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SCUBA
is an acronym for Self-Contained Underwater Breathing Apparatus.
These initials originated in 1939 in the US Navy to refer
to US military diver's rebreather sets. As with radar, the
acronym has become so familiar that it is often not capitalised
and is treated as an ordinary word: for example, it has been
taken into the Welsh language as "sgwba". A scuba
set provides a scuba diver with the breathing gas necessary
to breathe underwater. The Naval SCUBA was invented by Dr.
Christian J. Lambertsen, to aid the Navy in underwater warfare.
Dr. Lambertsen, who currently works at the University of Pennsylvania,
is considered by the Navy as "the father of the Frogmen."
Types of scuba set
Modern scuba sets are of two types:
• open-circuit (or Aqua-Lung™, very often written
"aqualung"). Here the diver breathes in from the
set and out to waste. This type of equipment is relatively
simple, making it cheap and reliable. The duration of open-circuit
dives is shorter than a dive with a rebreather, in proportion
to the weight and bulk of the set. It is uneconomic in its
use of expensive gas mixes such as heliox and trimix. The
cylinder is nearly always worn on the back. "Twin sets"
with two backpack cylinders were much more common in the 1960s
than now. Submarine Products sold a sport air scuba with 3
backpack cylinders. Sometimes cave divers have cylinders slung
at their sides instead.
• closed-circuit (or rebreather). Here the diver breathes
in from the set, and out back into the set where the exhaled
gas is reprocessed to make it fit to breathe again. They use
gas very economically, making long dives easy and special
mixes cheaper to use, at the expense of more complicated technology
and extensive experience and training requirements. The mechanisms
that control some types of rebreather are complex making them
expensive and more difficult to use and maintain.Both types
consist of a means of supplying air or other breathing gas
nearly always from a high pressure diving cylinder, and a
harness to strap it to the diver's body. Most open-circuit
scuba and some rebreathers have a demand regulator to control
the supply of breathing gas. Some rebreathers only have a
constant-flow regulator like in blowtorches. Some divers use
the word "scuba" to mean open-circuit sets only.
Open circuit scuba sets
Newspapers and television news often describe open circuit
scuba wrongly as "oxygen" equipment, probably by
false analogy from aeroplane pilots' oxygen cylinders.
At partial pressures over about 1.2 to 1.6 atmospheres, oxygen
becomes toxic. Open circuit scuba may supply various breathing
gases; but rarely pure oxygen, except during decompression
stops in technical diving.
Constant flow
Constant flow scuba sets do not have a demand regulator; the
breathing gas flows at a constant rate unless the diver switches
it on and off by hand. They run out of air quicker than aqualungs.
There were attempts at designing and using these before 1939,
for diving and for industrial use. Examples were "Ohgushi's
Peerless Respirator", and Commandant le Prieur's breathing
sets: see Timeline of underwater technology.
With a demand regulator
This type of set consists of one or more diving cylinders
containing breathing gas at high pressure (typically 220-300
Bar) connected to a diving regulator. The regulator supplies
the diver with as much of the gas as needed, at a pressure
suitable for breathing at the depth of the diver.
See diving cylinder for more information about the cylinders
and how they are arranged.
See Diving regulator for more information about diving regulators.
Colloquially this type of breathing set is often called an
aqualung, however, the word Aqua-Lung is correctly a tradename
protected by the Cousteau-Gagnan patent.
"Twin-hose" open-circuit scuba
In this type of set the two (or occasionally the one or the
three) stages of the regulator are in a large circular valve
assembly mounted on top of the cylinder pack.
It consists of two wide breathing tubes similar to those on
many modern rebreathers. The return tube was not for rebreathing
but because the air exhaust needed to be at the same depth
as the regulator's second stage diaphragm to avoid pressure
differences, which would cause a free-flow or resistance to
breathing according to the diver's attitude in the water.
