The PPM-3 from C.A.P. is the most affordable and simple CO2 controller
available. Its' proven technology is very accurate at measuring levels
of Carbon Dioxide in your growing area from 0 to 5000 ppm
(Parts-Per-Million). Using the PPM-3 is quite simple. Just plug your
CO2 device (CO2 valve or generator) into the "piggyback" cord and it
will automatically maintain the CO2 level that you specify. The PPM-3
works with any make of CO2 valve or generator that is 120 volt.