Unit 1: What is NITROX? Is it better than air? Do you need it?

What is Nitrox? Nitrox is air in which fraction of nitrogen is reduced (PO2 table) Nitrox is also called: Oxygen-enriched air Enriched air nitrox EANX Nitrox Nitrox has some partial solutions to offer. As the term is used in recreational diving, nitrox is air in which the fraction of nitrogen has been reduced. Commonly, this is accomplished by adding oxygen to air, but it can just as easily be done by removing some of the nitrogen. As the nitrogen percentage goes down, the oxygen percentage goes up. The resulting gas mixture has been given several names: oxygen-enriched air, enriched air nitrox, and nitrox. EANX You will also see it called EANX, where the “x” is usually replaced by the percentage of oxygen. EAN32 would be a nitrogen-oxygen mixture that is 32% oxygen. Nitrox is the most widely used term. Actually, “nitrox” can refer to any mixture of nitrogen and oxygen, so plain, ordinary air would be nitrox, and in the past nitrox has even been used to refer to nitrogen-oxygen mixtures with less oxygen than air that were used in undersea habitat diving. Reducing Nitrogen Reducing the amount of nitrogen in what you breathe underwater addresses some of the problems caused by nitrogen. When using nitrox, it is possible to dive longer to a given depth or to reduce the required surface interval between dives. Nitrox may have other advantages too. Whether it is true or not, many nitrox divers report that they feel less fatigued after their dives. However, there are trade-offs. Increasing Oxygen As we reduce the nitrogen fraction, the oxygen fraction increases. As you will learn in this course, the higher oxygen content presents its own limitations. Oxygen breathed under high pressures can become poisonous. So for example, when diving with nitrox, we must limit our maximum depth in order to reduce the possibility of oxygen toxicity. Today, nitrox is almost universally accepted as a breathing gas for recreational diving. The benefits and advantages far outweigh the problems and possible disadvantages. As a diver, you need a certain amount of knowledge, awareness, caution, and sensibility to use nitrox safely, but this is just as true of diving in general.

A Bit of History Development of nitrox use Early research and use
Controversy NAUI endorsement of EANX NAUI Standards for EANX training Early Development: 1878 Paul Bert shows nitrogen to be cause of DCS. 1908 J. S. Haldane publishes first diving decompression tables. 1935 Behnke et al. attribute narcosis to nitrogen. 1959 U.S. Navy Diving Manual introduces oxygen-enriched air. 1979 NOAA Diving Manual publishes NOAA Nitrox I as standard mix. 1985 IAND formed–Rutkowski expands nitrox to recreational diving. Controversy: 1991 Nitrox training agencies almost barred from DEMA Show. 1992 aquaCorps/SDRG pre-DEMA Workshop. 1992 NAUI sanctions teaching enriched air nitrox. 1994 Rodale’s Scuba Diving supports nitrox training. 1995 Other recreational training agencies accept nitrox. NAUI endorsement of EANX In 1992, NAUI endorsed EANX and officially accepted certifications from the two specialized nitrox training agencies. As late as 1993, Skin Diver Magazine took an editorial position against the use of nitrox by recreational divers as certain to lead to dangerous misuse and accidents. Then, in 1994, Rodale’s Scuba Diving ran an article on becoming nitrox certified, the first mainstream recreational diving periodical to support nitrox training. In 1995, PADI, British Sub-Aqua Club, and other recreational training agencies announced that they would soon offer enriched air nitrox instruction programs. Also in 1995, Skin Diver Magazine declared use of nitrox acceptable. NAUI Standards for EANX training In 1996, NAUI codified standards for Enriched Air Nitrox training and certification in its revised NAUI Standards and Policies Manual. Shortly thereafter, NAUI published the first nitrox training textbook from a mainstream recreational dive training agency. Today, enriched air nitrox has become a viable and valued part of recreational diving, and “nitrox fills” are available to qualified divers at diving destinations around the world. In this text and in your EANX training course you will learn the facts about enriched air nitrox and how to use it safely.

Why Dive With Enriched Air Nitrox
Extended Dive Time Nitrox and some of the benefits of its use in diving. Here are a few of them. You will gain a greater appreciation of both the benefits and the necessary precautions as you learn more about diving with oxygen-enriched air. First and most notably, nitrox allows divers to extend their bottom time and enjoy longer dives without having a required-decompression obligation. (See Example, next page.)

Why Dive With Enriched Air Nitrox continued
Repetitive dive times and surface intervals Safety margins and post dive fatigue Nitrox has advantages in repetitive dives. Looking at a typical recreational dive profile, let’s compare a series of two dives: one diver is breathing air, and the other diver is breathing EAN36. The first dive is to 24 meters/80 feet for 30 minutes. After a one-hour surface interval, the second dive will be to 17 meters/55 feet. A comparison of dive plans showing the dive time advantage using nitrox. The diver breathing EAN36 has significantly greater no-decompression time than the air diver. Diver one, using air and NAUI Dive Tables, emerges from the first dive with a Letter Group of G. One hour later, entering the water as an “F diver,” the diver has an adjusted maximum dive time of 19 minutes for the 17 meter/55 feet dive. If the diver is using the NAUI RGBM Tables, he has a maximum dive time of 55 minutes for the second dive. Diver two, using EAN36 and NAUI EAN36 Dive Tables, finishes the first dive with a Letter Group of F. One hour later, she begins the second dive as an “E diver.” She has an adjusted maximum dive time of 62 minutes for her 17 meters/55 feet dive. If using the NAUI RGBM EAN36 Tables, she would have a maximum dive time of 115 minutes! On the second dive, the diver using enriched air nitrox has a 43-minute advantage over the diver using air if using the NAUI Dive Tables or a 60-minute advantage if using the NAUI RGBM Tables. Another benefit of nitrox is the possibility of a shorter required surface interval. In the example above, the nitrox diver has 62 minutes available dive time after a one-hour surface interval if using the NAUI Dive Tables. If the air diver using the same tables wanted to have a dive time of 30 minutes, he would have to attain a Letter Group of D before beginning the second dive. To move from “G” to “D” requires a surface interval time of at least 2 hours 59 minutes! The NAUI RGBM Tables are based on a one-hour surface interval, so those numbers do not change. Technical divers will breathe a highly oxygen-rich nitrox mixture during their shallower decompression stops or long exposures in the shallow zones in order to more efficiently eliminate nitrogen from their bodies. Another very common use of nitrox in diving is simply as a safety margin. Divers who choose to dive conservatively will often breathe nitrox but continue to use air dive tables or an air computer. Over a series of dives, they will absorb significantly less nitrogen than their tables or computers indicate, effectively lowering their risk of decompression sickness. Finally, there may be truth in the claim of divers that they are less physically tired after a series of dives on enriched air nitrox. With less nitrogen in their bodies than they would have had diving on air, the divers may indeed feel less fatigue at the end of their diving day or diving trip.

Common Misconceptions About Nitrox
Myth: “Nitrox is for technical diving” Myth: “Nitrox is for deep diving” Myth: “If you dive with nitrox you won’t get bent” Myth: “Nitrox is safer than air” Myth: “If you dive with nitrox you won’t get narcosis” Q: What causes narcosis? (PO2 table) Myth: “It is hard to dive with nitrox” As was said earlier, many misunderstandings developed about nitrox as it began to be known (but not well understood) in the recreational diving industry. Before you move on to the next topic in this course, you should be aware of some of the persistent myths about nitrox and be able to sift the fact from the fiction. Myth 1: “Nitrox is for technical diving.” Although technical divers use oxygen-enriched air when appropriate, such as during in-water decompression stops, the use of nitrox does not make a dive “technical.” Technical dives are planned decompression dives, deep dives where both nitrogen and oxygen in the breathing gas may be diluted with helium (trimix), dives in which one changes between gas mixes during the dive, dives that use an oxygen percentage greater than 40%, or dives into overhead environments such as caves or wrecks. Myth 2: “Nitrox is for deep diving.” Quite the opposite. Because of the increased concentration of oxygen in nitrox, there are stringent depth limits imposed to prevent oxygen toxicity. Nitrox is a mid-range breathing gas and provides the greatest advantages for dives in the meters/ feet depth range. Beyond that depth the decompression advantage gain is minimal, and the risk of oxygen toxicity problems increases rapidly. Later in this book you will learn how to determine maximum operating depth (MOD) for various nitrox mixes. Myth 3: “If you dive with nitrox you won’t get bent.” Enriched air nitrox only reduces the fraction of nitrogen in the gas you breathe; it does not eliminate it. Moreover, no breathing gas mixture, dive table, or dive computer can absolutely eliminate the possibility of decompression sickness. Nitrox has advantages over air because the partial pressure of the nitrogen is less for any given depth. Nitrogen ingassing is slower, but you must still monitor depth and time–as well as the specifics of your breathing mixture–to avoid excessive nitrogen accumulation in your body. The knowledge and procedures in this text will help you control the risks of decompression sickness while maximizing the safety and enjoyment of your dives. Myth 4: “Nitrox is safer than air.” Nothing that we do is entirely without risk. All diving involves some risk, and as noted above, there are risks associated with nitrox. In this course, you will learn procedures and techniques to responsibly manage and minimize the risks of using oxygen-enriched air. Nitrox has advantages over air in many diving applications, such as increased available dive time and decompression advantages, but with these advantages comes your added responsibility to be mindful of oxygen levels and depth and time limits, analyze you own gas mixture, properly maintain your equipment, and dive prudently. Myth 5: “If you dive with nitrox you won’t get narcosis.” It is easy to assume that the reduced nitrogen in nitrox should reduce narcosis just as it provides advantages in other areas. The fact is that narcosis involves many factors, some of which are psychological. Evidence on nitrox and narcosis is sparse, but it is best to regard the narcotic potency of nitrox as the same as that of air. While it has been theorized that unmetabolized oxygen may be as narcotic as nitrogen, recreational nitrox divers do not subject themselves to oxygen pressures where this could be a factor. In order to reduce narcosis on deep dives, technical divers use trimix to lower both the nitrogen and the oxygen content of their breathing gas. Myth 6: “It is hard to dive with nitrox.” This book will help you to learn the proper planning and procedures for conducting dives with enriched air nitrox. Most of the procedures are simple, and where there are calculations involved, the formulas are usually backed up with tables where you can look up the necessary derived information. Using nitrox involves an understanding of partial pressures and oxygen limits, somewhat more advance planning and preparation than air diving, and an acceptance of your added responsibility for safe diving. But the training to become a nitrox diver is far less than the initial training that prepared you to be a scuba diver. Once you are in the water, the rule of nitrox diving is the same as any other diving: Plan your dive, and dive your plan.

Unit 2: Gases & Gas Mixtures
Do you have the “v-planner” app?

What’s in Air? Composition of air Simplifying the numbers:
Oxygen (O2) Nitrogen (N2) Argon (Ar) Carbon dioxide (CO2) ~ (average) Others Simplifying the numbers: 21% oxygen / 79% nitrogen Composition of air Our atmosphere is a mixture of gases. The proportions of the gases in air, excluding water vapor, are nearly uniform around the globe. The composition of dry air, expressed as fractions is: Oxygen (O2) Nitrogen (N2) Argon (Ar) Carbon dioxide (CO2) ~ (average) Others Simplifying the numbers We normally simplify the numbers and say that oxygen is 0.21 or 21% of atmospheric air. We will also include argon, as an inert gas, with the nitrogen and say that nitrogen is 0.79 or 79% of air. The amount of carbon dioxide varies somewhat around the world (there is more present in the more industrialized northern hemisphere), and while carbon dioxide has a significant role in general human physiology as well as the physiology of diving, at only about 350 parts per million (ppm) of air, it does not enter into nitrox calculations. The “others” category includes neon, helium, krypton, sulfur dioxide, methane, nitrous oxide, etc.

