CO2 Pressure Loss?

[BobsTrail]

I have aired up a number of times with my CO2 tank and I'm sure the mix of CO2 to air is very high. I noticed that after 2 or 3 weeks my tires are 3 to 4 pounds lower, even with the same tempature. It appears that CO2 is excaping even though it's more dense than air.

Is it time to get on-board air with Viar or ARB instead?

 

[Jeremy556]

Not likely. When you air up, are you airing up to your cold or hot pressure?

I run 38psi cold, which is 42psi hot, so when I air up after a trail, I air up to 42psi. If I only air up to 38psi, when the tires cool, I'll be down around 33 or so cold.

I air up/down too often to mess with refilling CO2 and run dual Viair compressors. That said, I would recommend the ARB dual compressor setup.

 

[marshal]

c02 molecules are much larger than air. but also don't forget that when you're airing up your tire from your tank the c02 that is coming out is closer to absolute zero than it is to ambient temp. its very, very cold. that is your main problem in pressure loss

 

[Swarty]

c02 molecules are much larger than air. but also don't forget that when you're airing up your tire from your tank the c02 that is coming out is closer to absolute zero than it is to ambient temp. its very, very cold. that is your main problem in pressure loss

Nope. Nowhere near absolute zero.
Here's why:
Dry ice forms at about -56C and absolute zero is -273C. So it's pretty much guaranteed to be less than 90C from ambient and more than 217C from absolute zero. Either way the molecules are not so small that they can easily escape since temperature changes the space between molecules much more than the size of the molecules themselves. Besides this the gas would quickly equalize in temp with the tire before getting even a couple microns through the tire since the heat capacity of a gas is minuscule when compared with a solid.

IMO
The OP either has a tire/valve issue or is simply seeing the affect of temp drop as mentioned earlier. Probably temp drop due to the consistency he's reporting but never count out tire shops using crap valves that start to fail from constant use very easily and would be slightly hastened by the water vapor in his tire that is now carbonic acid due to the co2.

 

[Intruder]

Get a valve tool and try tightening the cores up. If that doesn't do anything it's probably the temp change.

 

[Chsbrgr]

I have aired up a number of times with my CO2 tank and I'm sure the mix of CO2 to air is very high. I noticed that after 2 or 3 weeks my tires are 3 to 4 pounds lower, even with the same tempature. It appears that CO2 is excaping even though it's more dense than air.

Is it time to get on-board air with Vair or ARB instead?

Same CO2 results. Definitely have to top off the tires more often since they are mostly CO2 from airing down and airing up. Proved it to myself recently when I had a flat repaired. The repaired tire was filled with all air at the tire shop. After the flat, topped my other three off so they had the same pressure. Two weeks later, only the tire filled with all air had not lost 5 pounds.

CO2 at the trailhead is so much faster that it's worth it to me, but I have no question it loses pressure faster than tires with just air. If I was to make a cross country trip with no off roading, I'd drain my tires and fill them with all air.

 

[Bootleg]

I know for a fact that CO2 does not hold pressure as well as normal air. Being a cyclist, both mountain and road, there have been plenty of times I've repaired a flat and used pressurized CO2 to re-inflate the tire. Under normal conditions my tires lose between 1-2 psi over 3 days but the tires that have been re-inflated using CO2 can lose 15 psi in a mountain bike tire which normally holds 27 psi and 60 psi in a road bike tire which normally holds 110 psi. That's a dramatic difference.

I always deflate my tires once I get home and re-inflate them with my floor pump. When I don't I arrive to ride and they're flat. :mmph: CO2 should be used to get you home, not keep you rolling in the coming days and weeks.

 

[Swarty]

I know for a fact that CO2 does not hold pressure as well as normal air. Being a cyclist, both mountain and road, there have been plenty of times I've repaired a flat and used pressurized CO2 to re-inflate the tire. Under normal conditions my tires lose between 1-2 psi over 3 days but the tires that have been re-inflated using CO2 can lose 15 psi in a mountain bike tire which normally holds 27 psi and 60 psi in a road bike tire which normally holds 110 psi. That's a dramatic difference.



I always deflate my tires once I get home and re-inflate them with my floor pump. When I don't I arrive to ride and they're flat. :mmph: CO2 should be used to get you home, not keep you rolling in the coming days and weeks.

The rubber used in bicycle tubes is butyl rubber. Butyl and CO2 are soluble so CO2 will both absorb into the butyl and make it weaker causing leaks. Never leave CO2 in inner tubes. This is apples and oranges to a vulcanized rubber tire. Also CO2 molecules are larger than N2 molecules so they should hold pressure better than air. But anecdotal evidence is king on forums so if anyone is looking to sell their CO2 tanks for cheap send me a pm ;-)

 

[Ol'Baditude]

I run 38psi cold, which is 42psi hot, so when I air up after a trail, I air up to 42psi. If I only air up to 38psi, when the tires cool, I'll be down around 33 or so cold.

This is precisely what I do with my Powertank and my compressor and it works like a charm. Exact same pressures too, incidentally.

 

[Canazes9]

Not likely. When you air up, are you airing up to your cold or hot pressure?

I run 38psi cold, which is 42psi hot, so when I air up after a trail, I air up to 42psi. If I only air up to 38psi, when the tires cool, I'll be down around 33 or so cold.

I air up/down too often to mess with refilling CO2 and run dual Viair compressors. That said, I would recommend the ARB dual compressor setup.

Agreed - Pretty much my exact experience.

Temp of the gas filling the tires has squat to do with it - the mass difference between the gas charge and the tires/rims dictates that the gas fill will rapidly assume the temp of the tires, not the other way around. Hot tire pressures are higher than cold air pressures - fill your hot tires to your cold air inflation pressure and you are 100% guranteed to have them be ~3-5psi low when checked cold. Compensate for the difference when filling and the pressures will be right when checked cold.

David