Scuba skills

The certified scuba diver is expected to be able to assess what type of diving exposure suit is suitable for the planned dive, and to check that it is in safe usable condition, that it is the right size, and to dress correctly in it.Scuba diving with that suit will be more comftorbale. Entry level skills usually cover wet suits, but in countries where the water and/or weather conditions are very cold, dry suit skills may be considered an entry level skill. In other parts of the world, dry suit skills are considered a specialty skill. Where dry suits are used, the skills of using the dry suit safely during a dive are also necessary. These include equalizing, buoyancy control, inversion recovery, emergency venting and blowup recovery. Recreational divers trained in warm tropical waters may have no skills in the use of diving suits.[6][7]

The open circuit scuba set is usually stored and often transported as separate major components - harness, cylinder(s) and regulator(s), and usually buoyancy compensator, and assembled shortly before use. The scuba set is life support equipment and correct assembly and function is critical to the success of the dive, and in some cases to the survival of the user. The equipment is robust and reliable, is easily tested for correct function, and assembly is simple enough for the user after basic instruction and some practice. Some service providers will assemble scuba sets for their clients, particularly if it is rental equipment, but all certification agencies require the diver to be competent to assemble their own set. Scuba assembly generally entails mounting the cylinder(s) on the harness, connecting the regulator(s) to the cylinder valves, ensuring an uncontaminated and pressure-tight seal, and connecting the low pressure hose to the buoyancy compensator inflation valve. These operations usually require no tools, or at most a wrench, used to bolt twin cylinders to a backplate. Validating the function of the regulator and inflation valve is usually considered part of scuba assembly, but may also be considered part of pre-dive checks, and if there is a significant interval between assembly and use, is commonly done twice.[6][8]

Pre-dive check before a surf entry

Pre-dive checks range from inspection and testing of personal diving equipment, to review of the dive plan, with the dive team.

Recreational divers are personally responsible for the function of their own equipment, and when diving as buddies with other divers, they are expected to ensure that they are at least familiar with the operation of any part of the buddy's equipment that they might need to operate in an emergency.

Responsibility for pre-dive checks for professional divers is more complex, based on duty of care, and is usually defined in their organisational operations manual, which may stipulate recorded checklists for the equipment in use and the participation of other members of the diving team.[9][10]

Stride entry or front step entry from a low dockside

Backward roll entry from a small boat

Scuba diver exiting water by stern platform ladder on large dive boat

Getting into and out of the water with scuba gear in a moderate range of circumstances appropriate to the certification is considered a necessary skill set for both recreational and professional divers. Divers with disabilities or otherwise physically unable to make a safe entry or exit are expected to recognize the conditions for which they need help, and to arrange for assistance, or to refrain from diving in those conditions.[6][8][11]

The default condition for water entry is with positive buoyancy, but there are situations where a negatively buoyant entry is an advantage, for example when a current is running at the surface. This skill is not listed as an entry level skill as it is generally considered a higher risk procedure, and requires greater care and more precise control of weighting and pre-dive deflation of buoyancy compensator and dry-suit, confidence in the ability to equalise the ears during rapid descent and the ability to control descent rate and achieve neutral buoyancy without delay if needed. An acceptably safe negative entry requires adequate pre-dive checks on regulator and BC inflation function, and reasonably accurate balance of BC and/or suit inflation to ballast weight excess. This becomes more complex when large amounts of breathing gas are carried, as the weighting must allow neutral buoyancy at the shallowest decompression stop when the gas is expended, and the diver is therefore relatively overweighted at the start of the dive.

Common conditions where entries and exits may be made include:

• From the poolside
• From a small boat
• From a large boat
• From a beach or rocky shoreline
• From a jetty or dockside
• Into/out of deep water
• Into/out of shallow water
• Through a surf line

Standard entry procedures which are generally taught to entry level divers may include:

