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


JoltSucker
03-28-2000, 10:07 PM
I know this sounds crazy, but I wonder if the stuff they put in "helium-filled" balloons is not just pure helium.

I had a birthday recently, and the balloons used for the decorations eventually lost their helium. My eldest child asked me a simple question, "OK, if the reason why it no longer floats is because it's lost its helium, why is it still inflated?"

Why indeed? Is it because there is still helium in the balloon, but not enough to float the balloon? Or is it because they don't put pure helium in the balloon in the first place, but instead mix it with air: thus when the helium escapes it leave ordinary air behind?

It's a stupid question, but one I can't answer.

Yossarian
03-28-2000, 10:42 PM
Maestro! A little music to WAG by, please...

I'd suspect that even if the balloon is filled with pure helium, as it slowly "leaks" (maybe "diffuses out" would be better), there reaches a point where there just isn't enough He to sustain flight. You've got to consider that the system in question isn't just He, but He + balloon. The balloon totally filled with He is lighter than air and floats; with just a little He, and relatively more of the system consisting of balloon, it ceases to float.

If you think about it backwards, suppose you started filling the balloon. Would it float when there was one molecule of pure He in the balloon? Two? How about a whole mole of He atoms/molecules? Eventually it will have enough and start floating, until then, it's still to heavy.

Padeye
03-28-2000, 10:54 PM
As atoms go the two most common lighter than air gasses, Helium and Hydrogen, are pretty small. Small enough to easily slip through a permeable membrane like a toy balloon more easily than Nitrogen and Oxygen. As far as not floating Pantellerite is right on the money.

Yossarian
03-28-2000, 10:57 PM
still too heavy
still too heavy
still too heavy

Oh, well. I must've been thinking about the verb "to heavy"... use it all the time.

Kat
03-28-2000, 10:59 PM
Also, helium is not the only gas that can "diffuse" through the latex. Blow up a balloon and let it sit for a few days. It'll still shrink, (presumably until the air pressure is equalized(?)). So, it's entirely possible that, as the helium is flowing out, other gases (oxygen, nitrogen, carbon dioxide, etc.) will also flow in, thus preventing the balloon from completely deflating.

------------------
Your Official Cat Goddess since 10/20/99.

Hey, I like that hat, man. They sell men's clothes where you got that?

Scylla
03-28-2000, 11:02 PM
If the membrane is thin enough to pass helium perhaps it's a two way street and other gasses also contaminate the helium in accordance with Entropy (God, I love that word,) until the balloon reaches (drumroll please,) equilibrium with the surrounding air's latex permeable gases.

Scylla
03-28-2000, 11:04 PM
Damn Kat, you beat me to the punch.

Rysdad
03-28-2000, 11:14 PM
Originally posted by Kat:
So, it's entirely possible that, as the helium is flowing out, other gases (oxygen, nitrogen, carbon dioxide, etc.) will also flow in, thus preventing the balloon from completely deflating.

As long as the balloon maintains positive interior pressure (greater than 1 atmosphere caused by the resistance of the stretchy material compressing the gas inside) other gases won't flow in. That would be like ocean flowing up a river (never mind tides--that's something else). If the inside of the balloon and the outside of the balloon are in equilibrium, the balloon would be flat.

Joe_Cool
03-28-2000, 11:21 PM
Originally posted by Scylla:
If the membrane is thin enough to pass helium perhaps it's a two way street and other gasses also contaminate the helium in accordance with Entropy (God, I love that word,) until the balloon reaches (drumroll please,) equilibrium with the surrounding air's latex permeable gases.

Interesting theory, but I can't see how gas (air) at normal atmospheric pressure can flow INTO a balloon that contains gas at higher pressure, even if the balloon were 100% gas permeable. Once you reach a pressure equilibrium, I would tend to think the flow would mostly come to a stop.

Since I've never seen a balloon INflate due to spontaneous gas penetration (blow up a balloon 1/3 of the way and see if it grows...), I'd venture a guess that there is a one-way flow outward of helium and virtually no inflow of other gases. Especially since the balloon is still slightly pressurized from the elasticity of the skin. It seems that there is a minimum pressure threshhold for gas to leak.

Experimentation is in order...somebody needs to take a bunch of old "stale" He2 balloons, cut the stems, and HUFFFFFFFFF. If you talk like a munchkin, then they are filled with He2 and you know that it leaks out until it reaches a near pressure equilibrium. If your voice sounds normal, then you know that the gas flow is 2-way, and the balloon refills with air as it empties itself.

