The Case For Heliox : A Matter of Narcosis and Economics

by Dr. Bill Stone

During the past year there has been substantial discussion in the high tech community concerning the use of mixed gas to deal with the problems associated with deep diving on compressed air, particularly that of narcosis. The fact is, compressed air is a mixed gas, specifically a nitrox mixture consisting of roughly 21% oxygen, so some intial nomenclature is in order.

Nitrox mixtures are composed of a binary combination of oxygen and nitrogen. When the oxygen content is greater than that found in air, the mix is referred to as "enriched air nitrox", or EAN. Mixtures with a lower oxygen content than found in air, used in (shallow water saturation) diving, are referred to simply as nitrox, though "nartrox" may be more appropriate since these mixtures lead to enhanced narcosis. At the opposite end of the narcotic spectrum are such binary gases as heliox (oxygen-helium), hydrox (oxygen-hydrogen) and neox (oxygen-neon). None of these mixes exhibit neurological side effects within the depth ranges likely to be visited by surface-based technical divers in the near future.

Between the two binary gas mix "extremes" of nitrox and heliox there is a sliding scale of mixtures which contain oxygen, helium and nitrogen. These latter blends are known as trimix, which as of late have become the darling of the high tech community. In view of this state of affairs, the question that many people are asking today is, "Why use heliox at all?" The answer is simple: the complete elimination of narcosis.

When conducting scientific or exploration work at substantial depth, there is a high premium on staying frosty, both because of the inherent risks associated with deep diving and the fact that divers are generally operating under a very tight time constraint in which to accomplish the maximum amount of productive work. Under these conditions any psychomotor impairment is undesireable. There is a further advantage in that heliox is less dense and therefore leads to reduced breathing resistance and the associated CO2 build-up.

Though the issue, " to be, or not to be narced," is fairly straight forward, many between-the-lines questions still surround the use of heliox and trimix. Some of these gray areas include thermal considerations, decompression requirements, the issue of acceptable narcosis level, availabiltiy, access and cost.

Heat Loss

The issue of thermal considerations arises from the fact that helium has six times the ability to conduct heat than compressed air. Used in the wrong fashion, high-content helium mixes can push a diver into hypothermia in quick order, but not in the way that many people think. It's because of the misconceptions along these lines that some still hold the belief that trimix is warmer than heliox. Clearing up this confusion requires a brief discussion of the thermodynamics of heat transport.

In the underwater environment, heat is drawn away from the diver by two primary means; conductive and convective heat transfer, though it has not been determined exactly how the actions of these two physical principles are divided.

Conductive, or radiant heat transfer, is a function of the conductivity of the medium surrounding an object and the object's surface area and is the primary source of diver heat loss for surface-oriented dives (in the 0 - 600 fsw range). For the diver, conductive heat loss occurs through the skin; body heat is ultimately radiated away through the thickness of the exposure suit and into the surrounding water. The rate of this heat loss depends on the conductive heat transfer capacity of the medium immediately in conduct with the skin. In the case of a wet suit, the surrounding medium is water; conductive loss is unaffected by the choice of diving gas. However when wearing a dry suit, the immediate surrounding medium is the gas used to inflate the suit. This is where the high conductive capacity of helium mixes can cause a problem. When breathing a high-content helium mix, divers should use an auxillary suit inflation gas to avoid rapid heat loss, preferably one with as low a heat conductive capacity as possible such as argon or carbon dioxide, though the latter can cause skin irritation when moisture is present. For that reason argon is probably a better choice.With a heat transfer capacity of about 50% that of compressed air, argon is an attractive inflation gas for cold water as well, whether or not mix is used.

The second form of heat transfer is convective and is typically a minor component of diver heat loss except at very great depth. This latter form of heat loss occurs as a result of normal respiration using open circuit scuba since each breath that is expelled into the surrounding water carries with it heat from the body. However the heat removed through the respiratory tract is more a function of the heat capacity (density) of the gas being breathed than its heat conductivity. A good case can be made that because of the greatly reduced density (and consequently thermal capacity) of helium, convective heat losses for helium mixtures are probably less that that for compressed air (and other nitrogen mixtures) at any given depth i.e. because air has a greater density, it will remove more heat when equilibrated.

