C.A.P. PPM-1c CO2 Monitor 

The PPM-1c is an extremely accurate CO2 sensor. It measures the exact level of CO2 in your growing area and displays this value from 0 to 5000 Parts-Per-Million (PPM) on its' digital readout. 

Even though the PPM-1c can be used as a simple way to measure your CO2 level, the true potential of the PPM-1c is not realized until it is connected to one of the controllers C.A.P. has designed to work with it.

When the PPM-1c is connected to either the CO2-2, CO2-4 or CGC-1 it commands the controllers to release or produce more CO2 only when required.

The timers built into the atmosphere controllers continue to coordinate with the PPM-1c to give you the precision control not possible with other manufacturers CO2 control systems using only ON/OFF control.

You enter the exact level of CO2 you want to maintain (usually 1500 PPM) and you're done.

More info from the Manufacturer (PDF format - Lots of detail.)