Today I am reviewing the TPI 721 Combustible Gas Detector. The TPI 721 is designed to detect gases such as Acetone, Acetylene, Alcohol, Ammonia, Benzene, Butane, Ethanol, Ethylene Oxide, Gasoline, Hexane, Hydrogen, Methane, Naphtha, Natural Gas, Paint Thinners, Propane, and Solvents with 10-ppm methane sensitivity.  The TPI 721 Combustible Gas Detector is great for checking gas pipelines, valves, regulators, and meters on gas equipment, propane filing stations, liquid or gas fired heating systems, exhaust or fuel leaks, arson residue, fuel in marine bilges, and more.