SCUBA DIVER'S DICTIONARY

Barrier reef - A reef separated from shore by a lagoon or other relatively shallow water with deeper water beyond. A fringing reef grows along the shore.

Benthic - Pertaining to life in or at the ocean floor; bottom dwelling. Pelagic creatures are free-swimming (nektonic), living in the open water. Sessile organisms remain rooted in one spot for their adult lives (e.g., corals, sponges).
Biological Oceanography - The study of marine organisms, how those organisms interact with their environment and what controls their distribution in the ocean.

Bioluminescence - The illumination produced by tiny marine organisms when disturbed. Best seen when diving at night by quickly moving the arms or legs.

Cephalopods - Mollusks with the most advanced nervous system of all invertebrates. This class includes octopus, squid, cuttlefish and the chambered nautilus.

Coelenterates - A name still often heard when referring to invertebrates of the phylum Cnidaria (formerly Coelenterata) distinguished by a hollow or sac-like body and dart-like stinging cells called nematocysts. Includes corals, jellyfishes, hydroids and sea anemones.

Coriolis effect - The deflection of winds and currents caused by the Earth's rotation. Deflection is to the right in the Northern Hemisphere, causing major ocean currents to flow clockwise. The opposite is true in the Southern Hemisphere.

Diurnal tides - One high tide and one low tide in a 24-hour period. Semidiurnal tides have two high and two low tides a day.

Endemic - Species found in only one place. For instance, almost one-third of Hawaiian reef fishes are found only in the waters surrounding those islands.

Flood tide - Water flowing toward shore with the incoming tide. Water flowing offshore as the tide falls is called the ebb tide.

Geological Oceanography - A discipline of ocean science that seeks to explain a lot about how the oceans came to be, their physical expanse and how they continue to change.

Halocline - A sharp division between layers of fresh and salt water. Moving from one to the other can cause an abrupt change in buoyancy.

Littoral - Pertaining to a seashore.

Neap tides - Occur twice a month when opposing gravitational forces of the sun and the moon cause tidal ranges to be lower than normal (i.e., higher low tides and lower high tides). When the moon and sun are aligned, as at new and full moons, higher than normal spring tide ranges occur.

Photic Zone - A lighted layer of water, the depth of sunlight sufficient to sustain photosynthesis. In tropical oceans, this zone can extend to a depth of 660 feet (200 meters). In midlatitudes, due to more plankton and other small scattering particles, the zone typically reaches no more than 330 ft (100m).

Physical Oceanography - Looks at ocean currents, air-sea interactions, waves, tides and global water circulation.

Phytoplankton - Microscopic algae that produce more than 85 percent of the Earth's oxygen and are the basic food source (i.e., the base of the food chain) for all oceanic life. The next step up in the chain, zooplankton feeds on phytoplankton.

Refraction - The bending of light waves due to a change in speed as they pass from one medium to another of different density (e.g., air to water). Refraction accounts for objects appearing closer and larger underwater. Reflection is the turning back of angled light rays by the water's surface. These effects, plus diffusion (scattering of light) and absorption (absorbing light waves of different lengths at varying rates) result in reduced light levels and color underwater.

Sea Level - The standard definition that you know is only a concept. In fact the sea surface possesses a distinct, though transient, topography. It warps into expansive mounds and depressions. The pressure gradients caused by those hills and valleys have a tremendous effect on the circulation of the earth's surface currents.

Set and Drift - Refer to the direction and speed of a current, respectively.

Stipes - Strands of kelp.

Taxonomy - The scientific system of arranging living organisms by their similarities and differences into a hierarchy of categories: kingdom, phylum, class, order, family, genus and species. Species, the most specific, is defined as a grouping of closely related organisms that interbreed and produce fertile offspring.

Thermocline - A sharp division between layers of warmer and colder water of from a few degrees Fahrenheit to 10 F or more. The epilimnion is the warmer layer above the thermocline.

Tubules of Cuvier - The sticky, spaghetti-like strands ejected from the anal opening of sea cucumbers as a defense mechanism.

Turbidity - Reduction in underwater visibility caused by suspended particles that absorb and diffuse light waves. These particles may include algae, plankton, silt and pollutants. They are responsible for backscatter, the snow-like effect in many underwater photos.

The World Ocean - Contains 97 percent of all water on Earth, and holds a volume of about 139 million square miles/360 million square km. Polar ice caps account for another 1.9 percent, all the lakes and rivers of the world another 0.02 percent, and 0.001 is held in the atmosphere as water vapor. The average depth of the world's ocean is 12,230 feet (3,730 m) and reaches its deepest depth at 35,802 feet (10,833 m).

Zooxanthellae - Algae that live in the tissues of corals and other organisms and provide nutrition for the organism via photosynthesis, the process by which plants turn sunlight and nutrients into energy-rich organic compounds.

Burst disc - A thin metal disc in the valve stem of a scuba cylinder that is designed to prevent the cylinder from exploding by breaking if the tank's internal pressure rises beyond a safe level.

