What is the purpose of the bag breathdown?

• A. To inflate the breathing bag for ascent.
• B. To purge CO2 from the gas.
• C. To vent gas to the environment.
• D. To reduce the volume in the breathing bag to control ascent.

Answer: (D)

Bag breathdown is about using the breathing loop’s gas to fine-tune buoyancy. The gas inside the breathing bag contributes to how buoyant you are. As you ascend and ambient pressure drops, that gas tends to expand, making you more buoyant. To keep the ascent controlled, you reduce the gas volume in the bag—the breathdown. By deflating the bag, you offset the expansion and prevent an uncontrolled or too-rapid rise. This is a deliberate buoyancy-management technique, not a CO2 purge or a venting action to the environment, and it’s not about inflating the bag for ascent. So reducing the bag’s volume to control ascent is the purpose.