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Nitrox diving
Practiced for many years in the civilian and military fields, Nitrox diving has been offered to the general public since the early 1990s.
Nitrox, why?
The term “Nitrox” comes from the contraction of the wordsNITRogen (nitrogen in English) andOXygen.
A Nitrox dive is performed with air enriched in oxygen (O2) and therefore depleted in nitrogen (N2), also noted EAN for “Enriched Air Nitrox”.
The interest of such dives is to reduce the nitrogen load, in order to:
- increase dive safety by reducing the risk of desaturation accidents;
- decrease end-of-dive fatigue, largely due to the removal of excess nitrogen.
Nitrox: selective colors
The increase in the oxygen level leads to precautions being taken and in particular to the use of specific equipment in order to prevent the risk of heating of fats and impurities in contact with oxygen (especially if the percentage of oxygen exceeds 40% ). Without this being an absolute rule, “Nitrox” material is generally differentiated from “air” material by the use of yellow and green.
Where can you dive with Nitrox?
Nitrox diving is now widespread and can be practiced all over the world. The inflation of bottles with Nitrox requiring specific and expensive equipment, an additional cost is generally requested for this type of dive.
In addition, all organizations offer Nitrox training.
Rating
A Nitrox mixture is characterized by two numbers. The first indicates the percentage of oxygen, the second that of nitrogen (example 32/68).
If only one number is given, it is the O2 percentage (eg Nitrox 32 or EAN32).
Nitrox diving in France
French regulations (sports code) provide for two levels of Nitrox divers:
- Nitrox Diver (PN);
- Advanced Nitrox Diver (PN-C).
The "Nitrox Diver" level is accessible from the baptism or the first dives.
PN: Ability to evolve in a group with Nitrox whose oxygen content does not exceed 40%
To evolve in a supervised or autonomous group.
- Mastery of skills corresponding to the area of development concerned.
- Mastery of the management and use of its Nitrox equipment, of the analysis of the mixture whose oxygen content does not exceed 40% and of the information on the identification sheet of the bottle.
- Control of maintaining its balance and managing its profile in relation to the “floor” depth of its mixture.
- Mastery of decompression means (table or Nitrox computer).
- Knowledge of the hyperoxic risks associated with the use of Nitrox.
PN-C (confirmed Nitrox diver) Ability to evolve in a Nitrox dive and to perform decompression with pure oxygen
To evolve in a supervised or autonomous group.
- Mastery of the skills in the air corresponding to the area of evolution concerned.
- Mastery of PN skills.
- Mastery of the use and choice of equipment with several mixtures of Nitrox at the bottom and in decompression and the use of pure oxygen.
- Control of balance and stabilization at the depth of stops when changing mixtures.
- Knowledge of the principles of making mixtures.
What prerogatives for a Nitrox diver (PN)?
In France, a Nitrox diver (PN) can use Nitrox containing up to 40% oxygen.
The accessible space of evolution is defined both by the aptitudes (12 m, 20 m, 40 m) and by the floor depth which is a function of the oxygen content of the Nitrox used.
Art. A322-77(sport code, reworded summary): Nitrox divers, like all divers, must prove their skills to the dive director.
Art. A322-95(sports code): “The practice of diving with nitrox mixtures is subject to the justification of nitrox aptitude for the divers and the person supervising the dive group in accordance with the table appearing inappendix III-17 a.
The practice conditions for diving with nitrox mixtures are specified by theAppendices III-17 b and III-17 c.“
Art. A322-73(sports code, excerpt): “When a dive is made up of divers breathing different mixtures, it must not exceed the maximum evolution conditions accessible to the diver using the most restrictive mixture.“
The notion of “floor depth”
The increase in oxygen concentration in the breathed mixture leads to limiting the maximum depth of the dive in order to prevent the risk of hyperoxia (neurotoxicity of oxygen).
Art. A322-76from the sports code (excerpt): “The oxygen content of the nitrox determines the space of evolution. " [Do not confuse, the PpO2 of nitrox sets the depth limit, whereas for air dives the 60m limit is set by the partial pressure of nitrogen which dictates the narcosis threshold: PpN2 <= 5.6 bar].