In practice, heliox doesfeel cooler to breathe than air ( a phenomena probably due to radiant heat loss at the surface of the lungs) and some divers report it makes them colder. However, as discussed above the reputed chilling effect of helium mixes is misunderstood, and trimix offers no superior thermal characteristics over heliox.

Decompression Requirements

Another misconception regarding heliox, and an argument used by many in defense of trimix, is that heliox leads to unreasonably long decompression schedules and that trimix can be used to significantly shorten decompressions. This belief is based upon the common knowledge that nitrogen is absorbed by body tissues at a slower rate than helium, and seems to be supported by a comparison of decompression times from the US Navy Heliox and Extreme Exposure Air decompression tables. There are several points to be addressed here.

First, it should be noted that both the USN Heliox and Extreme Exposure Air tables were generated under the assumption that dives are conducted on a single (bottom) mix which is used for both the working and decompression phases of the dive, though in the case of the heliox tables O2 is used for the final phase of decompression. As a result, the comparison is a bit like matching up apples and oranges.

It has been known for some years that decompressions on deep dives can be dramatically improved by the selection of decompression mixtures with a high oxygen content. Indeed, it is now a common field practice in technical circles to use pure oxygen at the 20 and 10 fsw stops on most deep dives, and based on the development work that was carried out at Wakulla in 1987 and subsequent projects, many cave and wreck explorers have now begun to use enriched air mixtures at substantial depth.

Typically, for missions in the environs of 300 fsw, EAN 32 is utilized as a decompression gas beginning at 130 fsw, and in some cases EAN 30 for slightly deeper decompressions, although air (nitrox 21) has been used successfully as a safety hedge at stops as deep as 220 fsw. These mixtures were selected such that the PO2 levels did not exceed 1.4 - 1.6 atmospheres (atm) during decompression. Note that a PO2 of 1.4 atm represents the maximum allowable for sustained surface-based diving, though this restriction is relaxed somewhat during the decompression phase of the dive in which divers are completely at rest. This procedure of using EAN mixtures followed by oxygen has become a defacto community standard for special mix diving among cave, wreck and scientific user groups, and should be factored into any comparison between heliox and trimix.

Figure 1 compares the performance of three different bottom mixes from a decompression perspective. The decompression schedules were generated by the Hamilton Research DCAP computational program1. Each profile represents a 20 minute bottom time at a depth of 300 fsw for each of three mixes; nitrox -14, a.k.a. nartrox (14% O2, balance N2), trimix 14/34 (14% O2, 34% helium, balance N2), and heliox-14 (14% O2, balance He)2. In all cases, it is assumed EAN 32 is breathed during decompression, from 130 to 70 fsw, compressed air from 60 to 40 fsw, and oxygen from 30 fsw through surfacing.

Note, these profiles and decompression procedures were dive-tested at Wakulla Springs using heliox 14 with bottom times varying from 30 to 80 minutes. No bends incidents ocurred over 35 person-dives. The dives were conducted using a "microbell" during decompression which explains the somewhat unusual procedures. Air was used at the 60-40 fsw stops in the bell instead of a higher PO2 mix (ex. EAN 50) to give divers a "mask break," reduce their oxygen tolerance units (OTUs) and to purge the microbell atmosphere. The use of the bell also permitted oxygen to be breathed safely at 30 fsw versus the recomended 20 fsw limit for inwater decompression. The profiles for the other bottom mixes are calculated using identical procedures but have not been tested.

(FIGURE 1)

Fig. 1: Decompression profile for a 20 minute bottom time to 300 fsw using a) nitrox 14; b) trimix 14/34; and c) heliox 14

As can be seen in Figure 1, the use of nitrox-14 results in a total dive time (runtime) of 165 minutes, compared to 174 minutes for heliox, a 5% reduction (9 minutes) with trimix falling in between. Note that in this example the iso-narcotic depth of nitrox-14 (or equivalent narcotic depth, END , see technically speaking ) would be 327 fsw making it highly undesireable bottom mix from an operational perspective. Figure 2 shows a similar comparison for bottom times of 80 minutes. In this case a very different scenario appears; heliox-14 yields a total dive time of 668 minutes ( 11 hours and 8 minutes) versus 818 minutes for nitrox-14, a reduction of 18% (a substantial 150 minutes) in favor of heliox. Trimix yielded a dive time of 698 minutes. What's going on here?

