Scuba diving 1

Scuba diving

Scuba diver

Scuba diving is a form of underwaterdiving in which a diver uses a scuba set tobreathe underwater.[1]

Unlike earlier diving, which relied either onbreath-hold or on air pumped from thesurface, scuba divers carry their own sourceof breathing gas, (usually compressed air),[2]

allowing them greater freedom of movementthan with an air line. Both surface suppliedand scuba diving allow divers to stayunderwater significantly longer than withbreath-holding techniques as used infree-diving.

A scuba diver usually moves aroundunderwater by using swimfins attached tothe feet, but external propulsion can beprovided by a diver propulsion vehicle, or asled pulled from the surface.

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History

Original Aqualung scuba set.1: Air Hose, 2: Mouthpiece, 3: Regulator,

4: Harness, 5: Back plate, 6: Tank

The first commercially successful scuba sets were the Aqualung twinhose open-circuit units developed by Emile Gagnan and Jacques-YvesCousteau, in which compressed air carried in back mounted cylindersis inhaled through a demand regulator and then exhaled into the wateradjacent to the tank[3]. The single hose two stage scuba regulators oftoday trace their origins to Australia, where Ted Eldred developed thefirst example of this typeof regulator, known as the Porpoise, whichwas developed because patents protected the Aqualung's twin hosedesign. The single hose regulator separates the cylinder from thedemand valve, giving the diver air at the pressure at his mouth, not thatat the top of the cylinder.

The open circuit compressed air systems were developed afterCousteau had a number of incidents of oxygen toxicity using anoxygen rebreather, in which exhaled oxygen is passed through anabsorbent chemical to remove carbon dioxide before being breathedagain. Modern versions of rebreather systems (both semi-closed circuitand closed circuit) are available, and form the second main type ofscuba unit, mostly used for technical and military diving.

Etymology

The term "SCUBA" (an acronym for self-contained underwaterbreathing apparatus) originally referred to United States combat frogmen's oxygen rebreathers, developed duringWorld War II by Christian J. Lambertsen for underwater warfare.[2][4][5]

"SCUBA" was originally an acronym, but is now generally used as a common noun or adjective, "scuba".[6] It hasbecome acceptable to refer to "scuba equipment" or "scuba apparatus"—examples of the linguistic RAS syndrome.

Diving activities associated with scubaScuba diving may be performed for a number of reasons, both personal and professional. Recreational diving isperformed purely for enjoyment and has a number of distinct technical disciplines to increase interest underwater,such as cave diving, wreck diving, ice diving and deep diving.Divers may be employed professionally to perform tasks underwater. Some of these tasks are suitable for scuba.There are a fair number of divers who work, full or part time, in the recreational diving community as instructors,assistant instructors, divemasters and dive guides. In some jurisdictions the professional nature, with particularreference to responsibility for health and safety of the clients, of recreational diver instruction, dive leadership forreward and dive guiding is recognised by national legislation.[7][8]

Other specialist areas of diving include military diving, with a long history of military frogmen in various roles.They can perform roles including direct combat, infiltration behind enemy lines, placing mines or using a mannedtorpedo, bomb disposal or engineering operations. In civilian operations, many police forces operate police divingteams to perform search and recovery or search and rescue operations and to assist with the detection of crime whichmay involve bodies of water. In some cases diver rescue teams may also be part of a fire department, paramedicalservice or lifeguard unit, and may be classed as public service diving.Lastly, there are professional divers involved with the water itself, such as underwater photography or underwater filming divers, who set out to document the underwater world, or scientific diving, including marine biology,

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geology, hydrology, oceanography and underwater archaeology.The choice between scuba and surface supplied diving equipment is based on both legal and logistical constraints.Where the diver requires mobility and a large range of movement, scuba is usually the choice if safety and legalconstraints allow. Higher risk work, particularly commercial diving, may be restricted to surface supplied equipmentby legislation and codes of practice.Diving activities commonly associated with scuba may include:

Type of diving activity Classification

aquarium maintenance in large public aquariums commercial, scientific

boat and ship inspection, cleaning and maintenance commercial, naval

cave diving technical, recreational,scientific

diver training professional

fish farm maintenance (aquaculture) commercial

fishing, e.g. for abalones, crabs, lobsters, scallops, sea crayfish, commercial

frogman, manned torpedo military

media diving: making television programs, etc. professional

mine clearance and bomb disposal, disposing of unexploded ordnance military, naval

pleasure, leisure, sport recreational

policing/security: diving to investigate or arrest unauthorized divers police diving, military, naval

search and recovery diving public safety, police diving

search and rescue diving police, naval, public service

spear fishing recreational

stealthy infiltration military

surveys and mapping scientific, recreational

scientific diving (marine biology, oceanography, hydrology, geology, palaeontology, diving physiology andmedicine)

scientific

underwater archaeology (shipwrecks; harbors, and buildings) scientific, recreational

underwater inspections and surveys (occasionally) commercial, military

underwater photography professional, recreational

underwater tour guiding professional, recreational

underwater tourism recreational

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Breathing underwater

Scuba diver on reef

Water normally contains the dissolved oxygen from which fish andother aquatic animals extract all their required oxygen as the waterflows past their gills. Humans lack gills and do not otherwise have thecapacity to breathe underwater unaided by external devices.[2]

Although the feasibility of filling and artificially ventilating the lungswith a dedicated liquid (liquid breathing) has been established for sometime,[9] the size and complexity of the equipment allows only formedical applications with current technology.[10]

Early diving experimenters quickly discovered it is not enough simplyto supply air to breathe comfortably underwater. As one descends, inaddition to the normal atmospheric pressure, water exerts increasingpressure on the chest and lungs—approximately 1 bar (14.7 pounds per square inch) for every 33 feet (10 m) ofdepth—so the pressure of the inhaled breath must almost exactly counter the surrounding or ambient pressure toinflate the lungs. It becomes virtually impossible to breathe unpressurised air through a tube below three feet underthe water.[2]

By always providing the appropriate breathing gas at ambient pressure, modern demand valve regulators ensure thediver can inhale and exhale naturally and without excessive effort, regardless of depth.Because the diver's nose and eyes are covered by a diving mask; the diver cannot breathe in through the nose, exceptwhen wearing a full face diving mask. However, inhaling from a regulator's mouthpiece becomes second nature veryquickly.

