Abstract: A method is disclosed for changing the inert gaseous content of a volume including hydrocarbon vapors to air without producing an explosive mixture. First a sample is obtained from the volume and then mixed with air to form a gas mixture. During the mixing, the mixture is monitored for values including percentage in tenths of lower explosive limit and percentage of oxygen. Then the values are evaluated to see if the mixture forms an explosive mixture; and, if so, the volume is purged with an inert gas prior to ventilating the volume with air to remove the inert gaseous content including hydrocarbon vapors from the volume without producing an explosive mixture of the hydrocarbon vapors and air.Apparatus is also disclosed for use with the method. A sampling hose is located in a gaseous sample volume of inert gas and hydrocarbon vapors. It is connected to a valve to regulate the flow of gas through it. A second valve is connected to a source of air.