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If you say it, mean it. If you mean it, do it.
If you do it, live it. If you live it, say it.
-----
Joe Cool

Joe_Cool
03-28-2000, 11:23 PM
Originally posted by Rysdad:
As long as the balloon maintains positive interior pressure (greater than 1 atmosphere caused by the resistance of the stretchy material compressing the gas inside) other gases won't flow in. That would be like ocean flowing up a river (never mind tides--that's something else). If the inside of the balloon and the outside of the balloon are in equilibrium, the balloon would be flat.


CRAP. I hate when somebody makes my post while I'm still typing!

Qwisp
03-28-2000, 11:39 PM
Originally posted by Joe_Cool:

Experimentation is in order...somebody needs to take a bunch of old "stale" He2 balloons, cut the stems, and HUFFFFFFFFF. If you talk like a munchkin, then they are filled with He2 and you know that it leaks out until it reaches a near pressure equilibrium. If your voice sounds normal, then you know that the gas flow is 2-way, and the balloon refills with air as it empties itself.



Been there. Done that. You talk normal


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I have great faith in fools, self-confidence my friends call it.---
Edgar Allan Poe

Zor
03-28-2000, 11:39 PM
To answer the OP, the gas contained in floating balloons is, of course, not pure Helium if you want to get technical. I would guess that it's probably around 95% Helium when it was filled, although I wouldn't be surprised if it were 99% pure; Helium is a relatively cheap gas.

As has been mentioned, in order for a balloon to float, it's overall density must be less than that of the air surrounding it. Balloons are not impermeable to the effusion (geek term) of small molecules however. When you fill a balloon up with Helium and put it in air, the Helium inside will tend to escape outwards, trying to balance the concentration of Helium on both sides of the balloon. Similarly, the air outside will tend to leak into the balloon, as it wants to balance the concentration of air on both sides as well. Since Helium is a very light molecule, it moves around space at a higher average speed than air molecules. That means Helium molecules can penetrate the balloon more easily than air (just imagine them bumping into the balloon randomly), and you end up with a net movement of gas molecules in the outward direction. That's why your balloon will shrink as time passes; more gas is coming out of it then going in.

While all this is going on, your balloon is gaining weight because the small amount of air entering it weighs a lot more than the large amount of Helium leaving it. As the balloon shrinks and gets heavier at the same time, its density eventually surpasses that of the air surrounding it and it will just sink to the ground. At this stage, your balloon might still look pretty buff, but its density is just too high to float anymore.

Having a pressure gradient across the balloon surface will change things a little, but it can't stop effusion altogether. If the balloon was overfilled a little bit, you'll just start with more Helium leaking out, and less air getting in. The little positive pressure you had will soon be gone, and you'll end up in equilibrium as far as pressure is concerned.

So, in the end, the concentration of Helium and air will be balanced on both sides of the balloon. No more net movement of molecules will occur, and your balloon should stop shrinking as well. Then again, if your balloon was made of some elastic material, it will continue to exert a small pressure on the air within it until it just shrivels...

Steve-o
03-29-2000, 12:04 AM
Zor's right. A gas will have a net movement from a low pressure to a high pressure if the concentration of said gas is higher in the low pressure area. I work in a chem. lab, and see this happen all the time. Our high pressure gas lines or high pressure reactors sometimes get (slow, inaudible) leaks, resulting in air getting into the gas lines or reactors even though the gas lines or reactors can be as high as 2500 psi (that's over 150X atmospheric pressure).

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Things are random only insofar as we donít understand them.

Olentzero
03-29-2000, 02:44 AM
Everybody, all together now!

huffffffffffffffffffffffffffffffff

We represent
the Lollipop Guild
the Lollipop Guild
the Lollipop Guild....

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All I wanna do is to thank you, even though I don't know who you are...

scr4
03-29-2000, 03:29 AM
Actually, heliums is a very expensive gas. There are only a few natural gas mines in the world which contain any usable amount of helium. The "helium" sold for inflating balloons contains far less than 95% helium. More like 50%, if I remember correctly - just enough to make the balloon float. I'd guess the rest is air. In fact, they probably contain a fair amount of oxygen, so people who inhale them don't suffocate. At least that's what I'd do if I were selling balloons.

DSYoungEsq
03-29-2000, 10:32 AM
Grant.

JoltSucker
03-29-2000, 02:01 PM
OK, as the author of the OP, I judge scr4 to be the winner.

