Human factors in diving equipment design

Breathing apparatus must allow the diver to breathe with minimum added work of breathing, and minimise additional dead space. It should be comfortable to wear, and not cause stress injury or allergic reactions to component materials. It must be reliable and should not require constant attention or adjustment during a dive, and if possible performance should degrade gradually in the event of malfunctions, allowing time for corrective action to be taken with minimum risk.

Holding the scuba mouthpiece between the teeth can case jaw fatigue on a long dive. Jaw loads are a combination of apparent weight, or buoyancy, of the second stage, drag forces due to water or diver movement, hose forces, and inertia due to movement of the head. Smaller second stages can reduce some of these loads.

Allergic reactions to materials in contact with lips, gums and tongue are less common with silicone rubber and other hypoallergenic mouthpieces than with natural rubber, which was commonly used in older equipment.

Some divers experience a gag reflex with mouthpieces that contact the roof of the mouth.(DAN Europe surveys), but this can be corrected by fitting a different style mouthpiece.

Purging the second stage is a useful function to clear water from the interior. The purge button should function only when pressed, sufficiently to clear the chamber without blowing its contents down the diver's throat. There is a risk of regulator freeze when purging in cold water. Purge flow rate should not be unnecessarily high.

Cracking pressure is the pressure difference over the diaphragm needed to open the second stage valve. This should be low but not excessively sensitive to water movement. Once open, with gas flowing, the gas flow often produces a slight increase in the pressure drop in the demand valve which helps hold it open during inhalation, effectively reducing the work of breathing, but making the regulator more susceptible to free-flow. This can be adjusted by user-operated sensitivity settings in high-performance models.

The exhaust valve should offer the minimum resistance to exhalation, including a minimum opening pressure difference, and low resistance to flow through the opening. It should not easily block or leak due to foreign matter such as vomit.

Exhaust gas flow should not be unduly distracting or annoying to the diver in normal diving posture. flow should be directed away fom the faceplate of the mask, which is a combined effect of DV and mask design. Bubbles are noisy, and flow directly over the ears is undesirable, though a neoprene hood can help muffle the noise

Breathing effort should be reasonable in all diver postures. The diver can rotate in three axes, and may need to do so for a significant period including several breaths from any arbitrary orientation. The DV should continue to function correctly throughout the maneuvers, though some variation in breathing effort is inevitable.

Rebreather equipment removes carbon dioxide from exhaled gas and replaces the oxygen, then let's the diver breathe the gas again. This can be done in a self contained system carried out by the diver, in a system where the scrubber is carried by the diver and gas is supplied from the surface, or where the gas is returned to the surface for recycling, and the power to circulate gas in the loop can be lung power of the diver, energy fom the supply gas, or externally powered booster pumps. Scuba rebreathers tend to be circulated by lung power and the work of breathing can be a significant part of diver effort at depth.

Diving suits are worn for protection from the environment. In most cases this is to keep the diver warm, as heat loss to water is rapid. There is a trade-off between insulation, comfort, and mobility.

Wetsuits rely on a good fit to work effectively. The insulation relies on the low heat conductivity of the gas bubbles in the neoprene foam of the suit slowing heat loss from the water inside the suit to the water outside. If the water inside the suit can be flushed out and replaced by cold water, this insulating function is bypassed. Movement of the diver tends to move the water in the suit around mostly where it is present in thick layers, and if this water is forced out it will be replaced by cold water from outside. A close fit reduces the thickness of the of the layer of water and makes it more resistant to flushing. Semi-dry suits attempt to address this issue by making it more difficult for water to enter and leave the suit. They are also most effective when close fitting. A different problem can be caused by a suit which is too tight. If it restricts breathing this can aggravate work of breathing issues due to depth and gas density, which may not be noticeable at the surface, but can become a serious problem at depth. The insulation of foamed neoprene is mostly in the gas bubbles, which are compressed by the ambient pressure, so the insulation reduces with depth. Heavier duty neoprene compresses less but is less flexible, so it may be a better insulator, but may also encumber the diver more. A number of inner surface finishes, from bare rubber to plush have been tried. Each has advantages some of the time.

