The book so far has covered how gas behaves under pressure, what it does to your body, and how models plan a safe ascent. This part turns to equipment: how a diver actually gets gas to breathe under water.
Every piece of diving equipment solves 2 problems. It must deliver gas at the same pressure as the water around your chest, or you cannot breathe it in. And it needs a supply of that gas, either from the surface or carried by the diver.
Divers measure pressure in ATA (atmospheres absolute, about the same as bar). At the surface you are under 1 ATA of air. Every 10 m of sea water adds 1 more ATA. So at 30 m the ambient pressure, the total pressure around you, is 4 ATA. Gas at that pressure is squeezed into a quarter of its surface volume (chapter 1). So a diver at depth needs more gas for each breath.
The diving bell
A diving bell is a heavy chamber, open at the bottom, lowered on a rope. The water below traps air inside it, so people in the bell can breathe while they work on the seabed. Think of it like pushing an upturned glass straight down into a sink: the air stays trapped at the top.
In 1691 Edmond Halley designed a bell with a supply of fresh air. In 1716 he reported that he and 4 others had stayed at about 18 m for more than an hour and a half.
- Air
- Sea surface
Air squeezed by depth
As the bell goes down, the water presses harder on the trapped air and squeezes it. At 10 m the pressure is 2 ATA, so the air takes up half the space it had at the surface.
Water rises inside the bell to fill the rest. This is Boyle’s law: double the pressure and a gas takes half the space (chapter 1).
At Halley’s depth
At 18 m the pressure is 2.8 ATA, so the air takes up about 36% of the bell (1 divided by 2.8). The people inside breathe it at that pressure.
Each breath uses up oxygen and adds carbon dioxide. Without fresh air, the air in the bell would soon be unsafe to breathe.
Barrels of fresh air
Halley sent down weighted barrels of air from the surface. Each barrel hung below the bell, with a leather hose from its top into the bell.
Water came in at the bottom of the barrel and pushed its air up the hose into the bell. That pushed the water level in the bell back down. Used air went out through a tap at the top of the bell. With a steady supply of barrels, the air no longer limited the time in the bell.
Helmets supplied from the surface
A bell cannot move along the bottom. The next step put a small air space on the diver’s head and fed it from the surface through a hose.
In 1823 Charles Deane patented a smoke helmet for fire-fighters, fed with air through a hose. By 1828 he and his brother John had adapted it for diving. A hand pump on the surface pushed air down the hose. The air escaped under the helmet’s lower edge, which rested on the diver’s shoulders. The diver had to stay upright: if they bent over or fell, the helmet flooded.
Around 1837 to 1840 Augustus Siebe sealed a helmet to a watertight suit, with a valve to let used air out. This helmet could not flood when the diver leaned or knelt. It became the standard diving dress for the next century.
With air pumped from the surface, the air supply no longer limited the dive. The hose and the pump did. As divers worked deeper and longer, so did decompression sickness (chapter 5), the illness caused by gas bubbles forming in the body after a dive.
The first demand regulator
A regulator turns high-pressure gas into gas at the pressure around you. A demand regulator gives gas only when you breathe in. Think of it like a tap that runs only while you drink from it.
In 1864 Benoît Rouquayrol and Auguste Denayrouze patented the first one. The diver carried a tank of compressed air on their back, filled from a surface pump through a hose. The regulator fed the diver from that tank. It held enough air for the diver to unhook the hose for a short time. The French Navy bought the first sets in 1865, and Jules Verne gave them to Captain Nemo’s crew in Twenty Thousand Leagues Under the Seas.
Carrying all the gas
The last step was to carry the whole supply. In 1926 Yves Le Prieur, with Maurice Fernez, showed a set with a cylinder of compressed air carried by the diver. The diver opened a valve by hand, and air flowed to the mouth all the time. This is a free-flow set.
In 1943 Jacques-Yves Cousteau and Émile Gagnan patented a set that joined a high-pressure cylinder to a demand regulator. From 1946 it was sold as the Aqua-Lung. It is open circuit (OC): each breath is breathed out into the water. Every open-circuit scuba set since works the same way.
- Gas from cylinder
- Diver inhales
- Lost to the water
Free flow
In a free-flow set, gas leaves the cylinder at a steady rate set by the valve. The flow must be high enough for your deepest breath.
But you only breathe in for part of each breath. The gas that flows while you breathe out goes straight into the water. So most of the cylinder is wasted.
Demand
A demand valve opens only when you breathe in. Breathing in lowers the pressure in the mouthpiece. That moves a flexible disc called a diaphragm, which opens the valve. When you stop breathing in, it closes.
The cylinder supplies only the gas you breathe, at the pressure around you. The same cylinder lasts much longer than on a free-flow set.
Demand at depth
At 30 m the ambient pressure is 4 ATA. Each breath fills your lungs to the same size as at the surface. But the gas in it is squeezed to 4 times the density.
So the regulator takes 4 times as much gas from the cylinder for each breath, and the cylinder lasts a quarter as long as at the surface. Open circuit and rebreathers use gas in different ways; chapter 20 compares them.
- Gas from the surface
- Gas carried by the diver
What to remember
- The air in a diving bell is squeezed as it goes down, following Boyle’s law. Halley kept it fresh with barrels of air from the surface.
- Helmets fed from the surface let the diver leave the bell. Siebe’s sealed helmet and suit could not flood when the diver leaned over.
- A demand regulator gives gas only when you breathe in, at the pressure around you. The first was patented in 1864; Cousteau and Gagnan joined one to a high-pressure cylinder in 1943.
- A free-flow set wastes the gas that flows while you breathe out.
- On open circuit, each breath at depth takes more gas from the cylinder: 4 times as much at 30 m as at the surface.
Gas planning in DiveLogic
The gas needed calculator works out how much gas an open-circuit dive uses at each depth, from your breathing rate. Your RMV (respiratory minute volume) is the volume of gas you breathe in a minute at the surface. Many courses call it your SAC rate (surface air consumption rate). The planner does the same for every depth and stop of a full plan.
Sources
- Halley E. (1716). The art of living under water. Philosophical Transactions of the Royal Society 29: 492-499.
- Davis R. H. Deep Diving and Submarine Operations. Siebe Gorman, London (editions from 1935). (History of the Deane, Siebe and Fleuss equipment.)
- Verne J. (1870). Vingt mille lieues sous les mers. Hetzel, Paris.
- Cousteau J.-Y., Dumas F. (1953). The Silent World. Harper, New York.
- US Navy (2016). US Navy Diving Manual, Revision 7. Naval Sea Systems Command. (Open-circuit gas consumption at depth.)