UEL stands for Upper Explosive Limit. What does this indicate?

• A. The minimum concentration of fuel vapor in air that can ignite
• B. The maximum concentration of fuel vapor in air that can ignite
• C. The concentration at which the fuel begins to vaporize
• D. The concentration at which the fuel has completely burned

Answer: (B)

The upper explosive limit defines the highest concentration of fuel vapor in air at which a flame can still be ignited and propagate. In the flammable range, which lies between the lower explosive limit and the upper explosive limit, there’s just enough fuel and oxygen for combustion to occur. If the fuel concentration goes above this limit, the mixture becomes too rich to ignite because there isn’t enough oxygen to support combustion, even with an ignition source. So the correct interpretation is that this limit marks the upper boundary where ignition is still possible, not the minimum that can ignite, not the vaporization point, and not the point at which burning completes.