Ambient Air

A term for air normally found in the environment, roughly 20.9% oxygen, 78% nitrogen and 1.1% other compounds (argon, carbon dioxide and trace gases), excluding water vapour (the concentration of which varies by location, altitude, temperature, weather and which is normally constant enough to be disregarded for gas detection purposes). In gas detection, ambient air is used to give instruments their zero calibration or fresh air calibration, provided they are free of contaminants.