Rebreathers After Diving Open Circuit Scuba but I Want One
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tech talk Edited by Michael Menduno Switching to rebreathers after diving open circuit scuba But I want one... ed.—Technical Diving Instructor Mark Powell looks at switching A rebreather is simply a way to from traditional open circuit SCUBA to closed circuit rebreathers. reuse the gas breathed out by the diver in conjunction with a method of Many of the pieces of equipment used by technical divers removing the carbon dioxide produced look different to the equipment used by recreational divers. by the diver. The main advantage of a rebreather is that it is much more efficient However, for most of the time, the basic principles are the same. on gas usage. When breathing normal air However, the use of rebreathers in technical diving completely at the surface we use up about 4% of the changes many of the basic principles that govern recreational oxygen in the air for our metabolism and diving. Some limits are removed almost completely but other breathe out the rest of the oxygen as wasting ten times as much gas with each of the gas. well as all the nitrogen. This means that breath in order to obtain it. This is one of By reusing our gas this provides three complications are introduced. 96% of the gas we breathe escapes with the reasons why for open circuit divers key benefits to the technical diver. each breath. At depth this is made even gas planning and breathing rate is so Text by Mark Powell worse by the fact that we are breath- critical. The limiting factor for an open cir- 1 - Gas Duration ing gas under much higher pressure and cuit dive to 90m is almost certainly going Firstly we don’t need to use anywhere so each breath contains a much higher to be the amount of gas that needs to near as much gas as an open circuit volume of air. At 40m we are breathing 5 be carried for the dive. A rebreather diver. The amount of times the volume of air with each breath avoids this problem by re-using the gas Oxygen required compared to the surface breathed out by the diver, as the major- by the body is and at 90m we are ity of the gas isn’t being wasted the diver roughly the breathing ten times the doesn’t need to carry as much gas in same at any volume that we would order to do the same dive. A rebreather depth and so be on the surface. works by catching this exhaled breath our Oxygen With every breath this and instead of allowing it to escape it will last the gas escapes from our reuses it in the next breath. The small same at second stage and bub- fraction of Oxygen that was used up by 100m as it will YMES S bles to the surface. This is inefficient as the body is replaced with more Oxygen. at 20m. The ETER the body still only needs the same This means that rather than wasting the balance of the P amount of oxygen but we are vast majority of each breath we reuse all gas in the breath- 73 X-RAY MAG : 51 : 2012 EDITORIAL FEATURES TRAVEL NEWS WRECKS EQUIPMENT BOOKS SCIENCE & ECOLOGY TECH EDUCATION PROFILES PHOTO & VIDEO PORTFOLIO Despite taking these large cylinders they will still be used tech talk up very quickly and so the amount of available gas becomes the most critical part of our dive planning A Twinset of trimix can cost anything from ing loop, known as the diluent, is reused 2 - Gas Costs compared to open circuit rather than wasted and so again we use One of the biggest factors £30 for a relatively diving. This can look very the same amount at 100m as at 20m. This that comes into the decision week mix for use in attractive when you are vastly decreases the amount of gas we to switch to a rebreather is spending considerable will need to complete the dive and rather costs. This has two aspects, 40m to over £100 amounts on each open than twin 20L cylinders we can use two 3L the costs of the rebreather, for a mix suitable for circuit fill. However this cylinders. including buying and main- must be set against the On open circuit we taining it, diving deeper than costs of the rebreather. breathe much more set against 100m. Depending on the model at depth due to the the cost a rebreather is likely to effect of pressure and of open circuit diving. cost between four and eight thousand so as we go deeper One of the biggest dis- pounds. If you go for an older second and deeper we advantages of open hand model then you might get one for have to take larger circuit trimix diving is less than this. In addition to the initial cost and larger cylinders. the cost of the gas. A of the rebreather you will need to