Which statement correctly contrasts LEL and UEL?

• A. LEL is the minimum ignitable concentration; UEL is the maximum ignitable concentration.
• B. LEL is the maximum ignitable concentration; UEL is the minimum.
• C. LEL and UEL describe energy transfer, not concentration.
• D. LEL and UEL are the same value.

Answer: (A)

The key idea is the flammable range in air. LEL, or Lower Explosive Limit, is the lowest concentration of a flammable vapor in air that can ignite if an ignition source is present. UEL, or Upper Explosive Limit, is the highest concentration of the vapor in air that can ignite. Below the LEL, the mixture is too lean to ignite. Above the UEL, the mixture is too rich to ignite. Only between these two limits is there a combustible mixture capable of ignition. So the statement that LEL is the minimum ignitable concentration and UEL is the maximum ignitable concentration is the correct way to contrast them. The other options misstate these relationships or what the limits describe.