Open-circuit regulator

Aqualung Legacy regulator

Gekko dive computer with attached pressuregauge and compass

The most commonly used scuba set today is the "single-hose" opencircuit 2-stage diving regulator, connected to a single high pressure gascylinder, with the first stage connected to the cylinder valve and thesecond stage at the mouthpiece.[1] This arrangement differs from EmileGagnan's and Jacques Cousteau's original 1942 "twin-hose" design,known as the Aqua-lung, in which the cylinder pressure was reduced toambient pressure in one or two stages which were all in the housingmounted to the cylinder valve or manifols. The "single-hose" systemhas significant advantages over the original system for mostapplications.

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Aqualung 1st stage

Suunto pressure gauge close up

In the "single-hose" two-stage design, the first stage regulator reducesthe cylinder pressure of up to about 240 bar (3000 psi) to anintermediate level of about 10 bar (145 psi) above ambient pressure.The second stage demand valve regulator, supplied by a low pressurehose from the first stage, delivers the breathing gas at ambient pressureto the diver's mouth. The exhaled gases are exhausted directly to theenvironment as waste. The first stage typically has at least one outletport delivering breathing gas at unreduced tank pressure. This isconnected to the diver's submersible pressure gauge or dive computer,to show how much breathing gas remains in the cylinder.

Rebreather

An Inspiration electronic fully closed circuitrebreather

Less common are closed circuit (CCR) and semi-closed (SCR)rebreathers,[11] which unlike open-circuit sets that vent off all exhaledgases, process each exhaled breath for re-use by removing the carbondioxide and replacing the oxygen used by the diver.

Rebreathers release little or no gas bubbles into the water, and usemuch less stored gas volume for an equivalent depth and time becauseexhaled oxygen is recovered; this has advantages for research,military,[1] photography, and other applications. The first modernrebreather was the MK-19 that was developed at S-Tron by RalphOsterhout and used the first electronic control system. Rebreathers aremore complex and more expensive than open-circuit scuba, and specialtraining and correct maintenance are required for them to be safelyused, due to the larger variety of potential failure modes.[11]

In a closed-circuit rebreather the oxygen partial pressure in therebreather is controlled, so it can be increased to a safe continuousmaximum, which reduces the inert gas (nitrogen and/or helium) partialpressure in the breathing loop. Minimising the inert gas loading of thediver's tissues for a given dive profile reduces the decompressionobligation. This requires continuous monitoring of actual partialpressures with time and for maximum effectiveness requires real-timecomputer processing by the diver's decompression computer. Decompression can be much reduced compared tofixed ratio gas mixes used in other scuba systems and, as a result, divers can stay down longer or decompress faster.A semi-closed circuit rebreather injects a constant flow of a fixed nitrox mixture into the breathing loop, or changesa fixed percentage of the respired volume, so the partial pressure of oxygen at any time during the dive depends onthe diver's oxygen consumption or breathing rate. Planning decompression requirements requires a more

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conservative approach for a SCR than for a CCR, but decompression computers with a real time oxygen partialpressure input can optimise decompression for these systems.Because rebreathers produce very few bubbles, they do not disturb marine life or make a diver’s presence known atthe surface; this is useful for underwater photography, and for covert work.

Gas mixtures

A cylinder decal to indicate that the contents are aNitrox mixture

Nitrox cylinder marked up for use showingmaximum safe operating depth (MOD)

For some diving, gas mixtures other than normal atmospheric air (21%oxygen, 78% nitrogen, 1% trace gases) can be used,[1][2] so long as thediver is properly trained in their use. The most commonly used mixtureis Nitrox, also referred to as Enriched Air Nitrox (EAN), which is airwith extra oxygen, often with 32% or 36% oxygen, and thus lessnitrogen, reducing the likelihood of decompression sickness orallowing longer exposure to the same pressure for equal risk. Thereduced nitrogen may also allow for no stops or shorter decompressionstop times and a shorter surface interval between dives. A commonmisconception is that nitrox can reduce narcosis, but research hasshown that oxygen is also narcotic.[12][13]

Several other common gas mixtures are in use, and all need specializedtraining for safe use. The increased oxygen levels in nitrox help reducethe risk of decompression sickness; however, below the maximumoperating depth of the mixture, the increased partial pressure of oxygencan lead to an unacceptable risk of oxygen toxicity. To displacenitrogen without the increased oxygen concentration, other diluents canbe used, usually helium, when the resultant three gas mixture is calledtrimix, and when the nitrogen is fully substituted by helium, heliox.

For technical dives, some of the cylinders may contain different gasmixtures for the various phases of the dive, typically designated asTravel, Bottom, and Decompression gases. These different gas

mixtures may be used to extend bottom time, reduce inert gas narcotic effects, and reduce decompression times.

