What is trimix and why do divers use it?

In the early 1990s, deep sport divers were let into a secret that enabled them to carry out deep dives with a clear head, reduced risk and a light, easily breathable gas in their lungs.

That secret was trimix, a mixture of three gases, oxygen, nitrogen, and helium. Helium is a very light, non-toxic, minimally narcotic gas. Adding it to their breathing gas enables divers to reduce the percentage of oxygen and thus reduce the risk of oxygen toxicity that is always present when you dive below 60m or so using air. Trimix is easier to breathe, as it is less dense than air. It is also much less narcotic.

There are two disadvantages to using helium. First, it is expensive. Second, at high partial pressures, it has unwanted effects on the diver’s central nervous system.

Commercial diving and military operations have known for a long time that using helium has huge physiological advantages for a diver working at depth, although, rather than trimix, the breathing gas of choice for professional deep divers is a mixture of oxygen and helium only, known as heliox.

They use no nitrogen at all and very high percentages of helium because their primary concern is to minimize narcosis in the working diver and the consideration of cost is secondary to getting the job done. Also, as they are in constant communication, divers and their surface supervisor can work together to manage any nervous system problems.

Sport divers are not so fortunate. We do not have the luxury of vast funds nor do we dive with a surface supervisor. But, then again, when we dive, we are usually just sightseeing and are not engaged in work that requires intense concentration. So deep sport divers tend to add only as much helium as they need to keep the amount of oxygen in their breathing mix within safe limits and maintain a manageable level of narcosis. It is also thought that a little nitrogen in the mix tends to offset nervous system issues.

Successful trimix diving involves far more than just changing the gas in the cylinders and plunging in. While the rewards of going to depths hitherto beyond the range of sport divers are substantial, this is not a level of diving to be taken lightly. You need to be prepared for longer decompression times, carefully controlled ascents and total dependence on decompression gases. Good training, self-discipline, attention to detail and a great deal of in-water experience are essential to success in trimix diving.

I tell the full story of how trimix was introduced to sport divers in my book Technically Speaking: Talks on Technical Diving Volume 1: Genesis and Exodus.