Preparing and dressing in the diving suit

A certified scuba diver should be able to assess what type of diving exposure suit is preferable for the planned dive, to confirm that it is in safe, usable condition and the right size, and to put it on correctly.

Entry-level skills usually include the use of wet suits, but in countries where the water and/or weather conditions are cold, beginners may need to add dry suit training. However, at the other end of the complexity spectrum, recreational divers trained in warm tropical waters may not even need to acquire any diving suit skills at all. Using a dry suit safely requires special skills, including equalizing the ears and sinuses, buoyancy control, inversion recovery, emergency venting, and blowup recovery.^[8][9]

Preparing the equipment

Divers are each personally responsible for the function of their individual equipment. When diving as buddies with other divers, they are expected to familiarize themselves with every aspect of the buddy's equipment as well, to be able to operate for each other in an emergency.

Assembly of the scuba set – connection of the regulator to the cylinder valve

Pre-dive checks

Pre-dive check before a surf entry

See also: Buddy check and Diving safety § Use of checklists

Pre-dive checks include equipment inspection and function testing, and review of the dive plan with the team. Such checks can reveal problems that could make it necessary to abort the dive, including some which could potentially be fatal.

Some pre-dive checks are done while donning the dive equipment. Establishing a routine for the order of donning and checking can help avoid skipping critical checks; a written checklist may be more reliable. The risk of skipping a check is increased if the process is disrupted, and it is good practice never to distract a diver unnecessarily during a check. The value of a written checklist increases with the complexity of the equipment used, and even more so if there are unavoidable distractions.

For a shore entry, kitting up may be broken up into stages, with the suit, scuba set, and weights fitted at a convenient place, and the mask and fins added upon entering the water. In this case, some of the equipment may be checked both when it is donned and again just before committing to the water. If a long surface swim is necessary, the scuba set function and pressure should be checked again just before descent.

Responsibility for pre-dive checks for professional divers is more complex, based on the concept of the duty of care. It is usually defined in an organizational operations manual, which may stipulate recorded checklists for the equipment in use, and norms for the participation of other diving team members.^[11][12]

Entries

Standard water entries that are generally taught to entry-level divers include:

• Stride entry:^[6] 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 will hit it first. Because the regulator and mask are vulnerable to water impact in this entry technique, they are held in place with one hand. If the buoyancy compensator is inflated, the buoyancy and drag will limit the depth of penetration. The diver may need to quickly clear the area below the 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, such as the side of a swimming pool, a floating jetty, or the swim platform of a large boat. Seated entry requires sufficient upper-body strength to support the diver's weight on the arms while rotating to face the platform, then lowering oneself into the water in a controlled manner.^[6] Diver sit at the waterside in full equipment with both feet over the side in the water, place both hands palm down both on the deck on the same side of the body, and take the body weight on straight arms as they rotate their bodies to face the platform before lowering themselves into the water.^[14]
• Backward roll:^[6] this entry is used from small boats where divers complete preparations while seated on the side of the boat with their feet on the deck and the water behind them. It is particularly suited to inflatable boats and other small, open vessels with gunwale freeboards 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 divers seated along the gunwale. The backward roll is suitable when the distance to the water is short, when it is safe and comfortable to sit on the side deck, tube or gunwale, and when 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 it 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 delicate equipment, are shielded from the initial impact by the diver's body. Care must be taken not to fall on any divers already in the water. A multiple diver backward-roll entry is usually coordinated by a crew member who voices a countdown so all divers go at the same time. All 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 alternate entry that can be used from a low- to moderate-height standing position. It is seldom used, however, as the risk of injury, damage, or loss of equipment is greater than for the stride entry, and it 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: this is a relatively controlled and low-impact entry method. Few ladders are suitable for descending while wearing fins, so fins 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 styles.
• Surf and beach entries: these are ofteen complicated by breaking waves. A small break is not usually a problem, but a wave with sufficient energy to knock a diver over is usually best avoided, or addressed by going under if it cannot be avoided. The safe limit differs from diver to diver, and it depends on the equipment used. Going under the break requires wearing the mask and fins while the diver breathes through the regulator. It is easier to wear fins while walking backward through a broken wave, 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: proper approach depends on the details of the shoreline topography, water depth, and wave action. The diver may be able to jump in fully kitted, sit at the water's edge to fit mask and fins, and then slide or step in. In certain circumstances a diver may have to climb down into the water and fit the fins there. It is important both to know how to time the entry to minimize the effects of waves, and to know when entry is simply unacceptably risky. Risk is a measure of both the skill and fitness of the diver, and the quality of the equipment being used.
• Jump entries: some agencies teach jumps from heights of 3 meters or more.^[6] Relatively high jumps require the diver to strike the water upright with overlapped fins to reduce the risk of knocking them off, while simultaneously holding loose equipment in place, particularly the mask and demand valve. Demand valves should be desensitized where possible to reduce the risk of inducing a free-flow. High-jump entries may be unsuitable for rebreathers, sidemount configurations, or those 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.

