Human factors in diving equipment design


The underwater diver relies on various items of diving and support of equipment to stay alive and in reasonable comfort and to perform the planned tasks during a dive. The design of the equipment can strongly influence its effectiveness in performing the desired functions.
Divers vary considerably in anthropometric dimensions, physical strength, joint flexibility etc. Diving equipment should allow as full a range of function as reasonably practicable, and should be matched to the diver, the environment and the task. The interface between equipment and diver can strongly influence functionality. Diving support equipment is usually shared by a wide range of divers, and must work for them all.
The most difficult stages of a dive for recreational divers are out of water activities and transitions between water and the surface site such as carrying equipment on shore, exiting from water to boat and shore, surface swimming, and dressing into the equipment. Safety and reliability, adjustability to fit the individual, performance, and simplicity were rated the most important features for diving equipment by recreational divers.
The professional diver is supported by a surface team, who are available to assist with the out-of-water activities to the extent necessary to reduce the risk associated with them to a level acceptable in terms of the governing regulations and codes of practice. This tends to make professional diving more expensive, and the cost tends to be passed on tho the client.

Breathing apparatus

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., 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

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:
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.
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.

Harness

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. 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.
Weight of the harness including cylinders can be problematic for putting the set on for some divers.
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.

Buoyancy control equipment

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.
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.

Fins

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, so this style of fin is popular for cave and wreck penetration diving.
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.
Attachment to the foot follows two basic options. An integral foot pocket enclosing the heel, or an open heeled foot pocket with an elastic heel strap, which may be of adjustable length. Both systems allow full mobility of the ankle joint for bi-fins, but limit the motion for monofins.
Full foot-pockets are softer and more comfortable on bare feet, and spread the loads more evenly, but are often unsuited to wearing over a thick or hard-soled boot. Fin retainers may be necessary for security if the fit is a bit loose. Open heel foot pockets can be matched with foot width when wearing a boot, and the heel-strap selected or adjusted to fit. Fin straps may be of fixed or adjustable length. Fixed length straps are always the right length for a single user, and have fewer snag points, moving parts and other things that can fail. Adjustable straps are quickly adaptable to feet of different users.

Masks and helmets

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. Spherical port surfaces are generally used in recent atmospheric suits for structural reasons, and work well when the interior volume is large enough. They can be made wide enough for adequate peripheral vision.
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 from the eyes to the lenses 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.
Open circuit breathing apparatus produces exhalation gas bubbles at the exhaust ports. Free-flow systems produce the largest volumes, but the outlet can be behind the viewports so it does not obscure the diver's vision. Demand systens must have the second stage diaphragm and exhaust ports at approximately the same depth as the mouth or lungs to minimise work of breathing. To get consistent breathing effort for the range of postures the diver may need to assume, this is most practicable when the exhaust ports and valves are close to the mouth, so some form of ducting is required to direct the bubbles away from the viewports of helmet or mask. This generally diverts exhaust gases round the sides of the head, where they tend to be rather noisy as the bubbles rise past the ears. Closed circuit systems vent far less gas, which can be released behind the diver, and are significantly quieter. Diffuser systems have been tried, but have not been successful for open circuit equipment, though they have been used on rebreathers, where they improve stealth characteristics.
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. There is no supply of dry air to a half-mask, but rinsing is easy and need only momentarily interrupt breathing. A spitcock may be provided on standard helmets for rinsing. Demand helmets generally have a free-flow supply valve which directs dry air over the inside of the faceplate. Full-face masks may use either rinsing or free-flow, depending on whether they are intended primarily for scuba or surface-supply.
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 have larger internal volume, but they are strapped on more securely and the load is carried by the neck. This load is small enough to be easily accommodated by most divers, though it may take some time to get used to it.
The weight of a lightweight demand helmet in air is about 15 kg. Underwater it is nearly neutrally buoyant so it is not an excessive static load on the neck. The helmet is a close fit to the head and moves with the head, allowing the diver to aim the viewport using head movement to compensate for the restricted field of vision.
Free-flow helmets compensate for a potentially large dead space by a high gas flow rate, so that exhaled gas is flushed away before it can be rebreathed. They tend to have a large internal volume, and be heavier than demand helmets, 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. To look around further, the diver must rotate the torso. The view downwards is particularly restricted, and requires the diver to bend over to see the area near the feet.

Gloves

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.

Weights

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

Instrumentation

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.
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.

Cutting tools

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

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.

Buddy lines

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. 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

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.
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. 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.

Diver propulsion vehicles

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.

Cameras

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.

Distance lines and line markers, reels and spools

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. It must be possible to fit, interpret and remove a line marker by feel in total darkness with the line under moderate tension without dislodging the line. 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.
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.

Equipment storage on dive boats and dive preparation areas

While it is possible for a diver to put on and take off some items of equipment in the water, there is a greater risk of fitting them incorrectly or losing them, particularly when the water is a bit rough. Doing this in the surf is even more risky, and delays at the surface on a boat dive can let the divers drift off site. When possible, kit-up and pre-dive checks should be completed on shore or on the boat, and the kit-up area should facilitate this, or at least make it possible. For recreational diving charter boats, this gives preference to arrangements where each diver can safely and securely stow all their personal dive gear at the same place where they will be putting it on, and where it is not necessary for it to be handled by anyone else except at the diver's request, as unauthorised handling of another person's life-support equipment could have legal consequences if something goes wrong.
Boarding the boat after a dive may require equipment to be removed, and this presents another set of hazards, and the associated risks of injury and damage to or loss of equipment, some of which may be avoided if the diver does not have to take off equipment in the water, and heavy equipment does not have to be lifted over the side of the boat with fragile dangling components exposed to snagging and crushing hazards. The requirement to remove fins before climbing some ladders reduces the diver's ability to swim back to the boat if they drift away. When boarding an anchored boat, some way of keeping within reach of the boarding area while removing equipment is required, and it may be necessary to use both hands to ensure secure removal of some equipment.

