What's SCUBA?

Self Contained Underwater Breathing Apparatus (SCUBA) is the term given to equipment that provides the diver with a life support system which is carried independently by the diver without reliance on help from the surface. The development of cheap and convenient SCUBA starting in the 1940s lead to the huge growth recreational diving of the last 40 years.

There are two types of SCUBA variants:

What's an Aqualung (or open circuit SCUBA)?

An aqualung consists of two main components:

• a cylinder holding air at 200 to 300 times atmospheric pressure. This allows us to store air and transport it under water.
• an air pressure regulator. This has two jobs:
  • it reduces the very high pressure air in the cylinder to a pressure slightly higher than the water pressure at the depth at which we are diving. This is done by the part of the regulator that is clamped onto the cylinder, called the first stage of the regulator
  • it provides us with air when we inhale. This is done by the part of the regulator that the diver puts in his or her mouth, called the second stage or demand valve of the regulator.

Often regulators also have other, subsidiary components such as:

• a secondary demand valve for emergency use - called an octopus
• a contents gauge - to measure the air pressure in the cylinder
• direct feeds to the air inlets of buoyancy compensators and dry suits

When we exhale, using SCUBA, the air in our lungs is released into the surrounding water. For that reason SCUBA is called open circuit breathing equipment.

What's a rebreather (or closed circuit SCUBA)?

The main feature that distinguishes a rebreather from an aqualung is that the rebreather works by recycling the diver's exhaled gas.

Rebreathers are designed to take advantage of an interesting phenomenon of respiration: if you inhale a breath of air (79% nitrogen and 21% oxygen),  you:

• only use 4% of the oxygen to support your life processes
• exhale 17% oxygen. In SCUBA this would be lost to the surrounding water
• also exhale 4% carbon dioxide. High concentrations of this are poisonous

The basis of rebreather design is that the 17% oxygen in each exhaled breath is captured and recycled. This dramatically reduces the amount of gas consumed during a dive.

A rebreather recycles the exhaled air like this:

• it captures the exhaled breath. A twin hose with one valves links the rebreather to the diver's mouth :
  • when the diver inhales gas is drawn from one side of the breathing loop
  • and exhales into the other side of the breathing loop
• the 4% carbon dioxide in the air is removed by passing the breath through a chemical scrubber in the loop
• 4% more oxygen is injected into the loop from an oxygen cylinder to replace the oxygen removed by breathing

There are various types of rebreathers :

• oxygen only - for 6 metre maximum diving (only used by navies for shallow work around ships)
• semi-closed circuit rebreathers, which constantly inject fresh gas and vent stale gas from the loop
• closed circuit rebreathers, which have a control mechanism that measures the oxygen in the mix and injects more oxygen when the concentration becomes weak

Except for oxygen rebreathers, rebreathers can use a variety of gases:

• nitrox - for mid range (40 metre maximum) diving
• other mixtures such as trimix, heliox and neox - for deeper diving

Compared to open circuit scuba, rebreathers:

• allow much longer and deeper dives by recycling exhaled gases
• reduce decompression times by minimising nitrogen in the loop 
• use much less gas - good for big dives, remote locations and expensive gases
• create fewer bubbles - good for photography and for stealthy military divers
• are more expensive to buy and to run
• are more complex - they need a lot of day to day maintenance

What's nitrox?

Firstly, lets start by describing air: air is a mixture of oxygen (21%) and nitrogen (79%).

Nitrox is also a mixture of nitrogen and oxygen but it has a higher proportion of oxygen. Its sometimes called oxygen enriched air or enriched air nitrox.

The main reasons for using nitrox are:

• it allows longer mid-range (30m - 40m), no-stop diving
• it allows shorter decompression times
• it reduces the risk of decompression illness

Nitrox mixtures with 32 or 36% oxygen are commonly used for diving.

Nitrox mixtures with 40, 50 or 80% oxygen mixtures are commonly used for decompression.

There are two problems with breathing oxygen concentrations that are greater than that in air:

What's Technical Diving?

There are two main features that distinguish a technical dive from a normal sport dive:

• the use of hypoxic gas mixtures, such as trimix and heliox, to allow deeper diving than is safe when breathing air
• the use of rich nitrox and oxygen to accelerate decompression

There are two problems with using air for deep diving:

• nitrogen (79% of air) becomes poisonous at depths greater than 60m
• oxygen (21% or air) becomes poisonous at depths greater than 80m

Trimix is a blend of helium, oxygen and nitrogen, that reduces the proportions of nitrogen and oxygen, allowing deeper dives with reduced effects of these two gases. 

A problem with these designer gases is that they are only safe to breath at certain depth ranges:

• All gases mixes have maximum depths at which the high oxygen concentration, caused by the increase in water pressure, poisons the diver. The solution to this problem is to reduce the proportion of oxygen in the mix.
• Unfortunately, if you reduce the proportion of oxygen below 17% when you dive in shallow water (where the water pressure is low and therefore the gas concentration is low) there is not enough oxygen to support consciousness. 

Also, at the end of a deep and long dive, to acclerate decompression times, divers breathe gas containing 50% or 100% oxygen.

The Technical Diving answer to these problems of gas concentrations is to use different mixes of gas during each  phase of  the dive:

• travel gas: a nitrox (oxygen enriched air mixture) for shallow water - is used for the start of the descent, say from the surface to 28 metres (the maximum depth for 36% nitrox) and the ascent
• bottom gas: a trimix (lower nitrogen and oxygen levels that air) is used at depths of between say, 25 metres and 100 metres
• deco gas: a rich nitrox (50% oxygen) or even pure oxygen is used in very shallow water on the ascent (Pure oxygen is poisonous at depths below 6 metres, 50% nitrox is poisonous at depths below 18 metres)

That's why trademarks of technical diving are 

• the large number of cylinders and regulators the divers carry with them
• the large number of cylinders and regulators the divers leave at their decompression points for the return to the surface.
• the high level of planning involved in this type of diving

A further problem with breathing high concentrations of oxygen is that if you do it for long periods it becomes poisonous. So, divers need to plan and stay within safe oxygen consumption limits.