"The AS-4 CO2 Alarm Station with an Blue light alarm module comes with a Blue light LED display indicator and a LCD digital ppm level reading. It is designed to be mounted outside of the space to be monitored, and will display the CO2 ppm level. The AC-4 can be linked to either the low or the high level CO2 sensors, or it can be linked to both sensors for complete redundancy. The AS-4 will sound an audible alarm and blink the Blue LED indicator in the event of the CO2 level going above the user's preferred level set in the main controller."