Scuba set
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SCUBA is an acronym for Self-Contained Underwater Breathing Apparatus. These initials originated in 1939 in the US Navy to refer to US military diver's rebreather sets. As with radar, the acronym has become so familiar that it is often not capitalized and is treated as an ordinary word: for example, it has been taken into the Welsh language as "sgwba".
A scuba set provides a scuba diver with the breathing gas necessary to breathe underwater during scuba diving.
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[edit] Types of scuba sets
Modern scuba sets are of two types:
- open-circuit (In Europe, but not the U.S., often called an "aqualung", see Aqua-Lung™). Here the diver breathes in from the set and out to waste. This type of equipment is relatively simple, making it cheap and reliable.
- closed-circuit/semi-close 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 reprocessed to make it fit to breathe again.
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.
[edit] 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 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 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.
[edit] 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.
[edit] 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.
- See Diving regulator for more information about diving regulators.
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.
[edit] "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 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.
[edit] "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.
[edit] 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.
[edit] Rebreathers
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 metre / 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.
[edit] 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).
[edit] Underwater alternatives to scuba
There are alternative methods that a person can use to survive and function while underwater, including:
- free-diving - swimming underwater on a single breath of air.
- snorkelling - a form of free-diving where the diver's mouth and nose can remain underwater when breathing, because the diver is able to breathe at the surface through a short tube known as a snorkel.
- surface supplied diving - originally used in professional diving for long or deep dives where an umbilical line connects the diver with the surface providing breathing gas, and sometimes warm water to heat the diving suit, and usually nowadays voice communications. Some tourist resorts now offer a surface supplied diving arrangement, trademarked as Snuba, as an introduction to diving for the inexperienced. Using the same type of equipment as scuba diving, the diver breathes from compressed air tanks, which float on a free floating raft at the surface, allowing the diver only 20-30 feet (6-9 m) of depth to travel.
- Atmospheric diving suit - an armored suit which protects the diver from the surrounding water pressure.
- Liquid breathing - so far, in the real world, liquid breathing for humans is only laboratory experiments, and (one lung at a time) medical treatment. It has possibilities of being used for very deep diving. It is memorably portrayed in the film "The Abyss".
- Artificial gills (human) - these are mostly science fiction. In the real world they have to process a massive amount of water to extract enough oxygen to supply an active diver, and processing this much water takes a great deal of energy (possible for cold-blooded fish, but harder for humans with higher metabolic rates). But see Like-A-Fish for an attempt to develop real artificial gills for divers.
[edit] Breathing sets used out of water
Breathing sets operating on the above principles are not only used underwater but in other situations where the atmosphere is dangerous (little oxygen, poisonous etc).
- Firefighting
- Other jobs out of water, e.g. welding in a confined space
- Mining, especially mine rescue
- Operations in enclosed or poorly ventilated areas, e.g. large fluid or gas containers.
These breathing sets are nowadays called SCBA (Self Contained Breathing Apparatus) (The initials SCBA have had other meanings). The first open-circuit industrial breathing sets were designed by modifying the design of the Cousteau aqualung.
Industrial rebreathers have been used since soon after 1900.
Rebreather technology is also used in space suits.
[edit] History
A predecessor to scuba gear, the Momson lung, was used as emergency escape gear by WWII submariners.
Jacques-Yves Cousteau and Emile Gagnan invented the first commercially successful open circuit type of SCUBA diving equipment, the Aqua-Lung (often spelled "aqualung") in 1943. Among the things that prompted Cousteau to develop efficient air-breathing diving free-swimming diving gear, were two oxygen toxicity accidents that he had with rebreathers.
Another SCUBA pioneer was John Haven "Jack" Emerson, who also developed the iron lung and other breathing apparatus.
Before 1971 all breathing sets including scuba came with a plain harness of straps with buckles like on a rucksack or spray-tank-pack. The buckles were usually quick-release. Many did not have a backpack plate, but the cylinders were held directly against the diver's back. Sport scuba usually had quick-release fastenings instead of ordinary buckles. The harnesses of many diving rebreathers made by Siebe Gorman included a large back-sheet of strong reinforced rubber.