Some Facts About Individual Gases
Oxygen (O2) Nitrogen (N2) Argon (Ar) Carbon Dioxide (CO2) Helium (He) Neon (Ne) Oxygen (O2) Oxygen is one of the most abundant elements on earth. It is present not only as a gas–as free oxygen in the atmosphere and as an element in atmospheric carbon dioxide; it is a constituent in many other compounds, such as silicon dioxide (quartz and sand). Much of what you will learn in this course is about the risks of oxygen, the proper use of oxygen, setting appropriate limits by controlling the oxygen fraction in the nitrox we use and the depth to which we dive, safety considerations in oxygen handling, and care of scuba and air-fill equipment that will be exposed to high concentrations of oxygen. Nitrogen (N2) Nitrogen is a largely inert gas. It is not used in metabolism and serves essentially to dilute the oxygen in the air we breathe. It is also colorless, odorless, and tasteless. Although it does not combine easily with other elements, it is a component in many organic compounds and is in all living organisms. When breathed at higher pressures, it has a pronounced anesthetic effect referred to as nitrogen narcosis. Argon (Ar) Argon makes up about 1% of air. As noted above, it is included as part of the nitrogen component in enriched air nitrox calculations. Argon is one of the “noble gases” meaning it is virtually completely inert, not combining with other elements. (The other noble gases are helium, neon, krypton, xenon, and radon.) Carbon Dioxide (CO2) Carbon dioxide is colorless, odorless, and tasteless in small quantities. It combines readily with water to form carbonic acid, and at higher concentrations it has an acidic taste and odor, which you can sometimes sense when you sniff fizzing carbonated beverages. At high concentrations (above about 10%), it can be extremely toxic and cause convulsions and death. Carbon dioxide is a natural byproduct of our respiration and combustion of organic compounds. Helium (He) & Neon (Ne) Both helium and neon are less dense than nitrogen. The have a very low narcotic potential and so are possible candidates to reduce both the nitrogen content and the oxygen content of the breathing gas in deep diving. Helium is the mixing gas of choice for diving deeper than the air depth range. You will learn more about helium if you venture into technical diving. “Trimix” refers to a mixture of oxygen, helium, and nitrogen. Neon is not only too expensive for general use; it is also very slowly released from the body tissues and so requires long decompression times.

How Gases Behave Boyle’s Law: Pressure, Volume, and Density
Review of gas laws The interrelation of pressure, volume, and temperature of gases are described by the “gas laws” or the Ideal Gas Laws. You met them in your entry-level scuba course. The separate laws were formulated experimentally by many investigators over a period beginning in the seventeenth century. Scientific laws are frequently associated with the names of persons who first observed and formulated them, and the rules of gas behavior are no exception. Boyle’s Law: Pressure, Volume, and Density The relationship between pressure, volume, and density of gases was studied by Sir Robert Boyle in the seventeenth century. There are four variables that can be altered in a gas sample–pressure, volume, temperature, and quantity. Boyle fixed the amount of gas and its temperature during his investigations. He found that when he changed the pressure, the volume responded in the opposite direction. Boyle's Law states: “At constant temperature, the volume of a gas varies inversely with absolute pressure, while the density of a gas varies directly with absolute pressure.” Expressed as a formula, for a given gas sample: PV = K where P is the absolute pressure; V is the volume; and K is a constant. As a working formula, we usually use: P1V1 = P2V2 where the P and V are the absolute pressure and the volume for any two sets of conditions (the “before” and the “after”).

How Gases Behave continued
Henry’s Law: The Solubility of Gases Henry’s Law: The Solubility of Gases Gases dissolve in liquids. Solids dissolve in liquids too. You know that sugar dissolves in water because you can see it happen. Most persons are less aware that gases also dissolve, although carbonated beverages are a perfect everyday example of gas solubility. Some gases are more soluble in a liquid than other gases, and some liquids are better solvents of a gas than other liquids. When the pressure of a gas on a liquid is increased, more of the gas will dissolve in the liquid until the partial pressure of the dissolved gas (or gas tension) equals with the impinging gas partial pressure. In other words, the amount of a gas that will dissolve in a liquid is directly related to the pressure of the gas on the liquid. This is described in Henry’s law: “The amount of any given gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas that is in contact with the liquid and the solubility coefficient of the gas in the particular liquid.” According to Henry’s law, the relationship is linear. If one quantity of gas will dissolve at one atmosphere of pressure, then three quantities of gas will dissolve at three atmospheres. Temperature also affects the quantity of a gas that will be absorbed by a liquid. The solubility of a gas is inversely related to the temperature–the higher the temperature, the lower the solubility and vice versa. The solubility of a gas in a liquid depends on temperature and the partial pressure of the gas over the liquid. It also is governed by the nature of the solvent and the nature of the gas. Nitrogen, for instance, is about five times more soluble in fatty tissue than in watery tissue. This difference in solubility is something that must be considered in decompression theory. Henry’s law addresses the quantity of a gas that will dissolve, but it does not describe the rate at which the gas will dissolve. Whenever the pressure of a gas on a liquid is increased, molecules of gas begin to diffuse into the liquid, and we say ingassing occurs. In the beginning, the gas moves rapidly into solution, driven by the high partial pressure of the gas on the liquid compared to the gas tension of the dissolved gas–the high pressure gradient. As the gas tension increases, the pressure gradient becomes less, and the gas dissolves less rapidly until equilibrium is reached and ingassing stops. This is known as saturation. When the pressure of the gas on the liquid is reduced, offgassing occurs, again more rapidly at first, then slowing until equilibrium is reached. This can be seen by looking at the surface interval credit table on a set of standard dive tables. The diver offgasses rapidly for the first two hours or so after surfacing, then progressively more slowly until offgassing is complete.

How Gases Behave continued
Dalton’s Law: Partial Pressure in Gas Mixtures Dalton’s Law: Partial Pressure in Gas Mixtures An understanding of partial pressure and its consequences is probably the most important concept to grasp for safe diving with enriched air nitrox. The partial pressure of a gas in a mix is the portion of the total pressure exerted by that gas. Whether we are at the surface or diving, our body responds to each gas in a gas mixture according to its partial pressure. With oxygen-enriched air as well as with the more exotic mixtures of technical diving, we are manipulating the gas percentages, and therefore the partial pressures, of the gas mixtures that we choose to breathe. We must know what we are doing and be able to plan safe limits to our diving. In the balance of this chapter, you will learn about partial pressure and how to determine the partial pressure of any gas in your breathing mixture at any depth. In the next chapter, we will explore how nitrogen and oxygen affect your body at different pressures. For any single, pure gas, the pressure of the gas is the total pressure. Its effect upon us or in chemical reactions, its solubility, and so on are directly related to its pressure. Oxygen, for instance, supports our life and also combustion. If there is too little oxygen, we will lose consciousness and die, and materials will burn poorly if at all. If the oxygen pressure is high, it can be toxic to us and objects will burn furiously. In any mixture of gases, such as air, the total pressure of the mixture is equal to the sum of the individual pressures exerted by each individual gas. This was first observed in the early nineteenth century by the English chemist John Dalton. Dalton’s law states: “The total pressure exerted by a mixture of gases is equal to the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the volume.” In other words, the whole is equal to the sum of the parts. The pressure exerted by each component gas is termed the partial pressure of that gas. Expressed mathematically: Ptotal = P1 + P2 + P Pn where Ptotal is the total pressure of the gas mixture, and P1, P2, etc. are the partial pressures of each component gas. Dalton’s Law can be expressed another way: “The partial pressure of any component gas in a mixture is the fraction of that gas in the mixture times the total gas pressure.” or, expressed as a formula: Pg = Fg x Ptotal where Pg is the partial pressure of the component gas; Fg is the fraction of the component gas in the mixture; and Ptotal is the total pressure of the gas mixture. A useful way to state this for recall purposes is simply: “The part is a fraction of the whole.” (In formulas, “is” translates into “equals” and “of” into “times.”) You will meet this statement and its formulation several times and in many guises in this book.

Converting Between Depth and Pressure
Absolute vs. gauge pressure Recall from your entry-level scuba course the difference between absolute pressure and gauge pressure. Depth is a gauge pressure. Your depth gauge reads zero at the surface even though you are actually under one atmosphere of air pressure. Each 10 meters/33 feet that you descend in the ocean adds another atmosphere of pressure to the one atmosphere of surface pressure. The absolute pressure is “absolutely everything,” and it is one atmosphere more than what your depth gauge is telling you. Atmospheres absolute is usually abbreviated “ata” or “ATA” to distinguish it clearly from “atm” which could also mean gauge pressure. Many short-handedly use “ata” as a word in itself–usually in the plural form “atas.” Don’t be taken aback if someone asks: “What’s the Pee Oh Two of Eee A En thirty-two at four atas?” As you’ll discover shortly, the answer is 1.28 atmospheres.

Converting Between Depth and Pressure continued
Converting by formula To find absolute pressure: P ata = (D fsw / 33 fsw/atm) + 1 atm = (D fsw + 33 fsw) / 33 fsw/atm To find depth: D fsw = (P ata – 1 atm) x 33 fsw/atm Converting Between Depth and Pressure by Formula Converting depth to pressure uses a simple equation in which you first find the water (hydrostatic) pressure in atmospheres and then add one atmosphere for the sea-level air pressure to convert to the absolute pressure. Written as an equation using the U.S./Imperial system: P ata = (D fsw / 33 fsw/atm) + 1 atm = (D fsw + 33 fsw) / 33 fsw/atm Where P = the pressure in atmospheres absolute D = the depth in feet of seawater Using the S.I./metric system: P barabsolute = (D msw / 10 msw/bar ) + 1 bar = (D msw + 10 msw) / 10 msw/bar where P = the pressure in bars absolute D = the depth in meters of seawater Conduct a class exercise using real numbers. Converting absolute pressure to depth is the reverse. First subtract the atmospheric pressure and then multiply the water pressure by 33 fsw per atmosphere. In the U.S./Imperial system: D fsw = (P ata – 1 atm) x 33 fsw/atm In the S.I./Metric system: D msw = (P barabsolute – 1 bar) x 10 msw/bar

Converting Between Depth and Pressure continued
Converting by table Converting Between Depth and Pressure with a Table You can also use a table for conversions

Calculating Partial Pressures
If you know the absolute pressure: The basic formula: Pg = Fg x Ptotal Using a graphical figure Calculating partial pressures by formula To determine the partial pressure of gas in a mixture, multiply the gas fraction (the gas percentage expressed as a fraction of one) by the absolute pressure. The equation can be rearranged to find the gas fraction/percentage if you know the partial pressure and the total pressure (Fg = Pg/Ptotal) or to find the absolute pressure if you know the partial pressure and fraction/percentage of the gas (Ptotal = Pg/Fg). Calculating by graphical figure Some people find it easier to use a graphic mnemonic device rather than a mathematical formula or a mnemonic phrase. To use the figure, cover the item you want to know or solve for. The mathematical expression will be shown by the two exposed items. If “partial pressure” is covered, the side-by-side terms indicate that they are to be multiplied. Similarly, if you want to know what fraction will provide a desired partial pressure on the dive, cover “fraction.” The uncovered terms (“partial pressure” over “total pressure”) tell you to divide the desired partial pressure by the absolute pressure at depth. Suggested class activity: The instructor can lead the participants in a variety of calculations using formula and graphical figure.

Calculating Partial Pressures continued
Moving between partial pressure and depth using formulas: Depth to partial pressure First find the absolute pressure at depth. Then find the partial pressure of the component gas at that absolute pressure. Partial pressure to depth First find the absolute pressure of the gas mixture from the partial pressure and fraction of the component gas. Then find the depth for that absolute pressure. Calculating partial pressure of a gas in a mixture at a given depth: First find the absolute pressure at the depth using In U.S./Imperial system: P ata = (D fsw / 33 fsw/atm) + 1 atm = (D fsw + 33 fsw) / 33 fsw/atm In S.I. metric system: P barabsolute = (D msw / 10 msw/bar ) + 1 bar = (D msw + 10 msw) / 10 msw/bar Then find the partial pressure of the component gas at that absolute pressure using P gas = F gas x P absolute

Calculating Partial Pressures continued
Using a table Calculating partial pressure by table To save the effort of calculating, one can employ an oxygen partial pressure table, which can quickly show the partial pressure of oxygen for various mixes over a range of depths. To find the oxygen partial pressure for a nitrox mixture, move along the top row to the oxygen fraction of the mix. Then move down that column to the row of the desired depth. If the exact depth is not shown, round up to the next higher value. Suggested class activity: The instructor can lead the participants in a variety of calculations using the table.

