CO2Gro C2 EXT CO2 ControllerCO2GRO C2 EXT CO2 Controller with External Sensor Probe Take full control of your grow room environment with the CO2GRO C2 EXT CO2 Controller, available at Holland Horticulture. Designed for precise CO2 monitoring and dosing, this advanced controller allows growers to accurately regulate CO2 levels and optimise plant growth through every stage of development. Accurate CO2 Monitoring with External Probe The C2 EXT features an external CO2 sensing probe