Safety limits established for recreational scuba diving in conventional leisure diving have set the bar at 40 metres, with 60 metres for air diving in France, as a boundary not to be exceeded to prevent excessive risk of accidents.
The recent enthusiasm for technical diving (TEK) (deep diving) has led to increased risks, particularly for inexperienced divers. For these dives, choosing the right gas mixture is crucial. Sometimes Nitrox, Trimix or Heliox are used. Let's examine them.
Before diving into adventure, it's important to understand the risks involved. These depend on:
- Depth
- Dive duration
The first harmful effects we may experience at depth are caused by gas accumulation in the body. Indeed, the gases breathed underwater differ significantly from surface air. Consequently, this absorption is responsible for narcosis or decompression sickness:
Decompression Sickness
The likelihood of misusing or miscalculating decompression tables* increases with depth. For US Navy tables, this risk becomes significant (i.e., above 5%) beyond 30 metres during 60-minute dives.
For safer tables like DCIEM, this error margin is reached at 33 metres.
When dive time doesn't exceed half an hour, the risk of error with modern tables (Bühlmann, DCIEM, BSAC) is negligible if staying within 40 metres.
* Decompression tables, also called dive tables, are reference charts indicating required decompression stops based on your dive's depth and duration.
Narcosis
If we consider dangerous narcosis as causing loss of rational diver control, the high-risk range falls between 35 metres (under poor physical/mental conditions) and 70 metres (for experienced divers in good condition).
A reasonable limit for most divers, even those unaccustomed to deep diving but in good physical and mental condition, is between 45 and 55 metres.
Narcosis is caused by gas dissolving in neuronal fat layers and affecting nerve synapses (Meyer-Overton hypothesis): A gas's narcotic effect depends on its molecular size and fat solubility.
Oxygen Toxicity or Hyperoxia
The partial pressure of oxygen in breathed air increases with depth. Beyond a certain threshold (1.6 bar), it becomes dangerous.
What are the main risks? One major concern is symptoms of "High Pressure Neurological Syndrome", including convulsions that may cause drowning. Seizure risk becomes very high beyond 56 metres during dives exceeding 50 minutes with strenuous activity or cold conditions.
Maximum Physical Exertion Capacity
Physical capacity depends on circulatory and respiratory efficiency - essentially heart and lung performance. For someone with average fitness, cardiovascular and respiratory limits are reached roughly simultaneously.
Underwater, as depth increases, breathed gas density rises, increasing the effort required for ventilation.
At 60 metres, ventilation differs significantly from surface breathing. Thus, seemingly minimal exertion can cause breathlessness leading to suffocation.
So, Trimix, Heliox or Nitrox?
Nitrox Diving
Nitrox diving is widely discussed. Nitrox is oxygen-enriched air (O2 + N2) containing more than 21% oxygen. Increasing oxygen reduces nitrogen content, decreasing nitrogen saturation in the diver's body and limiting its negative effects.
This allows for more enjoyable, longer and safer dives.
However (there's always a but!), remember that oxygen partial pressure increases with depth and must remain below 1.6 bar.
Thus, oxygen-enriched mixtures don't permit diving as deep as Trimix, Heliox or even standard air.
Trimix Diving
Trimix combines three gases - typically Oxygen (O2), Nitrogen (N2) and Helium (He) in varying proportions.
Trimix's main drawback is nitrogen's strong narcotic effect at depth, requiring lengthy decompression stops.
This led to creating mixtures without nitrogen, using only Oxygen and Helium - known as Heliox!
Heliox Diving
Helium has negligible narcotic effects and avoids hydrogen's explosive risks when mixed with oxygen above 4%.
However, industrially-produced helium is expensive and complicates decompression procedures! It's best left to professionals.
This overview highlights different diving gas mixtures' characteristics.
Most importantly, remember that whether technical diving or shallow exploration, the priority is enjoying the thrill of diving.
Technical diving (TEK) grows increasingly popular as divers seek inaccessible sites and explore wrecks resting eternally underwater.
The key is choosing a reputable dive centre that follows safety protocols and provides quality supervision.