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Diver mobilityThe diver needs to be mobile underwater. Streamlining dive gear will reduce drag and improve mobility. Personalmobility is enhanced by swimfins and Diver Propulsion Vehicles.

Controlling buoyancy underwater

Diver under the Salt Pier in Bonaire.

To dive safely, divers must control their rate of descent and ascent inthe water.[2] Ignoring other forces such as water currents andswimming, the diver's overall buoyancy determines whether he ascendsor descends. Equipment such as diving weighting systems, diving suits(wet, dry or semi-dry suits are used depending on the watertemperature) and buoyancy compensators can be used to adjust theoverall buoyancy.[1] When divers want to remain at constant depth,they try to achieve neutral buoyancy. This minimizes gas consumptioncaused by swimming to maintain depth.

The buoyancy force on the diver is the weight of the volume of theliquid that he and his equipment displace minus the weight of the diverand his equipment; if the result is positive, that force is upwards. Thebuoyancy of any object immersed in water is also affected by thedensity of the water. The density of fresh water is about 3% less thanthat of ocean water.[14] Therefore, divers who are neutrally buoyant atone dive destination (e.g. a fresh water lake) will predictably bepositively or negatively buoyant when using the same equipment atdestinations with different water densities (e.g. a tropical coral reef).

The removal ("ditching" or "shedding") of diver weighting systems can be used to reduce the diver's weight andcause a buoyant ascent in an emergency.Diving suits made of compressible materials decrease in volume as the diver descends, and expand again as the diverascends, causing buoyancy changes. Diving in different environments also necessitates adjustments in the amount ofweight carried to achieve neutral buoyancy. The diver can inject air into dry suits to counteract the compressioneffect and squeeze. Buoyancy compensators allow easy and fine adjustments in the diver's overall volume andtherefore buoyancy. For open circuit divers, changes in the diver's average lung volume during a breathing cycle canbe used to make fine adjustments of buoyancy.Neutral buoyancy in a diver is a metastable state. It is changed by small differences in ambient pressure caused by achange in depth, and the change has a positive feedback effect. A small descent will increase the pressure, which willcompress the gas filled spaces and reduce the total volume of diver and equipment. This will further reduce thebuoyancy, and unless counteracted, will result in sinking more rapidly. The equivalent effect applies to a smallascent, which will trigger an increased buoyancy and will result in accelerated ascent unless counteracted. The divermust continuously adjust buoyancy or depth in order to remain neutral. This is a skill which improves with practiceuntil it becomes second nature.

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Underwater vision

A diver wearing an Ocean Reef full face mask

Water has a higher refractive index than air – similar to that of thecornea of the eye. Light entering the cornea from water is hardlyrefracted at all, leaving only the eye's crystalline lens to focus light.This leads to very severe hypermetropia. People with severe myopia,therefore, can see better underwater without a mask thannormal-sighted people.

Diving masks and hemets solve this problem by providing an air spacein front of the diver's eyes.[1] The refraction error created by the wateris mostly corrected as the light travels from water to air through a flatlens, except that objects appear approximately 34% bigger and 25%closer in salt water than they actually are. Therefore total field-of-viewis significantly reduced and eye–hand coordination must be adjusted.

(This also affects underwater photography: a camera seeing through a flat port in its housing is affected in the sameway as its user's eye seeing through a flat mask viewport, and so its operator must focus for the apparent distance totarget, not for the real distance.)Divers who need corrective lenses to see clearly outside the water would normally need the same prescription whilewearing a mask. Generic and custom corrective lenses are available for some two-window masks. Custom lenses canbe bonded onto masks that have a single front window or two windows.A "double-dome-ported mask" has curved viewports in an attempt to cure these faults, but this causes a refractionproblem of its own.Commando frogmen concerned about revealing their position when light reflects from the glass surface of theirdiving masks may instead use special contact lenses to see underwater.As a diver descends, he must periodically exhale through his nose to equalize the internal pressure of the mask withthat of the surrounding water. Swimming goggles are not suitable for diving because they only cover the eyes andthus do not allow for equalization. Failure to equalise the pressure inside the mask may lead to a form of barotraumaknown as mask squeeze.[1][15]

Light underwaterWater preferentially absorbs red light, and to a lesser extent, yellow and green light, so the color that is leastabsorbed by water is blue light.[16]

Table of Light Absorption in pure water

Color Average wavelength Approximate depth of total absorption

Ultraviolet 300 nm 25 m

Violet 400 nm 100 m

Blue 475 nm 275 m

Green 525 nm 110 m

Yellow 575 nm 50 m

Orange 600 nm 20 m

Red 685 nm 5 m

Infra-red 800 nm 3 m

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Underwater communication

Two divers giving the sign that they are "OK" ona wreck in the Dominican Republic.

A diver cannot talk underwater unless he is wearing a full-face mask,but divers can communicate, using hand signals.

Table of Hand Signals

No. Signal Meaning Comment

1. Hand raised, fingers pointed up, palm toreceiver.

STOP Transmitted in the same way as a traffic police officer’s STOP

2. Thumb extended downward from clenched fist. GO DOWN orGOING DOWN

3. Thumb extended upward from clenched fist. GO UP or GOINGUP

4. Thumb and forefinger making a circle withthree remaining fingers extended (if possible).

OK! or OK? Divers wearing mittens may not be able to extend 3 remaining fingersdistinctly.

5. Two arms extended overhead with finger tipstouching above head to make a large O shape.

OK! or OK? A diver with only one free arm may make this signal by extendingthat arm overhead with finger tips touching top of head to make the Oshape. Signal is for long-range use.

