Message text written by "Francis L. Precht"
>I'm not a chemist either, but 'air' regular or otherwise, does not hold
>moisture !!! This is a vary old notion that, well, doesn't hold any air
>(ok, bad pun). A simple demonstration can be had by placing a pan of
>water in a vacuum chamber, evacuating the chamber (i.e. no more
>'regular' air in it) and the water WILL evaporate. Water vapor is a
>gas, just as all other component gasses in the atmosphere. Evaporation
>is controlled primarily by energy availability, the vapor pressure
>gradient (difference in forces exerted by the 'number' of gas molecules
>present on either side of an interface), and water availability.
Perhaps we are talking semantics here but the effect is the same. If you
expose air to a vacuum the vapor pressure of the water at that temperature
is greater than the ambient pressure and it will not remain in a liquid
state. You can do the same thing by leaving the ambient pressure at one
atmosphere raising the temperature of the water to 212 F (100C). t one
atmosphere
>Using an air compressor to fill tires would simply use ambient
>atmosphere pumped under pressure into the tire. This air already has
>some water vapor in it (even air over the Sahara has a little water
>vapor in it). Using a bottled purified gas (like nitrogen) there is
>likely to be no water vapor in the mixture. Thus, less chance for rust.
The net effect of running the air through an ari compressor (the typical
way of inflating tires) is to reduce the amount of water. Since the higher
ambient pressure will reduce the amount of air remaining in vapor stage
(the reason there are drain taps on compressor tanks) the air coming from
the tank which is then reduced in pressure will likely have less water
vapor than straight ambient air. I say likely because one could propose a
situation where ambient humidity is so low that nair preciptates out in the
compressor. But this is not a typical situation for most of us.
>Same idea for the car capsule enclosure - merely displacing the ambient
>atmosphere that already contains some water vapor with a purified gas.
Quite the contratry. A tire is a sealed system. Once it is inflated there
is a limited supply of water vapor and, unless water has acatalytic effect
on rust, once the water is used up the process will stop. Heck, even if
water DOES have a catalytic effect, there is a limited supply of oxygen and
once that is used up the precess will stop. Contrast that with storing the
car in a cocoon as opposed to just parking it in the garage. In the garage
there is a constant supply of fresh oxygen and water vapor whereas in the
cocoon there is no such fresh supply.
<snip>
>Hope this helps, rather than confuse. But please remember, the air
>doesn't really hold mositure !!
>Bud Precht
I was taught that water (or other liquids for that matter) disolves into
air in a similar manner as CO2 disolves into water. Maybe the mechanisms
are different but the practical upshot is the same.
But filling tires with nitrogen is still snake oil!
Dave
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