Diving theory: What is trimix?

Since humans have set their eyes on the depths and have attempted to explore deeper and deeper, there have been many obstacles that have slowed or limited our progress. As we encounter them, through trial and error, we usually develop tools or techniques to get around those obstacles: to understand what trimix is, we first need to understand where it comes from and which obstacle it helps us overcome.

In this article, we'll go over the origin of trimix, explaining what trimix is and what it is used for, then go over its advantages and disadvantages, and talk about planning a dive using trimix.

What is trimix?

Trimix is a breathable gas mixture composed of Oxygen, Helium, and Nitrogen. The addition of helium (compared to air or nitrox) reduces the narcotic potential of the mix. It is used for deeper dives, where narcosis can potentially be a limiting factor. As Nitrogen (and potentially Oxygen) are narcotic gases, simply using air is not sufficient for deeper dives as the narcotic potential of the mix increases with depth.

More generally, trimix is used to reduce the effects of Oxygen and Nitrogen by reducing their proportion in the mix compared to air or nitrox: Reducing narcosis is one issue it solves, but another one is reducing the risk of oxygen toxicity by being able to reduce the ppO2 of the mix. As regular air contains 21% Oxygen, it reaches a ppO2 of 1.4 atm at 56.7m and a ppO2 of 1.6 atm of 1.6 atm at 66.2m (1.4 is usually the limit for the bottom part of a dive, while decompression stops can be completed at 1.6).

We can differentiate between different types of trimix, depending on the type of blend: Trimix is a mix of Oxygen, Helium, and Nitrogen, usually blended with helium, oxygen, and air; Heliair, also called "poor man's mix" is a type of trimix that is blended using only helium and air, which keeps the ratio between oxygen and nitrogen at 0.21:0.79; Heliox is only a blend which only contains Oxygen and Helium. Due to the high cost of helium, technical divers generally only add a fraction of helium and use trimix.

We can also differentiate the blends depending on the percentage of oxygen: Normoxic trimix is a blend that contains between 17%-21% Oxygen. It is called "normoxic" because the oxygen composition is similar to that of air, and there is enough oxygen to sustain human life if breathed on the surface. Hypoxic trimix is a blend that contains less than 17% Oxygen (hypo=less). The oxygen partial pressure (ppO2) on the surface of such a mix would not be sufficient to sustain human life, and is only suitable to breathe at depth, when the ppO2 has increased enough: this blend not only has a Maximum Operating Depth, but also a Minimum Operating Depth! The use of Hypoxic trimix is limited to technical divers with special training. Hyperoxic trimix is a blend that contains more than 21% Oxygen (Hyper=more). This blend not very common, because at the depth you want to add helium to your mix, you wouldn't necessarily want to increase the oxygen proportion, although at shallower depths for technical diving (in the 40-50 meters range), it is still possible to combine a weak nitrox with helium (example: diving at 45 meters can be done using a 25/20 trimix - 25% Oxygen, and 20% Helium).

Similar to Nitrox, Trimix cylinders need to be analyzed and marked before a dive, with the difference that there is helium to take into account. To analyze the blend, multiple Oxygen+Helium analyzers are available on the market, and similarly to oxygen, the Helium cells will have to be calibrated before analyzing (by using air as a reference, which has 0% Helium). To mark the cylinders, we describe the Oxygen/Helium content of the blend: for example, a trimix blend containing 21% Oxygen and 35% Helium would be marked (or referred to as) 21/35.

A historical perspective

At the turn of the 20th century, John Scott Haldane had established the first decompression tables, allowing workers and early divers to stay at greater depths for longer periods of time. For a long time, instances of DCS were the limiting factor, as workers were often sent hours at a time at depths which would be considered recreational by today's standards, and due to their inadequate return at the surface (decompression), they would get DCS. Although it wasn't a perfect model yet, Haldane's table significantly reduced the incidence of DCS and set the path for further exploration.

Now that the risk of DCS was mitigated, various organizations, including the US Navy, attempted to test the limit of these new models by going deeper and staying longer. However, they soon observed a new limiting factor: the occasional dizziness and drunkenness that workers would encounter at moderate depth now turned into incapacitating stupors, intense confusion, and other symptoms- this was, of course, nitrogen narcosis (keep in mind that this was all done while diving on air). A notable instance was in 1915, during the salvage operation of the USS F-4, a submarine that sank off the coast of Hawaii due to a technical failure during training: Divers were tasked to attach lifting chains under the hull of the submarine. To do that, they had to descend to a maximum depth of 90 meters, and although the operation was successful, it was noted that the divers were experiencing severe narcosis.

As it was noted that adding helium to the breathing mix seemed to alleviate these symptoms, the U.S. Navy began trials of deep dives using a mix of Oxygen and Helium (Heliox). The NEDU (United States Navy Experimental Diving Unit) was established in 1927, and the experimental dives combined with additional salvaging operations of the S-51, S-4, and USS Squalus led to the first publication of surface-supplied Heliox diving tables in 1939.

In the second part of the 20th century, Albert Alois Bühlmann, basing himself on previous work, refined decompression models and included helium, allowing decompression tables to be generated using any trimix blend. These models are still used to this day to plan technical trimix dives.

