Basic open circuit equipment
Preparing and dressing in the diving suit
A certified scuba diver is expected to be able to assess what type of diving exposure suit is suitable for the planned dive, to check that it is in safe and usable condition, to check 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 cold, dry suit skills may be considered to be entry-level skills. Using a dry suit safely during dives requires special skills, including equalizing, buoyancy control, inversion recovery, emergency venting, and blowup recovery. Recreational divers trained in warm tropical waters may not acquire diving suit skills.Preparing the equipment
Divers are personally responsible for the function of their equipment. When diving as buddies with other divers, they are expected to familiarize themselves with every aspect of the buddy's equipment with the purpose of being able to operate in an emergency.Scuba assembly
The set is usually stored and transported as separate major components: harness, and , buoyancy compensator, and assembled for each use. Correct assembly and function are critical to the dive, and in some cases to the survival of the diver. All certification agencies require the diver to be competent to assemble their own set. Scuba assembly generally entails mounting the on the harness, connecting the to the cylinder valves, ensuring an uncontaminated and pressure-tight seal, and connecting the low-pressure hose to the buoyancy compensator inflation valve. Validating the function of the regulator, and inflation valve is part of scuba assembly, and reviewed as part of pre-dive checks. Considering that there is a significant interval between assembly and use, the check is commonly done twice.Pre-dive checks
Entry and exit
The certification requires one being able to get in and out of the water in a wide range of circumstances. Divers with disabilities or otherwise physically unable to make a safe entry or exit are expected to identify 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 positive buoyancy, but negatively buoyant entry is appropriate in some circumstances, for example, given a strong surface current. Negative buoyancy is generally considered a higher-risk procedure. It requires the buoyancy compensator and dry suit to be deflated before entry, more precise control of weighting, confidence in the ability to equalize during rapid descent, and the ability to control descent rate and achieve neutral buoyancy without delay. An acceptably safe negative entry requires pre-dive checks on the regulator and BC inflation function, and a sufficiently accurate balance of BC and/or suit inflation to ballast dive weights. 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 more heavily weighted at the start of the dive. Common entry and exit points include: *poolside *small boat *large boat *beach or rocky shoreline *jetty or dockside *Into/out of deep water *Into/out of shallow water *surf lineEntries
Standard water entries that are generally taught to entry-level divers include: *Stride entry: This is the standard method of entry from a standing position at a moderately low height above sufficiently deep water. The diver simply steps forward and remains upright during the short drop into the water. The fins strike the water first and reduce impact. Depth of immersion can be limited by performing a scissor kick immediately after striking the water. If an unexpected obstacle is present or the water is shallower than expected, the feet hit it first. The regulator and mask are vulnerable to water impact in this entry technique, so they are held in place with one hand. If the buoyancy compensator is inflated, the buoyancy and drag limit the depth of penetration. The diver may need to quickly clear the area below the entry point so that other divers can follow when there is a large group or a current. *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. 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 rotate their body to face the platform before lowering themselves into the water. *Backward roll: 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 meter or less. Falling backward 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 back-mounted diving cylinder makes first contact with the water surface, and the more sensitive mask and demand valve, and any other sensitive equipment, are shielded from the initial impact by the diver's body. Care must be taken not to fall on divers already in the water. A multiple diver backward toll entry is usually coordinated by a crew person, who voices a count-down so all go at the same time. The more delicate and loosely fitted equipment should be held securely in place while rolling. In the case of sling or side-mount cylinders, these should be held in place by the arms to prevent them from hitting the diver in the face. *Forward roll: This is an alternative entry to the stride entry which can be used from a low to moderate height standing position. It is seldom used as the risk of injury, damage or loss of equipment is higher than for the stride entry, and rarely offers advantages. The technique is to stand at the edge of the deck, fully kitted, with the fins beyond the foot pocket overhanging the edge. The diver holds the mask and DV with one hand and bends forward at the hips, keeping the legs straight, curls in the head, and falls forward, rotating so that the top of the back-mounted cylinder strikes the water first. *Ladder descent: A ladder descent is a relatively controlled and low-impact entry method. Few ladders are suitable for descending while wearing fins, so they are usually carried over an arm or clipped to the diver, and put on once in the water. Putting fins on in the