Flash point measurement

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Whether or not there is a risk of fire or explosion is generally determined by the flash point and/or the explosion limits. The flash point is the lowest temperature of a liquid at which it still gives off enough vapour to be capable of ignition. Liquids of the group Evaporators can give an ignitable vapour at a specific temperature.

The flash point of the liquid is an indication of the hazard and the source strength of vapour at local temperature. Generally speaking the flash point is the key value taken as a basis for the legal requirements applicable to transport, trans-shipment and storage.

Figure 114 The Seta flash tester

Picture 115.Electronic seta flash tester Copyright photo ASCC
Measurement Type Substance properties
Output Flash point reading (ºC)
Principle Ignition of the vapours
Suitable for substance types N/A
Measurement range
Measurement units ºC
Operator experience level Low
Operation - Laboratory (ex-situ)
Dimensions (LxHxW)
Mobilisation Time Minutes
Mobilisation requirements None
Secondary equipment None
Environmental requirements None

The flash point is the simplest, quickest and most widely used method of demonstrating the degree of the fire hazard presented by a given substance. The flash point is a good yardstick for the risk that explosive or flammable mixtures may form.

The flash point is not a fixed scientific parameter for a given liquid. It is to some extent dependent on the design of the testing equipment and the test method. The type of testing equipment must therefore be specified with the indication of a flash point.

Operating principle

At present, various forms of standard equipment are in use, all based on the same principle: a given quantity of a liquid is heated while at intervals a small flame is placed close to the liquid surface.

The flash point is the lowest temperature at which an ignition is observed. The testing methods can be divided into two main groups, depending on the cup used being open or closed.

In the open cup method the sample is heated in an open cup until a given temperature is reached. A test flame is then exposed at the surface and the sample is observed to see whether ignition has occurred. If this has not happened the temperature is raised and the procedure is repeated until ignition takes place.

In the closed cup method, the flash point is determined in a cup covered with a lid, through which the vapours cannot escape. Therefore, flash points determined in a closed cup will always be lower (more dangerous) than those of the same substances determined with an open cup method.


The Seta flash tester (see Figure 114 and Picture 115) is an example of a flash point meter that determines the flash point by the "flash -no flash" method using the Seta flash tester (see The adjusted temperature (depending on ambient temperature) is set on the apparatus. When the cup is at the correct temperature, a sample of the pollutant is inserted through a small opening in a closed cup. A timer is then started, which sounds a bell after one minute. In the meantime the test flame has been ignited and, after the ringing signal, it is carefully brought into position above the liquid surface of the pollutant sample in the cup. If the vapour ignites, the flash point is lower than or equal to the adjusted temperature: if not the flash point is above the adjusted temperature.