Children in a swimming pool experiment with trapping air in a bucket by quickly submerging it upside-down. While seemingly insignificant, this simple observation forms the basis of a diving bell. While we may think of swimming pools as somewhat modern, the idea of a diving bell dates back as far as Aristotle in the 4th century BC.
Not only is the idea of a diving bell simple, but the actual device is quite rudimentary, even in modern implementations. Any watertight container (except for the opening) can function as one. By submerging it underwater and keeping it vertical, a pocket of air becomes trapped inside by the water pressure.
This container usually takes the shape of a bell, hence the name diving bell. A bell is particularly effective for trapping air during submersion, although such a shape is certainly not required.
Of course, as the container grows larger, submersion becomes more challenging. As you know from your understanding of buoyancy, the larger an object is, the heavier it has to be to become negatively buoyant and sink. If you don't believe me, try to pull a large plastic tub underwater while it is upside down.
Large containers can hold large amounts of air without drastic increases in weight. Air is essentially weightless, and thus makes the diving bell incredibly buoyant. The solution is to make a diving ball out of heavy, dense material, and to even add weights to the rim to aid in sinking.
Dropping a large metal container underwater serves little purpose, so naturally the idea is to put people inside it. This can be for any number of reasons, but is usually for exploration (watching things underwater) or commercial endeavors (recovering sunken items, intensive work, etc.). Putting people in a diving bell introduces two problems:
- Depth. To do anything useful, the bell will usually have to go fairly deep. However, with every 10 meters of depth, pressure increases and decreases the volume of air. What this means is that by the time the bell has reached a suitable depth, the volume of breathable air has shrunk considerably, leaving most of the bell filled with water.
- Respiration. People need to breathe. In comes air, out comes carbon dioxide. Put a person in a fixed volume of air, and it won't take long for carbon dioxide levels to reach a dangerous level. The diving bell needs fresh air.
The solution to both of these problems is the same: while the bell is submerged, continuously supply it with fresh air. This is commonly achieved by various hoses leading to the surface. Air is pumped from the surface to the bell at depth, keeping it full of fresh, breathable air, with excess easily escaping out of the bottom.
Diving bells have a long recorded history. In addition to Aristotle, Alexander the Great is said to have utilized them during the siege of Tyre, in 332 BC.
Fast-forwarding quite a bit, there are recorded uses of a diving bell, or a device conceptually identical, by Francis Bacon in 1597, Greek divers in 1538, and by various inventors in 1551 and 1616.
In 1687, Sir Williams Phipps used one to recovered treasure from a sunken Spanish ship off the coast of San Domingo. In 1640, a diving bell was used for a similar purpose in Charlton.
Before long, led by the likes of Denis Papin and Edmund Halley, users became quite adept at keeping the air in diving bell's replenished, allowing long periods of uninterrupted diving.
Modern diving bells
By the 1800s, diving bells were used quite extensively in various recoveries and civil engineering tasks, from the construction of bridges to the repair of the Thames Tunnel in 1827.
One small variation is opt for smaller bells that essentially fit over the head only, rather than large constructions with room enough for several people. The trickiest part of these underwater "helmets" is keeping the air inside, since any significant tilt of the head will flood the device with water.
Even today, many destinations offer bell diving experiences. with modern versions that overcome many of the limitations that plagued early iterations.
Diving bells and decompression sickness
While submerged in a diving bell, the operator is breathing air at ambient pressure, and is therefore subject to the usual risks of decompression sickness. In fact, it was the early use of diving bells and their DCS-causing brothers, like the caisson, that eventually led to the discovered of "the bends" by bridge builders in the late 1800s.
With all the practical uses for bell diving, a cure for this curious disease became a priority, leading to the initial research by John Haldane, which forms the basis of decompression theory still in use today.
Bell diving is not so common for the recreational diver. Nevertheless, it is interesting to learn about its history, both from an engineering perspective as well as a precursor to modern scuba.