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Some additional stuff for the nitrox FAQ. Thanks for all the feedback. I am hoping to post an updated version at the end of the week. Alan [20] How are nitrox cylinders identified? [21] What's in the cylinder? [22] Should I use nitrox for decompression? [23] Is it possible to get an oxygen bend? [24] What effect does the CO2 content have? -------------------------------------------------------------------- Subject: [20] How are nitrox cylinders identified? It is important to mark nitrox cylinders in a distinctive way due to the risks of diving without being sure of the contents of the cylinder. The standard colour markings are: a yellow body with a 4" wide green band near the top, the green band may include the neck portion of the cylinder. The cylinder should also be tagged with the nitrogen and oxygen percentages. As an added safety assurance you may also want to mark it with the MOD, the fill pressure and the fill date. Appropriate labels, tags and stickers are available from ANDI. Any shop which supplies nitrox fills should also have these markers and should insist on adequate markings on the cylinder. _ | |___ /XXX\ /\ <- Contents tag 4" green -> |XXXXX|\/ band | | | N | | I <---- Nitrox label | T | | R | <- Yellow body | O | | X | |_____| -------------------------------------------------------------------- Subject: [21] What's in the cylinder? Due to the danger of exceeding the maximum operating depth, nitrox fills should always be checked after filling and again before diving with the cylinder. You should never dive without being absolutely sure what is in your cylinder. -------------------------------------------------------------------- Subject: [22] Should I use nitrox for decompression? Use of nitrox in preference to air is advantageous. It may be used to reduce the length of the decompression penalty by following a nitrox decompression schedule, or it may be used as padding to increase the safety of the decompression by following an air schedule. This is similar to the use of pure oxygen during decompression. -------------------------------------------------------------------- Subject: [23] Is it possible to get an oxygen bend? Yes, but in practical terms it can be ignored. To get an oxygen bend you'd have to go well beyond all of the guidelines, omit a substantial amount of decompression obligation and be lucky enough not to have had an acute oxygen toxicity attack during the dive. Experiments carried out on goats at the Admiralty Experimental Diving Unit (AEDU) in 1945 demonstrated that oxygen bends are possible. The tests were based on immediate decompression (at 75 feet/min) to atmospheric pressure after one hour at the maximum depths (PO2 > 2.0 ATA). Severe bends resulted including pulmonary oedema and bubble embolism - identical to those caused by nitrogen. The symptoms disappeared within 10 to 15 minutes demonstrating that these were indeed oxygen bends. The oxygen was metabolised by the body. One out of seven occurences did not clear up naturally and required recompression for a full cure. Note that this procedure included substantial amounts of missed decompression and was at partial pressures well above the maximum recommendations for nitrox diving. It was concluded that the maximum PO2 that can be added safely to the tolerable PN2 lies between 2.0 and 3.5 ATA for immediate decompression. Since this is well above the maximum recommended PO2, due to the risk of acute oxygen toxicity, there is effectively no risk of an O2 bend in nitrox diving. Even in therapeutic recompression where the PO2 may be as high as 3.0 ATA there is no risk as the decompression rate is carefully controlled according to a well defined schedule. -------------------------------------------------------------------- Subject: [24] What effect does the CO2 content have? As yet there is no conclusive evidence that the CO2 level contributes to hyperbaric acute O2 toxicity in hyperoxic nitrox mixes. Extensive tests were carried out in the 1940's and 1950's and compared divers, non-divers and ex-divers in the same test scenarios. The researchers tried to find a means of identifying CO2 retainers and to find out if divers build up a tolerance to CO2. Nothing conclusive was found and this area requires further study, but it is highly unlikely to affect recreational SCUBA nitrox diving. Nitrox closed circuit rebreather designers will have to address this issue in order ensure sufficient expired CO2 absorption. There are some individuals whose breathing is regulated by oxygen levels rather than CO2, although most of these individuals probably shouldn't be diving anyway. This is known as "hypoxic drive", and often occurs in COPD (chronic obstructive pulmonary disease, ie emphysema) patients. The body adjusts to chronic elevated CO2 levels by ignoring that as a breathing stimulus, at which point the body's normal "backup" stimulus - lowered O2 levels - takes over as the primary means of regulating respiration. This, in itself, is not really a problem, although some of these people may stop breathing if exposed to high concentrations of oxygen.
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