To be more clear on this I should have said that it is well known from BikeForum anecdotal evidence that CO2 diffuses out of tubes faster than air. This comes up a lot. The polymers that are discussed in the paper you cite are much different animals than the elastomers that are used to make tubes.

After numerous, and I mean numerous, discussions on this topic many of us have come to the conclusion that the CO2 isn't just diffusing through the rubber but it is dissolving in the rubber and passing out. The N2 and O2 in air follow a diffusion pathway and thus take longer to get through the rubber.

I know of no one who has done a controlled experiment, however it wouldn't be hard to do as a homebrew measurement.

I have my doubts that the military does their own oxygen concentration. Like most other stuff the military does, it probably buys it from some one like Airliquide. Those guys process millions of tons of the stuff per year.

I'm also dubious that they would use compressed or liquid O2 on aircraft. They probably use O2 concentrators like old people use. Much less likely to go Boomba! Boomba! if you happen to put a hole in the aircraft;)

As for the "Nitrogen is a by-product of Oxygen" comment. I understand what he meant but other people might not. Lots of people have goofy ideas about chemistry and where stuff comes from. I just wanted to clarify it. I'm going to go clarify the He from natural gas thing next;)

And, no, I don't hate you! I lub my little wroomwroomoopsgums! Yes I do!:D

This is quite true for Ar and N2 and O2, however the CO2 molecule is huge compared to any of those (don't have the numbers right here and don't have time to look them up;)), yet we have ample anecdotal evidence that CO2 diffuses out of a rubber tube faster than N2 or O2 (I'll agree on Ar.) For that there is another mechanism going on that is independent of molecular size. It can't have to do with linearity since all the above are linear molecules. It can't have to do with electronegativity, since all of the molecules are nonpolar. It can't have to do with reactivity since most of the molecules above are nonreactive. The mechanism is still up in the air but solubility looks more likely than anything else I can think of.

See above. CO2 diffuses faster than air. Try it at home. Fill one with air and one with CO2. The CO2 one will be very soft by morning.

There is enough of a shortage of He that people are starting to notice. It's becoming a problem.

Again to be precise, helium is 'isolated' from natural gas. Natural gas is a mixture of hydrocarbon gases that can not 'produce' an element like helium. Helium is 'produced' from radioactive decay of higher elements...or by squishing two hydrogen atoms together.

[Edit] I'm sorry to harp on this but the term 'production' implies reaction to make something. Burning natural gas 'produces' CO2 and water. The oxygen, carbon and hydrogen are still there but their form has changed. To say that you have 'produced' nitrogen from oxygen or helium from natural gas, implies that, somehow, you have done a reaction and made these elements from other stuff. Elements aren't made from anything short of nuclear reactions.

Except to that **** Fluorine:D

By severe effect they probably mean that it will swell the elastomer and may make it weaker. It's not a chemical reaction but a physical change. In an o-ring, swelling is not good...especially if you are trying to seal a moving part. Swollen o-rings on valves, for example, won't let them reseat and seal. That makes them leaky...not usually a good thing:D

And I still lub my littl' wroomwroomoopsgums!

There's more to the story. Natural gas is a fossil fuel, meaning it used to be carbon-based lifeforms. Most Helium left the atmosphere (it's so light that it just drifts away) long before the proto-gas compost got smooshed and buried. But, He is also formed by the radioactive decay of elements such as uranium and we think this is how it gets into the gas.

At one time, the sole commercial source of Helium was a field in the Panhandle of Texas. For some reason that gas is extremely high in He. During the 30s the US imposed an embargo on Germany and it could no longer get He for its zeppelins. So they filled Hindenburg with hydrogen instead -> boom!

Extraction from other sources is a relatively recent occurrence. I remember talk when I was in school about what would happen when the He ran out. In retrospect it was a rehearsal for the same talk about oil. 'Course, without natural gas we can still produce power and probably plastics, but we'll have a heck of a time getting Helium!

