Speedyham has the right of it...Rule of thumb #1 for thermodynamics, air is not an ideal gas and does not behave according to the ideal gas law (my professor used to mark problems with air wrong if we used the ideal gas law!) Too many impurities, like moisture which expands faster than gas when heated. That's why something like pure nitrogen works better...
JP Rowland jeremyrowland -at- mac.com
Co-Driver, Wazoo Racing Subaru WRX, PGT