Extending The Envelope: A Primer On Self-Contained Diving Technology
© 1991 Michael Menduno. All rights reserved.
MIX Diving
" Doing deep dives on air is like having sex without a condom. The technology is there. It's stupid not to use it."
Dick Rutkowski
Int'l Association of Nitrox Divers
In many respects the revolution in self-contained diving could be called the "mix revolution." In the few short years since its introduction to the sport and scientific communities, the use of special mix technology has greatly extended the reliable range of self-contained diving and has served to spur the development of other supporting technologies and methods.
As a result, mix&endash; the software of technical diving&endash; is becoming a standard tool among the vanguard and is making comparably rapid inroads into mainstream diving as well. For good reason. Based on extensive commercial and military experience, mix technology offers significant safety and performance benefits in comparison to diving air. ( and represents a major opportunity for growth within the market).
Though it will likely be some years before mix technology achieves its full potential&endash; a matter of establishing the necessary infrastructure &endash; the truth is that the advent of this and other technologies has already made a major impact on sport and scientific self-contained diving signaling the end of the era of single mix &endash;"air"&endash; technology. What's more is that in time, the technology and methods being pioneered today in these communities, will likely effect changes in military and commercial diving as well, much as the growth of the PC eventually redefined the world of mainframe computing.
In order to appreciate the safety and performance benefits offered by mix technology and its application to self-contained diving it's useful to begin with a discussion of the basic physiological constraints that limit humans ability to breathe compressed gas underwater.
Figure 1: Breathing Gas Constraint Diagram
Physiology Primer
We live in a compressed gas environment. The atmospheric air we breathe is a mixture of gases composed of about 78% nitrogen, 20.9% oxygen, 0.9% argon, and a balance of carbon dioxide and rare gases at a total pressure of 14.7 p.s.i. at sea level, conveniently defined as one atmosphere (atm). Of these gases, oxygen is the only one required to sustain human life. The other gases breathed from the atmosphere or in a diver's breathing mix serve as a carrier and diluent for oxygen in order to maintain it within tolerable physiological limits defined by it's partial pressure1 .
Sidebox: What's an atm?
An atm, or atmosphere, is a unit for measuring pressure, that is used a great deal in technical diving. The total atmospheric pressure at sea level is defined as 1 atmosphere (atm), sometimes called an "ata", an atmosphere absolute. At sea level, the fraction of oxygen in air is 0.21 or 21%, so the partial pressure of oxygen, PO2 (total pressure x gas fraction = gas partial pressure) is 0.21 atm. One atmosphere = 1.013 bars (another common unit of pressure). For physiological purposes atms and bars are more or less interchangeable.
Another pressure unit with great utility is the kilopascal, a metric unit denoted kPa. A kPa is 1/100 of a bar -- very close to 1/100 atm. In time most physiological pressures will be defined in kPa. Some other common pressure units are as follows:
1 atm = 14.7 Psig
1 fsw = 1/33 atm (feet of seawater)
1 ffw = 1/34 atm (feet of fresh water)
1 msw = 1/10 bar (meters of seawater)
1 msw = 3.2568 fsw
1 kPa = 1/100 bar
Humans function optimally at an oxygen partial pressure of about 0.2 atmospheres (atm) and without too much CO2. Below this level, at about 0.12-0.14 atm, hypoxia (oxygen starvation) becomes an issue. At much above this level, for example at the elevated pressures encountered in diving, oxygen acts as a drug and can induce a variety of physiological effects depending on the dose, measured by its PO2, and exposure time. These can range from "whole body toxicity ", characterized by temporary lung and pulmonary damage for very prolonged exposures greater than 0.5 atm, to central nervous system (CNS) toxicity which can result in an epilectic-like grand mal seizures at relatively short exposures to high elevated oxygen pressures above about 1.4-1.5 atm. Though many animals are able to adapt to low oxygen partial pressures, not too many have evolved mechanisms for managing high PO2s, diving mammals being one exception.
For humans, oxygen uptake is as easy as taking a breath, no doubt an evolutionary adaption to wide ranging environments; it's surprising how little oxygen the body actually needs. Oxygen requirements vary from about 0.5 liters/minute at rest to about 1.0 l/m (0.05 cubic feet/min.) for moderate work, about up to 3.0 l/m for heavy workloads. That means that the oxygen content in a single 80 cubic foot scuba cylinder is enough to sustain an individual for close to 30 hours if it could be utilized efficiently. Conversely, the human body has to work hard to get rid of excess CO2, a waste product of it's metabolic processes. As a result, the body acts to control CO2 through the rate of ventilation.
When CO2 levels in the blood rise above optimal, for example as a result of increased exercise or workload, increased breathing gas density or as a result of marginally performing breathing equipment, ventilation increases, facilitating CO2 removal. For the most part. Though the body will try to regulate arterial CO2 to level it wants, it's not above making compromises. If it has to work too hard to reach its goal it will simply change it's mind, balancing how hard it has to work with how much excess CO2 it's willing to put up with. This can have significant ramifications when diving. Elevated CO2 levels, or hypercapnia, are believed to increase the body's sensitivity to high oxygen levels and speeds the onset of oxygen toxicity.
Even though oxygen is the most vital gas physiologically speaking, inert gas is important to any discussion about diving because oxygen must be diluted with some other gas when at an absolute pressure much beyond 1.4 - 1.6 atm, in terms of depth about 20 fsw or 6 msw. In diving the two basic diluents used are nitrogen and helium, though hydrogen has been used for very deep dives and some work has been done with neon and argon as well.
Of course inert gas is important in it's own right because it is the source of decompression problems and a substantial amount of effort is involved in it's reduction and management.
Humans are saturated at birth at a partial inert gas pressure of about 0.79 atm. At the increased absolute pressures encountered in diving, the inert gases in a diver's breathing mix are absorbed by the blood and other body tissue at an elevated partial pressure until they are eventually equilibriated with the increased ambient pressure, a process that can take as long as 72 hours. Decompression is a matter of reducing the elevated inert gas partial pressure in the blood and body tissue in a slow controlled manner during ascent to avoid the formation of bubbles in the blood or other body tissues which can result in decompression sickness (DCS). The methods used to accomplish this and the difficulty involved depend on the specific exposure and types of inert gases used. The properties which are believed to govern the decompression characteristics of a gas are it's diffusivity, solubility in water and fat, and it's thermodynamic properties.
The metabolic gases, oxygen and CO2 also play a role in decompression. Oxygen acts as a vasoconstrictor at high partial pressures and as such likely influences gas transport as does CO2 which acts as a vasodilator. What's more is that experience has shown that pressure reductions while breathing oxygen can result in bubble formation at very high partial pressures (above about 3.0 atm) though these are typically above the range encountered in diving. Some evidence and anecdotal experience also suggests that high CO2 levels may be associated with increased incidence rates of DCS (cite reference), although other studies refute these conclusions.
Under certain conditions it is also possible to evoke a form of decompression sickness without a change in pressure. This phenomenon, known as isobaric counterdiffusion, was first observed in the early 1970s when gas-containing skin lesions were found in divers exposed to 1000 fsw pressure breathing an oxygen-neon-helium mixture when placed in an oxygen-helium environment. These skin lesions were eliminated when the diver was placed in a sealed suit and surrounded by the same gas as the breathing medium(cite ref.). Later work reported further observations and described the mechanism believed responsible for this phenomenon(cite ref.). Gas bubbles can form when a gas with a high diffusion coefficient is substituted for a gas with a lower coefficient in a diver's breathing mix. Under these conditions the "fast" gas diffuses rapidly into tissue while the lower coefficient gas diffuses out more slowly. The net result is that the total inert gas concentration will rise and supersaturate tissues. This supersaturation can reach levels that can cause clinical decompression sickness. Counterdiffusion effects can be separated into superficial effects involving the skin and deep effects involving other organs and tissues, such as the inner ear, not affected by surface contact with the gases in question. Gas switches that can cause gas phase formation include air or nitrox to helium, hydrogen to helium and neon to helium, though these typically require a significant "soak" in the initial gas to evoke the effect. In commercial operations, gas switches suspected of causing a counterdiffusion problem are accompanied with a small increase in pressure to avoid supersaturation.
Breathing inert gases at elevated pressures can result in other physiological problems as well. With the important exception of helium, other metabolically inert gases such as nitrogen, hydrogen and to a lesser extent neon used to dilute oxygen in a diver's breathing mixture act as a narcotic at elevated partial pressures with a potency related to their affinity for lipids or fat in the body, a supposition known as the Meyer&endash;Overton hypothesis, which was first proposed in the early 1900s to explain the action of anesthetic gases (cite reference). This hypothesis was later extended to state that " all gaseous or volatile substances induce narcosis if they penetrate cell lipids in a definite molar concentration of about 0.03-0.07 moles of drug per kg of membrane which is approximately the same for all narcotics depending on the type of animal."
Though this narcotic potency of inert gases may be related to many physical constants including molecular weight, absorption coefficients, thermodynamic activity,Van der Waal's constants and other factors, the lipid solubility of a gas seems to provide the best prediction of potency though there are anomalies. Table 1 displays the correlation between the narcotic potency of various gases and their lipid solubility and other physical characteristics. Note that some gases such as Krypton and Xenon, a surgical anesthetic have sufficient potency to be narcotic at atmospheric pressure while hyperbaric exposures are needed to induce narcosis for diving gas diluents.
TABLE 1: Narcotic Properties of Gases
In the case of nitrogen, narcosis is probably measurable at partial pressures as low as 2-3 atm, the equivalent of breathing air at about 66 fsw, and increasingly becomes an issue at partial pressures above 3 atm, beyond about 100 fsw while breathing air, though with experience it is generally manageable at partial pressures of up to about 5 -6 atm (180 - 220 fsw on air). Nitrogen is also believed to interact with high levels of both carbon dioxide and oxygen increasing a diver's susceptibility to narcosis and the intensity of it's effects, an important consideration at depth. Neon has been shown to be about three times less narcotic than nitrogen. Hydrogen has also been shown to act as a "light" narcotic at very high partial pressures encountered at great depth. All other inert gases are more narcotic than nitrogen which is one reason few are used in diving mixtures. Though argon has been used experimentally, it is about twice as narcotic as nitrogen, making it not very suitable for breathing applications.
Though this narcotic property of some inert gases is highly undesirable from a diving perspective it's measured absence can sometimes result in another problem encountered in deep sea diving; the effects of rapid increases of hydrostatic pressure to about 600 - 800 fsw and beyond, referred to as High Pressure Nervous Syndrome (HPNS) and usually characterized by tremors. Oxygen- helium (heliox), and to some extent oxygen-neon (neox) breathing mixtures are associated with HPNS because they allow high hydrostatic pressures to be applied without a compensating narcotic effect, hence it's early designation, "helium tremors." HPNS can become a critical limiting factor for very deep dives, beyond about 800 fsw, though with the rapid descent rates used in technical diving this phenomena may occur at depths as shallow as 400 - 600 fsw.
Still another issue during deep dives is breathing gas density, which increases linearly with depth (absolute pressure) and with inadequate or poor performing equipment can cause difficulties as shallow as 160-180 fsw while breathing air. During a laminar gas flow encountered in breathing, the pressure required to move a gas is a linear function of it's density which is roughly independent of both the type of gas and depth. However with turbulence present to some degree in normal breathing and in all underwater breathing equipment, the pressure required increases approximately with the square of the flow. As a result the work of underwater breathing increases with depth and when denser gases are breathed. The effect is exaggerated during exercise when respiratory rates are increased.
High gas densities act to limit the diver's ability to ventilate his or her lungs and increase the work required. Typically in diving, oxygen partial pressures are sufficient to meet metabolic needs, and restrictions on ventilation result in CO2 build-up. In addition, during forced expiration there is a maximum rate of flow beyond which extra effort will not increase gas flow but instead will cause a temporary constriction of the diver's intra-thoracic airways. As a result, greater pressure is required to generate the same flow with a denser gas, a kind of vicious circle. At the increased densities encountered in diving, these two effects tend to interact. The result is to limit the amount of useful work a diver can do at depth (beyond about x? ). The performance characteristics of breathing equipment can offset this effect.
