The 121 is a full-featured, compact, two- to four-sensor confi ned space gas detector. Key features include easy-to-change, externally accessed battery packs (available in both alkaline and rechargeable versions) and a water-resistant case. The rechargeable, Lithium-ion battery pack provides up to 20 hours of continuous operation. The 121 includes 16,000 datalogging points of storage capacity for download to any PC compatible with Windows 98, NT, 2000 and XP. Key Features One to four plug-in smart  sensors  Combustibles: Oxygen , Hydrogen sulfi de , Carbon monoxide Optional toxic sensors , Sulfur Dioxide (SO2) , Nitric Oxide (NO) , Nitrogen Dioxide (NO2) , Chlorine (Cl2) , Hydrogen Cyanide (HCN) , Ammonia (NH3) , Phosphine (PH3) Rechargeable Lithium-ion battery pack provides up to 20 hours of continuous operation Rugged housing with stands use in harsh environments . Water- and dust-resistant case . Strong protective rubber boot