page_title: Breathing gas
section_title: Helium
hierarchical_section_title: Breathing gas / For diving and other hyperbaric use / Individual component gases / Helium
caption_attribution_description: English: High pressure gas storage unit
context_page_description: A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common, and only natural, breathing gas. But other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed habitats such as scuba equipment, surface supplied diving equipment, recompression chambers, high-altitude mountaineering, high-flying aircraft, submarines, space suits, spacecraft, medical life support and first aid equipment, and anaesthetic machines.
  Oxygen is the essential component for any breathing gas, at a partial pressure of between roughly 0.16 and 1.60 bar at the ambient pressure. The oxygen is usually the only metabolically active component unless the gas is an anaesthetic mixture. Some of the oxygen in the breathing gas is consumed by the metabolic processes, and the inert components are unchanged, and serve mainly to dilute the oxygen to an appropriate concentration, and are therefore also known as diluent gases. Most breathing gases therefore are a mixture of oxygen and one or more inert gases.
context_section_description: Helium (He) is an inert gas that is less narcotic than nitrogen at equivalent pressure (in fact there is no evidence for any narcosis from helium at all), so it is more suitable for deeper dives than nitrogen. Helium is equally able to cause decompression sickness. At high pressures, helium also causes high-pressure nervous syndrome, which is a central nervous system irritation syndrome which is in some ways opposite to narcosis.
  Helium mixture fills are considerably more expensive than air fills due to the cost of helium and the cost of mixing and compressing the mix.
  Helium is not suitable for dry suit inflation owing to its poor thermal insulation properties – compared to air, which is regarded as a reasonable insulator, helium has six times the thermal conductivity. Helium's low molecular weight (monatomic MW=4, compared with diatomic nitrogen MW=28) increases the timbre of the breather's voice, which may impede communication. This is because the speed of sound is faster in a lower molecular weight gas, which increases the resonance frequency of the vocal cords. Helium leaks from damaged or faulty valves more readily than other gases because atoms of helium are smaller allowing them to pass through smaller gaps in seals.
  Helium is found in significant amounts only in natural gas, from which it is extracted at low temperatures by fractional distillation.
caption: 2% Heliox storage quad. 2% oxygen by volume is sufficient at pressures exceeding 90 msw.
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