What is open circuit in diving?
Open circuit is standard scuba: you breathe gas from the tank through a regulator, and every exhaled breath vents into the water as bubbles. Simple, robust, and affordable, it is what nearly every diver learns on.
Open circuit is the breathing system behind almost all recreational scuba. A regulator drops the high tank pressure and delivers gas at ambient pressure the moment you inhale; every exhaled breath vents into the water as bubbles and is gone. Nothing gets recycled, and that is exactly the point: few parts, few failure points, simple servicing, and a price any diver can reach. It is the gear you use in every entry-level course and on the vast majority of dives worldwide.
How is open circuit different from a rebreather?
A rebreather (CCR) recirculates the gas you exhale: it scrubs out the CO2, tops up the oxygen, and hands the mix back to you, which makes it nearly silent and extremely gas-efficient. Open circuit throws every breath away, so gas consumption climbs fast with depth. In exchange you get simplicity and reliability no rebreather can match.