Why aren’t helium tanks lighter when full and heavier when empty? It seems to make sense if helium is lighter than air.
As they are encased fully in steel, the bouyancy they have in air is negligable given the weight of the canister itself.
Short answer: when the tank is full of helium, the helium is under great pressure, and thus much more dense (and weighty) than air.
When the tank is empty, it’s no longer under pressure. There’s less matter in it. Net effect: the tank weighs less.
A) You’re forgetting the point of being “lighter.” He isn’t lighter than the absence of itself. It does, of course, have mass, just not much of it. Fewer molecules in the same space has to be less dense, and “lighter.”
B) So it is lighter, you just can’t notice the difference between (any)compressed gas in a huge metal tank like that and a comparitive vacuum in said tank.
Dave Swaney said "B) So it is lighter, you just can’t notice the difference between (any)compressed gas in a huge metal tank like that and a comparitive vacuum in said tank. "
Have to disagree Dave. For example a 80 CF scuba tank weighs about 4 pounds more when full of compressed air than when empty.
Think of it this way:
Most gases, if compressed enough, will change to liquid. LOX, liquid oxygen, is a staple for fighter pilots. Liquid helium is denser than gaseous helium. Ergo, given they have the same volume (the size of the tank), the helium under the most pressure is heavier (more helium in the same space).
Enough helium compressed into a tank is denser than the surrounding air (or the air which would fill an “empty” tank).
Or I could just be blowin’ smoke up yer ass…