Diving gases have thermal consequences as well and can also effect vocal communications, the obvious example being a "Donald Duck" voice that results when breathing helium mixtures. There are also fire safety issues to consider when dealing with oxygen-rich mixtures and with combustible gases such as hydrogen. All of these aspects must be taken into account when considering the use of diving gases.
The Problem With Air
As a diving gas, air has several real limitations from a safety and performance perspective. These are illustrated in Figure 2. For shallow water surface-based dives in the 30-130 fsw range typical of most sport diving applications, the high nitrogen levels found in air result in long decompressions (e.g. short no-stop times); the oxygen levels being far below optimal2 . In addition, for long dives in excess of about 80 - 120 minutes beyond about 60 fsw, field experience has shown air decompressions to be excessive. Nitrogen can be a difficult gas to decompress from in comparison to diluent gases such as helium.
FIGURE 2: The Problem With Air
Conversely, for deep dives beyond about 190- 220 fsw, the partial pressure of oxygen becomes excessive (above about 1.6 atm), increasing the likelihood of CNS oxygen toxicity. However, the most immediate problem is usually narcosis. Nitrogen, the primary diluent gas in air, becomes increasingly narcotic beyond 100 fsw impairing the divers ability to perform and respond to events in a timely fashion. Density is also an issue on deep dives; air is difficult to breathe at depth and can effect regulator performance at depths greater than about 160-180 fsw, resulting in CO2 build-up .
In the midrange, from about 150 to 190 fsw, oxygen levels found in compressed air are close to optimal for open circuit surface-based diving, and the narcosis is manageable with experience. In practice this is a workable range for air, though it is sometimes extended deeper for short duration dives. Air is also useful as an intermediate or decompression gas for deep dives and is sometimes used as a part of a planned program of nitrox and oxygen decompression when diving with helium-based mixtures.
As a result of these limitations, air is generally considered to be inefficient as a diving gas for most applications, except from an availability and cost standpoint.
Special Mix Technology
First conceived in the early 1900's, and gradually applied to the problems of underwater breathing, the use of custom or special mix diving gases&endash; mix technology &endash;was developed by government and later commercial diving operators to overcome the inherent problems and limitations associated with diving compressed air. Today mix technology offers the capability of reliably supporting divers to depths beyond 2000 fsw and some believe the technology may eventually be extended as deep as 5000 fsw.
Though commercial divers universally refer to oxygen-helium mixtures called heliox&endash; as "mixed gas," the description is not very accurate; air is a mixed gas as well. Instead the term special mix is used in self-contained technical and scientific diving applications to refer to diving gases other than air including binary mixtures of oxygen with nitrogen; helium and neon, and trimixes, for example oxygen, helium and nitrogen. The mixtures delivered by closed circuit systems are also referred to as special mix.
Sidebox: Mix Technology Time Line
1879: First closed circuit oxygen rebreather (Fleuss)
1878 " Barometric Pressure" published by Dr. Paul Bert discussing oxygen toxicity, hypoxia and decompression sickness.
1908: Development of the first staged decompression techniques (Haldane)
1919 Patent for oxygen-helium breathing mixtures (C.J. Cooke)
1925 US Bureau Of Mines experiments with helium decompressions (Hidebrand, Sayers, Yant)
1926: First self-contained open circuit SCUBA system (Fernez-Le Prieur)
1928: British air with oxygen decompression tables to 320 fsw
(Davis, Damant)
1935: Nitrogen narcosis attributed to elevated nitrogen partial pressures (Behnke)
1937: First Successful Heliox Dive (420 fsw) (End, Nohl)
1939: USN Heliox Tables published (Behnke).
Successful Rescue & Salvage U.S.S. Squalus (240 fsw)
1943: Development of the Gagnan-Cousteau "Aqualung" demand valve regulator.
1943: Enriched air nitrox first proposed as a diving gas to reduce decompression problems (C. Lambertson).
1953: USN Air Tables Published
1957: Development of saturation techniques: The Genesis Project (Bond)
1959: Nitrox diving methods first published in USN diving manual
1962: First commercial heliox dive to 420 fsw (Wilson)
Buhlmann successfully demonstrates "gas sequencing" techniques on a 1000 fsw dive.
First saturation dive to 200 fsw (Link)
1965: First commercial saturation Dive to 250 fsw using the Westinghouse Cachalot System.
1970: First recorded incident of High Pressure Nervous Syndrome (HPNS).
NOAA launches enriched air nitrox program.
1977: First (Unsuccessful) open circuit special mix dive in the sport diving community(Smith, Holtzendorff&endash;Wakulla Springs)
1979: NOAA nitrox diving methods published in NOAA Diving Manual 2nd Ed.
1980: First successful special mix sport dives at Diepolder II to 360 fsw, (Sweet)
1982: ORCA Industries launches the first consumer dive computer, the "EDGE"
1987: International Association of Nitrox Divers Inc formed to provide enriched air training to sport divers.
1987: Jochen Hassenmeyer special mix open circuit dive to 656 Fsw.
The Wakulla Springs Project (320 fsw) successfully completed using heliox and Hamilton Research Ltd. tables
1988: Sheck Exley's mix dive to 780 fsw at Naciememto Del Rio Monte using trimix.
1989: Sullivan Connection Project (240 fsw) successfully completed using trimix.
1990: aquaCorps: The Journal For Technical Diving founded.
Opening of first special mix training center for sport divers, Key West, Florida.
1991: First organized sport dives using special mix on the Andrea Doria (250 fsw).
Successful completion of the Eagles Nest Project (290 fsw) using trimix.
Until recently, mix technology was limited to industrial and military applications utilizing surface supply and closed circuit breathing systems due to it's cost and availability and the increased planning, logistics and support involved. Now with growing user base sophistication and numbers, and an increasing demand for safety and performance, this technology is being re-applied to sport and professional open circuit diving applications with justification. All divers breathe a compressed gas underwater; the real issue is, are they breathing the "best" mix from a safety and performance point of view given their application and budget?"
The fundamental idea behind special mix technology is to improve diver safety and performance by"optimizing" a diver's breathing mix for a planned exposure with respect to three critical physiological requirements. These are:
1) To maintain reliable and efficient oxygen levels during the course of the dive.
2) To reduce and or eliminate undesirable inert gas effects such as narcosis and HPNS and to facilitate offgassing.
3) To reduce breathing gas density in order to minimize the work of breathing and prevent excessive CO2 build-up.
Essentially this optimization can be thought of as a physiological balancing act between the amount of oxygen that can be safely tolerated and the types and fractions of inert gases used as a diluent, the two counterbalancing components that make up any breathing mix.
Operationally, this is accomplished by selecting an appropriate mix for the exposure and operation, and in many cases, switching gases during the various phases of a dive according to a predetermined plan.
Fig.3 : Balancing Act Illustration
Sidebox: Gas Laws
The ideal gas laws, in particular, Dalton's law which states," the total pressure exerted by a gas mixture is equal to the sum of the partial pressures of the component gases in the mixture," has great utility for technical divers and is used frequently in calculating the essential operational characteristics of a special mix. A common form of this equation can be expressed as:
Pg = P x Fg
where Pg is the partial pressure of a gas,g , and is equal to the total pressure, P, of the gas mixture times the proportion or fraction, Fg of the gas in the mixture. Uses of this formula include; 1) Calculating the partial pressure of a gas in a mix, for example PO2 or PN2, given the working depth in pressure, and the fraction in the mix.
2) Calculating the proper fraction of gas, Fg to be used in a in a mix, given the working depth, and required partial pressure, and
3) Calculating the maximum reliable operating depth (pressure) of a specific mix on the basis of its component gas fractions. See discussion below.
Anatomy Of A Dive
From a physiological perspective, any dive can be thought of as consisting of two distinct phases; a compression (descent) and working phase in which gas loading commences and where CO2 levels are generally high due to high exercise levels, and the decompression or pressure reduction phase of the dive where the diver is usually at rest. These are shown graphically in Figure 4. Each of these phases has it's own set of physiological requirements and issues that must be addressed.
Figure 4: Dive Anatomy
The most important issue during the working phase of a dive is to insure that the diver maintains a reliable and efficient oxygen level measured by it's partial pressure based on the duration of the exposure and work requirements (exercise level). Of course oxygen pressures must be sufficient to avoid hypoxia. However if the working partial pressure of oxygen (the average PO2 encountered during the working phase of the dive) is too high, oxygen toxicity, whether immediate or cumulatively over the course of the dive, becomes a problem. Conversely if oxygen partial pressures are too low the counterbalancing pressures of the inert gas in the breathing mix are increased, increasing gas loading and resulting in increased decompression requirements.
A second issue is to insure that undesirable gas effects such as narcosis, or HPNS on very deep dives, are minimized or eliminated, again depending on the specific operation and environment3 . On deep dives, reducing breathing gas density is also important as it improves the ability to ventilate the lungs facilitating CO2 removal thus allowing increased work capacity.
During decompression, which generally represents the longest phase of many technical dives, the objective is to carefully facilitate offgassing and safely return the diver to the surface.
By managing oxygen levels judiciously and carefully selecting the inert gases used during these various phases of a dive, dive safety and ability to perform at depth can be greatly increased. In order to appreciate how this can best be accomplished, it is important to understand oxygen tolerance and the properties of the metabolically inert gases used as a diluent.
Oxygen Tolerance
Though humans function optimally at oxygen partial pressures of about 0.2 atm, during the limited duration exposures encountered in diving, oxygen levels can be extended upwards. In fact it's beneficial to raise oxygen levels so because it displaces inert gas which is the source of decompression problems. What's more is that the use of extra oxygen is the key to effective decompression. Increasing oxygen levels during decompression not only speeds up the decompression but makes it more reliable as well. The problem is that too much oxygen can be toxic. As a result exposures to elevated partial pressures of oxygen must be managed within strict physiological tolerances.
Generally, at the elevated pressures encountered in diving it is relatively easy to maintain sufficient oxygen levels (above about 0.12-0.14 atm) necessary to avoid hypoxia. The more relevant issue is that above about 0.5 atm oxygen acts as a drug and induces a variety of physiological effects based on the dose (PO2) and duration of exposure. These effects have been classified into two general types or levels of oxygen toxicity.
Central Nervous System (CNS) toxicity (the "Paul Bert effect") develops from short exposures (from a few to many minutes) to PO2s above about 1.6 atm, and may result in an epilectic-like convulsion, which though is not necessarily dangerous in itself under normal circumstances can result in drowning or physical injury, for example an embolism, to the diver, particularly when using self-contained technology. Generally experience has shown that PO2s below about 1.4-1.5 atm are out of the CNS range.
Susceptibility to CNS toxicity is exasperated by many factors including anxiety, cold and especially factors that cause CO2 build-up such as exercise, breathe conservation, regulator breathing resistance and increased gas density. Conversely, an established technique for reducing or postponing CNS toxicity is the method of intermittent exposure. If "breaks"&endash; periods of lowered (normoxic) oxygen levels&endash; are taken during oxygen breathing, tolerance is greatly improved. This method has been demonstrated to avoid convulsions and to postpone other forms oxygen toxicity at high PO2s (reference??). This is the motivation behind the use of 'air breaks," the practice of breathing air for five minutes every 20-30 minutes of oxygen breathing in the US Navy tables for recompression treatment.
Fig. 5: Oxygen Limits Range Illustration
A second form of classic toxicity called pulmonary toxicity (the "Lorraine Smith effect")&endash; oxygen's effect on the lungs&endash; results from very prolonged exposures to PO2s above 0.5 atm (ranging from many hours to days) and is usually characterized by temporary lung damage including chest pain, coughing, fluid in the lungs, the inability to take a deep breath and a reduction in vital capacity and can eventually lead to death. In recent years these effects have been more generally grouped into a broader category of toxicity resulting from long term exposures to elevated oxygen pressures called "whole body" toxicity which in addition to lung problems includes paresthia (especially numbness in fingertips and toes), headache, dizziness, nausea, effects on the eyes and a reduction in aerobic capacity. For practical purposes these these effects are fully reversible once PO2s are reduced and leave no long term damage.
