Lower Explosive Limit (LEL) is defined as:

Lower Explosive Limit (LEL) is defined as:

• A. the ratio of the density of a substance to the density of water
• B. the minimum temperature where a liquid fuel produces enough vapor to burn
• C. minimum concentration of vapor to air below which a flame will not propagate in the presence of an ignition source
• D. maximum vapor to air concentration above which a flame will not propagate

Answer: (C)

LEL is the lowest concentration of fuel vapor in air that can still sustain flame propagation when an ignition source is present. It marks the lean side of the flammable range; if the vapor-to-air mixture is below this level, it’s too lean to ignite or continue burning. This is different from the upper limit (UEL), which is the highest concentration where a flame can propagate before the mixture becomes too rich to burn. The other descriptions don’t fit: density relative to water isn’t about flammability, and a minimum ignition/flash point temperature describes when vapor is produced or can ignite, not the concentration in air.

LEL is the lowest concentration of fuel vapor in air that can still sustain flame propagation when an ignition source is present. It marks the lean side of the flammable range; if the vapor-to-air mixture is below this level, it’s too lean to ignite or continue burning. This is different from the upper limit (UEL), which is the highest concentration where a flame can propagate before the mixture becomes too rich to burn. The other descriptions don’t fit: density relative to water isn’t about flammability, and a minimum ignition/flash point temperature describes when vapor is produced or can ignite, not the concentration in air.