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SDUA Standard Gas Mixes

SDUA Standard Gas Mixes

Dr. Bill Creasy

I created San Diego Underwater Adventures to be a friendly, welcoming home for San Diego’s divers, a home which offers the highest quality education, equipment and experience for all our clients, from novice recreational divers through the most advanced technical divers and deep cave explorers.

More like a private club than a retail store, SDUA membership offers a host of benefits, including free courses, discounts on equipment and free gas fills, including air, Nitrox and Trimix. Of course, we will fill cylinders for any certified diver, but SDUA membership provides free fills, enabling our members to dive more frequently, using the proper gas for each dive?��Ǩ��all at affordable prices.

Our gas fills include the following:

As you can see, our pricing encourages standard gas mixes and discourages custom, or ?��Ǩ?�best mixes.?��Ǩ�� Many have asked why.

The Basics

We have all learned as beginning divers that breathing compressed gas at depth has profound physiological implications. We understand from Boyle’s Law that ?��Ǩ?�as pressure increases, the volume of a gas decreases; conversely, as pressure decreases, the volume of a gas increases.?��Ǩ�� For a diver, that means that for every 33 ft. of seawater (fsw) we descend , the pressure increases by 14.7 psi, and for every 33 fsw we ascend , the pressure decreases by 14.7 psi. A simple chart illustrates:

We also know from Dalton’s Law that ?��Ǩ?�in a mixture of different gases, the sum of the partial pressures of all the gases equals the total pressure.?��Ǩ�� Again, a simple chart illustrates:

And finally, we know from Henry?��Ǩ�Ѣs Law that ?��Ǩ?�the amount of gas which dissolves into a liquid is proportional to the partial pressure of that gas.?��Ǩ��

As we descend deeper and deeper, our tissues absorb more and more of the gases we breathe, and as we ascend and pressure decreases, our tissues release the absorbed gases. Since 78% of the gas in air is nitrogen, our standard recreational diving tables are constructed to manage that nitrogen.

Every diver learns such basic physics in his/her Open Water class.

The Problem

Nitrogen

Further study, however, brings additional issues to light. For example, nitrogen is a highly narcotic gas. Breathed at depth, its narcotic affect mimics that of alcoholic intoxication, which we refer to as ?��Ǩ?�nitrogen narcosis.?��Ǩ�� As early as 1835 researchers noted that when breathing compressed air ?��Ǩ?�the functions of the brain are activated, imagination is lively, thoughts have a peculiar charm and, in some persons, symptoms of intoxication are present?��Ǩ�� (Junod, 1835). It wasn?��Ǩ�Ѣt until 1935, however, that researchers isolated nitrogen as the component in compressed air that produces the narcotic effect, noting that at 100 fsw ?��Ǩ?�divers experienced a feeling of stimulation, excitement, and euphoria, occasionally a slowing of mental activity; responses to visual, auditory, olfactory, and tactile stimuli were delayed; and there was a limitation of the powers of association and a tendency for fixation of ideas.?��Ǩ�� At 300 fsw, ?��Ǩ?�the signs and symptoms amounted to stupefaction, with greatly impaired muscular activity.?��Ǩ�� Significantly, the signs and symptoms occurred at the beginning of exposure, and they did not change with more prolonged exposure (Behnke, et al., 1935).

In 1937 Shilling and Willgrube examined the effects of compressed air between 90 and 300 fsw on forty-six men performing addition, multiplication, subtraction and division, recording the time taken and the number of errors, as well as tests for reaction time and letter cancellation. Their results may be summarized in the following chart:

Effects of Compressed Air on Psychometric Tests

More recently, Kiessling and Maag (1962) devised a much more complex test of both manual dexterity and conceptual reasoning, carried out at 100 fsw. The results showed decrements of 33.46% in reasoning ability, 20.85% in reaction time, and 7.9% in manual dexterity. Again, the length of time spent at depth did not affect the degree of narcosis.

