C.A.P. PPM-2A - "The Fuzzy™" CO2 Controller 

People who need the absolute best CO2 controller available today, use the PPM-2a from C.A.P. Fuzzy Logic(tm) gradually brings the CO2 level up to the desired setting without over-shooting the desired CO2 level. 

The PPM-2a automatically "dials-in" the exact parameters for your area. It learns how your grow area uses CO2 and adapts to keep the CO2 levels constant.

The PPM-2a can be connected to a temperature or environmental controller, disabling the CO2 function while your exhaust fans are running to reduce CO2 waste.

The PPM-2a is capable of being used to control both compressed CO2 and gas-burning CO2 generators.

When used with a CO2 generator, the user can change the PPM-2a mode to ON / OFF control which eliminates the rapid cycling of the CO2 generator. The PPM-2a will control any make of CO2 valve or generator that is 120 volts.

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