• Stride entry:[4] This is the standard method of entry from a standing position at a moderately low height above reasonably deep water. The diver simply steps forward and remains upright during the short drop to the water. The fins strike the water first and reduce in-pact. Depth of immersion can be limited by performing a scissor-kick stroke immediately after striking the water, and if there is an unexpected obstacle or the water is shallower than expected, the feet will hit first. The regulator and mask are vulnerable to water impact in this entry technique, and it is generally recommended to hold them in place with one hand. If the buoyancy compensator is inflated, the buoyancy and drag will limit the depth of penetration. The diver may need to quickly clear the area below the gate so that other divers can take their turn when there is a large group, or a current running.
• Seated entry: Sometimes also known as a controlled seated entry or silent entry. This technique is suitable from a platform where the diver can sit facing the water with the legs hanging into the water, like the side of a swimming pool, a floating jetty, or a swim platform on a large boat. It requires sufficient upper body strength to support the divers weight on the arms while rotating to face the platform, then lowering oneself into the water in a controlled manner.[4] The diver sits at the waterside in full equipment with feet over the side in the water, places both hands palm down both on the deck on the same side of the body, and takes their weight on straight arms as they swing their body to face the platform before lowering themself into the water.[12]
• Backward roll:[4] The backward roll entry is used from small boats where the diver completes the preparations to dive while seated on the side of the boat with their feet on the deck and the water behind them, and is particularly suited to inflatable boats and other small, open vessels with a gunwale freeboard of about half a metre. Falling backwards into the water from this position is relatively safe and easy, and can be done simultaneously by all the divers seated along the gunwale. The backward roll is suitable when the distance to the water is short, it is safe and comfortable to sit on the side deck, tube or gunwale and the diver will not rotate more than about 120° during the roll. It eliminates the need to stand up and walk to an alternative entry point on a moving platform while encumbered by equipment, and can be done with back-mount or side-mount equipment. The heavy diving cylinder makes first contact with the water surface, and the more sensitive mask and demand valve, and in some cases camera equipment, are shielded from the initial impact by the diver's body. Some care must be taken not to fall on top of other divers already in the water. so a multiple diver backward toll entry is usually coordinated by a crew-person, who will give a count-down so all go at the same time. The more delicate and loosely fitted equipment should be held in place for security while rolling, or in the case of sling or side-mount cylinders, these should be held in place by the arms to prevent them from riding up and hitting the diver in the face.
• Forward roll:
• Ladder descent: A ladder descent is a relatively controlled and low impact entry method, but most ladders are not suitable for descending while wearing fins, so they are usually carried over an arm or clipped to the diver, and put on while in the water. Putting fins on in the water can be tricky in a seaway or current.
• Surf and beach entries
• Jump entries: Jumps from heights of 3 m or more may be taught by some agencies.[4] Relatively high jumps require the diver to strike the water upright with overlapped fins to reduce the risk of knocking them off, and to hold loose equipment in place, particularly the mask and demand valve. Demand valves should be desensitised where possible to reduce the risk of inducing a free-flow.

Standard exit procedures may include:

• From a pool side
• From a pool by ladder
• Into a small boat (over the side)
• Into a large boat (ladder)
• Onto a jetty (steps or ladder)
• Surf and beach exits

Breathing from a demand valve is the fundamental and definitive skill of scuba diving, and it must be done correctly to make effective use of a limited air supply, and to avoid drowning. Most recreational scuba diving is done with a half mask and the demand valve is held in the mouth, gripped by the teeth, and sealed by the lips. Over a long dive this can induce jaw fatigue, and for some people, a gag reflex. Various styles of mouthpiece are available off the shelf or as customised items, and one of them may work better if either of these problems occur. The air is breathed through the mouth, and the diver must be able to seal off the nasal passages from the pharynx so that breathing remains possible with a flooded or dislodged mask.[13][11][8] Breathing from scuba is mostly a straightforward matter. Under most circumstances it differs very little from normal surface breathing. In the case of a full-face mask, the diver may usually breathe through the nose or mouth as preferred.

The demand valve adds a little respiratory dead space to the airway, and there is added work of breathing due to hydrostatic pressure differences between the depth of the demand valve and the lungs, and due to cracking pressure and flow resistance in the demand valve. These factors make breathing from a demand valve more effort than normal breathing out of the water, and the additional work of breathing at depth due to increased density and viscosity of the compressed gas, make a slow deep breathing cycle more energy efficient and more effective at carbon dioxide elimination. The diver learns to breathe more slowly and deeply with practice, and this usually improves endurance on a given quantity of gas. Part of the skill is learning to relax under water, and part is to minimize effort by learning good buoyancy, trim, maneuvering and propulsion skills. The breathing rate should not be slowed down too much, or there is a risk of hypercapnia (carbon dioxide buildup).[13][11]

Scuba divers are often taught never to hold their breath underwater, as in some circumstances this can result in lung overpressure injury. In reality, this is only a risk during ascent, as that is the only time that a fixed amount of air will expand in the lungs, and even then, only if the airways are closed. A relaxed and unobstructed airway will allow expanding air to flow out freely.[13][11] Holding the breath at constant depth for short periods with a normal lung volume is generally harmless, providing there is sufficient ventilation on average to prevent carbon dioxide buildup, and is done as a standard practice by underwater photographers to avoid startling their subjects. Holding the breath during descent can eventually cause lung squeeze, and may allow the diver to miss warning signs of a gas supply malfunction until it is too late to remedy.

Skilled open circuit divers can and will make small adjustments to buoyancy by adjusting their average lung volume during the breathing cycle. This adjustment is generally in the order of a kilgram (corresponding to a litre of gas), and can be maintained for a moderate period, but it is more comfortable to adjust the volume of the buoyancy compensator over the longer term.

