Why Divers Use Helium in Trimix: END and Choosing a Mix

What narcosis costs a diver at depth

Beyond 30 m (100 ft) on air, impairment is not a possibility, it is the expected state. Clark's 2015 review puts it plainly: breathing compressed air at pressures above 4 ata, which is 30 m, with a nitrogen partial pressure of about 3.5 ata, will invariably result in nitrogen narcosis. Unrehearsed tasks such as sorting cards are already measurably impaired as shallow as 10-20 m. That is why most international sport diving qualifications stop at around 30 m, and why the arithmetic in our trimix calculator exists at all.

The accident data is harder to read but points the same way. The Australian diving fatality database estimates narcosis contributed to roughly 9% of reported deaths, and Clark notes DAN attributed 3.6% of UK deaths to inert gas narcosis in 2010. Depth alone, without direct evidence of narcosis, was involved in 54.3% of advanced open water training fatalities worldwide that year.

Helium is the fix because of what it does not do. In Clark's table of narcotic strength, helium sits at 0.2 against nitrogen's 1, with a fat solubility of 0.015 mg/ml against nitrogen's 0.067. The review describes narcosis as preventable by staying shallower than 30 m or by reducing the nitrogen partial pressure, by replacing some nitrogen with helium, which has no narcotic effect. For the symptoms themselves, see nitrogen narcosis symptoms and depth.

Equivalent narcotic depth, and the two ways it is calculated

END is the depth on air that would feel as narcotic as your mix. The catch is that planners disagree about oxygen. One convention counts oxygen as narcotic as nitrogen, so the narcotic share of the gas is everything except helium. The other counts only nitrogen and compares it with the 0.79 in air. Both are in use and they give different mixes for the same dive.

The gap grows as helium goes up. Tx 23/34 at 50 m has an END of 29.6 m if oxygen counts and 22.7 m if it does not. That sounds academic until you blend to the leaner answer. A diver who works to an END of 30 m under the oxygen-not-narcotic model ends up with Tx 23/25 for that 50 m dive, and the same gas measured the conservative way has an END of 35 m. At 70 m the same mistake is worse: Tx 17/44 reads 29.5 m under the model it was blended for and 34.8 m under the other one.

So the choice of model is not a rounding preference. It decides how much helium you buy and how narcotic the dive really is. Counting oxygen as narcotic is the conservative side, and it is the side to take if your team, your course and your planning software do not all agree.

Why the oxygen fraction shrinks with depth, and when the mix stops being breathable

Helium has nothing to do with the oxygen side of the plan. The oxygen fraction is the ppO2 limit divided by the absolute pressure at your deepest point, and the MOD follows from the oxygen alone, exactly as it does with nitrox. CMAS caps the bottom phase at 1.4 bar and allows 1.6 bar on stage decompression, the same pair of limits used in our nitrox MOD and best mix examples.

Work that limit down a depth profile and the oxygen gets thin. At 60 m the best mix at 1.4 bar is 20%. At 67 m it is still 18%. At 68 m it drops to 17%, and at that point the gas can no longer be breathed at the surface. Our calculator uses 0.18 bar as the lowest breathable oxygen partial pressure, which is also the factory default for the low ppO2 warning in Shearwater's Teric: for open circuit gases the display flashes red when the active gas falls below 0.18 bar.

The sources do not fully agree on where that line sits. DAN calls a mix below 16% oxygen hypoxic and warns it can cause immediate unconsciousness without warning if breathed at the surface or shallow. CMAS defines normoxic as oxygen content that will support life at the surface, usually 21% but never lower than 18%, and sub-normoxic as anything under 18% that must not be used at the surface. The site takes the stricter number. Shearwater's manual makes the same point about a 10/50 diluent: hypoxic mixes require special training since they can be deadly near the surface.

A hypoxic bottom mix therefore arrives with a second cylinder attached to the plan. DAN describes the travel gas as a gas with a higher oxygen content used to reach a depth where the hypoxic mix can be switched in, and TDI's Advanced Trimix equipment list requires a travel mix cylinder sized for the planned dive. The 0.18 bar floor is the physical minimum, not the switch depth you would choose: Tx 15/56 reaches 0.18 bar at 2.0 m but only gives 0.45 bar of oxygen at 20 m.

Normoxic or hypoxic trimix: which depths use which

Normoxic trimix keeps enough oxygen to breathe at the surface, so one cylinder covers the whole dive. That is the gas class for the 40-60 m band. TDI's Trimix Diver standard runs to 60 m (200 ft) and says gas mixes are not to have any less than 18 percent oxygen. CMAS sets the same 60 m ceiling for its Normoxic Trimix Diver, with 1.4 bar at the MOD and 1.6 bar on stops.

Below that, the oxygen limit forces the mix under 18% and the dive changes shape. TDI's Advanced Trimix Diver runs to 100 m (330 ft) and the course requires a travel mix cylinder; CMAS runs its advanced trimix training dives to 80 m. Both agencies also cap narcosis, and this is where they part company. TDI writes a maximum END of 30 metres/100 feet on all trimix dives. CMAS allows a maximum END of 40 m. That 10 m of disagreement is the single biggest lever on how much helium a dive needs. The course ladder itself, with prerequisites and costs, is in how to get into technical diving.

