20-30 ℃
The difference between the open flash point and the closed flash point mainly depends on the different measurement methods. Usually, the open flash point is 20-30 ℃ higher than the closed flash point. This is because the open flash point tester allows oil vapor to freely diffuse into the air, making it relatively difficult to reach a temperature that can flash. The closed flash point measurement is carried out in a closed container, which limits the diffusion of oil vapor. In addition, the open flash point is mainly used for the determination of heavy lubricating oil and dark lubricating oil, while the closed flash point is used for the determination of light lubricating oil and fuel oil.