What Is Trimix?
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.
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.
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.
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!
Helium and diving were made for each other. You don't have to be a super tech diver to enjoy the benefits of helium. Breathing high partial pressures of Nitrogen is dangerous and hard on the body, so why do it? Replacing nitrogen with helium is a reasonable thing to do for single tank dives as shallow as 100'. What helium does for the 140-170 foot range (previously "deep air") is nothing short of miraculous. You remember the dive, you perform well under stress and you feel better after the dive.