On 2/7/07, Wayne <wmc_st@xxiii.com> wrote:
>
> Randall wrote:
> > Not true, the pressure will still go up as the nitrogen warms. The pressure
> > rise is just more predictable with the (dry) nitrogen than with (possibly
> > wet) air.
>
> Well, yeah. Any gas is going to expand somewhat. I just meant it's not
> as drastic. I've only seen 1-3 psi on nitro' vs 2-6 on air.
Controlled test? Measure the temperatures of the tires, before and after?
At the temperatures and pressures a tire sees, water is going to act
as much like an ideal gas as nitrogen. The people who sell nitrogen
tire filling equipment make lots of claims. Many of them are just
plain false, the rest impossible to measure. First, they claim that
nitrogen diffuses through tires more slowly than oxygen. Ask the next
person who tells you that what they know about Graham's law. Because,
see, gases diffuse at rates inversely proportional to the square root
of their molar masses. Nitrogen has a molar mass of 28; oxygen is 32.
So, hmm, nitrogen is likely to diffuse faster. Then they make claims
that water vapor is magical, and expands at a rate different from
other gases. Hint: it's not; it doesn't.
The nitrogen generators that Ingersol-Rand and the like are pushing
are using a membrane, and result in a gas purity of about 95%. When
they're new, and maintained properly. If they break, who's going to
know? Since
the atmosphere is 78% nitrogen to start with, it's not terribly impressive.
The one claim that may have merit is that it reduces oxidation in the
tire. But that's not going to matter to most people driving cars.
The tires will fail due to tread wear, nail holes, or other hazard
before the innerliner breaks down. If you're running a truck fleet,
where you hope to get a million miles out of a carcass, it might make
a difference. I've not seen data that says this is true, just claims
from truck company maintance, and people selling N2 equipment.
--
David Scheidt
dmscheidt@gmail.com
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