These sets came with a mouthpiece as standard, but a fullface
mask was an option. Another optional extra was a mouthpiece
that also had a snorkel attached, and a valve to switch between
aqualung and snorkel.
single
hose aqualung |
"Single-hose"
open-circuit scuba
Most modern open-circuit scuba sets have a diving regulator
consisting of a first stage pressure reducing valve that is
sealed over the diving cylinder's output valve, and the second
stage "demand valve"; at the mouthpiece, with a
thin pressure hose linking the two stages. This type is called
"single hose". Many modern scuba sets have a spare
second stage demand valve on its own hose, which is called
an "octopus" or "alternate air source",
which is typically yellow in colour.
Captain Trevor Hampton in the 1950's or 1960's designed an
early single-hose aqualung with a fullface mask with a circular
window which was a very big and thus very sensitive demand
regulator diaphragm. But when he patented it, the Navy requisitioned
the patent, and by the time the Navy found no use in the patent
and released it, the market had moved on and he got no use
from the patent.
rebreather |
Rebreathers
With rebreathers, the gas the diver exhales is stored between
breaths in a "counterlung". In some rebreathers
one-way valves direct the gas through the mechanism. In others
the gas goes back and forth along the same route: this is
called the pendulum system. The oxygen consumed by the diver
is replaced, nearly always from a cylinder, The exhaled carbon
dioxide generated by the diver is removed by passing the gas
through a "scrubber", a canister full of soda lime.
Then the gas is fit to be re-inhaled. This type of scuba equipment
is known as 'closed circuit'.
Duration of a dive
The duration of an open-circuit dive depends on factors such
as the capacity (volume of gas) in the diving cylinder, the
depth of the dive and the breathing rate of the diver. An
open circuit diver whose breathing rate at the surface (atmospheric
pressure) is 25 litres per minute will consume 100 litres
of gas per minute at 30 metres. ( (30 m / 10 m per bar) +
1 bar atmospheric pressure ) ¥ 25 liter/min = 100 liter/min
). If a 15 litre cylinder filled to 200 bar is used until
there is a reserve of 25% there is (150 ¥ 15) = 2250 litres.
At 100 liter/min the dive will be a maximum of 22.5 minutes
(2250/100).
A semi-closed circuit rebreather dive is about three times
the length of the equivalent open circuit dive; gas is recycled
but fresh gas must be constantly injected and used gas vented.
Although it uses gas more economically, the weight of the
rebreathing equipment means the diver carries smaller cylinders.
So, if the diver uses a 5 litre cylinder filled to 200 bar
who leaves 25% in reserve will be able to do a 22.5 minute
dive (10l ¥ 200 bar ¥ 0.75 / 33.33). A fully-closed
circuit rebreather diver consumes about 1 litre of oxygen
per minute and virtually no diluent. So, if the diver has
a 3 litre cylinder filled to 200 bar who leaves 25% in reserve
will be able to do a 450 minute dive (3l ¥ 200 bar ¥
0.75 / 1). The life of the soda lime scrubber is likely to
be less than this and so will be the limiting factor.
In practice, dive times are more often influenced by other
factors such as water temperature and the requirement for
safe ascent (see decompression sickness).
Alternatives to scuba
There are alternative methods that a person can survive and
function while underwater, including:
* free-diving - swimming underwater on a single breath of
air.
* snorkeling - a form of free-diving where the diver's mouth
and nose can remain underwater when breathing, because the
diver is able to breathe at the surface through a short tube
known as a snorkel.
* surface supplied diving - originally used in professional
diving for long or deep dives where an umbilical line connects
the diver with the surface providing breathing gas, and sometimes
warm water to heat the diving suit, and usually nowadays voice
communications. Some tourist resorts now offer a surface supplied
diving arrangement, trademarked as Snuba, as an introduction
to diving for the inexperienced.
* Atmospheric diving suit - an armored suit which protects
the diver from the surrounding water pressure.
* Liquid breathing - so far, in the real world, liquid breathing
for humans is only laboratory experiments, and (one lung at
a time) medical treatment. It has possibilities of being used
for very deep diving. It is memorably portrayed in the film
"The Abyss".