water can be tricky in a seaway or current, especially for less buoyant fins. *Surf and beach entries: These entries are complicated by breaking waves. A small break is not usually a problem, but a wave with sufficient energy to knock the diver over is usually best avoided by going under it if it cannot be avoided. The safe limit differs among divers and depends on the equipment used. Going under the break requires the mask and fins to be worn while the diver breathes through the regulator. It is easier to walk backwards through the broken wave if the fins are worn, and the type and height of the break can be important. A plunging breaker is much more difficult and dangerous than a spilling breaker for both entries and exits. *Entry from a steep rocky shore: Depending on the details of the shoreline topography, water depth, and the wave action, the diver may be able to jump in fully kitted, sit at the water's edge to fit mask and fins, then slide or step in, or may have to climb down into the water and fit their fins in the water. Equally important are knowing how to time the entry to minimise the effects of waves, and knowing when entry is unacceptably risky. The skill and fitness of the diver, and the equipment in use all influence the risk, and must be considered. *Jump entries: Jumps from heights of 3 m or more may be taught by some agencies. 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. High jump entries may be unsuitable for rebreathers, sidemount configurations or other configurations where a cylinder or other equipment is mounted in a way that may allow it to swing and strike the diver when it hits the water, or where the impact may damage equipment.Negative entry
Negative entries are entries where the diver has negative buoyancy established before entering the water, allowing immediate descent, which can be useful when it is important to get below the surface as quickly as possible, in the presence of a strong surface current and a small descent target. This procedure requires all pre-dive checks to be done before entering the water, and the consequences of getting buoyancy settings wrong or neglecting a breathing gas setup check can be serious. Failing to connect inflator hoses, zip up a dry suit, or open the cylinder valve sufficiently can quickly lead to an emergency. Other problems can arise if the diver is too negative and has trouble equalising, or sinks so fast that the inflator valves cannot fill the dry suit or BCD fast enough to compensate for the compression of descent.Exits
Standard exit procedures include: *poolside *pool by ladder *small boat (over the side): heavy equipment is usually removed first and pulled onto the boat by a person onboard. Fins are commonly left on to help boost the diver out of the water. Depending on the height and shape of the boats topsides, the presence and type of handholds, and the skill and strength of the diver, it may be possible to climb out unaided, or may be necessary for one or more persons on the boat to help haul the diver in over the side. *large boat (ladder): An exit by ladder may require the diver to remove their fins first. Fins may then be passed up to someone on the boat, clipped off to the diver, carried hanging over the wrists by the straps, or lifted on board in a net or basket or clipped to a rope with other gear. Depending on circumstances, such as the height of the climb and weight of equipment, the diver may remove other equipment and pass or send it up, or climb the ladder while wearing it. The procedure may be complicated by motion of the boat, waves, or current, as the diver may have difficulty getting back to the ladder if they drift away after removing their fins. *jetty (steps or ladder). *Surf/beach exit *Rocky shore exits.Breathing from the demand valve
Breathing from a demand valve 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, so 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 mouthpiece styles are available off the shelf or as customised items, and one of them may work better if either of these problems occurs. Air is inhaled and exhaled 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. Under most circumstances scuba breathing differs little from surface breathing. A full-face mask may allow the diver to breathe through the nose or mouth as preferred. The demand valve adds a little respiratory dead space to the airway, and the is greater 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 a slow and deep breathing cycle more energy efficient and more effective at carbon dioxide elimination. Part of the skill is learning to relax under water, part is to minimize effort by learning good buoyancy, trim, maneuvering and propulsion skills and part is to breathe more slowly and deeply. Breathing too slowly or too shallow does not ventilate the lungs sufficiently and risks hypercapnia (carbon dioxide buildup). Breathing effort increases 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 the task, leaving none for other purposes. This may cause carbon dioxide buildup. If this cycle is not broken, panic and drowning may 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). Scuba divers are typically taught to not to hold their breath underwater, as in some circumstances this can result in lung overpressure injury. This is a risk only during ascent, as that is when air expands in the lungs. During ascent the airways must remain open. 