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It should be determined according to the product quality specifications whether to measure the closed-cup flash point or the open-cup flash point for each type of oil. Generally, petroleum products with high volatility are tested for their closed-cup flash point, as when the open-cup flash point is determined, the vapor formed when the oil is heated continuously disperses into the surrounding air, resulting in a higher measured flash point. For most lubricants and heavy oils, due to their low volatility, the open flash point is commonly measured. The principle behind determining the closed-cup flash point is to place the sample in the oil cup up to the ring mark, then heat the sample at a slow, constant rate while stirring continuously. At specified temperature intervals, with stirring stopped temporarily, a small flame is introduced into the cup; the lowest temperature at which the flame causes the vapors on the sample to ignite is considered the closed-cup flash point. The principle of open-cup flash point determination is to fill the sample into the test cup up to the specified mark. First, raise the temperature of the sample, then increase it slowly; when approaching the flash point, maintain a constant rate of temperature increase. At specified temperature intervals, a small test flame is passed horizontally through the test cup; the lowest temperature at which the vapor on the surface of the liquid is ignited by this test flame is taken as the result for determining the open flash point.