Diving Bell

Lore & Background

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 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. 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.

Reader's Guide

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: Fresh gas is available for breathing by the occupants, and volume reduction of the air in an open bell due to increasing hydrostatic pressure as the bell is lowered is compensated.

Adding pressurized gas ensures that the gas space within the bell remains at constant volume as the bell descends in the water. Otherwise the bell would partially fill with water as the gas was compressed. A wet bell, or open bell, is a platform for lowering and lifting divers to and from the underwater workplace, which has an air filled space, open at the bottom, where the divers can stand or sit with their heads out of the water. The air space is at ambient pressure at all times, so there are no great pressure differences, and the greatest structural loads are usually self weight and the buoyancy of the air space.

A fairly heavy ballast is often required to counteract the buoyancy of the airspace, and this is usually set low at the bottom of the bell, which helps with stability.

Did You Know?

• In 1687, Sir William Phipps used a diving bell to salvage 30 tons of silver from the wreck of Nuestra Señora de la Concepción, worth over £200,000.
• In 1775, Charles Spalding improved on Halley's design by adding a system of balance-weights to ease raising and lowering the bell.