LEL and UEL in relation with the Minimum Ignition Energy

LEL: The Lower Explosive Limit (LEL) is the lowest concentration of a gas, vapor or dust that will burn in air.
UEL:
 The Upper Explosion Limit (UEL) is the highest concentration of a gas, vapor or dust, that will burn in air.

These values for gases and vapors are mostly expressed as a volume-% (vol%) in air. For dust mostly as the mass-concentration in air in gr/m3. When the concentration of the substance in air is between these values there is an explosive atmos[here. When the concentration is beneath the LEL, the mixture is too lean and when the concentration of above the UEL the mixture is too rich to ignite.

Within this figure it is shown that always a Minimum Ignition Energy is needed for combustion. More energy is needed when a mixture is getting quite lean or quite rich. Below the LEL or above the UEL no combustion is possible.

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