Helium and Scuba Diving -Trimix and other diving gases.
What are trimix and heliox?
Trimix means a mix of three components (“tri” and “mix”), and usually when people talk about trimix, they mean the mix of oxygen, nitrogen and helium. Trimix is used in very deep dives instead of air to reduce the partial pressure of oxygen (to avoid oxygen toxicity) and nitrogen (to avoid nitrogen narcosis).
Heliox is a mix of helium and oxygen (“heli” and “ox”).
The percentages of gas components vary depending on the dive. The deeper one go, the less there will be oxygen and nitrogen, and the more there will be helium.
What are heliox, heliair or trimix?
Heliox is a mixture of helium and oxygen (HELIum-OXygen). Heliox is used on deeper dives than it’s possible to dive with regular air.
Trimix is also being used as travel gas or deco gas, depending on the situation.
The percentages of gas components vary depending on the dive. The deeper one go, the less there will be oxygen and nitrogen, and the more there will be helium.
Trimix mixes are labeled for example as “Trimix 10 50” or “Trimix 10/50”, where 10 represents the percentage of oxygen in the mix, and 50 is the percentage of helium.
Terms for different trimix gases and labels:
- Hyperoxic trimix: Oxygen content is more than 21%.
- Normoxic trimix: Oxygen content is 21%, same as air has.
- Hypoxic trimix: Oxygen content is less than 21%.
- Heliair is a mixture of helium and air (HELIum-AIR). Heliair contains always less oxygen than air (in percentage, so heliair has less than 21% oxygen).
- Triox is an other name for hyperoxic trimix.
Question: Why is helium used in diving tanks?
Air, which was first used in diving tanks, contains roughly 20% oxygen, and 80% nitrogen. The trouble with nitrogen in this situation is that nitrogen is a fairly heavy gas, and is soluble in blood at high pressure. Long term use of nitrogen can cause a strange sense of euphoria, or well being called nitrogen narcosis. This is a bit like being drunk, and makes the diver unable to assess dangers. Just as working drunk on the surface is not a good idea; working drunk at the bottom of the sea can be extremely hazardous.
Divers working at depth would have nitrogen dissolved into their blood, and it becomes necessary to decompress (surface) very slowly, to allow the nitrogen to come back out of the blood to be breathed out. Divers who come up too quickly have a huge risk of the nitrogen being released from the blood too quickly and forming bubbles in the bloodstream. This causes blockages, as the bubbles, although tiny, cannot pass through the fine capillaries. The condition is called the Bends, is very painful, and life threatening.
One way to avoid all these problems is to avoid the nitrogen. Divers who work at depth or for long periods use a mixture of 20% Oxygen and 80% Helium. Some recreational divers use a slightly cheaper mix of Helium, Oxygen and air. It still has nitrogen, but less than air. Helium is used for a number of reasons – It is light, cheap, and does not dissolve in blood the same way that nitrogen does. Being inert it cannot be toxic to the diver or corrosive to equipment.
“Air” tanks are used by divers at various depths of water. This higher pressure dissolves more “air” the deeper the diver goes. If the diver ascends too quickly, the equilibrium of the dissolved “air” in the lungs does not have sufficient time to equilibrate with the pressure at the decreasing depths. Like opening a carbonated beverage, if this happens in the diver’s blood, a painful hang-up of gases occurs, causing bubbles to form in the blood stream. The name of this event is called “the bends” (you can Google this for details). Of the atmospheric gases, nitrogen is the least soluble but highest concentration (about 80% by volume), so this is the gas that tends to form “bubbles” first.
In diving tanks, pure oxygen is mixed with helium rather than with nitrogen (which is removed by purification).
Helium being almost totally insoluble in water (blood) does not tend to form these bubbles. Dissolved and gaseous oxygen is rapidly exchanged so it does not present such a problem. In very deep dives the ratio of oxygen to helium is optimized to even minimize this problem even with oxygen.
For normal, no decompression dives, helium is not used in scuba tanks, just normal air( 21% Oxygen and 79% Nitrogen).
Technical divers who go beyond recreational depths (30m – 40m) employ the use of trimix. Trimix is a mixture of oxygen nitrogen and helium. Under pressure nitrogen and oxygen will build up in the body which is not a problem at normal recreational limits. Beyond 100 feet nitrogen will start to cause a condition called nitrogen narcosis that causes decreased motor skills and a euphoric state. Beyond 190 and 220 feet oxygen will become toxic, resulting in sensory distortions and seizures. Helium is used to dilute the oxygen and nitrogen to reduce these affects. Helium is the gas of choice to use because it is an inert gas, is thinner, therefore more compressible than air, and its narcotic properties are negligible in comparison to nitrogen. In a nutshell it allows a diver to go deeper for longer periods of time.