Dry suits rely on staying dry inside and maintaining a limited volume of gas distributed through the thermal undergarments. The volume of gas needed is fairly constant, but it expands and contracts in response to the ambient pressure variations as the diver changes depth. Squeeze and overinflation are dangerous. Suit squeeze is caused by insufficient gas in the suit, and will reduce flexibility of the suit and restrict the diver's freedom of motion. This could prevent the diver from reaching critical equipment in an emergency. Gas is added manually by pressing a button to open the inflation valve , which is customarily sited in the central chest area where it can easily be reached by both hands and is clear of the harness and buoyancy compensator. High flow rates are neither necessary nor desirable, as they could lead to overinflation, particularly if the valve sticks open due to freeze. Overinflation will cause an uncontrollable rapid ascent if not corrected. Dumping of suit gas is only possible in a restricted range of orientations, where the dump valve is above the gas to be dumped.

Problems caused by flooding of dry suits:

• Loss of thermal insulation and body chilling will generally make it necessary to abort the dive, but an immediate ascent may not be possible, or safe.
• Loss of buoyancy can be mitigated by deploying a DSMB, use of a large volume buoyancy compensator, or small lifting bag, or ditching ballast weight. It depends on details of the leak whether the suit will retain any gas, and where such gas can be held.
• The added mass and bulk of water in the suit will make exit from the water difficult. Mitigation is possible by fitting ankle dumps or cutting the suit at each ankle to allow the water to drain.

The ability of the diver to reach the cylinder valve can be constrained by the suit and personal joint flexibility of the diver. Back-mount configurations with valves up are difficult for many divers to reach to open or close the valves. This can cause delays in reacting effectively to some emergencies. This is partly a suit issue and partly a cylinder configuration issue.

The combination of suit and helmet can constrain movement more than either on its own. Dive suits can produce physiological strain in the user and considerble effort may be necessary to overcome the encumbrance of the suit. This can result in longer periods required to complete complex tasks, in an environment which is already non-conducive to dexterity or heavy labour. This was particularly noticeable on the standard diving suit.[7]

Dexterity problems with manipulators on atmospheric diving suits reduce their effectiveness for many tasks. The joints of atmospheric suits allow walking but are not suitable for swimming.

The surface-supplied diver's harness is an item of strong webbing, and sometimes cloth, which is fastened around a diver over the exposure suit, and allows the diver to be lifted without risk of falling out of the harness.[8]^:ch6 It also provides support for the bailout gas cylinder, and may carry the ballast weights, a buoyancy compensator, the cutting tool, and other equipment. Several types are in use. Recreational scuba harness is mainly used to support the gas cylinders, buoyancy compensator and often the weights and small accessories, but is not normally required to function as a lifting harness. These functions require distribution of the loads to the diver securely and comfortably.

Weight distribution of the harness can cause discomfort and nerve pressure injury out of the water.[9] Weight of the harness including cylinders can be problematic for putting the set on for some divers.[1]

A harness which must support side mounted cylinders should not unduly encumber the diver or make it difficult to access critical equipment like buckles, weights, cylinder valves, demand valves and pressure gauges. More than one cylinder may be mounted at each side.

In professional diving the harness must also function as a lifting harness, and must be strong enough to support the diver and attached equipment if lifted by the lifeline or umbilical, without causing direct or indirect injury. Some discomfort is considered acceptable as this is an emergency procedure.[3]

Buoyancy control by adjustable volume of gas in an environment where pressure varies rapidly with depth is inherently unstable and requires continuous monitoring and control input from the diver. The instability is proportional to the volume of the gas required for neutral buoyancy.

