The flash point can be determined if an ignition source is introduced to the lubricant. Flash point defines the lowest temperature at which the lubricant becomes vaporized into a flammable gas.
Closed cup methods are performd in a restricted environment and the results are not hampered by the exterior elements found in laboratory where the test is conducted.
Closed cup method gives lower values of flash point because the heat is enclosed in a vessel, which is not opened to atmosphere, and the lubricant is more likely to become incendiary at an earlier stage. Because of the lower values provided, the closed cup methods are more used as industry standards since they guarantee safer practices.
It is important to emphasize the fact that both methods are empirical, hence, the results can be affected by a large amount of disturbing factors, such that: the apparatus and materials used when the test is conducted, the laboratory where the test is performed and the testing technique.