274 NAUI Master Scuba Diver GLOSSARY during a dive. Downstream valve: A valve that opens in the direction of gas flow. Drag: The force of resistance to movement. Dry suit: A variable volume exposure suit worn in cold water. Dump valve: A manually operated exhaust valve for a buoyancy compensator that allows air to be quickly expelled. E Ebb tide: An outgoing tide. Ecology: The study of living organisms in their environments. Emergency Swimming Ascent: An out of air ascent where the diver ascends at as close to the proper ascent rate as possible, exhaling or attempting to inhale the entire way up. Energy: The capacity to do work. Enriched air nitrox (EAN or EANx): A breathing gas consisting of a mixture of nitrogen and oxygen other than that found in air. Epilimnion: The warmer layer of water above a thermocline in freshwater. Equilibrium: A state of balance between opposing forces. Euphoria: A feeling of elation and well-being. F Feeder zone: A region in the surf zone that feeds water to a rip current. Fetch: The distance over which wind blows in the process of generating waves. Fix: The intersection of two lines of position with an angle greater than 60°. Flood tide: An incoming tide. Fluid: A state of matter, either gaseous or liquid, in which matter is capable of flowing. Food chain: The transfer of energy along a chain of animals in the marine community. FSW: Feet Sea Water. G Gas tension: The partial pressure of a gas in a liquid. Gauge pressure: Pressure in excess of atmospheric pressure or which uses atmospheric pressure as a zero reference. Gay-Lussac’s Law: See Charles’ Law. General gas law: A combination of Boyle’s and Charles’ Laws used to predict the behavior of a given quantity of gas when changes may be expected in any or all of the variables. Global Positioning System (GPS): Electronic navigation system which uses satellites for positioning. H Haldanian theory: A theory by John S. Haldane, a British physiologist. His theory of gas absorption and elimination by body tissues forms the basis for some decompression tables. Half-times: The rate of absorption or elimination of gas in tissues at an exponential rate. A “5-minute” tissue is 50% saturated in 5 minutes, 75% saturated in 10 minutes, etc., until essentially saturated after six half-times.
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