The Use of Trimix in Deep Sea Diving

People who are inexperienced with scuba diving tend to assume that such divers carry oxygen in the cylinder on their back to breathe it through a regulator when diving underwater. Nothing can be further from the truth as scuba divers never use pure oxygen.

The reason for not using oxygen is that under the increasing pressure as you dive deeper, oxygen gives rise to "oxygen toxicity" that harms the cell lining of the lungs and the central nervous system.

In fact, what the recreational divers use is ordinary air that is a mixture of 79% nitrogen and 21% oxygen. But under the pressure of water at increased depth, the high level of nitrogen in air creates another problem which is called decompression sickness or "bends" in divers' terminology. Further, elevated nitrogen levels can cause nitrogen narcosis.

The deep sea divers who dive to much greater depths than the recreational scuba divers use a gas mixture called Nitrox that contains 32% or 36% of oxygen, the rest (68% or 64%) being nitrogen. The lower content of nitrogen reduces the risk of nitrogen sickness but now the increased oxygen introduces the problems with pure oxygen, though to a much lesser degree. Even so, the effect of increased percentage of oxygen starts having its effect as the diver goes deeper, so there is a limit to the depth to which the diver can descend when Nitrox is used.

To dive deeper, oxygen needs to be reduced to avoid oxygen toxicity but then the increased nitrogen brings on narcosis and decompression sickness. Question naturally arises as to how these two conflicting issues can be resolved. The answer is the use of another gas mixture known as Trimix. To reduce both the oxygen and nitrogen percentages, helium is used to replace some nitrogen. Trimix can have varying composition and is designated by the percentage of oxygen and the percentage of helium in the mixture, the rest being nitrogen. For example, 10/70 Trimix has 10% oxygen, 70% helium and the balance 20% nitrogen.

The advantage of helium is that it does not cause narcosis and when under pressure, it does not enter the tissues to the same extent as nitrogen so that "bends' are not as severe. Because of its low density, it reduces breathing resistance, but higher vibrations of voice chords can produce a squeaking effect at depth.

Helium conducts heat five to six times faster than nitrogen and so becomes much colder with the drop in water temperature as one goes deeper. This is one disadvantage with helium, the other being hyperbaric arthralgia, an arthritic-like stiffness suffered by divers during descent.

You can see that deep sea diving is considerably different from recreational diving and requires extensive study to avoid the hazards associated with it. Even the matter of breathing requires a lot of attention and divers have to follow the charts for the types of gases to be used corresponding to the depths to which they want to descend.