Glossary of Scuba Diving Terms
The International System of Units (ISO) is used throughout the world for engineering and scientific calculations. Other applicable metric units are also used during Dive Industry Technician Training. Sadly though, with the metric system being invented by the French, it’s unlikely the USA will be taking it up for public use any time soon, although all American scientists and engineers have been using it for decades, since high-end engineering and scientific math is simply too mind numbingly complex and confusing using imperial units to ever get anything done.
Officially, the United States is one of only three countries (Libya and Burma being the other two) that still, officially use the imperial system of measurement, but with an officially declared intention to convert to the metric system ‘sometime in the future…’
| 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.
Note: This term has been used throughout the text for simplicity and uniformity, though manufacturers may confusingly use a different one.
Abbreviations
A.B.L.J.
Adjustable Buoyancy Life Jacket: Usually of the “horse collar” type. Incorporating emergency air cylinder in addition to a low-pressure inflator. See also BCD.
A.F.
Across Flats: Applied to a spanner, describes the width between jaws, e.g. a ‘/z” AF spanner will fit a nut which measures ‘/z” across two parallel flat faces.
A.O.D.C.
Association of Offshore Diving Contractors.
A.S.S.E.T.
The Association of SCUBA Service Engineers and Technicians.
B.C.G.A.
British Compressed Gases Association.
B.A.
Breathing Apparatus: abbreviation for non-Scuba breathing apparatus. (See also SCBA).
B.C.
Buoyancy Compensator. See BCD.
B.C.D.
Buoyancy Control Device.
B.C.A.S.
British Compressed Air Society.
B.S.
British Standard.
B.S.I.
British Standards Institution.
BSP-F
British Standard Pipe – Fine (thread). Obsolete designation, now called the “G” series thread.
CDGCPL-2
Carriage of Dangerous Goods (Classification, Packaging and Labelling) and Transportable Pressure Receptacles Regulations.
C.F.M.
Cubic Feet per Minute: Still commonly used to measure compressor output.
C.O.S.H.H.
Control Of Substances Hazardous to Health.
C.P.
Charge Pressure: Occasionally found marked on SCUBA Cylinders instead of WP
D.I.N.
Deustche Institut fur Normung: German standards, commonly associated with regulator to cylinder connections.
E.A.N.x
(or EAN) Enriched Air Nitrox.
E.N.
Euro Norm (European Standard).
H.A.S.A.W.A.
Health and Safety at Work Act.
H.M.S.O.
Her Majesty’s Stationary Office.
H.S.E.
Health and Safety Executive.
H.P.
High Pressure.
I.D.E.S.T.
Inspectorate for Diving Equipment Servicing and Testing.
I.E.C.
International Electro-technical Commission.
I.M.C.A.
International Marine Contractors Association.
I.S.O.
International Standards Organisation.
L.P.
Low-pressure.
N.D.T.
Non Destructive Testing.
N.G.T.
National Gas Taper. Designation for a taper thread form manufactured to USA Standards not to be confused with N.P.T.
N.P.S.M.
National Pipe Straight Mechanical. Designation for a parallel thread form commonly found on SCUBA Cylinders manufactured to USA Standards.
N.P.T.
National Pipe Taper. Designation for a taper thread form manufactured to USA Standards not to be confused with N.G.T.
N.R.V.
Non-return Valve.
N.V.Q.
National Vocational Qualification, (in Scotland SVQ)
O.S.H.A.
Occupational Safety and Health Authority (USA) similar organisation to HSE (UK).
P.I.A.T.
Periodic Inspection and Test.
P.M.V.
Pressure Maintaining Valve.
P.S.I.
Pounds per Square Inch.
P.T.F.E.
Poly Tetra Fluro Ethylene: A plastic material, one use of which is as a tape for sealing taper valve threads into cylinders.
PP
Partial Pressure: e.g. ppN; = partial pressure of Nitrogen.
PPE
Personal Protective Equipment.
PPM
Parts Per Million.
R.I.D.D.O.R.
Reporting of Injuries, Diseases and Dangerous Occurrences Regulations.
R&D
Research and Development.
S.C.U.B.A.
Self Contained Underwater Breathing Apparatus: more appropriate description than the term “Aqualung”
S.C.B.A.
Self Contained Breathing Apparatus: USA abbreviation for non-Scuba breathing apparatus. (See BA).
S.I.
Statutory Instrument
S.l.T.A.
SCUBA Industry Trade Association.
S.R.V.
Safety Relief Valve.
T.P.R.
Transportable Pressure Receptacle.
W.C.
Water Capacity.
W.P.
Working Pressure. Occasionally C.P. meaning “Charge Pressure” can be seen used as an alternative.
Based on a list of terms originally prepared for DITC trainees by Mr C.S Meinert, Founder of ASSET.
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