(FIGURE 2)

Fig.2: Decompression profile for a 80 minute bottom time to 300 fsw using a) nitrox 14; b) trimix 14/34; and c) heliox 14

Note that for both the 20 and 80 minute dive profiles, bottom time represents only a small fraction of the total runtime (about 10% on average), the majority of the dive being taken up by decompression during which identical nitrox mixtures are used followed by O2. The conclusion, as can be seen in Figure 3, showing the complete range of trimix mixes (from 0%He or nitrox to 100% helium, heliox), is that for a bottom time of 20 minutes in the 300 fsw range, there really is no significant difference in decompression between any of the possible bottom mixes3.

(FIGURE 3)

Fig. 3: Total decompression times for a spectrum of gases ranging from nitrox 14 (14% oxygen, balance nitrogen) through trimix to heliox 14 (14% oxygen, balance helium) for 20 minute and 80 minute bottom times at 300 fsw.

Figure 3 indicates however, that there is a definite trend in favor of heliox and or a high-percentage helium mix for longer (deeper) dives. This may be partly explained by the fact that while nitrogen dissolves in tissues at a slower rate than helium, it also outgases more slowly; helium, while reaching a higher percentage of saturation, offgases at a faster rate.

While the use of nitrox mixtures (EAN 32, air) for decompression actually accelerates helium offgassing through, there is no corresponding acceleration when nitrox-14 is used as a bottom mix. As bottom times are increased, the heliox ( 0% N2) profile eventually cuts through the trimix profile leading to an overall reduced dive time (decompression time). It may be noted from Fig. 3 that the majority of the decrease in decompression time occurs by the time that the helium content reaches 50% of the bottom mix.

In summary, for short dives in the 300 fsw range, the choice of bottom mix appears to have little effect on total decompression time3. On long dives with a bottom time of about an hour or more, decompression can be substantially reduced by using a bottom mix with a helium content of 50% or more.

The Cost of Mix

Based on the discussion above, it would appear that trimix offers no special advantage over heliox from either a thermal or decompression perspective. So why shouldn't everyone use heliox on deep dives? The answer can be summarized in one word: cost.

Figure 4 (and corresponding Table 1) shows the operating cost per hour of open-circuit scuba for various bottom mixes as a function of working depth. (Note that only helium costs are considered as the cost for decompression gas is assumed to be roughly the same in all scenarios.) For example, an hour long swim at 300 fsw using heliox-14 as a bottom mix would cost approximately $163 for helium costs alone, or nearly three times the estimated $64/hour costs of trimix 14/34, which would produce the same narcosis level at 300 fsw as diving on compressed air at 165 fsw. If Air Products Corp. is planning to sponsor your next deep diving project the above discussion is irrelevant; use heliox. If not and the money for gas is coming out of your own pocket, then you can begin to wrestle with the essential trade-off of special mix diving; how much narcosis can you afford to eliminate?

(FIGURE 4)

Fig.4: Open-circuit scuba operating costs per hour as a function of the helium content of the mix. Given commercial gas costs of approximately $75 per K-bottle or about $0.008/ liter, the cost of operating an open-circuit rig at depth is given by:

Cost per hour ($) = ( Dw + 33 )• VO2 • RMT • Fhe • $0.008 /liter • 60 min./hr.

33 VO2

Depth (atm) He Consumption He Cost Conversion

per atm to hours

where:

Dw = working depth in fsw

VO2 = 1.0 metabolic oxygen consumption in liters/ minute per atm.

RMT/VO2 = 26 mean respiratory rate in liters of mix/ minute per atm, divided by the metabolic oxygen consumption.