(1) It confirms what I suspected, that pure helium was expensive, and that balloon-grade helium contains a lot of plain old air. That alone explains why the balloon is still inflated.

Questions for Zor and Steve-o: if you have a semi-permiable barrier between two different gasses at the same pressure/temperature, is Brownian motion the only force that causes the two gasses to mix? I can't imagine that the gas "knows" that is hasn't spread to the other side of the barrier - it just bumps around, and gets to the other side that way.

In the case of unequal pressure between the two gasses (a balloon), more gas leaks out out than leaks in because there more atoms, right? So not only does helium leak out, but it becomes less pure over time. This might explain the still-inflated non-floating balloon, but not some other behaviors I've noticed.

The balloon loses perhaps third of its volume when it gets to the stage that it stops floating (over the space of very few days), but once it gets to this state, its volume seems to stabilize. It can be weeks before it completely deflates. So that behavior lends additional credence to the theory that there are two gasses in the balloon.

Riboflavin
03-29-2000, 09:36 PM
Zor,

Questions for Zor and Steve-o: if you have a semi-permiable barrier between two different gasses at the same pressure/temperature, is Brownian motion the only force that causes the two gasses to mix? I can't imagine that the gas "knows" that is hasn't spread to the other side of the barrier - it just bumps around, and gets to the other side that way.

The gas doesn't have to 'know' whether it has spread to the other side of the barrier. What happens is that since there is a higher concentration of helium atoms in the balloon, there are more helium atoms near the surface of the balloon inside of it than outside of it. Only those atoms that hit the balloon can go through, and since there are more helium atoms inside of the balloon than out, more helium atoms will leave the balloon than enter it. When the concentration of helium is the same inside and out, then the number of atoms hitting the outside and coming in will be equal to the number of atoms hitting the inside and coming out.

(You can also see that the temperature will affect the rate of diffusion, since there will be more helium atoms hitting the ballon when the temp is higher).



------------------
Kevin Allegood,

"At least one could get something through Trotsky's skull."
- Joseph Michael Bay

Dystopos
03-29-2000, 10:05 PM
Judging scientific explanations by appeals to reason went out with Galileo...

Rysdad
03-29-2000, 10:17 PM
I believe that those half-filled, semi-limp balloons tend to stay that way because they've been stretched out. They're not as "elastically tight" as they used to be. You could poke a hole in one, and it wouldn't regain its original shape because it's been deformed. As a result of that deformation, it's no longer exerting as much pressure on the gas inside, and the leakage slows thereby allowing the balloon to remain in it's saggy, semi-filled state for a long time.

Rysdad
03-29-2000, 10:23 PM
Originally posted by Steve-o:
A gas will have a net movement from a low pressure to a high pressure if the concentration of said gas is higher in the low pressure area.

I'm wondering how this would happen.

If I had a "hermetically sealed" container holding pure helium at 5 atmospheres (A), and placed it inside another similar container holding pure neon at 1 atmosphere (B), then, at some point, would the little spectrometer I hid inside A start to register neon?

How?

douglips
03-29-2000, 11:41 PM
Just because it feels good doesn't make
it right. You've been spoonfed good information from smart people and you
deny it because it goes against your preconceived notions of what answer
you really want.
JoltSucker:
It confirms what I suspected, that pure helium was expensive and that
balloon-grade helium contains a lot of plain old air. That alone explains why
the balloon is still inflated.
Sorry, not true.

The only site I could find giving a cost for helium is The case for heliox (http://tekniskdykking.org/tekniskdykking/artikler/aqua/HelioxCase1.htm),
referring to helium as costing $75 per K-bottle, or $.008/liter.
$ 0.008 = the average commercial cost of helium per liter

So, my ~10 cubic foot cylinder of "balloon helium" contains about 280 liters
of helium, for a cost of less than $3. I paid $20 for the container, leaving
plenty of room for profit.

Helium is plenty cheap, though expensive compared to other gases.

JoltSucker:
['balloon' helium contains lots of air] explains why
the balloon is still inflated.

... if you have a semi-permiable barrier between two different gasses at the same
pressure/temperature, is Brownian motion the only force that causes the two gasses to mix? I can't imagine that the gas "knows"
that is hasn't spread to the other side of the barrier - it just bumps around, and gets to the other side that way.