• For the least sensitivity to depth variation, the volume of the of gas required for neutral buoyancy should be kept to a minimum averaged over the dive. This implies ballasting that is just sufficient to allow neutral buoyancy at the minimum weight of diver and equipment that is reasonably likely to occur during the dive. The obvious case is where a delay pushes the diver further into decompression obligation than planned, and it my be necessary to decompress at a shallow stop with the last remaining gas. This is not a time to be struggling to stay down, using more gas than otherwise necessary with the effort. The weight change in most dives is due to gas use, and unless equipment is lost or abandoned, the maximum weight change is the consumption of all the gas in all the cylinders carried. This can be calculated easily. The diver needs enough buoyancy volume to remain comfortably afloat before the dive starts. At the end of the dive there will be more buoyancy in reserve as a result of the gas consumption. Any large amount of reserve volume in the buoyancy compensator has potential for contributing to an uncontrolled buoyant ascent.[10]
• Gas in the dry suit is primarily intended for thermal insulation. The additional buoyancy is a normally undesirable but unavoidable side effect. When there is sufficient gas to relieve compression of the under-suit, any excess will gather at a local high point and contributes little to insulation. Removal of excess gas from the suit is only possible when the path of the excess gas to the dump valve or other vent point is continuously upward.[11] Automatic dump valve position is conventionally on the upper left sleeve, clear of the harness, but in easy reach of the diver at all times and at a natural high point for the most useful and likely trim positions for swimming, work, and particularly ascents. Other less popular options are on the forearm or cuff dumps, and most cuff seals will vent if raised high enough.[11] Any upward excursion by the diver will cause the gas in the suit to expand in proportion to the pressure change. Diver trim which guarantees easy venting of the suit during a normal, head-up ascent is inefficient for horizontal propulsion. The diver in a position trimmed with feet high is at risk of losing control of buoyancy in the event of a significant upward excursion.[11] This can be mitigated by ankle dump valves, but they are not fitted as standard equipment and are not used by most divers. Finning downwards against the buoyancy of excess water gas trapped in the legs of a dry suit is energetic at best and futile at worst. The problem can be exacerbated if the legs are baggy at the ankles and the boots are loose, as if they slip off the feet all control of the fins is lost.[11] Gaiters and ankle straps can reduce the volume of this part of a suit.
• The dry suit should not unduly restrict the diver's freedom of movement, but should not be excessively baggy, which can trap gas in unwanted places, and can increase drag as well as causing buoyancy and trim problems. The inherently baggy rubberised canvas standard diving dress was available with lace up legs to manage this problem. This option is no longer available, and divers must have suits tailored for a good fit.[11]

Female divers are reported to have more difficulties with buoyancy and trim. This may be a consequence of a buoyancy distribution not well catered for by most harness, buoyancy compensator and weighting systems, possibly exacerbated by dry suit buoyancy distribution. Many manage with available equipment, but it may take longer to learn to use less ergonomically matched equipment effectively. A similar problem is reported with unusually small divers.[1]

Fin design is a compromise between propulsive efficiency and maneuverability. Monofin s are the equipment of choice for deep apnea diving and both speed and endurance competition. Breath hold spearfishers need more maneuverabity, while retaining the best reasonably practicable efficiency, and they mostly choose long bifins. Professional and recreational scuba and surface-suppied divers will sacrifice more efficiency for better maneuverabity. Comfort issues, stressing of muscles and joints, particularly with less physically fit divers, may bias the choice towards softer fins that produce less thrust and maneuverabity. Divers needing maximum maneuverabity will usually choose stiff paddle fins which can be effective for reversing out of a tight spot but are inefficient for cruising using flutter kick. These fins work well with the frog kick, which is also less likely to shed vortices downward and disturb silty bottoms.[10]

Experimental work suggests that larger fin blades are more efficient in converting diver effort to thrust, and are more economical in breathing gas for similar propulsive effect. Larger fins were perceived to be less fatiguing than smaller fins.[12]

The mask must form a watertight seal around the edges to keep water out of the mask, regardless of the attitude of the diver in the water. Fit of mask affects the seal and comfort and must account for variability of face shapes and sizes. This is achieved for half masks by the very wide range of models available, but in spite of this some faces are too narrow, or noses too large to fit comfortably. This is less of a problem with full-face masks and less again with helmets, but other problems affect these, like overall head size, and neck length and circumference, so there is still a need for adjustment and a few size options. Ability to equalise mask space and ears is relatively easy to accomplish with half-masks, where the diver can usually pinch the nostrils closed through the rubber of the mask skirt. Helmets and most full-face masks do not allow the diver finger access to the nose, and various mechanical aids have been tried with varying levels of comfort and convenience.