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 a standard practice by underwater photographers to avoid startling their subjects. Breath holding during descent can eventually cause lung squeeze, and may allow the diver to miss warning signs of a breathing gas supply malfunction until it is too late to correct. Skilled open circuit divers make small adjustments to buoyancy by adjusting their average lung volume during the breathing cycle. This adjustment is generally in the order of a kilogram (corresponding to a litre of gas), and can be maintained for a moderate period, although it is more comfortable to adjust the volume of the buoyancy compensator over longer periods. The practice of shallow breathing or skip breathing should be avoided as it may cause carbon dioxide buildup, which can result in headaches and a reduced capacity to recover from a breathing gas supply emergency. It is not an efficient method to conserve breathing gas. The skills appropriate to single and twin hose scuba regulators differ sufficiently to require relearning for a change from one to the other, but twin hose open circuit is obsolescent, and single hose skills are portable between models.Demand valve clearing and recovery
Divers may remove their demand valves from their mouths under water for several reasons, both intentionally and unintentionally. In all cases, the casing may fill with water that must be removed before the diver can breathe again. This is known as clearing or purging the demand valve. The two clearing techniques for single hose regulators are: *Exhaling through the demand valve with the exhaust valve at the low point displaces the water with exhaled gas. Normal exhalation clears the demand valve with each exhalation provided that no parts of the internal volume are below the exhaust valve. *Blocking the mouthpiece (usually with the tongue) and pressing theMask clearing
Half mask
A half mask is not directly connected to the air supply. The only available source of air to displace the water is the diver's nose. The procedure involves exhaling through the nose into the mask until the water has 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 the top of the skirt remains sealed, the diver must press the upper part against the face. A half mask is held in place by a single strap, which though generally reliable and easy to inspect, has been known to fail. The skills are portable between models.Full-face mask
Several types of full-face mask exist, and the procedure for clearing them depends on the construction. In models that use an internal mouthpiece, the procedure is the same as with a half mask. Others automatically drain through the exhaust port of the demand valve provided the water can get to it. Models that use an oral/nasal internal seal 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 clear during normal breathing for small leaks, and may be cleared of major flooding by using the DV's purge button 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, using weights and a buoyancy compensator to control buoyancy. In the water the diver adjusts the BC's volume to increase or decrease buoyancy, in response to various effects that alter the diver's overall density. *Negative buoyancy: to descend or settle on the bottom. *Neutral buoyancy: when the diver wants to remain at constant depth. *Positive buoyancy: when the diver wants to float on the surface. A small amount of positive buoyancy my be used to ascend, but it must be constantly and continuously monitored to ensure that ascent does not accelerate to an excessive rate. To achieve negative buoyancy, divers may need to carry supplemental weight to counteract any excess buoyancy of the diver and buoyant equipment. Neutral buoyancy matches the average density of the diver and equipment to that of the water. This is achieved by increasing the buoyancy when the diver is too heavy, usually by adding gas to the BC, or decreasing buoyancy when the diver is too buoyant, usually by venting gas from the BC. Any uncompensated change in depth from a position of neutrality accelerates the change, making buoyancy control a continuous procedure—the diving equivalent of balance, in a positive feedback environment. It is always necessary to vent gas during ascent to maintain a neutral or moderate level of positive buoyancy and control the ascent. Similarly, during a descent, gas must be added to prevent a runaway descent. Buoyancy control compensates for changes of volume of the diving suit with changes of depth, and changes of mass due to using up the breathing gas.Trim
Diver trim is the orientation and posture of the body in the water, determined by the distribution of weight and buoyancy along the body as well as by the other forces acting on the diver. The stability and static trim of aMobility and maneuvering
The scuba diver usually uses legs and fins to move in the water, occasionally walking on the bottom as required by circumstances. Hands are occasionally used to grasp solid objects to remain in a position in a current, but are generally not used for propulsion and maneuvering by a competent diver, as they are often needed for other purposes while finning. Techniques for effective propulsion using fins include: *Flutter kick and modified flutter kick:Ascents and descents
Ascent and descent are the phases of a dive where ambient pressure is changing, and this comes with hazards. Direct hazards include barotrauma, while 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. The skill of equalization is essential to avoid injury during both activities.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. More complex, but also more straightforward in practice, is buoyancy control and descent rate. The diver must control descent rate by adjusting the buoyancy compensator and, if worn, the dry suit. The diver must be able to limit descent rate to match the ability to equalise, particularly the ears and sinuses, and must be able to stop the descent quickly without going into an uncontrolled ascent. In most cases the bottom provides a physical limit to descent, but this is not always the case, as in a wall dive orAscent