Clearly, the narcotic affect of nitrogen is a significant factor in diving, beginning at around 100 fsw and worsening as one dives deeper. Although some divers claim that one can acclimate to deep air diving through prolonged or repetitive deep air exposure, empirical research suggests otherwise. Although a diver may feel fine at 200 fsw on air, his/her reasoning and physical abilities are, in fact, significantly impaired. A diver who claims he is not ?��Ǩ?�narced?��Ǩ�� on air at 200 fsw, is like a driver who insists he is not drunk after four double scotches: he may believe it, ?��Ǩ?�but it just ain?��Ǩ�Ѣt so.?��Ǩ��

Researchers do not fully understand why or how inert gases such as nitrogen produce this narcotic affect. Many attempts have been made to correlate narcotic potency to a variety of factors, including lipid solubility, partition coefficients, molecular weight, absorption coefficients, thermodynamic activity, and the formation of clathrates. By far, though, lipid solubility offers the most satisfactory correlation, as Meyer and Overton suggested long ago (Meyer, 1899; Overton, 1901). The chart below illustrates their findings:

Correlation of Narcotic Potency of Inert Gases

Although the narcotic potency of an inert gas correlates well with a gas?��Ǩ�Ѣs lipid solubility, there is not a direct cause/effect relationship between the two. Notice, for example, that oxygen has a higher lipid solubility and relative narcotic potency than nitrogen. Both are narcotic, but the observed narcotic effects of oxygen at depth are not as great as that of nitrogen. This is probably due to the fact that oxygen is metabolized in the body, whereas nitrogen is not. But this is only a guess. With organic hydrocarbons such as alcohols, increasing the chain length increases the lipid solubility and the anesthetic potency, but only to a point: when the chain length reaches ten to fourteen carbon atoms, there is a sudden loss of anesthetic effect. Although the compound is lipid soluble, it does not produce narcosis. Clearly, there are other factors at work here, but we really do not know what they are. For our purposes, however, the correlation between lipid solubility and narcotic potency suggested by Meyer-Overton is a useful tool for reducing narcosis and its associated diving risk.

Oxygen

Like nitrogen, oxygen is also absorbed into our tissues at depth, but unlike nitrogen, oxygen quickly becomes toxic. Early investigations by Behnke et al. (1935, 1936) measured circulatory, respiratory and visual responses to breathing oxygen at ambient pressures of 1.0-4.0 ATA (sea level-99 fsw). Later researchers studied the effects of breathing oxygen at depth for the Royal Navy and the U.S. Navy, whose divers were using closed circuit oxygen rebreathers for covert military operations (Lambertsen, 1947, Larson, 1959; Donald, 1947, 1992; Yarbrough, et al., 1947). Using this work as a foundation, Lanphier and Dwyer (1954) established oxygen depth/time limits for U.S. Navy divers as a tool to manage oxygen exposure. And more recent studies performed at the U.S. Navy Experimental Diving Unit update those tables (Butler, 1986; Butler and Thalmann, 1984, 1986).

Breathing compressed oxygen at depth exposes a diver to two types of risk: 1) Central Nervous System (CNS) oxygen toxicity and 2) Pulmonary oxygen toxicity. CNS toxicity results from breathing elevated partial pressures of oxygen. Unfortunately, individual oxygen tolerance varies wildly among divers. Using the first neurological signs to appear as an indication of CNS oxygen toxicity, Donald (1947, 1992) found an enormous variation in oxygen tolerance among subjects exposed to the same conditions. Among thirty-six divers breathing oxygen at 89 fsw, signs of CNS oxygen toxicity occurred as early as six minutes, and as late as ninety-six minutes. He found no correlation whatsoever between oxygen tolerance and a diver?��Ǩ�Ѣs age, weight, fitness or a host of other factors. What?��Ǩ�Ѣs more, the same diver showed equally variable oxygen tolerance from day to day.

