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09-10-06, 04:51 PM
#36
khuon

Join Date: Aug 2002
Location: Catching his breath alongside a road near Seattle, WA USA
Posts: 12,234

Bikes: 1999 K2 OzM, 2001 Aegis Aro Svelte

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Originally Posted by humble_biker
Incidentally CO2 is compressed air, meaning that when it exits the cartridge and enters the tube it expands to larger than the normal CO2 molecule(!). It usually takes the molecules around 24 hours to shrink to normal size. That is why you will have a flat or soft tire the next day after using CO2.
I suppose I should say something insightful rather than simply reacting...

PV=nRT

Where:
• P = Pressure
• V = Volume
• n = number of moles
• R = Universal gas constant
• T = Temperature
We assume for now that this is a closed system which is not entirely true as we will see later. Thus we have:

Pcyl Vcyl / ncyl R Tcyl = Ptube Vtube / ntube R Ttube

Now n is a constant and is fixed (ncyl = ntube) as of course is R, thus our relationship becomes:

Pcyl Vcyl / Tcyl = Ptube Vtube / Ttube

Initially Pcyl > Ptube. As the gas from the cylinder flows into the tube, Ptube starts to go up and Pcyl starts to decrease. However, bear in mind that Vtube also increases. Because Vcyl is fixed, in order to preserve the relationship, Tcyl must decrease. This is why the cylinder gets cold.

The gas expands from a smaller volume in the cylinder to a larger volume in the tube. The ratio of this expansion combined with the initial pressure in the cylinder directly governs the final pressure inside the tube when everything has come to equilibrium. The molecules of the gas do not expand or shrink. The space between them does. All molecular sizes are governed by the composition of their atoms and their atomic bonds.

Now we come to why tyres filled with CO2 go flat faster than with air. CO2 molecules are more permeable and soluable in butyl rubber than other molecules in air. Thus when a tube is filled with air, the CO2 molecules will tend to permeate and leak through the rubber faster than the other components of other gases in the air. This leaves other gasses such as Nitrogen and Oxygen (amongst others) to linger around longer. When a tube is filled with just CO2, the rate of leakage is the same as that of the CO2 leaking out through the tube filled with air but since there's only CO2 to leak, the tube will go flat faster.

BTW, CO2 doesn't necessarily leak through rubber faster because of its size but because of how the molecules in rubber attract CO2 better than Oxygen or Nitrogen. As a result, the CO2 permeates the rubber which then swells and thus allows more molecules to escape.

Note - Writing equations with vBcodes sucks!
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