Demystification of Nitrox, Understanding Mixtures

Transcription

1 Demystification of Nitrox, Understanding Mixtures In a very real way everyone of us has breathed nitrox. Nitrox means a mixture of nitrogen and oxygen and the word "nitrox" doesn't reveal the percentages of those elements. So air is nitrox too because it basically contains nitrogen (79%) and oxygen (21%). The base of word nitrox is the expression Nitro(gen) and Ox(ygen). N is the chemical symbol of nitrogen and O is the chemical symbol of oxygen. So basically nitrox can be any mix of nitrogen and oxygen containing 1 to 99 % of oxygen with the remainder being nitrogen. Enriched air oxygen means a nitrox mix which has more oxygen than air has, so that the mixture has 22-99% of oxygen. Enriched air nitrox is shortened to EANx, where x is the percentage of oxygen, i.e. EAN40 (40% oxygen in the mix). Also the EANx40 label is used sometimes (the letter "x" is preserved in the label). Usually people mean oxygen enriched air when they use the term "nitrox", and we will continue to use them interchangeably. Theoretically nitrox could also be a "hypoxic" mixture containing less oxygen than air! Although to my knowledge no one has bothered with this type of mixture since the heyday of UW habitats. When we use the term hypoxic mix or hypoxic nitrox, we mean it has less oxygen than 21%. Hyperoxic nitrox means having more than normal level of oxygen and this is what the term normally means and some organizations advocate the use of the term EANx, because the x demands filling in the percentage number. So a normoxic nitrox is basically air, a mixture 21% of oxygen and 79% of

2 nitrogen. Hypoxic breathing mixtures are not used in ordinary recreational diving, but air and Nitrox/EANx are. Heliox which is a mixture of helium and oxygen, Trimix is a mixture of helium, nitrogen and oxygen. Heli-air is a mixture that begins as air to which helium is added initially and to which oxygen may be added to bring the oxygen up to a specified percentage. This was an earlier way to achieve Triple mix, today some gas blenders begin with Nitrox and add helium to achieve the desired mixture. Hypoxic mixtures are usually used for commercial, military and research diving. Hypoxic Heliox and Trimix are commonly used for deeper diving and hyperoxic or enriched mixtures are used to shorten decompression time upon return to shallower depths. A full discussion of mixed gas diving is beyond the scope of this brief article, but it is covered in the USN diving manual. The simple term Mixture with the gases being specified in the order of Oxygen, Helium and Nitrogen was used by the RN and most of the worlds navies for over 30 years. It is still used my many diving researchers. Other conventions are also used, but what is most important is to clearly label the mixture! The original British term Mixture referred to what we call Nitrox and was primarily for use in Semi-Closed Rebreathers (the CDBA). The US dive industry is interested in marketing product and recreational divers and training agencies are interested in keeping it simply. Needless to say there is a lot of quibbling among the Stake Holders of the Dive Industry.

3 How and when did all this interest regarding breathing mixtures begin. In about the year 1890 scientists proposed the benefits of using an air enriched with oxygen in diving rather than compressed air or just pure oxygen in rebreathers and also theorized that enriching air with oxygen could decrease decompression time which was still not systematically understood. The first time such a mixture was prepared in advance and used is thought to be 1904 but earlier dates are proposed by some. Clearly air was mixed in breathing bags or left in the loop at earlier dates but without a systematic understanding of what was occurring. Potentially this could increase the risks associated with both hypoxia and oxygen toxicity. The first really large diving tests were conducted immediately before and during World War I. Later in the 1930s and during WW II it was used extensively, primarily in rebreathers (SCRs), and was also experimented with in helmet diving during this period. Draeger, Davis and the Siebe Gorman Company did extensive work in this area and during the run-up to WW2 much of this information became classified although most of it was already known among rebreather divers and among scientists. Amateurs and rebreather clubs were experimenting with oxygen-air mixtures and even helium air and oxygen mixtures in the early 1930s until the beginning of the War and again after the war until the rebreather sub-culture which was never very large was swept off the dive scene by the sudden availability of

4 really usable open circuit SCUBA. CCRs and SCRs became marginalized in US diving circles. The rapid growth of interest in diving and the dive industry during this period is an interesting history in itself! In the year 1979 the National Ocean and Atmospheric Administration (NOAA) published Nitrox/EANx diving tables. For safety and convenience of use by diverse divers NOAA divided Nitrox into two primary mixtures following earlier Navy practice by advocating a 32% and a 36% Oxygen concentration. These are designated Nitrox 1 and Nitrox 2 and can be referred to EANx32 and EANx36 respectively. These mixtures are suitable for open circuit use and by many rebreathers. Many military divers and SCR divers prefer Nitrox 40 and Nitrox 50. Even higher concentrations of O2 have their place in specialty rebreathers and for decompression. So called technical diving advocates have popularized the use of enriched mixtures, especially Nitrox 50 and pure O2 to shorten decompression time and decrease risk of DCS. This is to be strongly commended and encouraged as long as the maximum safe depths limits for these mixtures are observed. MOD means Maximum Operating Depth or maximum recommended depth for a given mixture of Nitrox or Trimix. The dominant determinant of MOD is the Oxygen concentration and the resultant partial pressure of Oxygen that occurs as pressure increases with depth. A partial pressure of 1.3 (and below) is considered preferable and 1.6 is considered too high for prolonged exposure, although it is frequently used during

