Which device reduces the O2 cylinder pressure to 55-75 psi from the operating range of 10-207 BAR?

• A. the oxygen reducer
• B. the pressure regulator
• C. the regulator valve
• D. the gas mixer

Answer: (D)

In a closed-circuit dive, the oxygen supply must be delivered at a steady, safe low pressure and in the correct mixture with diluent to hold the breathing gas at the desired FiO2. The device that does this is the gas mixer. It not only lowers the high-pressure oxygen from the O2 cylinder down to a working pressure in the tens of psi, but it also blends that oxygen with diluent gas to achieve the precise gas composition for the loop. Delivering oxygen at 55–75 psi provides a stable, controllable flow into the loop, which is essential for maintaining a safe and constant PPO2.

Other components that only reduce pressure, like simple regulators or valves, don’t by themselves set or maintain the correct oxygen fraction. They handle pressure reduction, but without the mixing function, you can’t reliably control the breathing gas composition. The gas mixer is purpose-built to perform both tasks—low-pressure delivery and accurate gas blending—making it the correct choice for this scenario.