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 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.
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, 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.
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. 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 more than 50 cannons 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.
Once when asked to give a lecture, Salvador Dalí showed up in a diving bell, and insisted on speaking from inside it.
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.
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
(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 20 metres (64 feet).
Relationship to hyperbaric chambers
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.