Are you sure? I ask, because latex helium filled balloons slowly lose air as the helium leaks out - but after some time they stop, and stay a fixed size.
But they don't float when they reach that point!
If it were pure helium, wouldn't a: all the helium leak out, and b: still float if for whatever reason the helium did not leak?
Personally I suspect helium balloons have just enough helium to float when full and the rest is regular air.
Latex balloons deflate due to helium permeating the latex. IIRC this is proportional to the difference in pressure between the inside and outside of the balloon. (Because the rate is proportional to the difference in pressure.)
The balloon isn't going to deflate completely; eventually the pressure on the inside and outside will equilibrate. The balloon doesn't return to its original pre-inflated size because, I suspect, of some amount of irreversible deformation of the rubber as it stretches when inflated.
But anyway, balloon helium is quite pure, it's typically "Class 4" or "four nines" so 99.99% pure. At the local gas supply place, balloon grade and the stuff used for welding shielding gas are the same thing. While a lower purity might be fine for balloons, it presumably isn't worth it to them to either dilute it with cheaper gas or maintain separate product lines.
Welder here. I don't buy helium for welding any more because it's too expensive; most welders use argon nowadays. But in any case the whole point of using inert gases in welding is to keep oxygen away from the hot metal. If welding-grade helium tanks contained any significant amount of oxygen they would defeat the whole purpose. And yes, for filling balloons, you just buy a tank of welding helium and screw a special balloon regulator onto the outlet port. The gas is identical. Makes you talk in a squeaky-high voice if you breathe a little of it; kills you dead if you breathe a lot of it.
(Edit: I know somebody is going to ask why it kills you. Helium is not toxic, but it suffocates you if you breathe a lot of it without a break for -- you know -- real air.)
Cheap "balloon gas" is 50% helium, 20% oxygen and 30% nitrogen. That's cheaper overall, and also safer when people decide to breathe it in to make silly voices.
As androidgirl said already, you need a certain amount of helium for floatation. Intuitively, you wouldn't expect a balloon with just a few molecule of helium to magically float.
But next time you come across such a deflated helium balloon, maybe try inhaling its gasses? If your voice changes, you know there's at least enough helium for that.
I don't know how safe that actually is, but it seems to me to be a common thing without much reporting of ill effects. Still, do your own research before.
I don't know how safe that actually is, but it seems to me to be a common thing without much reporting of ill effects
I've been doing that regularly for decades w/o apparent ill effect. I've even taken it straight out of the tank. Over-do it, and you can get woozy from oxygen displacement. But I can't imagine how the poster-child for inert gases is otherwise going to hurt you.
Careful. If you're breathing pure helium, the time between feeling woozy and passing out is much shorter than you think. I helped a friend set up and tear down an art project involving a 12-foot weather balloon filled with helium. At some point, I felt it was pretty wasteful to be just venting the helium without even playing with it. I sat down on the rubber gym floor for safety and had a friend watching. Two breaths of pure helium in a row was enough to make me pass out. I felt woozy and said "whoah", and I heard my head thump the rubber floor of the gymnasium. I was out just long enough for my friend to realize I wasn't joking around.
Had I been using a heavier gas (lower effusion rate and tendency to pool in the lungs) or not exhaled and inhaled when I started to feel woozy, I very easily could have required resuscitation.
Are you sure? I ask, because latex helium filled balloons slowly lose air as the helium leaks out - but after some time they stop, and stay a fixed size.
But they don't float when they reach that point!
If it were pure helium, wouldn't a: all the helium leak out, and b: still float if for whatever reason the helium did not leak?
Personally I suspect helium balloons have just enough helium to float when full and the rest is regular air.