The field of vision of the diver is reduced by the helmet or mask. Peripheral vision is particularly reduced in the lower areas due to the bulk of the demand valve. Helmet design is a compromise between low mass and inertia, with relatively small interior volume and viewports affording restricted field of vision, and large viewports with large interior volume. Siting the viewport close to the face helps provide a better view but is complicated by the need for sufficient clearance in front of the face for a wide range of divers. Curved viewports can introduce visual distortions that can reduce the effectiveness of the diver at judging distance, and almost all viewports are made flat. Even a flat viewport causes some distortion, but it takes relatively little time to get used to this, as it is always the same.

Divers who need optical correction have choices. Contact lenses can be worn under all types of masks and helmets. Regular spectacles can be worn in most helmets, but can not be adjusted during the dive. Corrective lenses can be glued to the inside of half-masks and some full-face masks, but the distance is from the eyes may not be optimal. Bifocal arrangements are available, mostly for far-sightedness, and are popular with older divers to allow them to read their instruments. Defogging of bonded lenses is the same as for plain glass. Some dive computers have relatively large font displays, and adjustable brightness to suit the ambient lighting.

The inside surface of the viewport of a mask or helmet tends to be prone to fogging, where tiny droplets of condensed water disperse light passing through the transparent material, blurring the view. Treating the inside surface with a defogging surfactant before the dive can reduce fogging, but it may occur anyway,and it must be possible to actively defog, either by rinsing with water or by blowing dry air over it until it is clear

Security of fit. Masks held in place by adjustable straps can be knocked off or moved from the correct position, allowing water to flood in. Helmets are more securely attached.

Dead space. Important for full-face masks and helmets, not relevant to half masks

Internal volume: large internal volume half-masks tend to float up against the nose, which is uncomfortable, and becomes painful over time. The trend is towards low volumes and wide fields of vision, which requires the viewport to be close to the face. This makes it difficult to design a frame and nose pocket that will accommodate the full range of face shapes and sizes. Wide and high-bridged noses and very narrow faces are a particular problem, but the range of masks available will provide for most people. Full-face masks, helmets. The weight of a lightweight helmet in air is about 15 kg. Free-flow helmets compensate for a potentially large dead space to by a high gas flow rate. They tend to be heavier, and usually rest on the shoulders, so do not move with the head. As there is no need for an oro-nasal inner mask, they usually have a large viewport or several viewports to compensate for the fixed position.

There is a conflict between insulation and dexterity, and the reduction of tactile sense due to thick gloves or chilled hands. The diver can tolerate greater heat loss through the hands if the rest of the diver is warm, but in some cases such as diving in near freezing water or where the air temperature at the surface is below freezing, the risk of frostbite or non-freezing cold injury necessitates the use of gloves most of the time. Suitable design of equipment can help make the work of correct operation easier. For safety critical equipment, dexterity can make the difference between managing a problem adequately, or a situation deteriorating beyond recovery. Simple, large control interfaces such as oversize knobs and buttons, large clips, and tools that can be gripped by a heavily gloved hand can reduce risk significantly.

In very cold water there are two problems causing loss of dexterity. The chilling of hands and fingers directly causes loss of feeling and strength of the hands, and thick gloves needed to reduce chilling also reduce the sensitivity of the fingertips, making it more difficult to feel what the fingers are doing, and also making the fingertips wider and thicker and a poorer fit to components designed to be used by the naked hands. This is less of a problem with gloves where the fingertips have a reduced thickness of cover over the contact surface, but few gloves have this feature. The fingertips of the thumbs and forefingers are most affected, and also wear out faster than the rest of the glove. Some divers wear a thinner, tougher, work glove under the neoprene insulating glove, and cut the tips off the thumbs and forefingers of the neoprene gloves to expose the inner gloves as a workable compromise.

Long term grip strength is reduced by fatigue. If the glove requires effort to close the hand to hold an object, this will eventually tire the muscles involved, and grip will weaken sooner than when affected by cold alone. This is mitigated by gloves with a preform to fit a partly closed hand, and by more flexible glove materials. With dry gloves the inner glove can be tailored to be thicker where the insulation will not interfere with dexterity, while the outer, watertight glove can be chosen to provide the necessary toughness and wear resistance.

