Scuba diving
Last updated: April 25, 2007.
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.
Divers who want to breathe underwater and swim freely (outside a
submarine or submersible) have to wear masks and breathe nitrox
(an air-like mixture of nitrogen and oxygen) from tanks that they carry
with them. This sort of equipment is called
scuba (self-contained underwater breathing apparatus),
or an aqualung,
and it comes in two basic kinds.
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.
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 oyxgen 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 snorkelling, 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 deep 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 judgement
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 a diver's
body, protecting him 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 also
need torches to see where they're going. Goggles
or masks
keep the seawater away from their eyes and are typically fitted with
lenses to magnify
the underwater world.
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.
A very brief history of 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 breating 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
Books
Websites
