Z Gardâ„¢ is a series of gas monitoring sensors, controllers, and monitors. The S sensor is designed to detect the presence of carbon monoxide (CO) or nitrogen dioxide in air. A 4-20mA analog output version and an addressable RS485 digital signal version are available. The RS485 digital output version is designed to mate with a Z Gard C controller, operating as a system. The C controller is designed to interface with up to 40 remote S sensors. The COmbo gas monitor with integrated CO sensor is available in two versions: a metal oxide solid-state sensor and an electrochemical plug-in sensor. The manufacturer says the electrochemical plug-in sensor is well suited for facility monitoring.