SDTechDiving

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—all at affordable prices.

Our gas fills include the following:

As you can see, our pricing encourages standard gas mixes and discourages custom, or “best mixes.” 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 “as pressure increases, the volume of a gas decreases; conversely, as pressure decreases, the volume of a gas increases.” 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 “in a mixture of different gases, the sum of the partial pressures of all the gases equals the total pressure.” Again, a simple chart illustrates:

And finally, we know from Henry’s Law that “the amount of gas which dissolves into a liquid is proportional to the partial pressure of that gas.”

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 “nitrogen narcosis.” As early as 1835 researchers noted that when breathing compressed air “the functions of the brain are activated, imagination is lively, thoughts have a peculiar charm and, in some persons, symptoms of intoxication are present” (Junod, 1835). It wasn’t until 1935, however, that researchers isolated nitrogen as the component in compressed air that produces the narcotic effect, noting that at 100 fsw “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.” At 300 fsw, “the signs and symptoms amounted to stupefaction, with greatly impaired muscular activity.” 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:

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 “narced” on air at 200 fsw, is like a driver who insists he is not drunk after four double scotches: he may believe it, “but it just ain’t so.”

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:

Although the narcotic potency of an inert gas correlates well with a gas’s 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’s age, weight, fitness or a host of other factors. What’s 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.



		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—all at affordable prices. Our gas fills include the following: As you can see, our pricing encourages standard gas mixes and discourages custom, or “best mixes.” 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 “as pressure increases, the volume of a gas decreases; conversely, as pressure decreases, the volume of a gas increases.” 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 “in a mixture of different gases, the sum of the partial pressures of all the gases equals the total pressure.” Again, a simple chart illustrates: And finally, we know from Henry’s Law that “the amount of gas which dissolves into a liquid is proportional to the partial pressure of that gas.” 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 “nitrogen narcosis.” As early as 1835 researchers noted that when breathing compressed air “the functions of the brain are activated, imagination is lively, thoughts have a peculiar charm and, in some persons, symptoms of intoxication are present” (Junod, 1835). It wasn’t until 1935, however, that researchers isolated nitrogen as the component in compressed air that produces the narcotic effect, noting that at 100 fsw “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.” At 300 fsw, “the signs and symptoms amounted to stupefaction, with greatly impaired muscular activity.” 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: 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 “narced” on air at 200 fsw, is like a driver who insists he is not drunk after four double scotches: he may believe it, “but it just ain’t so.” 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: Although the narcotic potency of an inert gas correlates well with a gas’s 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’s age, weight, fitness or a host of other factors. What’s 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.
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		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