The PPM-4 is the newest addition to C.A.P.ís CO2 metering lineup, offering indoor gardeners the most economic and simplest method of monitoring and controlling CO2 dispersal throughout their growing area. The simplicity of this unit will make it attractive to both the beginning hobbyist and the seasoned grower alike. Operating the PPM-4 is as easy as mounting the meter to the wall, plugging the meter into the power interface and plugging in your CO2 generator or regulator. The PPM-4 utilizes a highly visible and intuitive LED indicator display to let you know generalized CO2 levels at a glance from across the room. PPM-4 also features an easy to use calibration function.