IEC 60079-29-4:2009 specifies performance requirements of equipment for the detection and measuring of flammable gases or vapours in ambient air by measuring the spectral absorption by the gases or vapours over extended optical paths, ranging typically from one meter to a few kilometres. Such equipment measures the integral concentration of the absorbing gas over the optical path in units such as LFL.meter for flammable gases. This standard supplements and modifies the general requirements of IEC 60079-0. Where a requirement of this standard conflicts with a requirement of IEC 60079-0, the requirement of this standard shall take precedence.