The lowest temperature of a fuel at which the vapors of that fuel catches fire without any ignition source called fire point. The fire point is measured using an open cup apparatus.
Method
To measure the fire point using an open cup method is conducted in a vessel that is exposed to the atmosphere outside. The temperature of the substance (Say Oil) is gradually raised and an ignition source is passed over the top of it after certain intervals until it reaches a point at which it ignites.
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