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 boat topsides, the presence and type of handholds, and the skill and strength of the diver, it may be possible to climb out unaided, but one or more people on the boat may need to help haul the diver in over the side.
• Large boat (ladder): this may require divers to remove their fins first. Fins may then be passed up to someone on the boat, clipped off to each diver, carried hanging over the wrists by the straps, lifted on board in a net or basket, or clipped to a rope with other gear. Depending on the circumstances, such as climb height and equipment weight, divers 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 divers 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.

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:^[15][13]

• 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 the purge button displaces the water with gas from the cylinder. If the exhaust valve is at the low point, the water will flow out through the valve. This method is more typical if the diver does not have sufficient breath left to clear by exhalation, as it consumes significant breathing gas, particularly when done frequently.

Divers may become nauseous and vomit underwater. Vomit left inside the DV must be cleared before breathing can resume. In this case it is usual to remove the DV from the mouth, flood it to rinse, and clear using the purge button. The process may be repeated as necessary. If the DV breathes wet after purging, something may be stuck in the exhaust valve. Flooding the DV and clearing again with the mouthpiece blocked usually clears the exhaust valve.

If the DV is dislodged from the diver's mouth unintentionally, it may end up in a place out of view of the diver. Three or more methods aid recovery:^[8]

• The reach method, (or hose trace method), works in all cases where the DV is not snagged. The diver reaches back over the right shoulder to the hose feeding the DV, loops thumb and fingers round the hose, then slides the hand along the hose, pulling it forward and over the shoulder until the DV arrives in the right hand, after which it can be placed in the mouth.
• The sweep method is quick and works in the common scenario in which the DV drops down on the diver's right side, while still draped over the right shoulder. In this situation the diver is usually between face down and upright, and sweeps the right hand across the waist from left to right, and around to the back, remaining in contact with the body or scuba set. Reaching as far back as possible, the diver then straightens the arm backwards and swings it around outwards and forwards in an arc until it is pointing forwards. This captures the hose, moving it where the left hand can find it by sweeping along the arm from the right hand to the neck. This method fails if the DV is on the left side of the cylinder.
• The inversion method works best when the DV has swung over to the left side of the cylinder behind the diver's back. The diver simply rolls forward into a head-down position, with the body near vertical, and relies on gravity to bring the DV down to where it can be captured.

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 gets snagged in such a way that it cannot be easily recovered. In some cases it may be prudent to abort the dive and surface, but this may not be practicable and it may be necessary to remove the harness partially or completely to recover the primary, after which the harness can be readjusted. A dive buddy can usually find the DV easily. If the DV cannot be reached, it is prudent to terminate the dive, as a free-flow could empty the cylinder in minutes.

Half mask

This is a mask 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.^[10] 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

The procedure for clearing these depends on the construction, as several types exist. In models that use an internal mouthpiece, the procedure is the same as with a half mask. Other models 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. They may be cleared of major flooding by using the DV's purge button to fill the mask with air.^[8]

Buoyancy control

The diver needs to be able to establish three states of buoyancy at different stages of a dive,^[17] 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.^[17]

• Negative buoyancy: to descend or settle on the bottom.
• Neutral buoyancy: when the diver wants to remain at a 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 the diver monitor it constantly and vigilantly to keep from ascending too quickly.

