Most modern open-circuit scuba sets have a diving regulator consisting of a first-stage pressure-reducing valve connected to the diving cylinder's output valve or manifold. This regulator reduces the pressure from the cylinder, which may be up to , to a lower pressure, generally between about 9 and 11 bar above the ambient pressure. A low-pressure hose links this with the second-stage regulator, or "demand valve", which is mounted on the mouthpiece. Exhalation occurs through a rubber one-way mushroom valve in the chamber of the demand valve, directly into the water quite close to the diver's mouth. Some early single hose scuba sets used full-face masks instead of a mouthpiece, such as those made by Desco and Scott Aviation (who continue to make breathing units of this configuration for use by firefighters).