Which of the following is NOT a factor in the MK 25 UBA's operational duration?

• A. Depth
• B. O2 supply
• C. Canister duration
• D. Diver workload

Answer: (A)

The main idea is that a MK25 UBA’s operational duration is limited by three things: how much oxygen remains in the loop (O2 supply), how long the CO2 scrubber can remove CO2 before it needs changing (canister duration), and how fast the diver uses gas (diver workload). The unit is designed to maintain a safe partial pressure of oxygen in the breathing loop by adding O2 as needed, and it relies on the scrubber to keep CO2 under control. If you exert yourself more, your metabolic rate rises, so you consume O2 faster and produce CO2 more quickly, shortening both the O2 life and the scrubber life. While depth does affect ambient pressure and can influence comfort or breathing effort, it does not directly change these three limiting factors. The rebreather keeps the target pO2 across depths, so duration is governed by O2 supply, canister life, and workload, not depth.