Trimix is a combination of oxygen, helium and nitrogen. The idea behind trimix is to displace nitrogen with helium so that you can avoid the drawbacks of breathing high partial pressures of nitrogen.

Commercial divers omit nitrogen entirely for mid-range depths, using helium/oxygen mixtures called heliox. While certainly doable for tech divers, the logistics and costs are usually beyond most sport/tech divers capability.

For mixes in the deeper ranges, the required 02 content of your mix drops to the point that you can mix helium and air, called heliair, and not have to add 02.

To bring the advantages of replacing nitrogen with helium to every day sport/tech divers, it is advantageous to use partial pressure fills of oxygen and helium and then to top off with air.

The voodoo surrounding trimix

There is a misconception about trimix. Trimix is frequently promoted as the holy grail of scuba diving, so horrifyingly complex that you must take 4-8 levels of training before you can use it. In reality mixing a tank of trimix is no more complex than mixing a tank of nitrox.

What is complex and potentially dangerous is diving to extreme depths using multiple bottles of different mixes of gas, where choosing the wrong regulator can end your life in a hurry.

But it is possible to create a normoxic tank of trimix and treat it similar to air, without multiple mixes, bottles and other complexities, and enjoy the benefits of helium.

A fable about trimix is that you can't breath it on the surface. Generally speaking, the human body can handle lowered 02 percentages down to 16%. So, if you did a 17% mix, you can breath it as long as you want without fear of hypoxia. The breathability of a mix is solely dependant on it's 02 percentage, helium is complete inert and does not figure in.

Why use trimix?

The air we breath is composed of about 20.9% oxygen, 79.1% nitrogen and .033% carbon dioxide plus various inert trace gases. This is fine until we start breathing air under water. As we go deeper the partial pressures of these gases increase and we start suffering from their side effects.

Side effects of nitrogen include:

Narcosis - This is a mental function imparement which ranges from a mild euphoric feeling (60'-90'), slowing of mental activity (100'-130'), memory imparment and task fixation (140'-160') tingling in lips, legs and feet, severe drop in intellectual capacity (170'-200'), Voice reverberation, stupor and a sense of impending doom (200'+).

Bends - Nitrogen absorbed into tissues and fluids of the body reverting into gas bubbles.

Physiological - Damage to tissues leading to domino effects on immune system.

What about Nitrox?

Nitrox is an attempt to replace nitrogen with oxygen. Oxygen is metabolized by our bodies, so it is not absorbed into the tissue. Adding oxygen effectively reduces your nitrogen uptake, but there are some problems:

Side effects of oxygen include:

Central nervous system toxicity- CNS causes a seizure which can prove fatal while underwater. Partial pressures of over 1.6 can be extremely dangerous. 1.4 should be considered the maximum for a working dive, and it is reasonable to use 1.1 or 1.2 as your standard.

Inflammation of lung tissue from long term exposure- You can calculate your exposure to minimize this, but reports are coming in from the field of "lung burn out" caused by high levels of 02 which are well under the standard limits.

So why helium?

Helium is a non-toxic, colorless, odorless, tasteless, inert, lightweight and nonexplosive gas. To quote George Irvine head of the WKPP: "Helium is our friend".

Advantages of helium:

-Narcotic effects are nil.

-Due to its lower density, breathing resistance at depth is significantly reduced.

-Helium off-gasses rapidly and it does not enter slow tissues as readily as nitrogen.

Disadvantages of helium:

-Helium conducts heat 5 times faster than air.

-Hyperbaric arthralgia, an arthritic-like stiffness, can occur during descent with some divers.

Helium has been used for diving as far back as 1938. The first true test of heliox diving was the rescue of the submarine Squalus in 1939 in 240 fsw. Since then helium has been used in dives to more than 2000 fsw.

As helium is less dense than nitrogen, it enters and leaves the tissues faster than nitrogen. Paradoxically helium requires a little more decom time with short dives than air, but less decom time on long dives than air. The key to using helium is slow descents and slow ascents. Additional deep stops are required when ascending on helium mixes.

Due to helium's ability to leave tissues rapidly, having 2 or more stages, for example 50% nitrox and 100% O2, allows you to off-gas helium faster than you could nitrogen.

There is a situation which can occur in depths past 400ft. called High Pressure Neurological Syndrome (HPNS) . This manifests itself with tremors, muscle twitching and coordination difficulties. Adding a small amount of nitrogen to your mix can alleviate these symptoms. But then, what are you doing down there, anyway?

Go ahead and use it!