Diving bell
654092
214034818
2008-05-21T21:09:48Z
Canis Lupus
5517044
Salvador Dali → Salvador Dalí
[[Image:Diving bell.jpg|thumb|Diving bell]]
A '''diving bell''', also known as a '''wet bell''', is a cable-suspended airtight chamber, open at the bottom like a [[moon pool]] structure, that is lowered [[underwater]] to operate as a base or a means of transport for a small number of divers. The [[Hydrostatic pressure|pressure of the water]] keeps the air trapped inside the bell. They were the first type of [[diving chamber]]. Unlike a [[submarine]] the diving bell is not designed to move under the control of its occupants, nor to operate independently of its tether.
==Mechanics==
[[Image:L-Taucherglocke.png|thumb|right|200px|Occupied diving bell illustrated]]
Diving bells are used as underwater rescue vessels and by [[professional diving|working divers]] doing underwater work and [[salvage]]. The bell is lowered into the water by cables from a [[crane (machine)|crane]] attached to a [[ship]] or [[Dock (maritime)|dock]]. The bell is ballasted so as to remain upright in the water and to be negatively [[buoyancy|buoyant]] so that it sinks even when completely full of air.
Hoses, fed by [[gas compressor|pumps]] on the surface, provide compressed [[breathing gas]] to the bell, serving two functions:
* Fresh gas is available for breathing by the occupants, and excess gas leaks out under the lip of the wet bell, where it bubbles naturally to the surface.
* As a wet bell is lowered, increasing [[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 was 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.
The physics of the diving bell applies also to an [[underwater habitat]] equipped with a [[moon pool]], which is like a diving bell enlarged to the size of a room or two, and with the water–air interface at the bottom confined to a section rather than forming the entire bottom of the structure.
==History==
[[Image:Alexander the Great diving NOAA.jpg|thumb|16th century painting of [[Alexander the Great]] lowered in a glass diving bell.]]
The diving bell is one of the earliest types of equipment for underwater work and exploration.<ref>{{cite journal |last=Bevan |first=J. |title=Diving bells through the centuries |journal=South Pacific Underwater Medicine Society Journal |volume=29 |issue=1 |date=[[1999]] |issn=0813-1988 |oclc=16986801 |url=http://archive.rubicon-foundation.org/5991 |accessdate=2008-04-25 }}</ref> 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."<ref>Arthur J. Bachrach, "History of the Diving Bell", ''Historical Diving Times'', Iss. 21 (Spring 1998)</ref> 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 [[sea sponge|sponge]] fishing. A diving bell was used to [[Marine salvage|salvage]] more than 50 [[cannon]]s from the Swedish warship ''[[Vasa (ship)|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.
Once when asked to give a lecture, Salvador Dalí showed up in a diving bell, and insisted on speaking from inside it.
==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]].
==Underwater habitats==
{{Main|Underwater habitat}}
As noted above, further extension of the wet bell concept is the moon-pool-equipped 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 stops|decompression]] (gradual reduction of pressure), required to avoid problems with nitrogen bubbles releasing from the bloodstream (the [[Decompression sickness|bends]], also known as caisson disease). Such problems occur at a pressure over two atmospheres, experienced below a depth of 20 metres (64 feet).
==Relationship to hyperbaric chambers==
[[Professional diver|Commercial diving operators]] now generally use the [[hyperbaric chamber]], a more modern type of sealable diving chamber based on a [[pressure vessel]] which is pressurised by an [[air pump]] rather than by the ambient water pressure. These have safety advantages and allow decompression to be carried out after being raised to the surface and taken back to base on a diving support vessel. They are used especially in [[saturation diving]] and undersea rescue operations. However this kind of diving chamber is often used in conjunction with a separate diving bell, or may be connected via an [[airlock]] to another compartment which uses the diving bell principle for access to the water.
== References ==
{{reflist}}
==See also==
*[[Diving chamber]]
* [[Timeline of underwater technology]]
* [[Bathysphere (vessel)|Bathysphere]]
* [[Benthoscope (vessel)|Benthoscope]]
*[[Moon pool]]
== External links ==
*[http://physics.kenyon.edu/EarlyApparatus/Pneumatics/Diving_Bell/Diving_Bell.html Diving Bell: Kenyon College Department of Physics]
*[http://www.thehds.com/publications/bell.html Historical Diving Society: The History of the Diving Bell]
*[http://www.divingheritage.com/chambers.htm Historic diving bells]
*[http://www.divingheritage.com/bells.htm Modern diving bells]
[[Category:Diving equipment]]
[[cs:Keson]]
[[da:Dykkerklokke]]
[[de:Taucherglocke]]
[[fr:Cloche de plongee]]
[[it:Campana subacquea]]
[[he:פעמון צלילה]]
[[nl:Duikerklok]]
[[ja:潜水鐘]]
[[ru:Водолазный колокол]]
[[sk:Potápačský zvon]]
[[fi:Sukelluskello]]
[[sv:Dykarklocka]]