Piston type:

Piston-type first stages are easier to make and have a simpler design than the diaphragm type. This may help to improve reliability. They need more careful maintenance because some of the internal moving parts are exposed to water and contaminants in the water.

With the piston-type first stage, the piston is rigid and acts directly on the seat of the valve. When the pressure in the medium pressure drops because the diver has used gas from a second stage valve, the piston lifts off the valve seat and slides towards the medium pressure chamber. This brings high pressure gas into the medium pressure chamber until the pressure in the chamber has risen enough to push the piston back onto the seat and close the valve.