What is the difference between open circuit and closed circuit in diving, and which applies to ADCI?

• A. Open circuit gas is not used in surface-supplied operations
• B. Open circuit gas is normally used in surface-supplied diving; closed-circuit rebreathers are specialized and require additional approvals, equipment, training, and supervision
• C. Closed circuit is used in all underwater dives
• D. Open circuit is illegal in ADCI operations

Answer: (B)

Open-circuit versus closed-circuit diving is about how breathing gas is handled after it’s inhaled. In open-circuit systems, the diver breathes gas from a surface or onboard supply and exhales it directly into the water; the gas isn’t recycled. In a closed-circuit rebreather, the gas is recycled, CO2 is removed, and oxygen is managed so the same gas mix can be reused, which keeps gas use efficient but adds complexity and safety considerations.

For ADCI operations, the standard setup is surface-supplied diving using open-circuit gas. This arrangement is simpler, safer to supervise, and aligns with typical commercial diving practices. Closed-circuit rebreathers are specialized equipment that require extra approvals, gear, training, and oversight. That’s why this choice best captures how open-circuit is normally used in surface-supplied diving and why closed-circuit gear is considered specialized.