First Stage

The part of the regulator that attaches to the tank valve and reduces high tank pressure to an intermediate pressure for the hoses and second stages.

The first stage is the metal heart of your regulator, the piece that clamps or screws onto the scuba tank valve. Its job is the first big pressure drop: it takes whatever is in the tank, often 200 bar or more, and reduces it to an intermediate pressure of roughly 8 to 10 bar above the surrounding water pressure, feeding that gas into the hoses for your second stages, inflator, and pressure gauge.

First stages come in two connection styles, the yoke or A-clamp system common in recreational diving and the DIN system that screws directly into the valve and is standard in technical diving and for 300 bar tanks. Internally they use either a piston or a diaphragm design. Environmentally sealed diaphragm models resist freezing and are the usual choice for cold water diving.

Ports matter when configuring a set: high-pressure ports feed the SPG or transmitter, while low-pressure ports feed second stages and the BCD or drysuit inflator. Keep the inlet dry, replace the dust cap after every dive, and have the intermediate pressure checked at each service.