85 replies to this topic

[craig333]

I'm curious to see what you get as the guys that run CO2 that I know haven't brought this issue up. I just went out and checked the spare on the Jeep which was last aired up with CO2 after losing the bead. 15 psi. Unfortunately I don't recall what it was aired up to then but likely wouldn't have been much more. 

[smlobx]

Not to rain on anyone's parade but I'm having a hard time following the logic of using CO2 over other gasses...

The best gas to run in tires is actually Nitrogen which is about 70% of what generic air is made up of.
People who race cars/trucks for a living from the Baja 1000 to Formula 1 all fill their tires with Nitrogen because it is inert and not affected (as much) by temperature fluctuations.

The reason the OP can fill more of his tires with CO2 is probably due to the cylinder more than anything else.

[Shadyapex]

Not to rain on anyone's parade but I'm having a hard time following the logic of using CO2 over other gasses...

The best gas to run in tires is actually Nitrogen which is about 70% of what generic air is made up of.
People who race cars/trucks for a living from the Baja 1000 to Formula 1 all fill their tires with Nitrogen because it is inert and not affected (as much) by temperature fluctuations.

The reason the OP can fill more of his tires with CO2 is probably due to the cylinder more than anything else.

I understand the superiority of using Nitrogen, it is a better gas for inflating tires. CO2 is handy just because you can store more of it in a bottle. It's my understanding that it's more compressible.

[Wallowa]

Well, why CO2 indeed. If I interpreted what my limited search produced here might be a reason. Volume of compressed gas vs size and pressure of cylinder. The aluminum 50 cf SCUBA tank has a rated and working pressure of 3,000 psi and contains 48+ cf of air. The CO2 cylinder in the article looks to be the same size cylinder as a "50cf" SCUBA cylinder, but this CO2 cylinder is filled to 100 psi and contains 87 cf of C02 gas. In short form, CO2 has more molecules per mole [amount of gas per volume compressed] so you could fill more tires with that cylinder of CO2 @ 100 psi than you could with it filled with air at 3,000psi... 55+% more.

 

But hey, I could very well be wrong! Phil

 

 

Ps..What Shady said..

Edited by Wallowa, 28 November 2015 - 06:47 PM.

[JHanson]

 

The reason the OP can fill more of his tires with CO2 is probably due to the cylinder more than anything else.

 

 

Exactly. As Wallowa said, it's strictly about available volume in a reasonably sized cylinder. No benefit to the gas itself, and possibly a disadvantage if it really does permeate through the tire that quickly. I know it will do so through a butyl bicycle inner tube.

 

Powertank, one of the pioneers in marketing CO2 tanks as a replacement for a compressor, claims that a tire filled with it will deflate more slowly than one filled with air, because "the CO2 molecule is larger than a nitrogen molecule, which comprises 78 percent of air." My understanding is the CO2 is actually soluble in butyl and thus does not have to permeate. But I'm going to experiment.

[Shadyapex]

This is interesting. I had always thought that the CO2 was a smaller particle than N but some quick online research seems to refute this. My experience has always been that I have to adjust the pressure in my tires much more frequently after airing up with the CO2. But I have never done a side by side comparison.

[Shadyapex]

This is from a cycling page but I thought it might be relevant. Many cyclist use a small CO2 cartridge to refill after a flat rather than a pump.

Dear Lennard,
Since a CO2 molecule is larger than either an oxygen or nitrogen molecule, why does it leak out of a bicycle tire faster?
Glenn

Dear Glenn,
Upon receiving your question, I put CO2 in a clincher tire with a Michelin butyl inner tube (latex tubes leak air quickly, as you’re probably aware). This particular tire and tube hold air pressure faithfully for weeks on one of my road bikes without needing pumping. And sure enough, within three days after inflating with CO2 to 90psi, the pressure had dropped to 45ps
...
Permeation by diffusion predicts gas leakage rates proportional to the inverse of the square root of their molecular weights. Using air as a reference the predicted leakage rates for common gases are: helium 2.7, air 1.0, nitrogen 1.02, oxygen 0.95, argon 0.85, carbon dioxide 0.81.

It turns out however that the leakage rate of CO2 is huge, and the reason is that it is actually soluble in butyl rubber and is thus not constrained to normal permeation loss, it can transfer straight through the bulk rubber resulting in severe tire pressure loss on the order of a single day.

 

Here's the link: http://www.baltimore...090207131052655

 

Thoughts? Does this sound legit? I think JHanson mentioned the possibility of solubility as well.

[JHanson]

 

Thoughts? Does this sound legit? I think JHanson mentioned the possibility of solubility as well.

 

 

 

 

It seems to be well accepted in the bicycling community, where small CO2 cartridges are popular for flat repair. I'll be back where I can compare two BFG AT tires side by side in a week or so. I'll report.

[ntsqd]

In skimming the recent posts I think a very important aspect about CO2 tanks was missed. The reason that you can get a LOT more tire fills out of a CO2 tank than a comparable sized N2 tank or air tank is rather simple. Phase change due to pressure. N2's phase change pressure isn't reached in a normal tank so even at 2500 psi it is still a gas, but CO2's is and it goes to a liquid state. Change that gas to a liquid and you can get a whole lot more volume of CO2 in the tank than you can get N2 in the same tank. Propane also phase changes to a liquid in the tank, but propane filled tires is too scary to think about. shudder......

Edited by ntsqd, 30 November 2015 - 01:38 AM.

[Wallowa]

ntsqd,

 

Very interesting.  You are you saying that when his CO2 tank is filled to the 100 psi rated pressure it goes from a gas to a liquid inside the cylinder?  Wow, I had no idea that such a low pressure could transform CO2 gas into a liquid.  Very interesting.

 

Thanks for insight,

 

Ps...I just re-read the Outback specs on the cylinder charging...they fill to 100psi 'cold' [no mention of temp] and then state that at 70 F the pressure is 837 psi....not certain how all this folds into the possible phase change for gas to liquid.

Edited by Wallowa, 30 November 2015 - 01:56 AM.