Glossary of Scuba Diving Terms
| Quantity | Name | Symbol | Equivalents |
| Length and Depth | metre | m | 1 m= 100cm |
| Mass | kilogram | kg | 1kg=1000g |
| Volume | litre | litre* | 1 litre of water weighs 1kg |
| Time | second | s | 60 s = 1 min |
| Electric Current | ampere | A | |
| Thermodynamic temp. | Kelvin | K | 0 K- = -273.15 C ‘Absolute Zero’ |
| Temperature | Celsius | C | 0 C = freezing point of water |
| Force | Newton | N | |
| Torque | Newton metre | Nm | |
| Pressure and Stress | Pascal | Pa | 1 Pa =1Nm2 |
| Work, Energy | joule | J | 1 J = 1 Nm |
| Pressure | bar | bar | 1 bar = 1.02 kg cm |
Absolute pressure
A gauge reading absolute pressure will only read zero when subjected to a vacuum and therefore will read approximately 1 bar at sea level, i.e. atmospheric pressure. All calculations involving the diver normally involve absolute pressure.
AMBIENT PRESSURE
Ambient means surrounding, therefore ambient pressure is the pressure of our surroundings, the term is often used in connection with the pressure acting upon the diver and his equipment underwater.
Anti – friction washer
A washer, usually of Nylon or PTFE fitted between the valve spindle and cap nut, thus preventing metal to metal contact and thereby reducing wear. Sometimes called a Thrust Washer (see Thrust Washer).
back up ring
A split, spiral ring usually made of PTFE fitted next to an 0 Ring to prevent it from being extruded from its groove.
BOTTLE
Archaic colloquial name (UK) for a Transportable Pressure Receptacle.
CIRCLIP
A “C” shaped spring clip used to retain filters and various other components, requiring special pliers for installation and removal.
CYLINDER
Common name (UK) for a Transportable Pressure Receptacle.
DECANTING
Term describing filling SCUBA cylinders by topping up from high pressure storage vessels.
ENRICHED AIR
A breathing gas mix consisting of Nitrogen and Oxygen with an Oxygen percentage in the range 21-50%
GAUGE PRESSURE
The majority of pressure measurement systems in every day use measure pressure relative to ambient pressure, therefore the majority of pressure gauges read zero when the system is at atmospheric pressure, not zero pressure. This is termed “gauge pressure”.
HELI AIR
See Tri-Mix.
HELIOX
A breathing gas mixture of Helium and Oxygen.
NITROX
Any Oxygen/Nitrogen mixture from 1% 02 and 99% N:, to 99% 02 and 1% N2.
NYLOC NUT
A self-locking nut with a nylon insert which prevents the nut from unscrewing.
ORIFICE
A hole through which, in this case, air passes.
OXYGEN CLEAN
Equipment or components that have been prepared for use in pure Oxygen or Oxygen enriched atmosphere, i.e. absence of contaminants.
OXYGEN COMPATIBLE
A material suitable for use with Oxygen at a specified pressure and temperature.
OXYGEN MIXTURE
An Oxygen/Nitrogen mixture whose Oxygen content is greater than 50% and less than 95% Oxygen.
OXYGEN SERVICE
A component or components which are both Oxygen compatible and Oxygen clean and therefore ready for use with Oxygen at a specified pressure and temperature.
PARTIAL PRESSURE
The pressure of an individual gas within a mixture of gases expressed as a fraction of the total pressure.
PARTIAL PRESSURE FILLING
Describes a method of mixing gases by decanting them directly into a SCUBA cylinder or storage bank where the gases diffuse into a mixture.
ROLL PIN
A retaining pin made from a thin rolled metal strip.
SEAT
A sealing surface made of rubber, nylon, or other synthetic material against which a valve orifice seals. Note: This term has been used throughout the text for simplicity and uniformity, although manufacturers may use a different one.
SELOC PIN
A split tubular pin manufactured from spring steel or stainless steel.
SHIM
A spacer of metal or plastic used as a packing, or to adjust the length of a spring.
SINTERED FILTER
Sintering is a process by which metal granules are fused together without melting to form a porous homogenous mass. Sintered metal is the most common type of regulator 1st stage inlet filter material.
TANK
Common name (USA) for a Transportable Pressure Receptacle.
THRUST WASHER
A metal or plastic ring which is used to transmit an applied force e.g. a thrust washer under a 2nd stage front cover compresses the diaphragm but prevents the turning action from wrinkling it.
TRANSPORTABLE PRESSURE RECEPTACLE
The name given in CDGCPL-2 to any vessel, including SCUBA cylinders in which any pressurised fluid may be transported. (See also Bottle and Tank).
TRI-MIX
A breathing gas mixture of Helium, Oxygen and Nitrogen. •
VALVE ORIFICE
A smooth round opening ground to a fine edge so that when in contact with a seat, a seal is created preventing airflow.