The optional CO2 sensor (MBS-S8) measures the CO2 PPM concentration in the grow room. Users can connect and control an additional CO2 system through the Hydro-X device. The sensor should be kept dry and clean to avoid premature failure. Package includes: 1 x CO2 sensor (MBS-S8), 1 x 16ft RJ12 cable, 1 x RJ12 T-splitter. Technical specifications: Dimensions: 160x86x86mm, Weight: 0.37kg.