Worked examples: best mix for 50 m and 70 m

Two dives, one ppO2 limit of 1.4 bar, and the two END targets the standards disagree about. Take the 50 m dive first. The absolute pressure is 6.0 bar, so the oxygen is 1.4 divided by 6.0, which is 0.2333, rounded down to 23%. Counting oxygen as narcotic, an END of 30 m needs the narcotic share cut to the pressure ratio 4.0 over 6.0, leaving 33.3% helium, rounded up to 34%. That is Tx 23/34, with an END of 29.6 m, just inside the target. Accept an END of 40 m instead and the helium falls to 17%.

The 70 m dive is the interesting one. At 8.0 bar the oxygen is 1.4 divided by 8.0, which is 0.175, rounded down to 17%. The mix is now hypoxic: it reaches 0.18 bar only at 0.6 m, so it cannot be breathed on the surface even though the number looks close. An END of 30 m needs 50% helium under the conservative model, giving Tx 17/50. Plan the travel gas around the switch, not around the 0.6 m figure. EAN32 works to 33.7 m at 1.4 bar, and at 30 m the bottom mix is already delivering 0.68 bar of oxygen, so a switch in that region has margin on both sides.

Every number in the table comes from the trimix calculator, which rounds oxygen down and both helium and END up. The mix it suggests is never leaner in oxygen or richer in narcotic gas than the limits you typed.

What helium costs, and why trimix fills are expensive

Helium is priced by the litre, and a deep mix wants thousands of litres of it. Two named UK shops show the shape of the bill. The ScubaDiving Gear Store in Manchester prices trimix as oxygen litres at 3p, helium litres at 8p and a £5.50 standing charge. Marine Quest in Eyemouth lists helium at 6.5p per litre and oxygen at 3.5p, with an air twins fill at £14 on 232 bar. Prices differ by country and by shop, so treat these as the arithmetic rather than the going rate.

Apply the Manchester rates to a twin 12 L set filled from empty to 200 bar, using the partial pressure blend our calculator gives. The helium for Tx 17/50 alone is 100 bar in the tanks, which is 2,400 L, or £192. The same shop's pre-banked EAN32 twins fill is £22.50.

That is the real reason END targets get argued about on the dock. Accepting a 40 m END instead of 30 m on the 70 m dive drops the helium from 100 bar to 76 bar, about £50 off the fill, and on the 50 m dive it roughly halves the bill. DAN lists the same two drawbacks in one sentence: the cost and difficulty of getting fills, plus an increase in heat loss.

The trade-offs helium brings with it

Cutting narcosis is one benefit bought with several costs. None of them is a reason to avoid helium on a dive that needs it, but each is a planning item.

• Decompression modelling. Shearwater's Teric runs Bühlmann ZHL-16C with gradient factors and tracks helium alongside nitrogen: each of the 16 tissue bars is the combined sum of the nitrogen and helium tensions. DAN notes that the duration of decompression associated with breathing helium has to be considered when choosing a mix.
• Gradient factor settings. The same computer ships with different defaults per mode: 40/85 in open circuit recreational mode, a more conservative 30/70 in the technical and rebreather modes. Dive computer algorithms explained covers what those numbers do.
• Isobaric counterdiffusion. DAN describes ICD as one inert gas entering tissues while another leaves, which can make bubbles form or grow with no change in depth. Shearwater's manual is explicit that its planner does not validate for this: it does not check isobaric counter-diffusion risks due to sudden helium switches, and the user is responsible for a safe profile.
• High pressure nervous syndrome. Clark describes HPNS as neurological symptoms at very high pressures, above 100 msw, with headache and tremor among the indications, and considers it a separate mechanism from narcosis. Helium is not a free pass to any depth.
• Heat. DAN lists an increase in heat loss as a drawback of helium fills, so thermal protection belongs in the gas decision rather than after it.
• Breathing effort. Shearwater's computers show a gas density figure that turns yellow at 6.3 grams per litre on open circuit, because a dense gas raises the work of breathing. Helium is the lightest of the three gases in the cylinder, so it works in your favour here.

When helium is not worth it for a recreational diver

If your dives stop at the recreational limit, the calculator will often tell you to skip the helium. Ask it for a 40 m dive at 1.4 bar with an END of 40 m and it returns EAN28 with no helium at all, because the target is already met by the depth itself. The helium only starts earning its price when you want the dive to feel shallower than it is.

Where that begins is a judgement, not a threshold. DAN says it has become standard practice to use helium deeper than about 150-165 feet (46-50 m), and notes that some divers prefer helium on dives as shallow as 80 feet (24 m). Neither number is a rule. What is firm is the training and the equipment: a recreational computer may not accept the gas at all, since Shearwater's open circuit recreational mode is nitrox only with no helium, and the TDI trimix courses ask for 100 logged dives and a minimum age of 18.

Trimix diving sits outside recreational limits and needs technical training and certification. The calculators here are planning aids, not a substitute for a course, a team briefing or the computer on your wrist, and every cylinder has to be analysed before it is breathed.