Diving Issues: This section looks at some of the physiological issues posed by diving. Breathing underwater:Water normally contains dissolved oxygen from which fish and other aquatic animals extract all their required oxygen as the water flows past their gills.
Humans lack gills and do not otherwise have the capacity to breathe underwater unaided by external devices.
Early diving experimenters quickly discovered it is not enough to simply supply air in order to breathe comfortably underwater.
As one descends, in addition to the normal atmospheric pressure, water exerts increasing pressure on the chest and lungs -
approximately 1 bar or 14.5 psi for every 33 feet or 10 meters of depth -
so the pressure of the inhaled breath must exactly counter the surrounding or ambient pressure in order to safely and efficiently inflate the lungs.
By always providing the breathing gas at ambient pressure,
modern demand valve regulators ensure the diver can inhale and exhale naturally and virtually effortlessly, regardless of depth.
Typically the diver's nose and eyes are encapsulated in a diving mask,
such that the nose cannot participate in inhalation except when wearing a full face diving mask.
However, inhaling from a regulator's mouth-piece becomes second nature very quickly.
The most commonly used Scuba set today is the open circuit 2-stage diving regulator, coupled to a single pressurized gas cylinder.
This 2-stage arrangement differs from Emile Gagnan's and Jacques Cousteau's original 1942 design, known as the Aqua-lung,
in which the cylinder's pressure was reduced to ambient pressure in a single stage.
The 2-stage system has significant advantages over the original single-stage design.
In the 2-stage design, the first stage regulator reduces the cylinder pressure of about 200 bar (3000 psi) to an intermediate level of about 10 bar (145 psi).
The second stage demand valve regulator, connected via a low pressure hose to the first stage,
delivers the breathing gas at the correct ambient pressure to the diver's mouth and lungs.
The diver's exhaled gases are exhausted directly to the environment as waste.
Less common (but becoming increasingly so) are the closed and/or semi-closed rebreather units .
Unlike the open circuit arrangements which vent all exhaled gases to the surrounding environment,
rebreathers capture each exhaled breath and recycle it for re-use by removing the carbon dioxide buildup and replenishing the oxygen used up by the diver.
Rebreathers release few or no gas bubbles into the water which has advantages for research, military, photography and other applications.
On deeper or more prolonged dives, gas mixtures other than normal atmospheric air are used.
Gases such as air with enriched oxygen content (nitrox) , oxygen with helium in order to decrease the percentage of nitrogen is known as trimix .
In cases of technical dives multiple cylinders will carried, each containing a different gas mixture for each distinct phase of the dive.
The distinct phases are usually designated as Travel, Bottom and Decompression.
Injuries due to changes in water pressure:The diver must avoid injury caused by changes in water pressure. Pressure injuries are called barotrauma .
They are caused by pressure differences between the outside and trapped air spaces inside the diver or the diver's equipment.
To avoid them, the diver equalizes the pressure in all air spaces with the surrounding water pressure when changing depth.
Decompression Sickness. Decompression sickness must be treated promptly, typically in a
recompression chamber , and may still result in fatality or permanent disability.
Decompression Sickness is now more commonly known as Decompression Illness (or 'DCI').
Need to see underwater: Light travels about 30% slower in water than air, which also means that the index of refraction of light in water varies by the same percentage from air. Since the lens in the diver's eye is designed to focus light arriving from air into a sharp image on the retina, light arriving from a water medium such as that encountered in an underwater dive will not be properly focused by the eye's lens, and underwater scenes will appear blurry. Diving masks and diving helmets solve this problem. Due to the mask's air partition between the diver's eyes and the water, the light rays entering the lens after travelling inside the mask's (or helmet's) air space are now properly focused, as they would be outside the water. Divers who require corrective lenses to see clearly outside the water would normally require the same prescription while wearing a mask. Some masks can be ground to the diver's prescription to avoid the need for contact lenses. Controlling Buoyancy Underwater. To dive safely, divers need to be able to control their rate of descent and ascent in the water.