This depth not to be exceeded or “floor depth” is calculated according to two elements:
- the percentage or fraction of oxygen (FO2) of the Nitrox used;
- the maximum partial pressure threshold (see box next page), which must never exceed 1.6 bar and which most often is reduced to a lower value of the order of 1.5 to 1 bar.
These two elements can be configured in your dive computer, which automates the desaturation calculations and the “floor” depth alarm.
Floor depth calculation: Pabs = PpO2 max / FO2
- Pabs = Maximum absolute pressure
- PpO2 max = Maximum partial pressure of O2 accepted (1.2 to 1.6 bar generally).
- FO2 = Fraction of O2 in the breathed mixture (ex. 0.4, 0.36, 0.32).
Calculation example for FO2 = 0.4 and PpO2 max = 1.6
- Pabs = 1.6 / 0.4 = 4 bars or 3 bars of water pressure at sea level, or 30 m max.
Setting up your computer
In the context of Nitrox diving, recent computers are valuable tools, used by nearly 100% of divers.
Once the “Nitrox” mode has been selected, two elements must be configured:
- The oxygen percentage of the mixture used (ex. 32);
- The accepted toxicity threshold, which can range from 1.6 to 1 bar, knowing that the median value of 1.4 bar is the one generally recommended. These two elements are enough to:
- Calculate desaturation;
- Set the floor depth and activate an alarm.
Take the time to read your computer's manual, it contains valuable information. Your safety is at stake.
Risk of hyperoxia
Indispensable to life, oxygen must be breathed in the right proportions. Usually, we breathe air composed of approximately 21% oxygen, which corresponds to a partial pressure at sea level of around 0.21 bar or “normoxia”. Beyond that, we enter a zone of hyperoxia which becomes dangerous when diving from 1.6 bar. This is the "Paul Bert effect", named after the first researcher to have studied this phenomenon: oxygen in too high a concentration stimulates the nerve centers and causes an epileptic-type seizure with the risk of drowning.
One rule: prevention
Since the warning signs are most often absent, the prevention of this risk is fundamental.
For this you need:
- do not exceed the floor depth depending on the mix used;
- and, of course, before diving, analyze the percentage of oxygen contained in your PE40 bottle.
Inflating bottles
See the Pleasure Diving article on scuba tanks
Oxygen percentage control
In France, the sports code provides that cylinders are identified according to the gases contained by indicating:
- the percentage of oxygen analyzed and the theoretical composition of the gas mixture;
- the date of the analysis;
- the name of the manufacturer or distributor.
- Before diving, as an end user, you must complete the form by mentioning:
- the pressure of the mixture;
- the percentage of oxygen and the composition of the mixture;
- the maximum depth of use;
- the date of the analysis;
- your name and initials.
- Turn on the analyzer;
- Calibrate it in air indicating that the oxygen level is 20.9% (if you are at sea level);
- Connect the analyzer to the bottle to be analyzed;
- Carefully open the tap until you hear a slight flow of gas (too strong a flow can damage the cell analyzing the O2);
- Wait until the O2 percentage indicated on the analyzer no longer changes, which usually takes 10 to 15 seconds.
- Note the percentage;
- Close the bottle;
- Turn off the analyzer.
Art. A322-93(sport code)
“Bottles are identified according to the gases contained.
The manufacturer or distributor of a breathing mixture other than air mentions onthe identification sheet for each bottle and on the register of the establishmentthe following information :
- the percentage of oxygen analyzed and the theoretical composition of the gas mixture;
- the date of the analysis;
- the name of the manufacturer or distributor.
Before diving, the end user completes the identification sheet for each bottle with the following information:
- the pressure of the gas mixture in the cylinder;
- the percentage of oxygen analyzed and the composition of the mixture;
- the maximum depth of use of the mixture;
- the date of the analysis;
- his name or initials.”
© Extract fromPleasure Diving booksby Alain Foret at Editions GAP.
Any reproduction prohibited on any medium whatsoever without the written consent of the publisher and the author.