Scuba skills

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Scuba skills are the skills required to dive safely using self-contained underwater breathing apparatus, (scuba). Most of these skills are relevant to both open circuit and rebreather scuba, and many are also relevant to surface-supplied diving. Those skills which are critical to the safety of the diver may require more practice than is usually provided during training to achieve reliable long-term proficiency

Some of the skills are generally accepted by recreational diver certification agencies as necessary for any scuba diver to be considered competent to dive without direct supervision, and others are more advanced, though some diver certification and accreditation organizations may consider some of these to also be essential for minimum acceptable entry level competence. Divers are instructed and assessed on these skills during basic and advanced training, and are expected to remain competent at their level of certification, either by practice or refresher courses.

The skills include selection, functional testing, preparation and transport of scuba equipment, dive planning, preparation for a dive, kitting up for the dive, water entry, descent, breathing underwater, monitoring the dive profile (depth, time and decompression status), personal breathing gas management, situational awareness, communicating with the dive team, buoyancy and trim control, mobility in the water, ascent, emergency and rescue procedures, exit from the water, unkitting after the dive, cleaning and preparation of equipment for storage and recording the dive, within the scope of the diver's certification.

Video Scuba skills

Basic open circuit scuba equipment skills

Preparing and dressing in the diving suit

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. 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.

Preparing the scuba equipment

Scuba assembly

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.

Pre-dive checks

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.

Entries and exits

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.

The default condition for water entry is with positive buoyancy, but there are situations where a negatively buoyant entry is an advantage. 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.

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
• Seated entry
• Backward roll
• Forward roll
• Ladder descent
• Surf and beach entries

Jump entries from heights of 3m or more may be taught by some agencies

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 the demand valve

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. 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.

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).

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.

Demand valve clearing and recovery

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 the demand valve, and there are two ways this can be done.

• 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.
• 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.

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:

• 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. 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 is flipped over to the right side of the cylinder. 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.

Mask clearing

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.

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.

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.

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Buoyancy control, trim and stability

Buoyancy control

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

• 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.

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 displaces equals the total weight of the diver. The diver uses a BC 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.

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 result 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.

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 a diver prematurely surfaces without a safety (decompression) stop.

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

Trim

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, 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.

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Underwater mobility and maneuvering

The scuba diver usually moves around in the water column, but may occasionally walk on the bottom when this is required by the task or other circumstances. Using the hands for propulsion and maneuvering is usually restricted to holding on to solid objects in a current. The general use of hands for propulsion and maneuvering by swimming motions is widely considered inefficient and the mark of incompetence. There are several techniques for effective propulsion using fins.

• Flutter kick and modified flutter kick: Flutter kick is the finning style most frequently used. In its basic form it resembles the flutter kick of surface swimmers, but slower and with a larger stroke to make effective use of the large surface area of the fins. The modified flutter kick is done entirely with bent knees, pushing water up and behind the diver to avoid stirring up sediment on the bottom.
• Scissor kick
• Frog kick and modified frog kick: The frog kick is like the swimming action of a frog or the kick used in breaststroke. Both legs operate together and produce thrust which is directed more consistently backwards than flutter kick, and is suitable for finning near a soft silty bottom as it is less likely to stir up the silt and degrade the visibility. The modified form is done with bent knees, and though less powerful, produces almost no down-thrust, and is often used in cave diving and wreck diving where silt out can cause dramatic loss in visibility and may compromise the ability of the divers to navigate out of the overhead environment.
• Dolphin kick is a powerful stroke where both feet are kept together and moved up and down. This is the only stroke possible with monofins, and can be very effective for the skilled practitioner. It is not good for precise maneuvering.
• Reverse, back, or backward kick is used to swim backwards along the main body axis. It is probably the most difficult finning technique, and is not suited to some styles of fin. The stroke starts with the legs extended backwards at full stretch, heels together and toes pointed . The power stroke involves flexing the feet to extend the fins sideways, feet splayed outwards as much as possible, close to right angles to the legs and pulling the fins towards the body by flexing the legs at the knees and hips in a motion that pulls the diver backwards. Part of the thrust is due to flow across the width of the fins, as they are swept outward and forward. The fins are then pointed backwards to reduce drag, heels moved together, and legs extended to the start position. Fairly stiff, wide bladed fins are reputed to be most suitable for this stroke, which generally produces very little thrust for the effort expended, but is the only method of finning which moves the diver backward. Reverse kick is generally considered an advanced skill.
• Rotation on the spot about a vertical axis is achieved by the helicopter turn: The diver bends the knees so that the fins are approximately in line with, but raised slightly above the body axis, and ankle movements are used to scull water sideways. The fin is rotated to maximize sideways projected area, then a combination of rotation of the lower leg and knee is used to produce sideways thrust. The fin is feathered to reduce drag for the return stroke. Thrust away from the centreline is more effective for most divers.

