The safety limits established for scuba diving in conventional leisure frame has defined the 40 meter mark, and 60 meters by plunging in the air in France , as a border not to be exceeded to prevent the risk of accident to increase excessively.
The recent craze for tek dives ( deep dives ) gives rise to an increase in risk, especially for divers inexperienced. For these dives, it is important to choose the right gas. Sometimes nitrox , trimix or Heliox is used . Let's take stock.
Before diving into the adventure , it is important to know the risks that await us. They depend both on:
- Depth
- Immersion time
The first harmful effects that we can suffer with depth are due to the accumulation of gases in the body. Indeed, the breathed gases are very different from the air that is usually breathed. Therefore, it is also this absorption that is responsible for narcosis or decompression accidents:
decompression accident
The probability of doing misuse or calculation error in decompression tables * increases according to the depth reached. For the tables of the US Navy, this risk begins to be high (that is to say above 5%) from 30 meters deep in immersion of 60 minutes.
For safer tables, such as DCiem, this margin of error is reached from 33 meters.
When the immersion time does not exceed half an hour, the risk of error on modern tables (Bülhmann, Dciem, BSAC) is imperceptible if we do not exceed the 40 meter mark.
* decompression tables, also called diving tables, are reference tables giving you the decompression levels to be respected according to the depth and immersion time of your Diving.
narcose
If we consider as a dangerous narcotic state that which causes the loss of rational control of the diver , a range of high risk can be fixed between 35 meters (during adverse physical or moral conditions) and 70 meters (for regulars being in good conditions).
A reasonable limit for most of the divers, even if they are not used to deep immersion, but presenting a favorable physical and moral condition, oscillates between 45 and 55 meters.
Narcosis is caused by the dilution of gases in the fatty layer of neurons and by its effects in nerve synapses (Meyer-Overton thesis): the narcotic effect of a gas depends on its molecular size and its solubility in fats.
oxygen or hyperoxia intoxication
The increase in the partial pressure of the oxygen contained in the breathable air increases with the depth. If it exceeds a certain threshold (1.6 bar), it can become dangerous.
What are the major risks? One of the most feared problems is the appearance of certain symptoms of " high pressures neurological syndrome , such as the convulsive attack which can cause death by drowning. The risk of attack begins to be very high, for immersions that exceed 50 minutes with unfavorable effort and cold conditions, from 56 meters deep.
Maximum effort capacity
The capacity for effort is conditioned by the yield of the circulatory system and the respiratory system. Basically, by the heart and the lungs. For a person who has a normal physical level of physical form, the cardiovascular and respiratory limits are more or less affected at the same time.
In immersion, when the depth increases, the density of the breathable gas increases, and with it, the effort necessary to carry out the ventilation function .
60 meters deep, ventilation is Modified significantly compared to surface ventilation. This is why an effort that seems minimal can lead to a shortness of breath resulting in suffocation.
then, Trimix, Heliox, Nitrox?
Nitrox diving
We talk a lot about Nitrox diving. The nitrox is an air mixture (O2 + N2) over-oxygenated with respect to the air that is breathed on the surface, that is to say that it will contain more than 21% oxygen. By increasing the amount of oxygen, the amount of nitrogen is reduced. The latter then less saturates the diver's organism , which makes it possible to limit the negative effects.
It therefore has the advantage of allowing pleasant, longer and more secure dives.
But (there is always one but!), Remember, the partial pressure of oxygen increases with depth, and that it must always be below 1.6 bar.
A over -oxygenated mixture will therefore not allow you to dive as deeply as with a trimix or heliox mixture, or even in the air.
Trimix diving
The trimix is actually a mixture of three gases. In diving, common acceptance wants these gases to be oxygen (O2), nitrogen (N2) and helium (HE), in variable proportions according to divers.
The main drawback of the Trimix is nitrogen which is a gas whose narcotic effect is strongly felt during the descent into the depths. It imposes fairly long decompression levels.
From there came the idea of removing this gas, to create a mixture only composed of oxygen and helium , otherwise called heliox !
The case of Heliox diving
Helium is one of the least dangerous gases in terms of narcotic effects, almost zero. In addition, it does not present the problems of manipulation of hydrogen which produces a spontaneous explosion when it mixes with oxygen, if the latter present in a proportion greater than 4%.
However, helium is not within the reach of all budgets since it is produced industrially. In addition, it also complicates procedures for Decompression ! It is therefore judicious to leave it to professionals.
## V1 ##
Here is a small overview of the different characteristics from different mixtures for diving.
But if we want to emphasize the most important, we will say that what should not be forgotten is above all to take pleasure during dives whether in tek diving or in little waters deep.
The diving Tek is more and more great success. Divers want to discover inconvenient places, to explore the wrecks that sleep eternally at the bottom of the waters.
The important thing is to choose your diving center : the one who will respect the safety rules and give you a supervision of quality.