5 decompression. NOAA Nitrox 1 attains a partial pressure of O2 of approximately 1.3 at a depth of 130 feet. This is considered it s MOD. Decompression diving is also beyond the scope of this brief article but should become an area of regular study for serious students of diving and all divers actually. Breathing Nitrox is generally safer than breathing air as long as the mixture is prepared correctly and it s MOD is correctly labeled. Testing after preparation is really necessary and now easily done. Preparation methods have been streamlined and there really has never been a good excuse for not labeling tanks clearly. Mark the MOD in large letters and the mixture percentages large enough so that a far sighted diver can read them during preparation and can read them off his dive buddies gear while diving. This is just basic safety. Accidents with the use of Nitrox have proven to be lower and DCS rates are considerably lower as has been proven over time. Increased total accident rates due to failure to prepare Nitrox properly have never been high and fortunately have decreased over the past decade. Accidents due to poor labeling routines still occur, but fortunately at a lower rate. Because of improved methods many resorts catering to divers routinely offer Nitrox at the same price as air fills. Many small dive shops around Florida and the Gulf charge about 1.5 times the cost of air for Nitrox, although higher for Nitrox above 40%.

6 Looking at current dive accident statistics from DAN and other mainstream sources, the Total Incidence of DCS does seem to decrease with the use of Nitrox. This has also occurred with the widespread use of improved dive computers and an overall improved emphasis on dive safety in general. Clearly dive accidents are much more than DCS incident rates. Many errors are possible and do increase risk when preparing breathing gases. Nitrox preparation should not be taken lightly, someone is going to breathing that gas and their life depends on it! But an accident doesn't have to be DCS only, and when diving with nitrox there are several other concerns too that air diving doesn't have. Gas blending, analyzing errors, oxygen clean equipment preparation and potential Oxygen toxicity(o2-cns Clock) are also sources of risks potential dive related accidents. Loose use of terms has led to some of the misunderstandings regarding Nitrox. Sometimes around dive shops I hear people state that Nitrox is for deeper diving. Not true, what is true is that Nitrox is rarely used below 130 ft. It is used on the deeper end of relatively shallow dives in the 60 to 130 foot (20 to 40 meter) range to increase bottom time and or decrease nitrogen on-gassing. The more oxygen you have in the mix, the shallower the MOD (maximum operating depth). Very usable depth and time advantages can be had using SCRs and OC- SCUBA in this range with Nitrox. Nitrox does offer some advantage in avoiding narcosis in the 80 to 130 foot range although it may be a mild advantage it is

7 noticeable particular when a diver who has been diving in the 90 to 100 foot range with Nitrox does dives on subsequent days with plain air. This has been confirmed by performance testing. There is some legitimate questioning of this but there was and still is an anti Nitrox camp in the dive industry and certifying agencies that are grasping at straws to emphasize what a serious and dangerous business this Nitrox is. There is no increased risk for air embolism with nitrox and the wider availability of surface oxygen and immediate medical evacuation have improved the clinical outcomes of the fortunately rare embolism cases. For the last decade many divers who basically dive the No-Deco Limits, have been diving Nitrox/EANx for the simple reason to gain more bottom time and to have shorter surface intervals. Many divers at resorts today use 32% or 36% Nitrox for dives that rarely exceed 100 ft depth as well as use dive computers set conservatively, often to air to decrease DCS risk. I have met many such divers. A short review of the improved dive times available at these intermediate depths can be immediately had by comparing the NOAA or NAUI No Deco Air Tables with the NOAA Nitrox 1 and Nitrox 2 No Deco tables. The commonly available EAN40 No Deco table is also very illustrative and it is very easy to construct examples by comparing these various Nitrox tables with commonly used No Deco air tables. The bottom time improvements are clear.

8 Divers who plan to dive deep mountain lakes found at altitudes above 700 meters also gain a time/depth advantage and lower their risks of DCS while diving at altitude. Enriched mixtures/ EANx commonly used in technical and commercial diving to shorten the decompression times. Nitrox diving in most of the western world requires certification which is fairly easily obtained and relatively low cost. Many of these courses can at least partially be done on-line and are not problematic for the experienced diver. I feel that a thoughtful diver with 15 dives behind him is a perfect candidate for Nitrox diving. BSAC was somewhat of a hold-out and late comer to Nitrox certification despite the long history of British use of this breathing gas. IANTD International Assoc. of Nitrox and Technical Divers ANDI and TDI quickly entered the marketplace and are well regarded by their students and they encourage the diver to advance their education in meaningful ways. NAUI sponsors Nitrox instruction and encourages shops to provide Nitrox. In 1995 PADI adopted nitrox training as a part of it's program, and the course certifies diver to dive nitrox dives within the recreational limits and with blends of 22%-40% of oxygen.

9 Recommended Reading for Divers Who Love Diving Deco for Divers Deeper into Diving USN Diving Manual NOAA Diving Manual BSAC Nitrox Instruction Manual The New Science of Skin and SCUBA Diving. Even the first edition of this book still makes my must read list, even though it s information regarding Helium is dated and misleading. Richard Pyles - Learners Guide to Rebreather Diving, an article available on-line that will greatly increase the non-rebreather diver s knowledge of these important devices and hopefully encourage others to explore the possibilities of offered. Actually everything Richard has written is worth reading even for us nonichthyologists. Norwegian Dive Manual and Deco Tables NAUI Dive Tables, especially NAUI No-Deco air table on plastic card. DCIEM Manual