NITROX AND TECHNICAL CLASS on line
Welcome to ProTech Scuba’s Enriched Air Nitrox and Technical Diver class on line. This class is designed to give you a supplemental look at the nitrox and technical diving. It is in no way designed to tell you all you will ever need to know about doing such dives, but it will provide you with helpful information that can be used in pursuing specialty classes in the subject areas. This class is not designed to persuade you to either dive nitrox (or technically) or not, but is merely used as a tool to enhance your understanding of each area of diving. It is strongly recommended that you take a complete nitrox specialty course or any of a number of technical diving specialty courses in order to have the practical experience and necessary skills to safely partake in the specialty.
OBJECTIVES:
1. Students will be able to explain the benefits and risks associated with nitrox diving.
2. Students will be able to describe the myths associated with nitrox diving and explain why they are considered myths.
3. Students will be able to describe the necessary procedures used to safely dive using Enriched Air Nitrox.
4. Students will be able to explain the factors that make technical diving more hazardous than recreational diving.
List all of the major components of the air we breathe and the percentages that they make up. What effects do these components have on the human body? How are they helpful? How are they harmful? Are they all really necessary?
Enriched Air Nitrox (EAN) made its way into the recreational scuba diving community during the late 1980s when Dick Rutkowski introduced divers to it in Key Largo, Florida. By the mid 1990s the controversy over diving with nitrox (another name for EAN) had simmered down and agencies world wide began offering certification classes in EAN (i.e., Oxygen Enriched Air). Today many divers enjoy the benefits of EAN, and more and more dive shops and dive destinations are offering EAN fills to their customers. As its name indicates, EAN is nothing more than a boost or enrichment to the air we breathe. In this case the boost comes from oxygen. Here is how it works. Typical air contains approximately 78% nitrogen, 21% oxygen, and 1% argon. Other gases (helium, carbon dioxide, carbon monoxide, methane, hydrogen, etc.) only make up a small percentage of the total mix and are therefore not officially recognized. Argon is considered an inert gas, just as nitrogen is an inert gas, and is typically figured in a total percent (79%). When air is enriched (boosted) with oxygen the amount of nitrogen in the mix decreases. For example, one of the most common EAN mixes is EAN32. This mix contains 32% oxygen and only 68% inert gas (nitrogen and argon). Remember from your basic scuba class that nitrogen is responsible for decompression sickness and is the limiting factor when planning the depth and time for a dive. This would mean that the less nitrogen in a mix the more time you could stay at a specified depth. This is considered to be a major benefit of diving with nitrox: More time at any given depth without the need of decompression. The best example of this is with a dive to 90 feet in salt water. On air a diver using NAUI dive tables has a maximum dive time (no decompression) of 25 minutes. The same dive on EAN36 has a maximum dive time of 50 minutes. Using EAN36 doubles the bottom time to a dive to 90 feet. Other benefits of diving with nitrox include shorter surface intervals, longer repetitive dives, less nitrogen absorption, and a lower risk of decompression illness (this can be misleading and is discussed later).
1. What causes nitrox to be such a beneficial gas to use while diving?
2. What are some problems that you can see with using nitrox? (Or, what are some problems that you may have heard of with using nitrox?)
There are five big myths circulating throughout the diving community dealing with nitrox. Theses myths are listed and discussed here in depth. The first myth is that nitrox is safer than air. This statement has some merit, but don’t be fooled. When used correctly and within the limits of the specific mix, the safety of nitrox is equal to that of air. There are procedures that can be followed that can make it safer than air; however, with increased concentrations of oxygen, oxygen toxicity becomes a problem that is not usually a concern when diving with air. One common way to make nitrox diving safer is to dive a nitrox mix (i.e. EAN32) using an air computer. The computer will give you the most bottom time possible by continually calculating your residual nitrogen time at the specific depth you are diving, while the nitrox mix will decrease the amount of residual nitrogen by enriching the air you are breathing with oxygen. Most divers will run low on gas before their computers will make them surface; therefore, the decreased amount of nitrogen in the nitrox mix will make their dive safer than if they had done the same dive on air. Granted, by doing this you are not benefiting from the extended bottom time that nitrox offers, but it is safer.
