Mad Dog Scientist’s Corner:
Is CO2 Bad Air?
by Laurence D. Preble, D.C.
I was incredulous! But when long-time bike mechanic Tom Armstrong has something to say, it is wise to listen. We had just finished fixing a flat-I am still impressed how fast Tom can do that. I inflated the inner tube with a CO2 cartridge and was about to get underway when Tom reminded me to release all the CO2 that evening and refill with air before the next ride, because any residual CO2 would cause the tire to lose pressure faster than normal.
I had heard this before but thought it sounded like an urban myth. I asked Tom why CO2 should lose pressure faster than normal air. He was confident of his assertion, but wasn’t sure why it should be so.
One might ask why a tire loses pressure at all, even with normal air. That much I knew: bicycle inner tubes are generally made of synthetic butyl rubber which is slightly porous. Air molecules, mostly nitrogen (N2) and oxygen (O2), are small enough to fit through the miniscule gaps in the material so that over time, the tube loses pressure. Why should carbon dioxide (CO2) fit through the pores in the butyl rubber any faster than nitrogen or oxygen molecules? Is the CO2 molecule much smaller than nitrogen or oxygen molecules? Has anyone actually bothered to test the claim that tires filled with CO2 deflate more quickly than air-filled tires? I had questions and wanted some answers!
Designing an experiment would be easy. I would fill a tire with CO2, measure the pressure and let it stand for a day, check the pressure again, empty out the tire and repeat the procedure with normal air; then compare the loss in pressure for air versus CO2. I figured that if the measurements turned out to be close, I could repeat the experiment until I became certain whether the difference was real or not. A quick test showed that a 16 gram CO2 cartridge gave me 101 PSI; I would use that test pressure for plain air as well.
I ran the experiment and was immediately nonplussed! The measurements were nowhere near “close.” After 24 hours, the tire filled with CO2 had lost precisely 35 pounds of pressure. A day later, I had my result for plain air-a loss of only 8 PSI! I tried it again and got the same results. The difference was very real. What was going on here?
Nitrogen and oxygen molecules each contain two atoms. A carbon dioxide molecule contains three atoms, one carbon atom, and two oxygen atoms.Wouldn’t that require carbon dioxide to be a bigger molecule than either nitrogen or oxygen? Wouldn’t that imply that CO2 should have a harder time leaking through the pores in the rubber than regular air? Well, no, actually-it doesn’t.
All three molecules in question are very tiny. Their sizes are measured in angstroms-that is one ten-billionth of a meter! However, nitrogen dominates the molecular contest as the biggest with a size of 3.6 angstroms. Oxygen comes in second place at 3.5 angstroms. Poor little carbon dioxide is only 3.4 angstroms wide! Wow, it has more atoms but ends up being a smaller molecule!
It turns out that there is a complex interaction between the electrical forces binding the atoms together that affects the overall size of the molecules. Because of that, the atoms in CO2 are confined to a smaller space than either O2 or N2 even though CO2 has more atoms! But even more importantly, CO2 is electrically attracted to the rubber molecules, drawing them into the interstices while O2 and N2 are repelled and must be forced through. This means that carbon dioxide very effectively dissolves into butyl rubber and evaporates out the other side.
Manufacturers of gas products often use what is called a molecular sieve to separate nitrogen, oxygen and carbon dioxide from air. A bike inner tube makes a pretty good molecular sieve. Pure oxygen will leak out of a tire 3-5 times faster than pure nitrogen, because of oxygen’s smaller size. But carbon dioxide will leak out much faster than that, up to 12 times faster than nitrogen.
One manufacturer of cycling products has come up with a direct replacement for CO2 cartridges guaranteed to keep bike tires inflated for up to a year. It’s called “Stayfill” available from http://stayfill.com. It is uses a proprietary gas made with much bigger gas molecules than normalair. They recommend this product for mountain bikes with the motto, “Fill it and forget it.”
However, it is expensive, and in my opinion, not practical for routine road bike usage-the slowest leaking gas offers little benefit if frequent punctures allow all that expensive “Stayfill” gas to leak out through a hole in the tire. Mountain bike tires are more durable and should fare better. My recommendation for road bikes would be to do what most experienced road bikers already do: Use CO2 for quick repairs only. It will get you home, but not much farther. Pump daily before each ride, with fresh, free air. Free is a very good price.
Dr. Larry Preble is LBC’s resident Mad (Dog) Scientist and a graduate of both Vanderbilt University and Logan College of Chiropractic. He has headed research projects in computer science, psychology and biomechanics. After moving to Louisville, he wrote for both PCM and Rainbow Magazines for several years and continues in the practice of chiropractic in the Louisville area. Larry is an enthusiastic LBC Mad Dog cyclist and is often spotted riding either his Bacchetta recumbent or Trek upright bicycle.