A flammable mixture can be classified for explosion protection under several key characteristics including its Relative Density, Flash Point, Flammable Limits, Ignition Energy and Ignition Temperature. In order to ensure correct assessment in terms of risk and correct selection of apparatus to be used in the Explosive Atmospheres, it is important to understand all of the particulars of the flammable materials being handled in any process plant. For example, it may be vital to know the temperature of ignition by hot surfaces, the flash point, or the lower explosive limit of the flammable mixture that a particular process will become exposed to before accurate and proper risk assessments can be made. The data relating to the particulars of a given substance may be extracted from recognised standards that define the characteristics of the most commonly handled or known substances. The definitions of the main characteristics are shown below.

Relative Density : The density of a gas or a vapour relative to the density of air. The result would be displayed in reference to air having a value equal to 1.

Flash Point : The minimum temperature at which a liquid give off sufficient vapour to form an explosive atmosphere when tested in a set of standard test apparatus.

Flammable Limits : The upper and lower percentages of gas / air mixture at normal atmospheric temperature and pressure which will burn if ignited.

Ignition Energy
The spark energy which will ignite the most easily ignited mixture of a test gas with air at atmospheric pressure in a set of standard test apparatus. The spark energy of the ignition is also related to the intensity of explosion, i.e. a highly flammable gas mixture can be ignited with a low energy spark to create a high intensity explosion. Ignition Temperature (Auto Ignition Temperature) Ignition temperature, or AIT as it is also known, is defined as the lowest temperature determined by a standardised method, at which the most explosive mixture of the given substance and air will automatically ignite when in contact with, or in the presence of, a heated surface. In this situation the flammable mixture spontaneous combustion will occur without the presence of an independent ignition source.

The Ignition Temperature of flammable materials (gases, vapours and liquids) is defined in IEC 60079, along with the corresponding gas group. Examples of ignition temperatures for commonly occurring flammable mixtures are town gas (surface methane)/air, which ignites at over 600ºC, and petrol/air which ignites at approximately 250ºC.