Flash point refers to the temperature, expressed in degree C, at which a flame can flash over the mixture of steam and air escaping from the heated oil under specified heating conditions and at certain intervals, causing the oil surface to flash. The high or low flash point of an oil product indicates its flammability, content of volatile compounds, degree of gasification, and safety. The hazard level of oil products is also classified based on their flash point. Oil with a flash point below 61 degree is considered flammable, while oil with a flash point above 61 degree is considered flammable.

During storage, transportation, and use, it is prohibited to heat oil products to their flash point. The heating temperature should generally be 20-30 degree below the flash point. There are two methods for determining the flash point of oil products: the closed cup method and the open cup method. The main difference between the two is that the closed flash point instrument heats up oil and gas in a closed container, while the oil vapor in the open flash point instrument can freely diffuse into the surrounding air. Therefore, the flash point detection of the same oil product results in different flash point values measured by two instruments. The higher the flash point of the oil product, the greater the difference between the two. The closed cup method is used to determine the flash point of fuels and light oil products, while the open cup method is used to determine the flash point of heavy oil products.