Originally Posted By: Ursae_Majoris
Originally Posted By: CharlieJ
Since oxygen molecules are able to diffuse through tires easily and nitrogen cannot, does it mean that over time the nitrogen in your tires will increase as more oxygen escapes and decreases? Example: I keep my tires inflated over a period of a few years. Over that time period would the concentration of nitrogen increase as the concentration of oxygen decreases, eventually leading to a tire that is mostly nitrogen filled?
that's my understanding as well. So places that charge extra for nitrogen fill-up are ripping people off.
I don't think so. I think the Law of Partial Pressures of Gasses works in this case as well.
The law states that you should treat mixtures of gasses as though each gas where independent. So if you had 100 psi air, then you treat the Nitrogen part as though it were 78 psi and the Oxygen part as though it were 21 psi.
That means that if the diffusion rate through a membrane is the same for both Nitrogen and Oxygen, then the amount of nitrogen loss is 3.7 times (78/21) the amount for oxygen - and that maintains the percentage amounts.
It also means that when the diffusion rate is different for these 2 gasses (which is the usual case), the effect diminishes over time. In this case, the difference between the amount of Nitrogen and Oxygen would tend to diverge but would slow down. I think you would be hard pressed to show any significant affect on the percentage.
BTW, some studies have been done with nitrogen inflation showing that oxygen permeates back into the air chamber from the outside.