In the beginning scuba divers dived without any buoyancy aid. In emergency they had to jettison their weights. In the 1960s adjustable buoyancy life jackets for aqualung-type scuba became available; one early make was Fenzy. The ABLJ is used for two purposes, one to adjust the buoyancy of the diver to compensate for loss of buoyancy (chiefly due to compression of neoprene wetsuit) and more importantly as a lifejacket that can be rapidly inflated even at depth. It was put on before putting on the cylinder harness. The first were inflated with a small carbon dioxide cylinder, later with a small air cylinder. An extra feed from the first stage regulator lets the life jacket be controlled as a buoyancy aid.
[edit] Accessories
In modern scuba sets, a buoyancy compensator (BC) or buoyancy control device (BCD), such as a back-mounted wing or stabiliser jacket (otherwise known as a 'stab jacket'), is built into the scuba set harness. Although strictly speaking this is not a part of the breathing apparatus, it is usually connected to the diver's air supply, in order to provide easy inflation of the device. This can usually also be done manually via a mouthpiece, in order to save tank air while on the surface. The bladders inside the BCD inflate with air from the ‘direct feed’ to increase the volume of the SCUBA equipment and cause the diver to float. Another button deflates the BCD and decreases the volume of the equipment and causes the diver to sink. Certain BCD's allow for integrated weight, meaning that the BCD has special pockets for the weights that can be dumped easily in case of an emergency. The aim of using the BCD, whilst underwater, is to keep the diver neutrally buoyant, i.e. neither floating up or sinking. The BCD is used to compensate for the compression of a wet suit, and to compensate for the decrease of the diver's mass as the air from the cylinder is breathed away.
Diving weighting systems, ranging from 2 to 15 kilograms, increase density of the scuba diver to compensate for the buoyancy of diving equipment, allowing the diver to fully submerge underwater with ease by obtaining neutral or slightly negative buoyancy. While weighting systems originally consisted of solid lead blocks attached to a belt around the diver's waist, some modern diving weighting systems are now incorporated into the BCD. These systems use small nylon bags of lead shot pellets which are distributed throughout the BCD, allowing a diver to gain a better overall weight distribution leading to a more horizontal position in the water. There are cases of lead weights being threaded on the straps holding the cylinder into the BCD.
Many modern rebreathers use advanced electronics to monitor and regulate the composition of the breathing gas.
Some scuba sets incorporate attached extra stage cylinders, as bailout in case the main breathing gas supply is used up or malfunctions, or containing another gas mixture. If these extra cylinders are small, they are sometimes called "pony cylinders". They often have their own demand regulators and mouthpieces, and if so, they are technically distinct extra scuba sets.
The diver may carry two or more sets of breathing equipment to provide redundant alternative gas systems in the event that the other fails or is exhausted. Modern recreational rigs most often have two regulators connected to a single tank, in case the primary regulator fails or another diver runs out of air. Some divers instead connect their backup regulator to a smaller "pony cylinder" for extra safety, and there are also emergency systems which mount a simple regulator directly to the top of a small cylinder. Rebreather divers often carry a side-slung open-circuit "bail out" to be used in the event the rebreather fails.
In technical diving, the diver may carry different equipment for different phases of the dive; some breathing gas mixes may only be used at depth, such as trimix and others, such as pure oxygen, which only may be used during decompression stops in shallow water. The heaviest cylinders are generally carried on the back supported from a backplate while others are side slung from strong points on the backplate.
When the diver carries many diving cylinders, especially those made of steel, lack of buoyancy becomes a problem. High capacity buoyancy compensators are used to allow the diver to control his or her depth.
An excess of tubes and connections passing through the water tend to decrease diving performance by causing hydrodynamic drag in swimming.
Some diver training organizations and groups of divers teach techniques, such as DIR diving for configuring diving equipment.
[edit] Open circuit trivia
Normalair is a firm that is now part of the Honeywell Corporation based in Yeovil (UK). They made an early make of single-hose aqualung that had a fullface mask as standard. Normalair provided the Deep-Dive 55 rebreather sets used by James Bond 007 in the 1981 film For Your Eyes Only.
Captain Trevor Hampton in the 1950s or 1960s designed an early single-hose aqualung with a fullface mask with a circular window which was a very big and thus very sensitive demand regulator diaphragm. But when he patented it, the Navy requisitioned the patent, and by the time the Navy found no use in the patent and released it, the market had moved on and he got no use from the patent.
