Flame-Proof Gap Width

The inner explosion chamber and the outer vessel are filled with the explosive mixture to be tested. By igniting the mixture inside the spherical chamber is tested whether an ignition is transferred through the joint gap to the outer vessel. 

The maximum gap of a combustible substance/air mixture is the gap width at which an ignition through the gap does not occur just any more. The lowest value of all maximum gaps of a substance that was determined under atmospheric pressure in dependence on the substance concentration is the Afaximum Experimental Safe Gap (MESG). The appertaining combustible substance mixture is designated as the most readily ignitable mixture. The gap width depends on the temperature and the pressure. A rise in pressure and temperature leads to a reduction of the gap width. 

The MESG s determined for the individual substances serve exclusively for the classification of substances regarding their ability for transmission of an internal ignition. They are not a measure for the constructive dimensions of flame-proof gaps at the type of protection flame-proof enclosure or for gap widths of FAs. 

T-Class Ignition temperatures CC) Permissible surface temperatures for equipment of Category 2 and 3 (°C) Examples for combustible substances with ignition temperature (in brackets) 

T3 200-300 200 Motor gasoline, diesel fuel, n-heptane (215 °C) and isoprene (220°C) 

---

In the result of the determination of the safety characteristic Flame-proof gap width and Ignition temperature an explosive atmosphere can be assigned to an Explosion Group (Ex-Group) and a Temperature Class (T-Class). 

---