The practice of shallow breathing or skip breathing in an attempt to conserve breathing gas should be avoided as it tends to cause a carbon dioxide buildup, which can result in headaches and a reduced capacity to recover from a breathing gas supply emergency. The breathing apparatus will generally increase dead space by a small but significant amount, and cracking pressure and flow resistance in the demand valve will cause a net work of breathing increase, which will reduce the diver's capacity for other work. Work of breathing and the effect of dead space can be minimised by breathing relatively deeply and slowly. These effects increase with depth, as density and friction increase in proportion to the increase in pressure, with the limiting case where all the diver's available energy may be expended on simply breathing, with none left for other purposes. This would be followed by a buildup in carbon dioxide, causing an urgent feeling of a need to breathe, and if this cycle is not broken, panic and drowning are likely to follow. The use of a low density inert gas, typically helium, in the breathing mixture can reduce this problem, as well as diluting the narcotic effects of the other gases.[14]

There are several reasons why a demand valve may be removed from a diver's mouth under water, both intentionally and unintentionally. In all cases, it may fill with water and this must be removed before the diver can safely breathe from it again. This is known as clearing or purging the demand valve, and there are two ways this can be done.[13][11]

• By exhaling through the demand valve with the exhaust valve at the low point - this will displace the water by exhaled air, and the water will flow down and out through the exhaust valve. Normal exhalation will clear the demand valve with each exhalation provided that there are no parts of the internal volume below the exhaust valve.
• By blocking the mouthpiece (usually with the tongue) and pressing the purge button, which will displace the water by air from the scuba cylinder. If the exhaust valve is at the low point, the water will flow out through the valve. This method is more usual if the diver does not have sufficient breath left to clear by exhalation, as it is wasteful of breathing gas, particularly when done to excess.

It may happen that a diver becomes nauseous underwater and vomits. Clearly any vomit that is left inside the demand valve must be cleared before it is safe to breathe through it again. In this case it is usual to remove the DV from the mouth, allow it to flood with ambient water, and clear using the purge button before using it to breathe. The process may be repeated once or twice to rinse the interior as necessary. If the DV breathes wet after purging, there may be something stuck in the exhaust valve. If the DV is filled with water and cleared again with the mouthpiece blocked this will usually clear the valve.

If the demand valve is dislodged from the diver's mouth unintentionally, it may end up in a place which is not obvious to the diver, and it will be fairly urgent to get it back. At least three methods are taught for recovery of a demand valve:[6]

• The reach method, (or hose trace method), is the most reliable as it will work in all cases where the DV is not snagged somewhere. The diver reaches back over the right shoulder to the low pressure hose feeding the DV, and loops thumb and fingers round the hose, then slides the hand along the hose, pulling it forward and over the shoulder until the DV is in the right hand, at which stage it can be turned the right way round and replaced in the mouth.
• The sweep method is quick and works in most cases, as the DV usually just drops down on the diver's right side, while still draped over the right shoulder. In this method the diver is usually face down to upright, and sweeps the right hand across the waist from left to right, around to the back, in contact with the body or scuba set, and round as far back as possible against the cylinder, the straightens the arm backwards and swings it around outwards and forwards in an arc until it is pointing forwards. This will normally capture the hose to the front of the arm, where the left hand can find it by sweeping along the arm from the right hand to the neck. This method will fail if the DV has been swept over to the left side of the cylinder.
• The third method is the inversion method, which works best when the DV has been flipped over to the left side of the cylinder behind the diver's back. The diver simply rolls forward into a head down position with the body near vertical, and relies on gravity to bring the DV down to where it can be reached.

If the diver has difficulty locating the demand valve by these methods, the octopus DV or bailout set can be used in the interim. Occasionally the DV will get snagged in such a way that cannot be easily recovered. In some cases it may be prudent to abort the dive and surface, but sometimes this is not practicable and it may be necessary to remove the harness partially or completely to recover the primary, after which the harness can be readjusted. A dive buddy can usually fix this kind of problem quite easily. A primary DV should not be left in an inaccessible position in case it develops a free-flow out of the diver's reach. If it cannot be reached it is prudent to terminate the dive.

Practicing mask clearing during entry level training

It is quite common for water to leak into the mask, which can be annoying, or interfere with clear vision, and the diver needs to be able to get rid of the water quickly and effectively. Reasons for the leakage include poor fit or fitting, leaking via head or facial hair, movement of the facial muscles causing temporary leaks, or impact of external objects against the mask, which may distort it temporarily, or move it so that it leaks, or in extreme cases dislodge it entirely from the diver's head.[8]

The methods of clearing differ between the conventional recreational diver's half mask, which covers the eyes and nose, and the full-face mask, which also covers the mouth.

A half mask is not directly connected to the scuba air supply. the only available source of air to displace the water in the case of a leak or flood is through the diver's nose. The procedure involves exhaling through the nose into the mask until the water has all been displaced by air. During this process, the air must be prevented from escaping at a high point, or the water will not be expelled. If the mask does not fit in such a way that this happens automatically, it must be sealed at the high point by the diver pressing it against the face.[8]

Several types of full-face mask exist, and the procedure for clearing them depends on the construction. They will automatically drain through the exhaust port of the demand valve provided the water can get to it, but this is not always possible, and in models which use an internal mouthpiece, the procedure is the same as with a half mask. Models which use an oral/nasal internal seal will usually drain to the demand valve or an additional drain valve at a low point when the diver's face is roughly upright or face down, and these will clear during normal breathing for small leaks, and may be cleared of major flooding by using the purge button on the demand valve to fill the mask with air.[6] Most diving masks can fog up due to condensation on the inside of the faceplate. This is avoided in most cases by applying an anti-fog surfactant to the inner surface before the dive, but if the mask still fogs up, the diver can deliberately flood it slightly to rinse off the droplets, and then clear the mask. This is considered a routine procedure.

