Scuba skills

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.

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.

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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.

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.

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.

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.

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.

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.

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.

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.

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.

Source from Wikipedia