* Artificial gills (human) - these are science fiction only.
In the real world they would have to process an unrealistically
massive amount of water to extract enough oxygen to supply
an active diver. But see Like-A-Fish for an attempt to develop
real artificial gills for divers.
Accessories
In modern scuba sets, a buoyancy compensator, such as a back-mounted
wing or stabiliser jacket (otherwise known as a 'stab jacket'),
is built into the scuba set harness. Although strictly speaking
this is not a part of the breathing apparatus, it is usually
connected to the divers air supply, in order to provide easy
inflation of the device, this can usually also be done manually
via a mouthpiece. The bladders inside the BCD inflate with
air from the ‘direct feed’ to decrease the total
density of the SCUBA equipment and cause the diver to float.
Another button deflates the BCD and increases the density
of the equipment and causes the diver to sink. Certain BCD's
allow for integrated weight, meaning that the BCD has special
pockets for the weights that can be dumped easley in case
of an emergency.
Diving weighting systems, ranging from 2 to 15 kilograms,
increase density of the scuba diver, allowing the diver to
fully submerge underwater with ease. Many modern rebreathers
use advanced electronics to monitor and regulate the composition
of the breathing gas.
Some scuba sets incorporate attached extra stage cylinders,
as bailout in case the main breathing gas supply is used up
or malfunctions, or containing another gas mixture. If these
extra cylinders are small, they are sometimes called "pony
cylinders". They often have their own demand regulators
and mouthpieces, and if so, they are technically distinct
extra scuba sets.
The diver may carry two or more sets of breathing equipment
to provide redundant alternative gas systems in the event
that the other fails or is exhausted. For open-circuit divers,
the two most common types of redundant configurations are
the "twinset", consisting of two similar systems,
and the "main plus pony", consisting of a large
main gas source and a small "pony" set. Rebreather
divers often carry a side-slung open-circuit "bail out"
to be used in the event the rebreather fails.
In technical diving, the diver may carry different equipment
for different phases of the dive; some breathing gas mixes
may only be used at depth, such as trimix and others, such
as pure oxygen, which only may be used during decompression
stops in shallow water. The heaviest cylinders are generally
carried on the back supported from a backplate while others
are side slung from strong points on the backplate.
When the diver carries many diving cylinders, especially those
made of steel, lack of buoyancy becomes a problem. High capacity
buoyancy compensators are used to allow the diver to control
his or her depth. An excess of tubes and connections passing
through the water tend to decrease diving performance by causing
hydrodynamic drag in swimming. Some diver training organizations
and groups of divers teach techniques, such as DIR diving
for configuring diving equipment.
History
A predecessor to scuba gear, the Momson lung, was used as
emergency escape gear by WWII submariners.
Before 1971 all breathing sets including scuba came with a
plain harness of straps with buckles like on a rucksack or
spray-tank-pack. The buckles were usually quick-release. Many
did not have a backpack plate, but the cylinders were directly
against the diver's back. Sport scuba usually had quick-release
fastenings instead of ordinary buckles. The harnesses of many
diving rebreathers made by Siebe Gorman included a large back-sheet
of strong reinforced rubber.
In the beginning scuba divers dived without any buoyancy aid.
In emergency they had to jettison their weights. In the 1960's
inflatable diver's lifejackets (also called ABLJ for adjustable
buoyancy life jacket) for aqualung-type scuba became available.
The ABLJ is used for two purposes, one to adjust the buoyancy
of the diver to compnesate for loss of buoyancy (chiefly due
to compression of neoprene wet suit)and more importantly as
a lifejacket that can be rapidly inflated even at depth. It
was put on before putting on the cylinder harness. The first
were inflated with a small carbon dioxide cylinder, later
with a small air cylinder. The use of an extra feed from the
first stage regulator permits control of the life jacket as
a buoyancy aid.
Great diving locations: • Cozumel
This article is licensed under the GNU
Free Documentation License.
It uses material from the Wikipedia
article "Scuba".
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