6. Hand flat, fingers together, palm down, thumbsticking out, then hand rocking back and forthon axis of forearm.

SOMETHING ISWRONG

This is the opposite of OK! The signal does not indicate emergency.

7. Hand waving over head (may also thrash handon water).

DISTRESS Indicates immediate aid required.

8. Fist pounding on chest. LOW ON AIR Indicates signaler's air supply is reduced.

9. Hand slashing or chopping throat. OUT OF AIR Indicates that the signaler cannot breathe.

10. Clenched fist on arm extended in direction ofdanger.

DANGER

All signals are to be answered by the receivers repeating the signal as sent. When answering signals 7 & 9, thereceiver should approach to offer aid to signaler.[17]

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Hazards of scuba divingAccording to a 1970 North American study, diving was (on a man-hours based criteria) 96 times more dangerousthan driving an automobile.[18] According to a 2000 Japanese study, every hour of recreational diving is 36 to 62times riskier than automobile driving.[19] A big difference between the risks of driving and diving is that the diver isless at risk from fellow divers than the driver is from other drivers.

Injuries due to changes in pressureDivers must avoid injuries caused by changes in pressure. The weight of the water column above the diver causes anincrease in pressure in proportion to depth, in the same way that the weight of the column of atmospheric air abovethe surface causes a pressure of 101.3 kPa (14.7 pounds-force per square inch) at sea level. This variation of pressurewith depth will cause compressible materials and gas filled spaces to tend to change volume, which can cause thesurrounding material or tissues to be stressed, with the risk of injury if the stress gets too high. Pressure injuries arecalled barotrauma[2] and can be quite painful, even potentially fatal - in severe cases causing a ruptured lung,eardrum or damage to the sinuses. To avoid barotrauma, the diver equalizes the pressure in all air spaces with thesurrounding water pressure when changing depth. The middle ear and sinus are equalized using one or more ofseveral techniques, which is referred to as clearing the ears.The scuba mask (half-mask) is equalized during descent by periodically exhaling through the nose. During ascent itwill automatically equalise by leaking excess air round the edges. A helmet or full face mask will automaticallyequalise as any pressure differential will either vent through the exhaust valve or open the demand valve and releaseair into the low pressure space.If a drysuit is worn, it must be equalized by inflation and deflation, much like a buoyancy compensator. Most drysuits are fitted with an auto-dump valve, which, if set correctly, and kept at the high point of the diver by good trimskills, will automatically release gas as it expands and retain a virtually constant volume during ascent. Duringdescent the dry suit must be inflated manually.Although there are many dangers involved in scuba diving, divers can decrease the risks through proper proceduresand appropriate equipment. The requisite skills are acquired by training and education, and honed by practice.Open-water certification programs highlight diving physiology, safe diving practices, and diving hazards, but do notprovide the diver with sufficient practice to become truly adept.

Effects of breathing high pressure gas

Decompression sickness

The prolonged exposure to breathing gases at high partial pressure will result in increased amounts of non-metabolicgases, usually nitrogen and/or helium, (referred to in this context as inert gases) dissolving in the bloodstream as itpasses through the alveolar capillaries, and thence carried to the other tissues of the body, where they willaccumulate until saturated. This saturation process has very little immediate effect on the diver. However when thepressure is reduced during ascent, the amount of dissolved inert gas that can be held in stable solution in the tissuesis reduced. This effect is described by Henry's Law.As a consequence of the reducing partial pressure of inert gases in the lungs during ascent, the dissolved gas will bediffused back from the bloodstream to the gas in the lungs and exhaled. The reduced gas concentration in the bloodhas a similar effect when it passes through tissues carrying a higher concentration, and that gas will diffuse back intothe bloodsteam, reducing the loading of the tissues.As long as this process is gradual, all will go well and the diver will reduce the gas loading by diffusion andperfusion until it eventually re-stabilises at the current saturation pressure. The problem arises when the pressure isreduced more quickly than the gas can be removed by this mechanism, and the level of supersaturation risessufficiently to become unstable. At this point, bubbles may form and grow in the tissues, and may cause damage

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either by distending the tissue locally, or blocking small blood vessels, shutting off blood supply to the downstreamside, and resulting in hypoxia of those tissues.This effect is called decompression sickness[2] or 'the bends', and must be avoided by reducing the pressure on thebody slowly while ascending and allowing the inert gases dissolved in the tissues to be eliminated while still insolution. This process is known as "off-gassing", and is done by restricting the ascent (decompression) rate to onewhere the level of supersaturation is not sufficient for bubbles to form. This is done by controlling the speed ofascent and making periodic stops to allow gases to be eliminated. The procedure of making stops is called stageddecompression, and the stops are called decompression stops. Decompression stops that are not computed as strictlynecessary are called safety stops, and reduce the risk of bubble formation further. Dive computers or decompressiontables are used to determine a relatively safe ascent profile, but are not completely reliable. There remains astatistical possibility of decompression bubbles forming even when the guidance from tables or computer has beenfollowed exactly.Decompression sickness must be treated as soon as practicable. Definitive treatment is usually recompression in arecompression chamber with hyperbaric oxygen treatment. Exact details will depend on severity and type ofsymptoms, response to treatment, and the dive history of the casualty. Administering enriched-oxygen breathing gasor pure oxygen to a decompression sickness stricken diver on the surface is a good form of first aid fordecompression sickness, although death or permanent disability may still occur.[20]

Nitrogen narcosis

Nitrogen narcosis or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcoholintoxication in divers who breathe high pressure gas at depth.[2] The mechanism is similar to that of nitrous oxide, or"laughing gas," administered as anesthesia. Being "narced" can impair judgment and make diving very dangerous.Narcosis starts to affect some divers at 66 feet (20 m). At this depth, narcosis manifests itself as a slight giddiness.The effects increase drastically with the increase in depth. Almost all divers are able to notice the effects by 132 feet(40 meters). At these depths divers may feel euphoria, anxiety, loss of coordination and lack of concentration. Atextreme depths, hallucinogenic reaction and tunnel vision can occur. Jacques Cousteau famously described it as the"rapture of the deep".[3] Nitrogen narcosis occurs quickly and the symptoms typically disappear during the ascent, sothat divers often fail to realize they were ever affected. It affects individual divers at varying depths and conditions,and can even vary from dive to dive under identical conditions. However, diving with trimix or heliox dramaticallyreduces the effects of inert gas narcosis.