Equalising
Communications
Emergency procedures
Procedures for managing reasonably foreseeable emergencies have been developed and standardised, and are included in training when the scope of diving encompassed by the training programme may put the diver at risk of those emergencies, some of which are included in entry level training.Loss of breathing gas
The diver has a limited ability to survive without breathing gas. Any interruption of the supply for more than a few seconds constitutes a life-threatening emergency. The diver must be prepared to cope with any reasonably foreseeable loss. Temporary interruptions due to flooding or dislodging the demand valve are addressed by recovery and clearing of the demand valve. More extensive interruptions require other skills. Ending the dive with an emergency ascent is appropriate in some circumstances. Other solutions involve accessing an alternative gas supply, either from an alternative source carried by the diver, or from another diver.Managing regulator malfunctions
There are several ways a regulator can malfunction. Some can be managed to a greater or lesser extent by the diver, but others, like a burst hose are not recoverable during the dive, though on a manifolded twin or a single cylinder with dual regulators, some of the gas can be saved if the cylinder valve is closed in time. Divers should learn the skill appropriate to the equipment they plan to use, so that the best response is possible in case of a malfunction. *Free-flow can be managed by feather breathing (manual cylinder valve operation to control gas flow) if the cylinder valve is easily reached. This is usually limited to side or sling mounted cylinders. *Regulator icing is a problem in cold water, particularly with regulators not rated for cold water operation. Freezing of the water in contact with the first stage can lock the first stage open, initiating a free-flow which will further cool the regutator, The only way it can be stopped is to close the supply valve from the cylinder, at which point the diver must necessarily switch to an alternative regulator supplied by a different cylinder valve. A similar problem can occur with the second stage, and a heavy free flow will commonly cause icing of both stages even in water that is not particularly cold. *Water leaks and wet breathing are usually caused by damage to the second stage regulator exhaust valve, diaphragm, or mouthpiece. The diver should switch to alternative second stage if available, though careful breathing and placement of the tongue in the air path can minimise salt spray aspiration. *Gas leaks – Most leaks cannot be corrected underwater, and the diver should consider the risk of the leak getting worse, when deciding when to terminate the dive. *Excessive work of breathing – Some second stages can be adjusted to reduce breathing effort at depth, others cannot. The diver may have the option to switch to an alternative gas supply or alternative regulator, but should consider the consequences of the problem deteriorating further when deciding whether to terminate the dive. *Juddering, moaning and popping are signs of mechanical problems, some of which may get worse during the dive, but do not themselves constitute an emergency. Popping is usuially a sign of a slow first stage leak.Emergency air sharing
Emergency air sharing may involve sharing a single demand valve, or one diver providing a secondary air source to another. The gas 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 standard approach is "octopus donation" in which the buddy offers the secondary "octopus" demand valve to the diver in trouble, although this is not universal. A variation on this approach is for the buddy to offer their primary demand valve to the diver in trouble, while switching to the octopus. The reasoning is that this is more likely to calm a diver in trouble, as they will know that the gas will be appropriate for the depth. Alternatively, two divers can share a single demand valve. This is known as buddy breathing. Buddy breathing is no longer taught as widely, although some groups still teach it. The standard buddy breathing technique is for the divers to alternately breathe from the demand valve, each taking two breaths, although since the receiver is likely to initially be out of breath, he/she may need a few more breaths at first to stabilise. Once air sharing has been established, the dive terminates, unless the underlying problem can be resolved. Assisted ascents using a secondary demand valve are simpler than buddy breathing ascents, the risk to both divers is lower, gas consumption may be less, and this skill is quicker to learn.Emergency ascents
An emergency ascent happens when no procedure allows a dive to continue safely. Emergency ascents are independent ascents, where a single diver manages the ascent alone, or is assisted by another diver, who provides gas, propulsion, buoyancy, or other assistance. In an emergency ascent the diver initiates the ascent intentionally, and chooses the procedure. Ascents that are involuntary or unintentionally uncontrolled are classed as accidents. *In a buoyant ascent the diver is propelled by positive buoyancy. *In a controlled emergency swimming ascent (CESA) the ascent remains under control and is performed at a safe rate. *In an emergency swimming ascent (ESA) the diver swims to the surface at either negative or approximately neutral buoyancy. Other forms of ascent which may be considered emergency ascents are: *In a tethered ascent, the diver controls the ascent rate by use of a