Nitrox ( " N i t r " ) stands for Nitrogen and ( " o x " ) for Oxygen.

Nitrox is easy to blend and to understand. It`s all about increasing the oxygen contingent and thereby decreasing the nitrogen contingent. To distinguish one blend of enriched air from another, the dive community uses the abbreviation EANx (for enriched air nitrox) followed by the oxygen content percent. For example, EANx32 is enriched air containing 32% oxygen and the balance (68%) nitrogen.

Nitrox is used because it contains less nitrogen than normal air. Therefore the body absorbs less nitrogen. That fact reduces the risk to come down with decompression sickness. Moreover divers use enriched air to extend the no decompression limits beyond normal air no decompression limits.

Deep diving and Nitrox doesn´t suit. Oxygen has its own set of problems, some of which are significantly more hazardous than nitrogen! Diving with enriched air makes oxygen toxicity possible within the depths you´ll be diving. Therefore, you need to know the risks of oxygen toxicity and why it´s important to stay well within oxygen limits.

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 (1) nitrogen narcosis (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'+) and (2) Bends (nitrogen absorbed into tissues and body fluids reverting into gas bubbles).

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.

Helium is a non-toxic, colorless, odorless, tasteless, inert, lightweight and nonexplosive gas. The advantages of helium are to reduce narcosis effects to nil, significantly lowered breathing resistance, does not enter as readily into tissue as nitrogen. Some of the disadvantages are that it conducts heat 5 times faster than air and can cause hyperbaric arthralgia (an arthritic-like stiffness) 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 decompresion time with short dives than air, but less decompression 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?

Helium and diving were made for each other. You don't have to be a super technical 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.