What function does the demand valve serve?

• A. Adds O2 to breathing bag during inhalation
• B. Regulates the bag temperature
• C. Monitors gas purity
• D. Provides audible alarm

Answer: (A)

When you inhale in a closed-circuit system, the demand valve opens and delivers oxygen from the supply into the breathing bag. This on-demand delivery keeps the bag filled with the correct oxygen content so the gas you breathe remains breathable and at the intended partial pressure, matching your inhalation. It doesn’t regulate bag temperature, monitor gas purity, or provide audible alarms—those functions come from other components of the system.