But you would be wrong to think that the problem with the Hindenburg was the hydrogen: what actually burnt was the light-reflecting paint on the outer cotton coat. This paint contained, among other things, iron-oxide, aluminum powder and some flammable plasticy stuff. Anyhow, unbeknown to the chemists of the time, iron-oxyde + aluminum can work as an explosive and is extremely flammable. It's unknown what was the reason for the initial ignition of the ship, but at any rate, the paint in the coating caused the ship's doom. The hydrogen had nothing to do with it - in fact, the Germans had lots of experience with hydrogen-filled blimps, used for years without a single accident.

Actually, that's far from a generally accepted explanation:
http://en.wikipedia.org/wiki/Hindenburg_disaster

(My personal opinion is that a helium Hindenburg would have docked safely that day, without any problems at all.)

The viscosity of Carbon dioxide is lower; it'll diffuse faster.

Why not coat the inside of the tube with that stop leak stuff they use in helium balloons to keep them inflated longer?

Some thougts, from a former paintballer:

Some paintball fellows thought of using compressed helium instead of CO2 or Compressed air, or EVEN compressed nitrogen...

What they found, however, is that the helium escaped between the O-rings designed to keep other compressed gasses in. Gasses compressed at high pressures, as much as 800PSI.

Likely, a tire would not be terribly likely to hold 100 psi for a long time, and even if it did how much would you trust the valve?

The anecdote by cyccommute is correct. It turns out that my previous assertions were wrong.

I have done some further research on the subject. It appears that atomic diffusion is in fact a function of the solubility. Here's what I found from tables in a book (I'll reference on demand):

-carbon dioxide diffuses the fastest.
-oxygen diffuses slower than carbon dioxide.
-nitrogen diffuses slower than oxygen.
-argon diffuses slower than nitrogen.
-(inferential) neon diffuses slower than argon.
-(inferential) helium (possibly) diffuses slower than neon.

These values apply only for natural/isoprene rubbers.

speak for yourself

finally an answer to the question. smaller => leaks out faster. More expensive initially,and leaks out faster. Heavier => slower.

finally an answer to the question. smaller => leaks out faster. More expensive initially,and leaks out faster. Heavier => slower.
By the way. I think moisture from the air would slowly collect in the tube, after repeated refilling. I wonder how much would collect in a month if you pump up your tires every day?

DieselDan spoke for himself in 2007 and hasn't been active on BF since 2013.
Zombie Thread Alert!

The Olympic people experimented with various exotic gases for many years. They found out through scientific experiment there's little difference as far as efficiency in a bicycle tire. Hydrogen, they recommend to stay away from due to its flammable content. Does anybody remember their history from the zeppelin the Hindenburg? I would agree with most of the posts - air, it's free and readily available.

Josh Poertner either at Silca or Zipp found a definite advantage with heavier gasses for the hour record. https://marginalgainspodcast.cc/the-...-gains-part-3/

I should note that not all heavy molecules work. It has to be a large molecule with low solubility in latex.

Not reinflating at regular intervals. Imgine latex with no reinflation?

I dislike responding to a 12 year old thread but your statement kind of requires it. A heavier molecule doesn’t necessarily mean that it diffuses slower. There are other factors to consider like the solubility of the gas in the rubber of the tube. Think of it this way: If you made a container out of sugar, it would hold hexane forever. It would only old water for a little while. Hexane has a higher molecular weight and is much, much larger than water but it won’t dissolve the sugar.

Kind of. Pure N2 is used because tire pressure varies less when N2 is used over N2 and O2 (that is, air). Moisture is not much of an issue if you use a compressor because most of the moisture condenses out at the high tank pressure and the air that goes into the tire has a very low dew point at tire pressure.

The moisture in air can condense during compression, especiallly after the air (which is heated by the compressor) is cooled down. So compressor tanks have a water valve in the bottom. Big industrial compressors have heat exchangers to cool the compressed air which condenses the water more completely. The water valve in the sump is sometimes put on an automatic timer, or there is a level sensor in a small sump which tells a valve to open to discharge the condensate. The point is, once air is compressed properly (with heat exchangers and a tank to remove water), there is LESS humidity in that air. And when the air pressure is relieved from the tank (at say 110 psi) to a wheel (from about 26 to 42 psi, depending up car weight, tires, and conditions) the moisture is already "boiled". That is, its a gas and there's no liquid water to boil. And there's very little of it. So the "dew point" (temperature at which water condenses) is very low.