Advantages of trimix

Using trimix is generally safer and has many advantages over regular air (or nitrox) due to multiple factors:

• The primary reason for using trimix, as mentioned previously, is its reduced narcotic potential: for deep dives, and for technical dives, this is essential to ensure the diver stays rational and keeps their mental capacity. For instance, for cave diving or overhead environments, the diver needs to stay lucid to prevent them from making wrong or potentially endangering decisions.
• As helium is a lighter molecule than either Oxygen or Nitrogen, trimix has a lower density than air. Although it might seem insignificant, at depth, when gas density increases, it can make a big difference. This is especially important for deep dives, when high gas density can increase the Work of breathing up to a point where Carbon dioxide accumulates faster than the body can get rid of it; this can result in Hypercapnia.
• The half-times for helium in tissue compartments are significantly lower than for nitrogen. Although that means helium will saturate the tissue faster, it also means it will desaturate faster as well. The tolerance for helium used in modern decompression models is also higher than for nitrogen. This means that using trimix will yield shorter decompression stops.

Disadvantages of trimix

There are also some limitations that come with using trimix:

• The logistical implications of using trimix are a big disadvantage: First of all, sourcing a dive center that can blend helium can be tricky, depending on your location, as it is not common everywhere. Secondly, the blending process takes longer and is generally prepared the day before, as the gases need to mix homogeneously (which can take a few hours).
• The price tag for trimix is one of the biggest setbacks for many people: Helium, contrary to compressed air, is a finite resource and is expensive; on top of that, the deeper you go, the more of it you will need. To give you an idea, let's use an average price for helium of 10 cents per Litre: filling a 12L twinset to 200 bar with a mix containing 25% Helium would cost 120€. If you plan on going deeper and require a blend with more helium, the price can rise quite fast - if you were to fill the same twinset with 60% helium instead, the price would rise to 288€. For this reason, a lot of divers who plan to use trimix often choose to dive using CCR, as you need less gas volume when using a rebreather, and the initial price of the unit can offset the price of helium in the long run.
• As the thermal conductivity of helium is higher than air -about 5 times higher- breathing trimix will make the diver lose heat faster than air. For the same reason, using trimix to inflate the drysuit is not recommended. For dives in cold water, divers will rather use a mix without helium, or a mix containing argon to inflate their drysuit, as it has a lower thermal conductivity.
• Switching gas while using trimix (for example, switching from a trimix backgas to a 50% Oxygen + 50% nitrogen gas decompression mix) can lead to Isobaric Counterdiffusion (ICD). Isobaric counterdiffusion is a complication that occurs when an inert gas diffuses into your body faster than another inert gas can diffuse out; it can lead to DCS. ICD can also superficially occur when a diver is breathing a heavier mix (air, for example) while surrounded by a lighter gas mix (trimix) in a drysuit, which is another reason not to use trimix to inflate your drysuit.

Choosing the proper trimix blend

Planning a trimix dive is a higher-commitment process than planning a regular, recreational dive, as trimix dives are, by nature, deeper and more technical. The first step is knowing which kind of trimix blend will be optimal for your planned dive: This means choosing the oxygen fraction and choosing the helium fraction. You will need to look at which depth you go, and choose the appropriate Oxygen Partial pressure, and Equivalent Narcotic Depth you decide to plan your dive at. As a general guideline, the maximum depth of the dive shouldn't be at a ppO2 higher than 1.4 atm, and at an END deeper than 40m (some agencies will say the END shouldn't exceed 30m).

If you know the depth at which you plan to dive, you can start by calculating the optimal oxygen fraction:

FO2 = 10×ppO2/(Depth+10) (in meters)

FO2 = 33×ppO2/(Depth+33) (in feet)

Once the oxygen fraction has been chosen, we can then find the helium fraction, knowing the END (Equivalent Narcotic Depth), using the following formula:

FHe = 1 - (END+10)/(Depth+10) (in meters)

FHe = 1 - (END+33)/(Depth+33) (in feet)

Note: in the formula above, to find the Helium fraction, it is considered that Oxygen is narcotic; if you choose to consider oxygen as non-narcotic, the formula will be slightly different, and you can find it here.

For example:

I am planning a dive at 60 meters. I want the ppO2 at the maximum depth to be 1.3 atm (rather than 1.4 atm, which gives us a few meters of margin in case something happens); I want the Equivalent Narcotic depth to be a conservative 35 meters, as it is important to stay lucid at those depths.

I can start by calculating the required oxygen fraction to have a ppO2 of 1.3 atm at 60 meters:

FO2 = 10×ppO2/(Depth+10) = 10×1.3/(60+10) = 0.186 = 18.6%

Next, I can calculate the Helium fraction, given my END of 35m:

FHe = 1 - (END+10)/(Depth+10) = 1 - (35+10)/(60+10) = 0.357 = 35.7%

Therefore, to plan this dive, I would require a 19/36 Trimix blend (19% Oxygen, 36% Helium).

If you're interested in learning in more detail how to plan decompression dives, you can read this article.