Technical 9 minutes
Trimix: adding helium to get your head and your breathing back
Three gases, three problems solved, and logistics you can't improvise
When air stops working, it's not just one problem: you're narcotized, breathing is hard, and oxygen is approaching its ceiling, all at once. Helium solves two of those three problems in one stroke, and creates a few new ones that need managing.
In this article you'll see why helium is the gas of depth, how you calculate a mix's equivalent narcotic effect, what the difference is between normoxic and hypoxic trimix, what complications it adds (thermal, decompression, and financial) and why using it is inseparable from technical training.
Trimix is the border between recreational and technical diving, and understanding it retrospectively clarifies why air has the limits it has. This article is educational material and explicitly does not qualify you to dive with gas mixes: that requires specific technical training.
Why helium
Helium is a noble gas, metabolically inert, and it has two properties that make it ideal for depth: its narcotic effect is far lower than nitrogen's, and it's much less dense, which drastically reduces the work of breathing.
Those two properties tackle exactly two of the three ceilings of air described in its article. The third, oxygen toxicity, isn't solved with helium but by adjusting the mix's oxygen fraction, which is the other half of the design.
A trimix is named by two numbers: the percentage of oxygen and the percentage of helium, with nitrogen making up the rest. An 18/45 carries 18% oxygen, 45% helium, and 37% nitrogen; a 10/70 carries 10% oxygen, 70% helium, and 20% nitrogen.
The reason for leaving some nitrogen in the mix, instead of eliminating it entirely, is partly economic (helium is expensive) and partly physiological: a proportion of nitrogen helps blunt a phenomenon that appears at extreme depths with mixes very rich in helium, which the heliox article covers.
It's also worth saying what helium does not do: it does not shorten decompression. It's an inert gas that dissolves and is eliminated like any other, with kinetics different from nitrogen's, and decompression plans with trimix are not shorter. It's used for your head and your breathing, not for time.
Keep reading with full access
This article is part of the full Encyclopedia. A donation from USD 10 unlocks every article, every species photo and the downloadable app, and helps sustain the Scuba Sign Paperless Diving Community project.