Maximum Experimental Safe Gap (MESG) and Classification into Explosion Groups
Determination of the Maximum Experimental Safe Gap (MESG) and Classification into Explosion Groups
To assess the flame-propagating capability of a substance, its MESG is determined in accordance with ISO/IEC 80079-20-1 using an MESG test apparatus (Fig. 1). Based on the experimental results, the substance is then classified into one of the explosion groups IIA1, IIA, IIB1, IIB2, IIB3, IIB, or IIC. The explosion group of a substance is used to evaluate the suitability of flame arresters or other ATEX-compliant equipment for the given substance.
The core of the measuring apparatus is an adjustable, concentric annular gap that separates an inner volume from an outer volume. To determine the MESG of a substance, it is fed into the apparatus in gaseous form and ignited in the inner volume by a high-voltage discharge. If the set gap width exceeds the MESG, the flame propagates into the outer volume, which can be detected visually. To determine the MESG, the fuel concentration must also be varied, as the MESG depends on this concentration. The corresponding curve typically has a parabolic shape with a minimum near the stoichiometric composition (λ ≈ 1; Fig. 2).
As with other safety-related parameters, the determined value depends on the ambient conditions, particularly the influences of temperature, pressure, and oxidizing agents.
In general, the MESG is determined at atmospheric pressure and room temperature. When testing vapors of liquids, the apparatus is heated to prevent condensation,
taking into account the mole fraction of the substance and the vapor-pressure curve.
In addition to the measurement conditions, the distinction between substance classification and device design must be considered. A common misconception when using the MESG is the assumption that the determined MESG corresponds to the maximum permissible gap width of a flame arrester element. However, the gap widths used in a device are always derived from full-scale explosion tests with a reference gas for the respective explosion group, in accordance with EN ISO/IEC 80079-49 or EN ISO 16852. This is because a given substance exhibits different flame propagation characteristics depending on the combustion mechanism (deflagration or stable/unstable detonation) and the surrounding system (atmospheric or confined). Consequently, the gap widths of the flame arrester element must be adjusted based on these experimental results.
