Diving bell
From Wikipedia, the free encyclopedia
A diving bell is a cable-suspended airtight chamber, open at the bottom, that is lowered underwater to operate as a base or a means of transport for a small number of divers. Unlike a submarine it is not designed to move under the control of its occupants, nor to operate independently of its tether.
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[edit] Mechanics
Diving bells are used as underwater rescue vessels and by working divers doing underwater work and salvage. The bell is lowered into the water by cables from a crane attached to a ship or dock. The bell is ballasted so as to remain upright in the water and to be negatively buoyant so that it sinks even when completely full of air.
Hoses, fed by pumps on the surface, provide compressed breathing gas to the bell, serving two functions:
- Fresh gas is available for breathing by the occupants. Exhaled gas is expelled under the lip of the wet bell, where it rises naturally to the surface.
- As a wet bell is lowered, the extra pressure from the water compresses the gas in the bell. If the gas pressure inside the bell were not raised by adding gas to compensate for the outside water pressure the bell would partially fill with water as the gas is compressed. Adding pressurized gas ensures that the usable workspace within the bell remains constant as the bell descends in the water as well as refreshing the air, which would become saturated with a toxic level of carbon dioxide and depleted of oxygen by the respiration of the occupants.
A similar principle to that of the wet bell is used in the diving helmet of standard diving dress, where compressed air is provided to a helmet carried on the diver's shoulders. Additional weights are carried on the waist and feet to prevent overturning. The modern equivalent of this diving equipment is used in surface supplied diving.
A wet sub may also provide a dry viewing chamber for the operator's head, acting as would a diving helmet.
[edit] History
The diving bell is one of the earliest types of equipment for underwater work and exploration. Its use was first described by Aristotle in the 4th century BC:"...they enable the divers to respire equally well by letting down a cauldron, for this does not fill with water, but retains the air, for it is forced straight down into the water."[1] In 1535, Guglielmo de Lorena created and used what is considered to be the first modern diving bell.
The earliest applications were probably for commercial sponge fishing. A diving bell was used to salvage a cannon from the Swedish warship Vasa in the period immediately following its sinking in 1628.
In 1690 Edmund Halley completed plans for a diving bell capable of remaining submerged for extended periods of time, and fitted with a window for the purpose of undersea exploration. In Halley's diving bell, atmosphere is replenished by sending weighted barrels of air down from the surface.
[edit] In nature
The diving bell spider, Argyroneta aquatica, is a spider which lives entirely under water, even though it could survive on land.
Since the spider must breathe air, it constructs from silk, a diving bell which it attaches to an underwater plant. The spider collects air in a thin layer around its body, trapped by dense hairs on its abdomen and legs. It transports this air to its diving bell to replenish the air supply in the bell. This allows the spider to remain in the bell for long periods, where it waits for its prey.
[edit] Underwater habitats
A further extension of the wet bell concept is the underwater habitat, where divers may spend long periods in dry comfort while acclimated to the increased pressure experienced underwater. By not needing to return to the surface they can avoid the necessity for decompression (gradual reduction of pressure), required to avoid problems with nitrogen bubbles releasing from the bloodstream (the bends, also known as caisson disease). Such problems occur at a pressure over two atmospheres, experienced below a depth of 32 feet. By not requiring a pressure resistant structure the habitat can be constructed at lower cost.
[edit] Diving chambers
Simple wet diving bells have been largely replaced now with more sophisticated diving chambers (sometimes erroneously called diving bells) which may resist underwater pressures and which are for use in underwater work involving saturation diving and submarine rescue operations. These may comprise a single chamber designed for attachment to a submarine's rescue hatch, or may be composed of two sealed chambers, an upper chamber at normal or greater atmospheric pressure and an entrance lock. These often carry large diving cylinders to provide emergency breathing gas supplies and can be used as a base for surface supplied diving operations.
[edit] References
- ^ Arthur J. Bachrach, "History of the Diving Bell", Historical Diving Times, Iss. 21 (Spring 1998)