DIN connector - A means of connecting a regulator to a scuba cylinder that is popular in Europe and among technical divers. The regulator first stage screws directly into the tank valve instead of using a yoke screw. A DIN connection is more secure and can handle tank pressures above 3,000 psi.

Downstream valve - A valve that opens in the same direction as the flow of air. In second-stage regulators this design acts as a fail-safe, causing a malfunctioning valve to stick in the open position so air will free-flow instead of being cut off.

Hydrostatic test - A test of the elasticity of the metal in compressed gas cylinders. A cylinder placed in a tank of water is filled with water under pressure. If the expansion of the cylinder is within acceptable limits, it is stamped with a test date mark; if it fails it is destroyed. Scuba cylinders require a hydrostatic test (or hydro) every five years, or sooner if metal damage or deterioration is suspected.

Open-circuit system - The typical scuba regulator with a single hose that exhausts exhaled air into the water. Recreational rebreathers are classified as semiclosed systems because most of the exhaled air is recirculated through the system via a return hose and scrubbed of carbon dioxide so residual oxygen can be used. A closed-circuit scuba system is one in which all exhaled air is recirculated.

Oxidation - The chemical process by which oxygen molecules combine with materials such as steel, aluminum and rubber. Water, especially salt water, speeds this process in metals; ultraviolet rays and ozone are catalysts for oxidization of rubber. Thorough rinsing and storage in a dry place away from sunlight helps slow deterioration of scuba equipment through oxidization.

Shell suit - A type of dry suit constructed of waterproof materials and typically latex seals. A shell suit does not offer the thermal protection or inherent buoyancy of a neoprene dry suit, but can be adapted to a range of water temperatures by wearing undergarments of varying thickness and material.

Trimix - A mixture of three gases used for diving beyond the depth range of air, usually helium, oxygen and nitrogen.

Unbalanced first-stage regulator - Uses tank pressure to assist in opening the air valve that provides air to the second-stage regulator. As tank pressure decreases, breathing becomes harder. Simple unbalanced regulators are rare nowadays, having been replaced by newer designs or balanced first-stage regulators.

Alveoli - The microscopic air sacs in the lungs that exchange breathing gas with the blood capillaries through the process of diffusion. Connected to the windpipe (trachea) by branching tubes called bronchioles and bronchi.

Barotrauma - Pressure-related injury to body air spaces, including squeezes (mask, skin, tooth, etc.), reverse blocks (ears do not equalize on ascent) and overexpansion injuries: Pneumothorax occurs when air escapes from ruptured alveoli into the space surrounding a lung, causing it to collapse. Mediastinal or subcutaneous emphysema occur when air escapes into the cavity surrounding the heart or is trapped under the skin near the neck. Arterial gas embolism (AGE) is caused by air bubbles entering the veins, traveling through the heart and entering the arterial blood supply, where they block capillaries and reduce blood supply to organs such as the heart and brain.

Carotid arteries - Two large arteries in the neck that carry blood to the brain. If these are restricted by an overly tight wet suit or hood, the carotid sinus reflex may slow the heart, decreasing the blood flow to the brain and potentially causing unconsciousness.

Dead air - The portion of each breath that doesn't contribute to gas exchange because some air remains in the trachea, bronchi, sinuses and doesn't reach the alveoli. The addition of a scuba regulator or snorkel adds to the dead airspace. Slow, deep inhalations and exhalations help reduce the amount of dead air.

Eustachian tubes - Flexible tubes that connect the throat with the middle ear. Normally closed, divers must open the tubes to equalize the pressure behind the eardrum (i.e., in the middle ear) with ambient pressure. This is accomplished via various maneuvers: Valsalva (breathing out against pinched nostrils and closed mouth), toynbee (swallowing with closed mouth and nose) or frenzel (throat muscles and tongue compress air).

Heat exhaustion - A condition that occurs due to insufficient fluid intake to make up for losses due to perspiration and respiration. Symptoms include excessive sweating, rapid, weak pulse and pale skin. Heat stroke can follow if the body's heat regulation mechanisms fail, and can be life-threatening. Symptoms include dry, hot skin (sweating stops), rapid, strong pulse and high temperature.

Hypercapnia - Elevated carbon dioxide level in the blood, which can be caused by shallow, fast breathing, overexertion or skip breathing (holding one's breath while diving). It can lead to disorientation and eventually unconsciousness. Hypocapnia is an especially low level of carbon dioxide in the blood caused by excessive hyperventilation. This can reduce the urge to breathe to a point where shallow-water blackout occurs due to lack of oxygen in the tissues (hypoxia).

Dive reflex - A slowing of the heart rate (bradycardia) and decreased blood flow to the limbs in response to breath holding and other immersion effects, especially in cold water. Does not reduce oxygen use in humans as it does in marine mammals (mammalian diving reflex).

Patent foramen ovale - In some adults the small prenatal opening between the left and right upper heart chambers has not closed. This allows some blood to flow directly from the right to left side of the heart, bypassing the lungs, which normally remove small gas bubbles from the blood circulation.