Weighting systems are needed to compensate for the buoyancy of the diver and buoyant equipment, mainly thermally insulating diving suits. The load distribution of buoyancy and ballast affect diver trim.

• Weight-belts of conventional design are fastened around the waist and load the lower back when the diver is trimmed horizontal. This can cause lower back pain, particularly when heavy to compensate for the buoyancy of a dry suit with thick undergarments. Weights supported by the harness distribute the load more evenly.
• Ankle weights used to improve trim add inertia to the feet, which must be accelerated and decelerated with every fin stroke, requiring additional power input for finning and reducing propulsive efficiency.
• The facility to shed ballast weight is considered a safety feature for scuba diving as it allows the diver to achieve the best positive level of buoyancy in an emergency, but inadvertent loss of ballast when the diver needs to control ascent rate is itself an emergency that can cause decompression illness.
• The need to pull weights clear when ditching in some orientations is additional task loading in an emergency. Getting the weight belt caught up in the harness can compound the diver's problems if the need to establish positive buoyancy is urgent.
• Ditching all weights may be appropriate for some emergencies, but in others it just changes the nature of the emergency.

Scuba diving with back-mounted manifolded twins and sling mounted decompression gas

When using multiple gas sources with multiple gas mixtures it can be critically important to avoid confusion of gas mix in use and pressure remaining in the various cylinders. The cylinder arrangement must allow access to cylinder valves when in the water. Use of the wrong gas for the depth can have fatal consequences without warning. High task loading for technical divers can distract from checking the mix when switching gas. It is important to check that each cylinder is what it should be and is mounted in the right place, to positively identify the new gas at each gas switch, and to adjust the decompression computer to allow for each change in gas for correct decompression. Some computers automatically change based on data from integrated pressure transducers, but still require correct pre-dive setting of gas mixes.

Classical configurations:

• Back mounted single cylinder is stable on the diver in and out of the water, compact and acceptably balanced. Some divers have difficulty reaching the valve knob, which is behind the back, particularly when the cylinder is mounted relatively low on the harness, or the suit is thick or tight.
• Back mounted twins with isolation manifold are stable in and out of the water, compact, heavy, and acceptably balanced for most divers. Some divers have difficulty reaching the valve knobs behind the back. This can be a problem in a free-flow or leak emergency, where much gas can be lost due to inability to access knobs quickly to shut down. Weight and buoyancy distribution may be top heavy for some divers.
• Back mounted independent doubles. Gas is not available if a cylinder valve must be shut down. The side-mount emergency options (feather breathing, regulator switch) are not available.
• Flexible valve knob extensions on back mount sets are not very satisfactory and not very reliable,and are an additional snag risk.[10]
• Pony cylinders for bailout or decompression gas clamped to the main gas supply put the valve where it cannot be seen, and may be difficult to reach. They are reasonably compact and manageable out of the water.
• Sling mount bailout and decompression cylinders allow easy access to the valve and visual check of labels on during gas switching. Up to four sling cylinders are reasonably manageable with some practice.

Alternative configurations:

• Inverted single or manifolded twin cylinders with valves at the bottom which are more reachable, but more vulnerable to impact damage. Custom hose lengths are needed, and hose routing will be different. This arrangement works for firefighters, and has been used by military divers. Weight and buoyancy distribution may be bottom heavy for some divers, and may adversely affect trim.
• Side mount provides much easier valve access, and it is possible to see the top of each cylinder to check the label when switching gas, which allows confirmation of correct gas. It is possible to hand off a cylinder when donating gas to another diver, so a long hose is not needed. The configuration has a lower profile than back mount, and it is possible to unclip cylinders for access of small openings. The configuration is clumsy out of water for crossing uneven terrain, and it can be difficult to mount more than one large cylinder each side when kitting up. Carrying six cylinders is probably more difficult when all are side mounted than when two are back mounted. Lateral shift of centre of gravity as gas is consumed is noticeable. Buoyancy distribution of cylinders is top heavy due to the regulator and valve. Buoyancy changes when handing off or staging must be compensated, but this applies to all configurations. Buoyancy changes due to gas consumption can cause cylinders to hang awkwardly if the bottom ends start to float. Stuffing hoses under bungees on the cylinder can be awkward when the hands are full of other equipment like camera and reel, but necessary to avoid dangling regulators snagging on the environment. Adding buoyancy to the top of a cylinder to improve cylinder trim makes it necessary for the diver to carry extra weight.