The diver needs to be able to establish three states of buoyancy at different stages of a dive:[15]

• negative buoyancy: when the diver wants to descend or stay on the seabed.
• neutral buoyancy: when the diver wants to remain at constant depth, with minimal effort.
• positive buoyancy: when the diver wants to float on the surface.

To achieve negative buoyancy, divers who carry or wear buoyant equipment must be weighted to counteract the buoyancy of both the diver and the equipment.[15]

When underwater, a diver often needs to be neutrally buoyant so that the diver neither sinks nor rises. A state of neutral buoyancy exists when the weight of water that the diver and their equipment displaces equals the total weight of the diver and equipment. The diver uses weights and a buoyancy compensator to maintain this state of neutral buoyancy by adjusting the BC's volume and therefore its buoyancy, in response to various effects which alter the diver's overall volume or weight.[15]

To remain neutrally buoyant, gas is added to the BC when the diver is negative (too heavy), or vented from the BC when the diver is too buoyant (too light). There is no stable equilibrium depth for a diver. Any change in depth from a position of neutrality results in a force toward an even less neutral depth, so buoyancy control is a continuous and active procedure—the diving equivalent of balance, in a positive feedback environment.[15]

It may be necessary to add gas to the BC, and it is always necessary to vent excess gas during a controlled ascent to maintain a suitable volume of gas in the BC during depth changes. Much the same must be done with the gas in a dry suit. When a wet suit is worn, the gas in the buoyancy compensator must also compensate for the volume changes of the suit to maintain neutral buoyancy. When gas is not added to the BC during a descent, the gas in the BC decreases in volume due to the increasing pressure, resulting in decreasing buoyancy with increased depth, until the diver hits the bottom. The same runaway phenomenon, an example of positive feedback, can happen during ascent if gas is not vented at a suitable rate, resulting in uncontrolled ascent, until the diver prematurely surfaces without a decompression stop.[15]

Skill in buoyancy control is achieved mainly by practice,[15] but it is easier to learn if the principle is understood.

The stability and static trim of a scuba diver affect the convenience and safety of the diver both at the surface and under water during the dive. Underwater trim is at approximately neutral buoyancy, but surface trim may be at significant positive buoyancy.

When the buoyancy compensator of a scuba diver is inflated at the surface to provide positive buoyancy, the positions of the centre of buoyancy and centre of gravity of the diver are generally different. The vertical and horizontal separation of these centroids will determine the static trim of the diver at the surface. The diver can usually overcome the trimming moment of buoyancy, but this requires constant directed effort, albeit usually not a great deal of effort. This allows a conscious diver to adjust trim to suit the circumstances such as the choice between swimming face down or face up, or remaining vertical for best field of view or visibility. The position of the diver's centre of gravity is determined by the distribution of weight, and buoyancy is determined by the equipment in use, particularly the buoyancy compensator, which can significantly influence centre of buoyancy shifts as it is inflated and deflated. Stable trim implies that the centre of buoyancy is directly above the centre of gravity. Any horizontal offset will generate a moment which will rotate the diver until the equilibrium condition is restored.

In almost all cases the centre of buoyancy of a diver with an inflated buoyancy compensator is nearer the head than the centre of gravity, and buoyancy compensators are all designed to provide this as the default condition, as an inverted diver floating at the surface is at risk of drowning. The offset in the forward/backward axis is quite frequently significant, and is usually the dominant factor in determining static trim attitude. At the surface, it is generally undesirable to be trimmed strongly face down, but it is useful to be able to trim face down at will. Vertical trim is acceptable providing it can be overcome for swimming.

Underwater trim is the diver's attitude in the water, in terms of balance and alignment with the direction of motion. The free-swimming diver may need to trim erect or inverted at times, but in general, a horizontal trim has advantages both for reduction of drag when swimming horizontally, and for observing the bottom. A slightly head down horizontal trim allows the diver to direct propulsive thrust from the fins directly to the rear, which minimizes disturbance of sediments on the bottom,[16] and reduces the risk of striking delicate benthic organisms with the fins. A stable horizontal trim requires that diver's centre of gravity is directly below the centre of buoyancy (the centroid). Small errors can be compensated fairly easily, but large offsets may make it necessary for the diver to constantly exert significant effort towards maintaining the desired attitude, if it is actually possible. The position of the centre of buoyancy is largely beyond the control of the diver, though the cylinder(s) may be shifted in the harness by a small amount, and the volume distribution of the buoyancy compensator has a large influence when inflated. Most of the control of trim available to the diver is in the positioning of ballast weights. Fine tuning of trim can be done by placing smaller weights along the length of the diver to bring the centre of gravity to the desired position.

