Inert gas is produced on board crude oil carriers (above 8,000 tonnes)(from Jan 1, 2016) by using either a flue gas system or by burning kerosene in a dedicated inert gas generator. The inert gas system is used to prevent the atmosphere in cargo tanks or bunkers from coming into the explosive range. Inert gases keep the oxygen content of the tank atmosphere below 5% (on crude carriers, less for product carriers and gas tankers), thus making any air/hydrocarbon gas mixture in the tank too rich (too high a fuel to oxygen ratio) to ignite. Inert gases are most important during discharging and during the ballast voyage when more hydrocarbon vapor is likely to be present in the tank atmosphere. Inert gas can also be used to purge the tank of the volatile atmosphere in preparation for gas freeing - replacing the atmosphere with breathable air - or vice versa.