Originally Posted by Astro14
Originally Posted by 40w8
As you add air a few times the oxygen will have slipped out since much smaller molecule.
As the tires age they should tighten up, and lose less air.
My Hankook V12's were so hard at 4 years they held 35 psi for 2 more years.....not even kidding.
Do tell...
Please, what is the Molecular size difference and the effect on permeability?
I don't quite get how the tires ageing and tightening up would work. I could marginally understand at some point the materials in the inner liner of the tire "consuming" or being oxidized by what's in the tire, and from there the rates decrease with diffusion rates and lengths.
But for those interested, here's the answer:
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Literature reports of kinetic diameters for O2 and N2 molecules, derived from several different types of experimental measurements, give slightly different values, but all show that O2 has a slightly smaller diameter than N2. The following examples expressed in Angstrom units demonstrate this (one Angstrom unit is 10 to the -10th power meters, i.e., one-ten-billionth of a meter): from gas viscosity data, O2 2.96 and N2 3.16 (difference 0.20); from van der Waal's interaction data, O2 2.90 and N2 3.14 (difference 0.24); from molecular refraction data, O2 2.34 and N2 2.40 (difference 0.06). Other experiments, less applicable to transport situations, such as from closest packing, when the two molecules exist in a frozen solid state at very low temperatures, still show O2 to be a smaller size than N2 (O2 3.75 and N2 4.00, difference 0.25).
Also
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In Barrers, for a typical rubber material, the permeability coefficient P, is dependant on temperature, but at 25C (77F) for O2 is about 10 and for N2 is about 3.
No idea if that "typical rubber material" set includes tires with their materials choices, which I'd assume includes a liner thst is designed to prevent escape of any molecule.
https://www.getnitrogen.org/pdf/graham.pdf
Interesting read. Practical benefit to passenger tires is minimal, but it does give some traceable physical parameters that are interesting.
Dr. Murphy is legit, I had the chance to interact with him on a project once.
Not sure if the final paragraph is legit though.