Barotrauma of descent is caused by pressure differences between the increasing ambient pressure and the internal pressure of gas filled spaces of the diver's body and equipment. The skills of equalization are simple but essential to avoid injury. More complex, but also more straightforward in practice, is buoyancy control and the associated control of descent rate. The diver is expected to be competent to control, and particularly, limit, the descent rate by adjustment of buoyancy of the buoyancy compensator, and when applicable, the dry suit. The diver must be able to limit descent rate to match the ability to equalize, particularly the ears, and to stop the descent quickly without going into an uncontrolled ascent if there is a problem, or when the desired depth has been reached. In most cases the bottom provides a physical limit to descent, but this is not always the case, and it is generally considered bad form to hit the bottom at speed. A skilled diver will stop at the desired distance above the bottom and stay at that depth, neutrally buoyant, and ready to proceed with the dive. These skills require practice, and are not usually fully developed after typical entry level recreational certification.[8][13][11]

Safety stop during ascent

Barotrauma of ascent is caused by pressure differences between the decreasing ambient pressure and the internal pressure of gas filled spaces of the diver's body. The two organs most susceptible to barotraumas of ascent are the ears and lungs, and both will normally equalize automatically. Problems may arise in the middle ear if the Eustachian tubes become blocked during the dive, and the lungs can be injured if the diver forcibly holds his or her breath during ascent, which can occur during an emergency free ascent. As lung overexpansion injury is potentially life-threatening, entry level diver training emphasizes developing the habits of not holding one's breath while diving on scuba, and slow continuous exhalation during simulated emergency swimming ascents. Techniques for clearing blocked Eustachian tubes during ascent are also generally taught at entry level.[8][13][11]

Uncontrolled rate of ascent can increase risk of decompression sickness and lung overexpansion injury even when diving within the no-stop limits of the decompression tables, so the skills of buoyancy control during ascent are important for diver safety and are included to some extent in all entry level training, but the criteria for competence vary among the certification agencies. Most, if not all, agencies require the diver to be able to limit ascent rate and to be able to achieve neutral buoyancy at a specified depth during an ascent without significantly overshooting the target depth, while using only a depth gauge or dive computer as a reference to depth and ascent rate, but this is a skill which requires considerable practice to master, and few learners can achieve true competence in the short time provided for practicing the skill in recreational entry level diver training. The skills involve venting the buoyancy compensator and where applicable, the dry suit at a rate which provides neutral or slight negative buoyancy at all stages of the ascent, or for highly skilled practitioner, just sufficient positive buoyancy to cause ascent at the desired rate, and neutral buoyancy when a stop is required. Most dry suits intended for scuba diving are fitted with an automatic dump valve, which can be adjusted to provide an approximately constant volume of gas in the suit, so the diver can concentrate on controlling ascent rate by venting the buoyancy compensator. These skills become critical when decompression stops are required, and even divers with excellent buoyancy control will often make use of aids to ascent rate and depth control to reduce risk. Shot lines are used at all levels of diving, and are in common use during entry level training, as a visual aid to ascent rate and depth control, and as a fall-back physical aid. The skills of deploying and using surface marker buoys and decompression buoys are generally considered advanced skills for recreational divers, but may be considered entry level skills for professional divers.[8][13][11]

Clearing the ears on descent

The pressure changes during ascent and descent may affect gas spaces in the diver and diving equipment. A change in pressure will cause a pressure difference between the gas space and environment which will cause the gas to expand or compress if that is possible, and constraining the gas from expanding or compressing to balance the pressure may cause damage to the surrounding material or tissues by over-expansion or crushing. Some gas spaces, such as the mask, will automatically release excess gas as it expands, but have to be equalized during compression, others, such as the buoyancy compensator bladder, will expand until the over-pressure valve opens. The ears are a special case, as they will usually vent naturally through the Eustachian tubes, but these may be blocked. During descent they do not usually equalize automatically, and must be intentionally equalized by the diver, using one of several possible methods. Most of the physiological airway automatically equalizes as long as the diver is breathing normally, but holding the breath can prevent equalization of the lower airways and lungs, which can lead to barotrauma.[8][13][11]

Equalizing of the ears and mask are part of the essential skills for any form of diving, and equalizing of the airways is necessary for any form of diving where the diver breathes under pressure. This is provided for by breathing normally, and is the reason why divers are advised not to hold their breath while changing depth.[8][13][11]

It may be necessary for the diver to establish positive buoyancy if the buoyancy compensator fails. The following methods are available:

• If using a dry suit, it may be inflated to increase buoyancy. This increases the risk of inversion and an uncontrolled inverted ascent, so is less risky when done trimmed feet down. The automatic dump valve will generally have to be adjusted to keep more gas in the suit.
• Weights may be ditched. Ideally only enough weight to establish neutral buoyancy, but this is not always possible. At the surface, this method has fewer disadvantages, and more weight may be ditched without disadvantage other than the loss of the weights.
• Some buoyancy compensators have a double bladder system. The backup bladder may be inflated if the primary fails. This system has other associated risks, as if it is possible to accidentally inflate the backup bladder, it is possible that a runaway buoyant ascent may occur before the diver can recover control. This class of failure has been associated with fatal accidents.
• A decompression buoy or lifting bag can be deployed on a reel line and the line used to control depth. There are hazards in deployment, but once the buoy is at the surface, the method allows excellent depth control if the buoy is big enough, and the diver can control ascent by controlling the rate at which the line is would back on the reel. A ratchet reel is preferable for this procedure, as it will not unwind under load unless the ratchet is released. There may be problems keeping the buoy inflated once the diver has surfaced, depending on design details of the buoy. At this point weights may usually be safely ditched. If there are insufficient ditchable weights it may be necessary to ditch the scuba set.

