In mixtures of combustible gases or steams with air an independent burn can reproduce itself only within a certain concentration range.
The border concentrations, where a explosion is just no longer possible, are called lower and upper explosion limit.
With a concentration below the lower explosion limit the mixture is too "lean" (it contains too little fuel). With a concentration above the upper explosion limit the mixture is too "fat" (it contains too much fuel, i.e. too little oxygen), in order to continue the burn after ignition.
Different standard methods of measurement exist for the determination of explosion limits.
atmospheric pressure, temperature till 200 °C
DIN 51 649-1
atmospheric pressure, temperature till 150 °C
ASTM E 681-98
raised pressure and temperature
ASTM E 918-83
(For detailed Information see also Fire and Explosion Protection)
Gases, which possess an explosion range, i.e. a lower and upper explosion limit under atmospheric conditions, apply as high inflammatory in the sense of the dangerous material regulation. Also the appropriate classification of combustible gases after danger property right relies on the explosion ranges.
The explosion limits are relevant data for new material registrations and combined with the limiting oxygen concentration for the interpretation of the preventive measure "avoidance of combustible atmosphere" are needed. The temperature -, pressure and inert gas dependence are to be considered. Futhermore it has to be noted that the upper explosion limit is a function of the oxygen content.