Ignoring other forces such as water currents and swimming, diver's overall buoyancy determines whether a diver ascends or descends.
Equipment such as the diving weighting systems, diving suits (Wet, Dry & Semi-dry suits are used depending on the water temperature)
and buoyancy compensators (which go by many different names such as BC, Stability 'Stab' Jacket,
they are also know as a 'Wing' when used as part of a twin-set configuration) can be used to adjust the overall buoyancy.
When divers want to remain at constant depth, they try to achieve neutral buoyancy.
This minimises gas consumption caused by swimming to maintain depth.
The volumes and weights of the diver and all equipment attached to the diver, contribute to the diver's overall buoyancy.
Volume creates an upward force and weight creates a downward force. If the force due to volume is greater than the weight, the diver ascends.
If the force due to volume is less than the weight the diver descends.
Diving weighting systems can be used to reduce the diver's weight and cause an ascent in an emergency.
Diving suits , mostly being made of compressible materials,
reduce in volume as the diver descends and expand as the diver ascends creating unwanted buoyancy changes.
The diver can inject air into some diving suits to counteract this effect and squeeze.
Buoyancy compensators allow easy and fine adjustments in the diver's overall volume and therefore buoyancy.
For open circuit divers, changes in the diver's lung volume can be used to adjust buoyancy .
Avoiding losing body heat:Water conducts heat from the diver 25 times better than air, which can lead to hypothermia.
Except in very warm water, the diver needs the thermal insulation provided by wetsuits and drysuits.
In the case of a wetsuit , the suit is designed to minimize heat loss.
Wetsuits are generally made of neoprene that has small gas cells, generally nitrogen, trapped in it during the manufacturing process.
The poor thermal conductivity of this expanded cell neoprene means that wetsuits reduce loss of body heat by conduction to the surrounding water.
The neoprene in this case acts as a insulator.
The second way in which wetsuits reduce heat loss is to trap a thin layer of water between the diver's skin and the insulating suit itself.
Body heat then heats the trapped water. Provided the wetsuit is reasonably well-sealed at all openings (neck, wrists, legs),
this reduces water flow over the surface of the skin, reducing loss of body heat by convection,
and therefore keeps the diver warm (this is the principle employed in the use of a Semi-Dry).
In the case of a dry suit, it does exactly that... keeps a diver dry. The suit is sealed so that frigid water cannot penetrate the suit.
Drysuit undergarments are often worn under a drysuit as well, and help to keep layers of air inside the suit for better thermal insulation.
Drysuits fall into two main categories neoprene and membrane; both systems have their good and bad points but generally they can be reduced to:
Membrane: high level of diver manoeuverability due to the thinness of the material,
however that also means that heavy weight undersuit is required if diving in cooler water.
Neoprene: low level of diver manoeuverability due to the material being considerably thicker than membrane material
(even when dealing with compressed neoprene) however the neoprene provides a higher level of insulation for the diver.
Breathing mixes:There is no single optimal breathing gas mix for every type of dive. Each mix must contain sufficient oxygen to sustain life and consciousness.
Mixes may contain other gases such as nitrogen and helium.
As the concentration of gases increases with the depth of the dive, and some gases are toxic at high concentrations,
the design of breathing gas mixes depends on the depth of the dive.
Avoiding skin cuts and grazes:Diving suits also help prevent the diver's skin being damaged by rough or sharp underwater objects, marine animals or coral.
Diving longer and deeper safely:There are a number of techniques to increase the diver's ability dive deeper and longer:
technical diving - diving deeper than 130 feet and/or using mixed gases.
surface supplied diving - use of umbilical gas supply and diving helmets.
saturation diving - long-term use of underwater habitats under pressure and a gradual release of pressure over several days in a decompression chamber
at the end of a dive
Being mobile underwater:The diver needs to be mobile underwater. Personal mobility is enhanced by swimfins and Diver Propulsion Vehicles.
Other equipment to improve mobility includes diving bells and diving shots.
