Scuba diving
by Chris Woodford. Last updated: January 19, 2012.
It's hard to believe people once lived in the oceans, but it's true—well, sort of. Like other forms of land life, humans began their evolution from fish in the Devonian part of the Paleozoic era of Earth's history around 400 million years ago. Since then, we've largely forgotten all the oceanic tricks we used to know. Our bodies, are no longer suited to living beneath the sea. We don't have fins, we can't breathe underwater, and we find the cold, darkness, and pressure of undersea life almost impossible to cope with. Fortunately, technology has come to our aid. Scuba breathing apparatus helps us stay underwater for hours at a time, while protective wetsuits, masks, goggles and other equipment ensure we can survive in the harsh undersea world, just like the fish we evolved from!
Photo: An underwater photographer from the US Navy dives off the coast of Panama City, Florida. Photo by Andy McKaskle courtesy of US Navy.
Breathing underwater
Photo: Scuba breathing tanks. These heavy metal cylinders are designed to withstand high pressures underwater. They contain "breathing air", a mixture of oxygen and nitrogen.
Why we need air
The air around us is about 79 percent nitrogen and 21 percent oxygen (with tiny amounts of a few other gases thrown in for good measure). When we breathe in, our bodies use the oxygen in a process called respiration that allows us to turn the food we've eaten into useful energy to power our muscles. We gather oxygen with powerful lungs, inflating and deflating every few seconds like a pair of a natural bellows adapted to processing the air all around us. Although our natural "breathing apparatus" evolved from the gills that fish use for breathing underwater, we're no longer able to breathe like they do. A fish gulps in water through its mouth and lets it trickle past its gills. These sensitive gas gatherers extract most of the oxygen it contains, and then the water drains back into the sea. If we tried to do that, we'd suffocate or drown! Gills are incredibly efficient oxygen collectors. Where our lungs grab about 25 percent of the oxygen from the air we inhale, fish gills remove about 80 percent of the oxygen in each gulp of water.
Divers who want to breathe underwater and swim freely (outside a submarine or submersible) have to wear masks and breathe oxygen from tanks that they carry with them. This sort of equipment is called scuba (self-contained underwater breathing apparatus), or an aqualung. While many divers use ordinary compressed air in their scuba tanks, others use more specialized breathing mixtures including nitrox (an air-like mixture of nitrogen and oxygen), heliox (a mixture of oxygen and helium), and trimix (mixing three gases—oxygen, nitrogen, and helium).
Two kinds of scuba
Breathing equipment comes in two basic kinds. In the simplest kind of scuba, divers breathe in an air-like mixture from a tank on their back. Their lungs remove about 25 percent of its oxygen and breathe the other 75 percent back out again with the waste carbon dioxide their lungs exhale at the same time. The mixture of carbon dioxide and oxygen bubbles out into the water—and is wasted. Eventually, when all the gas in the tank has been used up, the diver has to resurface again. Nearly all scuba divers use this kind of basic apparatus, which is called open-circuit scuba. It's quite inefficient, because most of the oxygen the diver breathes in actually ends up in the sea!
A more elaborate kind of scuba apparatus, called a rebreather, has the diver breathing in and out of a closed-loop of tubes and tanks around which gas is constantly circulating. As with ordinary scuba, the diver breathes in a mixture of oxygen and other gases. Their lungs remove some of the oxygen, and breathe out the rest with carbon dioxide. But instead of the oxygen and carbon dioxide being wasted into the sea, they flow back around the diver's breathing system for recycling, in a completely closed-loop. The carbon dioxide is removed by a chemical scrubber and more oxygen is added from tanks on the diver's back to replace what the diver's lungs removed. A rebreather (which is also known as a closed-circuit scuba) is much more efficient than an ordinary scuba because no oxygen is being breathed out into the sea.
Photo: A US Navy diver wearing a MK-16 scuba rebreather. Photo by Andrew McKaskle courtesy ofUS Navy.
The pressure problem
Getting enough oxygen is not the only problem divers face when they plunge beneath the waves: the nitrogen they breathe in can also cause problems. It's all to do with pressure.
The deeper you dive, the more the water pressure increases and the harder it is to breathe. If you've ever been snorkeling, or you're used to diving in the deep end of a swimming pool, you've probably noticed how much harder it is to breathe underwater. Imagine how much harder it gets when you go deep down under the sea, where the pressure increases very rapidly. Every extra 10 m (33 ft) you descend, the pressure increases by another atmosphere (an "atmosphere" is the normal air pressure around us on land). In other words, 10 m (33 ft) below the surface the pressure is twice as great as it is at the surface and 20 m (66 ft) below, it's three times as great. No-one can dive deeper than about 120 m (400 ft) without a special deep-sea diving suit because the pressure is too great to breathe. When you get to great depths, the pressure becomes truly immense. At a depth of about 10,000 m (33,000 ft), water pressure would produce a force equal to the weight of an elephant standing on every square inch of your body. Not surprisingly, you'd be instantly crushed as flat as a pancake!
