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The flash point refers to the lowest temperature at which, under specified conditions, the mixture of vapors emitted by the oil and air comes into contact with a flame, resulting in an instantaneous ignition. The flash point is the lowest temperature at which a minor explosion occurs. When the concentration of flammable gases in the mixture reaches a certain level, it will explode when exposed to fire. The significance of determining the flash point of oils: (1) The flash point of an oil can be used to determine the lightness or heaviness of its distillation composition. The general rule is: the higher the vapor pressure of the oil and the lighter its distillate composition, the lower its flash point. Conversely, oils with a heavier distillate composition have a higher flash point. (2) The flash point can be used to determine the fire hazard of oils. Because the flash point is the lowest temperature at which a fire hazard occurs. The lower the flash point, the more flammable the oil is, and the greater the risk of fire. Therefore, flammable liquids are also classified according to their flash point. Liquids with a flash point below 45°C are called flammable liquids, while those with a flash point above 45°C are called combustible liquids. The various fire safety measures for its transportation, storage, and use can be determined based on its flash point. (3) For certain lubricants, simultaneously measuring the open and closed flash points can serve as an indicator of the presence of low-boiling-point contaminants in the oil, for use in production inspections. Generally, the open flash point is 20–30°C higher than the closed flash point, as some of the oil vapor evaporates during the measurement of the open flash point. However, if the results of the two are significantly different, it indicates that the oil contains light fractions; this may be due to cracking during distillation, or incomplete separation of solvents during dewaxing or refining. (4) For fuel oil, the closed-method is used instead of the open-method because the former is more similar to the condition of oil and gas present in the storage tank. When studying fire hazards, it is inevitable to consider the issue of how to define the temperature limits between safety and danger. Fuel oils with a temperature above their flash point are generally not stored or transported in open containers.