My reaction after returning from my first trimix dive in the early
1990s was, This is just what we need. In the past, Id
made working dives to 220 feet on air and had been fairly narced to
say the least. On this run, Id been clear-headed at 250 feet.
I knew that from then on, Id never dive to such extreme depths
During the 1990s, the tek diving community embraced trimix (oxygen,
helium and nitrogen) as the accepted gas for diving deeper than 200
feet, and diving on air to 240 feet went from macho to moronic. Unquestionably,
trimix has made deep tek diving safer. Today, the tek diving community
agrees almost universally on the following standards:
For general open water and overhead environments to 130 feet:
Air or enriched air nitrox (EANx) is acceptable. Qualified tek divers
switch to trimix as shallow as 100 feet for complex dives or dives in
For overhead/complex environments deeper than 130 feet: Air/EANx is not acceptable; these environments call for trimix to reduce narcosis.
For general open water environments deeper than 165 to 200 feet: The exact cutoff point varies from one group to the next, but below this range, air/EANx is not acceptable at all; the community practice is to use trimix at these depths.
You might notice a gap in the prevailing tek community practicethe gap for open water diving between 130 feet and 165 to 200 feet. Air or trimix? Most of the tek community considers air/EANx acceptable to at least 165 feet in open water for noncomplex dives, but a minority adamantly insists that all dives deeper than 130 feet (some even say 100 feet) require trimix. Since few would argue against the benefits of reducing narcosis, one might wonder why the whole community doesnt agree. The reason is that trimix, while a wonderful tool, isnt a cure-all. Just as air has drawbacks and risks, so does trimix.
One immediate argument is that trimix costs more. While trimix has some concerns, this isnt one of them. Cost is a factor, but not a good argument against using trimix. There are
real, practical reasons unrelated to cost.
The primary concern about trimix is that helium is a light gas that diffuses rapidly. It is far less forgiving of decompression procedure errors than nitrogen. More than a few tek divers can attest to heliums bending potential because theyve had a DCS occur before they hit the surface. This rarely happens when decompressing from an air /EANx run. Because helium diffuses rapidly, it loads your tissues more rapidly during the dive and more readily unloads during ascent and decompression. Using trimix, your ascent rate becomes more critical and so does maintaining stop depth because a rapidly diffusing gas more easily forms bubbles if you screw up. A diver who accidentally bounces 20 feet above stop depth and quickly returns is more likely to get away with it decompressing from an air/EANx dive than a trimix dive. Some may argue this shouldnt happen, but it does with new and experienced tek divers alike. And its in the 130 to 165-180 range that novice tekkers gain the deco diving training and experience that trimix demands.
At times, even highly experienced trimix qualified tekkers will opt
for air (or EANx) in the 130 to 165-180 window. If conditions are rough
enough to cause an erratic decompression, a more forgiving nitrogen-oxygen
mix may reduce DCS risks. In training tek divers, many instructors prefer
air/EANx so they can assist a student with depth control problems and
have less bubble trouble concerns. So while trimix reduces the narcosis
risk, it increases the DCS riskin some circumstances, substantially.
But what about the narcosis?
The always-trimix argument revolves around narcosis, but theres another side. First, the added narcosis (to no deeper than 165 feet) is not substantially more than 130 feet for most divers in many environments, especially those suited to new tekkers and training them. Its only one atmosphere more, and the European dive community has used 165 feet as the recreational limit (not 130 feet) for years without the safety equipment and higher-level training the tek community now calls for.
All-trimix proponents say narcosis creates a false sense of security, and it can in the untrained, unwary diver. However, trimix also creates a false sense of security for those who believe it will eliminate human error or who dismiss the added decompression risks. And, trimix reduces, not eliminates, narcosis.
Never Say Never
As mentioned, virtually the entire tek community agrees that overhead
environments (cave or wreck) deeper than 130 feet always call for trimix. And, complex environments magnify narcosis concerns. While its reasonable for most tek divers to use air/EANx to 165 feet in calm, open tropical water, a cold, high-current, low-viz dive to 140 feet in the North Atlantic may not only call for trimix, but not be a suitable dive at all for the novice tekker.
So, while most tek divers no longer use air/EANx for deep penetration or dives below 165-180 feet, they recognize an overlapping range where one might use air/EANx or trimix, depending on the diver, the dive demands and the risk/benefits. The wise diver uses the right tool for the job. An extreme position either for or against trimix or air or EANx not only disregards the relative risks/ benefits of using each in varying situations, but ultimately undermines tek diving safety by denying divers the appropriate tool for the task.
Karl Shreeves is VP, Technical Development, for DSAT and PADI. He made his first helium dive in a chamber in 1978, and now tek dives with trimix, enriched air or air, depending on what the dive calls for.