Photo: A scuba diver is treated for "the bends" (decompression sickness) in a decompression chamber onboard a ship. Photo by Jeff Viano courtesy of US Navy.
That's not the only reason pressure is a problem. Water pressure also changes the way the body responds to the different gases that divers breathe in. If they dive deeper than about 30 m (100 ft), they can suffer from a problem called nitrogen narcosis (also called the "Martini effect" and "rapture of the deep"), which is a bit like being drunk underwater: it can dangerously impair judgment and put divers at great risk. To avoid this, divers who want to go deeper tend to breathe a gas mixture containing less nitrogen, such as heliox (a mixture of mostly helium and oxygen).
Coming back to the surface presents problems too. If divers come up too quickly, they can suffer from a problem called "the bends" (also known as decompression sickness or caisson disease). As they rise up and the pressure falls, bubbles of nitrogen start to appear in their blood. It's exactly the same as unscrewing a fizzy drink bottle. Removing the top reduces the pressure and the gas forced inside the liquid suddenly reappears in the form of bubbles. Depending on where in the blood the bubbles form, the bends can cause anything from a mild pain in the joints to complete paralysis or even death. Divers avoid it by surfacing very slowly. If they must surface quickly, in an emergency, they are usually put in a high-pressure decompression chamber on a ship or on land straight afterwards to "decompress" (with the pressure gradually reduced to normal).
Surviving underwater
Photo: Assorted scuba gear. You need all kinds of equipment to dive safely, but you can hire all of it if you need to.
Oxygen is not the only thing we need to survive under the sea. Ocean water is usually colder than the air above it, especially as you go further from the tropics toward the poles. Wetsuits made from a synthetic rubber called neoprene help to stop the heat escaping from your body, protecting you against a life-threatening condition called hypothermia. Up to half our body heat is lost from our heads, so divers typically wear neoprene hoods as well (and gloves and boots too). Some divers prefer to wear drysuits instead. Unlike wetsuits, drysuits are oversized, fully sealed overalls worn on top of thermal underclothes to keep the water out. One problem with wetsuits is that neoprene contains air pockets, which makes it naturally buoyant. In other words, a wetsuit increases your floatation. For this reason, divers typically wear weights around the belts to make it easier for them to sink beneath the surface.
Since little light penetrates beneath the ocean surface, divers often need to use torches to see where they're going, especially at great depths. Goggles (covering just the eyes) or masks (covering the nose as well) keep the seawater away from a diver's eyes; for people who normally wear eyeglasses, they can be fitted with different kinds of lenses to correct for short- or long-sight. Another handy piece of equipment is a wristwatch dive computer that tells divers how long they've been underwater, how deep they're diving, what temperature the water is, and other useful information. Since divers always have plenty to carry, they swim by kicking their feet rather than hauling with their hands. Long-bladed dive fins act like levers, giving them extra propulsion.
Who invented scuba diving?
- 360 BCE: Greek philosopher Aristotle suggests kettles of air can be lowered into the water to help divers breathe.
- 300 BCE: Greek ruler Alexander the Great believed to have explored underwater in a diving bell.
- 15th century: Italian scientist and inventor Leonardo da Vinci (1452–1519) invents a leather diving helmet with an air tube running to the surface.
- 1808: Friedrich von Drieberg invents a diver's breathing back-pack called Triton,
- 1943: Jacques Cousteau (1910–1997) and Émile Gagnan (1900–1979) invent modern scuba breathing equipment.
- 1950s: Aqualung widely available commercially and scuba "dive shops" pop up around many coastal resorts. Jacques Cousteau makes books and films that popularize scuba diving and secure his worldwide reputation as one of the greatest ocean explorers of all time.
Further reading
On this website
Books
- Scuba Diving by Dennis Graver. Champaign, Illinois: Human Kinetics, 2009.
- Eyewitness Scuba Diving by Monty Halls and Miranda Krestovnikoff. New York/London: Dorling Kindersley, 2006.
- Fifty Places to Dive Before You Die: Diving Experts Share the World's Greatest Destinations by Chris Santella. New York: Stewart Tabori & Chang, 2008.
- Diving: The World's Best Sites. Edited by Jack Jackson. New York: Rizzoli, 1997.
- Encyclopedia of Recreational Diving by Alex Brylske et al. Santa Ana, California: International PADI, Inc., 1988.
Websites
- PADI: Professional Association of Diving Instructors: "PADI Divers carry the most respected and sought after scuba credentials in the world." .
- BSAC: British Sub Aqua Club: "The world's biggest single diving club".
- Scuba Diving: The online home of scuba diving magazine.
- Flickr: Scuba diving: A Flickr group for scuba divers who want to share their amazing underwater photos. At the time of writing, there are over 50,000 photos and over 3,000 members.
- About.com Scuba: A large collection of scuba resources hand picked by Melissa Rodriguez.
