Nitrox Class - Part 2

Part one.

This is the second and last part in what you need to know about nitrox. Nitrox is the standard gas used by DIR divers from 0-30 meters / 0-100 ft.

Oxygen toxicity

Even if we need oxygen to survive oxygen is toxic in higher concentrations. We can suffer something called oxygen toxicity, or "ox tox" or "tox" if you're lazy.

There are two major type of oxygen toxicity. The most serious one is called CNS related oxygen toxicity where CNS related means that it will affect the Central Nervous System. You could say that this is a form of immediate oxygen toxicity.

We are at higher risk when we are diving high partial pressures of oxygen. Symptome can be anything from involuntary muscel contractions, visual disturbances and dizzyness to serious cramps that often leads to drowning unless the diver get's help immediately.

Sometime you will find the mnemonic CONVENTID used to memorize the symptoms. It's CONvulsions, Visual disturbances, Ear disturbances, Nausea, Twitching, Irritability and Dizziness.

If you keep the partial pressure of oxygen, PO2, at 1.4 ATA or lower, the risk of getting oxygen toxicity extremely low for a regular recreational diver doing no decompression dives. Since we can tolerate a PO2 up to 1.4 ATA we can calculate our maximum depth.


Our maximum depth is often called MOD (Maximum Operating Depth).

If we use our formula to calulate PO2 we get:
MODATA = 1.4 / FO2

Let us calculate the MOD for EAN32 and EAN36:

MODATA = 1.4 / 0.32 = 4.375 ATA, so 33.7 msw or 111.4 fsw.
MOD is always rounded down so MOD for EAN32 is 33 msw or 111 fsw.

MODATA = 1.4 / 0.36 = 3.889 ATA, so 28.9 msw or 95.3 fsw
MOD for EAN36 is 28 msw or 95 fsw.

We can also turn the question around and ask ourselves if we intend to dive to 24msw/80ft what is the maximum oxygen fraction?

MODATA = 1.4 / FO2 => FO2 = 1.4 / MODATA

24 msw/80 fsw is 3.4 ATA.
Using the equation above:
FO2 = 1.4 / 3.4 = 0.41, so 41%.
In other words we could use a maximum of 41% nitrox to dive to 24 msw/80 fsw.

Finding the maximum nitrox mix we could use is called "best mix" and even if we as DIR diver dont use that it is good to know about it and calculate it. Instead we used standardized gas mixes and our standard is 32%. Deeper than 30m / 100ft we will used helium based gas mixes, so called trimix.

For decompression diving the maximum PO2 is 1.6 ATA. That is during the actual decompression stops when we don't swim around or exert ourselves.


There is another way to get CNS related ox tox and that is to expose ourselves to high PO2 during one or several longer dives.

You could say that you would absorb oxygen and when you get to much you will get ox tox. To track this we use something called the CNS clock or CNS%. When the CNS% has reached 100% you have reached max so to speak.

To calculate this we use a CNS table:

PO2Maximum Single Exposure
Maximum 24 hour Exposure

To calculate the CNS% do this:
CNS% = Time spent / Maximum Single Exposure (for used PO2)

For example let's say we did a dive to 21msw/70fsw for 50 minutes using EAN32. What's the CNS%?

Let's start by finding our our PO2.
PO2 = FO2 * Pata => PO2 = 0.32 * 3.1 ATA = 0.99 ATA
We always round the ATA up and look it up in the table.

Our maximum single exposure time is 300 minutes.
CNS% = 50 min / 300 min = 16.7%
So our accumulated CNS% is 16.7%

If we do multilevel diving we have to calculate CNS% for every segment of the dive and add them together.

Also doing a surface interval longer then 90 minutes will lower the CNS clock with 50%.

In reality it has been shown that the CNS clock is really not a good tool to track oxygen toxicity which is a complex subject that scientist still struggle to understand fully.


There is another type of oxygen toxicity. It is called pulmonary or "whole body" oxygen toxicity. We could look at this as being long term damange. The symptoms are often lung irritation or breathing difficulties. To get this we really have to do extreme dives often for several days. Something called OTU's (Oxygen Toxicity Units) is used to track our dose.


When we dive nitrox we have to know our maximum depth, MOD. If we have access to nitrox tables there is not that much more to think about. If we however only have air tables we have to calculate the EAD - Equivalent Air Depth.

Since nitrox contains less nitrogen than air we also get less nitrogen while diving. If we however dive deeper we will on gas more nitrogen. It is the nitrogen partial pressure that controls this. When the nitrogen partial pressure, PN2 is the same the amount of gas our bodies take up is the same.

If we dive to 18msw/60ft with EAN32 our PN2 = 0.68 * 2.8 ATA = 1.904 ATA.

If we would use an air table we would have to calculate at what depth air will have a PN2 of 1.904 ATA. We could then use that new depth to find out how long we could stay with an air table. Does it makes sense?

