You’ve probably noticed by now that we can’t breather underwater without some kind of breathing gas. The standard setup in recreational diving that gas is compressed air, but the air needs to be reduced to ambient pressure before we can breathe it.
This is achieved through two regulators; the first stage regulator is attached to the top of the bottle and reduces the air to an intermediate pressure that is higher than required for breathing.
The second stage contains the mouthpiece. This stage reduces the air pressure further so it can be breathed and is designed so that it only supplies air when you need it; when you breathe in it supplies air, when you breathe out the exhaled air is dumped into the water. A safety feature of scuba regulators means that if they fail they will supply continuous air rather than cutting off.
Standard practice is to carry a spare second stage regulator (usually known as an octopus), which is often marked with yellow hose connectors to make it stand out. PADI teaches divers to carry this on the right hand side so that in the event of a failure either the diver or a buddy can find the spare second stage easily.
However, many divers carry it somewhere more convenient for them. It is good practice to check where your buddy carries his or her octopus before entering the water – it’s a good idea to find out how to jettison their weights if they wear an integrated weight system too.