Signs and symptoms of CNS toxicity occur quickly, often without warning, and they include localized muscle twitching, seizures and, with continued exposure, progressive neural destruction, permanent paralysis and death (Donald, 1947; Lambertsen, 1965). A useful diagram presents CNS oxygen toxicity signs and symptoms (Clark,1974; reprinted in Clark and Thom, 2003):

Unlike CNS toxicity, which results from breathing elevated partial pressures of oxygen, Pulmonary oxygen toxicity results from prolonged exposure to oxygen. Clark, et al . (1991) demonstrated that subjects breathing oxygen at pressures of 0.78-0.88, 1.0 and 2.0 ATA developed Pulmonary oxygen toxicity symptoms as early as six, four and three hours, respectively; those breathing oxygen at 3.0 ATA experienced symptoms within one hour. These symptoms begin as a mild tickling that induces coughing, which becomes more frequent and intense as exposure continues. When extreme, the tracheal symptoms include a burning sensation, accompanied by uncontrollable coughing. Usually, the symptoms diminish within two to four hours after exposure to the oxygen ends, and complete resolution usually occurs within three days.

Gas Density

Gas density also affects diving risk and performance. As we dive deeper, gas density becomes a significant factor in our ability to ventilate our lungs fully: in practical terms, the deeper we dive the harder we have to work to breathe, due, in part, to increased gas density. The following chart demonstrates relative gas densities:

If we cannot fully ventilate our lungs during a dive, CO 2 builds up (the by-product of our metabolizing oxygen), producing a potentially fatal consequence. Normally, as CO 2 builds up our bodies respond by increasing our breathing rate to expel it. At depth, however, it becomes increasingly difficult to do so, since air at sea level has a density (gram/liter of gas) of 1.138, while at 99 fsw the density increases to 4.552. As density increases, so does breathing effort, and with increased breathing effort, CO 2 builds up rapidly. Since CO 2 is twenty-five times more lipid soluble than nitrogen, its narcotic affects become quickly evident, resulting in disorientation and ultimately in a loss of consciousness. The denser the gas, the more effort is required to breathe it at depth, and the harder we work to breathe, the more CO 2 builds up, trapping us in a vicious cycle.

The Solution

Clearly, given the narcotic effect of nitrogen and the toxic effect of oxygen, along with the relatively high densities of both gases, we should search for a more efficient breathing gas than air for our diving, a gas with a much lower narcotic effect and a lower density than air.

In our search for such a gas, we need simultaneously: 1) to reduce the narcotic effect of nitrogen, 2) to reduce the toxic effect of oxygen, and 3) to reduce the overall density of the gas.

For standard recreational dives no deeper than 100 fsw, the simplest solution is to reduce the nitrogen content of the gas we breathe by increasing the oxygen content: if we increase the oxygen content from 21% to 32%, we thus reduce the nitrogen content from 79% to 68%, reducing the gas’s narcotic effect; if we increase the oxygen content to 36%, we reduce the nitrogen content even more, to 64%. The following chart illustrates:

Diving to 100 fsw on Nitrox 32 has the same narcotic effect as diving air to 81 fsw; and diving Nitrox 36 has the same effect as diving air to 75 fsw, both within a range where the narcotic effect of the nitrogen is minimal. You can easily calculate the equivalent air depth (EAD) of a gas mix by the following formula:

EAD = [(N/.78) x (Dfsw + 33)] ?��Ǩ�� 33

Example: A diver dives to 92 fsw on Nitrox 32. What is his equivalent air depth?

EAD = [(N/.78) x (Dfsw + 33)] ?��Ǩ�� 33
EAD = [(.68/.78) x (92 + 33)] ?��Ǩ�� 33
EAD = 75.97 fsw

But, of course, increasing the oxygen content of air also increases the partial pressure of the oxygen, as the following chart illustrates:

Partial Pressures of Oxygen (PPO2) with Air and Nitrox

You can calculate the partial pressure of oxygen for any given depth by using the following formula:

