Scuba 101: Definitions & Primer

by: admin
February 2, 2010

This is a VERY basic primer to some of the terms and definitions you’ll see mentioned on this site often. The definitions here are “industry standards”, and are in common use no matter where you dive.

A dive instructor or your dive leader will be able to help you with most diving terms , but these are the basics that every diver needs to know before they attempt to get in the water.

The word scuba began as an abbreviation of Self Contained Underwater Breathing Apparatus. Originally, the word referred only to the cylinder of compressed gas that allows divers to breathe underwater, along with the equipment such as face masks, breathing regulators, gauges, and other devices attached to it (or required to use it).

Now, of course, it’s the term for the sport of underwater diving as well as all the gear that goes along with it.

Actual bottom time (ABT): Total time underwater (in minutes) between leaving the surface until the ascent back to the surface begins. Often called simply “bottom time”.

Air: This term refers to the compressed air used for recreational scuba diving containing a mix of 21% oxygen, 78% nitrogen, and 1% other gasses (mainly argon).

Air compressor: The machine that compresses or pressurizes air for scuba purposes. These machines compress air from the atmospheric level (14.7 pounds per square inch or psi at sea level) to the capacity of the tank (usually between 2,500 and 3,000 psi).

Air pressure: The force per unit area exerted by the weight of air. Air pressure decreases with altitude. At sea level, the air pressure is 14.7 psi.

Alpha flag: The international maritime signal flag that indicates that a vessel is unable to maneuver, so other vessels should stay clear. The blue and white double-tailed pennant is recognized as the signal for divers below the surface. (Note: In the U.S., a red rectangle crossed horizontally from upper left to lower right with a white stripe is used as a dive flag.)

Altitude sickness: An illness brought on by a sudden change in pressure, usually with rising altitude. Divers can experience it when diving down from the surface, but it is more common as divers return to the surface.

Ambient light: The sunlight available underwater.

Ambient pressure: The pressure surrounding a person’s body. On land or onboard a boat, the ambient pressure comes from the weight of the atmosphere. Below the water, ambient pressure comes from the weight of the atmosphere plus the weight of the water. Ambient pressure is measured in atmospheres. One atmosphere is about 14.7 pounds of pressure per square inch.

Anticoagulants: Medications (like aspirin, or prescription drugs like Coumadin or Warfarin) that reduce the blood’s clotting ability. (Note: If you take anti-clotting drugs, talk to your doctor before you book a dive trip, and tell your dive instructor or dive leader before you get into the water. Anticoagulants can be extremely dangerous to divers.)

Ascent Bottle: An extra cylinder of air used on deep dives to allow decompression stops without fear of running out of air. They come in many different sizes, and your dive coordinator should advise you on the correct bottle size for a group dive.

Ascent/Descent line: Divers use these ropes suspended from a boat or a buoy to control their rate of ascent or descent.

Buddy: A diving partner. Inexperienced divers should never dive alone.

C-Card: A diver’s certification card for a specific level of achievement.

Core Temperature: The normal internal temperature of the human body is 98.6 degrees Fahrenheit. Maintaining that temperature is vital to safe diving.

Deep diving: For recreational divers, a deep dive is any dive below 60 ft.

Depth gauge: A device that indicates how far below the surface  a diver is.

Dive computer: A device that constantly measures depth and time, based on pre-programmed algorithm, to calculate a number of safety factors and sound a warning to alert the diver to safety issues.

Dry suit: A water-tight garment that keeps divers warm when diving in waters that are too cold for wetsuit protection, usually where temperatures are below 65 degrees Fahrenheit.

Hang tank: An extra tank, staged at a decompression stop. Normally found at the rear of a dive boat in case of emergencies.

Heliox: Mixture of helium and oxygen, usually used for very deep diving by experienced divers.

Hydro: Short for “hydrostatic test” , a safety inspection required on scuba cylinders to determine whether the tank walls are strong enough for safe usage.

Logbook: A diary listing dive history that is used to provide evidence of a diver’s experience.

MOD/ODL: Maximum operating depth or oxygen depth limit. This is the deepest that a diver can safely go using a particular gas mixture.

Multilevel diving: Spending a period of time at different depths on a single dive.

Octopus Regulator: An alternate second stage air source used by a diver’s buddy in a situation where two divers are sharing air because one has run out of air, or a reserve 2nd stage regulator.

Open circuit scuba: Recreational divers use open circuit scuba gear which expels exhaled air into the water as bubbles. This means that no part of the exhaled air is breathed in again by the diver.

Pony Bottle: A small scuba cylinder strapped to a diver’s main tank for emergency use.

Purge Valve: A way to clear masks or snorkels without removing them.

Rebreather: A closed-circuit SCUBA system that filters exhaled air, and recirculates it for the diver to use again. Rebreathers require special training and maintenance.

Regulator: Any device that changes air pressure for a lower diving level.

Submersible pressure gauge: Gauge used to monitor air supply during a dive.

Surface interval: Sometimes called a “required interval”, this is the length of time a diver must stay out of the water between two consecutive dives.

Wet suit: Any suit that provides thermal protection underwater by trapping a layer of water between the diver’s skin and the suit.

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