Diving (underwater)

I

Introduction

Diving (underwater), act of entering water and remaining below the surface for such purposes as working or exploring. Diving without mechanical aids has been practised since ancient times in the harvesting of pearls and sponges. Various devices to supply the diver with air and thus permit a prolonged stay under water have been tried since at least the 4th century bc. Alexander the Great supposedly made a descent in a machine that was probably a primitive form of diving bell; and Aristotle mentioned devices that enabled divers to breathe under water. Practical apparatus was not developed, however, until the 18th century.

II

Diving Bells

In 1717 the British astronomer Edmond Halley devised one of the first practical diving bells, an open-bottomed wooden chamber with glass windows in the top to admit light. Air was supplied to people inside the bell through leather tubes connected to air casks that could be lowered into the water as needed. As water entered the casks, the air in them was forced through the tubes into the upper part of the diving bell, which was kept clear of water by the pressure of the air. A modern steel version of this device, supplied with compressed air pumped through a hose, is used for such underwater work as building bridges, piers, and jetties. See Bathyscaphe.

III

Underwater Habitats

Underwater habitats, or diving stations, enable scientists to conduct research on the sea floor at depths of up to 100 m (328 ft) for unlimited periods of time. Air pressure inside these stations is equalized with sea pressure outside, thereby keeping the station walls from collapsing. By means of a technique known as saturation diving, scientists are able to live in and work around such stations for extended periods without the risk of developing a disease known as the bends. Divers breathe compressed air mixed with light, inert gases, such as helium. When the diver's blood becomes saturated with helium, the time required for decompression, even if the diver returns to the surface after a period of weeks or months, is no greater than that required after a dive lasting just a few hours.

A 13-m (43-ft) habitat launched by the National Oceanic and Atmospheric Administration of the United States (NOAA) in the Caribbean in 1988 provides a crew of six with an underwater base from which they can work nine-hour days. The station, called Aquarius, is connected by umbilical cable to an unmanned surface support craft that delivers a steady flow of air and power. Such systems are not feasible for deep-sea exploration, where water pressures far exceed human endurance.

IV

Diving Suits

From the 17th century onwards, efforts were made to perfect a helmet or suit to give divers maximum protection and freedom of movement. One of the first such successful devices was created in 1819 by the Anglo-German inventor Augustus Siebe. Based on the principle of the diving bell, it consisted of a leather jacket fitted to a metal helmet into which air was pumped from the surface through a flexible hose. The helmet was not watertight, but air pressure kept water below the diver's chin. The modern closed diving suit, essentially the same as one introduced by Siebe in 1830, is made of rubberized fabric. It covers the diver completely and is airtight. Valves on the helmet allow the diver to regulate air pressure inside the suit and thus to control buoyancy. Auxiliary equipment of modern diving dress includes weighted shoes to keep the diver upright and leaden plates for weight on back and chest. A rope containing telephone wires connects the diver with the surface and permits conversation. For deeper diving, jointed metal suits heavy enough to withstand great water pressures are used. In these suits air pressure can be kept normal, and the diver experiences less stress than when diving in shallower water with an ordinary suit.

A troublesome feature of ordinary diving dress is the unwieldy lengths of air hose and lifeline divers must drag with them. This problem has been met by the development of diving suits with a self-contained air supply consisting of a pressure cylinder of mixed oxygen and air and a regeneration chamber filled with caustic soda. Exhaled air is passed through the regeneration chamber, carbon dioxide is removed, and the air is then combined with oxygen and air from the cylinder and rebreathed. The air supply of such units lasts from about 45 minutes to 2 hours.

Self-contained underwater breathing apparatus (scuba), independent of diving suits, has been developed for skin diving. It differs from the conventional diving suit in that it is designed for swimming, whereas the more cumbersome type is suitable only for walking on ocean floors. One form of this apparatus, a lightweight breathing mask operating on the same principle as the self-contained diving suit, was used during World War II by underwater demolition teams to clear away mines and other obstacles before amphibious landings. At present, the most popular form is the aqualung designed in France during World War II by the French naval officer and underwater explorer Jacques Yves Cousteau. It consists of one, two, or three cylinders (carried on the back) of compressed air fed to the diver's mouthpiece through valves that assure a constant flow at a pressure automatically equalized with outside water pressure.