Bailout sets for surface-supplied diving are usually back mounted to keep the driver's arms and front clear for working.

Diving instrumentation may be for safety or to facilitate the task. The safety-critical information such as gas pressure and decompression status should be presented clearly and unambiguously.

Lack of standardised dive computer user-interfaces can cause confusion under stress. Computer lock-out at times of great need is a potentially fatal design flaw. The meaning of alarms and warnings should be immediately obvious. The diver should be dealing with the with the problem, not trying to work out what it is. Displays should allow for variations in visual acuity, and be readable with colour-blindness. Ideally critical displays should be readable without a mask, or provide for safe surfacing without a mask. There should not be too much distracting information on the main screen, and return to the main screen should be automatic by default, or auxiliary screens should continue to display critical decompression data.

Straps of wrist-mounts should be adjustable to suit ambidextrous wearing with a range of suits, and a single point strap failure should not result in loss of the instrument. Straps should be secure against sliding off the wrist in the event of suit compression when worn on the fore-arm.

Control consoles represent a concentrated source of information, and a large potential for operator error.[7]

Head-up displays a are sometimes used to alert the diver to changes that may require prompt response, usually related to gas concentrations in a rebreather loop.

Audible alarms and warnings are commonly provided on dive computers, often as user options. These generally alert the diver to ascent rate and decompression ceiling violations, so they can be corrected promptly. They can also be used to inform the diver when the no-stop limit is reached, or for gas integrated units, when pressure is getting low.

The primary function of diver cutting tools is to deal with entanglement by lines or nets. Preferably the tool should be accessible to both hands, and should be capable of cutting the diver free from any entanglement hazard predicted at the dive site. Many divers carry a cutting tool as standard equipment, and it may be required by code of practice as default procedure.

When entanglement risk is high, backup cutting tools may be required.

Dive lights may be needed to compensate for insufficient natural illumination or to restore colour. They may be carried in several ways depending on the purpose.

Head mount lights are used by divers who need to use both hands for other purposes. With a head mount there is a greater risk of dazzling other divers in the vicinity, as the lights move with the diver's head, and this arrangement is more appropriate for divers who work or explore alone. Helmet mounts are appropriate for illuminating work which is monitored via a helmet mounted closed circuit video camera. A wide beam allows good illumination of the field of vision of diver and video camera.

Hand-held lights are directable by the diver independently of the direction the diver is facing and do not require any special mounting equipment, but occupy a hand, and are at risk of being dropped unless clipped on. They are most suitable for incidental lighting, and where precise direction is useful. Beam width and intensity preference depends on the application.

Glove or Goodman handle mount allows precise direction and other use of the hand, but not always both at the same time. Canister lights allow the light head to be held in optional ways, and the cable prevents the light from falling far if dropped, and can be looped over the neck to suspend the light to illuminate close-up work, but is an additional encumbrance. It is possible and fairly common to carry more than one of these options. Where light is important for safety, the diver will carry backup lights.

There are also special purpose light mounts, such as video and camera modelling lights, which must illuminate where the lens points, DPV headlights, internal and external bell lights, and ROV lights, which can be used to illuminate a work site to help the diver.

A buddy line is a line or strap physically tethering two scuba divers together underwater to prevent separation and as a means of communication in low visibility conditions.[13] It is usually a short length and may be buoyant to reduce the risk of snagging on the bottom. It doesn't need to be particularly strong or secure, but should not pull free under moderate loads, such as line signals. Divers may communicate by rope signals, but may just use the line to attract attention before moving closer and communicating by hand signals. The disadvantage of a buddy line is an increased risk of snagging and entanglement, and the risk is increased with a longer or thinner line. Divers may need to disconnect the line quickly at either end in an emergency, which can be done via a quick release mechanism or by cutting the line, both of which require at least one free hand. A velcro strap requires no tools for release and can be released under tension.

