1.1 This part of IEC 61779 specifies requirements for group II (as defined in part 1) portable, transportable and fixed apparatus for the detection and measurement of combustible gas or vapour concentrations with air. The apparatus, or parts thereof, may be installed or used in potentially explosive atmospheres, other than mines susceptible to firedamp (i.e. group I). The requirements and test methods applicable to the apparatus covered by this standard are specified in part 1. 1.2 This standard is restricted to apparatus intended for the detection and measurement of volume ratios of combustible gas or vapour in air in volume fractions from 0 % to 100 % . NOTE 1 – Apparatus covered by this standard will normally be intended to operate in volume ratios greater than 100 % LEL. NOTE 2 – Although apparatus of the types covered by this standard may be suitable for detecting a wide range of combustible gases, particular gases (e.g. methane or propane) are specified in part 1 as the components of the test gases for the purpose of practical convenience. The performance requirements specified in this standard should therefore be considered with caution when the apparatus is used to detect other combustible gases, as some parameters – such as time of response – will be modified.