Answer from Matthew Faulkner, dive instruction and PhD student in Immunology the Durdik lab
Jeannine M. Durdik
Professor and Vice Chair of Biological Sciences
University of Arkansas
As a diver goes deeper, the pressure increases and gasses breathed are forced into body tissues (that or the diver gets squashed).
Helium issued to replace the Nitrogen found in normal air because at high-pressures enough Nitrogen is forced into the body tissues to have or cause a narcotic effect. Nitrogen narcosis or “raptures of the deep” affects the ability to think clearly and that can lead to serious problems and even death. Stories abound of divers offering their regulators (the breathing mouthpiece) to passing fish or similar inappropriate behaviours. There are occasions where, using plain compressed air ( 21% Oxygen and 79% Nitrogen), people have dived deep and died because of the poor judgments they make under the effect of Nitrogen narcosis.
Helium is a Noble gas and is non-toxic at all pressures.
Also, Helium is much lighter than Nitrogen and bleeds out from the tissues that much faster and its use reduces decompression times. Helium there is less chance of getting the bends (originally called caissons disease).
Helium and Scuba Diving Misfacts:
God gave us helium for diving, but the devil replaced it with nitrogen. At least he tried replacing it and giving it a bad name.
Helium is a noble gas for deep diving, but was not always thought so. In the early days of technical and recreational diving, the use of helium for deep diving was discouraged, indeed, really feared. Based on misinformation and a few early problems in the deep diving arena, helium acquired a voodo gas reputation, with a hands off label.
Some misapprenhension stemmed from the Hans Keller trajedy on helium mixes in 1962, some from misconceptions about isobaric switches ala light-to-heavy gases, some from tales of greater CNS risk, and some from a paucity of published and reliable decompression tables. Some concerns arose because 80/20 heliox no-deco time limits (NDLs) for short and shallow dives were longer than air limits. So people assumed helium decompression was longer, and more hazardous, than nitrogen.
In short, helium was getting a bad rap for a lot of wrong reasons.
It was also religion that switches from helium bottom mixtures to nitrox or air should be made as early as possible, and that so doing, would reduce overall deco time the most.
Not exactly so, at least according to modern decompression theory, and even classical Haldane theory if deep stops are juxtaposed on the profile. If helium and nitrogen are decreased in roughly same proportions as oxygen is increased until a big isobaric switch is made in the shallow zone to an enriched nitrox mix, deco differences between early switches to nitrogen versus riding lighter helium mixes longer are small. Small according to modern decompression theory and practice, but more important, such helium protocols leave the deco diver feeling better. As witnessed under field conditions, the collective experiences of technical and scientific diving operations support that assertion today. And so do modern decompression theories that have seen field testing, like the RGBM, and ad hoc deep stop protocols used by saavy divers.
Indeed there may be no need to switch to nitrogen mixtures at all. Riding helium mixtures to the surface, with a switch to pure oxygen in the shallow zone can be deco efficient, and safer too. So much so, that NAUI Technical Diving Operations has built a training regimen for divers and instructors based on helium for technical diving, and even offers a helitrox (enriched heliair) course. And a full set of RGBM Tables supports helium based training and tech diving.
In the same vein, the operational experiences of WKPP and LANL dive teams underscore many years of safe and efficient helium based deco diving. And that couples to a modern revolution in decompression theory and practice. In fact, WKPP exploits on helium could fill a book. LANL too. NAUI Tec Ops has been utilizing helium-based training for the past four years, or so, without problems. All this means many, many 1000s of tech dives with helium based mixes.
Today, helium is proving its worth as a safe and reliable technical mix. Its use is changing technical and exploration diving. Exit deep air, and enter deep helium and deep stops. It seems about time. Plus time for modern decompression theory to flush the dissolved gas theory entrenching diving for a hundred years.
Let’s look at why. And begin with comparative gas properties as they affect divers.