Unit 3: The Physiology of Diving and Nitrox

Nitrogen Narcosis The mechanisms of nitrogen narcosis are similar to that of gases used in general anesthesia. Divers may not be aware that they are impaired. There is no appreciable benefit to breathing nitrox. Ascent to a shallower depth is all that is required. Nitrogen Narcosis The mechanisms of nitrogen narcosis are imperfectly understood, but they are thought to be similar to that of gases used in general anesthesia for surgical procedures. Narcotic effects are not limited to nitrogen. Any inert gas breathed under pressure can cause narcosis, and the malady is more correctly called inert gas narcosis. The narcotic potency of an inert gas is related to its solubility in lipid (fatty) tissues. Because helium has minimal narcotic potency, it is used as a diluting gas in trimix when diving to depths where nitrogen narcosis becomes a serious concern or incapacitating. The effects of narcosis can be measured at shallower depths, but they become more pronounced when the partial pressure of nitrogen is approaching four atmospheres, which is about 39 msw/130 fsw when breathing air. Symptoms increase with increasing depth. Other factors also affect the degree of narcosis. Anxiety and stress, fatigue, cold, hard work, high carbon dioxide levels in the body and alcohol have all been shown to enhance narcosis. On the other hand, positive motivation seems to reduce its effects. Some divers report acclimatization following repeated exposures, but studies have shown that any adaptation is largely subjective. Narcosis itself is not the danger, but the impaired judgment, loss of orientation, and reduction in problem solving capabilities are, and the “narked” diver is at increased risk. It may become difficult for a diver to monitor time, depth, and air supply, remember the dive plan, or concentrate on the task at hand. Perhaps the most insidious thing about nitrogen narcosis is that divers may not be aware that they are impaired. At first glance, you might assume that you should be less subject to nitrogen narcosis when diving with nitrox because you have replaced some of the nitrogen with oxygen. However, there is no hard or definitive evidence to support this, and it is safer to assume that there is no appreciable benefit to breathing nitrox. As you will remember from your entry-level scuba course, nitrogen narcosis is easily reversible. An ascent to a shallower depth is all that is required. The symptoms disappear as you ascend. It could also be that the stress or anxiety that you are experiencing at depth is as much due to psychological factors of being outside your personal “comfort envelope.” In this case, the obvious solution is also an ascent to a shallower, more comfortable depth.

Decompression Sickness
What causes it DCS signs and symptoms You learned about the basics of decompression sickness (DCS or “the bends”)–what causes it, its signs and symptoms, how to avoid it, and how it is treated–in your entry-level scuba course. A brief review will be useful, and we can now use some of the concepts you learned in earlier chapters of this book. Again, if you want to learn more about decompression theory, the NAUI Master Scuba Diver course will greatly increase you knowledge and understanding. During a dive, our bodies are exposed to increased pressure. While we are underwater, the increased partial pressure of the nitrogen in the air (or gas) we are breathing forces additional nitrogen into solution in the tissues of our body (Henry’s Law). Passing out of the lungs through the walls of the alveoli, dissolved nitrogen enters the blood and is carried to all parts of our body. There, the pressure gradient between the nitrogen dissolved in our blood and the nitrogen in the surrounding tissues causes dissolved nitrogen to move into the tissues. The greater the pressure (depth) and the longer we are submerged, the more nitrogen will dissolve in our bodies until eventually the gas tension (partial pressure) of the dissolved nitrogen in our tissues reaches equilibrium with the partial pressure of the nitrogen in our breathing mixture at that depth. This may take more or less time depending on many factors, including the type of tissue and the circulation to it. Because nitrogen is metabolically inert, it simply remains in our tissues while we are at depth. When we ascend properly from a dive, the reverse occurs. The partial pressure of nitrogen in our lungs is now reduced (to atmosphere at the surface), and what is now excess dissolved nitrogen migrates from the areas of higher nitrogen tension (tissues) and diffuses into the free phase nitrogen within the blood stream (that is, into nitrogen is already present as micro-bubbles). The interplay between the dissolved phase and the free phase (bubbles) determines the effectiveness of safe nitrogen elimination as well as possible damage to the body. Ideally, the free phase nitrogen is carried via the blood to the lungs where it passes into the alveoli and is exhaled. This elimination occurs over time until the nitrogen tensions in our body’s tissues are again in equilibrium with the partial pressure of nitrogen in the atmosphere. This absorption and elimination of nitrogen are termed ingassing and offgassing. However, if we dive too deep, for too long, and then ascend too fast, the pressure gradient between the dissolved nitrogen and the ambient pressure is too great for proper, controlled offgassing, and released nitrogen diffuses into the free-phase bubbles within the body. Once the bubbles enlarge and begin to aggregate they cause mechanical injury and also block the blood flow to tissues, and the trauma presents as decompression sickness. The signs and symptoms of DCS vary greatly depending on the location of the injury and the severity of the case. They can range from simple excessive fatigue to weakness and tingling in the limbs, limb and joint pain and dysfunction, skin rashes and itching, vertigo, loss of sensation, paralysis, and even death. Divers once referred to “niggles,” (from the word “niggling,” meaning troublesome or irritating) mild symptoms of DCS that would probably (or hopefully) resolve themselves and go away over time. But in reality, anyone with a suspected case of DCS should be administered 100% oxygen as immediate first aid, examined by a trained physician, and if decompression sickness is suspected, receive recompression chamber treatment. If in doubt, treat! Untreated, even the mildest case of decompression sickness may worsen over time, and permanent injury is a more likely outcome.

Decompression Sickness continued
How to avoid DCS Treatment for DCS You have probably heard that “all dives are decompression dives.” This is a truism, in that; any reduction in pressure is consistent with the literal definition of decompression. Every diver ascending from depth is experiencing decompression in this sense. Divers, however, usually use decompression to mean the controlled reduction of pressure on ascent in order to control bubble growth and prevent decompression sickness. It is the rate of ascent that will determine level of risk with regard to bubble growth in accordance with Boyle’s Law calculations. As divers, we plan our dives to prevent excessive nitrogen loading during the dive and to control bubble growth on ascent in order to avoid “getting bent.” To plan dives, we use dive tables or dive computers. Because we reduce the rate of nitrogen uptake when we breathe oxygen-enriched air, special nitrox dive tables or computers are used for nitrox diving. We can also use a calculated “equivalent air depth” with standard air tables. These tools are discussed later in their own chapter. As a responsible diver, you should be mindful of other predisposing factors that are likely to increase susceptibility to DCS. Dehydration, which reduces the efficiency of your circulation, is now believed to be a major contributor to the development of DCS. Divers are well advised to drink plenty of fluids and avoid excessive amounts of diuretic drinks, such as coffee, caffeinated soft drinks, and alcohol, which can contribute to dehydration instead of preventing it. During diving, heavy exercise will increase circulation to the exercised parts and speed up ingassing. After diving, exercise can increase likelihood of bubble formation, while resting and relaxing will give the body better opportunity to offgas properly. General physical fitness is also beneficial because it promotes more efficient muscle use and blood flow. A person’s general circulation becomes less efficient with increasing age, and therefore older divers are advised to dive more conservatively.

Oxygen: The Good and the Bad
Is necessary to sustain life Too high an oxygen level can be just as harmful as too low In your beginning scuba course, your instructor probably said: “At extreme depths, even the oxygen in the air you breathe can become toxic, but this only happens at depths far greater than recreational limits, so don’t worry about it.” End of subject. Now you are learning to dive with oxygen-enriched air, and oxygen toxicity and oxygen safety are very real concerns. Oxygen is vital to our being. It is our essential life-support element. Inhaled oxygen is metabolized in our tissues, producing the energy necessary to sustain life. If we are deprived of oxygen, our survival time is measured in just minutes. Still, our bodies operate well only within a rather narrow range of oxygen partial pressures. Too high an oxygen level can be just as harmful as too low. Moreover, because oxygen is so highly reactive, supporting combustion and combining aggressively with many substances, special care and precautions are required when handling pure oxygen or gas mixtures that are high in oxygen concentration. A later chapter of this book is devoted to oxygen handling and equipment considerations. “Love, is like oxygen... You get too much you get too high. Not enough and you're gonna die.”

Oxygen and Metabolism Oxygen is our life-support gas
The primary waste product is carbon dioxide Oxygen and Metabolism Oxygen is our life-support gas, the gas we extract from air in respiration and use in metabolism to generate heat and energy. It is absorbed through our lungs, combined with the hemoglobin in our red blood cells, and delivered to the tissues via the arterial system. There it is metabolized to produce our life-maintaining energy. The primary waster product is carbon dioxide, which- dissolved in the serum of our blood- is delivered back to the lungs via the venous side of the circulatory system and exhaled.

Physiological Effects of Low Oxygen Levels (Hypoxia)
Hypoxia means “low oxygen” symptoms begin to appear if the partial pressure of inspired oxygen falls below about atmosphere Onset of symptoms at a PO2 of about ata Signs and symptoms include impaired mental performance and defective memory, blueness of the lips (cyanosis), fatigue, visual disturbances, and dizziness Physiological Effects of Low Oxygen Levels (Hypoxia) Our bodies are optimized to operate at greatest efficiency when the oxygen partial pressure approximates that of standard sea-level air, 0.21 atmospheres absolute. People who live at altitude do become acclimated to lower oxygen levels by increasing the number of red blood cells in their bodies and other changes. But, if the partial pressure of oxygen falls too low, the oxygen supply is inadequate to the task of fully supporting us, the symptoms of hypoxia, which means “low oxygen,” begin to appear. The brain, which requires large amounts of energy, is among the first to suffer. Manifestations of hypoxia include impaired mental performance and defective memory, blueness of the lips (cyanosis), fatigue, visual disturbances, and dizziness. For most persons, symptoms begin to appear if the partial pressure of inspired oxygen falls below about 0.16 atmosphere.

Physiological Effects of Low Oxygen Levels (Hypoxia) continued
If the PO2 falls below about 0.10 ata more severe symptoms leading to unconsciousness will occur. Hypoxia must be monitored in some rebreather situations or deep diving applications. If the partial pressure of inspired oxygen falls below about 0.10 atmosphere more severe symptoms leading to unconsciousness will occur. Because the breathing stimulus is largely controlled by carbon dioxide levels rather than oxygen levels, a hypoxic diver rarely feels any warning of impending loss of consciousness. Hypoxia is not a problem for divers breathing either air or oxygen-enriched air, but it must be monitored in some rebreather situations or deep diving applications. Extremely deep dives may require low percentages of oxygen in the “bottom mix” (the breathing gas used at depth), and mistakenly breathing the bottom mix at shallow depth can lead to hypoxia. Although extremely rare (only one reported event since accident records have been kept) hypoxia has also occurred from internal rusting of a steel cylinder stored over many months with some water inside, which supported additional rusting that consumed the oxygen in the stored air.

Physiological Effects of High Oxygen Levels
Central Nervous System Toxicity has a wide range of signs and symptoms, the most dramatic being epilepsy-like convulsions CNS toxicity can result from relatively short exposures to high partial pressures of oxygen Pulmonary Toxicity or Whole Body Toxicity results from prolonged exposure to elevated partial pressures of oxygen (above about 0.5 atmosphere) not a concern for recreational nitrox diver single dose Physiological Effects of High Oxygen Levels Oxygen is essential to us, and it also plays an important role in the treatment of diving maladies such as decompression sickness. Divers planning stage-decompression dives will breathe high concentrations of oxygen during decompression stops in order to offgas nitrogen more quickly, but safely. As nitrox divers, we use oxygen-enriched air to safely prolong our dive time or to increase our nitrogen safety margins. However, because we are utilizing an oxygen-enriched mixture, we must control and monitor the inspired partial pressure of oxygen we are breathing as well as pay attention to the other parts of dive planning. If we did not do so, we could easily get into trouble by diving to depths that allow the oxygen partial pressure in our breathing gas to become dangerously high. With oxygen and nitrox, it is entirely possible to get “too much of a good thing.” There are two types of oxygen toxicity. Central nervous system toxicity has a wide range of signs and symptoms, the most dramatic being epilepsy-like convulsions. CNS toxicity can result from relatively short exposures to high partial pressures of oxygen. The other type is called pulmonary toxicity or whole body toxicity. Pulmonary toxicity results from prolonged exposure to elevated partial pressures of oxygen (above about 0.5 atmosphere). As the name implies, its most pronounced effects are on the lungs, producing signs and symptoms such as chest tightness, breathing discomfort and pain, shortness of breath, and coughing. Development of pulmonary toxicity requires long-term exposures such as encountered in saturation diving, certain military and commercial diving, or recompression chamber treatment. Pulmonary toxicity is not a concern of the recreational nitrox diver; CNS toxicity is a concern. several doses

Oxygen Partial Pressure Limits
The generally accepted limit for nitrox diving is: 1.4 ata PO2 1.6 ata PO2 as a contingency 1.4 ata PO2 is more than adequate for 99.9% of the dives you may want to accomplish For recreational nitrox diving, the generally accepted PO2 exposure limit is 1.4 atmospheres absolute, with 1.6 ata reserved for contingencies.

