Trimix is a mixture of helium, oxygen and nitrogen. Divers use Trimix for deep diving to depths where the effect of nitrogen narcosis is no longer acceptable and normal oxygen concentrations are toxic.
Air is mostly nitrogen (78%) and oxygen (21%). Humans don’t use any of the nitrogen we breathe; biologically speaking, it’s an inert gas. We need oxygen to live, but at high pressures, it causes seizures and unconsciousness.
As a diver goes deeper, the surrounding water pressure increases. The pressure in the diver’s lungs also increases. At the higher pressure, nitrogen starts to affect the diver’s nervous system, producing a narcotic effect similar to alcohol. Divers call this nitrogen narcosis.
Nitrogen narcosis slows the reaction times of a diver breathing air, even as shallows as thirty metres, and can cause unclear thinking, hysteria and even paranoia. Even greater depths multiply the effects until mistakes and problems quickly become serious.
Replacing some of the nitrogen with helium reduces the narcosis, allowing the diver to think clearly, prevent issues and react normally to problems.
Fifty-five metres is considered the maximum safe depth a diver should breathe gas with 21% oxygen (eg air). To go deeper, helium replaces some of the oxygen, reducing the risk of the oxygen causing a seizure.
With all these advantages, it would make sense to use Trimix for all dives, except while it solves some problems it creates others.
Helium sucks up heat energy like cold feet in a warm bed. A diver breathing Trimix feels colder than a diver breathing air. Deep waters are colder than shallow, so divers breathing Trimix need more insulation.
Helium is expensive. Breathing Trimix can cost ten or twenty times more than air, so divers tend to limit its use to very deep dives, or dives where avoiding narcosis is essential.
For technical divers with a need, who can afford its use, Trimix is a great tool.