Oxygen toxicity

Oxygen toxicity occurs when oxygen in the body exceeds a safe partial pressure (PPO2).[2] In extreme cases it affectsthe central nervous system and causes a seizure, which can result in the diver spitting out his regulator and drowning.While the exact limit is idiomatic, it is generally recognized that Oxygen toxicity is preventable if one never exceedsan oxygen partial pressure of 1.4 bar.[21] For deep dives—generally past 180 feet (55 m), divers use "hypoxicblends" containing a lower percentage of oxygen than atmospheric air. For more information, see oxygen toxicity.

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Hazards of the diving environment

Loss of body heat

Dry suit for reducing exposure

Water conducts heat from the diver 25 times[22] better than air, whichcan lead to hypothermia even in mild water temperatures.[2] Symptomsof hypothermia include impaired judgment and dexterity,[23] which canquickly become deadly in an aquatic environment. In all but thewarmest waters, divers need the thermal insulation provided bywetsuits or drysuits.[1]

In the case of a wetsuit, the suit is designed to minimize heat loss.Wetsuits are usually made of neoprene that has small closed gas cells,generally nitrogen, trapped in it during the manufacturing process. Thepoor thermal conductivity of this expanded cell neoprene means that

wetsuits reduce loss of body heat by conduction to the surrounding water. The neoprene, and to a larger extent thenitrogen gas, in this case acts as an insulator. The effectiveness of the insulation is reduced when the suit iscompressed due to depth, as the nitrogen filled bubbles are then smaller and conduct heat better.

The second way in which wetsuits reduce heat loss is to trap a thin layer of water between the diver's skin and theinsulating suit itself. Body heat then heats the trapped water. Provided the wetsuit is reasonably well-sealed at allopenings (neck, wrists, ankles zippers and overlaps with other suit components), this reduces flow of cold water overthe surface of the skin, and thereby reduces loss of body heat by convection, which helps keep the diver warm (this isthe principle employed in the use of a "Semi-Dry" wetsuit)

Spring suit (short legs and sleeves) and steamer(full legs and sleeves)

In the case of a drysuit, it does exactly what the name implies: keeps adiver dry. The suit is waterproof and sealed so that frigid water cannotpenetrate the suit. Drysuit undergarments are usually worn under adrysuit to keep a layer of air inside the suit for better thermalinsulation. Some divers carry an extra gas bottle dedicated to filling thedry suit. Usually this bottle contains argon gas, because of its betterinsulation as compared with air.[24] Dry suits should not be inflatedwith gases containing helium as it is a good thermal conductor.

Drysuits fall into two main categories: neoprene and membrane; bothsystems have their good and bad points but generally their thermalproperties can be reduced to:• Membrane or Shell drysuits: usually a trilaminate construction;

owing to the thinness of the material (around 1 mm), these requirean undersuit, usually of high insulation value if diving in coolerwater.

• Neoprene drysuits: a similar construction to wetsuits; these are oftenconsiderably thicker (7–8 mm) and have sufficient insulation toallow a lighter-weight undersuit (or none at all); however on deeperdives the neoprene can compress to as little as 2 mm thus losing aproportion of its insulation. Compressed or crushed neoprene may also be used (where the neoprene ispre-compressed to 2–3 mm) which avoids the variation of insulating properties with depth. These drysuitsfunction more like a membrane suit.

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Injuries due to contact with the solid surroundings

Diving suits also help prevent the diver's skin being damaged by rough or sharp underwater objects, marine animals,coral, or metal debris commonly found on shipwrecks.

Hazards inherent in the diver

Diver behaviour and competence

Inadequate learning or practice of critical safety skills may result in the inability to deal with minor incidents, whichconsequently may develop into major incidents.Overconfidence can result in diving in conditions beyond the diver's competence, with high risk of accident due toinability to deal with known environmental hazards.Inadequate strength or fitness for the conditions can result in inability to compensate for difficult conditions eventhough the diver may be well versed at the required skills, and could lead to over-exertion, overtiredness, stressinjuries or exhaustion.Peer pressure can cause a diver to dive in conditions where he may be unable to deal with reasonably predictableincidents.Diving with an incompetent buddy can result in injury or death while attempting to deal with a problem caused bythe buddy.Overweighting can cause difficulty in neutralising and controlling buoyancy, and this can lead to uncontrolleddescent, inability to establish neutral buoyancy, inefficient swimming, high gas consumption, poor trim, kicking upsilt, difficulty in ascent and inability to control depth accurately for decompression.Underweighting can cause difficulty in neutralising and controlling buoyancy, and consequent inability to achieveneutral buoyancy, particularly at decompression stops.Diving under the influence of drugs or alcohol, or with a hangover may result in inappropriate or delayed response tocontingencies, reduced ability to deal timeously with problems, leading to greater risk of developing into an accident,increased risk of hypothermia and increased risk of decompression sickness.Use of inappropriate equipment and/or configuration can lead to a whole range of complications, depending on thedetails.