Silent bubbles - Tiny bubbles in the bloodstream that do not cause symptoms of decompression sickness but can be detected by Doppler ultrasound tests.

Archimedes' principle - An object immersed in water is buoyed up by a force equal to the weight of the water displaced. Salt water weighs more than fresh water (it's more dense) and therefore objects are more buoyant in salt water.

Atmospheric pressure - The weight of the atmosphere at sea level, 14.7 pounds per square inch (psi). Gauge pressure is the reading on submersible pressure gauges (psig), which ignore atmospheric pressure. At sea level gauge pressure reads zero. Absolute pressure (psia) is equal to atmospheric pressure plus gauge pressure. At the surface atmospheric pressure equals 14.7 psia or one atmosphere absolute (atm abs or ATA). Ambient pressure is the pressure of the surrounding water (hydrostatic pressure) plus the atmosphere. Ambient pressure at 33 feet in salt water (fsw) is 29.4 psia or two ATA. Pressure increases by one atmosphere every 33 fsw or 34 feet of fresh water (ffw).

Boyle's law - As the pressure increases, the volume of gas in a flexible container decreases proportionally and vice versa (assuming the temperature remains the same). During descent the ambient pressure increases and therefore the volume of air in a diver's lungs, ears, sinuses, buoyancy compensator (BC), exposure suit, etc., decreases while the density of that air increases (the same number of molecules are packed into a smaller space). For example, at a depth of 66 fsw (20 msw) or three ATA the cells in a wet suit are squeezed to one-third the size they were at the surface.
To maintain the surface volume in an airspace at depth (to prevent squeezes), more gas must be added to equalize the pressure inside and out. That is why scuba regulators are designed to deliver breathing gas at ambient pressure and why we use proportionally more air the deeper we dive.
On ascent the denser gas delivered at depth expands proportionally. Therefore it must be vented by breathing continuously to prevent a lung overexpansion injury.

Charles' law - If temperature increases on a flexible gas container, for the pressure inside to remain constant, the volume must increase. By extension, as the temperature increases on an inflexible container, the pressure inside increases. This is most applicable to scuba cylinders, which should not be left in the sun since they cannot expand sufficiently to accommodate a significant rise in internal pressure due to excessive heating.

Dalton's law - Each gas in a mixture exerts pressure equal to its proportion of the mixture (partial pressure). Oxygen is about 21 percent of air and therefore 21 percent of air pressure is due to the oxygen molecules, for a partial pressure (ppO2) of 3.1 psi at the surface (14.7 psi x 0.21). This becomes significant because in the denser gas breathed at depth the number of molecules of each gas is multiplied (e.g., five times as many at five atmospheres) and individual gases may become toxic. Depending on the depth, various gas mixtures are blended to reduce the negative effects of such gases as carbon monoxide and oxygen. For instance, oxygen toxicity can result from breathing plain air (normoxic) at a depth where the partial pressure of oxygen exceeds 1.6 ATA (24 psi), or just over 200 feet (61 m).

Defence and Civil Institute for Environmental Medicine (DCIEM) - A decompression model and set of dive tables developed by this Canadian agency.

Enriched air - Also called enriched air nitrox (EAN) or simply nitrox. A mixed gas consisting of an increased proportion of oxygen and decreased nitrogen; 32 percent O2 (NOAA Nitrox I, EAN32) and 36 percent 02 (NOAA Nitrox II,EAN36) are common. Nitrox is breathed by recreational divers trained in its use; it decreases the amount of nitrogen absorbed in a given time and/or increases bottom time.

Equivalent air depth (EAD) -The EAD describes the depth on air that would produce the same accumulation of nitrogen as the actual maximum depth dived when breathing nitrox or other decreased nitrogen gas mixture. It is shallower than the actual maximum depth. The theoretical depth is a term used in altitude diving that pertains to the amount of nitrogen accumulated at the actual depth dived. It is greater than the actual depth.

Haldane - The scientist who in 1908 developed the first systematized model for predicting and preventing decompression sickness. Historically, the majority of dive tables have been based on this model.

Henry's law - The amount of gas that can dissolve into a liquid is proportional to the partial pressure of that gas. Since the partial pressures are greater in the denser gas we breathe at depth, nitrogen is absorbed into the body's tissues, which are primarily liquid, faster at deeper depths. A tissue is said to be saturated when it can absorb no more of a particular gas at a given pressure (depth). As we ascend, the pressure of the gas dissolved in the tissues becomes greater than the ambient pressure (supersaturation) and the gas comes out of solution. This creates the "soda bottle" effect if the gas is released too rapidly (i.e., if the diver ascends too fast gas, bubbles form in the tissues).

Reduced Gradient Bubble Model (RGBM) - One of several decompression models not based on Haldane's theory.

Tissue compartments - A theoretical construct in decompression models that takes into account that body tissues absorb (ingassing) and release (offgassing) dissolved gas at different rates. An individual compartment's half-time is the number of minutes it takes for it to absorb or release half its gas capacity (i.e., between normal pressure and saturated). "Slow tissues" have long half-times and "fast tissues" have short half-times.