Nitrogen is limited as an inert gas for diving. Increased pressures of nitrogen beyond 130 fsw can lead to euphoria, reduced mental awareness, and physical dysfunctionality, while beyond 500 fsw loss of consciousness results. Individual tolerances vary widely, often depending on activity. Symptoms can be marked at the beginning of a deep dive, gradually decreasing with time. Flow resistance and the onset of turbulence in the airways of the body increase with higher breathing gas pressure, considerably reducing ventilation with nitrogen-rich breathing mixtures during deep diving. Oxygen is also limited at depth for the usual toxicity reasons. Dives beyond 150 fsw requiring bottom times of hours need employ lighter, more weakly reacting, and less narcotic gases than nitrogen, and all coupled to reduced oxygen partial pressures.
A number of inert gas replacements have been tested, such as hydrogen, neon, argon, and helium, with only helium and hydrogen performing satisfactorily on all counts. Because it is the lightest, hydrogen has elimination speed advantages over helium, but, because of the high explosive risk in mixing hydrogen, helium has emerged as the best all-around inert gas for deep and saturation diving. Helium can be breathed for months without tissue damage. Argon is highly soluble and heavier than nitrogen, and thus a very poor choice. Neon is not much lighter than nitrogen, but is only slightly more soluble than helium. Of the five, helium is the least and argon the most narcotic inert gas under pressure.
Saturation and desaturation speeds of inert gases are inversely proportional to the square root of their atomic masses. Hydrogen will saturate and desaturate approximately 3.7 times faster than nitrogen, and helium will saturate and desaturate some 2.7 times faster than nitrogen. Differences between neon, argon, and nitrogen are not significant for diving. Comparative properties for hydrogen, helium, neon, nitrogen, argon, and oxygen are listed in Table 1. Solubilities, S, are quoted in atm 1 , weights, A, inatomic mass units (amu), and relative narcotic potencies, p, are dimensionless (referenced to nitrogen in observed effect). The least potent gases have the highest index, p.
The size of bubbles formed with various inert gases depends upon the amount of gas dissolved and hence the solubilities. Higher gas solubilities promote bigger bubbles. Thus, helium is preferable to hydrogen as a light gas, while nitrogen is perferable to argon as a heavy gas. Neon solubility roughly equals nitrogen solubility. Narcotic potency correlates with lipid (fatty tissue) solubility, with the least narcotic gases the least soluble. Different uptake and elimination speeds suggest optimal means for reducing decompression time using helium and nitrogen mixtures. Following deep dives breathing helium, switching to nitrogen is without risk, while helium elimination is accelerated because the helium tissue-blood gradient is increased when breathing nitrogen. By gradually increasing the oxygen content after substituting nitrogen for helium, the nitrogen uptake can also be kept low. Workable gas switches depend on exposure and tissue compartment controlling ascent.
While light-to-heavy gas switches (such as helium to nitrogen) are safe and common practices, the reverse is not generally true. In fact, all heavy-to-light switches can be dangerous. In the former case, decreased tissue gas loading is a favorable circumstance following the switch. In the latter case, increased tissue gas loading can be disastrous. This is popularly termed the isobaric playoff.
Mixed gas diving dates back to the mid 1940s, but proof of principle diving experiments were carried out in the late 1950s. In 1945, Zetterstrom dove to 500fsw using hydrox and nitrox as a travel mix, but died of hypoxia and DCS when a tender hoisted him to the surface too soon. In 1959, Keller and Buhlmann devised a heliox schedule to 730 fsw with only 45 min of decompression. Then, in 1962, Keller and Small bounced to 1,000 fsw, but lost consciousness on the way up due to platform support errors. Small and another support diver, Whittaker, died as a result. In 1965, Workman published decompression Tables for nitrox and heliox, with the nitrox version evolving into USN Tables. At Duke University Medical Center, the 3 man team of Atlantis III made a record chamber dive to 2250 fsw on heliox, and Bennett found that 10% nitrogen added to the heliox eliminated high pressure nervous syndrome (HPNS).
Nice work, guys.
All the above properties favor helium for deep diving, but what do divers report after actually using helium?
Consensus among helium divers is that they feel better, less enervated, and subjectively healthier than when diving nitrogen mixtures. WKPP, LANL, and NAUI Technical Operations strongly attest to this fact. Though a personal and subjective evaluation, this remains very, very important. Physiological factors cannot be addressed on first principles always, and for some, just feeling better is good justification and works for many. Postdive deco stress on helium appears to be less than postdive nitrogen stress.
