Steve is 100% correct. This whole inert gas discussion is simple. Nitrogen
(about 78% of air) is cheaper than getting certified dried air (scuba tank
air) from a SCUBA shop. Certainly the tongue in cheek remarks of Hydrogen
notwithstanding.
The comments of Argon are interesting... ever pick up a similar sized filled
tank of Argon vs. N2? Next time you do you will likely be surprised at how
much argon weighs.
Nitrogen suppliers pump 100% dried N2. I havn't called my local welding
shop to find out how much dried air costs.
The key is DRIED (water free) gas. You could use air if it is dry with the
same results as Nitrogen. For those of you ex-scuba divers out there. Next
time you get your tank filled ...see their setup. The air that they
compress is city air and it is dried and at times refrigerated .. more
volume for same pressure. Why pump water vapor into the tank.. it just
wastes space... less dive time.
Water-vapor pressure at about 100 degrees is 47 mmhg --. So, for ambient
barometric air pressures at one atmosphere (which is 760mm Hg) dry gas
pressure is 713. If you think of lbs /sq inch this is comes out to about
1lb/sq inch.. depending on the amount of water vapor/humidity.
The specific heat of water is much greater than a dry gas. Hence, it
absorbs heat well and expands (builds pressure) much faster than other
gases. Therefore in a tire that has been inflated with a non-dried air
source the water vapor will expand and cause increase pressure. When it
cools you will see less pressure and condensation within the tire.
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