Which option is not part of the described O2 flow sequence in the closed-circuit UBA?

• A. O2 leaves O2 cylinder
• B. O2 travels through the connecting line
• C. O2 goes into the inhalation hose
• D. O2 travels to the air compressor

Answer: (D)

In a closed-circuit UBA, oxygen flow stays within a recirculating loop. Oxygen leaves the O2 cylinder, travels through the supply line into the breathing loop, and enters the inhalation hose so the diver can inhale it. The gas then circulates, with CO2 removed and O2 added as needed to maintain the target PPO2. There is no step where O2 is sent to an air compressor as part of this breathing-gas path. An air compressor isn’t used to feed the closed-loop during operation; it’s typically only involved in filling cylinders or supplying gas in open-circuit systems. So the step describing O2 traveling to the air compressor isn’t part of the described O2 flow sequence in the closed-circuit UBA.