If you are using one of these systems : Cell Excellens, SP8-IN1 or LSM700, the main CO2 valve is attached to 3 individual ones: The main valve should always be opened, and you should only touch the individual valve which is controlling the system you booked. {F22946832, size=full} Please do not forget to close the individual CO2 valve when your experiment is finished, but please LET THE MAIN VALVE OPEN: someone may be doing live imaging as well!