You can't think of it as being a pressure gradient if the opening is
small enough. Basically, if there is an appreciable flow through the opening there
will not be contamination, but if the flow is very slow there will be. The way I
explain it to people is that the holes are
small enough that gas molecules are not steadily streaming out, but are occasionally
bumping into the hole and sneaking through. So, for a good portion of time there is
no molecule of gas blocking the hole, so a molecule going 'upstream' has no resistance
in crossing the barrier unless it gets unlucky.

The gas doesn't stop crossing the barrier once it has spread to the other side,
it's just that with the same concentration of gas on both sides the traffic in both directions is equal.

Steve-o
03-30-2000, 12:13 AM
Originally posted by Rysdad:
I'm wondering how this would happen.

If I had a "hermetically sealed" container holding pure helium at 5 atmospheres (A), and placed it inside another similar container holding pure neon at 1 atmosphere (B), then, at some point, would the little spectrometer I hid inside A start to register neon?

How?




Maybe I didn't phrase it well, but I intended the "(slow, inaudible) leaks" part to be associated with the "A gas will have a net movement from a low pressure to a high pressure if the concentration of said gas is higher in the low pressure area" part of my post. In other words, if there is no leak, this will not happen.

douglips gave a very good explanation of how a slow leak can result in "reverse" contamination of a high pressure line.

BTW, Iím not sure about helium (Iíd have to look up its atomic radius), but hydrogen will slowly seep through a non-leaking metal container. It is small enough to work its way through the interstitial spaces of the metal, and out to the other side. Pretty cool I think.


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Things are random only insofar as we donít understand them.

scr4
03-30-2000, 12:39 AM
Balloon helium from party supply stores is definitely a helium-air mixture. this site (http://www.balloonhq.com/faq/whyfly.html) says:

...here you must only use helium and not balloon gas (a mixture of air and helium).

Another site (http://www.partyworld.com.au/balloonbible.htm) says

Helium Balloon Gas is a mixture of helium and nitrogen, and available from gas suppliers, balloon and party
shops.

Other sites do explain that pure helium is readily available in welding supply shops.

douglips
03-30-2000, 12:24 PM
scr4: Thanks for the links!

I stand corrected re: the purity of balloon gas. I was a little irritable last night, and apologize for the tone of my post.

JoltSucker
03-30-2000, 10:26 PM
This calls for some careful scientific study, and some of you guys are in a position to take it on -- any takers?

You could test the infiltration theory this way:

(1) Buy a helium-filled balloon, and the rest of the helium tank as well
(2) Put the balloon in an helium-tight compartment
(3) Replace the air in the compartment with the rest of the helium, to a pressure of 1 atm
(4) Leave the balloon in the chamber for a week
(5) Remove the balloon, and see if it still floats

If it still floats, you can surmise that the infiltration theory was correct. If it sinks, you can surmise that the 2-gas theory was correct.

Zor
03-31-2000, 12:48 AM
JoltSucker, here's a much easier way to get things done...

1): Buy a balloon.

2): Weigh the balloon on a scale.

3): Fill the balloon up with water. You might want to do this with a bucket full of water beneath the balloon. This will prevent the balloon from bursting under too much pressure as you fill it up from a faucet.

4): Empty the water in the balloon into a beaker and measure the total volume of water there.

5): Divide the weight of the balloon in grams by 0.98, the lifting force of 1 liter of Helium in air. The resulting figure is the minimum volume of Helium this balloon must contain in order to float, assuming the balloon gas is a mixture of Helium and Nitrogen or air.

6): Compare the minimum volume you derived from step #5 to the volume of the balloon you obtained from step #4. The ratio of the former to the later is the minimum concentration of Helium required for you balloon to float, given some balloon gas with a mixture of Helium and Nitrogen or air.

There, that should save you quite some trouble...


p.s. Your proposed experiment would have proved nothing even if you had actually performed it. Do you want the details?

Steve-o
03-31-2000, 01:28 AM
Today I got a balloon and measured its mass. It was 3.06 grams. I then blew it up while measuring the gauge pressure. The pressure increased in a fashion that would fit a natural logarithmic curve (i.e. Ė it increased very rapidly when the balloon was just beginning to fill, and then leveled off once the balloon was about 1/8 full). The final gauge pressure was 0.95 psi. Thatís 0.95 psi above atmospheric pressure here in Houston (sea level - 14.7 psi). The balloon was 4 liters when full. Using the ideal gas law equation, along with the information gathered on my balloon, I figured out the following:

The density of my balloon full with a 50/50 mix of He and Air would be 1.50 g/L

The density of my balloon full with pure He would be 0.95 g/L

The density of air is 1.29 g/L

Looking at these densities, only the pure He filled balloon would float. I took it one step further, and figured out that the He/Air mixture needed in the full balloon to match the density of air would be 69% He, and 31% Air. My balloon (with no string attached to it BTW) will only float with a He concentration greater than 69%, making me think that the gas in the He tanks is fairly pure He, even when nitrogen is mixed in. This makes me wonder why they would even bother. So little nitrogen would be able to be used as a diluent that it does not seem as though the trouble would be worth the cost savings.

