C02 question

[eric1115]

I have a question for all you guys and gals with Powertank setups...

I use a C02 inflator for on-the-trail mountain bike fixes (a 16g cartridge will fill a bicycle tire perfectly), but after that fix, I need to deflate the tire and refill with air once I'm back home. If I leave the C02 fill in the tire, it deflates over the course of a day or two. It's more pronounced with a conventional tire and butyl tube on a road bike than it is with my tubeless/sealant mountain bike setup, but still there. Unless I'm misinformed, C02 molecules are smaller than nitrogen molecules, and therefore move through the walls of a tube or tire more quickly.

Do you notice needing to top off more often with C02? Do most of you air down and back up often enough that it doesn't come into play? Or, are we just dealing with thick enough material that it doesn't have any appreciable leak down?

Eric

 

[Errant]

I've never had that issue with CO2 in my tires. I've also used it to fill truck tubes for river floats and not lost any noticeable pressure over a season.

 

[GeoScum]

Unless I'm misinformed, C02 molecules are smaller than nitrogen molecules, and therefore move through the walls of a tube or tire more quickly.

Without checking some old chemistry textbooks, I doubt that the atomic radius of N2 is much greater than CO2

 

[Ohaul]

Could it be that the CO2 system you are using is freezing (or leaving ice crystals) in the valve stem? If so then maybe its just a slow leak that gets cleared by compressed air.
A regulator like the ones on the larger CO2 bottles would hide a bit of ice inside the diaphragm housing.

 

[01tundra]

I've been running CO2 in my tires (on Trailready beadlock rims) for years and the only time I have a tire drop pressure is when I intentionally air down. Mine always stay at set pressure for as long as I allow them to.

 

[eric1115]

It does freeze the valve. That's not the issue, though. Most bikes run Presta valves, which are a mechanical closure. They freeze partially open sometimes, but when I screw the valve closed it stops leaking. There's no regulator to speak of on an inflator... it simply seals and punctures the cartridge, then uses a trigger or simply backing the threaded cartridge out to start the flow of C02.

Bike tires do lose air as well. Between the high pressure, low volume, and thin tube walls, it's not uncommon for a road bike at 120 psi to lose 5-10 psi per day. They generally will go from 100 to under 20 in a couple days if the fill was from a C02 cartridge. An automotive tire will hold pressure indefinitely for all practical purposes... I guess the difference in thickness and maybe material is sufficient to hold nitrogen OR C02 with little to no appreciable loss.

I looked it up; C02 molecules are 3.3 angstroms in diameter, and N2 molecules are 3.6. Smaller difference than I thought, but still apparently enough to matter.

 

[GeoScum]

I looked it up; C02 molecules are 3.3 angstroms in diameter, and N2 molecules are 3.6. Smaller difference than I thought, but still apparently enough to matter.

I would disagree. Air, which is 20% O2 does not loose pressure in a tire, and O2 clearly has a smaller atomic radi than CO2

 

[eric1115]

I would disagree. Air, which is 20% O2 does not loose pressure in a tire, and O2 clearly has a smaller atomic radi than CO2

I meant enough to matter in a bike tire. I'm speaking from a couple decades' experience as a cyclist, and 10 years as a bike mechanic. C02 does leak down. Air does as well, but not as fast. This was actually something we sat around chatting about on a slow day one winter, people who pay for a nitrogen charge in their tires.

Air is 78ish percent nitrogen, 21ish percent oxygen, 1ish percent C02, argon, others. So, if you inflate with whole air, assuming you don't deflate and reinflate, you will gradually approach a nearly pure nitrogen charge in your tires, as the vast majority of what's leaving through the tube walls is non-nitrogen and you're replacing it with over 70% nitrogen. Numbers with some assumptions:

Assume that the tire casing is inelastic, so volume doesn't change. No temperature change. Assume low humidity or a dryer in your compressor system. Inflate to 100 psi. A couple days later, you're at 90 psi. Let's say for argument's sake that some nitrogen escaped, but 90% of the loss was non-nitrogen.

