Heliox is a mixture of helium and oxygen that replaces air in deep diving. It's used because the nitrogen in air causes narcosis at depth, and oxygen at its normal proportion becomes toxic under pressure. Helium avoids both problems, but it distorts the voice, cools the body very fast and is expensive, so it gets recycled.
A saturation diver spends weeks breathing a mixture that doesn't exist in nature. It's not a technical whim: it's the only known way to work for days at depths where air is simply toxic. Understanding heliox also explains why the voice sounds strange, why the chambers are kept hot, and why gas is one of the most closely watched cost items on a campaign.
What happens to air as depth increases
The air you're breathing right now is roughly 21% oxygen and 78% nitrogen. At the surface it works perfectly. Under pressure, those same percentages behave differently, because what determines a gas's physiological effect isn't its percentage but its partial pressure — that is, the proportion multiplied by the ambient pressure.
With depth, two problems appear at once. Nitrogen at high pressure produces narcosis: a state of impaired judgement and coordination comparable to drunkenness, incompatible with handling hydraulic tools or welding blind in murky water. And oxygen, essential for life, becomes toxic to the central nervous system when its partial pressure rises too far, with a risk of convulsion.
A diver in a helmet, unable to head for the surface, can't afford either one. Hence the replacement of air with an engineered mixture.
Why helium
Helium solves the first half of the problem. It's an inert, very light gas that produces no appreciable narcosis across the depth range of commercial work. Its low density also reduces the effort of breathing: at great depth the gas is compressed, and breathing dense air would be exhausting — something that really tells over a shift of several hours.
The second half is solved by adjusting the oxygen. Instead of the usual 21%, a smaller fraction is used, calculated to keep the oxygen partial pressure within a narrow band: enough to avoid hypoxia, and contained enough to avoid toxicity over exposures lasting weeks. That adjustment isn't fixed — it's monitored continuously from life support control, one of the responsibilities of the life support technician.
The result is heliox. Another well-known mixture, trimix, adds a small proportion of nitrogen and is mostly associated with technical sport diving and specific applications; in commercial saturation operations, heliox is the standard.
The three effects you notice every day
The voice
Helium's low density alters the resonance of the vocal tract and makes speech sound high-pitched and nasal, to the point of being unintelligible over the radio. The fix is a voice processor, called an unscrambler or helium speech descrambler, which corrects the pitch so the supervisor can understand the diver. It's not a minor quirk: clear communication between the diver, the bellman and the supervisor is a safety element, not a convenience.
The cold
Helium conducts heat far better than air, so the body loses temperature through breathing and through the skin much faster than usual. This means the chambers have to be kept at high temperatures, and it's why the diver works in a hot water suit, fed from the surface through the umbilical. In saturation, managing heat isn't about comfort: it's survival.
The cost
Helium is a limited, expensive resource subject to supply pressures. That's why saturation systems don't waste it: they recover it. Reclaim units capture the exhaled gas, scrub it of carbon dioxide and moisture, re-analyse it and return it to the circuit. Losing gas through a leak is losing money in a very direct way, hence the obsessive care over the tightness of the entire system.
High pressure nervous syndrome
Helium isn't a perfect solution. At great depths, what's called high pressure nervous syndrome appears — a set of symptoms including tremors, dizziness and disturbed sleep, linked to the direct effect of pressure on the nervous system and made worse by fast compressions.
It's managed mainly by controlling the compression rate: going down slowly greatly reduces how often it occurs. In very deep operations, mixtures with a small added proportion of nitrogen have also been used to dampen it. It's one of the reasons human diving has a practical working depth ceiling, and why beyond it the industry turns to remotely operated vehicles.
What gases are carried on board
A dive support vessel doesn't just carry heliox. An operation's gas inventory typically includes pure helium and oxygen to make up mixes for the working depth, air for the system and for surface tasks, oxygen for hyperbaric medical treatment, and the specific mixes needed during decompression.
All of this is stored in gas cylinder banks, mixed, analysed and logged. Gas analysis is continuous: oxygen and carbon dioxide sensors in every chamber, with alarms and redundancy. This equipment forms part of the core of a saturation system, and you can see where it fits within the commercial diving equipment and in the layout of dive support vessels.
