Upper Flammable Limit Mean?
The upper flammable limit (UFL) is the highest percentage of a vapor or gas in the atmosphere that will result in a flammable response when an ignition source is introduced. The concentration of a gas or vapor may be beyond the upper flammable limit, at which point it is too concentrated to burn. The upper limit is contrasted with the lower flammable limit, at which point the concentration is too low to ignite.
Safeopedia Explains Upper Flammable Limit
The upper or lower flammable limit for a particular vapor or gas may vary in accordance with the weather or air pressure. The limit is shown as volume percentage at 25 degrees celsius along with the atmospheric pressure. Upper and lower flammability limits are used to ensure the best engine combustion or to prevent and explosion from occurring. They are important measures that are used in gas, diesel and internal combustion engines.