If you run these calculations, you may get slightly different numbers (give or take 1%) due to my horrible use of significant figures, and some rounding that I did. Also, the use of nitrogen as the He diluent will not have much effect on these results.


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Things are random only insofar as we donít understand them.

Zor
03-31-2000, 01:36 AM
I still believe the concentration of Helium within a balloon is more likely to be in the 95% range. When Helium is extraced from natural gases, the first process already puts its purity at 98.2% (figure from this link (http://www.sciencenet.org.uk/database/Chemistry/9609/c00256d.html) in this thread (http://boards.straightdope.com/ubb/Forum3/HTML/007021.html)). Furthermore, at standard temperature and pressure, 1 liter of Helium will have less than a gram of lifting force in air (it's about 0.98 grams). The volume of a balloon the size of your head is around 5 liters, and that doesn't leave much room for the weight of the balloon itself. If you dilute that Helium with any significant amount of Nitrogen, which weighs just slightly less than air, then you're balloon is going to sink. Even in the link (http://www.balloonhq.com/faq/whyfly.html) which scr4 supplied, you can see that the balloon in the first example will just barely float with a mixture of 80.2% Helium and 19.8% Nitrogen....

ZenBeam
07-15-2000, 03:40 PM
Here's a (possibly) interesting story on the helium purity controversy.

My Mother-in-Law bought a six pack of empty balloons for my son's birthday at a party supply store. They had tanks there to fill them, so she asked them to fill the ballons, and they asked her if she wanted 12 hour or 24 hour balloons. Remember, these are the same balloons. They had different spigots to fill the balloon from. When she related this, I thought of this thread. "Aha!" I thought "The smoking gun! They do use different grades of helium!"*

Not so fast!

She had bought the "24 hour" helium. The next morning, after the balloons sank to the ground (not quite 24 hours, but close) one of my sons slipped, and fell on one of the balloons. And it popped. And pealing from the inside surface of the balloon was a thin, milky white coating. If you ever smeared Elmer's white glue on the palm of your hand in grade school and peeled it off (everybody did this, right?), it kind of looked like that. Apparently, the 24-hour helium has some kind of additive to coat the balloon inside after it is filled, to slow down the helium diffussion.

On another note, my mother had a bear with a set of balloons delivered, and those balloons were much higher quality. They could still float (barely) five days later.

*Actually, I also considered that the spigots attached to the same tank below the counter. Who'd know? But that seemed awfully cynical.

John Zirkle
07-15-2000, 11:27 PM
You guys are funny!

About that balloon....
My boy brought home a helium-filled mylar balloon last week. This morning it was floating 6 inches off the living room floor. This afternoon, I found it upstairs, still floating 6 inches off the floor. Howcome it didn't hang up on the 2nd stair riser? OK, our breeze here in Honolulu can be pretty brisk & the windows are always open so it was probably blown up there, but if it can't rise over 6 inches on the first floor, why is it not lying on its side on the upper level?

Aloha,
Jack

lee
07-16-2000, 12:23 AM
That film was probably Hi-float. It is a substance that coats baloons and makes them somewhat less permiable than plain latex so they last long. I looked up balloon animals on the net once and found this out.

Arjuna34
07-16-2000, 12:26 AM
Originally posted by John Zirkle
You guys are funny!

About that balloon....
My boy brought home a helium-filled mylar balloon last week. This morning it was floating 6 inches off the living room floor. This afternoon, I found it upstairs, still floating 6 inches off the floor. Howcome it didn't hang up on the 2nd stair riser? OK, our breeze here in Honolulu can be pretty brisk & the windows are always open so it was probably blown up there, but if it can't rise over 6 inches on the first floor, why is it not lying on its side on the upper level?

Aloha,
Jack
Does it have a string attached? If so, maybe it can only lift 6 inches of string.

