Champion Tester Calibration only at 125 psi once a year (use the nitrogen rig that is calibrated for that test)
Wait, you can't use pure nitrogen for calibration if the calibration specifies air!
Pure nitrogen is used BECAUSE it has different properties to air, due to the absence of other gases. That means that if you want to substitute nitrogen, you would need some sort of data to show that the different gas doesn't affect the calibration.