Trimix vs Heliox: What Is the Difference and Who Uses Each

What is heliox diving, and how is it different from trimix?

The difference is one gas. DAN defines trimix as a mixture of oxygen, nitrogen and helium, and calls a mix of just oxygen and helium heliox. Both use helium for the same reason: it does not cause the narcosis nitrogen does at depth. Why helium works and how a trimix is picked with our trimix calculator is covered in why divers use helium in trimix. This page is about the choice between keeping some nitrogen and removing all of it.

In practice the two gases belong to different kinds of diving. Trimix is a scuba gas. TDI's Trimix Diver course trains divers to use custom oxygen, helium and nitrogen mixes on staged decompression dives to 60 m (200 ft), and asks for 100 logged dives and a minimum age of 18. Heliox is a working gas. The US Navy Diving Manual describes surface-supplied mixed gas diving as helium-oxygen fed from the surface by hose, and its saturation divers live and work in helium-oxygen. The Navy also puts heliox in rebreathers: its electronically controlled closed-circuit sets go to 91 m (300 fsw) on an 88/12 helium-oxygen diluent, against 46 m (150 fsw) on air.

Commercial rules move offshore divers onto helium early. Ireland's Code of Practice for Offshore Diving, based on the UK HSE code for offshore projects, makes 50 m the maximum depth for compressed air or oxygen-nitrogen mixes and calls for closed bell methods deeper than that. The industry's medical committee, DMAC, writes its emergency gas guidance specifically for heliox saturation diving.

Heliox is also a hospital gas, and that use has nothing to do with diving: a Cochrane review of 10 trials with 544 patients concluded that the evidence does not support routine use of heliox in acute asthma.

Trimix vs heliox compared

Most of the differences follow from how much helium each gas carries. Removing the last of the nitrogen helps narcosis and breathing effort, and makes the cold, the voice and the bill worse.

Why technical divers keep some nitrogen in the mix

Mostly because helium is the costly gas and the dive does not need all the nitrogen gone. DAN lists the cost and difficulty of getting fills among helium's drawbacks, and a 2024 Shearwater article expects deep open circuit dives to stick with trimix because of the cost of helium. The narcosis target is modest as well. TDI sets an END of 30 m for its trimix course, and BSAC's recommended open circuit mixes keep the END at or shallower than 30 m while leaving nitrogen in the cylinder: its 20/50 for 60 m is 30% nitrogen, and its 15/65 for 80 m is 20%. BSAC now plans those mixes by oxygen partial pressure and gas density rather than by END.

High pressure nervous syndrome (HPNS) is the second reason, and it only applies far deeper than most technical dives. The US Navy manual lists adding nitrogen or hydrogen to the breathing mixture among the attempts to block HPNS, and says no method appears to be entirely satisfactory. A footnote in the Shearwater article places that use of nitrogen generally beyond 150 m.

Decompression was the third reason, and the evidence has moved. Decompression models gave helium a penalty, so trimix was widely believed to decompress faster than heliox on bounce dives. The US Navy Experimental Diving Unit tested it with 32 volunteers. Divers on MK 16 rebreathers spent 40 minutes at 61 m (200 fsw) on either trimix 12/44 or heliox 12/88, then decompressed for 119 minutes on the MK 16 trimix schedule, which is 15 minutes shorter than the matching heliox one. If the models were right, the heliox divers should have had more decompression sickness. Instead, 50 heliox dives produced none and 46 trimix dives produced two, and the researchers concluded that trimix decompression is not more efficient than heliox.

That does not make your computer's helium settings optional. Simon Mitchell, quoted by Shearwater, points out that the study does not answer whether divers can tell their computers they are on an air diluent when they are really breathing trimix. How stops, gas switches and gradient factors fit together on a real profile is in how to plan a decompression dive.

