RoadBikeReview.com's Forum Archives - General


Archive Home >> General(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 )


CO2 - true or false???(17 posts)

CO2 - true or false???Rich_Racer
Jun 20, 2003 1:52 PM
A guy I was riding with this morning said that he'd been told by his LBS that you have to let the CO2 out of a tube, post-ride, and replace with air as the CO2 degrades the inner tube?! Really?? This doesn't sound like it could be right. Anyone know??
re: CO2 - true or false???russw19
Jun 20, 2003 1:58 PM
I don't think it degrades the rubber of the tube, but there is a law in Chemistry that states that any pure gas will permiate a membrane faster than a mixture. The CO2 in a cartridge is just that, CO2, where as the air you pump in with a pump is Nitrogen, Oxygen, and CO2 as well as a few more. So in theory, I think a tube filled with air as we know it will stay up to pressure longer than one filled with just CO2.

But some chemist will be better at explaining this to me.. I just remember something of the sort from 11th grade Chem class about 14 years ago.

Russ
Brief explanation of physics behind this phenomenonctheronj
Jun 20, 2003 3:09 PM
This reasoning is right on the money but the explanation is wrong.

The bicycle tube is not entirely leakproof; it has microscopic holes that allow gas through. Everything wants to be in balance on one side of the tube (the "membrane") and the other. Not in total balance, but in individual balance. So, if you pump up your tube with air, it's about 78% nitrogen, 21% oxygen, and 1% carbon dioxide -- the same fractions as the outside air. Now, pump up the tire with CO2, and you've got 100% CO2 on the inside -- this doesn't balance very well with the 1% CO2 on the outside, so the CO2 crosses the membrane more rapidly, and your tire goes flat faster.

This is the same reason a helium baloon loses air faster than one filled with air.

I must say, though, I would be suprised if the tire goes flat THAT fast. I use a Zefal frame pump so I don't know.

As to damaging the tube, CO2 is fairly inert, although it can form "carbonic acid", a weak acid, if it gets mixed with water. Do you think seltzer water would hurt your inner tubes? If you put it in your stomach, it's safe for your bike.
The real reason from an expert in diffusionContinental
Jun 20, 2003 5:06 PM
The primary reason for the faster pressure loss for tires filled with CO2 compared to tires filled with air is that the CO2 passes through the rubber easier than oxygen and nitrogen. This is because the CO2 actually dissolves in the rubber. Or, in other words, for rubber CO2 has a permability coefficient about 5 times higher than oxygen and about 10 times higher than nitrogen. So if you have two identical tires, one filled with air and one filled with CO2 to identical pressures (say 100 psig), the tire filled with CO2 start to loose pressure nearly 9 times faster than the tire filled with air because the CO2 passes through the rubber more readily. This faster pressure loss for the CO2 tire would occur even if the tires were place in a room with no air, and even if the room were filled with C02 at atmospheric pressure.

If you filled the tire with pure methane instead of pure CO2, the tire would hold pressure about as well as it does when filled with air. This is because methane doesn't dissolve in and diffuse through rubber nearly as fast as CO2. The tire would loose pressure quickly with helium because helium is a very small molecule that can pass through the tangle of rubber molecules.

geeks can check the link below:

http://www.cus.cam.ac.uk/~jae1001/teaching/materials/M6_Lecture_6.pdf
Unadulterated rubbishMel Erickson
Jun 20, 2003 1:58 PM
Your tire will go flat within a day or two if it's filled with CO2 and you'll have to "replace" it with good old air anyway. Maybe the shops confusing ozone with CO2. Ozone will deteriorate tires and tubes so don't store them around electric motors or other ozone generators.
re: CO2 - true or false???xxl
Jun 20, 2003 1:59 PM
I haven't heard that, but I do know that CO2 will "let itself" out of the inner tube, as its molecular construction allows it to pass through the walls of the tube more easily than air. That's why you should refill your tires, apres-ride, with air.