All of these forms of oxygen toxicity show highly variable effects on different individuals and even significant differences in the same individual at different times. That is the one reason why the US Navy's practice of requiring oxygen tolerance testing in divers is no longer viewed as particularly relevant or useful today in the technical community.
Oxygen Management Strategies
During diving operations there is an incentive to run oxygen levels as high as possible in order to maximize inert gas displacement subject to the constraints imposed by whole body and CNS toxicity tolerance. Historically several operational methods have been developed for managing these exposures in the field.
From an operational perspective, whole body toxicity resulting from prolonged exposures to PO2s above 0.5 atm is rarely an issue for surface-based technical diving operations. Most single day exposures, even for exploration dives, are well within tolerable whole body limits though toxicity can become an issue on extended multi-day missions4 . Conversely, managing whole body toxicity is critical during the lengthy exposures encountered in commercial and government saturation diving. In most of these cases, working oxygen levels are generally maintained at about 0.2- 0.4 atm except for short duration exposures and increased slightly during decompression. However there are many specialized diving applications that fall somewhere in between where more flexibility is needed.
Originally developed for habitat saturation-excursion diving, the Repex method (Hamilton R.W. 1989 Dec., Tolerating Exposure To High Oxygen Levels: Repex and Other Methods. Marine Tech Soc J 23(4): 19-25),
offers an empirically based approach for quantifying and managing
daily hyperoxic exposures (doses) over a mission duration of several days or longer. It picks a CNS "threshold" level of 1.5 atm and allows only short fixed exposures beyond this limit; all other dives are limited by "whole body" criteria which takes care of pulmonary problems.
The unique aspect of Repex is that it sets a daily exposure limit as a function of the total number of days of exposure, the "mission duration." For a single day mission the dose can be much higher than if the exposure is planned for many days. In addition, to some degree, provisions for recovery are implicitly built in because for most exposure levels the daily dose is built up in less than 24 hours, and thus there is some time for recovery.
The Repex dose is measured in "oxygen tolerance units" (OTU) which are the same size as the University of Pennsylvania's "UPTD," (unit pulmonary toxicity dose, or when cumulative, CPTD) and are accumulated over the course of an exposure. One OTU is equal to an exposure of about one minute to one atmosphere of oxygen, but is slightly more when above 1 atm, and slightly less when below. OTUs can be calculated as:
OTU = t x ((PO2- .5 atm)/.5).83
where t is the duration of the exposure in minutes. These are accumulated over the course of a dive and compared to a set of empirical daily average dose limits based on the number of days of exposure as shown in Figure 6. For a single exposure lasting only one day, the tolerable dose is about 850 OTUs. For multiday missions, the daily allowable dose drops from about 700 OTUs per day for a two day mission down to about 300-350 OTUs per day or less for missions of 10 days or more. Note that these limits are empirically based and intended as operational limits. At any time a diver should be able to tolerate a standard USN Table 6 treatment table (about 600 OTUs) with only mild lung irritation and perhaps other mild symptoms.
Figure 6: Repex Limits Graph
Avoiding CNS Toxicity
CNS toxicity is the more immediate and potentially life-threatening issue in technical diving operations, particularly in the absence of a full face mask which would prevent immediate drowning in the event of a seizure.
It is also an issue of some confusion.
Many people cite the US Navy's practice of beginning their oxygen decompression at 50 fsw (PO2=2.5 atm) during "mixed gas" operations, and the high oxygen levels used in recompression therapy (up to about 3 atm) as justification for running high PO2s during the working phase of a dive. Unfortunately they fail to take into account all the factors involved. Exposures to high PO2s in the "dry environment" of a helmet and diving bell (used in USN decompression diving) or in a chamber, with support at hand are very different than the situation encountered in self-contained diving both physiologically and risk-wise. What's more is that during the working portion of a dive CO2 levels are generally higher than when the diver is at rest, increasing sensitivity to convulsions. As a result, running working PO2s much above 1.6 atm (the air equivalent of about 220 fsw) is somewhat akin to playing Russian roulette; statistically there's a price to be paid. Fortunately, several methods have been developed to manage high PO2 exposures from an operational perspective though none address the specific considerations involved in technical diving applications.
Originally developed for it's (non-saturation) surface-supplied and closed circuit mixed gas operations, the classical USN oxygen partial pressure limits (normal exposures) shown in Table 2 below, have been widely reproduced and incorporated into many operational standards but have also been criticized as being too conservative5 and have other limitations as well. Though designed for managing CNS toxicity they go well below the level where CNS toxicity is likely. Conversely, the USN "exceptional exposure" limits are probably not conservative enough for general use and in any case were never intended for routine diving operations as specified in their definition. The USN limits represent operational decisions not research results and work is currently underway to make them more physiologically and operationally more realistic. (For a discussion see: Hamilton 1989 above or Kenyon D.J. and Hamilton R.W., Managing Oxygen Exposure When Preparing Decompression Tables, Proceedings European Undersea Biomedical Society 1989).
Table 2: USN Oxygen Exposure Limits
Drawing on more recent data and less conservative than the USN charts, a new set of limits were proposed for use in the Third Edition of the NOAA Diving Manual as shown in the table below. Developed primarily for "no-stop" and short duration dives characteristic of NOAA's diving operations through a "expert consensus" process, these limits are presented in a similar style to the USN, but in addition to single exposure CNS P02/time limits they include limits for a 24 hour day as well in an effort to take into account both CNS and whole body tolerances.
Table 3: NOAA Oxygen Exposure Limits TABLE
Widely circulated and used by both the NOAA and American Academy of Underwater Sciences (AAUS) in their oxygen-nitrogen ("nitrox") diving programs, the NOAA limits have been adopted in part by the fledgling technical diving community in the absence of other sanctioned operational methods, notably by the enriched air nitrox training agencies. However limitations inherent in the NOAA method have lead to some confusion and controversy as to how high to run working PO2 levels and whether the NOAA "oxygen toxicity clock" concept &endash; a fixed time limit for a single exposure level &endash; is adequate for managing technical diving exposures..
The controversy centers around whether NOAA's exposure limits for preventing CNS toxicity, though physiologically realistic, are operationally sound for technical diving exposures; specifically whether running a PO2 of 1.6 atm (the maximum exposure recommended by NOAA for up to 45 minutes) should be used routinely as a operational standard for special mix dives and for training. Some people argue that 1.6 atm is the "standard" set by NOAA for it's nitrox program and is also used others, though they recommend that this may be modified downward depending on the operation. What's more they argue that there haven't been any problems to date; that 1.6 atm is already conservative enough and that any change in the "standards" will cause confusion. Other groups advocate that a more conservative baseline PO2 of 1.2- 1.4 atm be utilized for working exposures. Who's right?
The real issue is that like those of the the USN, the NOAA exposure limits suffer from inflexibility and though easily applied to "no stop diving" appear not well suited for many of the types of exposures encountered in technical diving applications. Some of the specific limitations are these.
First, the NOAA method provides no means to account for the "multilevel" PO2 exposures typically encountered in technical diving, or for recovery, which can create a real accounting dilemma for technical divers even on fairly routine dives possibly putting a diver at risk. The problem is there is no way to accumulate and compare exposures at varying PO2s and compare them to a single limit.
Consider a dive to 220 fsw on air for 30 minutes. According to the NOAA limits (a single exposure to 1.6 atm for 45 minutes), the working PO2 of 1.6 atm leaves 15 minutes remaining on the "toxicity clock." So far so good. The working portion of the dive completed, the diver ascends to her first stop at 100 fsw stopping the clock (PO2=0.8 atm) and begins her decompression . At 70 fsw the diver switches to an intermediate decompression mix, EAN 50 (an oxygen-nitrogen mixture with 50 % oxygen, balance nitrogen, PO2=1.56 atm) starting the toxicity clock again, and spends the next 20 minutes decompressing at PO2s ranging from 1.56 to 1.1 atm as she ascends to 40 fsw accumulating "toxicity time." At 20 fsw the diver switches to pure O2 for a final 40 minute soak, PO2 = 1.6 atm taking a five minute " air break" after 25 minutes. A limits violation?
The rule of thumb that is sometimes used with the NOAA method is to ignore the decompression phase of the dive from a toxicity clock perspective. While this is probably valid for dives involving minimal decompressions typical of NOAA diving, it is probably not the case in many technical diving exposures involving lengthy decompressions. Even though the evidence is that PO2 exposures can be greatly extended during decompression when the diver is at rest ( cite Vann work ???) and that intermittency, taking periodic "air breaks" to lower PO2s, increases oxygen tolerance, the question remains, "How much time has the diver accumulated on her clock?" Field experience suggests that the cumulative effects of running PO2s at 1.6 atm within the NOAA time limits during the working portion of a dive followed by extended high PO2s during oxygen decompression can result in CNS toxicity problems during in-water stops (Private Memorandum from Capt. R. R. Pearson, Royal Navy, 1991).
A second problem is that the NOAA limits don't take into account diver work levels which effect CO2 build-up and can increase sensitivity to convulsions, for example, distinguishing between the working phase of a dive and decompression where the diver is generally at rest and can therefore tolerate higher PO2s. A PO2 level which can be reliably sustained for a "light dive" or for decompression might be dangerously high during the working portion of a dive involving heavy swimming or manipulating tools.
What's more is that from a performance perspective, the benefits of running PO2s in the CNS toxicity range may simply not be worth the risk. The fact is that for most technical diving exposures the added decompression benefits of running a PO2 of 1.6 atm during the working portion of the dive versus 1.4 atm, which is below the CNS range, is marginal as illustrated in the example below. This is discussed in more detail in Chapter 4.
Figure X: Decompression Advantage @ PO2 1.6 atm vs 1.4 atm
.
A final problem is that in the effort to combine CNS and whole body tolerances, the NOAA limits places unrealistic operational limitations on exposures to PO2 levels below the CNS range, considered to be less than about 1.4 -1.5 atm. The consensus seems to be that PO2s of about 1.4 atm or less are "sustainable" for the practical duration of surface based dives from a CNS toxicity avoidance perspective.
CNS Toxicity Avoidance: An Evolving Strategy
The idea of running at a sustainable or working PO2 level below the CNS range forms the basis for another simpler strategy that seems to be evolving within the technical community for managing high oxygen exposures and is used by many exploration divers for avoiding CNS convulsions. Similar to the strategy used in commercial saturation diving for avoiding whole body toxicity, the concept is simple and best summed up by the metaphor," CNS toxicity is like the sand beside the road. If you stay on the road you won't get in trouble."
In this approach the diver maintains a working PO2 of about 1.2-1.4 atm, below the range where CNS toxicity is likely, during the working phase of the dive where CO2 build-up is an issue. The specific PO2 that is used depends upon the specific operational requirements of the dive and the environment. For example in a cold water overhead environment where heavy workloads are planned, PO2s might be adjusted downward to 1.2-1.3 atm for an added safety margin; the prospects and consequences of a seizure in this environment being too high. Conversely, PO2 might be adjusted up to about 1.5 atm, still regarded as below the CNS range, for limited workload exposures or under specific operational circumstances. In general the 1.4-1.6 atm range is regarded as a "caution or buffer zone" during the working phase of a dive.
During the decompression phase of the dive PO2s are extended up to 1.6 atm (the equivalent of breathing pure oxygen at 20 fsw) in order to improve decompression, though these are sometimes boosted to as high as 1.9 atm (eg. O2 at 30 fsw) for limited durations on "big dives" involving heavy gas loading or under special operational circumstances, for example while using full face masks. This approach is illustrated graphically in Figure 8.
Figure 8: CNS Avoidance Strategy(Show technical and commercial)
This ability to extend oxygen levels, the key to effective decompression, takes advantage of the fact that CO2 levels are generally low during decompression when the diver is presumably at rest. In addition, planned intermittency i.e. taking five minute "airbreaks" every 20-30 minutes during oxygen breathing (ex. during 20 and 10 fsw stops) which greatly reduces sensitivity to convulsions, is usually feasible during decompression as opposed to the working phase of the dive. What's more is that actually maintaining a PO2 of 1.6 atm for the entire duration of decompression is rarely achieved in practice using open circuit scuba6 due to the limited number of gas decompression switches that are operationally feasible, though the resulting PO2s rarely fall far enough (less than 0.5 atm) on ascent between switches to contribute much to recovery. This is shown in Figure 9 below.
