Originally Posted by Unkgd
Compressed air always contains moisture - which is the major contributer to rising tire pressure when the tire warms up.
Bottled nitrogen contains significantly less moisture (on the order of ppm if I remember correctly) hence - less pressure rise when heated.
When I raced karts, I carried a nitrogen bottle around because it was a readily available source of consistently low moisture inert air. The size of the nitrogen molecules was not a concern. The important thing was to see a low and consistent tire pressure change due to heat. Nitrogen provided that. Also, you can get it from Praxair for something like $20 + deposit for a reasonably sized cylinder at 2000 psi. Just add a regulator and you are off to the races.
The 2000 psi cylinders are much more convenient at a remote location than a compressor that requires power and is noisy or a 5 gallon 125 psi tank that runs out quickly.