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Lower Flammability Limit (LFL) = Lower Explosive Limit (LEL)
It is the lowest concentration (percentage) of a gas or vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat).

Upper Flammability Limit (UFL) = Upper Explosive Limit (UEL)
It is the highest concentration (percentage) of a gas or vapor in air capable of producing a flash of fire in presence of an ignition source (arch, flame, heat).

at 20oC & 1 atm,
the LFL & UFL of Ammonia is 15% & 28% respectively
the LFL & UFL of Carbon Monoxide 12% & 75% respectively
the LFL & UFL of Gasoline is 1.4% & 7.6% respectively
the LFL & UFL of Kerosine 0.7% & 5% respectively
the LFL & UFL of Methane is 5% & 15% respectively
the LFL & UFL of Propane 2.1% & 10.1% respectively

DIN 51649 specify the explosion limits & limiting oxygen concentrations have been measured in mixtures with air under ambient conditions & at elevated temperatures. EN 1127-1 defines the limiting oxygen concentration as the Maximum Oxygen Concentration (MOC) in a mixture of combustible, air & inert gas, in which an explosion will not occur.

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