The flammable range is defined as the gap between the lower flammable limit and the upper flammable limit.

• A. Not true
• B. False
• C. Not defined
• D. True

Answer: (D)

The key idea is that a flammable (explosive) mixture exists only within a specific concentration window of vapor in air. This window is bounded below by the lower flammable limit, the minimum vapor concentration needed for ignition, and above by the upper flammable limit, the maximum concentration at which ignition can still occur. Between these two limits lies the flammable range—the range of concentrations that can ignite if an ignition source is present. So the statement is true. The exact limits depend on temperature, pressure, and the oxygen level, and are often expressed as percent by volume in air (for example, gasoline vapor might have LFL around 1.4% and UFL around 7.6%). Outside this range, the mixture is either too lean or too rich to burn.