Introduction to Trimix Diving

In the third installment of our series on technical diving, we’re moving away from beginner-level courses and into the world of trimix diving.       

The phrase “technical diving” conjures images of deep descents, mysterious shipwrecks and undiscovered caves. In this, the third installment of our series on technical diving, we’re moving away from discussion of beginner-level technical diving and initial decompression diving. Today we enter the world of trimix diving.

What’s in a name?

As the name suggests, trimix diving involves a combination of three gases: oxygen, nitrogen and helium. Depending on blending methods and “ingredients” (individual gases or mixes), you will also find references to helitrox or heliair. We add helium to the diver’s breathing gas for a few reasons depending on the planned dive. For depths from 197 to 213 feet (60 to 65 m), we consider trimix diving appropriate to reduce nitrogen narcosis. Narcosis affects every diver below 100 feet (30 m). While many might not feel much in warm, tropical waters at that depth, things change in colder, darker waters.

Beyond 213 feet (65 m), we use helium not only to ward off narcosis, but also to decrease exposure to high oxygen partial-pressures. Anyone qualified to dive nitrox up to 40 percent should know the maximum recommended partial pressure of 1.4 bar and the contingency of 1.6 bar. Exposure to higher partial pressures means the diver risks central-nervous-system oxygen toxicity underwater, which can lead to convulsions and drowning. Consequently, divers choose a trimix with less oxygen for those dives.

We also want to consider the density of the gas the diver is breathing. The deeper the dive, the higher the pressure surrounding the diver. Thus, the denser the breathing gas will be. Even with premium regulators, breathing will become more labored than on the surface. As a very light gas, helium helps keep gas density low and breathing more comfortable.

How do you begin trimix diving?

Classic approaches to trimix diving usually include an introductory level of normoxic trimix diving. This is followed by another course qualifying students to dive hypoxic or full trimix. What’s the difference?

Normoxic refers to a gas with normal oxygen content, typically between 20 and 21 percent. A minimum of 18 percent is acceptable, however. Breathing this gas at sea level is safe as it contains enough oxygen to sustain life when surrounded by an atmospheric pressure of 1 bar. Hypoxic — too little oxygen —refers to gases with less oxygen than 18 percent. The lower the oxygen content of a gas, the deeper you can breathe it. If that’s the plan, divers must consider taking another gas that they can breathe from the surface until they descend to the point where their main trimix becomes breathable — a travel mix.

For TDI, those two courses are entry level Trimix Diver (197 feet/60 m) and Advanced Trimix Diver (328 feet/100 m). For PADI, they are Tec Trimix 65 (213 feet/65 m) and Tec Trimix.  The course dives to a maximum of 295 feet/90 m, although certification is ‘unlimited’.

While prerequisites vary slightly, both agencies require students to have training as technical divers when they enroll in the courses. Courses typically take four days or more, depending on previous experience and how a student is progressing.

More options

In recent years, however, training agencies have started introducing trimix diving options earlier in their dive training.

RAID offers an option for trimix use on their Deco 40 course, utilizing helium to reduce both narcosis and gas density during decompression dives to a maximum depth of 131 feet (40 m). TDI introduced the Helitrox Diver qualification. This allows students to use up to 20 percent helium in a breathing gas, with a minimum of 21 percent of oxygen on decompression dives to 147 feet (45 m).

These qualifications allow divers to learn about using restricted versions of trimix without qualifying for deep-air diving first. While previously the tec community thought of deep-air diving as below 196 feet (60 m), opinions have changed. Once we consider gas density, trimix becomes a good choice for dives below 164 feet (50 m), and even shallower. Add to that the cold, dark water of the North Sea, for example, which leads to a higher narcotic load, and we’ve made a good argument for trimix.

Why aren’t we all diving trimix all the time?

As you may have guessed, there are a few drawbacks to trimix diving. First, helium is expensive. Therefore, divers not only carefully consider suitable trimix mixtures, but also recycle leftovers to use for other dives wherever possible. That said, technical diving is an extreme sport, and one should never put cost considerations before diver safety.

Helium is also rare and can be hard to come by in some parts of the world. In many places in Southeast Asia, for example, divers must pre-order helium with plenty of time to spare.

Next, there is the insulation issue. Helium feels cold, rendering it impossible to use for drysuit inflation. Unfortunately, many deep dives, even in warm waters, require (tropical) drysuits for exposure protection. Divers therefore must utilize another gas for suit inflation.

And, finally, there is the so-called “helium penalty.” Current decompression theory suggests that breathing trimix increases decompression obligation. This is due to helium’s propensity to enter body tissues quickly (due to its lightness) and exit them quickly on ascent. This requires even more careful decompression management than air. Recent studies suggest that the additional decompression time may be unnecessary, but there is no sufficient evidence to this point to prompt changes in dive planning considerations.

Where does this leave us?

Anyone seriously considering exploring deeper waters will have to utilize trimix in some form. Many cold-water divers opt to dive ‘mix’ from as shallow as 114 feet (35 m) to ensure they keep a clear head. Where supply problems and ongoing financial considerations are an issue, closed-circuit rebreathers (CCRs) often provide an alternative. While initial investment and training is costly for a CCR, for those who regularly execute deep dives, the cost will be offset eventually.