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Nitrox Class Part 1
Nitrox Class Part 2

DIR II (from video)
DIR III (from video)

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Nitrox Class - Part 1

Part two.

This is part one of two covering what you need to know about nitrox. Nitrox is the standard gas used by DIR divers from 0-30 meters / 0-100 ft.

A common joke is that air is only used to fill tires. Which by the way is not entirely true as many vehicles in certain countries fill their tires with nitrogen :)


Air consists of 21% oxygen (O2), 78% nitrogen (N2) and 1% of other gases, mainly argon. Common practice is to simplify things and say 21% oxygen and 79% nitrogen.

If you add additional oxygen to air you get nitrox. So nitrox consists of NITRogen and OXygen, hence the name.


Nitrox is really called Enriched Air Nitrox. It usually abbreviated to EANx where x is the fraction of oxygen. For example is EAN32 the same as 32% nitrox and consists of 32% oxygen and the rest nitrogen (68%). There are also other names for nitrox like SafeAir but they are not as common. In technical diving we usually just refer to the gas as the percentage not mentioning the fact that it is nitrox we are talking about. For example if you hear a guy at a dive shop asking for 32 in one bottle and 50 in the other he is talking about 32% and 50% nitrox.

The use of nitrox began in the 19th century but was not common until the 1950s when US Navy published information about it in their US Navy Diving Manual. You can find the current US Navy Diving Manual in the Download section. In the middle of the 1990's the recreational diving organizations started teach nitrox classes. NAUI started in 1992 while PADI got on the bandwaggon in 1995.

NOAA that does oceanographic research in the US started top use nitrox early in thior diving. The two most common nitrox mixes in recreational diving are 32% and 36%. They are also called NOAA I (32%) and NOAA II (36%). These mixes are also the ones you will find all over the world.

NOAA also has published their nitrox tables and diving procedures. You can find them at

DIR divers use almost exclusivly 32% nitrox as their preferred breathing gas down to 30m /100ft.


The advantages of nitrox doesn't come from the increased percentage of oxygen but rather from the decreased percentage of nitrogen. With less nitrogen in the breathing gas we will absorb less nitrogen during the dive and we could stay longer at the same depth with a similar gasloading compared to air. The oxygen in the breathing gas is also used but is not accumulated as nitrogen are.

Some say that that nitrox increases your safety if you dive it like air but even if that seems logical it is not possible to know for certain because the rate of decompression illness (DCI) are too low to make any statistical conclutions.

Nitrox is also used when we do decompression diving because they will speed up the removal of inert gases and lead to shorter decompression stops. This is sometimes called accelerated deco but is in fact the de facto standard how to do deco.

Since oxygen is narcotic you will can't combat narcosis by using nitrox even if that was something people thought when nitrox was introduced to the general public.


The increased oxygen percentage increases the risk of oxygen toxicity because oxygen is toxic even when we dive air. That is why we need to stay within a maximum depth with nitrox. Some dive shops use partial pressure blending to make nitrox in your tanks and you commonly need oxygen cleaned tanks for this. These recommendations can differ a lot between countries though so you need to check what's required in your country. Nitrox do require some additional preparation and some training of course. A nitrox class is usually done with a few hours of theory and two dives. You get the essential parts of the theory for free here though!

Gas laws

Since we have to think about oxygen toxicity a little bit more when diving nitrox there are a couple of things we need to be able to calculate, for instance our maximum allowed depth. Top be able to do that you need to learn a little bit about the gas laws.

Pressure and depth

Water is 800 times denser than air. When we dive the ambient pressure increases as we go deeper. We often use the unit atmospheres absolut (ATA) to define that pressure. On the surface we have 1 ATA. At 10 meters/33 feet depth in saltwater we have twice the ambient pressure, 2 ATA. At 20m/66ft we have 3 ATA and so on.

Another unit that is sometimes used is bar and 1 bar is for practical use the same as 1 ATA. Other units are msw and fsw which are also pressure units (meter salt water and feet salt water). So when we say 66ft or 20 meters we really mean 66 fsw or 20 msw which is the ambient pressure in the ocean at that depth.

Often when we need to calculate we have to use ATAs so it's neccessary to know how to convert between depth in msw or fsw to ATA.