Arjuna34

jayron 32
07-16-2000, 01:31 AM
The rate of diffusion of a gas across a semi-permeable membrane (of which latex is a good approximation) is controlled by 2 factors:

1) size of molecule diffusing; as given by Graham's Law (rate of diffusion is inversely proportional to square root of molar mass)

2) Difference in pressure across membrane.



Now, helium has a molar mass of 4 daltons. Nitrogen (78% of air) has a molar mass of 28 daltons. Oxygen (21% of air) has a molar mass of 32 daltons. Helium will therefore diffuse out of the balloon at a rate of sqrt(28)/sqrt(4) times faster than will nitrogen. That's about 2.65 times faster.

Now, helium will leak faster from a fuller balloon. If we could maintain a constant pressure of helium inside of the balloon, the rate of diffusion would be constant. However, as helium leaks out, the total pressure inside of the balloon decreases, meaning that the helium will leak out slower. Eventually, you get to the point where the pressure on the inside and the outside is the same, and the gas will stop diffusing. This occurs at a point somewhere before the balloon is completely flat, but it will still take a REALLY long time to get to this point. At this point, some of the air will be leaking in, and some of the helium will continue to leak out, but VERY VERY VERY slowly (don't wait around for it).

annalamerino
07-16-2000, 01:12 PM
Hi-float, eh? It sounds like it would make a terrible mess when it breaks over the furniture.
It's bad enough with the talc they usually put inside to keep the sides from sticking together in storage.
That's one reason not to inhale the helium for the funny voice, since you also get a lungful of talc dust.

John Zirkle
07-16-2000, 08:30 PM
Arjuna34,

OK, I get it. It's the lift capability of the remaining bouyant. There was no string attached (heh), but there was the clamp holding closed the entry port. Thank you!

Aloha,
Jack

Chuck Miller
07-18-2000, 08:45 AM
Okay... I know I'm joining this late, but how's that silly free-floating balloon float 6" above the 2nd floor floor if it could only float 6" above the 1st floor floor? I can understand the reasoning of a trailing string which touches the floor... but not a balloon with no strings attached! If it could float 6" above the 2nd floor floor, why didn't it try to get there sans help from the wind, i.e. head for the 1st floor ceiling?

CurtC
07-18-2000, 09:46 AM
Maybe the balloon floating six inches above the second floor was doing so because the surrounding air was heavier at that time? Maybe there was a rise in barometric pressure or a drop in humidity? It wouldn't take much of a change, since the density of air at 20 feet above sea level is not that different from the density at 29 feet.

I once got a helium balloon to slowly oscillate up-and-down by placing it in a confined space near a window which was letting the sun in. The sun would heat up the balloon, the heat would cause it to expand and therefore have less density, so it would rise. But up higher, the sun didn't shine on it, so it cooled off and fell back down. Then it would repeat.

John Zirkle
07-18-2000, 09:55 AM
Curt,

That sounds good, but I don't think it's the answer. I brought the balloon back downstairs...yep, 6 inches off the floor is where it settled in.

Aloha,
Jack

Chuck Miller
07-18-2000, 11:17 AM
Curt... Well, since Jack/John has now stated that the balloon floats 6' off the floor after bringing it back downstairs, there's got to be something we are missing. I agree there's not much difference in pressure at 20 vs. 29 feet above sea level, but that balloon (if it is going to float) should settle in at one or the other... not both. I do concur with your rising/falling scenario with the sun heating the gas/air inside the balloon.

galt
07-19-2000, 09:54 AM
Zor gets very close, but I still think he's wrong:
While all this is going on, your balloon is gaining weight because the small amount of air entering it weighs a lot more than the large amount of Helium leaving it. As the balloon shrinks and gets heavier at the same time, its density eventually surpasses that of the air surrounding it and it will just sink to the ground.

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.

galt
07-19-2000, 10:11 AM
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. :-)

AWB
07-19-2000, 10:45 AM
The cost of helium fell from $2500/ft3 in 1915 to 1.5 cents /ft3 in 1940. The U.S. Bureau of Mines has set the price of Grade A helium at $37.50/1000 ft3 in 1986.

That's pretty cheap to me.

Chuck Miller
07-19-2000, 11:55 AM
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!

John Zirkle
07-19-2000, 02:41 PM
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

John Zirkle
07-19-2000, 02:51 PM
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

Chronos
07-19-2000, 04:01 PM
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.

John Zirkle
07-19-2000, 06:03 PM
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

Chronos
07-20-2000, 02:30 AM
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.