Why commercial and saturation divers use heliox

Commercial and Navy divers do heavy work at depth for long periods, so narcosis and breathing resistance outweigh the gas bill. The Navy manual says surface-supplied mixed gas is useful even in the air range when freedom from nitrogen narcosis is required, and that helium-oxygen should be used deeper than 46 m (150 fsw) to reduce breathing resistance. Its normal limit for surface-supplied heliox is 91 m (300 fsw) for 30 minutes, with a maximum working limit of 116 m (380 fsw). Longer and deeper jobs go to saturation, where Navy systems support divers to 305 m (1,000 fsw) for extended periods.

A saturation diver breathes mostly helium with very little oxygen. The chamber is held at an oxygen partial pressure of 0.44-0.48 ata, and the manual's own example needs only 4.36-12.39% oxygen in helium to cover 0.44-1.25 ata at 91 m (300 fsw). There can be a little nitrogen too: one of the two methods for raising chamber oxygen is a first compression on air to 11 m (36 fsw), and the manual notes that nitrogen falls over time as helium purges it. DMAC caps the bail-out gas for heliox saturation divers at 1.4 bar of oxygen, with a floor of 0.4 bar.

Decompression is where heliox saturation looks least like a technical dive. The Navy decompresses a saturation chamber at 6 fsw (about 1.8 m) an hour in the deep range, slowing to 3 fsw (about 0.9 m) an hour over the last 15 m (50 fsw), and only travels 16 hours in every 24. Surface-supplied heliox has its own rules: no repetitive dives except as the manual allows, and a 12-hour wait after a no-decompression dive or 18 hours after a decompression dive.

Worked example: trimix and heliox at 60 m

Take a 60 m (200 ft) dive planned at a bottom oxygen partial pressure of 1.3 bar. The oxygen depends on depth alone, so it is the same for both gases. Absolute pressure at 60 m is 7.0 bar, 1.3 divided by 7.0 is 0.186, and our calculator rounds down to 18%. That gives 1.26 bar of oxygen at 60 m whatever fills the rest of the cylinder. The familiar 21/35 trimix would give 1.47 bar at the same depth, above the 1.4 bar limit the offshore code recommends.

The equivalent narcotic depth formula is END = (narcotic fraction × absolute pressure - 1) × 10, in metres. The narcotic fraction depends on the convention. If oxygen counts as narcotic, it is 1 minus the helium fraction. If only nitrogen counts, it is the nitrogen fraction divided by 0.79, the nitrogen share of air.

Heliox 18/82 has no nitrogen. Counting nitrogen only, the narcotic fraction is 0, the result goes below zero and the END is the surface. Counting oxygen as narcotic, the fraction is the 0.18 of oxygen: 0.18 × 7.0 is 1.26, and the END is 2.6 m. So heliox removes nitrogen narcosis, but on the conservative convention its END is not quite zero, because the oxygen is still there.

The trimix for the same dive, from the calculator's best mix with a 30 m END target and oxygen counted as narcotic, is Tx 18/43. Its END is 29.9 m on that convention and 24.6 m counting nitrogen only, with 39% nitrogen still in the cylinder. You can repeat this in the trimix calculator: enter oxygen and helium that add up to 100% and it shows the heliox as Tx 18/82, a trimix with no nitrogen. The calculator is a planning aid; both gases need technical or commercial training and an analysed cylinder.

Helium voice and heat loss

Both come from helium, so trimix divers get them in smaller doses and saturation divers get them in full. The Navy manual traces the high-pitched "Donald Duck" voice to the acoustic properties and low density of helium. Its saturation system carries four helium speech unscramblers to turn divers' voices back into something the surface can follow, and the Irish offshore code requires a speech processing system for any diver breathing a gas that contains helium.

Cold is the bigger problem. Helium conducts heat away from the body and the lungs quickly. The Navy manual puts respiratory heat loss on heliox at 10% of the body's heat production at 1 ata, 28% at 7 ata and 50% at 21 ata, and says standard insulation is then not enough. Navy saturation divers wear hot water suits and breathe heated gas, and the offshore code requires active heating for divers on oxygen-helium, with heated breathing gas on dives deeper than 150 m. On a trimix scuba dive the effect is smaller but real: DAN lists the increase in heat loss as one of helium's drawbacks.