This is the first type of diving demand valve which came into general use, and the one that can be seen in classic 1960's SCUBA adventures such as TV's Sea Hunt.
In this type of set, the two (or occasionally the one or the three) stages of the regulator are in a large circular valve assembly mounted on top of the cylinder pack. This type has two wide breathing tubes like those on many modern rebreathers, one for intake and one for exhalation. The return tube was not for rebreathing, but because the air exhaust needed to be as near as possible to the regulator's second stage diaphragm, to avoid pressure differences, which would cause a free-flow of gas, or extra resistance to breathing, according to the which way up the diver is 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 sets came with a mouthpiece as standard, but a fullface mask was an option. Another optional extra was a mouthpiece that also had a snorkel attached and a valve to switch between aqualung and snorkel.Note the correct layout of this type, in the image to the right. In comics there have been thousands of drawings of two-cylinder twin-hose aqualungs shown wrongly, with one wide breathing tube coming straight out of each cylinder top with no regulator.
"Single-hose" open-circuit scuba
Most modern open-circuit scuba sets have a diving regulator consisting of a first stage pressure reducing valve fastened over the diving cylinder's output valve. This valve cuts the pressure from the cylinder, which may be up to 300 bar, to a constant lower pressure, often about 10 bar above the ambient pressure, which is used in the "low pressure" part of the system. A relatively thin low-pressure hose links this with the second-stage regulator, or "demand valve," which is located in the mouthpiece. Exhalation occurs out of a one-way diaphragm in the chamber of the demand valve, directly into the water quite close to the diver's mouth. This configuration type is called "single hose". The first make of this sort of scuba was the Porpoise (make of scuba gear) which was made in Australia.All modern scuba sets have a spare second-stage demand valve on its own second hose, a configuration called an "octopus" because it often has more hoses for other purposes coming out of the primary regulator on the cylinder top. This second "second-stage" regulator and hose, or "alternate air source", or "safe secondary" or "safe-second" for short, is typically yellow (signalling that it is an emergency or backup device). It is often worn secured into a special friction plug on a diver's chest, easily available to be grabbed by, or offered to, a second diver in trouble for need of air. In so doing, this second mouthpiece eliminates the need for two divers, who need to share a cylinder, to "buddy-breathe" by trading off the same mouthpiece. The original octopus idea was conceived by Sheck Exley as a way for single-file-swimming cave divers to share air in a narrow tunnel, but has now become the standard in recreational diving. Modern "octopus" type primary stage regulators also typically feature high-pressure ports for use by computer sensors, and additional ports for additional low-pressure hoses for inflation of dry suits and buoyancy compensator (BC) devices.
Increasingly, in the 21st century, "safety" secondary mouthpieces have been combined with the inflator and exhaust assembly of buoyancy compensator (BC) devices. This combination eliminates the need for a separate low pressure hose for the BC. Some diving schools now suggest that a diver offer another diver in trouble their primary mouthpiece (i.e. the one in their mouth), before going to their own safe-secondary. The idea here is that the diver not in trouble has much more time to sort things out with his/her own equipment after temporarily losing ability to breathe.