These factors are counterbalanced somewhat by the fact that long duration working runs at 1.4 atm burn up the body's enzymes that protect against oxygen toxicity, creating potential increased sensitivity to CNS toxicity when PO2s are raised within the CNS range during decompression. As a result, CNS toxicity is still an issue when using this approach particularly during the lengthy decompressions (4-8 hours) associated with today's exploration dives.
Figure 9: PO2 vs Gas Switches&endash; Typical Profile
One solution would be to develop a "standardized unit" and algorithm for managing CNS tolerance similar to that of the Repex method used for managing whole body tolerance. In order to be effective and reliable the algorithm would have to overcome the limitations discussed above in current CNS avoidance methods; specifically it would take into account multilevel exposures, differences in work levels (CO2 build-up) as well as recovery. Ultimately the real issue is the "level of statistical confidence" inherent in such a algorithm particularly in the light of the variability in individual oxygen tolerance. The problem is similar to that of developing a reliable decompression algorithm only there appears to be more uncertainty. An algorithm's success will depend on building on a good database of experience.
One can easily envision such an algorithm incorporated into the computational algorithms used for generating decompression schedules, and eventually with the advent of online O2 sensing, for example in conjunction with full face masks and manifold blocks (see Chapter 9), implemented as a realtime algorithm in a diver carried meter or DC (dive computer). Work is now underway at Duke University Medical Center and Hamilton Research Ltd. using maximum likelihood analysis techniques to develop such an algorithm. Meanwhile several manufacturers are in the process of incorporating online O2 sensing into their systems.
Sidebox: Oxygen Calculations
Given a breathing mix, M, and a working depth, D, the partial pressure of oxygen can be readily calculated using the common form of Dalton's gas equation:
PO2= P x FO2
where PO2 is the partial pressure of oxygen, P= D/33 + 1 is the total ambient pressure in atm. at working depth D and FO2 is the fraction or percentage of oxygen in the mix. For example, the PO2 of air at a working depth of 200 fsw can be calculated as:
PO2= (D/33 +1) x 0.209= (200/33 +1) x 0.209= 7.06 x 0.209= 1.46 atm
Conversely, given a desired PO2 at a working depth D, the required fraction of oxygen in a diver's breathing mix can be calculated using the formula above by substituting in the PO2 and depth and solving for FO2. For example, the fraction of oxygen, FO2, in a diving mix required to yield a working PO2 of 1.4 atm at a working depth of 300 fsw is given by:
1.4 atm= (300/33 +1) x FO2.
Solving for FO2 yields FO2= (1.4/(300/33 +1))= 1.4 atm/10.09 atm= .138 or about 14%.
The optimal FO2 for a gas mix based on fixed maximum PO2 is shown graphically in the figure below.
Fig. 10: Optimal O2 Vs Depth
Note that with open circuit scuba systems, the fraction of a component gas in a mix, for example oxygen, remains constant while its partial pressure pressure varies with depth. Conversely in closed circuit systems which blend a diver's breathing mix dynamically "on the fly," the partial pressure of oxygen, known as the "set point" is fixed at the beginning of a run, and the fraction of oxygen varies with depth.
Given a desired maximum oxygen partial pressure, the "maximum operating depth" or MOD, of any mix can also be calculated using Dalton's equation. The MOD of a mix is simply the maximum depth that the mix can be used and still maintain a safe working PO2. Substituting in the max PO2, and FO2 in the equation and rearranging terms we have:
MOD= ( max PO2 -1)x 33 fsw
FO2
For example the MOD of air at a maximum PO2 of 1.4 atm is 186 fsw. Note that all breathing mixes have a maximum operating depth from an oxygen perspective.
Diluent Gas Selection
Once the appropriate oxygen levels have been determined, the choice of an "optimal" gas to serve as a diluent in a divers breathing mix&endash; the type of breathing gas used&endash; is a matter of physiological constraints and economics and depends on the planned exposure. In addition to the issues of narcosis, decompression characteristics and gas density there are also thermal considerations, potential voice distortion&endash; a factor in underwater communications systems&endash; in some cases fire safety issues, and the cost and availability of the diluent gas itself. All of these considerations must be taken into account in selecting a diver's breathing mix.
Nitrogen and helium are the two basic diluents used in diving mixes today, though hydrogen is sometimes used for very deep saturation dives and some work has been done with neon as well and other specialized "add gases." The few other metabolically inert gases have properties that make them unsuitable as a diluent for diving applications.
Nitrox Mixes
Nitrogen, which is the primary component of air, is the most common diluent used for shallow water diving due to it's availability, low cost and broad base of experience. Indeed air, the most common oxygen-nitrogen mixture or nitrox, was the only gas readily available for diving until the introduction of helium in the late 1930s.
In practice when diving nitrox on open circuit scuba, the oxygen fraction or percentage in a mix is set according to the maximum working depth of the dive to yield an appropriate PO2. Conversely, given a specific mix, its maximum operating depth can be calculated based on its fraction of oxygen using the maximum operating depth (MOD) formula discussed above. As a result, nitrox mixes are generally grouped according to their oxygen fraction.
Normoxic mixes optimized for shallow water habitat and saturation work typically contain a lower percentage of oxygen than that found in air (21-23%) in order to maintain a working PO2 of about 0.2 -0.4 atm at depth and avoid whole body toxicity, though air is sometimes used in habitats from which excursions are made to deeper work sites. While air (specified as a nitrox mix with an FO2 ranging from 19-23%)7 will likely remain the most versatile form of nitrox, the most widely used special mixes are"enriched air nitrox, designated EANx , or simply "enriched air," or EAN , the mix of primary interest in self-contained diving and in recompression therapy with an oxygen fraction typically ranging from 30- 50%. In this terminology, "EAN X" specifies an enriched air mixture with X% oxygen, balance nitrogen, while "nitrox x" refers to a normoxic mix with x% oxygen. Though in U.S. commercial and government circles the inert gas fraction in a diving mix is usually specified first, we prefer the European convention of specifying the oxygen fraction first followed by that of inert gas when more than one is used (see trimix specification below) as oxygen awareness and safety is a key issue in most technical diving operations.
Enriched air nitrox is rapidly becoming the gas of choice for diving in the 60-130 fsw range. By replacing some of the nitrogen fraction found in air with oxygen, decompression requirements using enriched air can be greatly reduced in comparison to air dives of the same depth and duration. What's more is that enriched air diving is no more difficult operationally than diving air and the decompressions can be easily calculated from air tables using an "equivalent air depth," or EAD formula (see chapter 4) though specially generated tables or EAN-compatible dive computers are probably a better choice for most applications. In addition, enriched air mixes can be easily produced using atmospheric air and pure oxygen though there are fire safety issues involved in the blending process. Beyond about 100-130 fsw fsw, the decompression benefits of enriched air are marginal and offer minimal advantage in comparison to air limiting their real usefulness from about 40 to 100 fsw. From a cost and decompression perspective, air seems to be the optimal in about the 130-190 fsw range and offers a lot of versatility.
Sidebox: The EAN Experience
Though at the time of this writing, the current installed base of EAN users and supporting infrastructure is still in an embryonic stage representing roughly 40-50 facilities and about 4000-5000 certified users worldwide, enriched air technology is now poised to gain broad market acceptance and enter a substantial growth phase. Enriched air is unique in this aspect as it has potential applications in a wide range of diving from recreational to professional giving it broad market appeal.
A survey conducted in January 1992 in conjunction with the Enriched Air Workshop, Houston Texas, organized by aquaCorps and others identified approximately 28,000 enriched air dives or tank fills (some tanks of enriched air were used for other types of diving) conducted over the last five years. Most of these represented organized users who practice consistent procedures, use some sort of record-keeping, and have a special concern for safety in order to protect their operations as well as their divers. Included in the survey are about 5,000 dives performed by scientific diving organizations.
Though the results have no statistical significance, approximately four incidences of DCS were associated with this sample; one reported incident of DCS and unspecified symptoms of oxygen toxicity in the sport community and three DCS incidences involving scientific organizations (2 cases in 3200 dives, one case in 1100 dives). Interestingly, several respondents who did not report DCS or oxygen symptoms in their own activities said they knew of them occurring to others. In a rough comparison, the incidence of DCS appears to be less than would be expected from a similar set of straight air dives. No problems were reported with mixing and handling the gases.
Today, enriched air nitrox is used primarily as a bottom mix for shallow water diving by both advanced recreational and technical diving users (see discussion below), and is emerging as a standard intermediate or decompression gas for deep special mix dives. EAN 32 & 36 mixes (32 & 36% oxygen respectively, balance nitrogen ), sometimes referred to as NOAA Nitrox I& II, are probably the most common mixtures utilized along with EAN 50 for use during decompression. In addition there is a growing interest in EAN 40 (MOD= 80 fsw @ a PO2 of 1.4 atm) for sport diving.
The key pacing items that will determine the growth of EAN over the next few years are supply availability influenced by the development of defensible national pumping and quality assurance standards, improved standardized training courses and materials and applicable decompression management tools, particularly EAN-compatible DCs (dive computers).
Though nitrox mixes offer the advantages of availability, easily calculated decompressions, mixing ease and low cost, their primary limitation is that nitrogen acts as a narcotic at partial pressures above about 2-3 atm, the equivalent of breathing air beyond about 100 fsw, a consequence of it's high lipid solubility. What's more is that nitrogen is believed to interact with high levels of both carbon dioxide and oxygen to increase susceptibility and intensity of the narcosis. Though narcotic impairment is not necessarily an issue in a benign environment and in certain situations is even considered socially acceptable, the prospects of being of being impaired while diving, like piloting a plane while drunk, are at best hazardous.
While nitrogen pressures as high as 15 atm (equivalent to about 460 fsw on air)&endash; enough to cause severe narcosis, nausea and stupefaction&endash; have been found to be not incompatible with survival (Hamilton reference) operationally nitrox mixtures including air are pretty well limited to a depth of about 200 fsw from a safety perspective. This assumes the diver has had sufficient repeated exposures and experience to assure accommodation to the narcosis and managing the operational requirements of the dive. However the air range is sometimes extended as far as 250 fsw or more for limited duration dives with adequate surface support though these exposures are generally considered dangerous and unproductive.
Sidebox: The Slowed Processing Model of Narcosis
Breathing hyperbaric air causes a syndrome of behavioral and subjective effects known as nitrogen narcosis which limits the work efficiency of divers and is ultimately life threatening. The classic view of the progressive effects of narcosis based on subjective descriptions can be summarized as follows:
Range (fsw of air/PN2) Symptoms
About 100 fsw (PN2= 3 atm) Mild euphoria, delayed responses.
About 165 fsw (PN2 = 4-5 atm) Sleepiness, hallucinations, impaired
judgement; laughter and loquacity
may be overcome by self control.
About 230 fsw (PN2= 6-7 atm) Convivial group atmosphere, severe impairment of intellectual
performance, uncontrolled laughter
or fear reaction in some.
About 300 fsw (PN2 = 8 plus atm) Stupefaction, mental abnormalities,
euphoria, almost total loss of
intellectual faculties.
Fairly recently, a theory called the"Slowed Processing Model of Narcosis" (cite references) has been developed to explain the effects of nitrogen narcosis on divers which suggests that prior to unconsciousness, the primary effect of narcosis on performance arises from a decrease in arousal which slows response but does not cause perceptual distortion in either vision nor audition. Though it does not explain the observed amnesic effects of nitrogen narcosis i.e. the apparent "forgetfulness" that accompanies high nitrogen partial pressures, the model offers a number of sound training principles for divers who must perform while narcotic. These can be summarized as follows:
• Disorganized behavior is not necessarily part of narcosis and can be overcome by training. Errors can be avoided by slowing down.
• Divers should relay on memory as little as possible when memory must be relied on. Relevant material should be highly overlearned and memory clues used to minimize forgetfulness.
• Divers must become familiar and comfortable with the sensations of narcosis and learn to allocate attention between tasks and the symptoms of narcosis in a manner appropriate to the situation. Divers can learn to use intensity and the types of symptoms to estimate performance capability.