Diving longer and deeper safelyThere are a number of techniques to increase the diver's ability to dive deeper and longer:• Technical diving – diving deeper than 40 metres (130 ft), using mixed gases, and/or entering overhead

environments (caves or wrecks)• Surface supplied diving – use of umbilical gas supply and diving helmets.[1]

• Saturation diving – long-term use of underwater habitats under pressure and a gradual release of pressure overseveral days in a decompression chamber at the end of a dive.[1]

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Scuba diver training and certification agencies

Diving lessons in Monterey Bay, California

Recreational scuba diving does not have a centralized certifying orregulatory agency, and is mostly self regulated. There are, however,several large diving organizations that train and certify divers and diveinstructors, and many diving related sales and rental outlets requireproof of diver certification from one of these organizations prior toselling or renting certain diving products or services.The largest international certification agencies that are currentlyrecognized by most diving outlets for diver certification include:• American Canadian Underwater Certifications (ACUC) [25]

(formerly Association of Canadian Underwater Councils) –originated in Canada in 1969 and expanded internationally in 1984

• British Sub Aqua Club (BSAC) – based in the United Kingdom, founded in 1953 and is the largest dive club inthe world

• European Committee of Professional Diving Instructors (CEDIP) [26] based in Europe since 1992 (see Cedip onFrench Wiki pages)

• Confédération Mondiale des Activités Subaquatiques (CMAS), the World Underwater Federation• National Association of Underwater Instructors (NAUI) – based in the United States• Professional Diving Instructors Corporation (PDIC) – based in the United States• Professional Association of Diving Instructors (PADI) – based in the United States, largest recreational dive

training and certification organization in the world• Scottish Sub Aqua Club (SSAC or ScotSAC) [27] the National Governing Body for the sport of diving in

Scotland.• International Training SDI, TDI & ERDi [28] - based in the United States, TDI is the world's largest technical

diving agency, SDI is the recreational division focusing on new methods and online courses, and ERDi is thepublic safety component.

• Scuba Schools International (SSI) – based in the United States with 35 Regional Centers and Area Offices aroundthe globe.

• YMCA Scuba – based in the United States, provided by Young Men's Christian Association (YMCA);discontinued after 2008.

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References

Scuba diving, grouped

[1] US Navy Diving Manual, 6th revision (http:/ / www. supsalv. org/00c3_publications. asp?destPage=00c3& pageID=3. 9). United States: USNaval Sea Systems Command. 2006. . Retrieved April 24, 2008.

[2] Brubakk, Alf O; Neuman, Tom S (2003). Bennett and Elliott's physiology andmedicine of diving, 5th Rev ed. United States: Saunders Ltd. p. 800.ISBN 0-7020-2571-2.

[3] Cousteau J.Y. (1953) Le Monde du Silence, translated as The Silent World,Hamish Hamilton Ltd., London; ASIN B000QRK890

[4] Vann RD (2004). "Lambertsen and O2: beginnings of operational physiology"(http:/ / archive. rubicon-foundation. org/ 3987). Undersea Hyperb Med 31 (1):21–31. PMID 15233157. . Retrieved April 25, 2008.

[5] Butler FK (2004). "Closed-circuit oxygen diving in the U.S. Navy" (http:/ /archive. rubicon-foundation. org/ 3986). Undersea Hyperb Med 31 (1): 3–20.PMID 15233156. . Retrieved April 25, 2008.

[6] "Compact Oxford English Dictionary - scuba" (http:/ / www. askoxford. com/concise_oed/ scuba?view=uk). Oxford University Press. .

[7] HSE press release E061:05 - 5 May 2005 HSE issues warning overrecreational dive training http:/ / www. hse. gov. uk/ press/ 2005/ e05061. htm

[8] Statutory Instruments 1997 No. 2776 HEALTH AND SAFETY, The Diving atWork Regulations 1997, http:/ / www. legislation. gov. uk/ uksi/ 1997/ 2776/contents/ made

[9] Hirschl, RB; et al (1995). "Liquid ventilatory in adults, children, and full-termneonates". Lancet 346 (8984): 1201–1202.doi:10.1016/S0140-6736(95)92903-7. PMID 7475663.

[10] Sekins, KM; et al (1999). "Recent innovation in total liquid ventilation system and component design". Biomedical instrumentation andtechnology 33 (3): 277–284. PMID 10360218.

[11] Richardson, D; Menduno, M; Shreeves, K. (eds). (1996). "Proceedings of Rebreather Forum 2.0." (http:/ / archive. rubicon-foundation. org/7555). Diving Science and Technology Workshop.: 286. . Retrieved August 20, 2008.

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[13] Brubakk, Alf O; Neuman, Tom S (2003). Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. United States: Saunders Ltd.p. 304. ISBN 0-7020-2571-2.

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[15] NOAA Diving Manual, 4th Edition, Best Publishing, 2001[16] Hegde, M (30 September 2009). "The Blue, the Bluer, and the Bluest Ocean" (http:/ / disc. sci. gsfc. nasa. gov/ oceancolor/ additional/

science-focus/ ocean-color/ oceanblue. shtml). NASA Goddard Earth Sciences Data and Information Services. . Retrieved 27 May 2011.[17][17] Miller, James W. NOAA Diving Manual Diving for Science and Technology. 2nd ed. Washington, D.C.: U.S. Government Printing Office,

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pageindex=1#page)[19] Is recreational diving safe?, por Ikeda, T y Ashida, H (http:/ / archive. rubicon-foundation. org/ 6770)[20] Longphre, J. M.; P. J. DeNoble; R. E. Moon; R. D. Vann; J. J. Freiberger (2007). "First aid normobaric oxygen for the treatment of

recreational diving injuries" (http:/ / archive. rubicon-foundation. org/ 5514). Undersea Hyperb Med. 34 (1): 43–49. ISSN 1066-2936.OCLC 26915585. PMID 17393938. . Retrieved May 3, 2008.

