Heliox, a breathing gas consisting of helium and oxygen, is commonly used in deep sea diving for several reasons. The high percentage of helium makes heliox much less dense than air, reducing breathing resistance at depth and decreasing the risk of nitrogen narcosis, oxygen toxicity and decompression sickness.
What is heliox?
Heliox is a breathing gas mixture of helium (He) and oxygen (O2). While normal air contains around 79% nitrogen and 21% oxygen, heliox contains much less nitrogen or none at all. Helium constitutes the majority of the gas blend, typically over 90%.
For example, a common heliox mixture used in diving contains 80% helium and 20% oxygen (Heliox 80/20). Other standard heliox mixes include Heliox 70/30 and Heliox 50/50.
Why is heliox a good gas for deep diving?
Heliox has several properties that make it well-suited for deep diving applications below about 150 feet (45 m):
- Low density – Helium is much less dense than nitrogen at high pressures, resulting in lower breathing resistance.
- No narcosis – Helium does not cause nitrogen narcosis, allowing better mental function at depth.
- Less oxygen toxicity – Lower oxygen content reduces risks of seizures from oxygen toxicity.
- Faster decompression – Helium off-gasses faster than nitrogen, allowing safer, shorter decompression stops.
Let’s look at each of these advantages in more detail:
1. Low density of helium
The low molecular weight of helium makes heliox significantly less dense than air at ambient pressure. As a diver descends into deep water, the increasing pressure causes gas density to increase. Breathing dense gas at high pressure causes increased breathing resistance and requires more effort.
Because helium is only one-seventh the density of nitrogen, using heliox mixtures reduces this breathing resistance. Heliox breathing gases feel thinner, making it easier to breathe comfortably at depths below 150 feet where nitrogen narcosis typically begins.
2. No nitrogen narcosis
Nitrogen narcosis is a reversible alteration in consciousness caused by breathing compressed air at depth. The narcotic effects resemble alcohol intoxication, impairing judgement, focus and coordination. Nitrogen narcosis can begin at as shallow as 100 feet (30 m) in some divers.
Since helium does not cause narcosis like nitrogen, heliox allows divers to descend deeper while remaining clear-headed and avoiding poor decisions. Divers can function better and avoid risky behaviors caused by narcosis.
3. Reduced oxygen toxicity
The lower percentage of oxygen in heliox mixtures also helps reduce the risk of oxygen toxicity. Breathing pure oxygen below 20 feet can quickly cause seizures, but the diluted oxygen in heliox allows deeper diving before reaching high partial pressures of oxygen.
For example, the typical oxygen limit is considered 1.4 ATA. Breathing air at 131 feet reaches this limit, causing seizures after a short time. But with Heliox 80/20, divers can go over 230 feet before reaching the same oxygen partial pressure.
4. Faster decompression
After deep, long dives, divers must ascend slowly to off-gas excess nitrogen dissolved in the body under pressure. Slow ascent and safety stops are necessary to prevent decompression sickness.
But helium diffuses from tissues faster than nitrogen. Switching to heliox for the final ascent allows faster decompression stops. The total decompression time can be reduced by 25 to 50% compared to breathing air at the same depth.
Applications for Using Heliox
Because of its advantages, heliox is commonly used for:
- Saturation diving – Working at extreme depths for days requires living in pressurized habitats. Sat divers breathe heliox to avoid narcosis and oxygen toxicity when leaving the habitat and working on the sea floor.
- Commercial diving – Helium allows shorter, safer decompressions after deep commercial diving operations like underwater welding, drilling, construction and offshore oil work.
- Military diving – Navies use heliox for deep submarine rescue, salvage and underwater ship maintenance.
- Technical diving – Helium allows technical divers to descend hundreds of feet deep using closed-circuit rebreathers.
- Scientific research – Scientists at aquariums and marine research centers use heliox to study deep sea animals at their natural pressures.
What are the downsides of heliox?
While indispensable for deep diving, heliox does come with some disadvantages:
- Expense – Helium is a non-renewable gas that must be purified after extraction. It costs significantly more than nitrogen.
- Breathing resistance at shallow depths – While ideal at depth, heliox has higher density and breathing resistance at the surface. Air is preferred for the beginning and end of dives.
- Loss of speech and voice tone – The high helium content causes loss of speech intelligibility and a Donald Duck-like voice.
- Increased hypothermia risk – Helium conducts heat much faster than air, increasing heat loss in cold water.
- Specialized training needed – Due to risks like oxygen toxicity and narcosis, heliox diving requires comprehensive training to master.
Key properties of helium
To fully understand the benefits of heliox, it helps to review several important properties of helium:
Density
Gas | Density (g/L) at 1 ATA |
---|---|
Air | 1.29 |
Helium | 0.18 |
As this table shows, helium has a very low density, only 0.18 g/L compared to 1.29 g/L for nitrogen at normal pressures. This makes heliox much less dense than air at depth.
Narcosis
Unlike nitrogen, helium has absolutely no narcotic effects at any depth. This eliminates impairment of mental and motor functions caused by narcosis.
Oxygen tolerance
Helium has no impact on oxygen tolerance. At the same oxygen partial pressure, heliox and air have identical risks of seizures from oxygen toxicity.
Decompression
Due to lower solubility, helium off-gasses from tissues about 2.65 faster than nitrogen. This allows heliox divers to decompress more quickly after deep dives.
Heat conductivity
Helium conducts heat 6 times faster than air. Without adequate insulation, heliox drastically increases heat loss and risk of hypothermia in cold water.
Gas mixtures for different depths
The optimal heliox blend depends on the maximum depth of the dive. Typical gas mixes include:
Heliox Mixture | Max Depth |
---|---|
Heliox 80/20 | 250 feet |
Heliox 70/30 | 150 feet |
Heliox 50/50 | 100 feet |
For depths beyond 250 feet, trimix adds small amounts of nitrogen to reduce oxygen content and lower the risk of seizures. For example, Heliox 75/20/5 contains 75% helium, 20% oxygen, and 5% nitrogen.
Proper procedures for using heliox
To safely utilize the benefits of heliox while minimizing risk, divers must follow proper techniques, including:
- Calculate correct mix for planned depth
- Ensure adequate heliox supply for dive duration
- Allow time to adapt to heliox at shallower depths before descending
- Monitor depth and time closely to avoid oxygen toxicity
- Use nitrox or pure oxygen for faster decompression
- Have emergency air available in case of heliox interruption
- Stay warm! Wear well-insulated exposure suit
Conclusion
In summary, heliox is invaluable for deep diving applications because:
- Lower density reduces breathing resistance at depth
- No narcosis effects from helium compared to nitrogen in air
- Less risk of oxygen toxicity due to lower oxygen content
- Helium speeds up decompression stops during ascent
By displacing narcotic nitrogen and lowering gas density, heliox allows divers to work efficiently at depths beyond 150 feet, while reducing decompression time. Despite the challenges of cost, heat loss and training, heliox opens up deeper depths to scientific, military and commercial divers while improving safety.