Flammability limits refer to the fact that mixtures of gaseous fuels and air will only burn if the fuel concentration lies within well defined limits. The terms "flammability limits" and "explosive limits" are used interchangeably. Limits are determined experimentally. The standard reference work is that by Zabetakis using an apparatus developed by the US Bureau of Mines. Limits are normally expressed in terms of volume percentage at 25ºC and atmospheric pressure, but are functions of temperature and pressure. Attaining the perfect combustible or explosive mixture between a fuel and air (the stoichiometric proportion) is important in internal combustion engines, for example in gasoline or diesel engines. A deflagration is a propagation of a combustion zone at a velocity less than the speed of sound in the unreacted medium. A detonation is a propagation of a combustion zone at a velocity greater than the speed of sound in the unreacted medium. An explosion is the bursting or rupture of an enclosure or container due to the development of internal pressure from a deflagration or detonation as defined in NFPA 69