The second myth is that nitrox is for deep diving. This is absolutely not true. Each nitrox mix has a safe maximum operating depth (MOD) and all divers should observe the recreational diving maximum depth of 130 feet. Because nitrox has additional oxygen, oxygen toxicity becomes a potential problem. For this reason each mix has a maximum depth that should not be exceeded. For example, the two most common mixes EAN32 and EAN36 have maximum operating depths of 111 and 95 feet ( with a PO2 of 1.4), respectively, far less than the recreational dive limit. The best depth range for diving nitrox is actually 50 to 110 feet of sea water.
Myth number three is that you cannot get decompression sickness. It is important to remember that the dive tables used when diving nitrox are no more, or less, dependable than the dive tables designed to be used with air. If you exceed the maximum bottom time for a given depth on any dive table, nitrox or air, the chance of getting decompression sickness increases dramatically. The only way to decrease the chance of decompression sickness is to cushion your bottom times, or extend your surface interval (cushioning your bottom time can be done as mentioned earlier, by using a different table or air computer with a higher nitrox mix, but MODs must still be observed).
The fourth myth is that narcosis is eliminated. It is true that nitrogen narcosis is significantly reduced; however, oxygen narcosis begins to take over. Just like nitrogen narcosis, oxygen narcosis is a severe reduction in judgment and the ability to complete simple tasks. Additionally, narcosis of any nature should be avoided and is eliminated by diving to a shallower depth. The final myth of EAN is that using EAN is difficult. Yes, there are additional steps to follow in order to safely plan and execute your dive, but once you are familiar with these procedures, nitrox diving is a safe and enjoyable experience that can leave you less fatigued after a dive than when diving with air, due to increased oxygen contents. A few things that need to be considered when diving nitrox are: 1) What is the percent of oxygen in your mix? 2) What is the maximum operating depth of your mix? 3) What is the maximum bottom time for the depth you are diving given your nitrox mix? And, 4) What was your oxygen exposure time for the dive you just completed? Items three and four would need to be considered when diving with air.
3. Summarize how each of the five myths are just that: myths.
4. Using your dive table, what is the maximum, no-stop dive time for the second dive after completing a 90 foot dive for 20 minutes and then sitting out for one hour before completing the second dive?
The amount of additional time provided by nitrox diving can be significant. In the following example I will compare two identical dives: one using air and one using EAN36. On the air dive and according to the NAUI dive tables, two divers completing a dive to 90 feet for 20 minutes would exit the water with a letter group of F. After staying out of the water for one hour (as recommended by NAUI), the divers would reenter the water for their second dive with a letter group of G. A second dive without decompression could be completed to 80 feet for 12 minutes. Using the same dive profile and EAN36, then end of the first dive would give the divers a letter group of E. After sitting out for an hour the divers returning to the water would have a letter group of D, as compared to a letter group of F with air. The most significant change, however, would be on the second dive to 80 feet. Divers using EAN36 could complete a no-stop dive for 36 minutes. That is three times the length of the same dive on air (24 minutes longer).
The main concern with diving nitrox is in getting oxygen toxicity, especially central nervous system (CNS) oxygen toxicity. This can occur with prolonged exposure to high partial pressures of oxygen; therefore, it is important to track your oxygen exposure while diving numerous days at the edge of the nitrox tables. Signs and symptoms of CNS oxygen toxicity are included in the acronym C.V.E.N.T.I.D.: Convulsions; Visual disturbances, tunnel vision; Ear ringing; Nausea; Tingling, twitching (facial and muscle spasms); Irritability, restlessness, euphoria, anxiety; and Dizziness, dyspnea. Methods for management of CNS oxygen toxicity are straight forward. First, it is important to observe the partial pressure of oxygen (PO2) limit of 1.4 atmospheres. This can be calculated by multiplying the percent of oxygen in the mix by the depth of the dive in absolute atmospheres. For example, a dive to 95 feet using EAN36 has a PO2 of 1.4 (.36 PO2 in mix X 3.88 ata = 1.4 PO2 exposure). The National Oceanic and Atmospheric Administration (NOAA) and other agencies have set the maximum exposure limits for a single dive at a PO2 of 1.4 to 150 minutes. This limit is increased to 180 minutes for repetitive dives. If the previous example was a single dive for 20 minutes then the divers would have a 13% exposure (20 minutes ÷ 150 minute limit = 13.33% exposure). If the previous example was a repetitive dive for 20 minutes then the divers would have an 11% exposure. This exposure percent would need to be added to any other oxygen exposures that occurred in the last 24 hours (oxygen exposure is cumulative in a 24 hour period). As you can see the chance of reaching 100% oxygen exposure, even during repetitive dives, is minimal in recreational diving, but it does exist none-the-less.
