Flash point tests are used to show the lowest temperature that a volatile substance is vapourised into a flammable gas. This is done by introducing a source of ignition, then waiting for the “flash” where the substance is ignited.
There are a wide range of methods but most are available in open or closed cup formats – but what is the difference between these?
Open Cup Flash Point
In these types of tests the vessel is open to the air. The temperature is raised slowly and a source of ignition passed over the top, until it “flashes” or ignites.
A key variable in this test is the height of the ignition source over the cup.
One of the most common open cup methods is the Cleveland Open Cup (COC) flash point test, as described by ASTM D92.
Closed Cup Flash Point
In closed cup tests the sample is tested inside a closed vessel – the lid is sealed and the ignition source is brought into the vessel, so it is completely separate from the outside atmosphere. This type of test gives a good simulation of the conditions inside a fuel tank.
Closed cup tests are in a defined and separated vessel, so the results are less subject to interference by outside sources. They also give lower flash points as the heat is contained more than in an open vessel.
All flash point tests are affected by other criteria including the lab environment they are conducted in, the exact equipment and method used.