This Practice provides design criteria for fixed gas detectors installed in process areas to detect the presence of combustible, oxygen, or toxic gases. This Practice addresses combustible, oxygen, and toxic gas detectors permanently installed for ambient air monitoring (e.g., leak detection, oxygen depletion). The technologies included are catalytic bead, infrared, and electrochemical. This Practice does not cover portable gas monitoring equipment, process analyzers, or sampling systems. The systems used to perform data acquisition, alarming, and control action are not covered by this Practice because the solutions are diverse and can include stand-alone to advanced multisystem configurations.