The term "flash point" is used to determine the lowest temperature at which a volatile substance can evaporate into a flammable gas. To measure flash point, it is necessary to introduce an ignition source into a substance and then wait for "flash," the point at which it can ignite.
There are several methods of measuring flash point, which can be divided into two broad categories: open cup and closed cup flash point.
Open cup flash point
As the name suggests, flash point measurements using the open-cup method are performed in a container exposed to outside air. The temperature of the substance gradually increases and the ignition source moves over the top of the substance until it reaches a point where it "flashes" and ignites.
The flash point here will vary depending on the distance between the substance and the ignition source (the height of the ignition source above the cup). The most commonly used method of opening a cup is called the Cleveland Open Cup (COC).
Closed cup flash point
Again, as the name suggests, the flash point in the closed cup method is conducted in a closed container that is not open to the outside atmosphere. The lid is sealed and the ignition source is introduced into the container itself, thus closer to actual conditions (such as those inside a fuel tank).
The four main varieties of closed cup flash point are Pensky Martens, Abel, Tag and the most widely used Small Scale, often called Setaflash. However, new developments in flash point testing have led to the introduction of modified safety standards.
What's the difference between the two?
Because the closed-cup method is performed in a closed environment, the results are less likely to be affected by external factors in the laboratory. Additionally, it is understood that they generally produce a lower flash point because heat is contained and the substance is more likely to become flammable at an earlier stage. Therefore, they are often used as industry standards because they produce lower results, ensuring safer operations.
However, it is important to note that neither the open cup nor the closed cup flash point is a fundamental parameter of the substance, but is an empirical result of testing. They will be subject to various standards, including the laboratory in which they are conducted, the equipment used and the testing methods chosen.