An emergency ascent usually refers to any of several procedures for getting to the surface in the event of an out-of-air emergency, generally while scuba diving.

Emergency ascents may be broadly categorized as independent ascents, where the diver is alone and manages the ascent alone, and dependent ascents, where the diver is assisted by another diver, who generally provides breathing gas, but may also provide propulsion, buoyancy, or other assistance. Emergency ascent usually refers to cases where the distressed diver is at least partially able to contribute to the management of the ascent.

An emergency ascent implies that the diver initiated the ascent intentionally, and made the choice of the procedure. Ascents that are involuntary or get out of control unintentionally are more accurately classed as accidents.

Emergency ascents may be classified as independent action, where no assistance required from another diver, and dependent action, where assistance is provided by another diver.

• Buoyant ascent is an ascent where the diver is propelled towards the surface by positive buoyancy.
• Controlled emergency swimming ascent (CESA) is an emergency swimming ascent which remains under control and which is performed at a safe ascent rate, with exhalation at a rate unlikely to cause injury to the diver by lung over-expansion.
• Emergency swimming ascent (ESA) is a free ascent where the diver propels him/herself to the surface by swimming at either negative or approximately neutral buoyancy.

Other forms of ascent which may be considered emergency ascents are:

• Tethered-ascent – where the diver has unintentionally lost full control of buoyancy due to a loss of ballast weight, and controls ascent rate by use of a ratchet dive reel with the end of the reel line secured to the bottom.[22]
• Lost mask ascent – where the diver is unable to read instruments effectively due to loss of the mask. It may not be possible to accurately monitor depth, rate of ascent or decompression stops. This can be mitigated if a dive buddy can monitor control the ascent, or if the diver's computer has audible alarms for fast ascent and exceeding a ceiling. Ascent on a tangible reference such as a DSMB line, shotline or anchor line is also helpful.
• Lost buoyancy ascent – where the diver loses the ability to establish neutral or positive buoyancy without resorting to ditching weights. This can be due to a major buoyancy compensator failure or a major dry-suit flood.

Emergency ascent training policy differs considerably among the certification agencies, and has been the subject of some controversy regarding risk-benefit.

A leak dry suit leak can be anything from a trickle of water through the cuff seal to a rapid escape of gas through a torn neck seal or damaged (or open) zipper followed by ingress of a large volume of water. There are two aspects to a catastrophic flood which put the diver at risk.[7]

Damage to the lower part of the suit can cause a sudden inrush of very cold water for winter users, or an inrush of contaminated water or chemicals for hazmat divers. This may not materially affect buoyancy during a dive, and the urgency of dealing with the problem is mainly due to the hypothermia or contamination hazard. A normal ascent should be possible, but exiting the water may be difficult due to the weight of water trapped in the suit.[7]^:ch.3

Damage to the upper part of the suit can cause a sudden venting of the air, resulting in a loss of buoyancy and possible uncontrolled descent, followed by flooding. The buoyancy loss may be so much that it cannot be supported by the buoyancy compensator. In this case alternative measures must be taken. The simplest case is to ditch sufficient ballast weight to allow the buoyancy compensator to regain neutral buoyancy, but this is not always possible, as there may not be sufficient ditchable weight to drop. Some divers do not allow for this contingency in their weight distribution, and planning for this contingency is not covered by all training standards. The suit may allow enough gas to be trapped inside the suit above the leak in some positions, but this may compromise mobility.[7]

A badly flooded suit may contain so much water that the diver cannot climb out of the water because of the weight and inertia. In this case it may be necessary to cut a small slit in the lower part of each flooded leg to let water drain out as the diver rises out of the water. This will take some time depending on the size of the holes, and agility will be seriously compromised while draining. If the exit is urgent or dangerous, larger drain holes will let the diver exit more quickly. The damage should not be difficult to repair if the slits are cut with reasonable care.[7]

Emergency sharing of breathing gas may be done by sharing a single demand valve, or by the donor providing a demand valve to the receiver, and another for their own use. The gas supply for the second demand valve may be from the same scuba set or from a separate cylinder.[1] The preferred technique of air sharing is donation of a demand valve that is not needed by the donor.

The procedure of sharing a demand valve is known as buddy breathing. It is no longer considered the default method of sharing breathing gas as the use of a separate "octopus" demand valve is considered to reduce the risks involved sufficiently to justify it being rated the standard practice by most, if not all, diving certification agencies. As a consequence, buddy breathing is no longer taught as extensively as it was in the past, but some agencies and schools still teach buddy breathing as an entry-level or advanced skill, as the ability to perform the skill successfully is not only considered a potentially life-saving skill in special circumstances, but also demonstrates the self-control and rational behavior that are desirable in an emergency. The standard technique for buddy breathing is for the divers to alternately breathe from the demand valve, usually each taking two breaths before exchanging the DV, but it is common for the receiver to be out of breath at the start of the procedure, and they may need a few more breaths to stabilize. Once a rhythm has been established, it is usual to terminate the dive and start the ascent, so buddy breathing training will usually include assisted ascents.[8][13][11] Assisted ascents using a secondary demand valve are simpler than buddy breathing ascents, and this skill is quicker to learn.[1]

