OK, it's a stupid question, but what's in helium-filled balloons?

Factual Questions
41

Zor gets very close, but I still think he’s wrong:

First, since people seem to think it’s an important issue, I’ll say that I don’t know what percentage of the mixture in the balloon is helium, but I also don’t think it matters. All we care about is that it’s some mystery mixture that’s lighter than air, and some or all of its components can leak out slowly, leaving a mystery mixture whose makeup is also lighter than air, but possibly less so.

The “air entering the balloon as the helium escapes” business seems patently bogus to me. If you consider that the mystery mixture inside is at a higher pressure than the air outside, the porous balloon wall is only going to pass gas (har) in one direction: out.

That said, the behavior is still easily explainable. When the balloon is brand new, the combination of the balloon itself and the mystery mixture inside weighs less than the corresponding volume of air, so it floats. As the mystery mixture leaks out, the balloon+gas system loses weight, as opposed to gaining weight as you say. However, the size decreases rapidly enough that with the weight of the balloon material itself remaining constant, the density of the balloon+gas system eventually surpasses the density of air, and it sinks.

How small the balloon can get and still float is basically a function of the weight of the balloon material (I say basically because there is still a question as to whether the makeup of the gas mixture inside the balloon changes as it leaks, but I bet that’s not nearly as large an effect as others are making it out to be). If you could eliminate the heavy knot, you’d probably significantly increase the life of your balloon.

Note that this explanation still works if we believe that air actually seeps into the balloon (which I personally think is ridiculous), because the density of the whole system will increase even more rapidly.

As to why, if the mystery mixture can leak out, does the balloon not go completely flat, I think jayron32 hit it right on the head. In layman’s terms, it leaks out easier when it’s under more pressure, and as it comes closer and closer to atmospheric pressure, it leaks out more slowly and probably stops somewhere slightly above 1 ATM. In my head, I have a picture of a stretched out piece of latex whose holes get smaller as it unstretches, eventually getting to the point where the holes are too small.

42

I have a hypothesis about the balloon that floats 6 inches off the floor both upstairs and downstairs: Local variations in temperature in a room probably have more of an effect on the balloon than altitude, when you’re talking about the 15 or so feet of altitude change inside a house.

Measuring temperatures at various places in your house, you’d probably find that the bottom of a room, whether it’s upstairs or downstairs, has a layer of colder air, which would float a balloon better. Strange as it may seem, due to the way air flows, the air is probably slightly colder and denser 6 inches off the floor upstairs than it is 6 inches from the ceiling downstairs.

When I was a kid, whenever I’d get helium balloons, I’d tie little paper cradles to the strings and load them with tiny bits of paper to tune them so they’d float in the middle of the room, rather than on the ceiling. You’d be surprised how easy it is to control how high the balloon floats in a still room – there is a large range of air densities from the ceiling to the floor.

As for getting up the stairs, that was probably a draft. One thing I noticed when doing my balloon-weighting experiments was that when balanced, they follow drafts extremely well. In our house, I could get a balloon to float about 8 inches from the ceiling and follow the draft down the hall, around two corners without bumping the walls, and through a doorway, dipping slightly just then so that it wouldn’t bump on the top. Every single time. It was eerie. :slight_smile:

43

The cost of helium fell from $2500/ft[sup]3[/sup] in 1915 to 1.5 cents /ft[sup]3[/sup] in 1940. The U.S. Bureau of Mines has set the price of Grade A helium at $37.50/1000 ft[sup]3[/sup] in 1986.

That’s pretty cheap to me.

44

THANK YOU GALT! Given this rationale about temperature, I think you have the solution about the floating balloon 6" off the floor up- and downstairs. Altitude plays a part, but so does temperature. I think I’ll consider that part of this thread closed with that answer!

45

I dunno about that.
My son reminded me of a helium-filled mylar balloon we once had which was specifically designed as a free-floater. It was shaped & painted like a flying saucer. There were no strings attached, but there was a set of light weights attached to the bottom of this lighter-than-air craft which could be peeled off (carefully!) in a spiral to allow the thing to float higher. Re-stick one or more of the weights and it would float lower. It would float at approximately the same altitude with the same weight aboard whether upstairs or down, day or night, warm or cool, calm or windy.
Folks, we’re back to lift capacity, but I still have a question…and it’s my original question. If the darn thing lifts ‘x’ ounces of weights to a height of say 5 feet on the first floor AND it lifts the same weight to approx the same height on the second floor, then whyinell wasn’t it either bumping its head on the ceiling downstairs or dragging its bottom upstairs?
Aloha,
Jack

46

By the way…I once watched that ‘saucer’ change levels. It maintained its HaG (Height above Ground) all the way up the stairs.
'Splain THAT!
Aloha,
Jack

47

If you’ll re-read galt’s post just above, you’ll see that he did, in fact, 'splain that. In a centrally-heated house, the air temperature at any given point is primarily a function of height above the floor, since cool air can’t sink any lower than the floor, and warm air can’t float any higher than the ceiling. On the stairs, you’d get a sort of cascade effect, with sinking currents of cold air along the floor, and rising, warm currents at the ceiling. Since your ufo balloon floated up the stairs, I’m going to venture a guess that it was floating over halfway up the distance to the ceiling at the time?
As to it maintaining the same altitude day and night, this is probably because if the temperature changes slowly enough, the helium can maintain the same temperature as the outside air, and if there’s any slack in the balloon, it can expand and contract with the air.

48

Thanks, Chronos.
I liked your explanation and I’ll accept it and include thanks to galt as well.
Except…
Does it have to be a central-heating situation? Here in Honolulu, I don’t have it. Just louvered windows and balmy breezes.
Ya don’t HAVE to eat yer hearts out…you could live here too.
Aloha and mahalo,
Jack

49

Hmm, that’d probably do it, too… It just nneds to be something fairly uniform; a roaring fireplace in the middle of a blizzard wouldn’t work too well.