If we wanted to use formulas it would be:
msw = (ATA-1) * 10
ATA = msw / 10 + 1

fsw = (ATA-1) * 33
ATA = fsw / 33 + 1

Partial pressure

Right here at the surface we have an ambient pressure of 1 ATA. Since air consists of 21% oxygen the oxygen will exert 0.21 ATA on oss. We have 79% of nitrogen in the air so that means 0.79 ATA of nitrogen. Every gas exert their part of the pressure and that part is called partial pressure.

If we add up the partial pressures of one gas like air, we get the total pressure of the gas. That is Dalton's law.

If we dive with air to 10msw/33ft the ambient pressure is 2 ATA. The partial pressure of the oxygen is 0.21 * 2 = 0.42 ATA and the partial pressure of the nitrogen is 0.79 * 2 = 1.58 ATA. Together they equal the ambient pressure: 0.42 + 1.58 = 2 ATA.

Pressure is often defined by the letter P. So ambient pressure is usually written Pata. The oxygen partial pressure is written PO2 as O2 is the chemical name for oxygen. Nitrogen partial pressure is PN2.

Since nitrox and air consists of oxygen and nitrogen these rule applies:

Pata = PO2 + PN2

The letter F for fraction is used when you want to write how much of a gas mixture one particular gas is. So in air the oxygen fraction is FO2 = 0.21 and the nitrogen fraction is FN2 = 0.79.

If we want to calculate the partial pressure of oxygen we could do it like this:

PO2 = FO2 * Pata

Since it is an equation we can turn it around to calulate what we don't know. For example, at what depth do the oxygen have a partial pressure of 1.4 ATA while breathing air?

1.4 = 0.21 * Pata
Pata = 1.4 / 0.21 = 6.66 ATA
Let's then convert 6.66 ATA:
In metric (msw) it is (6.66-1) * 10 = 56.6 msw
and in imperial (fsw) it is (6.66-1) * 33 = 186.8 fsw.

Self assessment

1. The air consists of ____ percent nitrogen.

2. What does EANx stand for?

3. What are the two most common nitrox mixtures?

4. When did the recreational agencies start teaching nitrox?

5. What is the primary advantage of using nitrox?

6. What is the ambient pressure at the depth of 26 meter or 85 feet in the ocean?

7. How deep in msw/fsw is 2.5 ATA?

8. How many percent nitrogen do we have in EAN36?

9. If we breath a 30% nitrox at 21 msw / 70 fsw what is the oxygen partial pressure?

10. At what depth do we have a PO2 of 1.4 ATA when breathing 32% nitrox?

11. How deep can we dive with EAN32 if we want to keep the PN2 the same as with have on air at 24 msw / 80 fsw?


1. It's really 78% but we usually use 79%.

2. EAN is an abbrevation of Enriched Air Nitrox and the x is the oxygen content.

3. NOAA I and NOAA II which is 32% and 36% nitrox.

4. At the beginning of the 1990's.

5. We breath less nitrogen so the body absorbs less nitrogen.

6. 26 msw / 10 + 1 = 3.6 ATA or 85 fsw / 33 -1 = 3.6 ATA

7. (2.5 ATA - 1) * 10 = 15 msw or (2.5 ATA -1) * 33 = 49.5 fsw

8. EAN36 is 36% oxygen and the rest nitrogen so 100-36 = 64% nitrogen.

9. First we convert msw / fsw to ATA.
21msw / 10 + 1 = 3.1 ATA or 70fsw / 33 + 1 = 3.1 ATA
The oxygen partial pressure is:
PO2 = FO2 * Pata => 0.3 * 3.1 = 0.93 ATA.
So the answer is 0.93 ATA.

10. PO2 = FO2 * Pata so Pata = FO2 / PO2.
Pata = 1.4 / 0.32 = 4.38 ATA which is 33.8 msw or 111 fsw.
So the answer is 33.8 msw or 111 fsw.

11. First we need to calculate the nitrogen partial pressure at 24 msw / 80 ft:
24 msw or 80ft = 3.4 ATA
PN2 = FN2 * Pata => 0.79 * 3.4 = 2.68 ATA.
We know the partial pressure and we know the nitrogen content in EAN32 so we can calculate the depth:
PN2 = FN2 * Pata => Pata = PN2 * FN2
Pata = 2.68 / 0.68 = 3.94 ATA
3.94 ATA is 29.4 msw or 97 fsw.

Go on to Part two.

This page was last modified 24 August 2006

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