| Breathing underwater | | | | between the diver's eyes and the water. The |
| Water normally contains dissolved oxygen from | | | | distortion created by the water is effectively |
| which fish and other aquatic animals extract all their | | | | reversed as the light travels from water to air. |
| required oxygen as the water flows past their gills. | | | | Divers who require corrective lenses to see clearly |
| Humans lack gills and do not otherwise have the | | | | outside the water would normally require the same |
| capacity to breathe underwater unaided by external | | | | prescription while wearing a mask. Some masks can |
| devices. | | | | be ground to the diver's prescription to avoid the |
| Early diving experimenters quickly discovered it is not | | | | need for additional corrective lenses. |
| enough to simply supply air in order to breathe | | | | Occasionally commando frogmen use special contact |
| comfortably underwater. As one descends, in addition | | | | lenses instead, to see underwater without the large |
| to the normal atmospheric pressure, water exerts | | | | glass surface of a diving mask which can reflect light |
| increasing pressure on the chest and lungs - | | | | and give away the frogman's position. |
| approximately 1 bar or 14.7 psi for every 33 feet or | | | | Controlling buoyancy underwater |
| 10 meters of depth - so the pressure of the inhaled | | | | To dive safely, divers need to be able to control |
| breath must exactly counter the surrounding or | | | | their rate of descent and ascent in the water. |
| ambient pressure in order to safely and efficiently | | | | Ignoring other forces such as water currents and |
| inflate the lungs. | | | | swimming, diver's overall buoyancy determines |
| By always providing the breathing gas at ambient | | | | whether a diver ascends or descends. Equipment |
| pressure, modern demand valve regulators ensure | | | | such as the diving weighting systems, diving suits |
| the diver can inhale and exhale naturally and virtually | | | | (Wet, Dry & Semi-dry suits are used depending on |
| effortlessly, regardless of depth. | | | | the water temperature) and buoyancy compensators |
| Typically the diver's nose and eyes are encapsulated | | | | can be used to adjust the overall buoyancy. When |
| in a diving mask, such that the nose cannot | | | | divers want to remain at constant depth, they try to |
| participate in inhalation except when wearing a full | | | | achieve neutral buoyancy. This minimises gas |
| face diving mask. However, inhaling from a regulator's | | | | consumption caused by swimming to maintain depth. |
| mouthpiece becomes second nature very quickly. | | | | The volumes and weights of the diver and all |
| The most commonly used Scuba set today is the | | | | equipment attached to the diver, contribute to the |
| open circuit 2-stage diving regulator, coupled to a | | | | diver's overall buoyancy. Volume creates an upward |
| single pressurized gas cylinder. This 2-stage | | | | force and weight creates a downward force. If the |
| arrangement differs from Emile Gagnan's and Jacques | | | | force due to volume is greater than the weight, the |
| Cousteau's original 1942 design, known as the | | | | diver ascends. If the force due to volume is less than |
| Aqua-lung, in which the cylinder's pressure was | | | | the weight the diver descends. Diving weighting |
| reduced to ambient pressure in a single stage. The | | | | systems can be used to reduce the diver's weight |
| 2-stage system has significant advantages over the | | | | and cause an ascent in an emergency. Diving suits, |
| original single-stage design. | | | | mostly being made of compressible materials, reduce |
| In the 2-stage design, the first stage regulator | | | | in volume as the diver descends and expand as the |
| reduces the cylinder pressure of about 200 bar (3000 | | | | diver ascends creating unwanted buoyancy changes. |
| psi) to an intermediate level of about 10 bar (145 psi). | | | | The diver can inject air into some diving suits to |
| The second stage demand valve regulator, | | | | counteract this effect and squeeze. Buoyancy |
| connected via a low pressure hose to the first stage, | | | | compensators allow easy and fine adjustments in the |
| delivers the breathing gas at the correct ambient | | | | diver's overall volume and therefore buoyancy. For |
| pressure to the diver's mouth and lungs. The diver's | | | | open circuit divers, changes in the diver's lung volume |
| exhaled gases are exhausted directly to the | | | | can be used to adjust buoyancy. |
| environment as waste. | | | | Avoiding losing body heat |
| For more information, see diving regulator. | | | | Water conducts heat from the diver 25 times better |
| Less common (but becoming increasingly so) are the | | | | than air, which can lead to hypothermia. Except in |
| closed and/or semi-closed rebreather units. Unlike the | | | | very warm water, the diver needs the thermal |
| open circuit arrangements which vent all exhaled | | | | insulation provided by wetsuits and drysuits. See the |
| gases to the surrounding environment, rebreathers | | | | main article: Diving suit. In the case of a wetsuit, the |
| capture each exhaled breath and recycle it for re-use | | | | suit is designed to minimize heat loss. Wetsuits are |
| by removing the carbon dioxide buildup and | | | | generally made of neoprene that has small gas cells, |
| replenishing the oxygen used up by the diver. | | | | generally nitrogen, trapped in it during the |