To achieve negative buoyancy, divers may need to carry supplemental weights to counteract any excess buoyancy.^[17]

Neutral buoyancy matches the average density of the diver and equipment to that of the water. This is achieved by increasing buoyancy when the diver is too heavy, usually by adding gas to the BC, or decreasing buoyancy when the diver is too light, 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.^[17]

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.^[17]

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 a scuba diver are important both at the surface and under water. Divers must maintain trim under water at neutral buoyancy, while they must hold surface trim at positive buoyancy.

When the BC is inflated at the surface to provide positive buoyancy, the centre of buoyancy and centre of gravity of the diver are generally different. The vertical and horizontal separation of these centroids determines the static trim. The diver can usually overcome the trimming moment of buoyancy, which requires directed effort. The diver can adjust trim to suit circumstances such as swimming face down or face up, or remaining vertical. The diver's center of gravity is determined by the distribution of weight, and buoyancy is determined by the equipment in use, particularly supplemental weights and the buoyancy compensator, which can significantly influence the position of the centre of buoyancy as it is inflated and deflated. Stable trim implies that the centre of buoyancy is directly above the centre of gravity. Any horizontal offset generates a moment that rotates the diver until the equilibrium condition is restored.

In almost all cases, the center of buoyancy with an inflated BC is nearer the head than the center of gravity, and BCs are 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. 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 (orientation) 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,^[18] and reduces the risk of striking delicate benthic organisms with the fins. A stable horizontal trim requires the diver's center of gravity to be directly below the center of buoyancy (the centroid). Divers can compensate small errors fairly easily, but large offsets may make it necessary for the diver to constantly exert significant effort towards maintaining the desired attitude. The position of the center of buoyancy is largely beyond the diver's control, though the cylinder(s) may be shifted in the harness by a small amount, and the volume of the BC has a large influence when inflated. Most of the control of trim available to the diver is in the positioning of ballast weights. Divers can fine tune trim by placing smaller weights along the body length to bring the center of gravity to the desired position.

Descent

Barotrauma of descent. Pressure differences between the increasing ambient pressure and the internal pressure of gas-filled spaces of the diver's body and equipment can cause this. Buoyancy control and descent rate are more complex, but also more straightforward in practice. Divers must control descent rate by adjusting the buoyancy compensator and, if worn, the dry suit. They must be able to limit descent rates to match the ability to equalize, particularly the ears and sinuses, and must be able to stop any 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 or blue-water diving. A skilled diver can stop at a desired depth or distance above the bottom, adjust to neutral buoyancy, trim level, and proceed with the dive.^{[10][15][13][21]}

Ascent

Safety stop during ascent

Barotrauma of ascent. The same pressure differences as barotrauma of descent cause this, but during ascent. Sinuses and lungs are most susceptible, although both normally equalize automatically during ascent. Problems may arise in the middle ear if eustachian tubes become blocked. Lungs can be injured if a diver forcibly holds the breath during ascent, which can occur during an emergency free ascent. Because lung over-expansion is potentially life-threatening, beginning diver training emphasizes learning not to hold the breath, and to exhale slowly and continuously during emergency ascents. Divers learn to clear blocked eustachian tubes during ascent at the start of any training.^[10][15][13]

Uncontrolled ascent can increase risk of decompression sickness and lung over-expansion injury even when diving within the no-stop limits of the decompression tables. All entry level training includes skills of controlling buoyancy during ascent, but certification agencies differ in the criteria they use to assess competence. Most require divers to be able to limit ascent rates and achieve neutral buoyancy at a specified depth during ascents without significantly overshooting the target depth. Divers must do this using only a depth gauge or dive computer, but this is a skill that usually requires more practice than recreational, entry level training provides. Divers must vent the buoyancy compensator and dry suit at a rate that provides neutral or slight negative buoyancy at all stages of the ascent, while using a slightly positive buoyancy to assist ascent, and neutral buoyancy to stop.^[21]

