Mixed Gas: Breathing Gas for Diving Other Than Air, Usually Implies a Helium-Based Mixture


Mixed gas is a type of breathing gas used in scuba diving, distinct from regular air, which consists primarily of nitrogen (78%) and oxygen (21%). The most common mixed gases are helium-based mixtures that offer several benefits to divers, such as reduced narcotic effects and longer bottom times. This entry explores the history, composition, and applications of mixed gas diving, as well as its advantages, disadvantages, and safety considerations.

History of Mixed Gas Diving

The use of mixed gases in diving can be traced back to the early 20th century when researchers began investigating the physiological effects of breathing different gas mixtures at varying depths. In the 1930s, Swiss engineer and diving pioneer, Hannes Keller, introduced helium as an alternative to nitrogen, significantly reducing the risk of decompression sickness and nitrogen narcosis. This paved the way for the development of mixed gas diving, which has since become an essential technique for deep and technical diving.

Composition of Mixed Gases

Mixed gas is composed of varying ratios of oxygen, helium, and occasionally, nitrogen. The specific mixture depends on the intended diving depth and duration, as well as the diver’s experience and physiology. The most common mixed gases include:

  1. Nitrox: A mixture of nitrogen and oxygen, with a higher oxygen concentration (22-40%) than regular air. Nitrox increases the no-decompression limit, allowing for longer dives with reduced risk of decompression sickness.
  2. Heliox: A mixture of helium and oxygen, typically used for deep diving (greater than 130 feet or 40 meters). The reduced density of helium minimizes breathing resistance and the risk of nitrogen narcosis.
  3. Trimix: A mixture of oxygen, helium, and nitrogen, which balances the benefits of helium with the lower cost and availability of nitrogen. Trimix is commonly used in technical diving to depths between 130-330 feet (40-100 meters).
  4. Hydreliox: A combination of hydrogen, helium, and oxygen, used for extreme deep diving (greater than 330 feet or 100 meters). Hydrogen is added to reduce the high-pressure nervous syndrome (HPNS) associated with breathing helium at great depths.

Applications of Mixed Gas Diving

Mixed gas diving is employed in various situations, including:

  1. Deep Diving: Mixed gases, particularly heliox and trimix, are critical for deep diving, as they reduce the risk of decompression sickness and nitrogen narcosis.
  2. Wreck Diving: The increased bottom time afforded by nitrox allows divers to explore shipwrecks for extended periods without requiring decompression stops.
  3. Cave Diving: Mixed gases facilitate longer and deeper penetration into underwater cave systems, enabling divers to explore previously inaccessible areas.
  4. Scientific Research: Marine biologists, archaeologists, and geologists often utilize mixed gas diving to conduct research at greater depths and for extended durations.

Advantages of Mixed Gas Diving

  1. Reduced Narcotic Effects: The substitution of helium for nitrogen minimizes the risk of nitrogen narcosis, a dangerous condition that impairs cognitive function and motor skills.
  2. Longer Bottom Times: Mixed gases, particularly nitrox, enable longer dives without the need for decompression stops, maximizing dive time and productivity.
  3. Improved Breathing Efficiency: The reduced density of helium-based mixtures results in less breathing resistance, making it easier for divers to breathe at depth.

Disadvantages and Safety Considerations

  1. Increased Cost: Mixed gases, especially helium, are more expensive than regular air, making mixed gas diving a costly endeavor.
  2. Specialized Equipment and Training: Mixed gas diving requires specialized regulators, cylinders, and
  3. analyzers, as well as thorough training and certification to ensure safe and proper use.
  4. Oxygen Toxicity: Breathing gas mixtures with higher concentrations of oxygen, such as nitrox, can result in oxygen toxicity if not managed correctly. This condition can lead to convulsions, unconsciousness, and even death.
  5. High-Pressure Nervous Syndrome (HPNS): Divers breathing helium-based mixtures at extreme depths can experience HPNS, a neurological condition characterized by tremors, dizziness, and nausea. The addition of hydrogen in hydreliox mixtures helps to mitigate this risk.
  6. Gas Embolism: As with regular air, divers using mixed gases are still susceptible to gas embolism, a potentially fatal condition resulting from gas bubbles entering the bloodstream.

Safety and Training

Due to the inherent risks and complexities associated with mixed gas diving, proper training and certification are essential. Several organizations, including PADI (Professional Association of Diving Instructors), NAUI (National Association of Underwater Instructors), and TDI (Technical Diving International), offer courses on nitrox, trimix, and other mixed gas diving techniques. These courses cover essential topics, such as gas blending, decompression planning, equipment configuration, and emergency procedures.

Divers interested in mixed gas diving should also maintain good physical fitness, adhere to established safety protocols, and always dive with a buddy or within a team. It is crucial to recognize and respect personal limits and only undertake dives that match one’s skill level and experience.


Mixed gas diving has revolutionized the world of scuba diving, enabling divers to explore deeper depths and extend bottom times, while reducing the risks associated with decompression sickness and nitrogen narcosis. However, this advanced technique comes with unique challenges, including increased cost, specialized equipment requirements, and potential health risks. As such, it is imperative that divers undergo proper training and certification, maintain rigorous safety standards, and continually update their skills to ensure safe and enjoyable mixed gas diving experiences.