What is the degree difference between the open flash point and the closed flash point?

Flash point is the minimum temperature at which a combustible liquid or solid releases sufficient vapor to form an ignitable mixture with air at its surface within a given container. The flash point of a combustible liquid varies with its concentration.

As the temperature rises, more oil vapor evaporates from the fuel surface. When the mixture of oil vapor and air reaches a certain concentration and comes into contact with an open flame, a brief flash (instantaneous ignition followed by immediate extinction) occurs. The temperature at this point is referred to as the flash point. Two common methods for determining flash point are the open-cup method and the closed-cup method. The flash point measured by the open-cup method is typically 15–25°C higher than that measured by the closed-cup method. The flash point is influenced by the molecular composition of the oil and the pressure above the oil surface-higher pressure results in a higher flash point. Flash point serves as a critical indicator for preventing oil fires. In open containers, oil should be heated to at least 10°C below its flash point, whereas no such restriction applies in pressurized containers.

When the concentration of the oil vapor and air mixture above the oil surface increases, the minimum temperature at which sustained combustion (lasting at least 5 seconds) occurs upon contact with an open flame is known as the fire point. The fire point is higher than the flash point.

From a fire safety perspective, it is desirable for oils to have higher flash points and fire points, with a larger difference between the two. Conversely, from a combustion efficiency standpoint, lower flash points and fire points, along with a smaller difference between them, are generally preferred.