These flameproof gas sensor heads that can be fitted with a range of different sensor technologies (infrared, pellistor, electrochemical) for detection of Hydrocarbons, Hydrogen or toxic gas and oxygen. They are Ex d certified components and as such must be fitted into compatible separately certified apparatus such a Flameproof (Ex d) junction box, Increased Safety (Ex e) junction box or Flameproof (Ex d) gas detector instrument housing.