PPO2 = [(Depth /33) + 1] x O2%

Example:
Depth = 87 fsw
O2% = 32%

PPO2 = [(Depth /33) + 1] x O2%
PPO2 = [(87/33) + 1] x .32
PPO2 = 1.16

Although divers differ greatly in their tolerance to oxygen at depth, most authorities agree that the partial pressure of oxygen should be kept below 1.4 during the active part of a dive and below 1.6 when at rest, such as during decompression stops. Although this does not guarantee that one will not suffer from CNS oxygen toxicity, it greatly reduces the probability. SDUA, following Global Underwater Explorers (GUE), sets the PPO2 limit at 1.3 or lower during the active part of a dive, and 1.6 or lower during decompression stops, offering a more conservative safety margin. At the same time, we keep the equivalent air depth of any gas mix below 100 fsw, where the narcotic effect of nitrogen is minimal.

For dives no deeper than 100 fsw, then, Nitrox 32 offers an efficient and practical gas, having a PPO2 of 1.29 and an EAD of 81 fsw, both within our limits. Even though Nitrox 32 has a higher density than air, it is not a significant factor at depths less than 100 fsw. Nitrox 32, then, becomes an excellent choice for dives in the < 100 fsw category.

For dives deeper than 100 fsw, we need a strategy that also addresses gas density. Our choices are the following, with their associated PPO2s and EADs:

For dives in the 100-150 fsw range, keeping oxygen at 21% and adding 35% helium (thus reducing nitrogen in the mix to 44%) produces impressive benefits. First, helium is the least narcotic of the inert gases, with a lipid solubility of 0.015, compared to nitrogen?��Ǩ�Ѣs 0.067, producing a relative narcotic effect of 0.2, as opposed to nitrogen?��Ǩ�Ѣs 1.0. Second, helium?��Ǩ�Ѣs gas density of 0.1573 is the lowest of the inert gases. Compared to nitrogen?��Ǩ�Ѣs 1.1009, helium offers an 85.71% reduction in gas density over nitrogen, producing a mix that is much easier to breathe at depth, and offering a maximum EAD of 85.95 fsw, well within our goal of < 100 fsw. Further, by keeping oxygen at 21%, the maximum PPO2 at 150 fsw is 1.16, well within our < 1.3 goal. With a 21/35 mix for dives in the 100-150 fsw range, we address all of our goals for a practical and efficient breathing gas: 1) we reduce the narcotic effect of nitrogen, 2) we reduce the toxic effect of oxygen, and 3) we reduce the overall density of the gas.

The same thinking applies to our other bottom gas mixes.

For decompression gas, we set the PPO2 at a 1.6 maximum and the EAD at roughly 100 fsw (with 21/35 slightly higher in 190 fsw, at 111.95, but with a PPO2 of 1.4), producing practical and efficient decompression mixes. As GUE?��Ǩ�Ѣs Jarrod Jablonski observes (2001), ?��Ǩ?�Choosing decompression mixtures is based primarily on doing a cost/benefit analysis. Individuals must assess the difficulty and logistical feasibility of carrying a particular set of decompression gases against the benefits derived from those gases.?��Ǩ�� Our standard decompression mixes provide effective decompression choices within our chosen PPO2 and EAD parameters. One could select other, custom-blended mixes, but the benefits rarely outweigh the expense and logistics of doing so.

Why Not Choose the ?��Ǩ?�Best Mix??��Ǩ��

One could argue that the correct approach to choosing a diving gas for any given dive is to determine the maximum depth of the dive, the PPO2 limit and the maximum EAD. Once those are known, choosing a ?��Ǩ?�best mix?��Ǩ�� is simply a matter of mathematical calculation.

Example: A diver is planning to explore a wreck located in 185 fsw. Following standard protocols, he/she sets 1.4 as the maximum PPO2 for the dive and 100 fsw for the maximum EAD. What gas should he/she choose?

Step #1: Calculate the oxygen percentage that will produce PPO2 of 1.4 at 185 fsw.

(Fraction of Gas)
FG = 1.4 PPO2 /ATA
FG = 1.4/[(185/33) + 1]
FG = 21%

The oxygen content of our mix will be 21%.

Step #2: Calculate the nitrogen percentage that will produce an EAD of 100 fsw at 185 fsw.

