What Is Trimix?

The Benefits and Considerations for Technical Diving With Trimix

Trimix Diving
A scuba diver completes a trimix dive. © Vincent Rouquette-Cathala, 2014

Most experienced divers are probably already familiar with the concept of deep diving beyond recreational limits using a breathing gas known as "trimix." While this word may be cloaked in mystery for the average recreational diver, it needn't be -- there is nothing magical about it. Using trimix is simply a method of limiting the side effects of breathing a gas under pressure to increase a diver's safety and enjoyment.

What Does the Word "Trimix" Mean?

The word "trimix" has two parts: “tri" from Latin and Greek meaning "three," and “mix” which refers to the fact that a the blend of different gasses is used. Although it would be technically correct to refer to any blend of three different gases as a trimix, in the dive community the word refers only to a blend of oxygen, helium, and nitrogen. Any combination of these gases can be considered trimix.

When a diver refers to trimix, he usually usually names the blend of gasses according to the percentage of oxygen and helium in the mix, with the oxygen percentage first. Following this convention, a diver may refer to trimix 20/30, which would be a mix of 20% oxygen, 30% helium, and an (inferred) complement of 50% nitrogen.

When Was Trimix First Used?

The first experiments reporting the use of helium in diving gases appear to have taken place during World War 2 in the British and American navies. For many years, trimix remained a research subject and was not used outside of the military. Probably the first divers to use trimix in a practical application were cave divers in the 1970's, who used helium mixes to explore deep caves. The more recent expansion of the scuba diving industry, and the technical scuba diving industry in particular, has helped the used of trimix to become more accepted. Diving with trimix is now standard practice when dive objectives lay bellow 150 ft, and is common in deep wreck, cave, and ocean diving.

What Are the Advantages of Diving With Trimix?

As a diver descends, the pressure surrounding him increases in accordance with Boyle's Law. High pressure compresses the gasses in in a diver's body, pushing the gases into solution. This can cause undesired physiological effects.

One example of an undesired effect caused by dissolved gas is nitrogen narcosis. Divers who go deep while breathing air experience nitrogen narcosis caused by the increased concentration of nitrogen in their bodies. The effects of nitrogen narcosis increase with depth, limiting the depths a diver can safely reach breathing air.

A diver is also limited by the percentage of oxygen in his breathing gas. High concentrations of oxygen exceeding 1.6 ATA (the partial pressure of the gas in units of atmospheres) puts a diver at the risk of oxygen toxicity, which can lead to convulsions and drowning. When diving on air, an oxygen partial pressure of 1.6 ATA is reached around 218 feet.

As the combined effects of high partial pressures of nitrogen and oxygen can limit a diver, a those who pursue deep diving may benefit by using a breathing gas with lower percentages of nitrogen and oxygen. This is where trimix becomes useful. The concept behind trimix is to remove some of the nitrogen from the breathing gas to help divers keep a clear head, and to remove some of the oxygen to increase the depth at which oxygen toxicity becomes a risk. Of course, reducing the percentage of oxygen and nitrogen in a gas mixture wouldn't be possible without replacing the some of the oxygen and nitrogen with a different gas. The third gas used in trimix is helium.

Why Was Helium Chosen as the Third Gas for Trimix?

Helium makes a good breathing gas when used in combination with oxygen and nitrogen in trimix because it reduces the narcotic effects of the gas mixture and and increases the depth to which a diver can safely dive by lowering the percentage of oxygen in the breathing gas.

Helium is less narcotic than nitrogen. The narcotic effect of a gas depends directly upon its solubility in fat tissues, and that solubility is dependent on the density of the gas. Less dense gases are less soluble in fat tissues. Helium is seven times less dense than nitrogen, and theoretically seven times less narcotic than nitrogen.

Using helium to decrease the percentage of oxygen in a breathing gas also increases the depth at which the oxygen partial pressure in the gas will reach unsafe levels. For example, a breathing gas with 18% oxygen instead of the standard 20.9% found in air will have a partial pressure of 1.6 ATA at about 260 feet instead of 218 feet.

In addition, the low density of helium makes a gas mixture easier to breathe at depth. This increases diver comfort and safety by reducing the work of breathing and lowering the chance of over exertion on deep divers. Finally, helium is absolutely neutral. Helium does not interact with any other chemical compounds, which avoids the onset of additional side effects.

Why Don't Divers Use Helium on Every Dive?

Up to this point, it may sound as if trimix is the perfect diving gas, but the use of trimix does have some draw backs that make it unsuitable for mainstream everyday diving.

1. Helium is scarce and expensive. While helium is the second most abundant element in the universe [1] it is rare on Earth and cannot be manufactured. There are only a few exaction points for helium on the planet, which makes helium a rare and valuable resource.

2. Diving with helium requires special training and procedures. Helium is absorbed and released much more quickly than nitrogen, requiring a diver to use advanced dive planning and decompression profiles. Decompressing from a trimix dive is not as straightforward as decompressing from an air or nitrox dive. There is also some evidence for a slightly higher risk of decompression sickness when diving with trimix compared to diving with air or nitrox.

3. Breathing helium can make you cold. Helium has a high thermal conductivity, leading divers to cool down faster when breathing trimix than when breathing any other gas mixture. Depending upon the dive conditions, water temperature, and hang time, the fact that breathing helium makes a diver colder must be taken into account when dive planning.

4. Helium can trigger high pressure nervous syndrome. Helium has the potential to trigger a form of toxicity specific to helium, called High Pressure Nervous Syndrome (HPNS). This toxicity can theoretically manifest as at depths as shallow as 400 ft, even though there are no confirmed reports of divers experiencing HPNS above depths of 600 ft.

Using trimix is the safest and most enjoyable was to dive to depths beyond 150 feet, but the expense, additional training required, and potential risks of diving with helium make using trimix impractical for most diving applications at shallower depths.

Learning to Dive With Trimix

For a diver who is interested in expanding his depth limits safely and progressively, a trimix certification is a good goal. Learning to use trimix safely requires a series of prerequisite courses that familiarize a diver with decompression procedures, advanced dive planning, and the use of multiple tanks. Even though the use of trimix requires a serious and safety-oriented mind-set, trimix dives are fun and rewarding when performed safely. A solid background of theory and underwater skills will give a trimix diver the tools to dive deeper and longer, and to bring back memories from what was only impenetrable darkness before.

 Vincent Rouquette-Cathala is a cave and technical diving instructor at Under the Jungle in Mexico.


1. "Chemistry in its Element" Chemistry World, Royal Society of Chemistry. 2014