Central Nervous System Toxicity
Factors that can increase your susceptibility to CNS oxygen toxicity heavy exercise, increased carbon dioxide build-up, chilling or hypothermia, and water immersion One cannot predict oxygen toxicity Oxygen Partial Pressure Limits Susceptibility to CNS oxygen toxicity varies greatly between individuals and is affected by other factors and conditions as well. One’s oxygen tolerance has also been shown to vary from day to day. It is impossible to predictably relate CNS oxygen toxicity appearance to any definite PO2 and time exposure. Nevertheless, it is certain that the greater the oxygen partial pressure and the longer the time of exposure, the more likely it is that symptoms of CNS toxicity will develop. In setting oxygen exposure limits, it is best to err on the side of safety. Beginning with its first presentation of oxygen-enriched air for scuba diving in the second edition of the NOAA Diving Manual (1979), NOAA has recommended a maximum oxygen partial pressure of 1.6 ata. In the new fourth edition (2001), they add the cautionary comment: “A slightly lower level provides less oxygen exposure risk.” Diving to a PO2 higher than 1.6 ata has been likened to knocking on the door of a casino. Once you go in, you could win, or you could lose a little, or you could lose a lot, but in the long run the house always wins. For recreational nitrox diving, the generally accepted PO2 exposure limit is 1.4 atmospheres absolute, with 1.6 ata reserved for contingencies. With appropriate selection of your enriched air nitrox mixture, 1.4 ata PO2 is more than adequate for 99.9% of the dives you may want to accomplish. Among the many factors that can increase your susceptibility to CNS oxygen toxicity (an “Ox-Tox Hit”) are heavy exercise, increased carbon dioxide build-up from whatever cause, chilling or hypothermia, and water immersion (as opposed to “chamber diving”). One cannot predict oxygen toxicity. “the bell curve...”

Central Nervous System Toxicity continued
The mnemonic acronym “ConVENTID” is useful for remembering the most obvious of them: Convulsions Visual disturbances Ears Nausea Twitching or Tingling Irritability Dizziness or Dyspnea It is also impossible to predict a reliable sequence of toxicity signs and symptoms. The first noticeable sign may be the epilepsy-like convulsions. This may not be serious in itself, but it is most certainly a problem if it occurs at a depth of 30 meters/100 feet while breathing out of a scuba regulator. Drowning is a very likely result. Many preliminary manifestations of CNS oxygen toxicity have been reported, either singly or in combination. The mnemonic acronym “ConVENTID” is useful for remembering the most obvious of them. ConVENTID stands for: Convulsions, Visual disturbances, Ears, Nausea, Twitching or Tingling, Irritability, and Dizziness or Dyspnea.

Central Nervous System Toxicity continued
Convulsions are the most obvious and most serious signs. Possible precursors to convulsions are: Visual disturbances, tunnel vision, dazzle or seeing “fireflies.” Ear ringing, tinnitus, or sounds like an approaching train in a tunnel. Nausea, including vomiting. Twitching, especially of the lips and small facial muscles or the hands, or tingling (paresthesia) especially in the fingers. Irritability, restlessness, euphoria, dysphoria (uneasiness or feelings of impending doom), anxiety, or general confusion. Dizziness and vertigo or dyspnea (difficult or labored breathing). Convulsions are the most obvious and most serious signs. Possible precursors to convulsions are: Visual disturbances, tunnel vision, dazzle or seeing “fireflies.” Ear ringing, tinnitus, or sounds like an approaching train in a tunnel. Nausea, including vomiting. Twitching, especially of the lips and small facial muscles or the hands, or tingling (paresthesia) especially in the fingers. Irritability, restlessness, euphoria, dysphoria (uneasiness or feelings of impending doom), anxiety, or general confusion. Dizziness and vertigo or dyspnea (difficult or labored breathing). Other signs can include facial pallor, slowed heart rate (bradycardia), pupil dilation, hiccups, and hallucinations. Appearance of any sign or symptom of oxygen toxicity is reason to terminate the dive. But, because precursor symptoms are highly variable–as well as subjective–and they may not occur, it is doubly important that divers keep their PO2 exposure within an acceptable limit. In one study, convulsions were the first noticed manifestation in 40% of the subjects studied. In another study, nausea was the most common first manifestation, followed by muscular twitching and vertigo. If a convulsion were to occur underwater, there is little that can be done until the active phase of the seizure is over and the muscles relax. Muscle contraction may cause the diver to lose the regulator, but the victim also ceases to breathe during the active phase as the vigorous, uncontrolled muscle contractions interrupt breathing and the tongue blocks the airway. No attempt should be made to surface victims of an “Ox-Tox Hit” at this time because they are effectively holding their breath. Because the convulsion was precipitated by breathing a high partial pressure of oxygen, and oxygen tensions in the body are therefore high, the person remains well oxygenated during the convulsion, and hypoxia is not a problem. Carbon dioxide levels will also become very high because the muscles are exercising heavily while the victim is not breathing. When the post-convulsive, resting phase begins, the muscles relax, and the victim remains unconscious. At this point, the victim can be taken to the surface and first aid care begun.

Managing Oxygen Exposure
The best way to avoid oxygen toxicity problems is to stay within correct oxygen exposure limits. OR... Managing Oxygen Exposure The best way to avoid oxygen toxicity problems is to stay within correct oxygen exposure limits. As stated above, the generally accepted limit for diving is 1.4 ata PO2, with 1.6 ata PO2 as a contingency. These limits are not lines drawn with a sword in the sand–see the casino comments above. Many divers have dived to 200 feet/61 meters and even considerably deeper on air, and most have returned none the worse for wear. But prudence should be part of all dive planning, and if you have reason to dive beyond recreational limits, you should definitely be preparing for and taking a NAUI trimix training course as well as one in decompression techniques. In deep diving, trimix reduces not only your oxygen exposure but also your nitrogen exposure. (If you want to safely dive deep and remember what you saw and did down there, try trimix.) The oxygen exposure limits described in this book carry an extremely low risk of oxygen toxicity. They are well below any levels that might reasonably be expected to cause problems. In addition to a general PO2 limit, NOAA, in the third edition of its Diving Manual, introduced oxygen exposure time limits for a range of oxygen partial pressures from 0.6 ata to 1.6 ata (see Table 3-1). The table shows allowable time for a single dive at any PO2 as well as the maximum accumulated exposure time over any 24-hour period. For example, for a PO2 of 1.0 ata (equivalent to 124 feet/38 meters on air), the maximum dive time for a single dive is 300 minutes (and is the same for any 24-hour period). For a PO2 of 1.4 ata (111 feet/33 meters on EAN32) the time limit for a single dive is 150 minutes (and 180 minutes in any 24-hour period). What would you do?

Managing Oxygen Exposure continued
NOAA Oxygen Exposure Limits In addition to a general PO2 limit, NOAA has oxygen exposure time limits for a range of oxygen partial pressures from 0.6 ata to 1.6 ata In addition to a general PO2 limit, NOAA, in the third edition of its Diving Manual, introduced oxygen exposure time limits for a range of oxygen partial pressures from 0.6 ata to 1.6 ata (see Table 3-1). The table shows allowable time for a single dive at any PO2 as well as the maximum accumulated exposure time over any 24-hour period. For example, for a PO2 of 1.0 ata (equivalent to 38 meters/124 feet on air), the maximum dive time for a single dive is 300 minutes (and is the same for any 24-hour period). For a PO2 of 1.4 ata (33 meters/111 feet on EAN32) the time limit for a single dive is 150 minutes (and 180 minutes in any 24- hour period). In all cases, a recreational nitrox diver’s single dive time will be limited by the no- decompression limits as well as the diver’s gas supply. These will be less than the NOAA single dive oxygen exposure limit, and the controlling limit, if it arises at all, would be the 24-hour oxygen exposure limit. Even then, only the most dedicated and determined nitrox diver would possibly exceed the 24-hour limit. (A diver using EAN36 and diving to the maximum dive times allowed by the NAUI EAN36 Dive Tables would have to perform seven square-profile dives to 27 meters/90 feet with dive times of 50 minutes for the first dive and 24 minutes for each repetitive dive with a minimum surface interval of 2 hours 39 minutes between each dive.)

Avoiding CNS Toxicity CNS toxicity is avoided by abiding by easily managed limits. Remember that the recommended maximum PO2 for recreational nitrox diving is 1.4 atmospheres, with a PO2 of 1.6 atmospheres as a contingency amount. Plan your dives and choose a nitrox mix that is appropriate to the dive. Avoiding CNS Toxicity CNS toxicity is avoided by abiding by easily managed limits. Avoid excessive oxygen partial pressures. Remember that the recommended maximum PO2 for recreational nitrox diving is 1.4 atmospheres, with a PO2 of 1.6 atmospheres as a contingency amount. Plan your dives and choose a nitrox mix that is appropriate to the dive. In the next chapter you will learn how to find the maximum operating depth for any given enriched air nitrox mixture as well as how to determine the optimal mix when you know the planned depth of the dive.

Unit 4: Choosing the Best Nitrox Mix

Enriched Air Nitrox Mixtures
Two Standard Nitrox Mixes EANx 32 or NOAA Nitrox I EANx 36 or NOAA Nitrox II Partial Pressure of Oxygen is the Limiting Factor Limit P02 to 1.4 ATA 1.6 ATA as a contingency Enriched Air Nitrox Mixtures Historically, there are two standard nitrox mixes, 32% oxygen and 36% oxygen, also called NOAA Nitrox I and NOAA Nitrox II. Many basic nitrox courses teach only the use of these two mixtures. In this NAUI course, however, you will be qualified to plan dives using any oxygen enriched air mixture from air up to 40% oxygen (the upper limit that does not require special cleaning of all equipment–see Chapter 6). The partial pressure of oxygen is the limiting factor for all diving with oxygen- enriched air. If you are to limit your exposure to a PO2 of 1.4 atmospheres (1.6 ata as a contingency), then you must be able to establish the maximum operating depth for the gas mixture with which you are diving. On the other hand, if you know your planned maximum depth in advance, you should be able to pick the best mix for that dive from the range available to you (air to 40% oxygen) and request the optimal blend of oxygen and nitrogen when you have your cylinder filled by the blending technician.

Maximum Operating Depth
The maximum operating depth (MOD) is the maximum depth that should be dived with a given nitrox mixture. MOD should be derived from the recommended maximum oxygen partial pressure of 1.4 atmospheres MOD should be written prominently on the cylinder’s contents label The maximum operating depth (MOD) is the maximum depth that should be dived with a given nitrox mixture. You will have to know your MOD if you are only able to obtain a particular nitrox blend, such as EAN32. Regardless of the blend, you need to establish the maximum depth to which you can dive using that cylinder. If you are able to obtain a custom blend to your specifications, you can use the more flexible “best mix” and ask for an appropriate oxygen percentage in your mix. For normal recreational nitrox diving, the MOD should be derived from the recommended maximum oxygen partial pressure of 1.4 atmospheres, but it can be calculated for other oxygen partial pressures as well. When a cylinder is filled, the mixture in a cylinder is analyzed, logged, and a contents label is placed on the cylinder. At that time, the MOD should be written prominently on the cylinder’s contents label. If the reference limiting PO2 is other than 1.4 ata, this should be noted on the cylinder too.

Maximum Operating Depth continued
MOD by Table (imperial) To determine the maximum operating depth, go to the column that contains the percent oxygen in your mix. If your mix is not on the table, use the next greater percent. Move down the column to the row that is your chosen maximum oxygen partial pressure (probably 1.4). The maximum operating depth in feet of seawater is shown at the intersection of column and row. Example: You are given a cylinder of EAN36. To find the MOD, read across the top row to the column marked 36%. Then read down that column to the row for 1.4 ata. Your maximum operating depth is 95 feet. If you dive deeper, you will exceed 1.4 ata PO2. Example: You receive a cylinder that contains EAN33, and you choose to dive to a more conservative oxygen partial pressure of 1.3 atmospheres. To find your MOD, read across the top row to the column marked 34% (rounding up from 33%). Then read down the column to the row for 1.3 ata. Your maximum operating depth for your chosen oxygen partial pressure is 93 feet.

