Discussion about scuba diving, in terms of its development, is most often bound to begin by exploring the meaning and history of the term, “scuba”. Strictly speaking, the term is an acronym for the phrase “self-contained underwater breathing apparatus”, and should therefore be written S.C.U.B.A. Like many brand names, “scuba” has become descriptive of a particular activity (like “Xeroxing”, for example) and no longer refers to the specific apparatus.

The acronym does, however, allude to the defining characteristic of the type of diving it refers to: that is to say that scuba diving is diving where a compressed air-supply accompanies the diver, thus allowing them a great amount of freedom. This type of diving stands in contrast to breath-holding techniques developed for, and employed in, free diving and snorkelling, as well as to techniques developed for surface assisted diving. In surface assisted diving, the diver receives air from the surface, and is weighted down artificially to keep at a specified depth.

Self-contained breathing apparatus diving only came into existence in about 1942 when free-diving legend, Jacques-Yves Cousteau, and his partner, Emile Gagnan, designed and proceeded to build the first “Aqualung”. Aptly named, the device consisted of a cylinder containing compressed air. Connected to the cylinder were an airtight hose, a mouthpiece, and a hose that reconnected the mouthpiece back to the tank. In this design, the CO2 released by the body in respiration was filtered to extract oxygen from the mixture. This is known as a rebreather system, and is today only really used in specialised circumstances where stealth and efficiency are important.

The primary problem with a faulty rebreather system is that oxygen toxicity can occur, which can be lethal in underwater environments.

Compressed atmospheric air is the most common gas used in scuba tanks in contemporary times, but higher percentages of oxygen can be infused into the gaseous mixture when needed (to cut down on levels of nitrogen, for example). A typical sample of atmosphere will comprise of about 21% oxygen, 78% nitrogen, and 1% rare gases. In high oxygen mixtures, the oxygen content of the mixture is increased to up to thirty-eight percent.

The mouthpiece designed to expel CO2 into the water was designed by the Australian, Ted Eldred, and was known as the “porpoise”. The “porpoise” system enjoyed several advantages of the closed-system Aqualung, not least of which was a reduction in the chances of a diver suffering from oxygen toxicity.

Talking specifics, the Porpoise used a two stage decompression system, the first of which happened at the valve connecting the air-tank to the hose. The second decompression valve is situated in the mouthpiece itself. Both decompression valves ensure that the gas breathed in by the diver is at a pressure that will not harm respiratory system tissue (i.e. air pressure is equivalent to ambient water pressure).

The respiration of air at the same pressure as the surrounding environment means that a diver’s lungs inflate as easily as they would at normal atmospheric conditions. A possible downside to this, and something which divers are always taught about, is decompression sickness: this occurs when compressed air in the bloodstream and tissue of a diver is allowed to decompress too quickly. The result is that the gas expands, causing tiny bubbles. Symptoms of “the bends” include joint pain, headache, confusion, muscular pain, irregular heartbeat and problems breathing. In a worst case scenario, the bends may cause death, although this is very uncommon in modern times.