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[22] "Thermal Conductivity" (http:/ / hyperphysics. phy-astr. gsu. edu/ hbase/ tables/ thrcn. html), Georgia State University, accessed February15, 2008

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[24] Nuckols ML, Giblo J, Wood-Putnam JL. (September 15–18, 2008). "Thermal Characteristics of Diving Garments When Using Argon as aSuit Inflation Gas." (http:/ / archive. rubicon-foundation. org/ 7962). Proceedings of the Oceans 08 MTS/IEEE Quebec, Canada Meeting(MTS/IEEE). . Retrieved April 17, 2009.

[25] http:/ / www. acuc. org/

Scuba diving 16

[26] http:/ / www. cedip. org/[27] http:/ / www. scotsac. com/[28] http:/ / www. tdisdi. com

Further reading• Cousteau J.Y. (1953) Le Monde du Silence, translated as The Silent World, Hamish Hamilton Ltd., London; ASIN

B000QRK890• Ellerby D. (2002) The Diving Manual, British Sub-Aqua Club (BSAC); ISBN 0-9538919-2-5• Dive Leading, BSAC; ISBN 0-9538919-4-1• The Club 1953-2003, BSAC; ISBN 0-9538919-5-X• Richardson D. (2008) Open Water Diver Manual, PADI; ASIN B004JZYO0E• Free Scuba textbook by George D. Campbell, III called Diving With Deep-Six (http:/ / www. deep-six. com/

page50. htm)

External links• Divers Alert Network (http:/ / www. diversalertnetwork. org/ )—Diving Emergencies/Hyperbaric Chamber

Assistance• Scuba diving travel guide from Wikitravel

Article Sources and Contributors 17

Article Sources and ContributorsScuba diving  Source: http://en.wikipedia.org/w/index.php?oldid=508978724  Contributors: *drew, -Ozone-, 200footdrop, 2help, 2ocean7, ABF, AbsolutDan, AdamDavid, Akamad, Akrabbim,Alabasterclam, Alansohn, Alexa22, Alexander Phipps, Alvin-cs, Ambrish.shrivastava, Andy Marchbanks, Anthony Appleyard, Ardo191, Art LaPella, Artinge, Aspro, Avjoska, Babcocgf,BaroloLover, Basilicofresco, Bemoeial, Bencherlite, Benweatherhead, Bhadani, Biart studio, Biff-calliafas, Blarrrgy, BlinkingBlimey, Bobbyboybob, Boing! said Zebedee, CStyle, Caese,Cakunk123, Calwolf, CambridgeBayWeather, Canadian-Bacon, Capricorn42, Charlesjsharp, Chris.n, Chunshek, Civertan, Clayoquot, Clueless, Cmeide, Cmichael, Collins18, CommanderShepard, Corti, Crum375, Csrempert, DJ Clayworth, Daddy.twins, Danski14, Darthgriz98, Date delinker, Dave3141592, Dbenbenn, Ddejager, DeathSource, DennyColt, Deputy821, DerHexer,Dfrg.msc, Diliff, Dina, Diver71, Divingdoc36, Donald Albury, DoubleBlue, DougsTech, Doyley, Dr Noonien Soong, Duffman, Dukerobo, E0steven, Echuck215, Edlitz36, Elonka, Emdx,Epastore, Epbr123, Epson291, Ericorbit, Erud, Ethan2226, Everyking, Exir Kamalabadi, FF2010, Fagbag456, Finavon, Fitzsflies, Flessas, Fordan, Fram, Franamax, Freakofnurture, Fuzzball!,Gadfium, Gareth Griffith-Jones, Gawker, Gene Hobbs, Gene Nygaard, GeorgeLouis, Gettingtoit, Gjs238, Gr0ff, Graham87, GrahamBould, GregorB, GuruSteve, Hadal, Halmstad,Hamiltondaniel, Happibunny, Haukurth, HawaiiScubaDivers, Hbackman, Hektor, Henrikhoffa, Henry Flower, HeroOfTheSovietUnion, HexaChord, HeyStopThat, Hibsch, Hkremer, Hmains,Hmoul, Hotcrocodile, Huw Powell, Hydrargyrum, Imaswfan, Imnotminkus, Intelligentsium, Intothewoods29, J. Finkelstein, J.delanoy, JEJoyce, Jacek Kendysz, Jamal12491, Janemba13, Jarvik,JasonFox, Jawfish, Jax184, Jennavecia, Jhog1978, Jim.belk, Jjharvey8, Jnorton7558, Joe Bolt, Joeymoe, Jofijk10, Johnbelamaric, Johnlogic, Johnuniq, Jojhutton, Jon Kranhouse, Jon186,Jotimmsy, Jpatokal, Jtowler, Jzouksta, Jóna Þórunn, KVDP, Kablammo, Katalaveno, Kbdank71, Kevin Kidd, Khukri, Kimberlygretchen, Kingpin13, Kocio, Kraftlos, Krawi, Kubigula, Kuru,Lari-fari, Leafyplant, LeaveSleaves, Lectonar, Legis, Leonard^Bloom, Leuko, Ligand, Lightmouse, LilHelpa, Linkspamremover, LonelyBeacon, Longhair, Lotje, Lsi john, Luigi2, Luismiguens,Lukespires, Luna Santin, MIgartua, MZMcBride, Macintosh User, Maldivian, Malicart, Mangostar, Mark.murphy, Markovich292, Matthew Yeager, Maynard Hogg, Mbeatty, Melaen, Mets501,MickMacNee, Mion, Mit027*, Mochila69, Mojodaddy, Momi bear, MrFish, Ms2ger, Mushroom, Nathan Johnson, Nckiller2119, Niele, Nikolai, Noclevername, NoisyMe, Northamerica1000,Nuno Tavares, Nyttend, Obli, Ohnoitsjamie, Ohwell32, OneAhead, Otsykes, Owain.davies, PDH, Pbsouthwood, PeHa, Pennywisdom2099, Peter Horn, Petruchi41, PhilipO, Pmcg47580,Polylerus, PrestonH, Propars55, Prunk, Pumpumstealer, Pursey, Quintote, Quuxplusone, RHaworth, Raining girl, Rama, Randroide, RazorICE, Reader contributor, RexNL, RexxS, Riana, RichFarmbrough, Rincewind32, Risker, Robert P. O'Shea, Robert94704, Roberta F., Roman82, Satman, Saxman101, Scarian, SchreiberBike, SchuminWeb, Scuba diva, ScubaSteve77,Scubadiver-dad, Scubawebguy, Seaworks, Seinfreak37, Sfan00 IMG, Shaka, Shantavira, ShaunES, Shoessss, Ski1382, Skoobes, Skwa, Smallman12q, SmartGuy Old, Sneftel, Sokoban,Southfloridafusion, Sp, Spencer, Spetzna-, StaticGull, Station1, SteOsu, Storkk, Strabismus, StradivariusTV, SunDragon34, Suramik, Swpb, TDIPete, Tabletop, TeaDrinker, Tedickey, Tempshill,Tenbergen, Teoga, Terrillja, The High Fin Sperm Whale, The Original Wildbear, Thingg, Thisisjusthowweroll, Thy^Saint, Tide rolls, TigerShark, Tim@, Timothyarnold85, Tiptoety, Tlesher,Todd Vierling, Tomer yaffe, Tompot, Tony1, Trevor Wennblom, Trollderella, Truthspeaketh, Tvaughn, Ucanlookitup, Udaza, Useight, Vary, Vegetator, Velella, Vicarious, Virgil Valmont, VivHamilton, Vranak, Vsmith, VuKhoa, Wally Tharg, WarthogDemon, Wavelength, Webdinger, Weregerbil, Wheneverdone, Wiki4robert&me, Wikid77, Wildfire88, Wimt, Wkguide, Woohookitty,Wtmitchell, Www.scubamazing.com, XLerate, Xezbeth, Xose.vazquez, Yegor Chernyshev, Z10x, Zorbak5044, 627 anonymous edits

