Which statement best defines the Upper Explosive Limit?

• A. The lowest concentration of gas required for ignition.
• B. The average concentration of gas in air.
• C. The highest concentration of gas that will support combustion.
• D. The concentration at which oxygen becomes non-flammable.

Answer: (C)

Upper Explosive Limit defines the upper end of the flammability range for a fuel in air. A combustible gas or vapor will burn only if its concentration falls between the lower and upper limits. Below the lower limit, there's not enough fuel to sustain a flame; above the upper limit, the mixture is too rich and cannot propagate combustion. So, the upper limit is the highest concentration of gas in air that will still support ignition. The other statements describe the lower limit (minimum concentration for ignition), an irrelevant average, or an unsupported claim about oxygen, which aren’t correct definitions of this term.