Auto-Ignition Temperature (AIT) is the minimum temperature at which a gas or vapor will spontaneously ignite without an external ignition source, under standard atmospheric conditions. This process is called heat-ignition, as opposed to spark-ignition.
AIT is defined in IEC 60079-4 and is independent of pressure and temperature for volume concentration. It represents the surface temperature that can cause ignition, making it critical for equipment selection in hazardous areas.
Different gases have vastly different AIT values. Methane has an AIT of 595°C, while carbon disulfide has the lowest known AIT at less than 100°C. Gasoline, diesel, and kerosene have AITs around 200°C, while ethers and aldehydes have AITs below 200°C.
The safety relationship is clear: equipment temperature class must be lower than the AIT of gases present. For example, T3 equipment (200°C maximum) cannot be used for gases with AIT < 200°C. This relationship ensures equipment surfaces won't ignite surrounding gases.
AIT is determined under standard conditions and represents a critical parameter for hazardous area classification. Understanding AIT values is essential for selecting appropriate equipment and ensuring safe operation in potentially explosive atmospheres.