Crowcon T4 [T4-HOCA] 4-Gas Portable Multigas Detector, O2, CH4, H2S and CO T4-HOCA 4 Gas Sensor, Detects Oxygen (O2), Methane (CH4), Hydrogen Sulfide (H2S) and Carbon Monoxide (CO). T4 integrates innovative safety features and an intuitive, rugged design to provide advanced protection for those working in harsh environments. This portable multi-gas detector, which is exceptionally easy to use and service, protects against the four most prevalent gas hazards: carbon monoxide (CO), hydrogen sulphide (H2S), flammable gases and oxygen (O2) depletion.