Image Sources, Licenses and ContributorsFile:Buzo.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Buzo.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: SoljaguarFile:Aqualung (PSF).png  Source: http://en.wikipedia.org/w/index.php?title=File:Aqualung_(PSF).png  License: unknown  Contributors: Editor at Large, LA2, 1 anonymous editsFile:Scuba diver1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Scuba_diver1.jpg  License: Public Domain  Contributors: Original uploader was Scott Sanchez at en.wikipediaFile:Aqualung legacy regulator.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Aqualung_legacy_regulator.jpg  License: Creative Commons Attribution-Sharealike 3.0 Contributors: User:ArtingeFile:Dive computer,compass, and Pressure gauge.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Dive_computer,compass,_and_Pressure_gauge.jpg  License: Creative CommonsAttribution-Sharealike 3.0  Contributors: User:ArtingeFile:Aqualung 1st stage.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Aqualung_1st_stage.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: User:ArtingeFile:Pressure guage close up.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pressure_guage_close_up.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:User:ArtingeFile:Plongee-RecycleurInspiration 20040221-153656.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Plongee-RecycleurInspiration_20040221-153656.jpg  License: CreativeCommons Attribution-Share Alike  Contributors: EMDXFile:EANxDecal.png  Source: http://en.wikipedia.org/w/index.php?title=File:EANxDecal.png  License: GNU Free Documentation License  Contributors: AtonX, Man vyi, Mattes, Serguei S.Dukachev, 2 anonymous editsFile:Cylinder mod.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Cylinder_mod.jpg  License: Public Domain  Contributors: RexxSFile:Diver under the Salt Pier, Bonaire, Dutch Antilles.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Diver_under_the_Salt_Pier,_Bonaire,_Dutch_Antilles.jpg  License: CreativeCommons Attribution-Sharealike 2.5  Contributors: JohnnyMrNinjaImage:Full face diving mask - ocean reef.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Full_face_diving_mask_-_ocean_reef.JPG  License: Public domain  Contributors:Mark.murphyFile:9-24-2007 diving with wellington-05.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:9-24-2007_diving_with_wellington-05.jpg  License: Creative CommonsAttribution-Sharealike 3.0  Contributors: User:ArtingeFile:Dry suit front-entry.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Dry_suit_front-entry.jpg  License: Public Domain  Contributors: RexxSImage:wetsuit0806.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Wetsuit0806.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: David CorbyImage:SCUBA diving lessons.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:SCUBA_diving_lessons.JPG  License: Public Domain  Contributors: Intothewoods29 (talk)Image:Scuba-diving.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Scuba-diving.jpg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: BLueFiSH.as, Civertan,Diwas, Fschoenm, Man vyi, Wikipeder, 3 anonymous edits

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