5. What is the percent of oxygen exposure for a single 95 foot dive using EAN36 for 50 minutes? What about for a repetitive dive?
6. What do you think the limiting factor(s) would be for choosing the “best mix” for a particular dive sight?
Even though EAN32 and EAN36 are the most common mixes, some divers prefer to dive the “best mix” possible for a particular dive sight or depth. In order to choose the best mix a diver must have two pieces of information: 1) the maximum depth of the dive, and 2) the preferred oxygen exposure limit (in this case a PO2 of 1.4 is the maximum limit that NAUI recommends when diving nitrox. The PO2 of 1.5 and 1.6 are considered to be contingency, should something go wrong during the dive). With these two pieces of information a division calculation can provide the best mix. For example, at 90 feet and using a PO2 of 1.4 the best mix is EAN38 (1.4 ÷ 3.73 ata = .38 PO2). (Remember to use the formula for converting feet of depth to absolute atmospheres as discussed in the search and recovery section of the Master Scuba Diver class.) Every depth has a best mix, but divers often choose to build in a safety factor by using one of the two standard mixes. This is both for the convenience of having a preexisting dive table and because odd mixes are often not available from nitrox suppliers.
7. What is the best mix for a dive to 80 feet using a PO2 of 1.4?
8. What equipment modifications do you think would need to be made in order to dive using Enriched Air Nitrox?
The best mix from problem seven should have worked out to be EAN41. However, the limit for Enriched Air Nitrox for recreational diving is 40% oxygen. Beyond 40% the concentration of oxygen increases the likelihood of spontaneous combustion of petroleum type products. To better explain this it is important to look at what a fire needs to burn. Every fire needs heat, fuel and oxygen. If you take away any one of the three things a fire will not exist. For scuba, oxygen is part of the breathing gas. As you increase the concentration of oxygen, so do you increase the chance of fire. Petroleum products (hoses, plastics, o-rings, etc.) provide the fuel for the fire and friction from the gas moving through the regulator provides the heat necessary to ignite the fire. There is a story of a diver who had recently been certified to dive nitrox coming out on the beach fully geared and ready to dive. When the diver purged his regulator it burst into flames and began to melt against his wet suit. A nearby diver reached over and turned off the diver’s air supply which cut off the flow of oxygen to the fire and caused it to go out. Later reports indicated that the diver had a tank of air that contained 80% oxygen and that was the cause of the fire. This story is included not as a way to scare you, but as an example of how some people feel they know more than what they really do based on their level of training. It is also included to express to you the need of training and dive planning. For recreational dive purposes, divers must follow “The 40% rule.” This rule applies to equipment considerations, as well levels of oxygen exposure and nitrox safety. Through years of research and the laws of physics, NOAA has determined that the risk of fire is virtually non existent if the percent of oxygen is below 40%. Therefore, the EAN limit for recreational sport scuba diving is 40%. As mentioned in the Equipment class portion of the Master Scuba Diver class, all equipment considerations fall under the 40% rule. According to this rule BCs, dry suits and lift bags would not need to be oxygen cleaned before using them with EAN up to 40%. Regulators and submersible pressure gauges (SPG)/computers also need not be oxygen cleaned, but it is recommend that you follow the manufacturer’s recommendations so that the warrantee is not voided. SDI/TDI recommend that all scuba cylinders used for nitrox be dedicated and oxygen cleaned (then only used for nitrox mixes). The reason for this is that nitrox fills often expose the cylinder to 100% pure oxygen during the filling process. This can put the fill station operator at extreme risk if the tank has not been properly cleaned. It is equally important that once a piece of scuba gear has been oxygen clean that it not be recontaminated by exposing it to standard air (which may contain compressor oil).
9. What equipment can be used with nitrox up to 40% without the risk of fire? What does the manufacture of your regulator require with regards to nitrox diving?
10. When technical diving is mentioned to you, what is the first thing that comes to mind?
A few years prior to the introduction of nitrox diving to the recreational dive community, technical diving began surfacing. Technical diving got its start with cave divers who needed special decompression and gas management techniques. Although most tech divers use modified, conventional equipment some special equipment and techniques were needed to allow divers to safely explore the 150 to 350 fsw (50-100 meter) range. Most agencies define technical diving as meeting any one or more of four specific criteria: beyond 130 fsw, EAN of 41% or more, any mix that is not air, and/or planned decompression. Diving agencies recognize that technical diving requires more training and preparation, and that not every diver has the physical and mental capacity to safely partake in the sport. Because dives are deeper in technical diving conditions are usually colder and harsher than normal. Beyond 130 fsw the risks of personal injury increases exponentially. For those reasons divers should not consider technical diving unless they are ready and willing to make a commitment to doing it correctly.
