Page "Electrical equipment in hazardous areas" Paragraph 22

Explosive gases, vapors and dusts have different chemical properties that affect the likelihood and severity of an explosion.

Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight.

Empirical testing is done to determine parameters such as the maximum experimental safe gap, minimum ignition current, explosion pressure and time to peak pressure, spontaneous ignition temperature, and maximum rate of pressure rise.

Every substance has a differing combination of properties but it is found that they can be ranked into similar ranges, simplifying the selection of equipment for hazardous areas.