ratchet reel with the line secured to the bottom. This may be used if weights have been lost at depth and it will not be possible to maintain neutral buoyancy throughout the ascent, and the diver has a decompression obligation. The line must be long enough to reach the surface, and may have to be abandoned. *In a lost mask ascent, the diver surfaces without reading instruments. It may not be possible to accurately monitor depth, rate of ascent or decompression stops. Instead another diver can monitor the ascent, or the diver can use the dive computer's audible alarms to know when to slow down or a tangible indicator such as a DSMB line, shotline or anchor line can regulate the ascent. As a last resort, rising more slowly than the smallest exhaled bubbles is generally safe when decompression stops are not required. A technique for trapping an air bubble at an eye may be used to make occasional instrument checks. *In a lost buoyancy ascent, the diver loses the ability to establish neutral or positive buoyancy without dropping weights. This can be due to a buoyancy compensator failure or dry-suit flood. Emergency ascent training policy differs considerably among the certification agencies, and has generated controversy regarding risk-benefit. Some agencies consider it irresponsible to fail to teach a skill which could allow a diver to safely manage a foreseeable emergency, others claim the probability of ever needing the skill to be low enough to disregard, and the risk of injury during training to be higher. Accident statistics are inconclusive.Buoyancy compensator failure
It may be necessary for the diver to establish positive buoyancy if the buoyancy compensator fails. The following methods are available: *A dry suit may be inflated. This increases the risk of inversion and an uncontrolled inverted ascent, so is less risky when done trimmed feet down. The automatic dump valve must be adjusted to retain more gas. This is a preferred method when available, as no equipment is abandoned, and full control of buoyancy is retained throughout the ascent. * Weights may be dropped. Ideally only enough weight to establish neutral buoyancy, but this is not always possible. At the surface, more weight may be dropped at only the cost of the weights. If no weights can be dropped, it may become necessary to abandon the scuba set. This method is not reversible if too much weight is dropped. *Some buoyancy compensators have a backup bladder, which may be inflated if the primary fails. However, if it is possible to unintentionally inflate the backup bladder, it may lead to a runaway buoyant ascent. *A decompression buoy orBuoyancy compensator blowup
Given a continuous gas leak into the buoyancy compensator, the diver can continuously dump excess gas while disconnecting the low pressure supply hose. If upright or trimmed even slightly heads-up, this usually allows gas to exit faster than it enters. 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 severe risk of decompression sickness. Once disconnected, the diver can neutralise buoyancy by oral inflation or further deflation. If using a full-face mask, the hose can be temporarily reconnected to add gas when needed.Dry suit flooding
A dry suit leak can range from a trickle to a flood. Two aspects to a catastrophic flood put the diver at risk. Damage to the lower part of the suit can admit cold water for winter divers, or contaminated water or chemicals for hazmat divers. This may not materially affect buoyancy, and the risk is mainly hypothermia or contamination. A normal ascent is typically feasible, but exiting the water may be difficult due to the weight of trapped water. Damage to the upper part of the suit can cause a sudden gas venting, significantly or catastrophically reducing buoyancy and triggering uncontrolled descent and flooding. The buoyancy loss may exceed the buoyancy compensator's capacity. The simplest remedy is to drop sufficient weight to allow the buoyancy compensator to function effectively. This requires sufficient detachable weight. Some divers do not prepare for this contingency in their weight distribution, and such planning is not covered by all training standards. A flooded suit may hold so much water that the diver cannot exit the water because of the weight and inertia. It may be necessary to cut a small slit in the lower part of each flooded leg to drain the water.Dry suit blowup
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.Manifolded twins
Dive management skills
These are the skills of following the dive plan when undesirable events are avoided. They include planning and monitoring the dive profile, gas usage and decompression, navigation, communication, and modifying the plan to suit actual circumstances.Monitoring depth and time, and decompression status
Whenever a dive may require decompression stops, it is necessary to monitor dive depth and duration to ensure that appropriate decompression procedures are followed if necessary. This process may be automated via a dive computer, in which case the diver must understand how to read the output and respond correctly to the information displayed, and for more complex dive plans, to input the appropriate settings. The display and operation of dive computers is not standardised, so the diver must learn to operate the specific model of computer. Accurate monitoring of depth and time is particularly important when diving using a schedule requiring decompression according to decompression tables, when a diving watch and depth gauge are used. Undertaking dives with obligatory Decompression stops requires the diver to follow the appropriate decompression profile. This requires the ability to maintain depth for the necessary interval and to ascend at the correct rate. Ideally the diver should be able to do this without a static reference, referring only to instrument displays, but a decompression buoy or shotline may be used to indicate appropriate stopping points, thereby reducing the associated risk. Decompression stops are considered an advanced skill for recreational divers, but may be considered a basic skill for professional divers. Recreational divers are required to be able to avoid incurring a staged decompression obligation as a basic skill.Breathing gas management