EAD = [(N/.78) x (Dfsw + 33)] ?��Ǩ�� 33
100 fsw = [(N/.78) x (185 fsw + 33)] ?��Ǩ�� 33
0 = [(N/.78) x (218)] ?��Ǩ�� 133
0 = (N/.78) - .61
.61 = N/.78
.48 = N

The nitrogen percentage of our mix will be 48%.

Step #3: If the oxygen content is 21% and the nitrogen content is 48%, then the helium content must be 31% [1.0 - (.21 + .48)].

For a dive to 185 fsw, then, our ?��Ǩ?�best mix?��Ǩ�� is Trimix 21/31.

That?��Ǩ�Ѣs a lot of work to arrive at a ?��Ǩ?�best mix?��Ǩ��! Of course, one could use a computerized dive planner to get the ?��Ǩ?�best mix?��Ǩ�� faster, but what do we gain? The entire calculation assumes that the ?��Ǩ?�best?��Ǩ�� PPO2 is 1.4 and the ?��Ǩ?�best?��Ǩ�� EAD is 100 fsw. All the rest is just number crunching.

As we?��Ǩ�Ѣve seen, though, oxygen toxicity is a highly individual response, both among divers and within an individual diver at any given time. It is always best to err on the conservative side when dealing with oxygen toxicity, keeping PPO2 < 1.3. Likewise, with the narcotic effect of nitrogen, lower is better in most cases. Here we find the standard gas mixes to be a powerful tool:

For a 185 fsw dive, our standard mix would be 18/45, providing a PPO2 of 1.19 and an EAD of 70.41. With our standard mix we have significantly improved our safety margin, and we have eliminated the need for complex calculations.

Standard mixes carry other benefits, as well. The vast majority of dives in San Diego will be < 400 fsw; indeed, most will rarely exceed 250 fsw; and most ?��Ǩ?�recreational?��Ǩ�� divers will stay < 100 fsw. For such dives, our standard mixes offer a powerful tool: selecting a gas mix for any given dive is easy; it enhances team planning; it facilitates simple and consistent cylinder marking; and it makes decompression planning much simpler. In addition, standard mixes are much more affordable and convenient, since SDUA can create our standard mixes with banked Nitrox 32 and Helium, offering most fills while you wait.

One should never apply rules rigidly or unthinkingly, of course, and there are dives that require custom mixes. Long, deep exploration dives, such as those done on the WKPP cave project, require lower PPO2s due to the prolonged exposure to oxygen at depth, for example, and one may choose to ?��Ǩ?�bump?��Ǩ�� his/her mix up or down to the next category, say from 21/35 to 18/45 to address specific environmental or physical issues. But in San Diego, such dives are the exception, not the rule.

In diving, ?��Ǩ?�simple is good,?��Ǩ�� and standardized gas mixes greatly simplify our diving, offering enhanced safety, convenience and affordability. The more you understand gas mixes, the more useful standardized mixes become. You will quickly find that they offer an important tool in becoming a competent, confident, comfortable?��Ǩ��and safe?��Ǩ��diver.

References

Behnke, A.R., H.S. Forbes, and E.P Motley. ?��Ǩ?�Circulatory and Visual Effects of Oxygen at 3 Atmospheres Pressure.?��Ǩ�� Am J. Physiol 114 (1936), 436-442.

Behnke, A.R., F.S.Johnson, J.R Poppen , et al. ?��Ǩ?�The Effect of Oxygen on Man at Pressures from 1 to 4 Atmospheres.?��Ǩ�� Am J Physiol 110 (1935), 565-572.

Behnke, A.R., R.M. Thomas and E.P. Motley ?��Ǩ?�The Psychologic Effects from Breathing Air at 4 Atmospheres Pressure.?��Ǩ�� Am J Physiol 112 (1935), 554-558.

Bennett, P.B. and J.C. Rostain. ?��Ǩ?�Inert Gas Narcosis?��Ǩ�� in Bennett and Elliott?��Ǩ�Ѣs Physiology and Medicine of Diving, 5th ed. Edinburgh: Elsevier Limited, 2003.

