ATEX Gas detectors are available for 2 explosion classes as follows:

Zone 1 is available with or without display.

Gas detectors for ATEX Zone 1 are located in a flameproof enclosure, EEx d

The shell is such that ignition and combustion within the shell of a potential explosive gas mixture is not propagated to the explosive mixture outside the enclosure and the enclosure can thus endure the resulting explosion pressure without damage.

In Zone 1, one can expect an explosive atmosphere to occur during normal operation but the presence of it is relatively short.

The equipment shall not be a source of ignition in normal operation or when an error occurs.

Gas detectors for ATEX Zone 2 are of the design Non-sparking EEx nA

EEx nA is a non-sparking piece equipment.

In Zone 2, you need not count with an explosive atmosphere to occur during normal operation, and if it should occur, which is probably rare it is of short duration.

The equipment shall not be a source of ignition during normal operation.