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Originally Posted by usnavyretired
"Air" has moisture in it or as the weather man/woman would say, humidity. The amount of moisture/humidity changes the volume of the air based on temps. Place a tire with 30 psi in ithe sun and several hours later, it will read higher than 30 psi, place that same tire in a freezer and it will read lower than 30 psi. Thats why aircraft use dry nitrogen vice air to inflate the tires.
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Boyle's law applies to all gases equally. In other words they all lose volume with a temperature drop equally. They all expand at the same rate irrespective of the molecular composition..whether it is oxygen, nitrogen or plain water vapor. My guess is the military does not want water vapor in tires because it condenses from a gas to a liquid at relatively high temperatures versus other gases like nitrogen which have a very low boiling point. To have a gas like water vapor condense from a gas to a water would drop the pressure because of the big drop in volume when it condenses.
Nitrogen makes more sense not because other gases do not expand or contract with temperature, (they do) but because nitrogen has such a low boiling point that they never have to worry about the gas condensing at very low temperatures. Such as when a fighter is flying at 50,000 feet.
The atmosphere is 78% nitrogen so it is plentiful and therefore cheap plus it boils at -320 Fahrenheit...so there is no way it is going to condense in a tire.