.78 - .10*.10 = .77

77/78 or 98.7% of what's left is nitrogen. Fill back to 100 psi, and:

(.987 * 90) + (.78 * 10) = 96.6% nitrogen.

Do it again in another couple of days, and it's basically a pure nitrogen charge.

 

[Mc Taco]

I meant enough to matter in a bike tire. I'm speaking from a couple decades' experience as a cyclist, and 10 years as a bike mechanic. C02 does leak down. Air does as well, but not as fast. This was actually something we sat around chatting about on a slow day one winter, people who pay for a nitrogen charge in their tires.

Air is 78ish percent nitrogen, 21ish percent oxygen, 1ish percent C02, argon, others. So, if you inflate with whole air, assuming you don't deflate and reinflate, you will gradually approach a nearly pure nitrogen charge in your tires, as the vast majority of what's leaving through the tube walls is non-nitrogen and you're replacing it with over 70% nitrogen. Numbers with some assumptions:

Assume that the tire casing is inelastic, so volume doesn't change. No temperature change. Assume low humidity or a dryer in your compressor system. Inflate to 100 psi. A couple days later, you're at 90 psi. Let's say for argument's sake that some nitrogen escaped, but 90% of the loss was non-nitrogen.

.78 - .10*.10 = .77

77/78 or 98.7% of what's left is nitrogen. Fill back to 100 psi, and:

(.987 * 90) + (.78 * 10) = 96.6% nitrogen.

Do it again in another couple of days, and it's basically a pure nitrogen charge.

Whoa, whoa, whoa. Some of us were Anthropology majors in college.

 

[Mc Taco]

I will be getting a CO2 system in a couple of weeks. Will keep an eye on this.

 

[taggart_lumpy]

I always fill with c02 when leaving a trail head and have never noticed an issue between fillings.

 

[Ohaul]

I should have finished the thought…

...which are a mechanical closure. They freeze partially open sometimes, but when I screw the valve closed it stops leaking.

Yeah I understand this type of valve. I have bikes with Schrader and other with Presta valves. I guess part of what I was suggesting was that you might be freezing the sealing surfaces (ie o-ring or sealing seat) so that even when you close it, once the seal thaws/ softens it is not actually closed all the way.

...There's no regulator to speak of on an inflator... it simply seals and punctures the cartridge, then uses a trigger or simply backing the threaded cartridge out to start the flow of C02.

I was trying to compare between the CO2 bike time inflators and the “powertanks” set ups. They might not have the same issues, because having a regulator eliminates the issue of freezing. My “ultra-flate” gets *really* cold when I use it.

I bet a huge part of your issue is the type/brand/quality of the tubes you’re using. Having filled scuba tanks :snorkel: with HE for tri-mix diving, I am familiar with partial pressure gas loss. I have been able to keep a HE fill, for weeks (analyzed) for diving. After servicing several valves with new O-rings & seats from a non-OEM parts, I had a difficult time keeping a tri-mix fill for over a week. A lot of the HE had escaped and changed the mix. I rebuilt the valves again (different parts) and now I can store HE for a few weeks.

Heck I could be way off I graduated with a degree in Art…:Wow1:

 

[off-roader]

CO2 Tanks also get really cold even with a regulator. I frequently completely frost up my old 'Yellow' hose for several feet after the regulator, not so much with my new hose.

Not sure if it's an issue with vehicle tires. I've never seen an issue anyway.

 

[taggart_lumpy]

definitely avoid the thin plastic hose

ive seen them freeze up and well you can imagine what ~150psi will do in a thin brittle hose.

 

[off-roader]

Yeah, I believe the opaque yellow ones I had were Polyethylene hoses.

The clear Polyurethane ones I replaced them with would appear to be able to survive the freezing temps better.