and final stage diaphragm, which would cause a free-flow of gas, or extra resistance to breathing, depending on the diver's orientation in the water. In modern single-hose sets this problem is avoided by moving the second-stage regulator to the diver's mouthpiece. The twin-hose regulators came with a mouthpiece as standard, but a full-face diving mask was an option. Single-hose regulator 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). Modern regulators typically feature high-pressure ports for pressure sensors of dive-computers and submersible pressure gauges, and additional low-pressure ports for hoses for inflation of dry suits and BC devices. Secondary demand valve on a regulator Most recreational scuba sets have a backup second-stage demand valve on a separate hose, a configuration called a "secondary", or "octopus" demand valve, "alternate air source", "safe secondary" or "safe-second". The idea was conceived by cave-diving pioneer Sheck Exley as a way for cave divers to share air while swimming single-file in a narrow tunnel, but has now become the standard in recreational diving. By providing a secondary demand valve the need to alternately breathe off the same mouthpiece when sharing air is eliminated. This reduces the stress on divers who are already in a stressful situation, and this in turn reduces air consumption during the rescue and frees the donor's hand. Some diver training agencies recommend that a diver routinely offer their primary demand valve to a diver requesting to share air, and then switch to their own secondary demand valve. The idea behind this technique is that the primary demand valve is known to be working, and the diver donating the gas is less likely to be stressed or have a high