Types of scuba sets

Modern scuba sets are of two types:
open-circuit (In Europe, it is often called an "aqualung", see Aqua-Lung™, first invented by Jacques Yves Cousteau and Emile Gagnan). Here the diver breathes in from the set and out to waste. This type of equipment is relatively simple, making it cheaper and more reliable. The two-hose design originally used was the one designed by Cousteau and Gagnon. The single-hose design generally used today was invented in Australia by Ted Eldred.
closed-circuit/semi-closed circuit (also referred to as a rebreather). Here the diver breathes in from the set, and out back into the set where the exhaled gas is processed to make it fit to breathe again. These existed before the open-circuit sets and still exist today, but they are used less than open-circuit sets.
Both types of scuba provide a means of supplying air or other breathing gas, nearly always from a high pressure diving cylinder, and a harness to strap it to the diver's body. Most open-circuit scuba and some rebreathers have a demand regulator to control the supply of breathing gas. Some rebreathers only have a constant-flow regulator like in blowtorches. Some divers use the word "scuba" to mean open-circuit sets only.

Open circuit scuba sets
The duration of open-circuit dives is shorter than a rebreather dive, in proportion to the weight and bulk of the set. It can be uneconomic when used with expensive gas mixes such as heliox and trimix. Most divers use standard air (i.e. 21% Oxygen / 79% Nitrogen). The cylinder is nearly always worn on the back. "Twin sets" with two backpack cylinders were much more common in the 1960s than now; although twin cylinders (aka "doubles") are commonly used by technical divers for the increased duration and redundancy they provide. Submarine Products sold a sport air scuba with 3 backpack cylinders. Sometimes cave divers have cylinders slung at their sides instead.
See diving cylinder for more information about the cylinders and how they are arranged.
Newspapers and television news often describe open circuit scuba wrongly as "oxygen" equipment, probably by false analogy from airplane pilots' oxygen cylinders. Until Enriched Air Nitrox was widely accepted in the late 1990s, almost all sport scuba used simple compressed air. This allowed the scuba industry in the U.S. to bypass being supervised by the Food and Drug Administration (FDA), which defines non-air gas mixtures intended to prevent or treat diseases, as "drugs." Exotic gas mixtures presently used in scuba are intended to prevent decompression illness in diving, but officially, the FDA appears to continue to believe that scuba divers all use compressed air.
At partial pressures over about 1.6 atmospheres, oxygen becomes toxic. Open-circuit scuba sets may supply various breathing gases; but rarely pure oxygen, except during decompression stops in technical diving.
Some divers use Enriched Air Nitrox, which has a higher percentage of oxygen, usually 32% or 36% (EAN32 and EAN36, respectively). This lets them stay underwater longer, because less nitrogen is absorbed into the body's tissues. The most common Nitrox blending method needs an oxygen service tank, which is a tank that has had any non-oxygen-compatible grease or rubber removed, by cleaning and replacing parts.

Constant flow
Constant flow scuba sets do not have a demand regulator; the breathing gas flows at a constant rate unless the diver switches it on and off by hand. They run out of air quicker than aqualungs. There were attempts at designing and using these before 1939, for diving and for industrial use. Examples were "Ohgushi's Peerless Respirator", and Commandant le Prieur's breathing sets: see Timeline of underwater technology.

With a demand regulator
This type of set consists of one or more diving cylinders containing breathing gas at high pressure (typically 200-300 Bar) connected to a diving regulator. The regulator supplies the diver with as much of the gas as needed, at a pressure suitable for breathing at the depth of the diver.
Colloquially this type of breathing set is sometimes (depending on the country of the English speaker) often called an aqualung; however, the word "Aqua-Lung" (note spelling) is a tradename protected by the Cousteau-Gagnan patent. The trademark is now owned by U.S. Divers.

Old-type "twin hose" Cousteau-type aqualung

"Twin-hose" open-circuit scuba
This is the first type of diving demand valve which came into general use, and the one that can be seen in classic 1960s SCUBA adventures, such as TV's Sea Hunt.
In this type of set, the two (or occasionally one or 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 diver's orientation in the water: head-up, head-down, level. 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[citation needed] of two-cylinder twin-hose aqualungs shown wrongly, with one wide breathing tube coming straight out of each cylinder top with no regulator.

A "single-hose" aqualung with "octopus" primary regulator. See further detail in photo description.