Based on experience, it is generally agreed by divers that frequent exposure to narcosis leads to adaption, however the type and extent of this adaption is not clear. There is some evidence of adaption that is specific to narcosis, that is that over successive exposures performance while narcotic improves at a greater rate than a surface control, known as "true adaption." On the other hand, this kind of adaption has not been found in some experiments where the improvement in performance was identical for narcosis and the surface control. This is the case of non-specific learning. The differences in these two forms of adaption are shown in Figure 11 below.
Figure 11: Narcosis Adaption
At present though there is not enough information to determine exactly how adaption applies to narcosis an additional training principle in dealing with adaption can be summarized as follows:
• Divers should practice as much as possible prior to the dive on tasks to be performed underwater in order to maximize effectiveness.
Though there will likely always be circumstances where divers must deal with narcosis, with the technology available today, the focus of dive operations should be to eliminate or reduce it to acceptable limits rather than trying to simply tough it out. Braving narcosis may have been "in" during the macho era of the seventies and eighties but to the 'thinking diver' of the 90's it is definitely considered uncool.
In addition to narcosis, field experience has shown that nitrogen can be difficult to decompression from on long dives in comparison to "lighter," quicker diffusing diluents such as helium. This is one reason that decompressions from very long air dives beyond about 60-80 fsw are generally recognized as unreliable when using USN tables even when decompression times are doubled or tripled (see "Safer Than Air?", John Crea, aquaCorps Journal Vol.3, No.1, "MIX," 1992). Interesting enough this "slow" diffusibilty of nitrogen can sometimes be used to advantage during decompression making nitrox including both air and enriched air extremely useful as intermediate gases during decompression as discussed below.
Nitrogen is also a relatively "heavy" gas compared to helium with seven times the density, effecting a divers ability to ventilate (remove CO2) limiting work capability at depth. It was these considerations which lead to the development of helium for use in deep diving.
Helium Mixes
C.J. Cooke applied for the first patent on the use of oxygen-helium mixtures or heliox as a breathing gas in 1919, but apparently never put it to use. Credit for the development of heliox mixtures should probably go to the eminent chemist Dr. Joel H. Hildenbrand, who with R.R. Sayers and W.P. Yant of the US Bureau of Mines, first tried helium decompressions with small animals on the basis of it's low density and solubility. They reasoned that these properties would improve decompression time in comparison with nitrogen. Though their results were encouraging, it was not until a 26 year old intern at the Milwaukee County General Hospital Edgar End and diver Max Nohl actually dived a heliox mix to 420 fsw that it's principle advantage was discovered&endash;the absence of narcosis&endash; a consequence of its low lipid solubility. Shortly thereafter the U.S. Naval Experimental Diving Unit, led by Momsen, Behnke and Yarborough, began to explore the use of helium , and in 1939 used it successfully during the salvage of the USS Squalus (243 fsw). For the next 20 years, the US Navy was the only user of oxygen-helium mixtures (the US having only readily available sources of helium) until it's application to commercial diving in the early sixties.
Today heliox is the fundamental diving gas used for virtually all commercial and military diving operations beyond 190 fsw as a result of it's lack of narcosis and other advantages. Helium is light and easy to breathe thus reducing the work of breathing. In addition, it's high diffusibility offers decompression advantages in comparison to nitrogen.
Though used extensively in deep surface-supplied and closed circuit operations, until recently, the sport and professional self-contained diving community has had little experience with helium diving. As a result there is some confusion and lack of information regarding the use of helium in self-contained diving applications. In order to appreciate its applicability and use, it is helpful to discuss some of the misconceptions associated with helium mixes and review some of its advantages and limitations in comparison to nitrox.
Helium Decompression Characteristics
One area of confusion concerns the decompression characteristics of helium mixes in comparison to nitrox and the generally held belief that the use of helium results in excessive decompression times. While it is true that nitrox seems to require shorter decompressions for certain classes of dives, the results depend on the specific exposures considered and the type of decompression procedures used. Of course helium decompressions have their own problems and it's generally recognized that the use of oxygen is needed to produce reliable decompressions, though the same can be said of long nitrox dives8 .
In a saturation exposure, nitrogen requires nearly three times as long to eliminate than helium on the basis of its slower diffusivity, a fact reflected in nitrogen's "compartment half-times," the basic accounting scheme used to track gas loading/unloading in decompression algorithms, that are 2.65 times as long as those of helium. In addition, studies (cite reference) have shown that human tissue can tolerate a "higher overpressurization" of helium in comparison to nitrogen and that statistically, helium dives tend to produce fewer "CNS (central nervous system) bends" than nitrogen versus the "pain only" variety. For these reasons helium is often considered "superior" to nitrogen as a diluent. However because of helium's relatively faster uptake and offloading its decompression time advantage is usually lost in most surface-based diving operations.
Generally for dives up to about 80-120 minutes, nitrox mixtures require less decompression and shallower decompression stops than heliox though the transition or "crossover point" depends upon the depth of the dive and the decompression procedures used. This crossover phenomena is primarily the result of the way in which decompressions are calculated&endash; how the arithmetic works&endash; versus what is actually happening in the body9 . The concept is illustrated in Figure 12 below.
Figure 12: Cross-over HE Vs N2 Decompression
Figures 13. and 14. compare total decompression time for dives to 100 and 300 fsw over a range of bottom times using an "appropriate" heliox and nitrox mix from an oxygen perspective i.e. both mixes are calculated to provide a working PO2 of 1.45 atm at the planned working depth (calculating an appropriate mix is discussed below). In both of these examples it is assumed that oxygen levels are boosted to 1.6 atm at the first required decompression stop and that pure oxygen is breathed during the 20 and 10 fsw stops.
Figure 13: N2-O2 Vs He-O2 100 fsw
Figure 14: N2-O2 Vs He-O2 300 fsw
Note that in the case of the dive to 100 fsw, heliox decompression times are significantly greater than those of nitrox, though the relative difference tends to diminish for longer bottom times. In the case of the dive to 300 fsw, the heliox mixture, in this case a heliox 14 (14% oxygen, 86% helium)10 actually results in a shorter decompressions than nitrox 14 for bottom times greater than x minutes and this advantage increases with increased bottom time. Note that while nitrox 14, perhaps better named "nartrox 14" may be "appropriate" from an oxygen perspective in comparison to air which has PO2 of 2.1 atm at 300 fsw, its increased nitrogen content (86% versus 79% for air) would make it highly unsuitable as a diving gas from an narcosis perspective yielding an "air equivalent" of 342 fsw (see calculations below).
Gas Switches: An Opportunity For Accelerated Decompression
Operationally, these differences in decompression times are not as pronounced as shown above as they do not take into account the effect of accelerated decompression methods. This is another area where helium mixes offer a distinct advantage over nitrox; the potential to accelerate offgassing. To understand this advantage it's helpful to briefing discuss the effects of "switching" the inert gas in a divers breathing mix during the decompression phase of a dive.
It was once suggested on the basis of the "independent" partial pressures of gases in a mixture that a "porridge" of gases could be prepared which would greatly reduce or eliminate the need for decompression. For example, if seven different inert gases were used on a dive to 200 fsw (a total pressure of 7 atm), then the partial pressure of each gas would be less than one atmosphere and consequently no "supersaturation" would occur, eliminating the need for decompression. In practice this strategy does not work; inert gas partial pressures are additive with respect to their tendency to form a gas phase ("bubbles"). However experience has shown that strategically changing the gases breathed during decompression can substantially improve decompression times (Workman, 1969; Buhlman, 1969, 1983, 1987). First pioneered by Hans Keller in the early sixties, this technique has been incorporated into diving practices.
Today the standard practice in deep commercial and military heliox diving is to switch to air as a breathing mix during the intermediate stages of decompression (generally at decompression stops less than about 220 fsw) before eventually switching to pure oxygen to complete the decompression. The effect is to accelerate helium offgassing by maximizing its offgassing gradient since there is "no helium" in the intermediate mix being breathed.
What's more is that since nitrogen diffuses much slower than the helium it is displacing (by a factor of about 3 x) additional inert gas loading is slowed during the decompression phase of the dive which can be a significant factor on deep dives with corresponding lengthy deep decompression stops. The net effect is to accelerate the overall decompression.
Note that there is no corresponding "decompression trick" when using nitrox, as suitable"slower" diffusing inert gases are generally non-existent. Though argon been used experimentally it's high fat solubility makes it about twice as narcotic as nitrogen limiting its useful range. What's more is that it diffuses at a rate of only about 1.4 x times slower than nitrogen and consequently offers no real corresponding advantage in comparison to helium diving. Similarly neon which is non-narcotic diffuses at about the same rate as nitrogen resulting in no real net advantage.
The emerging technical diving community has taken this method one step further by switching to a series of one or more enriched air mixes sometimes including air, known as intermediate mixes, during helium decompressions, prior to breathing pure oxygen at 20 and 10 fsw (see chapter 4 and discussion below). In addition to facilitating helium offgassing this method serves to boost oxygen levels, the key to effective decompression. The result of these techniques is to greatly reduce the decompression differences between helium mixes and nitrox over the relevant technical diving ranges. Figure 15. and 16. below shows the effects of applying these decompression techniques on total decompression times to the dives discussed above. As can be seen, by applying these techniques, the differences in decompression times between heliox and nitrox are greatly reduced. Note that gas switches are also used sometimes during the descent or "travel" portion of a dive to minimize gas loading on descent. This technique is discussed further below.
Figure 15 &16: N2-O2 Vs He-O2 Decompression w/ Gas switches
(100-300 fsw)
Thermal Conductivity
Another common misconception is that the use of helium mixes result in excessive diver heat loss. This belief arises from the fact that helium has six times the ability to conduct heat than compressed air. Used improperly, helium mixes can push a diver into hypothermia in quick order, but not in the way that many people think. It's because of this misconception that many people believe that nitrogen mixes are warmer. A brief discussion of diver heat loss is needed to clear up this confusion.
In the underwater environment heat is drawn away from the diver by two primary means; conductive and convective heat transfer, though it has not yet been determined exactly how the actions of these two physical principles are divided.
For surface-based technical dives in the 0-600 fsw range, the primary source of diver heat loss seems to be conductive or radiant heat transfer. Conductive heat transfer is a function of the conductivity of the medium surrounding an object and the object's surface area. For the diver, conductive heat loss occurs through the skin; body heat is 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.
When diving with 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. Helium mixes can cause rapid heat loss when used as a suit inflation gas or in a saturation environment.
During saturation dives in which the divers live in a high pressure heliox environment, the ambient temperature in the bell and chamber system must be heated and maintained within a narrow range of typically plus or minus 2-5 °F11 depending on the depth, to avoid diver hypothermia on the downside and hyperthermia on the high side. For surface-based technical dives the solution is much simpler.When diving a helium mix, divers should use an auxiliary suit inflation gas such as air or preferably one with a lower heat conductive capacity such as argon with a heat transfer capacity of about 50% that of compressed air. Though carbon dioxide also has a low heat conductive capacity it can cause skin irritation when moisture12 is present making argon a better choice. Special gases such as sulfur hexaflouride having 4-5 the conductive capacity of air are also sometimes used.
Sidebox: Suit Gases
Developed in response to the need for an alternative drysuit inflation gas when diving helium mixes, the use of argon as a suit gas is growing in use. For good reason. With a heat transfer capacity of about 50% of that of compressed air, argon provides an important thermal protection advantage to the drysuit diver that has been described by one tek'nophile as "the equivalent of donning a warm sweater."
In practice, argon can be used to reduce as much as about 40% of the thermal undergarment requirements in warm water and or can be used as an addition to regular undergarments to provide a thermal margin on coldwater dives. Argon is typically carried in a small inflation bottle (5-8 cf), with regulator and inflation hose, mounted to a diver's backplate or tank or carried in a special drysuit pocket designed for that purpose, though 13 cf pony bottles are sometimes used. A typical 200-300 fsw dive with bottom times up to 30 minutes requires about 2-3 cf of inflation gas assuming the suit is not used for buoyancy.