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Ascents and descents

Ascent and descent are the phases of a dive where ambient pressure is changing, and this causes a number of hazards. Direct hazards include barotrauma, indirect hazards include buoyancy instability and physiological effects of gas solubility changes, mainly the risk of bubble formation by supersaturated inert gas in body tissues, known as decompression sickness.

Descent

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.

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.

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.

Equalizing

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.

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.

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Underwater communications

Divers need to communicate underwater to co-ordinate their dive, to warn of hazards, to indicate items of interest and to signal distress.

Most professional diving equipment such as full face diving masks and diving helmets include voice communication equipment, but recreational divers generally rely on hand signals and occasionally on light signals, touch signals and text written on a slate Through-water voice communications is available for recreational diving, but is restricted to full face masks and is not in general use.

Rope signals can be used if the diver is connected to another diver or tender by a rope or umbilical. There are a few partly standardised codes using "pulls" and "bells" (a pair of short tugs). These are mostly used as backup signals by professional divers in the event that voice communications fails, but can be useful to recreational and particularly technical divers, who can use them on their surface marker buoy lines to signal to the surface support crew.

Hand signals are generally used when visibility allows, and there is a range of commonly used signals, with some variations. These signals are often also used as an alternative by professional divers. There is also a set of instructional hand signals used during training. Recreational divers are expected to be familiar with the standard set of hand signals used by their certification agency, and these have to a large extent been standardized internationally and are taught on entry level diving courses. A few additional specialised hand signals are commonly used by technical divers.

Light signals are made using an underwater torch in dark places with reasonable visibility. There are not many standard light signals. The light can also be used to illuminate hand signals in the dark

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Emergency procedures

The diver has a very limited ability to survive without a supply of breathing gas. Any interruption to that supply must be considered a life-threatening emergency, and the diver should be prepared to deal effectively with any reasonably foreseeable loss of breathing gas. Temporary interruptions due to flooding or dislodging the demand valve are recoverable by recovery and clearing of the demand valve. More permanent interruptions require other strategies. An obvious response which is appropriate in some circumstances is to ascend to the surface. This response is appropriate when the consequences are acceptable. When the surface is near enough to easily be reached, and the diver has no significant risk of decompression sickness as a consequence of a direct ascent, an emergency free ascent may be a suitable response. If the surface is too far to reach with confidence, or if the risk of decompression sickness is unacceptable, other responses would be preferable. These involve getting an alternative supply of breathing gas, either from an alternative source carried by the diver, or from another diver.

Emergency ascents

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 transportation 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 continuous 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.

Another form of ascent which may be considered an emergency ascent is a 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.

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

Emergency air sharing

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. 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. Assisted ascents using a secondary demand valve are simpler than buddy breathing ascents, and this skill is quicker to learn.

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.

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.

Bailout to alternative gas supply

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.

Standardised emergency procedures used with manifolded twins

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:

• 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.

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Dive management skills

Monitoring depth and time

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.

Breathing gas management

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.

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.

Use of auxiliary equipment

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

• 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.

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Diver rescue skills

Diver rescue, following an accident, is the process of avoiding or limiting further exposure to diving hazards and bringing a diver to a place of safety where the diver cannot drown, such as a boat or dry land, where first aid can be administered and from which professional medical treatment can be sought. Rescue skills are considered by some agencies to be beyond the scope of entry level divers, but other agencies consider some or all of them an essential part of the entry level diving skill set, as this is more compliant with the concept of buddy diving, and a required part of the skill set for a stand-by diver.

Diver rescue skills include:

• Controlled buoyant lift - a technique used to safely raise an incapacitated diver to the surface from depth. It is the primary technique for rescuing an unconscious diver. It can also be used where the casualty has lost or damaged his or her diving mask and cannot safely ascend without help.
• Making the casualty buoyant on the surface.
• Attracting help
• Towing a diver on the surface
• Landing a casualty.
• In-water artificial respiration
• CPR on land or a boat
• Oxygen first aid on land or a boat
• General First aid

More than one technique may be taught for any of these skills, the choice depending on the standards of the training agency.

