Measures of Flammability

Flashpoint:

The flashpoint is the temperature at which a liquid gives off sufficient vapors for an external ignition source to cause a flame to flash across the surface of the liquid. However, if the ignition source is removed, the flame will go out because self-sustained combustion is not possible at this temperature.

Flashpoint is tested using apparatus like Cleveland Open Cup ASTM D-92, and Pensky-Martens Closed Cup ASTM D-93

Firepoint:

Firepoint is the temperature at which the material will begin self-sustained combustion if an external ignition source is used to initiate the process. This temperature is usually only slightly higher than the flashpoint.

Flammability Limit or Explosive Limit:

It is percentage mixture of flammable vapor or gas in air that can be ignited.  The flammable range is the area between the upper (UFL) and lower (LFL) flammable limits, also referred to as explosive limits
(UEL and LEL). Gasoline, for example, has a lower flammable limit of
approximately 1.5 and an upper flammable limit of approximately 7.5. This means that if its vapors are mixed in the surrounding air between 1.5 and 7.5%, and an ignition source is introduced, it will burn or explode. If the percent of vapors in air were 1%, the mixture would be too lean to burn because sufficient fuel would not be present. If the percent of vapors in air were 10%, the mixture would be too rich to burn because there would be too much fuel relative to the oxygen.

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