OC vs. CCR – Max's Blog

With OC (Open Circuit) you are breathing the gas from the tank(s) just once, but with CCR (Closed Circuit Rebreather) your exhaled gas goes into a pendulum or a loop.

           Lungs
           ↑   ↓
     --- Mouthpiece ---
           ↑   ↳ Water
      Gas cylinder

CCR

Oxygen (Loop)

           Lungs
           ↑   ↓
     --- Mouthpiece ---
         ↓        ↑
    Scrubber    Counter-
    canister ->  lung
                   ↑
                Oxygen
               cylinder

Mixed Gas (Loop)

           Lungs
           ↑   ↓
     --- Mouthpiece ---
         ↓        ↑
    Scrubber    Counter- <- Diluent
    canister ->  lung       cylinder
                   ↑
                Oxygen
               cylinder

The scrubber removes the CO₂ from the exhaled breathing gas. (Prevents a CO₂ buildup.)
Pendulum CCRs have a dead gas space in contrast to loop CCRs. (Like with a normal snorkel.)
Many loop CCRs have two counterlungs, which are called exhale and inhale counterlung.

Comparison

Variantion	Advantages	Disadvantages
OC	– Simple gear configuration – Well known procedures	– High gas costs – Complicated gas logistics
CCR	– Less gas consumption – Reduced deco time	– Hugh initial costs – Risk of CO₂ poisoning