Before a fire or explosion can occur, three conditions must be met
simultaneously. A fuel (ie. combustible gas) and oxygen (air) must
exist in certain proportions, along with an ignition source, such as
a spark or flame. The ratio of fuel and oxygen that is required
varies with each combustible gas or vapor.
The minimum concentration of a particular combustible gas or
vapor necessary to support its combustion in air is defined as the
Lower Explosive Limit (LEL) for that gas. Below this level, the
mixture is too “lean” to burn. The maximum concentration of a gas
or vapor that will burn in air is defined as the Upper Explosive
Limit (UEL). Above this level, the mixture is too “rich” to burn.
The range between the LEL and UEL is known as the flammable
range for that gas or vapor.