Another positive benny about helium diving scores the minimum-bends depth (MBD), that is, the saturation depth on a mix from which immediate ascension to the surface precipitates decompression sickness (DCS). For helium mixes, the MBD is always greater than that for proportionate nitrogen mix. For instance, the MBD for air (80/20 nitrox) is 33 fsw, while the MBD for 80/20 heliox is 38 fsw. This results from helium’s lesser solubility compared to nitrogen as it affects deeper and longer diving.
And (coming up last) helium decompression is efficient and fast. In fact, many helium deco dives are not possible with nitrogen mixtures. That should give us all good vibes.
On most counts, helium appears superior to nitrogen as a diving gas. Helium bubbles are smaller, helium diffuses in and out of tissue and blood faster, helium is less narcotic, divers feel better when they leave the water after diving on helium, and helium MBDs are greater than nitrogen MBDs.
That, plus efficient and maybe less deco time, are strong endorsements. Great. But how does this translate into actual diving practice? Here’s how.
Helium NDLs are actually shorter than nitrogen for shallow exposures, as seen comparatively in Table 2 for 80/20 heliox and 80/20 nitrox (air). Reasons for this stem from kinetic versus solubility properties of helium and nitrogen, and go away as exposures extend beyond 150 fsw, and times extend beyond 40 min or so. diving favors helium as a breathing gas.
In addition, modern decompression theory (like the RGBM) requires deep stops which do not fuel helium buildup as much as nitrogen in addressing both dissolved gas buildup and bubble growth. And helium deep stops, like nitrogen deep stops, usually couple to shorter and safer overall deco. Nice symbiosis and just one more reason to use helium.
That is another topic, so suffice it to close here with a comparison of helium versus nitrogen deco profiles. These are not academic, they have been actually dived (WKPP, LANL, NAUI Tech Ops). Profiles were generated with the RGBM/ABYSS software package, Abysmal Diving, Boulder). RGBM staging is always deeper, but shorter overall, than Haldane staging with Buhlmann ZHL or Workman USN parameters.
The first is a comparison of enriched air and enriched heliair deco diving, with a switch to 80% oxygen at 20 fsw. Dive is100 fsw for 90 min, on EAN35 and EAH35/18 (nitrox 65/35 and tmix 35/18/47), so oxygen enrichment is the same. The deco profile (fairly light by tech standards, but manageable and easy for training purposes) is listed in Table 3. Descent and ascent rates are 75 fsw/min and 25 fsw/min.
Overall the enriched heliair deco schedule for the dive is shorter than for the enriched air. As the helium content goes up, the deco advantage for enriched heliair increases.
This may surprise you. Now check out corresponding USN or ZHL deco requirements for these dives. In the enriched heliair case, ZHL deco time is 39 min versus 19 min above, and in the enriched air case, ZHL deco time is 33 min versus 22 min above. This not only underscores helium versus nitrogen misfact in staging, but also points out significant differences in modern deco algorithms versus the Haldane stuff of some 40 – 100 years ago. Recall that Haldane staging only addresses dissolved gases, while modern models track both dissolved gases and bubbles in staging.
Ludicrous differences? Maybe not so bad since differences are on the safe side.
Lastly consider a deep trimix dive with multiple switches on the way up. Table 4 contrasts stop times for two gas choices at the 100 fsw switch. The dive is short, 10min at 400 fsw on 10/65/25 tmix, with switches at 235 fsw, 100 fsw, and 30 fsw. Descent and ascent rates are 75 fsw/min and 25 fsw/min.
See Table 4.
Obviously, there are many possibilities for switch depths, mixtures, and strategies. In the above comparison, the oxygen fractions were the same in all mixes, at all switches. Differences between a nitrogen or a helium based decompression strategy, even for this short exposure, are nominal. Such usually is the case when oxygen fraction is held constant in helium or nitrogen mixes at the switch.
Comparative calculations and experience seem to suggest that riding helium to the 70 fsw with a switch to EAN50 is good strategy, one that couples the benefits of well being on helium with minimal decompression time and stress following isobaric switch to nitrogen. Shallower switches to enriched air (EAN) also work, with only nominal increases in overall decompression time.
Just a suggestion.
Helium Bottom Line
Helium has been a mainstay, of course, in commercial diving. But its emergence and use in the technical diving community has been more recent, within the past 10 years or so. Some of this is due to cost certainly. It’s not cheap to dive helium. But a lot of it is due to misconception. The activities of a very knowledgeable and vocal technical diving community are changing both.