Question 23Safety & Environmental - 1st Asst/Chief

The correct answer is B) that concentration above which the mixture is too rich to burn. The upper explosive limit (UEL) is the maximum concentration of a flammable vapor in air above which the mixture is too rich to burn. At concentrations above the UEL, there is too much fuel (flammable vapor) relative to the available oxygen, making the mixture incapable of supporting combustion. The other options are incorrect because: A) is incorrect as the UEL is the concentration above which the mixture is too rich to burn, not just enough to produce an explosion. C) is incorrect as the UEL is not defined as a specific percentage of flammable vapor, but rather the maximum concentration above which the mixture is too rich. D) is incorrect as the UEL is not defined by the percentage of oxygen present, but rather the maximum concentration of flammable vapor that can still support combustion.