A diving bell is a rigid chamber used to transport divers from the surface to depth and back in open water, usually for the purpose of performing underwater work. The most common types are the open-bottomed wet bell and the closed bell, which can maintain an internal pressure greater than the external ambient. Diving bells are usually suspended by a cable, and lifted and lowered by a winch from a surface support platform. Unlike a submersible, the diving bell is not designed to move under the control of its occupants, or to operate independently of its launch and recovery system. The wet bell is a structure with an airtight canopy which is open to the water at the bottom, that is lowered underwater to operate as a base or a means of transport for a small number of divers. Air is trapped inside the bell canopy by pressure of the water at the interface. These were the first type of diving chamber, and are still in use in modified form. The closed bell is a pressure vessel for human occupancy, which may be used for bounce diving or saturation diving, with access to the water through a hatch at the bottom. The hatch is sealed before ascent to retain internal pressure. At the surface, this type of bell can lock on to a hyperbaric chamber where the divers live under saturation or are decompressed. The bell is mated with the chamber system via the bottom hatchway or a side hatchway, and the trunking in between is pressurized to enable the divers to transfer through to the chamber under pressure. In saturation diving the bell is merely the ride to and from the job, and the chamber system is the living quarters. If the dive is relatively short (a bounce dive), decompression can be done in the bell in exactly the same way it would be done in the chamber. A third type is the rescue bell, used for the rescue of personnel from sunk submarines which have maintained structural integrity. These bells may operate at atmospheric internal pressure and must withstand the ambient water pressure.
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." There are apocryphal accounts of Alexander the Great (including some versions of the Alexander Romance), exploring underwater in an enclosed vessel, lowered from his ships. Their origin is hard to determine, but some of the earliest dated works are from the early Middle Ages. In 1535, Guglielmo de Lorena built and tested a diving bell, to explore a sunken vessel in a lake near Rome. De Lorena's diving bell only had space for enough oxygen for a few minutes. However, the air in his diving bell was reported to last for one to two hours with the limiting factor being a diver's ability to withstand cold and fatigue, not lack of oxygen. The mechanism he used needed to keep the pressure inside the bell continuous, supply fresh air, and remove air exhaled by the diver. To accomplish this, it is believed that de Lorena used a system similar to Edmond Halley's design, two and a half centuries later. An improved, but possibly unbuilt, design from 1616 by Franz Kessler featured a bell covering a diver down to their ankles, and added windows and a ballast to the bottom. This design supposedly no longer needed to be tethered to the surface, although there is no evidence it was actually built. In 1642, John Winthrop reported one Edward Bendall building two large wooden barrels, weighted with lead and open at their bottoms, to salvage a ship Mary Rose which had exploded and sunk, blocking the harbor of Charlestown, Boston. Bendall undertook the work on condition that he be awarded all the value of the salvage should he succeed in unblocking the harbor, or half the value he could salvage if he could not. In 1658, Albrecht von Treileben was permitted to salvage the warship Vasa, which sank in Stockholm harbor on its maiden voyage in 1628. Between 1663 and 1665 von Treileben's divers were successful in raising most of the cannon, working from a diving bell. A diving bell is mentioned in the 1663 Ballad of Gresham College (stanza 16):
In late 1686, Sir William Phipps convinced investors to fund an expedition to what is now Haiti and the Dominican Republic to find sunken treasure, despite the location of the shipwreck being based entirely on rumor and speculation. In January 1687, Phipps found the wreck of the Spanish galleon Nuestra Señora de la Concepción off the coast of Santo Domingo. Some sources say they used an inverted container for the salvage operation while others say the crew was assisted by Indian divers in the shallow waters. The operation lasted from February to April 1687 during which time they salvaged jewels, some gold and 30 tons of silver which, at the time, was worth over £200,000. In 1689, Denis Papin suggested that the pressure and fresh air inside a diving bell could be maintained by a force pump or bellows. Engineer John Smeaton utilized this concept in 1789. In 1691, Dr. Edmond 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 design, atmosphere is replenished by sending weighted barrels of air down from the surface.
In 1775, Charles Spalding, an Edinburgh confectioner, improved on Halley's design by adding a system of balance-weights to ease the raising and lowering of the bell, along with a series of ropes for signaling the surface crew. Spalding and his nephew, Ebenezer Watson, later suffocated off the coast of Dublin in 1783 doing salvage work in a diving bell of Spalding's design.
Mechanics
The bell is lowered into the water by cables from a crane, gantry or A-frame attached to a floating platform or shore structure. The bell is ballasted so as to remain upright in the water and to be negatively buoyant, so that it will sink even when full of air. Hoses, supplied by gas compressors or banks of high pressure storage cylinders at the surface, provide breathing gas to the bell, serving two functions:
… excerpt ends here. Continue reading the full article.