Clips and attachment points should be reliable and must generally be operable by one hand with gloves suitable for the water temperature, without needing to see what is being done, as it may be dark, low visibility, or out of view. Single-hand operation is necessary where only one hand can reach, and is always preferable, as the other hand may be in use for something important at the time. While unlikely, it is possible for most types of clip to jam closed, and if this may endanger the diver it should be possible to use an alternative method to disconnect, which does not involve special tools. Cutting loose using the diver's cutting tool is the standard.[10]

A reliable clip is one that does not allow connection to anything or disconnection by accident, but requires specific action by the operator to clip or unclip. Unreliable clips may cause loss of equipment or entanglement. Bolt snaps and screw-gate carabiners are examples of clips with a reputation for reliability.[10] The carabiners are more secure, and may be load rated, but are less convenient to operate. Carabiners are approved for attaching the umbilical to a surface supplied diver's harness.[3]

Two divers scootering with heavy duty DPVs

The most efficient position for towing behind is when the wake of the thruster bypasses the diver. This is usually achieved by using a tow leash from the DPV to a D-ring on the lower front of the harness, and a handle on top of the DPV with a dead-man switch, to turn off the power of the DPV as soon as the diver lets go of the handle.

The Nikonos V, a film camera designed specifically for underwater use

Underwater cameras are usually popular models encased in a watertight pressure housing, though there have been a few notable exceptions, such as the Nikonos and Sea & Sea ranges, in which the camera body was the pressure housing. Controls are generally operated by movable links penetrating the watertight case, each requiring reliable seals, and each a potential leak. Compact and lightweight camera bodies with multiple controls packed into a small space tend to transform into bulky, heavy and expensive units when repurposed for moderately deep diving. Controls must be operable using thick gloves in cold water. Lighting varies depending on conditions, subject, lens, and other variables, and the use of modelling lights and flash is common. These are usually supported by a camera tray and arms which allow the lightning to be aimed. This can make a camera setup very bulky and it may require most of the diver's attention. At the other extreme, a head mounted sports video recorder may be triggered at the start of the dive and thereafter ignored until it is time to stop recording.

For most underwater photography, a camera that is close to neutral buoyancy will be easier to handle and have less disruptive effect on diver trim. Strobe arms incorporating incompressible buoyancy compartments are the preferred system, as they do not need to be adjusted for changes of depth.

Several manufacturers produce compact cameras which are inherently water resistant to about 10 msw, and underwater housings rated to around 40 msw, which are small enough to fit into a pocket, have a fairly large zoom range, and a large preview screen. Automatic focusing allows divers with imperfect vision to take acceptable photographs, and a minor leak is more an annoyance than a catastrophe.

A cave diver running a distance line into the overhead environment to facilitate a safe exit

Distance lines are used for underwater navigation where it is either essential to mark the route out of the overhead environment, or to or to return to a specific point. Lines are deployed from reels and may be left in place or recovered on the return. Reels should be easy to use and lockable to prevent unintentionally unrolling, and have sufficient friction to prevent overwinds.

Line markers are generally used on permanent guidelines to provide critical information to divers following the line. The slots and notches provided are used to wrap the line to secure the marker in place. A simple passage of the line through the enlarged area at the base of the two slots will allow the marker to slide along the line, or even fall off if brushed by a diver. To more securely fasten the marker, an extra wrap may be added at each slot. The basic function of these markers is fairly consistent internationally, but procedures may differ by region, and between teams. The protocol for placement and removal should be well understood by the members of a specific team. Temporary line markers are only for the use of the team who placed them and are removed during exit to avoid littering a cave system with irrelevant and potentially confusing information.[14]

Design and construction of pressure vessels for human occupancy are regulated by law, safety standards, and codes of practice. These specify safety and ergonomic requirements, Lock opening sizes, internal dimensions, valve types and arrangement, safety interlocks, pressure gauge types and arrangements, gas inlet silencers, outlet safety covers, seating, illumination, breathing gas supply and monitoring, climate control and communications systems are covered, as well as structural strength, permitted materials, over-pressure relief, testing, fire suppression and periodical inspection.

