This European Standard specifies performance requirements for Group II portable, transportable and fixed apparatus for the detection and measurement of integral concentrations of combustible gas or vapour in air over a defined open path. The apparatus, or parts thereof, may be installed or transported for use in potentially explosive atmospheres. The general requirements and test methods applicable to the apparatus covered by this European Standard are specified in Part 1. This European Standard concerns apparatus intended to provide measurement or warning of specific combustible gases in air which constitute a potential hazard by virtue of their flammability. Open path detection apparatus as covered by this European Standard measures the path integral concentration. It is not capable of resolving the spatial variation of concentration along the optical path