When testing the flash point of liquids, two primary methods are used: the open cup flash point and the closed cup flash point. These methods are vital for determining the flammability of liquids and their safety during handling and transportation. Let’s explore the differences between the two and their applications.
The open cup flash point test involves placing a liquid in an open container and applying heat. A flame is passed over the surface of the liquid at regular intervals to detect the flash point—the temperature at which the liquid produces enough vapor to ignite.
This method is typically used for testing liquids with high flash points and is commonly applied in industries like petroleum and chemicals.

It measures the temperature at which a liquid’s vapors ignite in an open environment.
The test is more exposed to external conditions like air flow, which can influence the results.
It is suitable for liquids that tend to have a higher flash point.
In contrast, the closed cup flash point test involves testing the liquid in a sealed container. This method prevents the evaporation of volatile substances, giving a more accurate reading of the liquid’s flash point in controlled conditions. The closed system makes it ideal for measuring substances with lower flash points.
The test occurs in a sealed environment, reducing contamination or loss of vapor.
It is used for liquids with lower flash points, like solvents and alcohol.
Results are generally more consistent than the open cup method.
| Test Type | Open Cup Flash Point | Closed Cup Flash Point |
|---|---|---|
| Method | Liquid exposed to open air | Liquid sealed in a container |
| Flash Point Range | Higher flash points | Lower flash points |
| Accuracy | Less precise | More accurate |
| Applications | Petroleum, chemicals | Solvents, alcohols |
Choosing between an open cup flash point test and a closed cup flash point test depends on the properties of the liquid you are testing. If you are working with substances that have higher flash points, the open cup method may be suitable. For more volatile liquids, the closed cup test is preferred for accuracy and safety.