MESG (Maximum Experimental Safe Gap):

The distance through which an explosion can propogate if the gap between two machined surfaces exceeds a certain value.

MIC (Minimum Igniting Current):

The minimum level of current at which each gas or vapour will ignite when tested in a standard intrinsic safety apparatus.

Other factors include explosion pressure and time to peak pressure.

GROUP A GASES/VAPOURS:

Acetylene

GROUP B GASES/VAPOURS:

Butadiene, Ethylene Oxide, Acrolein (inhibited), Hydrogen, Propylene Oxide, manufactured gases containing more than 30% Hydrogen (by volume)

GROUP C GASES/VAPOURS:

Acetaldehyde, Allyl Alcohol, N-Butyraldehyde, Carbon Monoxide, Crontonaldehyde, Cyclopropane, Diethyl Ether, Djiethylamine, Epichlorohydrin, Ethylene, Ethylenimine, Hydrogen Sulphide, Morpholine, 2-Nitropropare Tetrahydrofuran, Isoprene, Unsymmetrical Dimethl Hydrazinc (UDMH)

GROUP D GASES/VAPOURS:

Acetic Acid (Glacial), Acetone, Acrylonitrite, Ammonia, Bensene, Butane, 1-Butanol (Butyl Alcohol), 2-Butanol (Secondary Butyl Alcohol), N-Butyl Acetate, Isobutyl Acetate, Di-Isobutylene, Ethone, Ethonol (Ethyl Alcohol), Ethyl Acetate, Ethyl Acrylate (inhibited), Ethylene Diamine (Anhydrous), Ethylene, Dichloride, Gasoline, Heptanes Hexanes, Isoprene, Isopropyl, Ether, Mesityl Oxide, Methane (Natural Gas), Methanol (Methyl Alcohol), 3-Methyl-1-Butanol (ISO-Amyl Alcohol), Methyl Ethyl Keytone, Methyl, Isobutyl Keytone, 2-Methyl-1-Propanol (Isobutyl Alcohol), 2-Methyl-2-Propanol (Tertiary Butyl Alcohol), Petroleum Naphtha, Pyridene, Ocianes, Pentanes, 1-Pentanol (Amyl Alcohol), Propane, 2-Propanol (Isopropyl Alcohol), Propylene, Styrene, Tolvene, Vinyl Acetate, Vinyl Chloride, Xylenes

GROUP E DUSTS:

Powdered metals such as Aluminum, Magnesium and their commercial alloys

GROUP F DUSTS:

Dusts such as Carbon Black, Coal or Coke dust

GROUP G DUSTS:

Dusts such as Flour, Starch or Grain dust