Closed bell design must allow for access by divers wearing bailout sets appropriate for the depth. The amount of gas in the bailout set is calculated for a return rate of 10 metres per minute from the reach of the excursion umbilical, At greater depths this may require twin sets of high pressure cylinders. It must also be possible for the bellman to hoist an unconscious diver through the lock. The internal volume must include enough space for divers and equipment including racks for the excursion umbilicals and the bell gas panel. On-board gas cylinders, emergency power packs, tools and hydraulic power supply lines do not have to be stored inside. Access while underwater is through a lock at the bottom, so that the internal gas pressure can keep the water out. This lock can be used for transfer to the saturation habitat, or a side lock can be provided, which does not need to allow passage with harness and bailout cylinders as these are not carried into the habitat area and are serviced at atmospheric pressure.

The splash zone is the region where the bell passes through the surface of the water and where wave action and platform movement can cause the bell to swing around, which can be uncomfortable and dangerous to the occupants. To limit this motion a bell cursor may be used.

A bell stage is a rigid frame which may be fitted below a closed bell to ensure that even if the bell is lowered so far as to contact the clump weight or the seabed, there is enough space for the divers to get in and out through the bottom lock. If all the lifting arrangements fail, the divers must be able to shelter inside the bell while awaiting rescue.

Medical and supply lock outer doors are fitted with safety interlock systems which prevent them from being opened with internal pressure above atmospheric to avoid the possibility of human error allowing them to be opened while the inner lock is not sealed.

Each compartment of a hyperbaric system for human occupation has an independent separate externally mounted pressure gauge so that it is not possible to confuse which compartment pressure is being displayed. Where physically practicable, lock doors open towards the side where pressure is normally higher, so that a higher internal pressure will hold them closed and sealed.

Internal diameter of hyperbaric living compartments and deck decompression chambers is constrained by codes of practice for reasonable comfort for the occupants. For emergency transfer, there may be overriding logistical constraints on size and mass.

Task loading, nitrogen narcosis, fatigue, and cold can lead to loss of concentration and focus, reducing situation awareness. Reduced situation awareness can increase the risk of a situation that should be manageable developing into an incident where damage, injury or death may occur.

(Multiple contingencies, incident pit. Reliance on team members who may not be in the right place, Ability of the diver to respond reliably to foreseeable incidents.)

A diver must be able to survive any reasonably foreseeable single equipment failure long enough to reach a place where longer term correction can be made. The solo diver can not rely on team redundancy, and must provide all the necessary emergency equipment indicated as necessary by the risk assessment, whereas a team can in many cases reduce risk to an acceptable level by distribution of redundancy among its members. However the effectiveness of this strategy is tied to reliability of team cohesion and good communication.

No gender-specific traits have been identified which require design of tasks and tools exclusively for female divers. Fit of diving suits must be tailored to suit the range of human shapes and sizes, and most other equipment fits all sized, is adjustable to suit all sizes, or is available in several sizes. A few items are designed specifically for female use, but this is often more a fine tuning for comfort or cosmetic styling than an ergonomically functional difference.[1]

Female divers are reported, on average, to experience greater difficulty in performing five tasks of recreational diving: Carrying heavy equipment on shore, putting on the scuba set, underwater orientation, underwater balance and trim and descent. The first two are related to lifting large, heavy and bulky equipment. Balance and trim could be related to buoyancy and weight distribution, but insufficient data is available to specify a remedy.[1] Buoyancy compensators may have been optimised for male buoyancy characteristics.

There is a relative growth in the older sector of recreational diver demographics. Some are newcomers to the activity and others are veterans continuing a long career of diving activity. They include older female divers. More research is needed to establish the implications of age and sex related variations on human factors and safety issues.[1]

• Human factors in diving safety – The influence of physical, cognitive and behavioral characteristics of divers on safety
• Human factors and ergonomics – Application of psychological and physiological principles to engineering and design
• History of underwater diving – History of the practice of descending below the water's surface to interact with the environment
• Diving equipment – Equipment used to facilitate underwater diving