Most dry suits are fitted with an automatic dump valve, which divers can adjust to provide an approximately constant volume of gas in the suit. This allows a diver to concentrate on controlling ascent rate via the buoyancy compensator. These skills become critical in decompression stops, and even divers with excellent buoyancy control use aids to reduce risk. Shot liness 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 fallback physical aid. Typically only advanced recreational divers learn to deploy and use surface marker buoys and decompression buoys, but professional divers consider these entry level skills.^[10][15][13]

Equalizing

Clearing the ears on descent

During ascent and descent, gas spaces in both diver and equipment undergo pressure changes that cause the gas to expand or compress, possibly causing damage. Some spaces, such as the mask, release excess gas when pressure breaks the seal to the face, but have to be equalized during compression to avoid mask squeeze. Others, such as the buoyancy compensator bladder, expand until the over-pressure valve opens. The ears usually vent naturally through the eustachian tubes, unless they are blocked. During descent they do not typically equalize automatically, so the diver must equalize deliberately.^[10][15][13]

Loss of breathing gas

Any interruption of air supply for more than a few seconds constitutes a life-threatening emergency. Divers address temporary interruptions due to flooding or dislodging the demand valve by recovering and clearing it. Even ending the dive with an emergency ascent is appropriate in some circumstances. Other solutions involve finding an alternative gas supply, either from the diver's emergency supply, or from another diver.^[8][10]

Emergency air sharing

This 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.^[1] The preferred technique of air sharing is donation of a demand valve from a buddy diver who does not need it.

The standard approach is "octopus donation," in which the buddy offers a secondary octopus demand valve, although this approach is not universal. A variation is for the buddy to offer a primary demand valve, while switching to the octopus. This is often more likely to calm a diver in trouble, because they will know that the gas will be appropriate for the depth.^[1]

Alternatively, two divers can share a single demand valve. This is known as buddy breathing. The technique is no longer taught as widely, although some groups still use it. Divers alternately breathe from one demand valve, each taking two breaths. Because the receiver is likely to initially be out of breath, they may need a few more breaths at first to stabilize.

Once air sharing has been established, the dive terminates, unless the underlying problem can be resolved.^[10][15][13] 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 the skill is quicker to learn.^[1]

Emergency ascents

An emergency ascent is made when no available procedure will allow a dive to continue safely. These are independent ascents, where a single diver either manages the ascent alone, or is helped by another diver who provides gas, propulsion, buoyancy, or other assistance. In an emergency ascent the diver initiates the action intentionally, and chooses the procedure. Ascents that are involuntary or unintentionally uncontrolled are considered accidents.

• In a buoyant ascent the diver is propelled by positive buoyancy. The level of control may vary considerably.
• 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:

• Tethered ascent, where a diver controls the ascent rate by using a ratchet reel with the line secured to the bottom.^[27] This works if weights have been lost at depth and it will not be possible to maintain neutral buoyancy throughout the ascent, and if the diver must decompress. The line must be long enough to reach the surface, and may have to be abandoned.
• Lost-mask ascent, where a 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. Alternatively, the diver can use a tangible indicator to regulate ascent such as a DSMB line, shotline, or anchor line. As a last resort, rising more slowly than the smallest exhaled bubbles is generally safe when decompression stops are not required. The technique of trapping an air bubble at an eye may be used to make occasional instrument checks.
• Lost-buoyancy ascent, where a diver can't establish neutral or positive buoyancy without dropping weights. Either buoyancy compensator failure or dry suit flooding can cause this.