Access equipment

This includes the equipment needed to get into and out of the water. In most cases this refers to diving from a floating platform, but also applies to shore dives where access requires equipment.

Diving stages and wet bells

Diving stages and wet bells are open platforms used to lower the divers to the work site and to control the ascent and in-water decompression, and to provide safe and easy entry and exit from the water. Design must provide space for the working diver and where appropriate, the bellman, in positions where they are protected from impact during transit and prevented from falling out when above the water. The divers may be seated, but more usually stand during transit.
A stage must have a way to guide the umbilical from the surface tending point to the diver so the diver can be sure of finding the right way back to the stage. This can be provided by having the diver exit the stage on the opposite side to boarding, with the umbilical passing through the frame, but this is not infallible in bad visibility, and a closed fairlead is more reliable. Running the umbilical via the stage may also be needed to ensure the diver cannot approach known hazards, such as the thrusters of a dynamically positioned vessel.
A wet bell has an open bottomed air space at the top, large enough for the diver and bellman's heads, intended as a place of refuge in an emergency, where some breathing problems can be managed. The air space must be large enough for an unresponsive diver to be suspended by their harness with their head in the air space. The bell is also provided with an on-board emergency emergency gas supply, sufficient to for any planned or reasonably foreseeable decompression, and a means of safely switching between surface and on-board gas supply. This necessitates an on-board gas distribution manifold and divers' umbilicals that are deployed from and stored on the bell, and someone to operate the panel and tend the working diver's excursion umbilical. The bellman does this, and also serves as standby diver. The buoyancy of the air space may have to be compensated by ballast.

Diver ladders

For some applications, dive boat ladders that allow the diver to ascend without removing the fins are preferred. When there is a lot of relative motion between the diver and ladder, it can become difficult to safely remove fins, then get onto the ladder, and not lose the fins. A ladder that can be climbed with fins on the feet avoids this problem.
If a ladder is to be climbed in full equipment, suitable handrails to brace the diver while climbing the upper rungs and moving onto the deck to the place where gear will be removed are necessary for safety. This also applies if the divers need to climb down a ladder wearing dive gear, and they may need to turn round at the top of the ladder.

Dive platforms

A dive platform, or swim platform, is a horizontal surface on a dive boat giving more convenient access to the water than the deck. It may be large enough for several divers to use simultaneously, or just enough for a single diver. The platform may be fixed, folding, or arranged to lower divers into the water and lift them out again. Most dive platforms are mounted at the stern, usually on the transom, at a height a short distance above the waterline. They are easily flooded by a following sea, and are self-draining. Fixed and folding platforms are generally provided with ladders which can be folded or lifted out of the water when not in use, and steps or ladders from the platform to the deck, while lifting platforms may be sufficiently immersible for the divers to swim directly over the platform and stand up to be lifted to a level where they can walk off onto the deck. Handrails for use while using steps and ladders, when crossing or waiting on the platform, or making adjustments to equipment are a valuable safety adjunct as the platform will often be moving when in use, and the diver's will usually be encumbered by heavy and bulky diving equipment.

Recovery of an incapacitated diver

Professional divers may be required to wear a harness suitable for lifting the diver out of the water in an emergency, and there will usually be an emergency recovery plan and the necessary extraction equipment and personnel available. Private recreational divers are not usually required to make any special provisions for an emergency, but recreational diving service providers may have a duty of care to their customers to provide for reasonably foreseeable emergencies with reasonably practicable facilities. There may be a regional or membership organisation standard or code of practice. Getting an incapacitated diver out of the water onto a boat or the shore for first aid would usually be considered an expected level of care. Recreational divers are not required to wear lifting grade harness, so other plans should be in place, and these often necessitate removing equipment from the diver, and the risk of losing the equipment. Details of methods to recover a diver into a boat will vary depending on the geometry of the boat. Simply dragging a diver over the pontoon of an inflatable hull may work in many cases. Larger boats with higher freeboard may have lifting gear that can be put to use with a rescue sling.

Tools

Tools which are intended for use by divers should take into account the handicaps of the underwater environment on operator stability, mobility and control, within the full range of conditions in which they are likely to be used. Buoyancy effects on tool and operator, water movement, and reduced sensory input can complicate underwater tool use.

Tool bags, pockets and lanyards

Lanyards and clipping points can prevent loss of tools and equipment like cameras, lights and cutting tools in mid-water or poor visibility, but can increase entanglement risk. Carrying heavy tools can compromise the diver's ability to accurately control ascent and descent rates, so it is common practice for professional divers to have their tools delivered in a bag lowered from the surface, or to transport them in a basket on the stage or bell which transports the diver to the underwater workplace. Tools do not have to be carried inside the pressurised volume of a closed bell, so the basket or rack can be on the bell stage or clump weight.
Pockets for small accessories are common on jacket-style buoyancy compensators. Wing buoyancy compensators generally do not have pockets, as the wing is behind the diver and the harness is usually fairly minimal, but pockets can be added to the waistbelt if there is space. They are supported by the webbing at the top and strapped around the thigh to prevent flapping. Pockets on the diving suit are more popular with technical divers, and may be glued to the front or side of the thighs of the suit, or attached in similar positions to wetsuit shorts or tunic worn over the main suit. Occasionally a chest pocket or internal key pocket may be provided. Sidemount divers may use a butt-pack, a clip-on bag worn behind the diver below the harness, and unclipped and brought forward for access.

Physiological variables

, 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.
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.
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. 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.