Let's calculate at what depth we have PN2 = 1.904 ATA for air.
PN2 = FN2 * Pata => Pata = PN2 / FN2
Pata = 1.904 / 0.79 = 2.41 ATA, which is 14.1 msw or 46.5 fsw or 15 msw/80fsw rounded up.

So if we look at our air table and it says we can stay 100 minutes at 15 msw/50ft, using air we could stay 100 minutes at 18msw/60ft with nitrox 32%. That's nice isn't it?

Instead of thinking hard we could use a predigested formula:
EAD (msw) = ((1-FO2) / 0.79 * (D+10)) - 10
where D is the depth in msw.
EAD (fsw) = ((1-FO2) / 0.79 * (D+33)) - 33
Where D is the depth in fsw.

If you are like me you will have forgotten the EAD formula as fast as you have finished reading it. But when I think about finding the depth that air will give the same partial pressure of nitrogen, like we did in our example above, I always manage to calculate the EAD.

For fast in the field calculations we can use a rule of thumb that say EAD for EAN32 is 20% of our depth. So if we dive to 30msw/100fsw it's 20% off to get the EAD => 24msw/80ft. Hey, why didn't I tell you that before...

Equipment considerations

The run of the mill nitrox classes taught by the recreational agencies allows you to use up to 40% nitrox. Often you need to have your tanks oxygen cleaned but can use your normal regulator. This will vary from country to country so you have to check up what the rules are where you life. Usually oxygen cleaning is only valid for one year.

Oxygen cleaning means that the bottle is cleaned inside and that oxygen compatible lubricants and often also other material (viton) in the tank orings are used.

As DIR divers we try to avoid everything that is not nessessary or can cause confusion. That is why we try to avoid using stickers that says nitrox or similar on it since that information is not relevant. MOD is however and stage bottles or deco bottles are marked with that in huge numbers on the side. But you better know the MOD on the tank you are wearing because you can't read it under water.

Analyze your gas

When you drop of your tanks to get nitrox fills you have to analyze the gas when you come back to pick them up. Some places also makes you fill out a log book where you verify that the gas mixture was as you have ordered.

To check the oxygen content you use an oxygen analyzer that measures the oxygen and shows the percentage usually with in decimal, like 31.8%. However the accuracy of the analyser is not better than +/- 1% so it might actually be 32.8% or 30.8% in there. The nitrox tables are also made for this inaccuracy so you don't have to worry about it.

Oxygen analyzers works a little different but normally you have to calibrate them before using them. That is most commonly done by setting them to 20.9% or 21% while having them in contact with air. Some of the newer instruments also have a calibrate button you could use.

When you do your analyzing, the gas need to pass the oxygen sensor with a flow of 2-4 liters per minute. Some analyzers have a flow restriction in their design while on others you have to open the valve really slowly. If the flow is too high the sensor will get too much oxygen and it will show a higher percentage than what is in the bottle.

When you analyzed your gas you write down the result on a piece of tape that you put on the bottle. Preferably just below the valve opening on the tank neck. There are dedicated tapes sometimes with a dive organization's or dive shop's name on it. You could also use Gaffa tape, preferably white or yellow instead of the silver.

You write down the oxygen percentage, the date, your initials and the fill pressure. If you want you can also put the MOD on there. When you've done that it is best to put a piece of tape over the valve opening. That way you know that you analysis is correct and that no one has used the bottled or tampered with it since.

A good idea is to also double check the analysis at the dive site before using the bottle.

Remember: Don't be sloppy with the analysis and above all not the marking of the bottle - you life depends on it!

When nitrox is not available

Of course nitrox is the gas of choice but sometimes that is not available or maybe you just want to top of a bottle with a tiny bit of air. In every case make sure you get air that is good enough for nitrox. Sometimes it's called double filtered and is run through an extra filter after the air compressor and is supposed to be cleaner than regular air. If you often fill at places that don't mix nitrox you can buy a personal filter that will make the air cleaner.


Nitrox has many advantages and is the obvious choice for situations where air could have been used. It is good to be able to calculate gases, depths, pressures and other things. It is also neccessary to know all of this if you have the ambition to get into technical diving since what you have learned here is the foundation for tech training and you are expected to know this stuff.

In real life diving nitrox is almost as easy as diving air. Just make sure you analyze the gas and stay within the maximum depth.

Good luck!


Self assessment

1. What are the two types of oxygen toxicity?

2. What is MOD for 28% nitrox?

3. If I want to dive to 30msw/100fsw what is the highest oxygen content I could use?

4. What is EAD for 32% while diving to 21msw/70fsw?

5. At what depth can I get oxygen toxicity when breathing air?

6. If there is as nitrox sticker on the bottle does it mean there is nitrox in it?

7. If the bottle has a nitrox sticker on it does it mean I can fill it with nitrox?

8. What should the analysis tape have written on it?>/p>

9. Is it OK to let your buddy analyze the gas for you?

Coming soon.

ⓒ 2002-2014 Peter Steinhoff