[VAL] Re: Nitrogen in tires

[John Sellers <sellersj@xxxxxxxxxx>]

Dr. J,

I'm 100% with you on this one. One of the local tire dealers is
extensively advertising nitrogen inflation on TV and radio.  As an
engineer who is very conversant with the physics of gases it seems to me
that the claimed advantages are greatly [pardon me] _overinflated_.

At the risk of being called pedantic, let me briefly review physics:

Dry air consists of 79% nitrogen, 21% oxygen, and traces of other gases
like carbon dioxide, argon, etc.

Now, the physical [as opposed to chemical] properties of a pure gas
depend on the number of atoms in the molecules (one, two, or more) and
the molecular weight.  Nitrogen, N2, is a diatomic gas having a
molecular weight of 28 and oxygen, O2, is a diatomic gas having a
molecular weight of 32.

So there's very little difference between the physical properties of
nitrogen and oxygen, and practically no measureable difference between
the physical properties of dry air and dry nitrogen, since air is 79%
nitrogen to begin with.

Nitrogen doesn't expand and contract with temperature?  Not true. 
Identical to air in that respect.

Tires run cooler with nitrogen inflation?  Oh? Why?  The physical
properties of the filling gas are the same in either case and everything
else is the same.  You might get your tires to run a little cooler if
you filled them with helium, a monatomic gas having a very high thermal
conductivity.  (But you couldn't keep them full very long--helium can
leak through almost anything!)

_Chemically_, nitrogen is less active than oxygen--not really an inert
gas but closer to it than oxygen.  But tires fail from tread wear and
sidewall deterioration from sunlight (and running underinflated), not by
oxidation from the inside out.  So where's the advantage?

I suppose the advantage of nitrogen inflation--if there is any--is that
nitrogen from a high pressure bottle is truly dry, while air from your
air compressor is saturated with water vapor.  As Dr. J. points out,
this affects the pressure versus temperature curve down to the freezing
point, at which point the water freezes out and no longer affects the
pressure.  You can secure the benefit of dryness with an air dryer in
the air line if you want to.

But we live in a world where people make money selling designer-label
bottled water, gold-plated speaker wires, and any number of other things
costing significant money in return for illusory advantages.  If people
want to pay extra for nitrogen inflation, it's OK with me.

But I think I'll stick with plain old air.

Best,
John Sellers
WBCCI/VAC #1587
1960 Pacer
Dayton, Ohio