Butler, F.K., Jr. Central Nervous Systems Oxygen Toxicity in Closed Circuit Scuba Divers, III. US. Navy Experimental Diving Unit Report, No. 5-86, 1986.

Butler, F.K., Jr. and E.D. Thalmann. ?��Ǩ?�Central Nervous System Oxygen Toxicity in Closed-circuit Scuba Divers,?��Ǩ�� in Proceedings on the Eighth Symposium on Underwater Physiology, ed. by A.J. Bachrach and M.M. Matzen. Bethesda, M.D.: Undersea Medical Society (1984), 15-30.

Butler, F.K., Jr. and E.D. Thalmann. ?��Ǩ?�Central Nervous System Oxygen Toxicity in Closed Circuit Scuba Divers, II.?��Ǩ�� Undersea Biomed Res 13 (1986), 193-223.

Clark, J.M. ?��Ǩ?�The Toxicity of Oxygen.?��Ǩ�� Am Rev Resp Dis 110 (1974) 40-50.

Clark, J.M., R.M. Jackson, C.J. Lamertson, et al. ?��Ǩ?�Pulmonary Function in Men after Oxygen Breathing at 3.0 ATA for 3.5 Hours.?��Ǩ�� J Appl Physiol 71 (1991), 878-885.

Clark, J.M. and S.R. Thom. ?��Ǩ?�Oxygen Under Pressure,?��Ǩ�� in Bennett and Elliott?��Ǩ�Ѣs Physiology and Medicine of Diving, 5th ed. Edinburgh: Elsevier Limited, 2003.

Donald, K.W. ?��Ǩ?�Oxygen Poisoning in Man, I & II.?��Ǩ�� Br. Med J 1 (1947), 667-672, 712-717.

Donald, K.W. Oxygen and the Diver. UK: The SPA Ltd; 1992.

Jablonski, Jarrod. Getting Clear on the Basics: The Fundamentals of Technical Diving. High Springs, Florida: Global Underwater Explorers, 2001.

Junod, T. ?��Ǩ?�Recherches sur les Effets Physiologiques et Therapeutiques de la Compression et de Rarefaction de l?��Ǩ�Ѣair, taut sur le Corps que les Members Isoles.?��Ǩ�� Ann Gen Med 9 (1835), 157.

Kiessling, R.J. and C.H. Maag. ?��Ǩ?�Performance Impairment as a Function of Nitrogen Narcosis.?��Ǩ�� J. Appl Physiol 46 (1962), 91-95.

Lambertsen, C.J. ?��Ǩ?�Problems of Shallow Water Diving. Report Based on Experiences of Operational Swimmers of the Office of Strategic Services.?��Ǩ�� Occup Med 3 (1947), 230-245.

Lambertsen, C.J. ?��Ǩ?�Effects of Oxygen at High Partial Pressure,?��Ǩ�� in Handbook of Physiology, Section 3, Vol. 2, eds., W.O. Fenn and H. Rahn. Washington, D.C.: American Pysiological Society, 1965, 1027-1046.

Lanphier, E.H. and J.V. Dwyer. Diving with Self-contained Underwater Operating Apparatus. U.S. Navy Experimental Diving Unit Report, No. 11-54, 1954.

Larson, H.E. A History of Self-contained Diving and Underwater Swimming (Publication No. 469). Washington, D.C.: Natl Acad Sci-Natl Res Council, 1959.

Meyer, H.H. ?��Ǩ?�Theoris der Alkoholnarkose.?��Ǩ�� Arch Exp Path Pharmak 42 (1899), 109.

Overton, E. Studien ?ɬ�ber die Narkose. Jena: Fischer, 1901.

Shilling, C.W. and W.W. Willgrube. ?��Ǩ?�Quantitative Study of Mental and Neuromuscular Reactions as Influenced by Increased Air Pressure.?��Ǩ�� U.S. Nav Med Bull 35 (1937), 373-380.

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