"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, which was made in Melbourne, Australia by Ted Eldred. 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).
The first scuba set tradenamed Porpoise was a rebreather, but when a demonstration resulted in a diver passing out, Ted began to develop the single-hose open-circuit scuba system. Its regulator's first stage and second stage had to be separated to avoid the Cousteau-Gagnan patent, which protected the double-hose scuba. In the process, Eldred also improved performance.
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 (signaling that it is an emergency or backup device). It is often worn secured into a clip on the stab-jacket or a special friction plug on a diver's chest, easily available to be grabbed by, or offered to, a second diver short 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. Diving instructors continue to teach buddy-breathing; then they show the new method that has superseded it. 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 dive-computer pressure-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.

Cryogenic open-circuit scuba
There have been designs for a cryogenic open-circuit scuba which has liquid-air tanks instead of cylinders.
Jordan Klein designed a cryogenic open-circuit scuba called "Mako" and made at least a prototype.
The Russian Kriolang (from Greek cryo- (= "frost") + English "lung") was copied from Jordan Klein's "Mako" cryogenic open-circuit scuba. Janwillem Bech's rebreather site shows pictures of a Kriolang that was made in 1974. Its diving duration is likely several hours. It would have to be filled immediately before use.
SCAMP (Supercritical Air Mobility Pack®) is an out-of-water liquid-air open-circuit breathing set designed by NASA by adapting space suit technology.

An Inspiration™ rebreather seen from the front

Rebreathers
Main article: Rebreather
With rebreathers, the gas the diver exhales is stored between breaths in a "counterlung". In some rebreathers, one-way valves direct the gas through a "loop". In other rebreathers, the inhaled and exhaled gas goes back and forth along a single tube: this is called the pendulum system. The oxygen consumed by the diver is replaced, nearly always from a cylinder. The exhaled carbon dioxide generated by the diver is removed by passing the gas through a "scrubber": a canister full of soda lime. Then the gas is fit to be re-inhaled. This type of scuba equipment is known as 'closed circuit'.
Since 80% or more of the oxygen remains in normal exhaled gas, and is thus wasted, rebreathers use gas very economically, making longer dives possible and special mixes cheaper to use at the expense of more complicated technology and more experience and longer training. There are three variants of rebreather: oxygen rebreathers, semi-closed circuit rebreathers, and fully closed circuit rebreathers.
The rebreather's economic use of gas, typically 1.6 litres of oxygen per minute, allows dives of much longer duration than is possible with open circuit equipment where gas consumption is typically 10 times higher. Oxygen rebreathers have a maximum operating depth of around 6 metres / 18 feet, but several types of fully closed circuit rebreathers, when using a helium-based diluent, can dive deeper than 100 metres / 330 feet. The main limiting factors on rebreathers are the duration of the carbon dioxide scrubber, which is generally at least 3 hours, and that the scrubber gets less efficient at depth because the scrubber's inside is more crowded with diluent molecules hindering the carbon dioxide molecules from reaching the absorbent as quickly.

Duration of a dive
The duration of an open-circuit dive depends on factors such as the capacity (volume of gas) in the diving cylinder, the depth of the dive and the breathing rate of the diver.
An open circuit diver whose breathing rate at the surface (atmospheric pressure) is 15 litres per minute will consume 3 x 15 = 45 litres of gas per minute at 20 metres. [(20 m/10 m per bar) + 1 bar atmospheric pressure] × 15 L/min = 45 L/min). If an 11 litre cylinder filled to 200 bar is used until there is a reserve of 17% there is (83% × 200 × 11) = 1826 litres. At 45 L/min the dive at depth will be a maximum of 40.5 minutes (1826/45). These depths and times are typical of experienced sport divers leisurely exploring a coral reef using 200 bar aluminum cylinders rented from a commercial sport diving operation in most tropical island or coastal resorts.
A semi-closed circuit rebreather dive is about three times the length of the equivalent open circuit dive; gas is recycled but fresh gas must be constantly injected to replace at least the oxygen used, and any excess gas from this must be vented. Although it uses gas more economically, the weight of the rebreathing equipment means the diver carries smaller cylinders. Still, most semi-closed systems allow at least twice the duration of open circuit systems (around 2 hours).
An oxygen rebreather diver or a fully closed circuit rebreather diver consumes about 1 litre of oxygen per minute. Except during ascent or descent, the fully closed circuit rebreather that is operating correctly uses no or very little diluent. So, a diver with a 3 litre oxygen cylinder filled to 200 bar who leaves 25% in reserve will be able to do a 450 minute = 7.5 hour dive (3 L × 200 bar × 0.75 / 1). The life of the soda lime scrubber is likely to be less than this and so will be the limiting factor of the dive.
In practice, dive times are more often influenced by other factors such as water temperature and the need for safe ascent (see decompression sickness).