Management of breathing gas during the dive is a critical skill to avoid potentially fatal consequences. For the basic case of no-decompression open-water diving, which allows a free emergency ascent, this requires ensuring sufficient air remains for a safe ascent (plus a contingency reserve) and for the possibility of an assisted ascent, where the diver shares air with another diver. Gas management becomes more complex when solo diving,Navigation
Use of auxiliary equipment
These are generally considered advanced techniques for recreational divers, but basic skills for professional divers. *Bailout to a redundant gas supply: Switching to a bailout cylinder in case of main gas supply failure. Techniques depend on how the cylinder is carried and mask type. *Logging the dive
The diver's log is a record of the diving experience of the owner, and professional divers are required to keep theirs up to date and correctly filled in. This requires the diver to take note of the data required in terms of the logbook data sheets, and fill it in correctly. It is usually required for the diving supervisor to countersign each page of a professional diver's logbook to maintain the it as a legal document. Recreational logbooks are generally not required outside of training, but are recommended as a record of experience, which may be inspected by a service provider as evidence of experience when applying to take further training or participate in dives requiring specific experience. Electronic logs are becoming more popular as the later generation dive computers will download the dive data they collect automatically in a reasonably user-friendly format. Some information must be input by the user, such as the venue, dive buddy and what was seen and done on the dive.Diver rescue
Dry suits
Skills necessary for the safe use of dry suits include: *Choosing a suit of appropriate size and fit. The suit should allow freedom of movement to work and reach all necessary accessories, valves etc. when worn over suitable undergarments. The seals should be tight enough to seal reliably, but not so tight as to restrict blood flow, particularly the neck seal. *Selecting appropriate undergarments for the water temperature. Undergarments should provide appropriate insulation for the water temperature, fit the diver without constraining movement, fit under the suit comfortably, and provide sufficient insulation in case of a major leak to allow the diver to safely ascend and exit the water. *Inspection of the suit for damage and defects before the dive, zip lubrication, and a check of seal condition. *Putting the suit on without damaging the wrist and neck seals, using a suitable lubricant if applicable, and adjusting the seals to lie smoothly on the skin of the neck and wrists, without folds or leak paths caused by hair or clothing. *Closing the zip correctly to avoid damage or leaks. A smooth pull while supporting the zip, and ensuring that the zip does not snag on ta fold of the suit, undergarments or hair. The sealing surfaces must be free of any contamination which could cause the zip to jam, dislocate or leak. *Removing excess gas after putting on the suit. Usually done by opening the dump valve and squatting down, hugging the knees to compress the suit as much as possible. *Choosing and distributing ballast weight to provide correct trim and buoyancy at the start and end of the dive. *Maintaining an appropriate gas volume in the suit during the dive to avoid suit squeeze or large bubbles of excess air. **Buoyancy control during descent, at constant depth, and during ascent, by adding gas when needed via the inflator valve, and dumping excess, usually by keeping the dump valve high and adjusted correctly. **Inflation and dumping excess gas - setting the auto-dump valve. A correctly set auto-dump valve will seldom require adjustment during ascent. **Maintaining appropriate trim and attitude underwater and at the surface. To a large extent this depends on correct weighting and weight distribution. *Undressing from the suit without damaging zip or seals. Withdrawing the hands through the wrist seals can take some practice. This is a time when seals are easily damaged. A lubricant that does not harm the seal can help. *Post dive cleaning and maintenance. *Managing contingencies: **Connecting and disconnecting a pressurised inflator hose: It is possible, with a little effort, to connect an inflator hose under pressure, as long as the locking balls are free to retract and the locking ring has been slid back to release the balls. It just requires being pushed on far enough that the locking system engages. However, in rare cases the hose end is not entirely compatible with the nipple on the valve, and the incompatible combinations will not connect regardless of whether they are pressurised, so it is important that compatibility is checked on hired or new equipment. If the dry-suit and buoyancy compensator use the same connector, they can be swapped out if one malfunctions. Buoyancy compemsators have an oral inflation option, so if the dry-suit inflator hose does not supply air for any reason during a dive the BC hose can