When oxygen from a cylinder is being used, the valve should be opened:

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Opening the oxygen cylinder valve fully is the correct approach because the downstream regulator and flow controls are designed to work with the full cylinder pressure. If the valve is opened only partially, the regulator may not receive enough pressure to deliver the intended flow, leading to unstable or insufficient oxygen delivery. Fully opening the valve ensures the system pressure is available for the regulator to meter accurately and respond promptly to the set flow. After opening, you can set the desired flow on the regulator and check for leaks to confirm safe operation.