I recently presented an argument, based on science, that CO2 should leak more slowly that air, as it is a bigger molecule than Nitrogen.
It was a very well made argument, as TEF members would expect based on my previous posts, however, I too pumped up my tyre with CO2 and I agree it does go down quicker.
Something weird is going, on so maybe we should ask Darra O'Brien, or the smiley Manc guy ?
Having said that, the CO2 pumps are brillinat as an emergency solution out on a ride
Found this on another forum...
Hmm, this has puzzled me. So much so I had to look into the science of butyl tube inflation My search of the web has located many misinformed articles and postings on triathlon/cycling forums. Indeed one of my earlier postings on the subject was ambiguous and plays right into sticking point 1:
jibby26 wrote:
CO2 is a relatively small molecule and will squeeze out through the holes in the rubber overnight.
CO2 is a relatively small molecule, but just to screw with your mind on a Saturday morning it is also a relatively big molecule. It all depends on the stick you use to measure it. If you think of nitrogen (N2), oxygen (O2) and CO2 as snooker balls wanting to hit one another then CO2 is bigger than N2 which is bigger than O2. If however you think of them as fast moving molecules that want to get out of the sieve they are in (just like in the example of an inner tube) then it is different. The "kinetic" diameter of CO2 is the smallest, followed by that of O2 followed by N2. Hopefully that clears up this misconception.
Now for the nitty gritty science bit. The reason the tyre goes flat is due to permeation of gases (gas diffuses through the rubber) as opposed to effusion where the gas goes directly through a small hole (a puncture). The different gases all have a different permeation rate (permeability) through the rubber, and different rubbers all have different permeabilities for the same gas.
Now hopefully that is a little clearer than an open water lake after a mass start and I can through in an equation
The amount of gas that moves through the rubber is known as the flux and given by:
Jm=[Rm x A x (Pi-P0) x Fm]/t
where Rm = Permeability of gas m through material (in Barrer)
A = Area (in cm)
Pi = Inflation pressure (in cmHg)
P0 = Outside pressure (in cmHg, normally 76)
Fm = Molar fraction of gas m (i.e. air is 78% N2 so FN2=0.78)
t = Thickness of inner tube (in cm, approximately 0.1)
Now, for a butyl tube RN2=0.323xE^[-10], RO2=1.30xE^[-10], RCO2=5.17xE^[-10],
If we put the numbers in for a 1cm2 patch in tube inflated with air (Fm: m=N2=0.7809, O2=0.2095, CO2=0.0004) to 8 bar (608cmHg) we get:
JN2=1.34E[-07] cm3/s i.e. one ten millionth of a millilitre per second per cm2 of tube
JO2=1.45E[-07] cm3/s
JCO2=1.10E[-09] cm3/s
So from this you can see that most of the gas lost from a normally inflated tube is actually O2. If you total up these losses then the tube is losing 2.80E[-07] cm3/s per cm2 of tube. My crude estimation of an inner tube puts it's surface area at 500cm2 so the actual loss is around 0.14 microlitres of gas per second.
But if you use a CO2 inflator there is no N2 or O2, so JCO2 now becomes 2.75E[-06] cm3/s per cm2 of tube. i.e. the rate of gas loss from the tire is about 10 times greater.
Hopefully some of you have read to this point and understood some if not all of it. There are a few caveats to these numbers, they are only true for the pressure and compostion of the gas initially described, as the tire deflates both of these will change and it gets too complicated to describe here.
As for why your tire is still infalted Andy, the only thing I think can explain it is Halfords have made the tube so thick the permeation rate is very slow, and/or they have added something to the rubber which reduces the permeation of CO2.