Some divers flush the air from their suit with a special valve before connecting the suit inflation system for added protection. Still others stage or carry a small pony filled with argon and begin their argon soak during decompression when hypothermia can become a problem, particularly on extended hangs. Argon can be readily obtained from any welding supply facility and downloaded into an inflation bottle using a special adapter and fill hose and or a booster pump. The inflation bottle should be clearly marked "Inert Gas" to avoid the situation where the bottle might be inadvertently mistaken for a breathing gas. Eventually special inert gas scuba connectors will become the standard.
The second form of heat transfer is convective and is typically a minor component of diver heat loss except at depths beyond about 400-600 fsw often achieved in surface-supplied diving. That's the reason breathing gases are heated in deep commercial diving operations. In open circuit scuba convective heat loss occurs as a result of normal respiration since each breath that is expelled into the surrounding water carries with it heat from the body. However the heat removed from the respiratory tract is primarily a function of the thermal capacity of a breathing gas, the amount of heat that can be carried out of the lungs13, rather than its heat conductivity. The thermal capacity of a gas is a function of it's density.
A good case can be made that because of the greatly reduced density and consequently lower thermal capacity of helium, convective heat losses for helium mixtures are probably less that that for air or other nitrox mixtures at any given depth i.e. because nitrox has a greater density, it will remove more heat in each breath when equilibriated though it takes longer to reach equilibrium. In practice, heliox sometimes feels cooler to breathe than air (a phenomena possibly due to radiant heat loss at the surface of the lungs) and some divers report it makes them colder. However, on a physiological basis its reputed chilling effects are greatly overstated. Nitrogen appears to offer no superior thermal characteristics over helium.
The Trimix Strategy
Given that helium offers the relative advantages of the complete lack of narcosis, reduced gas density, manageable decompressions and the lack of thermal disadvantages, why isn't heliox the gas of choice for long and deep open circuit dives? Though there are increased operational requirements to consider in comparison to nitrox mixes, the bottomline can be summarized in one word: cost.
Due to the inherent inefficiencies in open-circuit scuba systems14 , heliox is an expensive gas for most technical diving applications. Figure 17. shows helium costs for a 60 minute dive on heliox over a range of depths assuming a surface consumption rate of one cubic foot per minute and a one third gas reserve. Note helium costs are assumed to be about $50 per K-cylinder or about $.15 cf and does not include the costs of mixing and analysis which can add as much as 100% to the overall cost. Also note that the cost of decompression gas, i.e. air, enriched air and/or oxygen are not considered. As shown in the diagram, the cost of heliox ranges from about $80-$205 per hour for helium alone for dives in the 200-500 fsw range being broached by today's explorers, an expensive proposition.
Figure 17. Helium Costs Per Hour Vs Depth
The solution to this cost dilemma has been the development and use of trimix, or oxygen-helium-nitrogen mixes, for open-circuit deep dives beyond about 200 fsw. First developed in commercial circles in the the mid to late sixties and later utilized on an experimental basis by various military groups in the late seventies, trimix has become the "darling of the high tech community" since it's introduction to sport diving in about 1987. For good reason. Trimix seems to combine some of the best features of both nitrox and heliox diving and offers significant cost savings over heliox.
The basic idea behind trimix is simple and can best be understood by considering the limitations of air (nitrox) for deep dives. As discussed above the problem with deep air diving is twofold. First, the oxygen partial pressures found in air become excessive beyond about 190-220 fsw (PO2 = 1.4-1.6 atm). In addition, the nitrogen narcosis becomes increasingly hazardous with depth and gas density which can impair diver performance increases. The idea behind trimix is to first insure that oxygen levels are set (generally reduced) to provide a safe working PO2, and to reduce narcosis to manageable levels, and reduce gas density, by substituting helium, a non-narcotic, for a portion of the nitrogen. If a lot of helium is good then some must be better than none at all. The question becomes, how much narcosis can you afford to eliminate?
Sidebox: C2 Technology
The economics of helium use are highly dependent on the type of breathing system used. Because a divers expired gas is simply blown out into the water open circuit scuba is very inefficient from a gas utilization perspective. When using closed circuit (C2) technology (see Chapter 9 for a discussion) the economics look very different.
Closed circuit systems utilize a single inert gas diluent which is blended dynamically with oxygen in order to maintain a predetermined constant PO2, known as the "set point." Helium is the principal diluent used for deep diving systems. When using closed circuit, helium loss occurs when the counterlung which is maintained at ambient pressure, vents excess gas when the diver ascends. If the dive profile is roughly monotonic (square wave), i.e. there are few if any repetitve descents and ascents prior to surfacing, then the total volume of diluent gas used is approximately that required to maintain the counterlung volume at the maximum depth. Given that most counterlungs have a capacity of about seven liters, the diluent gas used up during a dive is roughly seven times the maximum depth in pressure units and is independent of bottom time.
Factoring in the cost of helium, the closed circuit operating costs can be easily calculated as a function of depth and are displayed in Figure 18. As shown in the figure, gas costs are inconsequential for closed circuit systems which offer roughly several hundred times the efficiency of open circuit scuba (i.e. an operating cost of roughly about $.20-.50 per hour versus the $80-$205 cost per hour for open circuit scuba).
Figure 18
This observation explains why the next generation of closed circuit systems, expected to be priced in the $10-15,000 range, offer a cost effective alternative to open circuit scuba for diving in the 200-500 fsw range. This can be easily seen from a breakeven analysis based on gas costs savings.
A simple method for estimating the cost of open circuit system diving is to estimate the number of cylinders required for a 60 minute open circuit dive to depth D and multiply by the "fully-loaded" cost per cylinder. The results can then be compared to the cost of a closed circuit system. This difference in hardware costs can then be divided by the savings in gas costs for a closed circuit dive and a "breakeven" number of dive hours can be calculated as shown below.
Open Circuit Hardware Costs= (SCR x 60 min./hr) x (D/33 + 1) x $500
80 cf
Where SCR= the surface gas consumption rate (= 1 cf/min). D is the working depth. We assume 80 cf cylinders are used as a reference at a "fully-loaded" cost (regulator, pressure guage and supporting hardware) of $500.
For a 60 minute dive to 300 fsw, the open circuit hardware cost is $3750. Assuming heliox 14 (14% oxygen balance helium) is used (see cost discussion in main text) and the cost of a closed circuit system is $10,000, the "breakeven" point is given by:
($10,000- $3750) = 36 one hour dives to 300 fsw
($172- $.34)
where $172 is the open circuit helium cost of a 60 minute dive to 300 fsw using heliox 14, $.34 is helium cost for a closed circuit system. The closed circuit system pays for itself in 36 one hour dives.
A more general form of breakeven analysis is given in Figure 19 which shows the number of one hour dives needed to breakeven as a function of working depth.
Figure 19
These calculations indicate that technical divers who expect to be diving regularly to depths beyond 200 fsw would do well to consider C2 technology. Other significant benefits of closed circuit technology includes range-enhancement and safety (due to its virtually unlimited gas supply), compactness (compared to the bulk of multiple cylinders), near optimal decompression and silence.
In summary, the issue of heliox economics is largely a matter of the inefficiency of open circuit scuba. Eventually as closed circuit technology becomes widely available, heliox will become the gas of choice for deep diving operations.
Equivalent Narcotic Depth (END)
Given a mixture of oxygen-helium-nitrogen, denoted trimix "x/y," where "x" denotes the oxygen fraction, "y" the fraction of helium, the balance of the mix being nitrogen, it's narcotic level at any depth can be simply calculated by determining the relative amount of nitrogen in the mix, the source of narcosis, compared to the nitrogen found in air. This "equivalent narcotic depth," or END, of a mix, measured in feet of seawater, can be calculated as follows:
END = ( (1-FO2-FHe ) x (D/33 + 1) ) x 33 - 33
.79
relative FN2 working conversion
in the mix depth(atm) to fsw
where 1- FO2-FHe (=FN2) is the fraction of nitrogen in the mix, 0.79 being the nitrogen fraction of air, and D is the working depth in feet of sea water. For example, at a working depth of 300 fsw, the equivalent narcotic depth of a trimix 14/50 (14% O2, 50% He, balance N2) is 118 fsw i.e. the narcotic equivalent of breathing air at 118 fsw. Conversely, given a planned working depth D, the narcotic level of a trimix can be set to a desired level ranging from "no narcosis" to "D fsw" by adjusting the amount of the helium in the mix. The cost advantages can be substantial.
Figure 20. shows helium costs per hour of diving at a working depth of 300 fsw utilizing trimix where the helium fraction, FHe, is allowed to vary from 0.0% (pure nitrox) to 86% (pure heliox), oxygen levels being set at 14% to yield a working PO2 of 1.4 atm. Costs can be seen to range from $0 to $117 per hour according to the formula:
Helium Cost/hour (trimix) =
(FHe x $.15/cf He x SCR cf/min. x 60 min./hr. x (D/33 + 1))/.66
where FHe is the helium fraction in the mix, $.15 is the cost of helium per cubic foot, SCR=1, is the surface consumption rate in cf per minute, D=300 fsw is the working depth and .66 is the reserve factor to reflect a one third gas reserve.
Figure 20. Helium Costs per Hr. (Trimix) @ 300 fsw
The Economics of Narcosis
Given a working depth of 300 fsw, an FO2 of 14% and using the equivalent narcotic depth formula above, the helium fraction, FHe, can be rewritten as a function of END. Substituting this into the cost equation, the helium cost per hour of diving trimix can be calculated as a function of the desired equivalent narcotic depth. The results are shown in Figure 21. Note that an END of "0" is equivalent to breathing air at the surface while an END of "-33" fsw indicates that there is no nitrogen, and hence no narcosis in the mix at all.
In practice, END's are typically set in the 75 to 150 fsw range depending on the application though they are sometimes extended as high as 200 fsw or more for specific operations. Assuming an END of 150 fsw for the dive to 300 fsw shown below, trimix helium costs would be $56 per hour, a significant savings over the cost of $117 for pure heliox.
Figure 21: Helium Costs/Hr (Trimix) Vs END
From a decompression perspective, trimix bridges the gap between nitrox and heliox, yielding decompression times generally falling somewhere in between resulting in shorter decompressions than heliox on short deep dives (20- 80 minutes bottom times beyond 200 fsw) typical of self-contained open circuit diving15 . In addition, because of its helium content, trimix offers the opportunity for accelerated decompressions through the use gas switches to air and or enriched air mixes on ascent.
Figure 22. shows total decompression times for a series of trimixes for a 20 and 80 minute dive to 300 feet ranging from 0.0% helium (pure nitrox) to 86.0% helium (pure heliox) with the oxygen fraction set at an optimal 14%, utilizing appropriate decompression methods16 . Note that decompression times increase over nitrox on the 20 minute dive once helium is added to the mix but remain roughly constant as the helium content is increased until the nitrogen is completely eliminated i.e. decompression is roughly independent of the helium content over the range of about 20-60%. Conversely, the addition of helium reduces decompression times for the 80 minute dive in comparison to nitrox, FHe = 0.0%, a consequence of helium's rapid diffusibility as discussed above. Helium offers an increasing decompression advantage as bottomtimes increase.
Figure 22: Decompression Times For Trimix (300 fsw)
In addition to cost and decompression advantages over heliox on short dives, trimix by virtue of its nitrogen content (heavier density) creates less voice distortion when using underwater communications systems which are growing in use in technical circles. As a result of these advantages, trimix is rapidly becoming the emerging standard for open circuit deep diving in the technical community, its growth being limited by available training, supply and to some degree cost. Though mix diving generally represents more work and expense in comparison to air, the reality is that it's the only way to safely conduct deep diving operations. Deep diving costs one way or the other and there is simply no way to get around it.
Sidebox: Trimix Today
Approximately 300-400 open circuit trimix dives ranging in depth from about 150-870 fsw have been conducted by explorers from the cave, wreck, and scientific diving communities at the time of preparing this manuscript. The majority of these dives were conducted in the 200-300 fsw range with bottom times up to about 45-60 minutes. Approximately 20-30 dives have been conducted in the 300-500 fsw range and only a handful in excess of 500 fsw conducted by Sheck Exley, Live Oak, FL and Jochen Hassenmayer, Switzerland.
