quote:Originally posted by PanteraDoug:Kelly. As long as you are in the calculating mood, how contaminated is the nitrogen going to be by the air left in the tire when drained as completely as you can get it? Then pumping it up with nitrogen?...I'm taking a guess and thinking around 5% contaminated with air?
Pretty close depending upon what pressure you fill to. In round numbers air is 78% N2, 21%O2 and 1% Argon (there is actually trace amounts of lots of stuff but close enough). It’s not exactly right, and hopefully Avogadro (Google that one for grins Doug) will forgive me, but as an approximation, in my example of the 335/35/17 that was about 2 cubic feet internal volume, if it’s completely flat at atmospheric pressure, there’s still about 0.42 ft3 of oxygen in there by volume. If you fill it to 29.4 psi (2 ATMs gauge), there will be 6 ft3 of gas and still 0.42ft3 of oxygen so by volume it’s now 7%. Every time you drain and refill the tire to 2 ATMs you dilute the oxygen to about a third of the previous concentration. The size of the tire doesn’t matter for this estimation but if you fill it to a higher pressure, say 45 psig (3ATM gauge), it dilutes to about ¼ the concentration each successive fill. Kapeesh?
quote:Is the tank of nitrogen going to be 100% free of water vapor because nitrogen retains no water vapor or just because of the manor the tank was filled with nitrogen?
It's a byproduct of how the industrial gas companies produce the gas. They super-chill and liquefy air, then put it in a distillation column where it stratifies in the column in order of molecular weight. Since N2 doesn’t liquefy until about -320F (and you thought it was cold outside tonight!), water just becomes another solid contaminate that gets removed with CO2 and a bunch of other BS. When they convert the liquid N2 back to gas, it’s pretty much just N2.
quote:That means no matter what I do there will still be a percentage of water vapor trapped in the tire?
Yah, but its presence gets diluted just like the oxygen if you fill and drain repeatedly with N2.
quote:The descant filter is a good idea though. Never thought to look for one? Doe!
McMaster Carr is your friend
Best,
K