Technical Diving Gas Mixes

What and Why?

In this post we will be exploring the different technical diving gas mixes breathed by divers who want to explore beyond the recreational time and depth limits. Technical divers utilise a variety of different gas mixes which we will look at (briefly) in this post.

This post does not constitute nor substitute formal training from qualified and experienced professionals. It is intentional that no formulas, limits or other details are provided which will enable the reader to plan dives based on this information alone. The purpose of this post is simply to de-mystify some of the witchcraft behind different diving gas mixes.


Most dives are done with the diver breathing exclusively Air. It is of course highly abundant, accessible and – best of all – free! Air is essentially a mixture of Oxygen (approx. 21%) and Nitrogen (approx. 79%). To be precise around 1% of the air we breathe is made up of dozens of other gasses in trace amounts, but there is no practical reason to take these into account.

The crucial component here is, rather obviously, Oxygen. We breathe in air and metabolise the oxygen that we need – around 1 litre per minute. We convert this Oxygen into Carbon Dioxide and exhale it with the remainder of the unused air.

Inert Gasses

The Nitrogen is not necessary in and of itself. It is an inert gas, meaning that we have no use for it nor reaction to it. It is necessary to have AN inert gas present in the mix because too much Oxygen can be toxic. You can have too much of a good thing!

Over hundreds of thousands of years our bodies have evolved in a relatively stable atmosphere. As we descend then the gas we are breathing increases in pressure – something new and unfamiliar to us. If we find ourselves breathing Oxygen at too high a pressure then we risk a severe reaction which could prove to be fatal. With respect to the Oxygen concentration, it is considered safe to breath Air to a depth of approximately 55 metres.


Any diver who has ventured into deep water in the past can probably attest to the effect of gas narcosis. When we breath Air deeper than around 35/40 metres we are likely to start experiencing a slight fuzzy feeling, often described by divers as the “Martini” effect. Indeed, the effects of gas narcosis are comparable to drunkenness: a lack of co-ordination and a dip in cognitive abilities. This is caused by the density of the gas we are breathing and it disappears completely and immediately when we ascend.


Nitrox is simply a name for any gas mix comprised of Oxygen and Nitrogen (Air is Nitrox). For diving purposes we generally consider Nitrox to be a gas mix with a higher (or ‘richer’) concentration of Oxygen. This is why it is often called Enriched Air, or Enriched Air Nitrox (EAN for short).

For recreational diving Nitrox is a wonderful thing! By breathing a mix with less Nitrogen we will absorb less of it than on a comparable dive breathing Air.  This means either we get to stay at depth for a significantly longer time (gas supply permitting); or we can make standard profile dives which factor in an extra margin of safety.

The drawback of decreasing the Nitrogen in the mix is that we increase the Oxygen. This means that we cannot go as deep breathing Enriched Air as we can when diving with standard Air. The most popular Nitrox blend is EAN32 (32% O2) since this is still considered safe to breath slightly beyond 30 metres, making it suitable for most recreational dives.


As stated before, Air is ‘safe’ up to around 55 metres.  The Narcotic effect at this depth will be fairly strong, however. This should only be attempted by experienced divers with suitable training and under acceptable conditions.

Oxygen and Nitrogen have a very similar molecular weight (density) so the Narcosis will not be noticeably affected by breathing Nitrox instead of Air.

If a diver wishes to go deep but does not want to experience the Narcotic effect of the gas then they would need to add a very ‘light’ gas to the mix.  On earth there is one very light gas which can be harnessed relatively easily: Helium.

By substituting some of the Oxygen and Nitrogen for Helium the user can dive to 55 metres and still feel (with respect to Narcosis) like they are at, for example, 35 metres.


A mix of the three elements is called Trimix. Typically, when learning to dive with Trimix you will start by breathing ‘Normoxic’ Trimix. This is a Trimix containing the standard 21% Oxygen which we are used to. The same depth limit applies for Normoxic Trimix as it does for Air. A benefit of Normoxic Trimix is it also means that this gas can be safely breathed at any stage of the dive.

Hypoxic Trimix

Since it is the Oxygen which limits the maximum depth which the gas can be breathed, divers wishing to go deeper still must switch to ‘Hypoxic’ Trimix – a mix containing less than 21% Oxygen.

With Trimix it would be possible to plan a dive to, say, 100 metres depth and only experience a ‘Narcotic depth’ of 40 metres. This gas would have such a low concentration of Oxygen that it may not sustain consciousness at or close to the surface. Therefore the diver must breath a ‘travel gas’ until they are deep enough to switch to their ‘bottom gas’ (Trimix). This gas would usually be the Nitrox which the diver will breathe upon ascent in order to accelerate their decompression.

Helium is also an inert gas – like Nitrogen our bodies have no use for it. When we breathe it we will absorb it into our bodies and when we ascend we must take into account the decompression of this gas as well as that of the Nitrogen.

Since Helium has different physical properties to Nitrogen the decompression of this gas will be different to the Nitrogen. Therefore a thorough understanding of the different decompression requirements is necessary to avoid injury and illness. A diver switching from Trimix to a Nitrox containing too much Nitrogen may experience and Inner-ear bend. This is a particularly unpleasant form of decompression illness.

Do divers ever breath pure Oxygen?

Pure Oxygen is quite simply the best way to off-gas inert gas from our bodies. As described in other posts, the rate at which we off-gas inert gas depends on the difference in pressure between that particular gas in our bodies v’s the pressure of that same gas in our lungs. By removing all inert gas from our lungs (breathing pure O2) we will make this pressure difference as great as possible, making it the fastest way to off-gas.

This is not to be attempted by anybody though: Oxygen becomes extremely toxic extremely quickly as we descend. A thorough understanding of the dangers combined with excellent skill at maintaining buoyancy control are essential for any diver wishing to finish a dive breathing Oxygen.

Is that all?

For the most part, deep divers will always be breathing some mix of these three elements (Oxygen, Nitrogen and Helium). Two other gas mixes which are less common but still in use are Heliair (Helium mixed with normal air, maintaining the normal ratio of Oxygen : Nitrogen) or Heliox (only Oxygen and Helium). These mixes have their own benefits  but the theory behind the gasses in use remain the same.

I hope this information has proven to be useful and interesting for you. If you wish to learn more about Technical diving then why not enrol in the PADI Tec 40 course. Just to try it you can even complete a one day ‘Discover Tec’ programme. For more information or to make a booking, please contact:

Safe diving!