An oxygen cylinder should be taken out of service and refilled when the pressure inside it is less than:

• A. 1,000 psi.
• B. 200 psi.
• C. 1,500 psi.
• D. 500 psi.

Answer: (D)

The key idea is maintaining a safe reserve of gas by not letting the cylinder get too low. Oxygen is stored under high pressure, and the gauge shows how much remains. You want to refill or replace the cylinder before the pressure falls too far so you have enough gas to deliver the prescribed flow and enough time to swap cylinders without interruption. The commonly accepted minimum service pressure for an oxygen cylinder is 500 psi. Below this, there may not be enough gas to safely complete the required delivery, and you risk running out during use. Pressures like 1000 or 1500 psi are comfortably full and would waste gas, while 200 psi leaves little margin. So, 500 psi is the practical cutoff that balances safety and practicality.