be used, and it will normally be swapped out under pressure, so the cylinder valves do not have to be closed and regulators purged for the swap, which is likely to leave the diver without breathing gas while making the swap. When a hose is connected underwater a small amount of water will be trapped in the connector and blown into the suit when the valve is operated. This is minor issue. **Managing a stuck inflation valve: Inflation valves are quite simple, but can jam open or closed. If the valve jams open and a few pushed don't loosen it up, it will be necessary to disconnect the inflator hose or constantly dump gas from the suit.Depending on the leak flow rate either of these options may work, though a disconnection will usually be safer, and it should not be necessary to add gas to the suit during ascent. **Recovery from inversion. **Recovery from uncontrolled buoyant ascent/blowup. **Managing a leak or flood.Rebreather
Rebreather skills are necessary when using a rebreather for recreational or technical diving. Due to the technical complexity of mixed gas rebreather design and construction, and the significantly larger number of possible failure modes compared with open circuit diving, the skill set required is more complex and generally requires more training and practice to master. As with most diving equipment, there are preparation skills, in-water standard operating skills, emergency skills and after use maintenance skills involved, all of which may have details specific to the model of rebreather in ise, though there are common principles involved. * Preparing the rebreather: Parts of the rebreather may require assembly before use, after which it must be tested for correct function. The scrubber canister must be filled with the correct amount of absorbent material. Positive and negative pressure tests are typically conducted. The positive pressure test ensures that the unit does not lose gas while in use, and the negative pressure test ensures that water does 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 ensures that the scrubber material warms toSpecial applications
Training, assessment and certification
Scuba skills training is primarily provided by practical instruction directed by a registered or certified diving instructor. Additional practice and skills maintenance are the diver's responsibility. Recreational divers may attend refresher courses, which may involve revisions to earlier practices. Service providers such as dive shops and charter boats may require a checkout dive for divers unfamiliar with the region, or who haven't dived for some time. The checkout dive allows the diver to demonstrate basic skills relevant to the expected conditions. It is the individual diver's responsibility to maintain sufficient skill and fitness to dive safely and not endanger themselves or others, and to judge whether they are ready to handle the anticipated conditions.Recreational diver training
Many recreational diver training organizations offer diver training. Successful completion is shown by the issuance of a " diving certification", 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 days to weeks, and require classroom sessions, confined water skills training and practice, and an open-water dives, followed by assessment of knowledge and skills. Accurate schedules are generally only available from the specific school or instructor who presents that course, as this will depend on local conditions and other constraints. The initial open water training for a person who isTechnical diver training
Technical diver training generally follows a similar pattern to recreational training, but provides more theoretical information, and in many cases, an exhaustive level of skill 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 typically provided by schools affiliated to or approved by one or more commercial, scientific or other professional diver certification or registration organisations Professional diver training standards require significantly higher skill level than recreational certification. The professional diver is expected to manage most contingencies and still perform the planned work under difficult conditions. Professional training may include confidence training or stress training, where simulated emergencies are enacted, or unlikely contingencies are simulated, to develop the diver's confidence in their ability to safely manage contingencies. The amount of time spent on skill and confidence development is generally proportional to the length of the training programme, as basic skills are usually learned fairly quickly.Skills retention
Although many scuba skills are safety critical, most are straightforward and are easily retained once learned, given occasional practice. The routine skills that are exercised on most dives or every dive are usually well retained, but emergency skills may be seldom practiced outside of the training environment, and are consequently often poorly retained and inadequate in the face of a real emergency. Even in the training environment they are not often overlearned to the point of integrating with the divers muscle memory. In many cases it is likely that if an emergency occurs during a dive under stressful conditions, the diver will be unable to manage the challenge safely. Refresher training is recommended by some training agencies when the diver has not dived for an arbitrary period, such as six months, but the actual need for relearning is not so easily identified, and checkout dives are a common requirement of service providers for divers without a convincing logbook showing recent appropriate experience.References
See also
* * {{authority control Underwater diving procedures