nitrogen in tires was: aluminum wheels

[Robert Myers]

WARNING!!!  Nerd alert!  Delete now to avoid a possible headache.

Andrew,

There will be _some_ difference in "expansion" due to either the presence
or absence of _liquid_ water.  The vapor pressure of water increases rather
significantly over commonly experienced temperature changes.  Since a tire
is a closed system if there is liquid water present then the delta-P with
corresponding to a certain delta-T will be larger.  Now the question
remains: just how significant is this difference.  I suspect it won't be of
huge importance.

Your reference to Boyle's Law is just fine but it isn't the one applying to
this situation.  Boyle's Law is PV = K.  There is no temperature term in it
since it applies to gases expanding at constant temperature.  Gay Lussac's
Law is the one you want here, P/T = K or P = KT, pressure is directly
proportional to temperature in a constant volume container (that is Kelvin
temperature,  BTW).

Also, assuming that the tire is essentially a constant volume container
(not totally true but a reasonable approximation) then pressure and
temperature will be directly proportional.  The presence of liquid water
will complicate things a bit since a reservoir of liquid water means that
the changing amount of water vapor caused by changes in temperature must be
added to the amount of gas in the tire.  At higher temperature more liquid
water will convert to gaseous water and the total amount of gas in the tire
will increase in accord with Avogadro's Law, P/n = K or P = Kn where n is
the number of moles of _gas_ present.

The total amount of gas pressure present, according to Dalton's Law of
Partial Pressures, will be P(total) = P(N2) + P(O2) + P(H2O)  (+ other
minor components of air such as CO2 and trace amounts of other gases such
as Argon, etc - all minor terms in the equation).  In a closed container,
such as our tire under consideration all of these terms, barring a leak in
the tire, are constant except for P(H2O) which will increase with
temperature  Dalton's Law, BTW, assumes constant temperature.  The total
number of moles of gas present will be n(N2) + n(O2) + n(H2O) +...  The
only variable term here is n(H2O).

All this assumes that the air mixed with water vapor inside the tire
behaves as an ideal gas and that the tire is a constant volume
container.  Since the tire is not a truly constant volume container and
there actually is no such thing as an "ideal gas" these relationships are
only approximations of the actuality of the situation.

I probably have overlooked something in a dashed off treatise but here ends
the nerd alert.

At 07:41 AM 3/14/02, Andrew Duane USG wrote:

>Hairy green toads from Mars made Dave Hord say:
>
> > Helium is pointless in tires..but yes, nitrogen CAN be used and DOES have
> > an advantage.  Filling your tires with pure nitrogen essentially ensures
> > there is no residual water moisture in the tires at the time they are
> > filled up.  Due to the fact that there is no water moisture, your tires
> > will not increase their pressure much with heat.  Thus, a tire filled at
> > 32psi with nitrogen when cold will be closer to 32psi when hot, then a tire
> > filled with just normal 'air'.
>
>Expansion of gasses at temperature has to do with Boyles law, not
>water vapor in the gas mixture. Why would nitrogen not expand as
>it gets warmer? Am I missing something?
>
>--
>
>Andrew L. Duane (JOT-7)                 duane at zk3.dec.com
>Compaq Computer Corporation             (603)-884-1294
>110 Spit Brook Road
>M/S ZKO3-3/U14
>Nashua, NH    03062-2698
>
>
>
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Bob
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