Normoxic Trimix Diving: Diving Deep With a Clearer Head

Nearly every deep diver has been there: you surface from an amazing dive, but the memories are a bit foggy. Divers who are trained in stage decompression diving on air will start to notice the adverse effects of nitrogen narcosis on deeper dives at some point during their diving careers. Normoxic trimix is a great solution for divers who experience nitrogen narcosis and want to dive with clearer heads. The normoxic trimix course introduces divers to the advantages of helium while staying within the same limitations as air. The course is can be a goal on its own, or a step on the way to deeper trimix training. And who knows, the course could lead you to pursue even deeper diving and the attractions to be found beyond the limits of air.

What Is Normoxic Trimix?

Normoxic trimix refers to a breathing gas made of any combination of oxygen, helium and nitrogen, in which the oxygen content is the same as it is in air (20.9%). In practice, oxygen percentages between 18% and 21% are commonly accepted as normoxic trimix. When referring to a normoxic trimix blend, the oxygen percentage is stated first, followed by the helium percentage. The rest of the blend is assumed to be nitrogen. Thus, one of the most common blends of normoxic trimix, 21/35, is 21% oxygen and 35% helium.

When Should a Diver Consider Using Normoxic Trimix?

Normoxic trimix does not allow a diver to go deeper than the limits of air. Because the percentage of oxygen in normoxic trimix is the same as the percentage of oxygen in air, the depth limit of 186 feet (PO2 of 1.4) still applies. The percentage of oxygen is a limiting factor because of the risk of oxygen toxicity on deep dives.

If normoxic trimix doesn't allow a diver to go deeper, what is the benefit of using it? The addition of helium reduces the percentage of nitrogen in the diver's breathing gas, which reduces narcosis and gives a diver a clearer head on deep dives. In addition, the gas a diver breathes is compressed by the surrounding water according to Boyle's Law. On deep dives, this compression “thickens” the breathing gas and increases the work of breathing. Helium is a less dense gas than both oxygen and nitrogen, and substituting it for some of the nitrogen in the diver's breathing gas reduces the overall density the gas and reduces the work of breathing on deep dives.

The main benefit of using normoxic trimix instead of air on deep dives is a clearer head, better awareness, and reduced work of breathing.

Does Diving Normoxic Trimix Reduce Decompression Times?

Diving normoxic trimix does not result in extended no-decompression limits, nor does it reduce decompression times. Although some of the nitrogen is removed from the breathing gas, the helium substituted in place of the nitrogen must still off-gas. Decompression schedules when diving with normoxic trimix are similar to the decompression schedules used when diving air.