I'm surprised nobody mentioned this yet, or maybe I just missed it. Here is an example of why we don't use air instead of CO2:

https://www.genuineinnovations.com/bi...px?prodid=1011

The product listed above will use either CO2 cartridges, or "Big Air" cartridges (which are filled with.... AIR). Notice how BIG the Big Air cartridges are.

So to answer the actual question: Because air cartridges would have to be a lot bigger than CO2 cartridges.

And to answer the question that wasn't asked: There ARE alternatives that use air, but they're not very popular because of the size.

From my years of paintball playing, this statement you have quoted is absolutely true. We used to use things called anti-siphon tanks to prevent liquid CO2 from entering the gun upon rapid firing. How does it happen? The universal gas law (PV = nRT) governs pressure, volume and temperature. Basically, as the pressure drops rapidly, so does the temperature. If you've used a CO2 inflater before you will notice as it fills the tire the cartridge becomes extremely cold. At these temperatures, liquid CO2 can exist at standard pressures (until it warms back up again of course).

To the OP, CO2 is cheap, compressible and easy to package in a small inflator cartridge. There is already a large market for CO2 in small packages already, so it is the logical way to go with inflators.

You probably get more bang for the buck with CO2 because it compresses and the fact that someone already makes it for something else. Also though nitrogen is fast on the rise as the new inert gas. no water, no increase in the heat.They are actually using it for auto painting and the hose heats it to reduce the viscosity of the paint without having to use solvents or less of them.

Those little CO2 cartridges were orginally used for soda water. You know those seltzer bottles, you probably last saw one in a Warner Bros cartoon, Bugs Bunny spraying Elmer down with a bottle with a lever on it. Opposite side of the lever is a little cartridge that gets unscrewed, the little tank gets put in, and it's screwed back together. Anyway, that's what the cartridges were designed for. Since they were made for that, many other uses have happened. There's a few homebrew beer kegging systems that use them, I've seen computer air sprayer thingies that use them, and of course, they're a ready source of compressed gas for things like bb guns and paintball guns and, yes, emergency bike inflators.

So the "Why CO2" is because there's already a market for it, and as cyclist we're probably soaking up a half percent of the total production. If we wanted air, or nitrogen, or some other reasonably inert gas (argon?) we'd have to pay for a custom production facility for 'em.

Thanks, I'll let the heavy drinkers pay for my speed inflators. Oh wait. I'm still paying for it. D'OH!

That does not make liquid CO2. You have to have it still compressed and cold. Below 75 psi there is no liquid CO2 and at 75 psi we are talking at -58.6C. You would have frostbite. However, I do believe that through rapid fire you get water condensing from the air, this is possible and probable.

You can get frostbite from compressed air. Hold a can of compressed air (computer duster) upside down and point it at anything, and it will freeze it instantly. That is how the new freeze away wart systems work, as it is a controled form of frostbite, all though it uses propane and ether.

The Big Air cartridges are propane. So be careful, MTBers, if you're smoking while you change a tube.

You got it right on the money. Molecular Nitrogen (which makes up the vast majority of plain 'ol air) is a much larger molecule than CO2, thus it's dang near impossible to compress a useful amount of it into an itty-bitty cylinder. This is also the problem with CO2, since it's a smaller molecule, it bleeds out through the rubber of an inner tube more easily.

So if you get a flat and use a CO2 pump to re-fill.. you should empty and re-fill your tire with the floor pump back in the garage after making it home?

Does anyone put nitrogen in their road tires? Like at the tire shops now you can have them inflate your tires with the stuff for extra. That would be overkill for a bike. I'm still waiting for compressed helium to fill my tires with.

I used to use nitrous oxide containers but I couldn't get home from parties with two flat tires.

helium is smaller than CO2.

So does the kinetic theory of gases not apply here, or is it liquid in the container?

There is a lot of false information being thrown around here. CO2 is not a 'smaller' molecule than N2, it simply diffuses through butyl rubber at a faster rate due to various subtle interactions.

The reason why CO2 is used in a cartridge is simple: when compressed to a certain pressure at room temperature it condenses into a liquid - this is what is present inside your little cylinder. Air (or nitrogen) would still be a gas at the same pressure/temperature; since a liquid is more dense than a gas, you can get 12 or 16 grams of CO2 in a small package. You could not do this were the substance to remain gaseous. When expanded to a lower pressure the liquid changes into a gas to inflate the tire (and this absorbs heat, which is why the inflator gets cold).

If you have taken chemistry or physics you may have been exposed to 'phase diagrams' for various substances: these show the state of the substance as a function of various variables (e.g. pressure, temperatue, etc.). CO2 is also common, non-toxic and will not harm rubber.

Yes

I'd instead suggest be ready to fill up before your next ride. Then you can learn for yourself how much pressure was lost in the time you waited to re-fill.

Al

I'm no expert but I do know that paintball CO2 tanks take higher pressures then 700psi. And the only time I ever play paintball is in the winter time and the cold weather only increases the pressure in the tank. And with that being said, it is very possible for liquid CO2 to enter the gun.

Finally!

Jeez people, ideal gas laws don't apply to phase changes. CO2 (and propane) is liquid in those canisters - compressed air or N2 are not liquid at any reasonable pressure.

If you don't hold the inflater properly (vertically), you will get liquid CO2 in the nozzle which will probably freeze into a solid and prevent any more CO2 from going into your tire.

That's an easy way to forget to do it...

Thank you for the clarification.

Google turned up this corroborating explanation as well.

Then next time one will not forget.

You are right though, some folks may forget, but for me checking tire pressure by hand before any ride is habitual.

Al

Usually, the tire that was filled with CO2 is completely flat in 16 hours or so, not likely to go unnoticed when you're preparing for the next ride.

Actually, I think what you are experiencing when a valve freezes up is ice formed from water vapor in the air. While some liquid CO2 might shoot out of an inflator, it quickly turns to gas under atmospheric pressure.

I'm no chemist or physicist but I know the answer.

The real answer is that CO2 can be stored under pressure in liquid form. When you release the pressure, it morphs into gas form and in the process absorbs a lot of heat, that's why the cartridges are so cold after you use one. If you just tried to compress enough air to inflate your tire it would take a lot bigger and stronger cartridge.

Actually I have never experienced it, only heard about it. I think it's a problem when people hold the inflator improperly - and/or - throttle the flow too much (little bursts instead of one quick shot).

What you're saying could happen of course, and if so, I would expect it to be a bigger problem in areas/seasons of higher humidity. But the drop in temperature would be the same no matter how you hold the inflator... so if holding the inflator vertically (valve up, liquid down) prevents the freezing, then it would seem it's really the CO2 that's icing up if you hold it sideways. Also, once you make the connection, there's no air (and no water vapor to freeze) in the pathway between cartidge and tube.

Easy to run some experiments on if I didn't mind wasting a few carts.

Right, which is why I like to check pressure before ride, not after.

Al

BINGO!!! Tmass is spot on target here.