What someone starting out needs to know
A trainee doesn't need to be a physiologist, but they do need to understand the logic: the mix is designed for the depth, partial pressure trumps percentage, and gas is an expensive consumable that gets recycled. In initial training these concepts are covered progressively, first applied to surface-supplied diving and later to saturation. You'll find the usual route in how to train as a commercial diver and the list of schools in commercial diving schools.
The reassuring part is that the diver doesn't decide the mix. It's calculated and controlled by the life support team, with written procedures and cross-checking. The demanding part is that you need to know how to read what the instruments are telling you, and react in time when something drifts out of range.
How the mix is prepared and controlled
The mix isn't improvised. Before a campaign, the composition needed for the planned working depth is calculated, and it's made up by blending helium and oxygen in the right proportions, with follow-up analysis to verify the result is what's expected. Every batch is labelled and logged.
During the campaign, the system's atmosphere changes on its own: the divers consume oxygen and produce carbon dioxide and moisture. So the system doesn't just contain the gas, it treats it continuously. Oxygen is added to replace what's consumed, the atmosphere is circulated through chemical scrubbers that strip out the carbon dioxide, moisture is removed and temperature is controlled.
That set of tasks is the permanent job of life support control, and it's why the system is never left unattended, day or night. A silent failure in carbon dioxide scrubbing is more dangerous than many a noisy breakdown, because its early symptoms — headache and drowsiness — are easily mistaken for tiredness.
Redundancy is the usual answer: duplicate sensors, cross-checking with independent instruments, alarms and periodic manual checks. The industry's culture, inherited from specific accidents, is systematically wary of trusting a single reading.
A note on gas during decompression
During final decompression, the atmosphere's composition is adjusted progressively as the pressure drops. Reducing the pressure without correcting the oxygen proportion would quickly lead to a hypoxic mixture, so the oxygen fraction is increased in a controlled way, following the planned profile.
It's a slow, tightly scripted process where nothing is improvised, and it's one of the reasons decompression from saturation takes days and is planned well in advance. You can see how it fits into the full cycle in how long a saturation campaign lasts.
Frequently asked questions
What exactly is heliox?
It's a breathing mixture made up of helium as the inert gas and oxygen at a fraction adjusted to the working depth. It replaces air in deep diving because it avoids nitrogen narcosis and lets the oxygen partial pressure be kept within a safe range over prolonged exposures.
Why do divers' voices sound high-pitched?
Because helium is much less dense than air and changes the resonance of the vocal tract, which raises the pitch of speech and makes it hard to understand. Communications go through a voice processor that corrects the distortion so the supervisor can understand the diver clearly.
Is helium reused?
Yes. Saturation systems include recovery units that capture the exhaled gas, scrub it of carbon dioxide and moisture, analyse it and return it to the circuit. Helium is expensive and in limited supply, so recycling it is both an economic and an operational necessity.
What is high pressure nervous syndrome?
It's a set of symptoms that appears at great depths with helium mixes: tremors, dizziness, nausea and disturbed sleep. It's linked to the effect of pressure on the nervous system and is controlled mainly by compressing slowly, along with mix adjustments on very deep operations.
Can you dive deep on compressed air?
Not safely, beyond a limited range. With depth, the nitrogen in air causes narcosis and oxygen reaches toxic partial pressures. That's why deep commercial diving uses engineered mixtures rather than air.
Who controls the mix during a campaign?
The life support team, from the saturation system's control panel. They continuously monitor the oxygen, carbon dioxide, pressure, temperature and humidity of every chamber, with redundant sensors and verification procedures.
Published 22 August 2026 · By Arthur More, professional saturation diver
Photo: infomatique · CC BY-SA via Openverse
This article is part of our in-depth guide to how deep a commercial diver can actually go — the complete picture, with sources.
This article is part of our in-depth guide to how deep a commercial diver can actually go — the complete picture, with sources.
This article is part of our in-depth guide to how deep a commercial diver can actually go — the complete picture, with sources.
This article is part of our in-depth guide to how deep a commercial diver can actually go — the complete picture, with sources.