Calculating MOD Begin by finding the total pressure that it takes to produce the maximum acceptable oxygen partial pressure Pg = Fg x Ptotal Then convert to a depth D fsw = (P ata – 1 atm) x 33 fsw/atm To calculate the maximum operating depth for any nitrox mixture, begin by finding the total pressure that it takes to produce the maximum acceptable oxygen partial pressure. Then convert this total pressure to a depth. You can do this in two separate steps: Example: What is the maximum operating depth for EAN36 (using 1.4 ata PO2 as your acceptable limit)? Step 1: Find how many total atmospheres of pressure will produce your target PO2. To help you remember the formula, use the mnemonic phrase: “The part is a fraction of the whole” or the mnemonic diagram that was presented in Chapter 2: P ata = 1.4 atmospheres / 0.36 = 3.9 atmospheres absolute Step 2: To find the depth at which the absolute pressure is 3.9 ata, use the formula from Chapter 2: = (3.9 atm - 1 atm) x 33 fsw/atm = 95 fsw (rounding down), or = (3.9 atm x 33 fsw/atm) - 33 fsw = 95 fsw Using S.I./metric measurements with a value of 10 msw/bar, the depth is 29 meters.

Combining the two steps The two step process can be combined into a single formula by replacing the total pressure in Step 2 with its equivalent “PO2 limit / FO2” that you used in Step 1: Or, if finding the absolute pressure first and then subtracting the number for fsw in one atmosphere: Using the first formula, the above example becomes: D fsw = ((PO2 limit/FO2) – 1 atm)) x 33 fsw/atm or D fsw = ((PO2 limit / FO2) x 33 fsw/atm) – 33 fsw Our example using one step D fsw = ((1.4 ata / 0.36) – 1 atm) x 33 fsw/atm = 95 fsw

Using the OCEANx to Establish MOD Do you have the app? Do you have a dive computer? The OCEANx calculator is a wheel-type tool that allows the oxygen percentage of the mix to be dialed in (See Figure 4-2). Oxygen percentages from 25% to 40% can be set on the OCEANx. With the appropriate oxygen percentage dialed into the window at the upper edge of the wheel, the calculator shows the actual depth for partial pressures of oxygen from 1.0 to 1.6 atmospheres. These actual depth values appear in the “AD fsw” window at the bottom. The OCEANx also displays the equivalent air depth for actual dive depths up to a PO2 of 1.6 and other oxygen exposure information, such as NOAA single dive oxygen exposure time limit. Equivalent air depth will be used in the next chapter’s discussion of using standard air dive tables with nitrox.

Best Mix “Best mix” is the nitrox mixture with highest fraction or percentage of oxygen that can be used at the target depth.

Choosing Best Mix Using the Best Mix Table Calculating the Best Mix
Using the OCEANx to establish best mix Choosing Your “Best Mix” The standard nitrox mixes of 32% and 36% oxygen are sufficient for most recreational oxygen-enriched air diving. The NAUI EAN32 and EAN36 dive tables, which are presented in the next chapter, have a tolerance of ±1% and are therefore useable for mixes of 31% to 33% and 35% to 37% oxygen. Yet there may be times when a diver wants to maximize no-required-decompression-stop dive time for a specific depth. To do this the diver needs to determine the highest fraction or percentage of oxygen (up to EAN40) that can be used at the target depth. The filling technician can then blend the specific mix for that dive. This mix is known as the “best mix.” Best mix can be determined from a table, from a formula, or by using the OCEANx Calculator.

Choosing Best Mix continued
The Best Mix Table Best Mix By Table Table 4-3 presents oxygen percentages (up to 40% maximum) that will provide oxygen partial pressures from 1.2 to 1.6 atmospheres at various depths. Except for contingency purposes, the diver should select a PO2 of 1.4 or less. Some extra- cautious divers always use a PO2 of 1.3 as their personal maximum. After selecting the PO2, move down the column to the row that shows the planned depth, rounding up if the exact depth is not on the table. The intersection of column and row shows the best mix oxygen percentage for the dive.

Calculating best mix is similar to the calculation for maximum operating depth in reverse. Step 1: Determine the absolute pressure at the target depth Calculating Best Mix Calculating best mix is similar to the calculation for maximum operating depth in reverse. First, convert the target depth into the absolute pressure. Next, determine the fraction of oxygen in the mixture that will produce the maximum acceptable PO2 at that absolute pressure. This can be done in two separate steps, or the two steps can be blended into a single formula. Example: What is the best mix for a dive to 99 fsw if the oxygen partial pressure is not to exceed 1.4 atmospheres? Step 1: Calculate the absolute pressure at 99 fsw. Divide the depth by 33 fsw/atm to determine the hydrostatic pressure; then add one atmosphere for the air pressure at the surface to find the absolute pressure. Use either of the formulas in Chapter 2: P ata – [99 fsw / (33 fsw/atm)] + 1 ata = 4 ata or

Step 2: Determine what fraction will produce the target partial pressure at that absolute pressure Fg = Pg / P total or FO2= PO2 / P total Step 2: Find what fraction of oxygen will produce the target PO2 at 4 atmospheres absolute. The basic formula (the part is a fraction of the whole) is: Pg = Fg x Ptotal Or, since you know the target partial pressure and the absolute pressure: Fg = Pg / Ptotal or FO2 = PO2 limit / P ata Enter the known values: FO2 = 1.4 ata / 4 ata = 0.35 EAN35 is the best mix for the dive. The procedure is identical in an S.I./metric calculation. What do these abbreviations stand for?

The two steps can be combined into a single formula. The two-step process can be combined into a single formula by replacing the absolute pressure in step 2 with its formulaic equivalent from step 1: Or, in the example: FO2 = (1.4 atm x 33 fsw/atm) / (99 fsw + 33 fsw) = 0.35

Using the OCEANx Calculator for Best Mix Using the OCEANx to Establish Best Mix The OCEANx calculator can be used to determine your best mix. Beginning with the O2% window set at 40%, rotate the upper wheel counter-clockwise while observing the bottom window area that shows actual depth. Turn the wheel until you see your target depth appear next to the value 1.4 (or your chosen maximum PO2) in the PO2 column. If the exact target depth does not appear, use the next window that will show the next greater actual depth adjacent to Your best mix for the dive is now displayed in the O2% window. When diving with oxygen-enriched air, it is essential that you know the maximum operating depth for the gas with which you are diving. The MOD should be determined for each cylinder independently at the time the contents are analyzed (see Chapter 8). It should be marked clearly on the cylinder contents label, and the cylinder should be checked before the dive. Best mix allows you to have more control over your diving by using the optimal nitrox blend for your dives. Perhaps gaining additional no-required-decompression dive time in the process.

Unit 5: Dive Tables and Dive Computers

Dive Tables There are many different dive tables in use today
NAUI Dive Tables NAUI RGBM Tables U.S. Navy Tables DCIEM Tables Buhlmann based tables Other Tables Do you know how to use “dive tables?” Dive Table In your entry-level Scuba Diver course, you learned how to use dive tables to plan and execute dives. You learned that dive tables are used to monitor and control the amount of nitrogen in our bodies in order to minimize the risk of decompression sickness. This chapter will build on that knowledge and acquaint you with using dive tables designed for diving with oxygen-enriched air. There are many different dive tables in use today. They are based on varying models of nitrogen absorption and elimination. If you were certified as a NAUI Scuba Diver, then you probably used either the NAUI Dive Tables or the NAUI RGBM Dive Tables to plan your dives. Or, you might have learned to use the U.S. Navy Tables (on which the NAUI Dive Tables are based), the Canadian Defence and Civil Institute for Environmental Medicine (DCIEM) Tables, or tables developed by Dr. A. A. Bühlmann of Switzerland (which were also designed to accommodate altitude diving). All three of these tables are endorsed for use by NAUI in NAUI courses. Another algorithm, which was developed under the auspices of the Diving Science and Technology Corporation, is used in the PADI Recreational Dive Planner. The British Sub Aqua Club has its own tables; the British Royal Navy has its; Stolt Offshore has its. In fact, many navies and most commercial diving operations have developed proprietary dive tables for their own use. Most have also developed tables for use with oxygen- enriched air. If you want to learn more about decompression theory and dive table development, you can advance your knowledge in a NAUI Master Scuba Diver course. There are also texts that present advanced decompression theory, and good information can be found on many web sites. In this course, we will consider only the classic NAUI Dive Tables and the NAUI RGBM Dive Tables, which use the more modern concept of dual-phase modeling and the Reduced Gradient Bubble Model developed by Dr. Bruce Wienke. We also assume that you already know how to use dive tables to plan your dives. If you are out of practice in the use of dive tables and dive table terminology, you should go back and review the information in your basic scuba textbook. Remember that if you are diving at higher altitudes (above about 300 meters/1000 feet), you will have to use special altitude dive tables or apply a conversion for theoretical depth. The NAUI RGBM Tables are available and cover three altitude ranges up to a maximum of 3,048 meters/10,000 feet above sea level. Also, planned required-decompression diving, whether breathing air or nitrox, is an advanced skill that requires additional training. Recreational divers use oxygen-enriched air to increase the safety margin of no-required decompression dives or to extend maximum dive time without encountering a mandatory decompression obligation.

Air Dive Tables So-called “standard dive tables” are designed for diving while breathing air. Air Dive Tables So-called “standard dive tables” are designed for diving while breathing air. The mathematics and the formulas that were used in their development assume the diver is breathing a mixture that is 79% nitrogen/inert gas and 21% oxygen.

No-required Stop Times
EANx Dive Tables give increased maximum dive times for standard mixes. When NOAA introduced procedures for diving with an enriched air nitrox mix of 32% oxygen/68% nitrogen (NOAA Nitrox I) in 1979, they also published a set of derivative nitrox dive tables that took into account the reduced percentage (and therefore partial pressure) of nitrogen. The tables were based on the concept of equivalent air depth; that is, because the NOAA Nitrox I diver would be breathing only 68/79ths of air’s partial pressure of nitrogen at any depth, the diver could enter the dive tables at a depth that is equivalent to only 68/79ths of the absolute pressure at the actual dive depth. (Note that the fraction is applied to the absolute pressure at depth, not just the depth.) The NOAA Nitrox I tables looked exactly like the U.S. Navy tables from which they were derived. Only the numbers changed because of the time credits for the reduced nitrogen partial pressures. EAN32 turned out to be a very convenient oxygen fraction because in the depth range from the surface to 110 feet, the NOAA Nitrox I tables exactly corresponded to the U.S. Navy air tables for the next shallower depth increment. In other words, an EAN32 diver diving to a depth of 70 feet had 60 minutes of bottom time, which was the USN no-decompression limit for an air diver diving to only 60 feet. An air diver descending to 70 feet would have only 50 minutes of bottom time. NOAA Nitrox I divers could, in effect, use standard U.S. Navy air tables to plan and execute dives, if they knew how to adjust for their equivalent air depth.

Enriched Air Nitrox Dive Tables
Enriched Air Nitrox Tables In the fourth edition of the NOAA Diving Manual (2001), two sets of EAN32 tables were presented. An “Abbreviated” version copies the NAUI EAN32 Dive Tables with a few changes; an “Expanded” version presents a fuller set of tables (with stage decompression information) modeled on the U.S. Navy tables format. When NOAA Nitrox II information was published by NOAA in the fourth edition of the NOAA Diving Manual, an Abbreviated version and an Expanded version of tables were presented for EAN36, similar to the EAN32 tables. Although the correspondence is not as striking as it is with NOAA Nitrox I, still for actual depths from 70 feet to 110 feet, the NOAA Nitrox II tables corresponded to dives that were two increments (20 feet) shallower on the U.S. Navy air tables. For example, an air diver diving to 21 meters/70 feet using the U.S. Navy air tables would have a no- decompression limit of 50 minutes and an ending repetitive letter group of “J.” An EAN36 diver could descend to 27 meters/90 feet for that same 50-minute bottom time and ending letter group. An air diver descending to 27 meters/90 feet would have only 30 minutes of no-decompression dive time. If an air diver were to dive 50 minutes at 27 meters/90 feet, he would face a required decompression stop of 18 minutes at 3 meters/10 feet and would emerge as an “L” diver.