I can't see why CO2 would degrade a tube, but I'm no chemist, either. Does it make a difference if the tube is polyurethane vs. rubber?
He just wants to sell you more CO2!! (nm)Alexx
Jun 20, 2003 5:17 PM
CO2 is non-reactiveMR_GRUMPY
Jun 20, 2003 5:59 PM
In theory, CO2 is slightly better for your tubes than air. Air always has just a little Ozone in it, and Ozone will degrade your tubes.
Yes, CO2 will leak out of your tubes just a little faster than plan air. I really don't know that a person could tell the difference. It might be as much as a few extra pounds a day.
Somebody here should do an experiment. Fill a tire with CO2, let it sit, check back in two weeks and give it the squeeze test. Then, pump up a tire to about 85-90 pounds, let it sit for two weeks, give it the squeeze test, and report on the results.
Think again53T
Jun 20, 2003 6:16 PM
I used to use CO2 a lot. Every time I changed a tube on the road it would be flat the next morning. No need to wait two weeks. The first time I thought I flatted the second tube, but after that I was a lot wiser and just pumped it up with air the next day. I use a frame pump now, since I saw a picture of Jan U. out training with a frame pump on his bike. (Hey, I might as well be honest)

In the air gun community, there is a belief that CO2 will disolve into your rubber o-rings used as seals on some air (actually CO2) pistols. This is blamed for the swelling and softening of the o-rings that sometimes occurs if the gun is stored with CO2 pressure still in it. I won't say this is true, but it is a belief.
Any rubber swelling, softening from CO2 is reversibleContinental
Jun 20, 2003 6:46 PM
CO2 does dissolve in rubber and cause it to swell slightly and soften. This is completely reversible and causes no permanant damage.
Not that it matters, but why do they compress CO2 not air?AllUpHill
Jun 21, 2003 5:58 AM
Why is CO2 the gas of choice for compressing into these little cartridges? Seems like all the applications (tires, guns, ...) just need some kinda gas, not a specific property of CO2. Why not plain air or some other cheap gas?

Just asking since we seem to have a lot of chemists on the board.
CO2 liquifies under moderate pressureContinental
Jun 21, 2003 7:37 AM
The cartridges contain liquid CO2 which vaporizes and creates a large volume of gas to fill the tire. A cartridge full of air at the same pressure would contain gas and would have to be larger than a pump to have enough air to inflate the tire.
440 psi is not moderate pressure...C-40
Jun 21, 2003 12:10 PM
Better take a look at the liquid/gas chart for CO2. It's a liquid at around 440 psi at 20 degrees C. The pressure in a C02 cartridge must be at least that much.
And what pressure does the chart give for liquid air?TFerguson
Jun 21, 2003 6:32 PM
I beleive that you will find that 440 psi is "moderate".
TF
Air will not liquify at ambient temp, moderate is relativeContinental
Jun 23, 2003 3:08 AM
At ambient temperature air will not liquefy at any pressure. Nitrogen needs to be cooled at least to -232 F (the critical temperature) and oxygen -181 F before they will liquefy. I didn't calculate, but it would would take about 5000 pounds per square inch to compress enough air into a cylinder to give the same volume of gas that you get from liquid CO2.

Moderate is certainly a relative term, but my definition is that if you can buy a cylinder at Walmart for $0.50, this is a moderate pressure that does not require special equipment for production, storage, or usage. You wouldn't buy a 5000 psig air cylinder at Walmart or carry it on your bike.
The infamous double flatMR_GRUMPY
Jun 21, 2003 11:32 AM
This morning's 50 miler almost ended early. A teammate of mine lead me right into a crater size pot hole. He says "hole", just as he is missing it by two inches. Both front and back are now filled with CO2. I'll use different wheels tomorrow, and report back on how the CO2 leaks out.
(I'll be able to give my teammate a hard time for the next six months because of his error. It will be well worth the time spent changing tubes) His new name will be "Pot Hole Man"
re: CO2 - true or false???Rob Sal
Jun 21, 2003 2:04 PM
I thought it was the traces of oil found in some cheap cartridges that affects the rubber rather than the CO2.