Hamilton Research Ltd. has been responsible for preparing the gas plans and decompression procedures for the majority of these dives using the DCAP computational algorithm, though schedules from Submariner Research Ltd., Underwater Applications Corp. and other sources have also been used as well. To date the experience has been good. In the four reported DCS-related incidents, two were "suspected" DCS and successfully treated with surface oxygen following the dive. One treated incident was due to operational error&endash; the diver had ran out of gas prematurely due to equipment unfamiliarity and was forced to surface omitting the required decompression. A final incident appears to have involved minor pain-only symptoms and the diver refused recompression treatment. One fatality during a trimix dive occured as a result of a freak flow reversal in a water filled cave causing the exit to be blocked.
From a technology perspective the focus of most of these pioneering dives has been to develop reliable operational procedures including gas mixes, reliable decompression methods, staging, and adequate diver support. Two groups, lead by Cpt. Billy Deans of Key West Diver Inc., Key West, FL and Jim King of Deep Breathing Systems, Sevierville, TN have been largely responsible for this effort drawing on the earlier work of the Wakulla Project conceived and led by Dr. Bill Stone (cite reference), and the Sullivan Connection conducted by Parker Turner, Bill Gavin and Bill Main. Key West Diver has developed many of the field methods and procedures for open water operations and has served as a training center for the majority of other technical diving operators and users in the US and abroad. In parallel, Deep Breathing Systems has been responsible for developing operational procedures and equipment for deeper exploration work. In addition, with the help of DCIEM, Toronto, Canada, Deep Breathing Systems has spearheaded the testing and validation of the decompression methods and tables generated by the DCAP algorithm using Doppler monitoring and blood tests to measure decompression stress. As a result, an emerging set of community operational guidelines has been developed with some success.
Today, most dives conducted in the 180-300 fsw range have standardized around the use of trimix 14-17/x, (14-17% O2 depending on the depth) with ENDs ranging from 85-175 fsw (FHe= 25-60%) depending on the exposure and application. Though "lean" trimixes with a helium content of 25% or less (an END of 175 fsw at 250 fsw), sometimes referred to as "poorman's mix" on the basis of their relatively low cost and mixing ease, are sometimes used to "take the edge off narcosis," the general thinking today is to eliminate as much narcosis as economically feasible. As a result mixes with at least 50% helium (an END of 85-118 fsw at depths of 250-300 fsw) are becoming the community standard. Decompressions for these operations utilize a single enriched air intermediate mix, typically an EAN 32-36, which is breathed beginning at the first or early decompression stops, typically in the 110-130 fsw range, followed by oxygen breathing at the 10 and 20 fsw stops. Based on the good success with these procedures, a provisional set of standardized decompression schedules, known as the Key West Consortium Tables, have been prepared by Hamilton Research Ltd for dives to 250 fsw to assist in standardizing operations and managing access to tables and have been adopted by the majority of technical diving operators. Schedules utilizing air as an intermediate gas have also been developed and used with success where access to sufficient onboard enriched air supplies is a problem.
For deeper dives in the 300-500 fsw, ENDs are generally set at 150 fsw or less and decompression procedures typically involve air as an intermediate gas beginning as deep as 220-230 fsw, followed by two enriched air intermediate mixes, for example, an EAN 30 for stops beginning at 150 fsw with a switch to EAN 46-50 at the 80-70 fsw stops followed by oxygen at 20-30 fsw. Doppler testing following these dives has suggested the use of air as a deep intermediate gas reduces decompression stress significantly.
In addition, in response to the desire to maximize dive time at remote sites, for example offshore while conducting wreck diving operations, repetitive trimix tables have been developed by Hamilton Research Ltd. for dives to 250 fsw. Though the potential for being able to utilize repetitive diving seems promising and will probably grow as trimix use becomes more wide spread within the community, these tables have received minor usage to date. Work has also been done by Submariner Research, Underwater Applications Corp. and others on altitude tables for specific applications.
Presently there are about a dozen or more operators/user groups regularly conducting trimix diving operations and offering limited training. Over the next five years, trimix diving will become more accessible through and will replace the use of air for extended open circuit deep diving operations as a result of its safety advantages
Exotic Mixes
Interesting enough trimixes offer a solution to another problem encountered in oxygen-helium (heliox) diving; the effects of rapid increases of hydrostatic pressure to about 600-800 fsw, referred to as High Pressure Nervous Syndrome, or HPNS, usually characterized by tremors, nauseousness, disorientation, derangement and in some cases major brain dysfunction. Symptoms and susceptibility vary widely among individuals. HPNS becomes a critical limiting factor on dives beyond about 800 fsw though this phenomena may occur as shallow as 400 to 600 fsw with the very rapid descents associated with self-contained diving.
The most efficient way to reduce HPNS is to greatly reduce descent rates with increased depth (from hours to days), providing the diver with time to adapt to pressure. However this method is impractical for most deep dives. A method that is frequently used today is to counterbalance the excitatory effects of pressure with inert gas narcosis. In this approach, nitrogen, and sometimes hydrogen, which is about four or five times less narcotic than nitrogen, are added in suitable concentrations to the basic oxygen-helium mix used in deep diving to make rapid descents (compressions) possible. These trimixes allow compressions to 1500 fsw in less than 40 hours and are reasonably effective in alleviating HPNS (cite reference). The addition of 10% percent nitrogen allowed exposures to pressures up to about 2,250 fsw in the Atlantis experiments conducted by Dr. Peter Bennett at Duke University, and experimental dives to 1,500 fsw and more in open water.
More recently, Comex has begun working with hydrogen mixtures as a means to alleviate HPNS as a part of its HYDRA program on the basis of hydrogen's low density, cost and ready availability. Since H2 is four to five times less narcotic than nitrogen, large concentrations can be used in diving mixes. To date, Comex has experimented with equal parts of hydrogen and helium at maximum depths ranging from about about 1600-2600 fsw with oxygen percentages of less than one percent. Though this formula has the merit of simplicity and yields satisfactory results, the optimal mixture&endash; a compromise between the potential narcotic effects of H2 and the efficient alleviation of HPNS &endash;is probably a function of depth and further research is needed.
Fig. 23: Comex H2-He-O2 Dive Profile
A principal physiological advantage of hydrogen is its density which is half that of helium. Gas density which effects ventilatory performance becomes a significant issue on very deep dives. By substituting H2 for half of the helium content of a diving mix, breathing gas density can be reduced by almost 25%, which should increase ventilatory performance by approximately 15% over standard heliox mixes based on XXXX (cite reference). Experience has shown that the improvements when using hydrogen are actually more than predicted (cite ref) suggesting that lower viscosity of H2 may offer some additional benefits.
What's more is that H2 decompressions seem to be as fast as those with helium, which generally requires about 15 minutes to decompress per 1 ft. of depth from saturation, or about three weeks from a depth of 2000 fsw, an observation that seems contrary to predicted times based on hydrogen's higher solubility in water and oil. H2 should be more soluble in body tissues and more potent in inducing DCS. These results suggest that chemical processes may be involved in hydrogen decompression in which H2 may be converted into water inside the body providing it enters oxidation pathways, a phenomena that would be unique to hydrogen. Such a process, referred to as "biochemical decompression," would avoid a diver's saturation by continuous conversion of hydrogen in the tissues. If the oxidation rate equaled the gas uptake rate, the diver might be able to rapidly decompress without DCS incidence (cite reference). This new concept of decompression appears to be a promising area in hyperbaric research. In addition, hydrogen appears to offer some thermal advantages over helium when using the heated gas systems required in deep saturation diving due to its high specific heat at constant pressure in comparison to helium. As result of these advantages, hydrogen mixes seems to be a promising gas for very deep diving.
Of course, the use of hydrogen raises fire safety concerns due to hydrogen's potential explosiveness in the presence of oxygen. Safety relies upon the limitation of the O2 content in the diving mixes involved and in the blending process. Normally, an O2-H2 mix cannot ignite when its oxygen content is less than 5%, however this may be altered by the presence of other gases such as helium or by pressure. As a result current practices limit O2 content to 2.5% in diving mixes and delivery or storage pressures to 2500 psi, and make use of special blending and handling equipment and procedures (cite ref.). As such hydrogen diving is expensive and well beyond the range of technical self-contained diving.
Neon is another diluent gas that has been used with some success in both animal experiments and human trials (cite ref). Because of the expense of pure neon, most trials have utilized a mixture of neon and helium known as crude neon or neon 75 (75% neon, 25% helium), which is a by-product of air distillation.
In comparison to other diluents, neon seems to produce no narcotic effects to about 1200 fsw ( Hamiliton, 1972) and presents no particular problems with decompression. Voice distortion is less than with helium or hydrogen and its lower thermal conductivity might prove to be advantageous in cold water diving. However neon's relatively high density which is about three quarters of that of nitrogen probably limits its useful working depth to about 600 fsw. However its real limitation in open-circuit diving is cost which ranges from about $1.50-1.80 per cubic foot, about 7-8 times the cost of helium though it might play a role in the future with closed circuit systems.
Sidebox: Clayton
Argon has also been used in limited applications but its properties make it not very suitable for diving. Argon has a high lipid solubility making it about twice as narcotic as nitrogen and a problem in decompression. Further more its density which is about 40% greater than that of nitrogen makes it difficult to breathe at depth though it does allow effective voice communications and also acts as a good insulator against heat loss.
Argon has been used experimentally as a decompression gas with the purpose of reducing the inspired pressure of both helium and nitrogen (Keller and Buhlmann, 1965, Keller, 1967). Its benefits in this application have not been fully explored but its use seems to be effective (Schreiner, 1969). However its immediate benefits in diving are the use of argon as a suit inflation gas. Argon offers about a 40% improvement in insulation capacity over air and as a result it is growing in use in conjunction with dry suit inflation systems.
Few other metabolically inert gases exist that appear suitable for use in diving mixtures and little data exists on their applicability. Heavier gases tend to exhibit the same limitations as argon including high lipid solubilities and density. Of the lighter gases, deuterium is not available in sufficient quantities to be considered. Methane is sufficiently inert and lighter than many gases but its lipid solubility should make it reasonably narcotic, and there are fire safety issues as well. Other gases may have some value, for example in improving voice communications when added as a small fraction of a breathing mix, one example being CF4, tetrafluoromethane, a heavy gas with a relatively low solubility. Still other gases have properties that make them useful as a thermal insulator (suit inflation gas), for example sulfur hexafluoride, which is five times denser than air.
A summary of the physiological properties of diluent gases used in diving are shown in Table 4. below.
Table 4: Diluent gas Summary
Determining An Optimal Breathing Mix
Having reviewed the key considerations in setting appropriate oxygen levels and selecting the diluent gases to be used in a diver's breathing mix, the
the type and content of the mixtures required for a specific open circuit diving operation can be readily determined.
As discussed above, from a physiological viewpoint, any dive can be divided two distinct phases each with it's own set of physiological requirements and issues that must be addressed. These are the compression (descent) and working phase&endash; typically representing the deepest portion of the dive&endash; in which gas loading commences and where CO2 levels are generally high due to increased exercise levels, and the decompression phase of the dive where the pressure is reduced in a controlled manner to facilitate inert gas offloading. In some cases, the compression or descent phase is treated separately from the working portion of the dive, for example when diving in a cave environment where significant travel or excursion time may be required at depths much shallower than the ultimate working depth in order to reach a specific destination.
Accordingly, mix technology is used to optimize the diver's breathing mix during each of these phases. With open circuit systems17, this is accomplished by determining the optimal mix(es) for each phase and switching gas mixes during the dive according to a predetermined sequence or plan usually provided as a part of the diver's decompression schedule. The types of gases used and the number of switches depend on the specific exposure and operation.
Note that while air tables and dive computers can sometimes be used directly for special mix diving, for example, following an air-based DC or table while breathing enriched air as a safety hedge, most mix diving requires special application tables that take into account the specific gases and methods used. These are discussed in detail in Chapter 4, Decompression Management.