The conventional technique, known as octopus donation, is to donate a secondary demand valve supplied from the donor's primary gas supply, known as an octopus DV, which is mounted ready for use in an easily accessible position in the donor's chest area, and is often yellow for easy recognition.[1]

The alternative is to donate the primary demand valve that the donor is currently breathing from, on the principle that it is known to be working and is immediately recognizable and accessible. The donor, who should be less stressed, will then switch to the secondary demand valve, which in this arrangement is generally mounted on a loop of bungee cord which hangs on the neck, and keeps the secondary demand valve tucked up under the diver's chin, where it can often be reached without the use of the hands, by bending the head forward and gripping the mouthpiece with the teeth.[1]

An alternative to relying on a dive buddy to supply breathing gas in an emergency, is to carry an independent supply of emergency breathing gas in a separate cylinder, known variously as an independent alternative air source, bailout cylinder or pony cylinder. This is necessarily the option used by solo divers, as they may not be anywhere near another diver in an emergency, but it is also the choice of many professional diving organisations and conventional recreational divers, who prefer not to rely on an unfamiliar buddy. The details of the technique vary depending on how the bailout cylinder is carried. This skill is generally not taught to entry level recreational divers, but may be part of the basic required skill set for professional divers.[1]

In the event of a continuous leak of gas into the buoyancy compensator, the diver can continuously dump excess gas while disconnecting the low pressure supply hose. If upright or trimmed even slightly head-up, this will usually allow gas out faster than it flows in. The ability to disconnect the inflation hose under pressure is an important safety skill, as an uncontrolled buoyant ascent puts the diver at risk of lung overpressure injury, and depending on decompression obligation, at what could be severe risk of decompression sickness. Once disconnected, the diver can neutralise buoyancy by oral inflation or further deflation of the BCD. If using a full-face mask, the hose can be temporarily reconnected to add gas when needed.

The possible consequences of a dry suit blowup are similar to a BCD blowup, and the method of management fairly similar. The instinctive reaction of trying to swim downwards is usually counterproductive, as it will prevent the automatic dump valve from releasing excess gas, while at the same time inflating the suit legs, making it difficult to fin, and if the boots slip off, impossible to fin. The diver must ensure that the dump valve is fully open, at the high point of the suit, and urgently disconnect the inflation hose. Many suits will release air at the neck or cuff seal if those are the highest point of the suit. It may be necessary to descend after this to compensate for rapid ascent, and to do this it may be necessary to dump gas from the BCD. After achieving neutral buoyancy, a normal ascent is usually possible, as it is seldom necessary to add air to the suit during ascent. The type of inflation hose connection can make a large difference to the urgency of the situation. The CEJN connector allows a much faster gas flow than the Seatec quick disconnect fitting, and the Seatec is considered safer by the DIR community for this reason.

The secondary second-stage demand valve can be kept in place just under the diver's chin by an elastic loop which goes around the neck.

One of the standardised configurations used with manifolded twins is that developed by the DIR movement for cave exploration. The procedures listed are those developed for this configuration, and are in general use by a large number of technical divers. The diver breathes from the primary second-stage regulator supplied from the right side first stage by a long (2-meter/7-foot) hose. A secondary second-stage regulator is carried just beneath the chin, suspended by a breakaway elastic loop around the neck, supplied from the left side first stage cylinder by a shorter (0.5-meter/2-foot) hose. The cylinder valves and manifold isolation valve are normally open:[16]

• If another diver experiences an out-of-air emergency, the donor diver hands over the primary regulator, which they both know is functioning properly. The donor then switches to the secondary regulator. The entire gas supply is available to both divers for the remainder of the dive and they are able to separate by a sufficient distance to pass through tight restrictions with the donor behind the recipient.
• If the primary regulator malfunctions, the diver closes the right-shoulder cylinder valve and switches to the secondary regulator. The entire gas supply is available for the remainder of the dive.
• If the secondary regulator malfunctions, the diver closes the left-shoulder cylinder valve, continuing to breathe through the primary regulator. The entire gas supply is available for the remainder of the dive.
• Cylinder to manifold connection malfunction, though rare, can result in an extremely violent gas loss. In case of the right side manifold connection leak, the diver closes the isolating valve to secure the gas in the left cylinder, and continues to use the gas from the right cylinder until it runs out, and then switches to the secondary regulator. At least half of the remaining gas volume is available for the remainder of the dive once the isolation valve has been closed.
• In case of the left side manifold connection leak, the diver closes the isolating valve switches to the secondary regulator to use as much of the gas in the left cylinder as practicable before it runs out, then switches back to the primary regulator. At least half of the remaining gas volume is available for the remainder of the dive once the isolation valve has been closed.