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Basic rebreather diving skills

• Preparing the Rebreather: The rebreather may require some assembly before use, and should be tested for correct function according to the manufacturer's recommendations. The scrubber canister must be filled with the correct amount of absorbent material, and the unit tested for leaks. Two leak tests are usually conducted. These are generally known as the positive and negative pressure tests, and test that the breathing loop is airtight for internal pressure lower and higher than the outside. The positive pressure test ensures that the unit will not lose gas while in use, and the negative pressure test ensures that water will not leak into the breathing loop where it can degrade the scrubber medium or the oxygen sensors.
• Prebreathing the unit (usually for about 3 minutes) shortly before entering the water is a standard procedure. This ensures that the scrubber material gets a chance to warm up to operating temperature, and works correctly, and that the partial pressure of oxygen in a closed-circuit rebreather is controlled correctly.
• Buoyancy control using the Rebreather
• Ascents and descents
• Monitoring the partial pressure of oxygen: Partial pressure of oxygen is of critical importance on CCR's and is monitored at frequent intervals, particularly at the start of the dive, during descent, and during ascent, where the risk of hypoxia is highest.
• Monitoring carbon dioxide level: Carbon dioxide buildup is also a severe hazard, and most rebreathers do not have electronic CO₂ monitoring. The diver must look out for indications of this problem at all times.
• Diving mask clearing and dive/surface valve draining
• Bailing out to an alternative breathing gas supply: Bailout to open circuit is generally considered a good option when there is any uncertainty as to what the problem is or whether it can be solved. The procedure for bailout depends on details of the rebreather construction and the bailout equipment chosen by the diver. Several methods may be possible:
  • Bailout to open circuit by switching the mouthpiece bailout valve to open circuit.
  • Bailout to open circuit by opening a bailout demand valve already connected to the full face mask, or by nose-breathing in some cases.
  • Bailout to open circuit by closing and exchanging the rebreather mouthpiece for a separate demand valve.
  • Bailout to rebreather by closing the mouthpiece and switching to the mouthpiece of an independent rebreather set.
• Bail out ascent: Unless the problem can be corrected fairly quickly and reliably, bailout will include aborting the dive and ascent.
• Diluent flush: Many diver training organizations teach the "diluent flush" technique as a safe way to restore the mix in the loop to a level of oxygen that is neither too high nor too low. It only works when partial pressure of oxygen in the diluent alone would not cause hypoxia or hyperoxia, such as when using a normoxic diluent and observing the diluent's maximum operating depth. The technique involves simultaneously venting the loop and injecting diluent. This flushes out the old mix and replaces it with a known proportion of oxygen.
• Draining the loop: Regardless of whether the particular rebreather has the facility to trap any ingress of water, training on a rebreather will feature procedures for removing excess water from the loop.

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Scuba skills for special applications

There are a range of special applications for scuba diving for which additional skill sets are required. In many cases the skills for one of these special applications may be shared by several others, with a few specific only to that application. There are also many underwater work and activity skills not directly related to the use of scuba equipment.

Some of these applications are listed here:

• Decompression diving
• Penetration diving

• Cave diving
• Ice diving
• Wreck diving

• Side mount diving
• Solo diving
• Underwater searches
• Underwater search and recovery

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Training, assessment and certification

Scuba skills training is primarily provided by practical instruction under the guidance of a registered or certified diving instructor, on the assumption that the instructor is both competent and willing to provide a quality of training and assessment according to the relevant training standards, and to ensure that the learner is competent according to the assessment criteria applied. Additional practice of the skills is the responsibility of the diver, and is generally necessary to reach and retain a level of competence sufficient to deal with the foreseeable contingencies which may occur during diving under the range of conditions in which the diver is certified to dive. Recreational divers may attend refresher courses when they have not dived for a significant period, in which the instructor ensures that they are still competent in the skills required by their certification, and it is not uncommon for service providers like dive shops and charter boats to require a checkout dive from divers unfamiliar with the region, or unable to show sufficient evidence of adequate current skill level. The checkout dive is usually a demonstration by the diver of basic skills appropriate to the expected conditions, and may be assessed by an instructor or divemaster. These refresher courses and checkout dives are usually informal, and may vary considerably.

It is the individual diver's responsibility to maintain sufficient skill and fitness to dive safely and not endanger themself or other divers, and to judge whether they are competent and fit to dive in any given circumstance, based on the information available and a realistic dive briefing by the service provider.

Recreational diver training

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

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

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. 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.

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References

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See also

Source of article : Wikipedia