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Glossary - P

PADI Professional Association of Diving Instructors
Parallels (of latitude) Angular distance north or south of the equator from 0° to 90°.
Partial Pressure The portion of the total gas pressure exerted by a single gas in a breathing mixture, expressed as Pg for an unknown gas or PO2 in the case of Oxygen.
Perigee Perigee Tides - The moon's orbit is elliptical and so its distance form the Earth varies throughout the lunar month.  When it is nearest to the Earth, the Moon is said to be in Perigee and when it is furthest away to be in Apogee.  Therefore the greatest attraction between the earth and moon occurs at perigee, and when this coincides with the time of a new of full moon, the resulting spring tides are greater than average springs.
PFO Patent Foram Ovale. A small hole between the two sides of the heart. It normally closes up shortly after birth. If it doesn't it can cause a predisposition to decompression sickness in adults.
Physiology The study of the body's actions and reactions. Diving physiology is primarily concerned with the effects of water pressure on the diver.
Pillar Valve The metal fitting on the neck of a cylinder. It consists of an on/off valve and a connection point for the regulator 1st stage.
Pilotage Navigation by reference to terrain features, both natural and artificial, usually with the aid of an appropriate chart.
Planing A boat is said to be planing when it is essentially moving over the top of the water rather than through the water.
Plotter Combined map or chart instrument having a protractor with ability to rotate and a scale rule. Sometimes called a Douglas Protractor.
Pneumothorax A lung over expansion injury that involves air bubbles escaping into the chest area and causing lung collapse. This can result from ascending to rapidly and/or breath holding.
Pony Bottle A small cylinder commonly used as an emergency breathing system, also known as a "Bailout Bottle".
Poor Mans Trimix Helium and Air
Port A connection for hoses on the first stage of a regulator. The first stage may contain 4-6 of these and they are usually marked with LP (low pressure) or  HP (High Pressure).  
Port The left side of a vessel when facing the bow.
PP Partial Pressure
Pressure Absolute Pressure - Absolute pressure is a measure of the pressure exerted on an object from all sources; includes water (hydrostatic) and air (atmospheric) pressure.
Atmospheric Pressure -The measure of the weight of the surrounding column of air. At Sea level this is about One bar.
Hydrostatic Pressure - The measure of the weight of the surrounding column of water.
Prime meridian The line of longitude passing through Greenwich, England. It is 0° and therefore neither east nor west
PRM  Practical Rescue Management (BSAC)
Projection Representation of the earth's surface drawn over lines of latitude and longitude on a flat grid.
Protection of Military Remains Act 1986 This piece of legislation is administered by the Ministry of Defence and provides for the protection of military remains of any nationality in UK waters. This includes vessels and aircraft lost at sea.
Protection of Wrecks Act 1973 The Protection of Wrecks Act 1973 is administered by DCMS and allows wrecksites of particular historic, artistic or archaeological importance to be protected under the law. Section 2 of the Act is administered by the Receiver of Wreck and provides protection for wrecks that are deemed dangerous by virtue of their contents.
PSI Pounds per square inch.
Pulmonary Oxygen Toxicity The cumulative effect (over time) of being exposed to Oxygen while the body is under pressure.
Purge Valve A valve on a snorkel, mask or regulator that allows it to be cleared of water.










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