Fhe = helium fraction in the breathing mixture

$ 0.008 = the average commercial cost of helium per liter

Note that RMT/VO2 is generally taken to be the constant 26, i.e. total gas consumption per minute per atm. is approximately 26 times the metabolic oxygen consumption based on empirical tests. Also note that the above costs assume the diver mixes and checks his/her own mix; blending costs are not included.

The Economics of Narcosis

Given a planned working depth and a desired narcosis level orequivalent narcosis depth (END), it is a relatively straightforward matter to calculate the required helium gas fraction in the breathing mix as a function of planned working depth and the narcosis level. Substituting this into the cost equation used in Figure 4, the operating cost per hour of open-circuit scuba can be calculated as a function depth and equivalent narcosis depth (END) at the working depth of 300 fsw, as shown in Figure 5. The graph summarizes the narcosis/cost trade-offs between heliox and various trimixes. The trade-off comes down to selecting a feasible operating cost and a tolerable narcosis level.

(FIGURE 5)

Fig. 5: Open-circuit scuba operating costs per hour as a function of equivalent narcosis depth (END), the depth at which the narcosis level would be the same if the dive were conducted on air. An effective narcosis level of "-33 " indicates that there is no nitrogen in the mix (i.e. heliox is being used); an equivalent narcosis depth of "0" is equivalent to breathing air at the surface (1 atm).

This discussion of operating cost assumes that the dive is to be conducted using open-circuit technology. If a fully closed-circuit breathing system (rebreather) is used, the numbers look very different. With closed-circuit, helium loss occurs only when the counterlung vents as a diver ascends. If the dive profile is monotonic, that is, there are no repetitive descents and ascents prior to surfacing, then the total volume of diluent gas used (generally a single inert gas, ex. helium, with a small fraction of oxygen added in order to serve as a bail-out at depth) is that required to maintain counterlung volume at maximum depth . Given that most counterlungs have a capacity of approximately 7 liters, diluent gas used is roughly 7 liters times working pressure in atmospheres, and is independent of bottom time(for a discussion of closed-circuit systems see Technologically Inspired by Dr. RW Hamilton, aquaCorps Journal, Vol.2, pg. 10).

Factoring in the cost of helium, Figure 6 shows closed-circuit operating costs as a function of depth. Note that the equivalent narcosis depth, END, is "-33" at all depth ranges i.e. there is no nitrogen in the mix. The conclusions that can be drawn is obvious; mix costs are inconsequential for closed-circuit operations, which offer roughly 100 times the efficiency of open-circuit systems. The observation regarding rebreather costs is worth discussion.

Currently there are two groups in the U.S.; Cis-Lunar Development Laboratories, and Carmellan Ltd., preparing to market a closed-circuit system in the near future with prices in the neighborhood of $10-15,000. Are these costs out of reach?

Assume that you expect to be diving in the 300 fsw range, not an unreasonable depth considering that many of the more advanced wreck and cave explorers are routinely working this depth range today. A simple method of estimating the equipment costs involved in open-circuit scuba is to calculate the gas required for a one hour dive at 300 fsw, expressed in terms the number of scuba tanks required, times an average capital cost per tank of $400 (which includes regulator, pressure guage and supporting hardware). Assuming 80 cf cylinders are used for the purpose of analysis, each containing approximately 2200 liters of gas, the open-circuit hardware cost (OCHC) per one hour dive is thus:

OCHC ($) = ( VO2 • RMT/VO2 • 60 min/hr • (Dw +33)/33 ) • $400

2200 liters/tank tank

where all of the terms have been previously defined.

Assuming heliox 14 is used on the dive, and that the capital costs of a closed-circuit system is $10,000, a rebreather will pay for itself in gas cost savings alone within:

Breakeven = ($10,000 - $4293) = 35 one hour dives to 300 fsw

($163 - $0.49)

Note that the cost and bulk of open-circuit hardware needed for such a dive is not inconsequential. Experience at Wakulla Springs in 1987 showed that the volume of tankage required could only be effectively moved with a diver propulsion vehicle (DPV).

