By B.R. Wienke and T.R. O’Leary
NAUI Technical Diving Operations
Excerpt from ADM issue 13
Helium 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.

Unjustly so.

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.

Helium Properties

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?

Helium Vibes

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 Staging

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.

Helium ingasses and outgasses 2.7 times faster than nitrogen, but nitrogen is 1.5 to 3.3 times more soluble in body aqueous and lipid tissue than helium. For short exposures (bounce and shallow), the faster diffusion rate of helium is more important in gas buildup than solubility, and shorter NDLs than nitrogen result. For long bottom times (deco and extended range), the lesser solubility of helium is a dominant factor in gas buildup, and helium outperforms nitrogen for staging. Thus, deep implies helium bottom and stage gas. Said another way, transient diving favors nitrogen while steady state 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.
Bruce Wienke is a Program Manager in the Nuclear Weapons Technology/ Simulation And Computing Office at the Los Alamos National Laboratory (LANL), with interests in computational decompression and models, gas transport, and phase mechanics. He authored Technical Diving In Depth, Physics, Physiology And Decompression Theory For The Technical And Commercial Diver, High Altitude Diving, Basic Diving Physics And Applications, Diving Above Sea Level, Basic Decompression Theory And Application, and some 200 technical journal articles. Most importantly he writes for Advanced Diver Magazine.