Whenever there is a possibility that the pressure exposure of a dive may incur a decompression obligation on the diver it is necessary for safety to monitor the depth and duration of the dive to ensure that either there is no decompression obligation, or that the appropriate decompression procedures are followed for a safe ascent. This process may be automated by using a personal dive computer, in which case the diver is required to understand how to read the output and follow the decompression instructions displayed. The display and operation of dive computers is not standardized, and the user is expected to learn the correct operation of the specific model of computer to be used before diving with it. Accurate monitoring of depth and time is particularly important when diving using a schedule requiring decompression according to decompression tables.[8][13][11]

Management of breathing gas is a critical skill for scuba diving, as the scuba diver must, by definition, carry all the required breathing gas for a dive, and running out unexpectedly is at best alarming, and at worst can have fatal consequences. For the basic case of no-decompression open-water diving, where a free ascent is acceptable in an emergency, this may be as simple as ensuring that sufficient air remains in the cylinder to allow a safe ascent at any time, usually allowing for a contingency reserve, and for the possibility of an assisted ascent, where the diver supplies breathing gas to a buddy. Gas management becomes more complex when solo diving, decompression diving, penetration diving, or diving with more than one gas mixture.[13][11]

A submersible pressure gauge is used to indicate the remaining gas pressure in a diving cylinder. The amount of available gas remaining can be calculated from the pressure and the cylinder internal volume, and the time that he diver can dive on the available gas depends on the depth and work load, and the fitness of the diver. Breathing rates can vary considerably, and estimates are largely derived from experience. Conservative estimates are generally used for planning purposes.[8][13][11]

These are generally considered advanced techniques by recreational certification agencies, but may be considered basic skills for professional divers.[3]

• Bailout to a redundant gas supply: Switching to a bailout cylinder in case of main gas supply failure. The techniques vary depending on how the cylinder is carried and what type of mask is used.
• Use of surface marker buoys: A surface marker buoy is useful to indicate the position of the diver to people on the surface. Control of line tension is important to prevent entanglement and snagging.
• Use of decompression buoys: Sub-surface deployable buoys allow the diver to signal that the ascent has begun, and indicate the position of the diver to people on the surface, often the crew of the boat which must pick the divers up after the dive. Deployment skills include controlled inflation, paying out line in a way they avoids snags and jams, maintaining appropriate depth control during the deployment and control of line tension during ascent.
• Use of distance lines
• Use of shot lines: Shotlines are used to indicate a position so that divers can reach the bottom at the right place, and ascend to the surface where the surface crew expects them. The choice, rigging and deployment of shotlines to suit the dive profile and environmental conditions is also a diving skill.
• Underwater navigation, using Compass and underwater pilotage
• In-water decompression stops: Divers who may develop a decompression obligation need to be able to follow the required decompression profile to avoid decompression sickness. This requires the ability to maintain fairly accurate depth for the required periods, and to ascend at the correct rate. Some divers have the skills to do this independently of a static reference, referring only to depth and time instrument displays, others rely on a decompression buoy or shotline to monitor and help control changes of depth.
• Analyzing nitrox mixtures for oxygen fraction: The safe use of nitrox mixtures depends on using them at depths where the partial pressure of oxygen is within acceptable limits, and this requires knowledge of the oxygen fraction, so the maximum operating depth can be calculated. Recreational divers are responsible for analysis of their own breathing gas.
• Switching gases for accelerated decompression: A critical skill for this procedure is positive identification of the breathing gas in use at any time, as decompression mixtures are generally extremely dangerous to breathe at the maximum depth of the dive
• Use of lifelines and buddy lines.

Many recreational diver training organizations exist, throughout the world, offering diver training leading to certification: the issuing of a "Diving Certification Card," also known as a "C-card," or qualification card.

Recreational diver training courses range from minor specialties which require one classroom session and an open water dive, and which may be completed in a day, to complex specialties which may take several days to weeks, and require several classroom sessions, confined water skills training and practice, and a substantial number of open-water dives, followed by rigorous assessment of knowledge and skills. Details on the approximate duration of training can be found on the websites of most certification agencies, but accurate schedules are generally only available from the specific school or instructor who will present that course, as this will depend on the local conditions and other constraints.

The initial open water training for a person who is medically fit to dive and a reasonably competent swimmer is relatively short. Many dive shops in popular holiday locations offer courses intended to teach a novice to dive in a few days, which can be combined with diving on the vacation. Other instructors and dive schools will provide more thorough training, which generally takes longer.

Diving instructors affiliated to a diving certification agency may work independently or through a university, a dive club, a dive school or a dive shop. They will offer courses that meet, or exceed, the standards of the certification organization that will certify the divers attending the course.

Technical diver training generally follows a similar pattern to other recreational diver training, but tends to provide a more comprehensive level of theoretical learning, and in many cases, a far more exhaustive level of skill over-training, with higher standards for assessment, as the risks are higher and the necessary competence to manage reasonably foreseeable contingencies is more complex.

Professional diver training is generally provided by schools affiliated to or approved by one or more of the commercial, scientific or other professional diver certification or registration organizations.[9][10][3] Professional diver training standards may require a significantly higher level of over-training than most recreational certification agencies, as the professional diver is expected to manage most contingencies and still perform the planned work under difficult conditions. Professional divers may also be provided with what is variously known as confidence training or stress training, where simulated emergencies are enacted, or unlikely contingencies are simulated, with the intended result of developing the diver's confidence in their ability to manage contingencies while in a controlled environment. The amount of time spent on skill and confidence development is generally proportional to the length of the training programme, as the basic skills are usually learned fairly quickly.