Enriched Air Nitrox Dive Tables continued
NAUI EAN Dive Table Rules Treat each dive as a square profile dive, with the deepest point reached on the dive being used as the depth for the whole dive. If the exact depth or time does not appear on the table, round up to the next greater number. The tables assume an ascent rate of 30 feet/9 meters per minute. Planning repetitive dives progressively shallower will yield shorter required surface interval times. The required surface interval between two separate dives is 10 minutes; the minimum recommended surface interval is one hour. The tables are designed to be used at sea-level atmospheric pressure, and adjustments must be made for altitudes above about 1000 feet/300 meters. If flying or ascending to altitude after diving, wait 12 hours after a single dive and 18 hours after a repetitive dive series. On the NAUI Nitrox Tables, Table 1 and Table 3 each have some additional information. They show the oxygen partial pressure for each depth (a column to the left on Table 1 and a top row on Table 3). Information for dives that exceed a PO2 of 1.4 atmospheres is shaded light green to make the planner aware that these dives exceed the normal oxygen exposure limit for recreational nitrox diving. The maximum depth on both tables is for a PO2 of 1.6 ata. Therefore, the NAUI EAN36 Dive Tables have a maximum depth of 33 meters/110 feet. The NAUI Nitrox Dive Tables have a tolerance of ±1%. The EAN32 Dive Tables can be used for nitrox blends from 31% to 33% oxygen; and the EAN36 Dive Tables, for blends from 35% to 37% oxygen. The usual dive table rules and precautions apply when using the NAUI Nitrox Dive Tables. Treat each dive as a square profile dive, with the deepest point reached on the dive being used as the depth for the whole dive. If the exact depth or time does not appear on the table, round up to the next greater number. The tables assume an ascent rate of 9 meters/30 feet per minute. Planning repetitive dives progressively shallower will yield shorter required surface interval times. The required surface interval between two separate dives is 10 minutes; the minimum recommended surface interval is one hour. The tables are designed to be used at sea-level atmospheric pressure, and adjustments must be made for altitudes above about 300 meters/1000 feet. If flying or ascending to altitude after diving, wait 12 hours after a single dive and 18 hours after a repetitive dive series. While using NAUI’s prepared tables is the easiest way to plan dives, the three NAUI standard tables–Air, EAN32, and EAN36 Dive Tables–do not always yield interchangeable results. Divers using these tables should not move from one table to a richer oxygen table in any repetitive dive series. The tables actually assume that the diver will be using the same gas mixture throughout. You can change from beginning with air to EAN32 or EAN36 and continue to use the air tables. You can switch from initial use of EAN32 to EAN36 and continue to use the EAN32 table. This is not the case with the NAUI RGBM Tables with which you can switch mixtures and tables from dive to dive. In order to effectively and safely switch between gas mixes or to use nitrox blends not encompassed by the EAN32 and EAN36 tables, you should use equivalent air depth (EAD). By applying the equivalent air depth formula to your diving, you can use any standard air tables as long as you also follow oxygen exposure limits.

Equivalent Air Depth and Standard Air Tables
Equivalent Air Depth is determined by the partial pressure of nitrogen that the diver is actually breathing Because nitrox has a lower fraction of nitrogen than air, the nitrogen partial pressure will also be less than with air for any given depth, and the diver’s equivalent depth for nitrogen absorption will also be less than with air. It is not the actual depth, but the partial pressure of nitrogen in the breathing gas that matters. Equivalent Air Depth and Standard Air Dive Tables As noted earlier, equivalent air depth (EAD) is determined by the partial pressure of nitrogen that the diver is actually breathing. Because nitrox has a lower fraction of nitrogen than air, the nitrogen partial pressure will also be less than with air for any given depth, and the diver’s equivalent depth for nitrogen absorption will also be less than with air. It is not the actual depth, but the partial pressure of nitrogen in the breathing gas that matters. The theory behind equivalent air depth is that an EAN diver’s exposure to nitrogen for any absolute pressure (depth) will be proportionally less than with air as the fraction of nitrogen in the mix is less than 0.79 (the fraction of nitrogen in air). Moreover, the rate at which nitrogen is absorbed by the various tissues is related to the pressure gradient between the inspired nitrogen partial pressure and the tension of nitrogen dissolved in the tissues. If the inspired nitrogen partial pressure is less, the pressure gradient is less, and nitrogen moves into the tissues more slowly. And, according to Henry’s Law the total amount of nitrogen that will dissolve in the tissues over time is directly proportional to the nitrogen partial pressure.

Equivalent Air Depth and Standard Air Tables continued
EAD Example: If you were breathing a mixture that is 36% oxygen, then the nitrogen percentage would be 64%, and the nitrogen fraction would be 0.64. When you dive with this mixture, you expose yourself to 64/79ths the nitrogen partial pressure that you would encounter if breathing air. Therefore, you can consider your depth to be 64/79ths (roughly 80%) of the absolute pressure that you would encounter at your actual depth if you were breathing air. If you find this difficult to conceptualize or understand, here is a concrete example. If you were breathing a mixture that is 36% oxygen, then the nitrogen percentage would be 64%, and the nitrogen fraction would be When you dive with this mixture, you expose yourself to 64/79ths the nitrogen partial pressure that you would encounter if breathing air. Therefore, you can consider your depth to be 64/79ths (roughly 80%) of the absolute pressure that you would encounter at your actual depth if you were breathing air. If you were breathing EAN40, your nitrogen partial pressure would be 60/79ths (about three-quarters) of what it would be if you were breathing air. You must relate your nitrogen advantage to absolute pressure and then convert that absolute pressure to a depth. If you were to try to relate it immediately to a depth, you would be neglecting the one atmosphere of air pressure at the surface. You should remember this from your beginning class in which you learned about pressure at depth and the volume of air in a flexible container. Equivalent air depth is especially useful if you want to change blends from one dive to the next, if you are requesting a custom blend based on “best mix,” or if for some reason you find yourself diving with a mix that is not standard EAN32 or EAN36. If you know equivalent air depth for your mix, you can use that EAD with the NAUI standard Air Dive Tables (or any air dive tables) to plan your dive or to plan repetitive dives. By determining the EAD prior to each dive, you can easily change mixes to either richer or leaner oxygen content from one dive to the next. You can find equivalent air depth by using a table, by calculating EAD, or with the NAUI OCEANx calculator. Let’s begin with using a Table.

Equivalent Air Depth by Table (imperial) The Equivalent Air Depth Table relates the percentage of oxygen in your nitrox mix to an actual dive depth to show an equivalent air depth. The actual dive depth is not usually a round number because it is relating to a round-number equivalent air depth that will be used on the dive tables. To use the Equivalent Air Depth Table (imperial), enter the table at the upper left and move to the right to the oxygen percentage that corresponds to the nitrox mix that you will be using. Move down that column to the maximum planned depth of your dive. If your exact maximum depth is not on the table, use the next greater depth. Move to the left along that row to the column at the far left. The equivalent air depth that you should use with air dive tables is shown in the left column. For example, for 32% oxygen, any dive between 86 feet and 98 feet would use 80 feet on an air table for dive planning. Contingency depths for which the PO2 is between 1.4 and 1.6 atmospheres have been shaded darker. Example 1: On the first dive of the day, your planned depth is 80 feet, and your cylinder is filled with EAN38. What is your maximum dive time for the dive? Enter the equivalent air depth table at the 38% column and move down. Eighty feet does not appear on the table, so continue down to the next depth–85 feet. Moving to the left, the equivalent air depth for this dive is 60 feet. Using standard NAUI Dive Tables for air, you will find that your maximum dive time for a 60-foot dive is 55 minutes. Example 2: Your actual dive time for the dive in Example 1 is 35 minutes. Two hours later, you re- enter the water to return to the same location. You are diving with EAN32. What is your adjusted maximum dive time for this repetitive dive? Determine your exiting letter group for the first dive. With an EAD of 60 feet for the first dive and a 35-minute dive time, you emerge from the water as a “G” diver. Two hours later you are a “D” diver. Since you are now diving to a depth of 80 feet with EAN32, your equivalent air depth from the EAD table is 70 feet. A “D” diver diving to 70 feet has an adjusted maximum dive time of 25 minutes on the NAUI Dive Tables.

Calculating Equivalent Air Depth Equivalent air depth can be calculated in discrete steps, or the procedure can be combined into a single formula. Step 1: Determine the absolute pressure at the depth to which you will be diving. Step 2: Apply the nitrogen “credit” that your nitrox blend gives you. Step 3: Convert this air-equivalent absolute pressure to an equivalent air depth As with MOD and best mix, equivalent air depth can be calculated in discrete steps, or the procedure can be combined into a single formula. As a step-wise process, you would first determine the absolute pressure at the depth to which you will be diving. Second, apply the nitrogen “credit” that your nitrox blend gives you. Finally, convert this air- equivalent absolute pressure to an equivalent air depth.

Step 1: P ata = (D fsw / 33 fsw/atm) + 1 atm Step 2: P ataEAD = (Fin mix / Fin air) x P ata Step 3: EAD fsw = (P ataEAD – 1 atm) x 33 fsw/atm Conduct a class exercise to calculate an equivalent air depth.

Calculating EAD Example: What is the equivalent air depth for a diver diving with EAN32 to a depth of 80 feet/24 meters? Calculating Equivalent Air Depth As with MOD and best mix, equivalent air depth can be calculated in discrete steps, or the procedure can be combined into a single formula. As a step-wise process, you would first determine the absolute pressure at the depth to which you will be diving. Second, apply the nitrogen “credit” that your nitrox blend gives you. Finally, convert this air-equivalent absolute pressure to an equivalent air depth. Example: What is the equivalent air depth for a diver diving with EAN32 to a depth of 24 meters/80 feet? Step 1: Find the absolute pressure at 24 meters/80 fsw using the formula from Chapter 2: or = 3.42 ata P ata = (D fsw / 33 fsw/atm) + 1 atm P ata = (D fsw + 33 fsw / 33 fsw/atm) P ata – ( 80 fsw / 33 fsw / atm) + 1 atm = 3.42 ata Step 1: Find the absolute pressure at 80 fsw using the formula from Chapter 2: or = 3.42 ata

Calculating EAD Step 2: Apply your nitrogen “credit.” Since you are diving with EAN32, the nitrogen fraction of your mix is (= ). Your air-equivalent absolute pressure is set by the ratio between the fraction of nitrogen in your mix and the fraction of nitrogen in air, or: Step 2: Apply your nitrogen “credit.” Since you are diving with EAN32, the nitrogen fraction of your mix is 0.68 (= ). Your air-equivalent absolute pressure is set by the ratio between the fraction of nitrogen in your mix and the fraction of nitrogen in air, or: = 2.94 ata P ata (air equiv) = (0.68/0.79) x 3.42 ata = 2.94 ata

Calculating EAD Step 3: Convert 2.94 ata to an equivalent air depth: D fsw = (P ata – 1 atm) x 33 fsw / atm D fsw = (2.94 ata – 1 atm) x 33 fsw / atm = 64 fsw You would use for 70 feet / 21 meters on your dive tables. Step 3: Convert 2.94 ata to an equivalent air depth: D fsw = (2.94 ata – 1 ata) x 33 fsw / atm = 64 fsw. You would use 70 feet/21 meters on your air dive tables.

Calculating EAD using a single formula EAD fsw = (((80 fsw + 33 fsw) x FN2) / 0.79) – 33 fsw = 64+ fsw

Calculating EAD with the OCEANx To find equivalent air depth with the OCEANx calculator, dial-in the oxygen percentage into the upper window. The long window immediately below shows the maximum actual depth to be used with each equivalent air depth, which is printed immediately to the right. Using the OCEANx to Establish Equivalent Air Depth To find equivalent air depth with the OCEANx calculator, simply dial-in the oxygen percentage into the upper window. The long window immediately below shows the maximum actual depth to be used with each equivalent air depth, which is printed immediately to the right.

Take advantage of the “deep stop” Rule of Halves When ending any no-decompression dive in excess of 12 meters / 40 feet, halve the distance from the dive’s deepest depth to the surface. Ascend to that depth and make a two to three minute stop (with two and one-half minutes being optimum). Then continue your ascent to the 3-6 meter/10-20 foot safety stop for one minute before surfacing. The Rule of Halves In 2003, NAUI introduced the “Rule of Halves” into its recommended diving procedures. The NAUI Rule of Halves is based on RGBM theory and provides an extra margin of safety for divers by further controlling both dissolved-phase and free-phase nitrogen in the body. It can be used with any dive deeper than 12 meters/40 feet. To use the Rule of Halves, when ending any no-decompression dive in excess of 12 meters/40 feet, halve the distance from the dive’s deepest depth to the surface. Ascend to that depth and make a two to three minute stop (with two and one-half minutes being optimum). Then continue your ascent to the 3-6 meter/10-20 foot safety stop for one minute before surfacing. For example, following a dive to 30 msw/100 fsw, you would perform a two and one-half minute stop at about 15 msw/50 fsw and then ascend to 5 msw/15 fsw for a one minute stop before surfacing.

