How many liters does the breathing bag hold during normal operation?

• A. 3 liters
• B. 4 liters
• C. 2 liters
• D. 5 liters

Answer: (B)

The breathing bag provides a gas reservoir in the closed-circuit loop, allowing inhalations to draw from stored gas rather than pulling directly from the cylinder each breath. Its size is a balance: enough volume to smooth breathing and handle normal tidal breaths, but not so large that it adds excessive dead space or buoyancy changes. In normal operation, a diver’s tidal volume and breathing rate are such that about four liters of gas in the bag can accommodate several breaths and maintain comfortable breathing while the scrubber and other loop components process the gas. If the bag were much smaller, you’d frequently draw from the main gas supply, increasing work of breathing and could destabilize the loop; if it were much larger, the extra volume would introduce unnecessary dead space and buoyancy shifts. Therefore the breathing bag capacity is about four liters during normal operation.