Human factors in diving equipment design is the application of human factors and ergonomics principles and data to the design of diving equipment and diving support equipment to facilitate diver-system interactions and provide for diver wellbeing and overall dive efficiency. The underwater diver relies on various items of diving and support equipment to stay alive, healthy and reasonably comfortable and to perform planned tasks during a dive.
Divers vary considerably in anthropometric dimensions, physical strength, joint flexibility, and other factors. Diving equipment should be versatile and chosen to fit the diver, the environment, and the task. How well the overall design achieves a fit between equipment and diver can strongly influence its functionality. Diving support equipment is usually shared by a wide range of divers and must work for them all. When correct operation of equipment is critical to diver safety, it is desirable that different makes and models should work similarly to facilitate rapid familiarisation with new equipment. When this is not possible, additional training for the required skills may be necessary.
The most difficult stages for recreational divers are out of water activities and transitions between the water and the surface site, such as carrying equipment on shore, exiting from water to boat and shore, swimming on the surface, and putting on 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 occupational safety and health regulations and codes of practice. This tends to make professional diving more expensive, and the cost tends to be passed on to the client.
Human factors engineering (HFE), also known as human factors and ergonomics, is the application of anatomical, psychological and physiological principles to the engineering and design of equipment, procedures, processes, and systems. Primary goals of human factors engineering are to reduce human error, increase productivity and system availability, and enhance safety, health and comfort with a specific focus on the interaction between the human and equipment.
General principles
Diving equipment is used to facilitate underwater activity by the diver. The primary requirements are to keep the diver alive and healthy, while secondary requirements include providing comfort and the capacity to perform required tasks. Safe operation requires correct equipment function as well as diver competence.
Fault tolerance is the property that enables a system to continue operating properly in the event of the failure of some of its components. If its operating quality decreases at all, the decrease may be proportional to the severity of the failure. Diving equipment, especially hardware elements that are high availability or safety-critical systems, must have a high fault tolerance. The ability to maintain functionality when portions of a system break down is referred to as 'graceful degradation', as opposed to a small failure causing total breakdown. The diver must also be fault tolerant, a state that is achieved by competence, situational awareness and fitness to dive, .
Physiological variables
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.
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. On the other hand, a team can reduce risk to an acceptable level in most cases by distributing redundancy among its members. However, the effectiveness of this strategy is tied to 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 sizes, 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.
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.
Breathing apparatus
The breathing apparatus must allow the diver to breathe with minimal added work of breathing, and minimum additional dead space. It should be comfortable to wear, and not cause stress injury or allergic reactions to its materials. It must be reliable and not require constant attention or adjustment during a dive, and performance should degrade gradually in the event of malfunctions, allowing time for corrective action to be taken with minimum risk. When more than one breathing gas mixture is available, the risk of selecting a gas unsuitable for the current depth must be minimised.
Demand valves Holding the scuba mouthpiece between the teeth can cause jaw fatigue on a long dive. The loads that cause this fatigue can be reduced by using smaller second stages, different hose lengths and routing, angled swivels, and improved mouthpiece design, which may include customised bite grips. Allergic reactions to mouthpiece materials are less common with silicone rubber and other Hypoallergenic materials 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 of mouthpiece.
Purging the second stage is a useful function to clear water from the interior. The purge button should function only when pressed, and should be powerful enough to sufficiently clear the chamber while not blowing its contents down the diver's throat. 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 or orientation. Once open with gas flowing, the gas flow often produces a slight increase in the pressure drop in the demand valve. This helps hold the demand valve open during inhalation, effectively reducing the work of breathing, but making the regulator more susceptible to free-flow. In high performance models, the user can adjust these sensitivity settings.
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 too distracting or annoying to the diver in a normal diving posture. Flow should be directed away from the faceplate of the mask and bubbles should not flow directly over the ears.
Work of breathing Breathing effort should be reasonable in all diver attitudes. 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. Breathing performance testing for compliance to standards is generally done facing forward and facing down. Manual adjustment of the inhalation valve spring may be available, and can help if an unusual orientation must be maintained for a long period.
Hose routing Scuba divers must be able to easily provide emergency gas to other scuba divers who are diving as part of the same group, which can be made easier or more difficult by the handedness of the regulator, the hose length, and the hose routing. Different configurations are used for specific circumstances. For example, the "long hose" arrangement is used to facilitate gas sharing when swimming in single file through a restriction. There are hose routings that have been standardised to a reliable system.
Rebreathers
Rebreather equipment removes carbon dioxide from exhaled gas and replaces it with oxygen, allowing the diver to breathe the gas again. This can be done in a self-contained system carried 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. The power to circulate gas in the loop can be the lung power of the diver, energy from the supply gas pressure, or externally powered booster pumps. Scuba rebreathers tend to circulate by lung power, and the work of breathing can make up a significant part of diver effort at depth; in extreme circumstances it may exceed the capacity of the diver without additional workload. The position of the counterlungs and the orientation of the diver in the water, can have a significant effect on work of breathing, as can the restriction of flow between the diver's teeth.
Using two scrubber canisters in series can provide a level of redundancy in that a fault in one that allows carbon dioxide breakthrough will not necessarily directly affect the other. It is also possible to repack just the first scrubber after a short dive, and may be possible to also change the order so that the freshest scrubber is last in the circuit. Mounting the counterlung across the diver between the scrubbers can eliminate transverse shifts in the centre of buoyancy during the breathing cycle, and also mounting it longitudinally in line with the lungs eliminates longitudinal buoyancy shifts during the breathing cycle. The counterlung could be mounted across the back or across the chest.
A wide variety of rebreather types are used in diving because of the highly variable requirements in different situations. A diving rebreather is safety-critical life-support equipment – some modes of failure can kill the diver without warning, some others require immediate appropriate response for survival.
Some rebreathers have control systems which will lock out if they do not complete a satisfactory pre-dive test, but others which may be used for cave diving, where the inability to start a dive may prevent the diver from exiting from a cave in which they have surfaced in a space isolated from the exit by a water-filled passage, will not prevent the diver from starting a return dive, but will warn them of the problems detected, as it may be possible to escape by operating the rebreather manually.
Mouthpiece retaining straps