Most diving gases are a mixture of oxygen and one or more other inert gases. Both oxygen and inert gases can become toxic when breathed under pressure, at depth.

Table of contents

1 Types of breathing gas 2 Individual gases 2.1 Oxygen 2.2 Nitrogen 2.3 Helium 2.4 Argon 2.5 Neon 2.6 Hydrogen

Types of breathing gas

Common diving breathing gases are:

• Air is a mixture of oxygen, 21%, and nitrogen, 79%, and is the most common diving gas being cheap and simple to use. As its nitrogen component causes nitrogen narcosis it has a safe depth limit of 40 metres/ 130 feet for miost divers.
• Nitrox is a mixture of oxygen, more than 21%, and nitrogen, less than 79%, and is mainly used instead of air, to reduce decompression requirements and to decrease the risk of Decompression sickness.
• Pure Oxygen is mainly used during the shallow phase at the end of a technical dive for allerated decompression stops.
• Trimix is a mixture of oxygen, nitrogen and helium and is used during the deep phase of a technical dive.
• Heliox is a mixture of oxygen and helium and is used during the deep phase of a technical dive.
• Heliair is a mixture of oxygen, nitrogen and helium and is used during the deep phase of a technical dive. It is easily blended from helium and air and so always has a 21:79 ratio of oxygen and nitrogen with the balance consisting of helium.

Oxygen

Oxygen the only gas that must be present in a diving, breathing gas. This is because it supports the human body's metabolic process sustaining life. The human body cannot store oxygen for later use as it does with food. If the body is without oxygen for more than a few minutes unconsciousness follows. The body without oxygen for four minutes can die.

The proportion of oxygen in a breathing gas determines the depth at which the mixture gas can safely be used:

• • hypoxic mixes have lower proportion of oxygen than air, 21%, or more strictly less than 16%, oxygen and are design only to be only safely be breathed at depth. Trimix, Heliox and Heliair are typical of hypoxic mixes and are used in Technical diving as deep breathing gases.
  • normoxic mixes have the same proportion of oxygen as air, 21%
  • hyperoxic mixes have a higher proportion of oxygen than air 21% and are designed only to be used in shallow water. Nitrox is a typical hyperoxic breathing gas.

The maximum safe partial pressure of oxygen in a breathing gas depends on exposure time, but for dives of less than 3 hours is 1.4 bar. Abaove this partial pressure or longer than this exposure, the diver risks oxygen toxicity involving a fit similar to an epileptic fit.

Oxygen is in the order of five times more expensive than air.

Nitrogen

Nitrogen is an inert gas that causes nitrogen narcosis in the diver, so its use is limited to shallower dives. Nitrogen can cause decompression sickness. Air is the cheapest diving, breathing gas.

Equivalent Air Depth is a calcuation often used to determine the maximum proportion of nitrogen in max for a particular depth of dive. Many divers find that the level of narcosis caused by a 30 metre / 100 feet dive, whilst breathing air, is a comfortabe maximum. The partial pressure of nitrogen at this depth on air is 3.16 bar (Fraction of nitrogen x Absolute pressure = 0.79 x 4 bar). So, what fraction of nitrogen would cause the same narcosis at 60 metres? 45% nitrogen. (3.16 / 7 bar)

Helium

Helium is an inert gas that is less narcotic than nitrogen, so is suitable for deeper dives than nitrogen rich gases. Helium can cause decompression sickness. It is not very suitable for drysuit inflation because of its poor thermal insulation properties. Helium is in the order of ten times more expensive than air.

Argon

Argon is an inert gas that is more narcotic than nitrogen, so is not suitable as a diving, breathing gas. It is used for drysuit inflation because of its good thermal insulation properties. Argon is much more expensive than air.

Neon

Is an inert gas used in commercial, deep diving but is very expensive.

Hydrogen