Technical diving is equipment intensive. Divers must prepare for high air consumption, colder temperatures, and long decompression stops. Some of the equipment that a normal tech diver might take along could include: two tanks for bottom breathing, two decompression tanks, one tank of argon for filling the dry suit, an extra long hose on the second stage for tandem octopus breathing, a lift bag and reel to be used as a personal ascent line for decompression stops, two computers (one for a back up), a high powered light and battery pack, and a separate regulator for each of the five tanks just mentioned. Normal scuba tanks are typically upgraded to include a “Y” or “H” valve for a back up regulator, or better yet, divers might carry a pony bottle with a separate regulator. Some of this equipment may never be used, but the tech diver needs to carry it in order for him/her to be a self-sufficient diver. Unlike recreational divers who rarely think about having to do a decompression stop, technical divers plan for decompression and, therefore, plan for decompression sickness and expect it. The additional equipment is a necessary evil when planning all aspects of the dive (good and bad).
Aside from the supernormal scuba set up, tech divers have to plan for contingencies that normal divers never think of. Deeper depths means that there is an increased risk of nitrogen narcosis, CNS oxygen toxicity, and decompression sickness. To help to alleviate some of the symptoms, tech divers often use helium to supplement the nitrogen (known as trimix). Helium helps reduce nitrogen narcosis and decompression sickness by reducing nitrogen absorption and is typically used at depths beyond 180 fsw. Nitrogen narcosis can effect the task loading that normally occurs when completing technical dives. A diver may forget which tank is suppose to be used next, or how long his decompression requirement is at a specific depth. Both errors could cost the diver their life and should be avoided at all costs.
Technical diving required more training, preparation and equipment. It also requires that the person be mentally and physically prepared for all possible outcomes. Divers should weigh the benefits and costs of such an activity prior to pursuing any technical diving opportunities.
11. What additional steps must be planned when completing a technical dive beyond 200 fsw?
12. What special rules apply to the dive tables for recreational divers?
Just like technical diving has special procedures and rules that must be followed, recreational diving has rules that reduce the risk of decompression sickness and increase the safety of each dive. Many of these rules are simple modification that should be observed when using standard dive tables. To begin with it is important to remember that dive tables are designed to be used at elevations under 1000 feet. Beyond that is considered to be altitude diving and special training and modifications are essential. Standard rules that you may not think about when using your dive tables include using the exact or next greater time or depth. Rounding down on a dive table will increase your chance of decompression sickness and remove the cushion that table makers have included. NAUI recommends that all divers stay out of the water between dives for at least one hour, even though most dive tables required only 10 minutes between dives to consider them to be separate dives. Also, it is important that when a dive is cold and/or strenuous the table user uses the next greater time on the table. Cold and strenuous dives increase the chance of decompression sickness and additional safety should be built in. Additionally, hypothermia increases the diver’s risks and is caused by a decrease in normal core body temperature. Lastly, divers should always observe an ascent rate that does not exceed 30 feet per minute. Having a slower ascent rate allows more outgassing and greatly reduces the chance of decompression sickness.
Suppose two divers make a no decompression dive to 60 fsw for 30 minutes followed by a 60 minute surface interval. Their second dive is to 50 fsw for 30 minutes. During their dives they complete a three minute precautionary safety stop at 15 feet (do not include the safety stop as part of the actual dive time). What is the end-of-dive letter group after the second dive?
Answer: 60 feet for 30 minutes results in a letter group of “F” on the NAUI dive tables. After a 60 minute surface interval the letter group improves to an “E.” An “E” diver using the dive tables and going to 50 fsw has an RNT (residual nitrogen time) of 38 minutes and an adjusted maximum dive time of 42 minutes. Adding the RNT to the actual dive time for the first dive gives the divers a TNT (total nitrogen time) of 68 minutes. Returning to table one shows that a 50 fsw dive for 68 minutes results in an en-of-dive group of “I.”
13. If the second dive was cold or strenuous what would have been the end-of-dive letter group?
14. What is the purpose of completing a slow ascent at the end of a dive to any depth?