A more general form of breakeven analysis is given in Figure 7, which shows the number of one hour dives needed to breakeven as a function of planned working depth. This graph indicates that serious sport divers who expect to be diving regularly at depths as shallow as 200 fsw would do well to consider the C2 technology option when it becomes generally available. Other substantial benefits of rebreathers, such as range-enhancement ( a virtually unlimited gas supply ), compactness (the elimination of open-circuit bulk), near optimal decompression and silence add further to its appeal.

(FIGURE 7)

Fig. 7: Number of dives to breakeven point, as calculated solely on the basis of helium usage cost between a closed-circuit (C2) system (assumed to cost $10,000 per unit) versus presently available open-circuit technology. Breakeven is calculated as:

Breakeven = CCHC - OCHC________________

( 18.78 - 7 liters • ( $0.008))( Dw + 33)( 1 - maxPo2 • 33)

33 Dw + 33

In summary, the heliox versus trimix debate presently rests with the cost trade-offs inherent in the ineffeciencies of open-circuit scuba at depth. Eventually, as C2 technology becomes widely available, the debate will come to an end and heliox will become the bottom mix of choice for deep diving.

Dr. Bill Stone is the chairman and founder of Cis-Lunar Development Laboratories which specializes in the design of advanced life support systems for undersea and space exploration. A cave diver and member of the Explorers Club, Dr Stone has organized and led over 26 speleological expeditions over the last twenty years and can be contacted at: Cis-Lunar Labs, 7739 Laytonia Dr., Derwood, MD 20855. Fax: 301-975-2128 x 6075.

Footnotes:

1. The DCAP program utilizes the Tonawanda II algorithm with ascent-limited matrix designated 11F6, based on the Haldane-Workman-Schreiner model. Use of a different model might yield different results (see footnote 3. below). The exact behavior of different inert gases in the body is not well established. The principles used here have been validated in several hundred diverse exposures, but the basic gas transport physiology is still somewhat uncertain.

2. Note that the oxygen fraction for the three bottom mixes is set at 14%. This yields the optimal PO2 of 1.4 atm at the working depth of 300 fsw (10 atm) i.e. PO2 = 10 atm x .14 = 1.4 atm.

3.Note that these results( i.e. the specfic crossover points) are dependent on the depth range considered and also the specific decompression regime used, though the conclusion that the use of heliox yields shorter decompressions for long dives (dives greater than 80- 100 minutes) is generally accepted.

For example, at 300 fsw using a decompression regime of EAN 32 from 120 fsw, EAN 50 from 70 fsw, and pure oxygen from 20 fsw, the deompression associated with heliox is significantly longer for a 20 minute bottom time and does not show a decompression improvement over " nartrox" until after about 100 -125 minutes of bottom time, based on Submariner Research Ltd's modified Buhlmann algorithm. Note that specific crossover points can vary somewhat with the computational algorithm used to generate the schedule but is not the major factor.

Sidebox:

To appreciate the full extent of impairment associated with compressed air divers should try heliox sometime, if only at a depth of 130 fsw. Those that do may be surprised. After breathing heliox on descent, switch over to a bottle of compressed air and you will instantly be convinced of something you probably never really believed: there is a real noticeable nitrogen impairment even at shallow depth. Most divers never notice it because they gradually come under the influence diving compressed air down from the surface.

When conducting scientific or exploration work at substantial depth, there is a high premium on staying frosty, both because of the inherent risks associated with deep diving and the fact that divers are generally operating under a very tight time constraint in which to accomplish the maximum amount of productive work.

If Air Products Corp. is planning to sponsor your next deep diving project the discussion is irrelevant; use heliox. If not and the money for gas is coming out of your own pocket, then you can begin to wrestle with the essential trade-off of special mix diving; how much narcosis can you afford to eliminate?

The conclusion that can be drawn is obvious; mix costs are inconsequential for closed-circuit operations, which offer roughly 100 times the efficiency of open-circuit systems.

Box:

Reccomended Reading

Stone, W.C., The Wakulla Springs Project, US Deep Cave Diving Team, Derwood, MD