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 skills which could allow divers to safely manage foreseeable emergencies. Others claim that you are more likely to injure divers while training these skills than they are ever likely to actually need them. Accident statistics remain 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 one uses 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. Be aware that 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, divers should not unintentionally inflate the backup bladder, as this may lead to a runaway buoyant ascent.
• Divers can deploy a decompression buoy or lifting bag on a reel, using the line to control depth. There are hazards in deployment, but it is a standard procedure. Once the buoy is at the surface, this allows excellent depth control if the buoy is big enough. The diver can ascend safely by controlling the rate at which the line is wound back on the reel. A ratchet reel is preferable for this procedure, as it will not unwind under load unless the ratchet is released. It is necessary to keep the buoy inflated once the diver has surfaced unless another method for providing positive buoyancy is used.
• Weights can be removed from the diver and fastened to the end of a reel line clipped to the diver. The diver then lets out line to control the ascent. This is the same procedure used to control surfacing when weights are lost. The reel line is tied off to the bottom in the CMAS self-sufficient diver qualification, but using the weights, which may be recovered later by using the reel.

Buoyancy compensator blowup

If there is a continuous gas leak into the buoyancy compensator, the diver can continuously dump excess gas while disconnecting the low-pressure supply hose. If the diver is upright or trimmed even slightly heads-up, this usually allows gas to exit faster than it enters. This 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 neutralize 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 of a catastrophic flood put the diver at risk.^[9]

• 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.^[9]: ch.3
• 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. ^[9]
• 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.^[9]

Dry suit blowup

These possible consequences are similar to those of a BCD blowup, and are managed in similar ways. The instinctive reaction of trying to swim downward 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. If the boots slip off it will be impossible to fin. The diver must ensure that the dump valve is fully open and at the high point of the suit. Then the diver should urgently disconnect the inflation hose. Many suits will release air at the neck or cuff seal if those are the highest points of the suit. It may be necessary to descend after this to compensate for rapid ascent, dumping 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.^[9] 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 for this reason the DIR community considers it safer.

Manifolded twins

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

The Doing It Right movement developed one of the standardized configurations with manifolded twins, specifically for cave exploration. Many technical divers use these procedures. The diver breathes normally from a right-side primary second-stage regulator, mounted on a long hose tucked under at the waist and looped behind the head for quick deployment. A secondary second-stage regulator is carried just beneath the chin, suspended by a breakaway elastic loop around the neck, and supplied from the left-side, first-stage cylinder by a shorter hose. The cylinder valves and manifold isolation valve are normally open:^[18]

• Should another diver experience an out-of-air emergency, the donor diver offers the primary regulator, because it is known to be working correctly. The donor then switches to the secondary regulator. The entire gas supply is available to both divers. The primary hoses are long enough that they are able to separate by a sufficient distance to pass through tight restrictions with the donor ahead of the recipient.^[18]
• If the primary regulator malfunctions, the diver closes the right-shoulder cylinder valve and switches to the secondary regulator. The entire gas supply remains available.^[18]
• If the secondary regulator malfunctions, the diver closes the left-shoulder cylinder valve, continuing to breathe through the primary regulator. The entire gas supply remains available.^[18]
• Cylinder to manifold connection malfunction can result in violent gas loss. Should 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, switching to the secondary regulator if needed. At least half of the remaining gas volume is available once the isolation valve has been closed.^[18]
• Should the left-side manifold connection leak, the diver closes the isolating valve and switches to the secondary regulator to use the gas in the left cylinder while possible, switching back to the primary regulator after the secondary is exhausted. At least half of the remaining gas is available once the isolation valve has been closed.^[18]

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 standardized, 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.^[10][15][13]

Undertaking dives with obligatory decompression stops requires the diver to follow the appropriate decompression profile. This includes maintaining depth for necessary intervals, and ascending 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 any 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

For the basic case of no-decompression open-water diving, which allows a free emergency ascent, breathing gas management requires ensuring sufficient air remains for a safe ascent (plus a contingency reserve). It also considers the possibility of an assisted ascent, where the diver shares air with another diver. Gas management becomes more complex when solo diving, decompression diving, penetration diving, or diving with more than one gas mixture.^[15][13]

A submersible pressure gauge indicates the remaining gas pressure in each diving cylinder. The amount of available gas remaining can be calculated from the pressure, the cylinder internal volume, and the planned reserve allowance. The time that the diver can dive on the available gas depends on the depth, gas mixture, work load, and diver fitness. Breathing rates can vary considerably, and estimates are largely derived from experience. Conservative estimates are generally used for planning purposes. The diver must know when to turn the dive and start the exit and ascent while there is enough gas to surface safely.^[10][15][13]

Navigation

Nav finder and underwater compass – basic underwater navigation tools

Suunto SK-7 diving compass in aftermarket wrist mount with bungee straps

The two basic aspects of scuba navigation are surface navigation to find the dive site, and underwater navigation, to find specific places and to reach the ascent point.

