SDTechDiving

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)] – 33

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

EAD = [(N/.78) x (Dfsw + 33)] – 33
EAD = [(.68/.78) x (92 + 33)] – 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:

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’s 0.067, producing a relative narcotic effect of 0.2, as opposed to nitrogen’s 1.0. Second, helium’s gas density of 0.1573 is the lowest of the inert gases. Compared to nitrogen’s 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’s Jarrod Jablonski observes (2001), “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.” 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 “Best Mix?”

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 “best mix” 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)] – 33
100 fsw = [(N/.78) x (185 fsw + 33)] – 33
0 = [(N/.78) x (218)] – 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 “best mix” is Trimix 21/31.

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

As we’ve 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 “recreational” 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 “bump” 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, “simple is good,” 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—and safe—diver.

References

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

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

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

Bennett, P.B. and J.C. Rostain. “Inert Gas Narcosis” in Bennett and Elliott’s 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. “Central Nervous System Oxygen Toxicity in Closed-circuit Scuba Divers,” 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. “Central Nervous System Oxygen Toxicity in Closed Circuit Scuba Divers, II.” Undersea Biomed Res 13 (1986), 193-223.

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

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

Clark, J.M. and S.R. Thom. “Oxygen Under Pressure,” in Bennett and Elliott’s Physiology and Medicine of Diving, 5th ed. Edinburgh: Elsevier Limited, 2003.

Donald, K.W. “Oxygen Poisoning in Man, I & II.” 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. “Recherches sur les Effets Physiologiques et Therapeutiques de la Compression et de Rarefaction de l’air, taut sur le Corps que les Members Isoles.” Ann Gen Med 9 (1835), 157.

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

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

Lambertsen, C.J. “Effects of Oxygen at High Partial Pressure,” 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. “Theoris der Alkoholnarkose.” Arch Exp Path Pharmak 42 (1899), 109.

Overton, E. Studien über die Narkose. Jena: Fischer, 1901.

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



		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)] – 33 Example: A diver dives to 92 fsw on Nitrox 32. What is his equivalent air depth? EAD = [(N/.78) x (Dfsw + 33)] – 33 EAD = [(.68/.78) x (92 + 33)] – 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: 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’s 0.067, producing a relative narcotic effect of 0.2, as opposed to nitrogen’s 1.0. Second, helium’s gas density of 0.1573 is the lowest of the inert gases. Compared to nitrogen’s 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’s Jarrod Jablonski observes (2001), “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.” 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 “Best Mix?”* 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 “best mix” 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)] – 33 100 fsw = [(N/.78) x (185 fsw + 33)] – 33 0 = [(N/.78) x (218)] – 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 “best mix” is Trimix 21/31. That’s a lot of work to arrive at a “best mix”! Of course, one could use a computerized dive planner to get the “best mix” faster, but what do we gain? The entire calculation assumes that the “best” PPO2 is 1.4 and the “best” EAD is 100 fsw. All the rest is just number crunching. As we’ve 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 “recreational” 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 “bump” 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, “simple is good,” 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—and safe—diver. References Behnke, A.R., H.S. Forbes, and E.P Motley. “Circulatory and Visual Effects of Oxygen at 3 Atmospheres Pressure.” Am J. Physiol 114 (1936), 436-442. Behnke, A.R., F.S.Johnson, J.R Poppen , et al. “The Effect of Oxygen on Man at Pressures from 1 to 4 Atmospheres.” Am J Physiol 110 (1935), 565-572. Behnke, A.R., R.M. Thomas and E.P. Motley “The Psychologic Effects from Breathing Air at 4 Atmospheres Pressure.” Am J Physiol 112 (1935), 554-558. Bennett, P.B. and J.C. Rostain. “Inert Gas Narcosis” in Bennett and Elliott’s 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. “Central Nervous System Oxygen Toxicity in Closed-circuit Scuba Divers,” 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. “Central Nervous System Oxygen Toxicity in Closed Circuit Scuba Divers, II.” Undersea Biomed Res 13 (1986), 193-223. Clark, J.M. “The Toxicity of Oxygen.” Am Rev Resp Dis 110 (1974) 40-50. Clark, J.M., R.M. Jackson, C.J. Lamertson, et al. “Pulmonary Function in Men after Oxygen Breathing at 3.0 ATA for 3.5 Hours.” J Appl Physiol 71 (1991), 878-885. Clark, J.M. and S.R. Thom. “Oxygen Under Pressure,” in Bennett and Elliott’s Physiology and Medicine of Diving, 5th ed. Edinburgh: Elsevier Limited, 2003. Donald, K.W. “Oxygen Poisoning in Man, I & II.” 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. “Recherches sur les Effets Physiologiques et Therapeutiques de la Compression et de Rarefaction de l’air, taut sur le Corps que les Members Isoles.” Ann Gen Med 9 (1835), 157. Kiessling, R.J. and C.H. Maag. “Performance Impairment as a Function of Nitrogen Narcosis.” J. Appl Physiol 46 (1962), 91-95. Lambertsen, C.J. “Problems of Shallow Water Diving. Report Based on Experiences of Operational Swimmers of the Office of Strategic Services.” Occup Med 3 (1947), 230-245. Lambertsen, C.J. “Effects of Oxygen at High Partial Pressure,” 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. “Theoris der Alkoholnarkose.” Arch Exp Path Pharmak 42 (1899), 109. Overton, E. Studien über die Narkose. Jena: Fischer, 1901. Shilling, C.W. and W.W. Willgrube. “Quantitative Study of Mental and Neuromuscular Reactions as Influenced by Increased Air Pressure.” U.S. Nav Med Bull 35 (1937), 373-380.
		Page 2 of 2		PREVIOUS PAGE
		This article was contributed by Dr. Bill Creasy and do not necessarily reflect the beliefs and/or opinions of SDTechDiving; they are the sole written opinion/expression of the author(s). SDTechDiving is not responsible for content contained within this article, including links which may take the reader to websites outside of our control