown in the table:

Contents of vessel with
internal volume 1 m3
Approximate mass/kg
Empty (ie evacuated) X + 0.00
Filled with hydrogen(*) X + 0.08
Filled with helium(*) X + 0.17
Filled with air(*) X + 1.20

(*) Filled to nominal atmospheric pressure.

Project: Try making a vessel that can withstand being evacuated whilst remaining sufficiently thin-walled and light that its average density (ie total mass/external volume) is less than the density of the air around you (a value that will probably be within ±10% of 1.2 kg/m3) even when wrapped as a parcel. Address the parcel to someone, take it to a Post Office under your arm and ask how much it will cost to send it. You will then be asked to place it on their balance.

If it is too difficult to make - and I don’t know anyone who has managed it - consider filling the vessel with helium (not hydrogen – it is too dangerous) and note that whilst the vessel contents are now heavier, the stresses in the box structure are very much less; the walls can be flimsier and the overall density will therefore be less – giving more upthrust. You have just designed a balloon, and probably a cuboid balloon.

Last Updated: 25 Mar 2010
Created: 8 Oct 2007

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