A flammability diagram shows the flammability region in mixtures of fuel, oxidant (usually oxygen) and an inert gas (usually nitrogen). If the oxidant concentration is reduced, the range of flammability is also reduced i.e. the difference between the upper explosible limit and lower explosible limit becomes smaller. This effect can be graphically represented by plotting a three component flammability diagram showing how the oxidant, fuel and inert gas interact.
Following ASTM E681, uniform mixtures of gases or vapours are created in a closed flask at the temperature of interest (up to 200 oC) and exposed to a central electrical ignition. Tests are repeated for different concentrations / mixtures, with visual examination to determine if flame propagation takes place.
Flammability diagram tests are useful for determining if a mixture is flammable and deciding the required control measures to prevent the formation of flammable (explosive) atmospheres.