What is the lowest concentration in air that will produce a fire or flash when an ignition source is present?

What is the lowest concentration in air that will produce a fire or flash when an ignition source is present?

• A. Lower explosive limit
• B. Upper explosive limit
• C. Flashpoint
• D. Autoignition temperature

Answer: (A)

The key idea is the flammable range of vapors in air, specifically the minimum concentration that can support ignition with a spark or flame. The lowest concentration in air that can ignite when an ignition source is present is the lower explosive limit. It marks the point where there’s just enough fuel vapor to form a combustible mixture with air; below this, the mixture is too lean to ignite. The upper explosive limit is the highest concentration that can still ignite; beyond it, the mixture is too rich. The flashpoint is a temperature-based concept for liquids—the temperature at which enough vapor is produced to form an ignitable mixture near the surface, not a concentration in air. The autoignition temperature is the temperature at which a material will ignite without any ignition source. So the lowest ignitable vapor concentration in air is the lower explosive limit.

The key idea is the flammable range of vapors in air, specifically the minimum concentration that can support ignition with a spark or flame. The lowest concentration in air that can ignite when an ignition source is present is the lower explosive limit. It marks the point where there’s just enough fuel vapor to form a combustible mixture with air; below this, the mixture is too lean to ignite. The upper explosive limit is the highest concentration that can still ignite; beyond it, the mixture is too rich. The flashpoint is a temperature-based concept for liquids—the temperature at which enough vapor is produced to form an ignitable mixture near the surface, not a concentration in air. The autoignition temperature is the temperature at which a material will ignite without any ignition source. So the lowest ignitable vapor concentration in air is the lower explosive limit.