For bikes that use tubes and are pumped up using a hand pump without any heat exchanger or tank with sump, you can be creating liquid moisture within the tube. A small amount. So if you are running tubes and don't use a compressor with a tank and drain valve, N2 might make a slight difference. For a bike using sealant, the added moisture is ridiculously small compared with the amount of sealant, and probably adds to sealant life by replacing sealant water that has evaporated. Also for bikes, the permeability of O2 vs N2 doesn't make much difference unless you are speaking of weeks of riding. During which you will probably be topping off the pressure anyway. So IMHO, unless you use tubes and don't use an air compressor to inflate, substituting dry N2 is a needless expense. I'm also of the opinion that it's a pretty ridiculous and fairly needless expense and conceit to use dry N2 even if you do have tubes and normally use a hand pump - the effect is just too small. And there's like a gazillion bikes out there that use hand-pumped air without issues.

N2 (mw 28) is a smaller molecule than O2 (mw 32) but IIRC the permeability of O2 through rubber is actually higher. So with air, you'd theoretically lose pressure faster as the O2 leaks out. A ridiculously small difference for passenger tires but it might make a difference for race cars. For passenger cars, its also ridiculous because most cars tires are inflated with compressed dry(er) air. So moisture is an issue. There's also a separation going on. Suppose O2 does leak out faster than N2. As an extreme example, assume that O2 leaks out, and N2 doesn't leak at all. Now suppose you put 100 psi in your road tires. 80% is N2, 20% is O2. Assume you lose 5 psi. Now you are at 80%/0.95 N2 and 15%/0.95 O2. 84.21% N2 and 15.79% O2. You top off with air. Do the math: you now have 84% N2 and 16% O2. Assume you've lost another 5 psi. Because you have less O2, this takes a lot longer. But it happens, and you top off. So now you're at 88% N2 and 12% O2. After a while, you get to about 99.2% N2! Every time the tire leaks down its leaking mostly O2 and the gas inside the tire keeps getting enriched in N2.

The point is, by topping off your car tires with compressed air, there is minimal moisture remaining. And because the O2 leaks out preferentially (which is the whole point of using N2), you automatically get a gas mixture that is enriched in N2 in your tire. For free!

I agree with cyccocommute on the Argon issue and the technical issues of diffusivity of a gas through rubber. It's not just a matter of molecular size. Compatibility (essentially solubility) must be considered. It's complex.

BTW, notwithstanding my blather above, If I were going to insist upon something other than air, I'd use (cheaper) dry N2 before I went with Argon. And if you're a gram counter, N2 is about 5 grams per wheel lighter than argon!

I know, I know... zombie thread.
FWIW: the Blackbirds stored oxygen in liquid form. From my favorite book on the subject:
https://books.google.com/books?id=De...olume&q=oxygen

If you like airplane engineering, this is a fascinating look at the development of a unique airplane:
https://www.amazon.com/Archangel-Sen.../dp/1563479338
You'd probably enjoy the information regarding the lubricants developed just for the Blackbirds.

Not exactly. There's a difference between "extracted from" and "produced from". He is an element and it takes pretty extreme conditions (like the core of the sun) to "produce" He from hydrogen. There is a small amount (on the order of 1%) of He in natural gas due to decay of elements like thorium and uranium.

The He is extracted from natural gas (mostly ethane and methane) by liquifying the hydrocarbons producing LNG, and then removing nitrogen from the remaining off gases.

It is key to understand that He can only be extracted, and not produced. There's a finite amount in natural gas reserves.

https://gasliquids.com/pdfs/2016_HeliumInNaturalGas.pdf

No, they use nitrogen because it has more a consistent contraction/expansion rate, making it more stable. The tire pressure remains more consistent than it would using regular air. It has nothing to do with the moisture in the gas mixture.