TPI 721 Combustible Gas Detector Designed to Detect Propane, Butane, Natural Gas, etc. Review By Max | 8:49 AM MST, Mon November 09, 2015 Today I am reviewing the TPI 721 Combustible Gas Detector. Tags tpi 721 tpi 721 combustible gas detector tpi 721 gas detector tpi combustible gas detector combustible gas leak detector Read more about TPI 721 Combustible Gas Detector Designed to Detect Propane, Butane, Natural Gas, etc. ReviewAdd new comment