Dive Computers and Nitrox
Dive computers perform real time dive calculations. Generally, their algorithms are quite conservative. Because they sample the diver’s depth and dive time every few seconds and recalculate nitrogen absorption over a range of theoretical tissue compartments, divers enjoy extended dive times when using a dive computer. In effect, the diver receives “credit” for the shallow portions of the dive, which is not possible with the “square-profile” assumptions of dive tables. Dive computers perform real time dive calculations. Generally, their algorithms are quite conservative. But, because they sample the diver’s depth and dive time every few seconds and recalculate nitrogen absorption over a range of theoretical tissue compartments, divers enjoy extended dive times when using a dive computer. In effect, the diver receives “credit” for the shallow portions of the dive, which is not possible with the “square-profile” assumptions of dive tables.

Dive Computers and Nitrox continued
Two basic options Use a Nitrox capable computer Use an Air capable computer to increase your safety margins. Note: Currently, many manufacturers have incorporated the NAUI RGBM algorithms as well as the NAUI Rule of Halves into their dive computers. Currently, many manufacturers have incorporated the NAUI RGBM algorithms as well as the NAUI Rule of Halves into their dive computers. More and more divers are buying and using dive computers, especially as prices are decreasing with increased production and after recovery of initial research and development costs. Many dive centers now include computers in their rental equipment inventory. Dive computers display not only current depth and dive time, they also show dive time remaining and usually the maximum depth thus far in the dive. They monitor ascent rate and will sound an alarm or flash in the display if a diver is ascending too fast. Additional bells and whistles may include temperature, scrolling of maximum dive times for the next dive or actual pre-dive planning software for repetitive dives, and dive recall display of not just a few but many previous dives and surface intervals. Many computers will allow you to download your dives to a logbook program on a computer. (General experience using desktop computers prompts the warning: keep a backup.) In addition, with a nitrox-capable computer you can set the oxygen percentage of your breathing gas on each dive. The computer then calculates nitrogen absorption for your dive based on the oxygen level that you have “dialed in.” Nitrox computers will also track single-dive and 24-hour oxygen exposure limits. If you use a nitrox computer, read the manufacturer’s instructions carefully. Some computers will revert to a default value of air or to an extremely conservative value if you do not dive with them within a certain time of setting the mixture. Also, different manufacturers use different gas absorption models, and dive computer brands and models vary from less to more conservative–in addition to variations in their display and download features. As with acquiring any diving equipment, you should establish your own personal needs and look into the range of computers available before buying. As a nitrox diver, you have two basic options. With a nitrox-capable computer, you can set the computer to exactly model the theoretical nitrogen loading and offgassing for your dives. On the other hand, many divers opt for an air computer, which will present an automatic safety margin when diving with nitrox. Another option is to acquire an enriched air nitrox computer and leave it set at 21% oxygen unless you want to use the advantages of nitrox on given dives. You now have enough information to use oxygen-enriched air for safe diving and dive planning. In the final chapters of this book, you will learn precautions that must be exercised when handling oxygen at high partial pressures, how diving equipment is cleaned and serviced for exposure to oxygen, how nitrox is prepared, and your responsibility for gas analysis and other unique aspects of nitrox use. Diving on air tables while using nitrox... Diving your computer on air mode while using nitrox...

Unit 6: O2 Precautions and Preparing EANx
Q: Is oxygen flamable?

Oxygen Handling Firefighters use the concept of the “fire triangle.” In order for a fire to occur or continue, three things must be present: fuel, oxygen, and heat. As the fraction and partial pressure of oxygen increase, many materials that do not burn under normal conditions will burn if ignited. Oxygen Handling As you learned in an earlier chapter, oxygen supports combustion and combines readily–sometimes aggressively or violently–with almost anything that is not already oxidized. Slower oxidation can destroy a material over time–solid iron turns into rust. Or, oxidation can be rapid enough to produce extreme heat and visible light, which we call burning or fire or combustion. Sometimes this combustion can be so violently rapid that it is an explosion. There are other elements and materials that are reactive enough to combine aggressively, produce extreme heat, and “burn,” but they are not so commonly found. Oxygen, on the other hand, is ever-present, and in enriched air nitrox it is present in higher than normal amounts. In the Earth’s atmosphere, with its oxygen fraction of 0.21 ata, and at normal temperatures, materials do not spontaneously ignite and burn. A source of ignition (heat) is required to initiate burning. After a material (fuel) is ignited, then the fire itself provides the heat to sustain burning. Firefighters use the concept of the “fire triangle.” In order for a fire to occur or continue, three things must be present: fuel, oxygen, and heat. If any one of these is absent, a fire will not start. If any of the three is removed, the fire will be extinguished. When a fire has consumed all available fuel, burning ceases. If the fire is sufficiently cooled, as by dowsing it with water, it will go out. If a carbon dioxide fire extinguisher is used to remove the oxygen around the fire, the fire goes out. But, as the fraction and partial pressure of oxygen increase, many materials that do not burn under normal conditions will burn if ignited. Also, any fuels will ignite more easily. Materials that are of little concern in air may become quite flammable in an oxygen-rich environment and even more so in a high-pressure oxygen-rich environment. Petroleum-based products and other hydrocarbons are of special concern because they ignite extremely easily in a pure oxygen environment. Other materials, although they may not become flammable, will oxidize and degrade much more rapidly in the presence of hyperbaric oxygen. When high pressures of oxygen are going to be present, extra care must be taken to prevent the fire triangle from occurring.

Oxygen Handling continued
Equipment that will be used with pure or very high concentrations of oxygen must be specially prepared to withstand the oxygen and to prevent fires and oxidation.

Equipment Considerations Hydrocarbons and petroleum-based products must be avoided. This includes not only petroleum-based compressor lubricants but also the silicone lubricants normally used in scuba air systems. Neoprene, silicone “rubber,” plastic and metal shavings, even finely divided particulate matter all become potential fuels for a fire in an oxygen-rich environment, especially one at high pressure. Equipment Considerations The ways that nitrox is prepared will be discussed later, but the most common mixing method involves introduction of pure oxygen at pressure. All equipment systems that will be so exposed must be specially prepared in order to withstand the oxygen. Hydrocarbons and petroleum-based products must be avoided. This includes not only petroleum-based compressor lubricants but also the silicone lubricants normally used in scuba air systems. Neoprene, silicone “rubber,” plastic and metal shavings, even finely divided particulate matter all become potential fuels for a fire in an oxygen-rich environment, especially one at high pressure. A spectacular laboratory demonstration is to burn steel wool in a high-oxygen environment.

Oxygen Cleaning The equipment must be “oxygen clean,” and “oxygen compatible” parts must be used in order to minimize the risk of fire or destruction by the oxygen. After cleaning, future contamination must be avoided. Oxygen Cleaning All equipment to be used with pure oxygen must be cleaned for oxygen service. The equipment itself must be “oxygen clean,” and “oxygen compatible” parts must be used in order to minimize the risk of fire or destruction by the oxygen. Oxygen clean means that any potentially flammable contaminants have been removed. Oxygen compatible means that the materials used in the various parts in the system are not flammable or readily oxidizable in the presence of high-pressure or pure oxygen. This special oxygen cleaning can be complex, involving an initial cleaning using solvents that remove all hydrocarbons and then the use of special oxygen-compatible lubricants, o-rings, seats, seals, flexible tubing, etc. as the system is reassembled. Even though certain materials may not be flammable in the presence of high-pressure oxygen, more durable materials that do not so rapidly degrade will be substituted. “Formal oxygen cleaning,” which is required in many industrial and governmental (e.g., NASA) applications, requires adherence to very strict procedures as well as careful documentation. After cleaning, future contamination must be carefully avoided, or the whole oxygen cleaning process will have to be repeated. Even skin oil from a finger can contaminate formally oxygen-cleaned equipment and necessitate re-cleaning. Care must also be taken to avoid sources of ignition, the third side of the fire triangle. A sudden increase in system pressure could elevate the temperature in the system sufficiently to cause ignition of any contaminants. Valves should be opened slowly to prevent a sudden increase in pressure with an accompanying rapid temperature rise. Even a static spark from a rug or opening a Velcro® closure could cause ignition in a pure oxygen environment.

The 40% Rule Any equipment that is to be used with pure oxygen or an oxygen level that is above 40% (and at a pressure above 200 psi) must be cleaned for oxygen service and have only oxygen-compatible parts. This is a “rule of thumb,” but it is generally accepted for oxygen handling. The 40% Rule When is oxygen cleaning necessary? As a recreational nitrox diver, you will dive with an ordinary, well-maintained regulator, but your cylinder will have been cleaned for designated service as a nitrox cylinder, and it will be clearly marked as an enriched-air nitrox cylinder. The general rule of thumb is that any equipment that is to be used with pure oxygen or an oxygen level that is above 40% (and at a pressure above 200 psi) must be cleaned for oxygen service and have only oxygen-compatible parts. This so-called “40% rule” has gained acceptance over time although there are no published test results to show that 40% is the absolute limit above which oxygen cleaning is unequivocally necessary. No definitive industry-wide standards exist for handling gas mixes intermediate between air and 100% oxygen. The U.S. Navy handles gas mixes up to 40% oxygen the same as air. The Occupational Safety and Health Administration has set a 40% break-point above which special cleaning is required. NAUI and the recreational diving industry as a whole have accepted the 40% rule, and EAN40 is the accepted upper-limit fraction of oxygen in recreational nitrox diving. Forty percent seems to be a valid limit in the sense that at or below this level oxygen compatibility problems have not occurred. Above 40% one is in a gray area for which there is seemingly no firm data. Flash combustion has occurred above 40% oxygen, so it is better to err on the side of safety. Technical divers using higher oxygen fractions, such as in a decompression gas mix, use scuba equipment that is specially serviced and is fully oxygen clean and oxygen compatible for these special applications.

Equipment Preparation Normal maintenance service is sufficient for your regulator. Your cylinder is a different matter. Equipment Preparation So what about your own regulator and your cylinder? Your regulator will not encounter an oxygen level above 40%, so the rule applies. Normal maintenance service is sufficient for your regulator. Of course, as with all of your equipment, you should take care of your regulator, rinsing it after dives, keeping it clean, avoiding extreme heat, and having it serviced regularly. The same general cautionary statement is true of all scuba equipment that will be used with nitrox up to EAN40– submersible pressure gauges, BC inflators, BCs, etc. For all scuba gear, cleaner is better, and the prudent diver takes good care of his or her life support equipment.

Equipment Preparation continued Cylinders must be prepared for designated nitrox service because most of the time pure oxygen will be used in preparing a nitrox fill. After being cleaned for use with oxygen-enriched air, the cylinder will be labeled to clearly identify it as a nitrox cylinder. Your cylinder is a different matter. The most common way to prepare nitrox is to introduce an initial quantity of pure oxygen into an empty cylinder, which is then topped off to service pressure with specially cleaned air. Every time your nitrox cylinder is filled, it is subjected to pure oxygen at high pressure. So, your cylinder must be “dedicated” to nitrox use. This means that it will be cleaned before it is used as a nitrox cylinder. To prepare a nitrox cylinder, the interior of the cylinder is first washed with a solvent to remove all hydrocarbons and particulates. All traces of the solvent are then flushed out. The cylinder valve, through which the oxygen must pass, is disassembled, thoroughly cleaned, and then reassembled using oxygen-compatible lubricants, o-rings, seats, and seals. After being cleaned for use with oxygen-enriched air, the cylinder will be labeled to clearly identify it as a nitrox cylinder. The original “standard” identification for an enriched air nitrox cylinder is a yellow cylinder with the top painted green down to four inches below the shoulder of the cylinder and with an identifying EANx label stenciled onto the cylinder. (If the cylinder has been more formally cleaned for service with higher than 40% oxygen mixtures, it will be labeled as such.) Many cylinders that are put into nitrox service are not yellow but some other color, so the more common standard identification labeling is a “nitrox” decal label that encircles the cylinder just below the shoulder. The label has a green band four inches wide that is bordered above and below with a narrow yellow band. An identifier such as “Nitrox” or “Enriched Air Nitrox” is printed in yellow on the green band. Cylinders must be prepared for designated nitrox service because most of the time pure oxygen will be used in preparing a nitrox fill. Although there are several ways to blend nitrox, some of which do not require using pure oxygen, all will use extra-clean air that meets a standard of greatly reduced hydrocarbon content. So, preparing a cylinder and designating it as “for nitrox service” is valid, even for those blending methods that remove nitrogen rather than adding oxygen. When the cylinder is hydrostatically tested, it will have to be re-cleaned before being returned to nitrox service.

