Not having a helium tank handy to toss on my bathroom scale, I found myself wondering when a tank of helium (or hydrogen, or some other less dense than air gas) is heavier… when it’s full, or when it’s empty? By empty, i mean with nothing inside, a vaccum. (Not Full of Air)
I would imagine that it would be lighter when it was full, but that seems counter-intuitive, because the gas has mass, and that should be added to the mass of the tank… so the whole system is more massive, and should weigh more.
On the other hand, if you imagine a balloon, it sure seems to be lighter when it is full of helium…
BTW If it were possible to construct a container as light and voluminous as a party balloon, but rigid enough to be evacuated, it would be more buoyant than a helium balloon of the same size.
The helium in the tank is under pressure, so there is a lot more of it than there would be if the tank was at pressure equivalent to the air outside it. Thus a full tank is heavier than one that’s empty. An empty tank does not normally have a vacuum inside - it’s empty when the pressure of the helium inside equalizes with that of the air outside.
Ah, **Mangetout[/] beat me to it. On his last point - any amount of helium weighs more than nothing (a vacuum).
An empty balloon is lighter than a helium-filled balloon, which is lighter than an air-filled ballon. The helium balloon floats while the empty balloon does not because it has a much lower density.
Anything when weighed in an fluid (liquid, gas, etc.) weighs less as per Archimedes principle :
Wf(Weight of Body in Fluid) = Wv(Weight of Body in Vacuum) - Wf (Weight of Fluid Displaced)
For a given cylinder the volume of air displaced by the cylinder is constant, so the cylinder (with contents) will weigh (Wf) the least when there is vacuum inside the cylinder. The cylinder (with contents) will weigh(Wf) the most when the density of the fluid inside is the most (that is compressed helium). Cylinder filled with Helium at the same temperature and pressure as outside air will weigh(Wf) less than that filled with air.
This is the classical “paradox”. What weighs more: a ton of steel or a ton of cotton? The answer is: depends where you weighed them. If you weighed them in air, then the true weight of 1 ton of cotton is more than the true weight of 1 ton of steel. If you weighed them in vacuum then both are the same.
Helium atoms weigh 4, nitrogen molecules weigh 14. Helium balloons rise, but if you could pump up the pressure in the balloon to seven atmospheres, a helium balloon would equal the density of the air and have no bouyancy.
The problem with the tank is easy: since all matter has mass, and since higher pressure gas packs more mass into a container than a lower pressure gas… a high pressure gas bottle will always weigh more than one at 1atm pressure.
What’s confusing is your definition of “empty.” If you open the valve on a helium tank, the pressure will drop to 1atm and the flow will stop… but the tank will still have 1atm helium inside, not air. It’s empty (the flow has stopped), yet it’s full (it contains helium, not air.) Is it empty or full? Depends on how you define the word “empty!” If you somehow drive out the remaining helium and replace it with air, THEN is the tank “empty?” But it’s full of air!
Let’s hear from someone with empirical experience – me. For several weeks one summer I filled helium balloons at a farmer’s market. I was also responsible for picking up the helium-filled tanks and dropping off the empty tanks at the supplier. I can tell you from experience that the tanks without the helium in them weighed more than the tanks with the helium in them.
Mass-wise or weight-wise? Mass-wise, tanks containing helium is “heavier” (with more mass) than empty ones. Weight-wise, unless the contents of the tanks are less dense than air, they will be heavier.
How can a container of helium that is liquefied (under pressure) be lighter than the same container of helium at 1atm pressure; that would mean that helium is lighter in liquid state than as a gas, which is absurd beyond belief; Walloon, are you sure you didn’t just write your post the wrong way around?
Anyway, aren’t bottled gases sold by weight? - if they are, and you’re right, then a half-full bottle would cost more than a full one!
Walloon-what was your method for measuring the weight, and what where the relative values of the weight difference compared to measurement error ranges?
This may be a bit of a nit-pick, since I agree with Magetout, but I’m pretty sure that compressed Helium gas is just that: compressed Helium gas, and not a liquid at all. Even under high pressure (but not too high, such as those found in a typical compressed gas pressure), it would still take extremely low temperatures–like, near absolute zero–to liquify Helium. Here’s the first He phase diagram I could find:
I was trying to find a P-T diagram for Helium too when I read that. I don’t know, but I suspect that it would take tremendous pressure to get He into liquid at room temperature, more than would commonly be neccesary for storage. Propane is liquid under not too much pressure but that’s a far cry from helium.
To clarify what I thought was obvious the first time – the exhausted, no-helium tank was more of a strain on my muscles to drag back to my car, than the helium-full tank was to drag out of my car. I did this for weeks. It satisfied my curiousity about the OP above.
I think your first post was clear, but it doesn’t seem to make sense. If no air got into the empty tank, it would have a residual amount of helium, which would have to weigh less than the amount in the fully charged tank, and thus it couldn’t be heavier. Now, if all or most of the helium in the used tank was displaced by air (which I assume would require leaving the valve open unobstructed), then for the “empty” tank to be heavier, that amount of air would have to weigh more than the full helium charge did. I guess it’s possible, but it’s not what I would have expected.
The critical temperature for helium is -267.9 C (5.2 K) – above that point, no amount of pressure will liquefy it. (By contrast, the critical temperature for propane is 97 C.
