absolute pressure

The pressure that includes atmospheric pressure.

ambient pressure

The ambient pressure is the pressure at a specific location. Ambient pressure includes atmospheric pressure. See atmosphere absolute and bar.

ata

Abbreviation for atmosphere absolute

atmosphere absolute

The pressure at a location that includes the pressure of the atmosphere. Pressure gauges typically do not show “absolute” pressure. Abbreviated “ata”. One atmosphere absolute = 1.01325 bar.

bar

A unit of pressure that is not a direct SI value, but used commonly by divers. The pressure of 10 meters (32.5 feet) of sea water is defined as 1 bar for purposes of diving. One standard atmosphere is equal to 1.01325 bar.

bends

A colloquial term for decompression sickness (DCS). The term came into use after people who were exposed to high pressures developed DCS and would walk in a contortion in order to alleviate some of the pain. Named after a woman’s fashion style termed the “Grecian Bends”.

best mix

Used to describe the Fraction of Oxygen that provides the maximum benefit of nitrox based on a specified oxygen exposure and dive depth. In equations typically shown as “fg”.

Boyle’s Law

Describes the change in gas volume with gas pressure. Stated as volume is inversely proportional to absolute pressure. Written mathematically as: P1 x V1 = P2 x V2

Buhlmann Tables

A group of dive tables formulated by Dr. A. A. Buhlmann, a prolific hyperbaric physiological researcher. There are many different tables but a select few are common in the sport diving world. Some dive computers are based on “Buhlmann Tables”.

CNS

Abbreviation for Central Nervous System. Used with other terms such as CNS Oxygen Toxicity, meaning the effects of oxygen toxicity on the central nervous system.

CNS Oxygen Toxicity

Central Nervous System oxygen toxicity characterized by a Short Term High Dose of oxygen (greater than 1.0 bar). Sometimes referred to as the “Paul Bert effect”. Diving exposures up to an oxygen pressure of 1.6 bar are time defined by the NOAA guidelines. May result in seizures if the guidelines are ignored.

Central Nervous System

As used in diving, relates to the brain, brain stem and in some cases the spinal cord.

ConVENTID

Acronym used for some of the common signs and symptoms of CNS oxygen toxicity. NOT to be relied on as most oxygen events are not recognized until too late. Plan your dive and dive your plan!

DCS

Abbreviation for Decompression Sickness.

Dalton’s Law

Describes that the total pressure of a gas is comprised of the sum of all the partial pressures of the constituent gases. Written mathematically as: Pt = Pgas1 + Pgas2 + Pgas3 ……

decompression sickness

A series of maladies associated with nitrogen coming out of solution of the blood, sometimes as bubbles. This may cause just simple pain or be associated with severe neurological effects.

dive computer

An electronic device that continuously monitors both time and depth to calculate remaining bottom time. Advanced computer may monitor and report tremendous amounts of related information.

EAD

Abbreviation for Equivalent Air Depth

Equivalent Air Depth

The equivalent nitrogen pressure depth corrected for a nitrogen content that is different from normal air. Can be thought of as “the depth the body thinks it is at due to the nitrogen content”.

EAN

Abbreviation for Enriched Air Nitrox. May be used with numeric values for specific oxygen content such as EAN32 or EAN40.

EANx

Abbreviation used with general nitrox values or where the specific oxygen value is not known or specified. NOT to be used with numerical values such as EANx28.

END

Abbreviation for Equivalent Narcotic Depth also known as the equivalent air depth (EAD).

Enriched Air Nitrox

A breathing gas that contains more oxygen than normal air and a corresponding decrease in the amount of nitrogen.

fg

Abbreviation for fraction of gas, where the specific gas is not specified.

fO2

Abbreviation for fraction of oxygen. The oxygen symbol may be replaced with any other gases symbol depending on use.

gauge pressure

Pressure that typically does not include the effect of the atmosphere. Most SPGs (submersible pressure gauge) as well as automotive tire pressure gauges are in this category. They do not show “1 bar or 15 psi” as the lower scale. To convert gauge pressure to Atmospheric Pressure Absolute, add the appropriate atmosphere unit such as 1.0132 bar for SI or 14.696 psi for imperial.

Heliair

A mixed gas that simply mixes helium and air. The major defect of heliair is that when diving at optimum oxygen concentrations of 1.4 bar, the END is 60m (195 feet). At 1.6 bar oxygen pressure, the END is 66m (214.5 feet) which may be well beyond the narcosis tolerance of mixed gas divers.