Underwater navigation^[28] includes observing natural features, operating a compass, estimating distance travelled, and setting distance lines used to navigate underwater. Basic navigation is normally taught as part of entry level certification. Advanced underwater navigation is usually part of advanced recreational diver training including rescue training.^[29]

Use of auxiliary equipment

These are generally considered advanced techniques for recreational divers, but basic skills for professional divers.^[5]

• 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.
• Surface marker buoys: a buoy indicates the position of the diver(s) to people on the surface. Control of line tension is important to prevent entanglement and snags.^[30]
• Decompression buoys: inflatable 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 boat crew who recover the divers. Deployment skills include controlled inflation, paying out line while avoiding snags and jams, maintaining appropriate depth control during deployment, and control of line tension during ascent.
• Distance lines: line carried on a reel or spool and laid out as a guide for the return.^[31]
• Shot lines: these allow divers to follow the line, reach the bottom at the right place, and ascend to the surface where the crew expects them. The choice, rigging, and deployment of shotlines to suit the dive profile and environmental conditions is another skill.^[32]
• Nitrox diving: the safe use of nitrox mixtures depends on remaining at depths where the partial pressure of oxygen is within acceptable limits. This requires knowledge of the oxygen fraction, which is necessary to calculate the maximum operating depth.^[13]
• Switching gases for accelerated decompression: this requires positive identification of the appropriate breathing gas in use at any time, as decompression mixtures are typically dangerous to breathe at the dive's maximum depth.^[18]
• Helium-based breathing gases (trimix and heliox): these gases are used to reduce nitrogen narcosis at greater depths, and advanced skills of dive planning, gas switching, dive monitoring and decompression are needed. These are considered advanced skills for both professional and recreational diving.
• Lifelines and buddy lines: the use of a lifeline is a team skill. The diver's line tender controls the deployment and recovery of the line, and ensures that slack is kept to a minimum. The diver informs the tender by line signals when to let out more and when to pull in. The diver must avoid getting the line snagged on underwater obstacles, or inadvertently passing under things which may prevent a direct ascent to the surface. Buddy lines may be used to prevent buddy separation in darkness or low visibility. The divers must be able to release the lines on their own end in an emergency, and must avoid snagging lines on underwater obstacles.^[8]

Logging the dive

The diver's log is a record of the diving experience, and professional divers are required to keep theirs up to date and correctly filled in. The diving supervisor is usually required to countersign each page of a professional diver's logbook to maintain it for legal use. Recreational logbooks are generally not required outside of training, but are recommended. They may be inspected by service providers as evidence of experience when applying for further training or applying to participate in dives requiring specific experience. Electronic logs are becoming more popular, and newer dive computers will download the dive data they collect automatically in a reasonably user-friendly format. The user must still input some information, such as the venue, dive buddy, and what was seen and done on the dive.

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 such as in a swimming pool, and open-water dives, followed by assessment of knowledge and skills.

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. Other instructors and dive schools provide longer and more thorough training.

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 offer courses that satisfy the standards of a certification organization.

Professional and technical 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 organizations ^[11][12][5] Professional and technical diver training standards require a significantly higher skill level than recreational certification. Professional or technical training may include confidence training or stress training, where simulated emergencies are enacted, or unlikely contingencies are simulated, to develop divers' confidence in their abilities to safely manage contingencies. The amount of time spent on skill and confidence development is generally proportional to the length of the training program, 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 over-learned to the point of integrating with the diver's 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 of time, such as six months, but the actual need for relearning is not so easily identified. Checkout dives are a common requirement of service providers for divers without a convincing logbook showing recent appropriate experience.