There are two regulators, which are used to reduce the gas pressure in the tank to a level that it can be delivered on demand to be breathed. The first stage regulator, which is attached to the top of the bottle, reduces the pressure in the tank to an intermediate pressure, slightly higher than is required for breathing.
The second stage regulator contains the mouthpiece and reduces the pressure from the first stage to the correct pressure for breathing. It is designed to supply air on demand, so that it only provides gas when the diver breathes in, while exhaled gas exits into the water. Regulators are designed to be failsafe so that in the case of a failure they continuously provide air rather than not supplying any air.
It is standard practise to carry a second regulator (usually known as
an octopus) to either use as a backup should the primary second stage
fail, or to provide to a buddy if required. The octopus often has a longer
hose than the primary second stage to allow easier access for a buddy
and marked with yellow hose connectors or tape.
View Simply Scuba's selection of regulators