Generally the starting point for planning a mix operation is to first calculate the optimal mix to be used during the working phase of the dive, known as"bottom mix" in technical parlance. The basic logic used is presented in flowchart form in Figure 24. Note that the flowchart does not explicitly detail all of the special operational considerations discussed above which could influence the choice of bottom mix, for example, extended bottomtimes, environmental constraints or special decompression considerations, which could effect desired oxygen levels. Similarly training and experience levels or budget may influence the choice or blend of diluent gases. These factors must be taken into account at the appropriate step in the calculation when determining a breathing mix.
Figure 24: Gas Calculation Flowchart
As shown in Figure 24, the first step is to determine appropriate oxygen levels of the mix based on the planned maximum working depth of the dive. Once oxygen levels have been determined, the diluent gases&endash;the type of breathing gas to be used&endash; can be determined based on the depth of the dive, the desired narcosis level and budget. In practice this calculation leads to several "standard" choices of bottom mix depending primarily on the exposure range (depth and time) of the dive though these can vary depending on the specifics of the operation.. These are shown in Figure 25 below.
Figure 25: Mix Range Chart
For most applications, air is the optimal breathing gas for most dives of less than about 40 fsw. In this range decompression constraints are not usually an issue and air is the lowest cost alternative. For dives in the 40-130 fsw range which encompasses the majority of self-contained diving (including recreational diving), enriched air nitrox is clearly the gas of choice by virtue of its decompression advantages though these are limited much beyond 100 fsw. In the range of about 130 fsw to 190 fsw, air is probably the optimal mix for most applications given its low cost and easy availability though "light" enriched air mixes (FO2= 25-30%) are sometimes used. For deep working dives beyond about 190 fsw, helium mixes, whether trimix, heliox or exotics are generally becoming recognized as a requirement in self-contained from the viewpoint of diver safety and productivity.
Sidebox: Which Mix
The flowchart logic shown in Fig. 24 can be readily applied to calculate various bottom mix. Some simple examples are as follows:
1) Calculate the "optimal" mix for a planned dive for 80 minutes to 70 fsw. As discussed above the optimal FO2 for the dive can be calculated from the ideal gas equation, PO2= (D/33 +1) x FO2. Substituting a PO2=1.4 atm, D= 70 fsw,and solving for FO2 we have: FO2 =.45. Because the PO2 is set for 1.4 atm, the oxygen toxicity clock can be ignored. Given that the planned working depth is less than 200 fsw, nitrogen is likely to be the diluent of choice. Hence an EAN 45 (45% oxygen, balance N2) is the optimal mix. Decompression requirements for the dive can be calculated using an EAD formula or calculated directly using a computational algorithm (see Chapter 4 for discussion.) In practice, this dive might be conducted using a "standard" EAN 36 mix due to the potential difficulty in obtaining a custom enriched air blend which would result in increased decompression requirements, the O2 level of the EAN 36 being below optimal for this dive (PO2= 1.12 atm). Note as planned bottomtime increased, for example to 120-200 minutes, the advantages of using the optimal blend in terms of reduced decompression requirements (in-water time) and hence reduced gas logistics and costs would provide a powerful incentive to utilize the custom blend. 2) Calculate the optimal mix for a dive to 165 fsw with a 30 minute bottom time.
In this case the FO2 calculation yields an optimal FO2 of 23% at a PO2 of 1.4 atm, while nitrogen is acceptable as a diluent. The resulting blend, an EAN 23 could be used, however the additional oxygen in comparison to air (FO2= 21%) would yield minimal decompression advantages and probably not be worth the expense. Similarly, the use of a helium mix, for example a trimix would probably not be worth the trouble or expense, the narcosis being manageable with experience. However if the planned bottomtime were greatly extended fro example to 80-100 minutes, a trimix might be considered both from a narcotic and decompression perspective.
3) Calculate the optimal mix for a dive to 325 fsw.
The optimal fraction of oxygen can be calculated as 13%. Because of the depth a helium mix would be used. The amount of helium, or FHe would depend on the desired narcotic level. In the case of an extended dive in an overhead environment or perhaps a training dive the planned END would probably be minimal. For example, for an END of 85 fsw, the optimal mix would be trimix 13/60. Conversely for a relatively short open water dive, requiring minimal diver workload (and a "tight" budget), the END might be set as high as 175 fsw, yielding an optimal mix of trimix 13/40.
Note many factors and considerations enter in to the calculation of an optimal mix from an operational perspective. For that reason, technical divers preparing to use special mix need to be able to "think and reason" in terms of what the technology can offer.
For many sport diving applications, in particular "no-stop" dives, the working phase of the dive represents all or the majority of the total dive time, the decompression phase being a slow ascent and often a safety stop. In this case, the dive is typically conducted on a single bottom mix which can be optimized for the exposure. However this is not the case for most technical dives involving extended decompression in which the working phase of the dive represents only a small portion of runtime. Accordingly multiple gas mixes including oxygen are required to accomplish the dive safely and efficiently.
For example a simple shallow to intermediate water dive requiring limited decompression may involve a single bottom mix such as air or an enriched air mix, followed by oxygen for decompression at the 20 and 10 fsw stops. A more complex dive in deep water typically involves switching to a number of specific mixes depending on the exposure and operation. These mixes are typically carried in both the divers "set" (back-mounted tank system) and in one or more diver-carried "stage bottles"&endash; often 80-100 cf cylinders clipped to the diver's harness&endash; and in some cases are staged in "gas depots19 " so as to be readily accessible to the divers. The types of possible gas mixes used during the various phases of a hypothetical dive are shown in Figure 26 and discussed in more detail below.
Anatomy Of A Special Mix Dive
Figure 26: Hypothetical Mix Dive Profile
Travel mix is sometimes utilized during specific operations involving
lengthy travel or excursion time at depths much shallower than the ultimate planned working depth in order to minimize gas loading during descent.
In this case, the oxygen level is generally set for the maximum depth encountered during the travel phase of the dive rather than the final working depth in order to minimize the inert gas fraction in the divers breathing mix during the travel portion. For most applications, a nitrox mixture, either air or enriched air is used depending on the travel depth and is carried in a stage bottle(s). The relatively slow diffusibility of nitrogen offers the additional advantage of slowing gas uptake prior to reaching the planned working depth for example where a helium-based mix is to be used.
Upon completing the travel portion of the dive, the diver switches to his or her bottom mix as indicated in Figure 26. Travel mixes are typically used in conjunction with special application tables that take into account the specific profile of the dive, though they might be used as a decompression safety hedge in the absence of a profile-specific table.
For very deep dives, for example, Sheck Exley's dive to 870 fsw at El Nacimento Mante (see Chapter 7, Applications), multiple travel mixes may be used on descent each with an oxygen fraction optimized for some portion of the excursion. Travel mixes also have application for deep open water dives where the oxygen fraction of the bottom mix may result in hypoxia if breathed for any length of time at the surface (a mix with an FO2 of 14-16%, resulting in a PO2 of 0.14-0.16 atm at the surface), for example in heavy sea states where divers must breathe from their sets at the surface prior to commencing their descent.
Bottom mix optimized for the working phase of the dive is usually what people associate with mixed gas diving and is typically carried in the diver's set, for example in back-mounted doubles. As discussed above, here the issue is setting an suitable oxygen level to yield a working PO2 of 1.2-1.4 atm in most cases, and selecting a diluent appropriate to the planned depth range. Because the working phase typically represents the deepest portion of the dive in most sport applications, bottom mix usually represents the largest volume of gas consumed by the diver when using open circuit scuba though this is not always the case in dives involving extended decompression.
The use of intermediate or decompression mixes is becoming a standard procedure for extended dives requiring decompression stops much below 50-70 fsw. Because of the relatively low fraction of oxygen set to yield a PO2 of about 1.4 atm at the maximum working depth, a diver's bottom mix is generally a very inefficient breathing mixture during the decompression phase of the dive.
Today, the standard procedure for most special mix (helium-based) dives is to utilize air and or one or more enriched air mixtures as intermediate mixes during decompression prior to switching to pure oxygen at 20 fsw. These are generally specified as a part of the diver's decompression schedules. The strategy here is to boost oxygen levels back up to an optimal PO2 of 1.6 atm as early in the staged decompression as possible, typically during the first few stops&endash; oxygen is the key to effective decompression, and to get off the helium-based bottom mix by substituting nitrogen, a slower diffusing gas, for helium in the intermediate mix in order to facilitate helium offgassing and minimize additional gas loading during decompression. Note that this opportunity for inert gas switching is for the most part limited to helium mix diving.
Sometimes, depending on the specific decompression plan, multiple intermediate mixes must be utilized to maintain PO2s as close to 1.6 atm as feasible by switching to increased oxygen-enriched mixes as the decompression progresses . For example on an extended deep mix dive, air might be utilized as an intermediate mix for stops in the 220 to 160 fsw range, followed by an EAN 30 from 150 to 80 fsw, EAN 50 from 70 fsw to 30 fsw, followed by pure oxygen at 20 fsw19 .
In practice this ability to optimize decompression by maintaining PO2 levels at or near 1.6 atm during the ascent must be weighed against the operational costs of carrying additional mixes. These considerations are discussed in more detail in chapter 4. In some cases, a single intermediate mix such as an EAN 50 (MOD = 72 fsw) might used to carry out the entire decompression from a special mix or deep air dive, including the 20 and 10 fsw stop in order to minimize the number of cylinders carried by the divers. In addition, intermediate mixes are sometimes used as a "hedge or pad" gas i.e. not calculated into the decompression schedules, to yield an additional safety margin.
Though long held commonplace in commercial and military diving, the use of oxygen for in-water decompression is rapidly becoming an emerging standard in the technical diving community for dives involving more than about 20-30 minutes of air decompression and is likely to become standard for all decompression dives in the future as oxygen becomes more readily available. In self-contained diving, oxygen breathing is normally initiated at 20 fsw (PO2 = 1.6 atm) during decompression prior to surfacing, though it is sometimes used as deep as 30 fsw (PO2=1.9 atm) under special circumstances.
Often both the 20 and 10 fsw stops are pulled at twenty feet to avoid wave surge in the case of open ocean diving, and to maximize the inert gas off-loading gradient and benefit from what is called the "oxygen window" effect discussed in Chapter 4. Five minute "air breaks" are usually taken every 25 minutes or so while breathing oxygen to decrease the divers sensitivity to elevated oxygen pressures and avoid CNS toxicity problems. Note that these breaks generally do not count towards the diver's decompression obligation unless calculated into the tables.
Though special tables are generally required to take the full performance benefits of in-water oxygen decompression i.e. to reduce decompression times, oxygen is often breathed as a pad or safety hedge in conjunction with following air tables and air-based dive computers. Oxygen is sometimes breathed on the surface as well, about two to three hours after long deep (beyond about 300 fsw) trimix dives. Doppler bubble scores for this type of dive seem to increase several hours after surfacing indicating potential decompression stress. Surface oxygen can also be used to shorten surface intervals, reduce time-to fly limitations, and has long been recognized as a requirement in treating dive injuries. Maintaining ample oxygen supplies is essential to dive safety while conducting technical diving operations.
A summary of the types of mixes used during the various phases of a dive is shown in Table 5 below, while specific examples of a range of special mix diving are provided in Chapter 7, Operations.
Table 5: Breathing Mix summary
Mix Operations
It's important to understand that the improved safety and performance offered by mix technology comes at the price of increased planning, logistics and expense. Even though making a simple enriched air dive is as easy as air, and involves minimum additional training, requirements on a big dive including; planning and set-up, gas and gear management and insuring adequate back-ups, can be significant and demand an experienced team, particularly for long and deep dives.
Conducting a deep mix dive safely requires caution, special training, knowledge and understanding, practice, the proper equipment, and diver support-- lots of it. Deep dives are "operations" and there is a lot more involved than simply jumping off the boat. Specific planning and operational considerations are discussed in chapter 7, Operations.
Expense is also an issue; special mix diving costs more. Though the primary expense is usually in equipment investment, eg. additional tanks, regulators, and special application tables, gas costs can be significant in open circuit technical diving operations. However this extra cost can usually be more than justified by the safety and performance it adds. Presently, enriched air fills in the U.S. are running about $8-14 per hundred cubic feet retail, about the same cost as an oxygen fill, while a trimix dive can cost $100 and up. Depending on the application, it can be a small price to pay for increased safety and the ability to do much more.