Trimix

When diving on compressed air, the partial pressure of nitrogen increases with depth, causing nitrogen narcosis—a state of mental impairment similar to alcohol intoxication. By 40 metres (130 feet), narcosis affects reaction times and situational awareness, and by 60 metres (200 feet), it poses extreme risk. Helium is an inert, non-narcotic gas under standard technical diving pressures. Replacing a designated percentage of nitrogen with helium lowers the overall narcotic charge of the gas mix.

Oxygen toxicity is the second limiting factor in deep diving. High oxygen partial pressures (above 1.4 bar during the working phase of a dive) can trigger CNS oxygen toxicity, leading to underwater convulsions without warning. On air, a PO2 of 1.4 bar is reached at 56 metres (184 feet). Trimix solves this by trimming oxygen levels down. For instance, a Trimix 14/55 mix contains 14% oxygen and 55% helium, keeping the PO2 at a safe 1.12 bar at a depth of 70 metres (230 feet).

Helium also possesses lower gas density than nitrogen. At depth, dense compressed air increases the physical work of breathing, which can cause hypercapnia (carbon dioxide buildup). The light molecular structure of helium makes breathing easier at high ambient pressures, protecting the diver's respiratory system and reducing exhaustion during strenuous bottom work.

Executing Trimix dives from a liveaboard requires careful planning beyond standard recreational trips. Because helium is expensive and limited in availability in remote areas, liveaboards demand pre-booking gas quantities weeks or months before departure. Blending is typically performed via partial-pressure filling or continuous-flow systems, requiring high-pressure gas boosters to extract every bar of gas from bank cylinders.

Trimix diving alters your deck routine and gear configuration. Open-circuit technical divers routinely carry twinsets or sidemount cylinders filled with Trimix bottom gas, along with one or two stage cylinders containing decompression gases such as Nitrox 50 (EAN50) and 100% oxygen. Closed-circuit rebreather (CCR) divers carry a small Trimix diluent cylinder alongside oxygen and scrubber canisters, significantly reducing total helium consumption.

Decompression obligations are mandatory on Trimix itineraries. Because helium dissolves into body tissues rapidly during the bottom phase, divers follow strict ascent profiles generated by decompression algorithms (such as Bühlmann ZHL-16C with Gradient Factors). Dive plans mandate exact bottom times, gas switches at specified depths, and surface-marker buoy (SMB) deployments for open-ocean drift decompression.

Myth: Trimix is only for extreme deep exploration below 100 metres. Fact: Trimix is routinely used at moderate technical depths between 40 and 60 metres (130 to 200 feet) to eliminate narcosis and provide clear-headed problem solving on deep wrecks and walls.

Myth: Breathing helium shortens your required decompression stops. Fact: Helium enters and leaves fast tissue compartments rapidly, but it does not eliminate decompression obligations. Decompression obligations remain substantial and must be managed using hyperoxic deco gases during ascent.

Myth: Any liveaboard offering Nitrox can easily supply Trimix blending. Fact: Helium requires specialized high-pressure gas boosters, partial-pressure blending equipment, digital helium analyzers, and tec-certified boat staff that standard recreational liveaboards do not carry.

Myth: Helium causes high-pressure nervous syndrome on standard tec dives. Fact: High-Pressure Nervous Syndrome (HPNS) is a neurological condition that only manifests at extreme depths, generally exceeding 130 to 150 metres (425 to 500 feet), far beyond standard open-circuit or rebreather technical profiles.