One hundred million tonnes of O are gotten from air for industrial uses every year. Industries use two main methods to make oxygen. The most common method is fractional distillation of liquefied air. N evaporates while O is left as a liquid. O is the second most important industrial gas.Because it is more economical , oxygen is usually stored and transported as a liquid. A small steel tank of 16 liters water capacity with a working pressure of 139 bar (2015 psi) holds about 2150 liters of gas and weighs 28 kilograms (62 lb) empty. 2150 liters of oxygen weighs about 3 kilograms (6.6 lb). 