How Nitrox is Made Partial-Pressure Mixing OXYGEN SAFETY
Tell me what you see... How Nitrox is Made Partial-Pressure Mixing In partial-pressure mixing, the blending technician first puts a measured amount (pressure) of oxygen into the cylinder and then fills the cylinder to its service pressure with air. OXYGEN SAFETY Partial-Pressure Mixing Partial-pressure mixing systems are the easiest to set up, requiring only a source of high-pressure oxygen and a source of clean, high-pressure air. The air used for nitrox blending must meet the higher standard of purity with a lower hydrocarbon level than ordinary scuba air. In partial-pressure mixing, the blending technician first puts a measured amount (pressure) of oxygen into the cylinder and then fills the cylinder to its service pressure with air. For simplicity, blending a nitrox fill normally begins with the cylinder emptied of any previous mix, but it could start with adding oxygen and then air to whatever remained in the cylinder from the previous use. Calculations for partial-pressure blending are simple. Using Dalton’s Law of partial pressures, a measured amount of pure oxygen (oxygen fraction equals 1.00) plus a measured amount of air (oxygen fraction equals 0.21) produces a full cylinder of nitrox at whatever oxygen fraction is desired. If the cylinder does not begin at empty, it is just a matter of blending three gas mixtures instead of two. Small adjustments may have to be made because Dalton’s Law calculations are “ideal gas” calculations, and the blending technician is mixing “real” gases. In practice, the blending technician will consult a table (rather than performing the calculation for each fill) to determine what pressure of oxygen should be added to the empty cylinder that will then be topped off with air. Because high-pressure oxygen is being introduced into the nitrox cylinder, the cylinder and its valve must be oxygen clean and oxygen compatible. The cylinder will be filled slowly to minimize temperature increases as the oxygen and then the air are compressed into it. After filling the cylinder, the mix will be allowed to cool, the nitrox blend will be analyzed, and the proportions adjusted as necessary. Partial-pressure blending is popular in dive centers both because it is easy and relatively inexpensive to set up and because it lends itself to preparing small quantities of a variety of different blends. Each cylinder’s fill is unique. With partial-pressure blending, the diver can request any nitrox blend for a single cylinder or several different mixes in several cylinders, and the dive center can easily supply the need.

How Nitrox is Made continued
Continuous-Flow Mixing The continuous-flow method injects a measured flow of pure oxygen into the air before it reaches the intake of the compressor Continuous-Flow Mixing This is the method that was first used by NOAA. The continuous-flow method injects a measured flow of pure oxygen into the air before it reaches the intake of the compressor. The oxygen-air mixture is then compressed. As the high-pressure nitrox exits the compressor, the oxygen fraction is analyzed on-line, and the oxygen flow at the low-pressure intake is fine-tuned, either manually or by an automatic feedback control, until the desired blend is attained. The high-pressure nitrox is routed to a bank of storage cylinders from which it will be drawn to fill individual scuba cylinders. In commercial diving, the mix may be delivered directly to the diver through a hose attached to the helmet or band-mask. The oxygen is well homogenized with the air before being drawn into the compressor and some oxygen injection systems advertise that the blend is so thoroughly mixed before entering the compressor that the compressor does not have to be hydrocarbon-free. Whether this is true or not, the nitrox will be used to fill a nitrox-dedicated cylinder, so the compressor should not introduce any hydrocarbons that might contaminate the cylinder, which at a later time may be filled by partial- pressure blending. Continuous-flow mixing is used when larger quantities of a given nitrox blend are needed. Although the method could be used to fill individual scuba cylinders, it would be inefficient and wasteful of oxygen.

Your Responsibility As a nitrox diver, you are responsible not only for your dive planning and execution but also for your equipment. You are also responsible for the correctness of what you will breathe. You are responsible for your cylinder’s contents being what you asked for. A final step before you take the cylinder away with you is verifying the contents. Your Responsibility As a nitrox diver, you are responsible not only for your dive planning and execution but also for your equipment. You should keep your gear well maintained, be careful in its use, and keep it clean. Your cylinder will be a dedicated nitrox cylinder, and although sometimes it may be filled with air (EAN21 as some call it), you should obtain your air only from a fill station with a compressor that supplies nitrox- compatible grade air. Another air source could possibly deliver air that would contaminate your cylinder for future partial-pressure blending. Your responsibility for your breathing gas does not end with keeping your equipment clean and obtaining your oxygen-enriched air from someone whom you trust. A unique feature of nitrox diving is that the end-user is responsible for the correctness of what he or she will breathe. You are responsible for your cylinder’s contents being what you asked for. A final step before you take the cylinder away with you is verifying the contents. The next chapter will discuss this last step– analyzing your gas and correctly labeling your cylinder.

Unit 7: Knowing What You Breathe
When should you analyze your gas? What do you do if “they” do it for you...?

How Oxygen Analyzers Work
Oxygen analyzers come in all sizes Most have digital readouts, but analog readouts are also available Analyzers should have an accuracy of one-tenth of one percent (e.g., 31.7% vs. 32%) There are many styles of oxygen analyzers available, ranging from large units to small analyzers designed for use in the field. Most of the analyzers used for nitrox diving have a digital readout, but analog readouts are also available. Ideally, an analyzer should display oxygen content to an accuracy of one-tenth of one percent. That is, the analyzer would display 31.7% rather than just 32%.

How Oxygen Analyzers Work continued
The sensor commonly used in nitrox analysis is electrochemical They are rugged, portable and relatively less expensive than other types An essential operation in using any analyzer is calibrating it Must be calibrated before each use Nitrox analyzers are normally calibrated using standard air (20.9% or 21%) Generally accepted that mix be within ±1% of target value The sensor commonly used in nitrox analysis is electrochemical. In this type of sensor, the oxygen in the gas diffuses through a membrane where it is ionized as it comes in contact with an electrode. A small electric current is generated by the oxygen ions, which is proportional to the amount of oxygen in the sample. The analyzer measures the resulting current and displays a readout in percent oxygen. Electrochemical analyzers have the advantage of being rugged and portable. They are also relatively less expensive than other types. An essential operation in using any analyzer is calibrating it. Analyzers may exhibit some instability or drift as they age, and so they should be calibrated before each use. In scientific use, calibration often includes zeroing the analyzer to a known inert gas that has no oxygen content. For nitrox use, an analyzer is normally calibrated using standard air. That is, with the analyzer exposed to air, it is set to read 20.9% or 21.0%. Although an analyzer should read in tenths of a percent, it is generally accepted that a mix need be only ±1% of the target value. NAUI EAN32 and EAN36 Dive Tables have a tolerance of ±1% in their use. If the mix is off more than 1% and tables are to be used, the blending technician should adjust the mix in the cylinder. A nitrox dive computer can be set to the actual percentage of the mix, so a greater error would be tolerable as long as the planned depth of the dive does not exceed the maximum operating depth of the mix.

Analyzing Your Gas Accurate analysis depends on the reliability of the analyzer and the flow rate through the oxygen sensor. Acceptable flow rate is about 1 liter per minute and should be between ½ and 2 liters per minute. Analyzing Your Gas When you pick up your cylinder of nitrox, the facility will ask you to analyze your mix and record your analysis in the fill station log. The facility that filled your cylinder will have an oxygen analyzer available for this purpose. You will also record the mix information on the cylinder itself or verify what has already been recorded on the cylinder by the blending technician. Accurate analysis depends not only on the reliability of the oxygen analyzer but also on the flow rate of the gas through the oxygen sensor. If the flow rate is too high, the analyzer will detect an excess amount of oxygen, and the displayed oxygen percentage will be too high. If the flow rate is too low, the displayed oxygen percentage may also be too low. The acceptable flow rate is about one liter per minute and should be between one-half and two liters per minute. Many analyzer manufacturers supply a specially designed flow restrictor or flow-rate regulator that attaches directly to a low-pressure inflator hose fitting on a regulator first stage. The flow restrictor is either self-setting or will have a meter that allows its valve opening to be set to an appropriate flow rate. The downstream side of the flow restrictor attaches to the oxygen sensor of the analyzer.

Analyzing Your Gas continued
Accurate oxygen analysis depends on accurate calibration of the analyzer. Use a known source such as an air cylinder to ensure proper calibration. Once calibrated, attach the analyzer to your nitrox cylinder, confirm that flow rate is the same as during calibration and allow analyzer to stabilize. Record this information on your cylinder contents label. Accurate oxygen analysis also depends on accurate calibration of the analyzer. Before analyzing the gas in your oxygen-enriched air cylinder, you must calibrate it using a known source. This is usually a scuba cylinder that is filled with air. Attach the flow restrictor/regulator to the air supply and the oxygen analyzer to the downstream side of the flow restrictor. Turn on the analyzer and the air supply. If the flow restrictor is not preset, adjust the flow rate to about one to two liters per minute. Allow the air to flow to the analyzer for a minute until you are sure that the analyzer has stabilized. Then, set the analyzer’s readout to 20.9% or 21.0%. Once the analyzer is calibrated, you can analyze the mix in your enriched air nitrox cylinder. Without disturbing the analyzer’s setting, shut down the air-supply cylinder, allow the analyzer system to depressurize or depressurize it manually, remove the setup from the air cylinder, and reattach it to your nitrox cylinder. Then, turn on the valve of the nitrox cylinder. You may need to be expeditious with this switch, as most analyzers will turn themselves off after a certain time of no use. Check the flow rate if it is adjustable and set it to the same as it was for your air analysis. Allow a minute for the reading to stabilize and then take a reading of the oxygen fraction or percentage of your mix. Record this information on your cylinder contents label.

Tracking Your Nitrox Cylinder
Cylinder Labeling Every nitrox cylinder must be properly labeled with contents and other pertinent information. Prepare cylinder label immediately after analyzing to avoid forgetful errors. Data should include fill date, oxygen percentage, maximum operating depth, cylinder pressure, and name of analyzer/end-user. Cylinder Labeling Every oxygen-enriched air cylinder must be properly labeled with its contents and other necessary information. The label should be prepared immediately after the contents are analyzed in order to avoid forgetful errors. The cylinder label may be a commercially prepared decal or tag that can be written on with a waterproof marker and subsequently erased or wiped off with alcohol and reused. Or, a label can be made using an indelible marker on tape that will be removed before the next refill. The data on the cylinder label should include: fill date, oxygen percentage, maximum operating depth, cylinder pressure, and the name of the analyzer/end- user. An evolving practice is to add a second label mounted vertically on the cylinder body and marked with maximum operating depth in numbers three inches high (for example, using house-number decals). This MOD label is positioned so that it is unobstructed and can be easily read by a dive partner who is swimming next to the user.

Tracking Your Nitrox Cylinder continued
Filling Out The Logbook Once you have analyzed your cylinder and labeled it, you will be asked to complete the permanent Fill Station Logbook and sign that you have received the cylinder. Enter your name, date, your certification, cylinder’s serial number, pressure, oxygen mix, maximum operating depth, signature. Logbook tracks all nitrox cylinders leaving facility. Logbook verifies that you either analyzed the contents or knew the particulars of the fill when you received your cylinder. Filling Out The Logbook Once you have analyzed your cylinder and labeled it, you will be asked to complete a permanent Fill Station Log and sign-off that you have received the cylinder. This logbook is kept by the facility that filled your cylinder. In the logbook, you will enter your name, the date, your certification, the cylinder’s serial number, the cylinder pressure, the oxygen percentage of the mix, and the maximum operating depth. You will also sign the logbook entry. The cylinder log is used to keep track of all nitrox cylinders that leave the facility. It also verifies, by your signature, that you either analyzed the contents or watched as a blending technician analyzed them for you, and that you knew the particulars of the fill and its limits when you received the cylinder. And so, at last, you have it all together, and you are ready to enjoy the world of nitrox diving. Armed with your cylinder of enriched air nitrox–perhaps blended to your personal specifications, your knowledge of the risks, limits, and advantages of the gods’ ambrosia/devil’s gas that you are about to use, an appropriate dive plan, and a compatible dive partner, you head off to the dive site. There you and your partner assemble and don your equipment, rehearse and review your dive plan (including a review of your MOD), mutually check each other’s gear, and begin your dive. And, once you make your descent, it really is “just like breathing air.”

What's next... 40% to 100% “Advanced Nitrox”...
“Decompression Procedures”... ...longer and deeper dives! Beyond the end of the dive tables...

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Unit 1: What is NITROX? Is it better than air? Do you need it?

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Unit 1: What is NITROX? Is it better than air? Do you need it?

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