| Rebreathers release few or no gas bubbles into the | | | | manufacturing process. The poor thermal conductivity |
| water which has advantages for research, military, | | | | of this expanded cell neoprene means that wetsuits |
| photography and other applications. | | | | reduce loss of body heat by conduction to the |
| On deeper or more prolonged dives, gas mixtures | | | | surrounding water. The neoprene in this case acts as |
| other than normal atmospheric air are used, such as | | | | an insulator. |
| air with enriched oxygen content, known as nitrox, or | | | | The second way in which wetsuits reduce heat loss |
| oxygen with helium and a reduced percentage of | | | | is to trap a thin layer of water between the diver's |
| nitrogen, known as trimix. In cases of technical dives | | | | skin and the insulating suit itself. Body heat then |
| multiple cylinders may be carried, each containing a | | | | heats the trapped water. Provided the wetsuit is |
| different gas mixture for a distinct phase of the dive, | | | | reasonably well-sealed at all openings (neck, wrists, |
| typically designated as Travel, Bottom and | | | | legs), this reduces water flow over the surface of |
| Decompression. These different gas mixtures may be | | | | the skin, reducing loss of body heat by convection, |
| used to extend bottom time, reduce inert gas | | | | and therefore keeps the diver warm (this is the |
| narcotic effects and reduce decompression times. | | | | principle employed in the use of a "Semi-Dry") |
| Injuries due to changes in water pressure | | | | In the case of a dry suit, it does exactly that: keeps |
| The diver must avoid injury caused by changes in | | | | a diver dry. The suit is sealed so that frigid water |
| water pressure. Pressure injuries are called | | | | cannot penetrate the suit. Drysuit undergarments are |
| barotrauma. They are caused by pressure | | | | often worn under a drysuit as well, and help to keep |
| differences between the outside and trapped air | | | | layers of air inside the suit for better thermal |
| spaces inside the diver or the diver's equipment. To | | | | insulation. |
| avoid them, the diver equalizes the pressure in all air | | | | Drysuits fall into two main categories neoprene and |
| spaces with the surrounding water pressure when | | | | membrane; both systems have their good and bad |
| changing depth. | | | | points but generally they can be reduced to: |
| Effects of breathing high pressure gas | | | | Membrane: high level of diver manoeuverability due to |
| Decompression sickness | | | | the thinness of the material, however that also |
| The diver must avoid the formation of gas bubbles in | | | | means that heavy weight undersuit is required if |
| the body, called decompression sickness or 'the | | | | diving in cooler water. |
| bends', by releasing the water pressure on the body | | | | Neoprene: low level of diver manoeuverability due to |
| slowly at the end of the dive. This is done by making | | | | the material being considerably thicker than |
| decompression stops and ascending slowly using dive | | | | membrane material (even when dealing with |
| computers or decompression tables for guidance. | | | | compressed neoprene) however the neoprene |
| Decompression sickness must be treated promptly, | | | | provides a higher level of insulation for the diver. |
| typically in a recompression chamber. Administering a | | | | Avoiding skin cuts and grazes |
| higher concentration of oxygen to a decompression | | | | Diving suits also help prevent the diver's skin being |
| sickness stricken diver on the surface is a good form | | | | damaged by rough or sharp underwater objects, |
| of first aid for decompression sickness, although | | | | marine animals or coral. |
| fatality or permanent disability may still occur. | | | | Diving longer and deeper safely |
| Nitrogen narcosis | | | | There are a number of techniques to increase the |
| Nitrogen narcosis or inert gas narcosis is a reversible | | | | diver's ability dive deeper and longer: |
| alteration in consciousness producing a state similar to | | | | technical diving - diving deeper than 130 feet and/or |
| alcohol intoxication in divers who breathe high | | | | using mixed gases. |
| pressure gas at depth. Being "narced" can impair | | | | surface supplied diving - use of umbilical gas supply |
| judgement and make diving very dangerous. It | | | | and diving helmets. |
| occurs at any depth, but in most cases doesn't | | | | saturation diving - long-term use of underwater |
| become noticeable until deeper depths; typically when | | | | habitats under pressure and a gradual release of |
| breathing air at around 30m/100 ft. Jacques Cousteau | | | | pressure over several days in a decompression |
| famously described it as the "rapture of the deep". | | | | chamber at the end of a dive |
| Need to see underwater | | | | Being mobile underwater |
| Water has a higher refractive index than air. Light | | | | The diver needs to be mobile underwater. |
| entering the eye from the water behaves differently | | | | Streamlining dive gear will reduce drag and improve |
| than light entering from air. This creates a distortion | | | | mobility. Personal mobility is enhanced by swimfins |
| that affects normal vision. Diving masks and diving | | | | and Diver Propulsion Vehicles. Other equipment to |
| helmets solve this problem by creating an air partition | | | | improve mobility includes diving bells and diving shots. |