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?
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.
Sorry, not true.
The only site I could find giving a cost for helium is The case for heliox,
referring to helium as costing 75 per K-bottle, or .008/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.
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.
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.
Things are random only insofar as we don’t understand them.
Balloon helium from party supply stores is definitely a helium-air mixture. this site says:
Another site says
Other sites do explain that pure helium is readily available in welding supply shops.
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.
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 in this thread). 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 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…
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.
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?
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. – 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. That’s 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.
Things are random only insofar as we don’t understand them.
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.
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
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.
Does it have a string attached? If so, maybe it can only lift 6 inches of string.
Arjuna34
The rate of diffusion of a gas across a semi-permeable membrane (of which latex is a good approximation) is controlled by 2 factors:
size of molecule diffusing; as given by Graham’s Law (rate of diffusion is inversely proportional to square root of molar mass)
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 [sup]sqrt(28)[/sup]/[sub]sqrt(4)[/sub] 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).
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.
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
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?
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.
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
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.