factor Despite taking these Twinset of trimix can in training on the rebreather which is likely large cylinders they cost anything from £30 to add on another thousand pounds. In will still be used up very quickly and so the for a relatively week mix for use in 40m to addition there will undoubtedly be addi- amount of available gas becomes the over £100 for a mix suitable tional costs to add on most critical part of our dive planning. On for diving deeper than 100m. Depending on the extra equipment to the a rebreather our gas supply is used up at This makes each trimix dive model a rebreather is basic rebreather. As a the same rate irrespective of depth and an expensive proposition. On result it is not uncommon so the amount of gas we are carrying is the other hand a rebreather likely to cost between for the initial start up costs no longer the limiting factor. This means uses much smaller cylinders four and eight on a rebreather to be that running out of gas on a rebreather is because the gas is reused between 6 and 10 thou- much less of a concern than for an open rather than wasted, as a thousand pounds. sand pounds. Obviously circuit diver. Instead the critical factors result we use much less and you will need to do a lot become the amount of decompression the gas costs are much lower. We might of trimix diving in order to save enough to we are incurring, the duration of our car- only be spending £5-£10 for the same justify this initial outlay. bon dioxide absorbent and our risk of mixtures discussed above. As a result The majority of divers do not do enough oxygen toxicity. there is a significant gas saving when diving to clearly justify buying a rebreath- er based on savings in gas costs. REBREATHER TERMS: Unless you are doing 20 or more trimix dives a year then it is not cost OC Open Circuit (traditional SCUBA) effective. If you also do a significant CCR Closed Circuit Rebreather amount of recreational diving then SCR Semi-closed Circuit Rebreather each dive may actually cost you eCCR Electronically controlled Closed Circuit Rebreather more on a rebreather. mCCR Manually controlled Closed Circuit Rebreather On open circuit you may only HUD Heads Up Display (LED display of oxygen partial pressure) need to pay for an air fill but on BOV Bail Out Valve (Open Circuit bail out used in an emergency) even the shallowest dive you will still Diluent Gas used to make up the balance of the breathing loop, can be air or trimix need to use pure oxygen in one of Scrubber Module used to remove carbon dioxide from the breathing loop the cylinders and use carbon diox- YMES ide absorbent in the rebreather. The S ETER P 74 X-RAY MAG : 51 : 2012 EDITORIAL FEATURES TRAVEL NEWS WRECKS EQUIPMENT BOOKS SCIENCE & ECOLOGY TECH EDUCATION PROFILES PHOTO & VIDEO PORTFOLIO PETER SYMES tech talk The majority of Rebreathers The fact that divers do not do a rebreather enough diving to uses smaller cylinders makes clearly justify buying the logistics of a rebreather based on filling much easier savings in gas costs. annual each dive. This the open circuit diver the The replace- requires lots of rebreather diver will not other ment of additional heli- use their stages unless there advan- the three um and oxy- is an emergency but will still tage of a oxygen gen each day need to carry them. However rebreather is that sen- and filling on open circuit the diver will it can reduce the sors and these large use the decompression stages decompression handset back gas on each dive and so they will obligation when compared batteries cylin- need to be filled each day. to the same dive on open also needs ders and circuit. For a diver using a twinset to be decompres- 4 - Decompression and a stage cylinder they may taken into sion cylinders Obligation plan their gas to have the account. each day is This means expensive, time that a shal- consuming low recrea- and labouri- tional dive may ous. only cost £5 for an The thing open circuit diver fact that a next day to but £10-£15 for a rebreather uses pick it up then the rebreather diver. smaller cylinders logistics of this can It is clear that makes the logistics of filling be significant. Similarly if you are unless you are much easier. The volumes diving somewhere without easy doing significant of gas needed are much access to a dive centre that can numbers of deep smaller when filling a 3L cylin- fill trimix the ability to take your trimix dives a year with little der rather than twins 12s and so it own gas simplifies the planning or no recreational diving then the becomes feasible to take enough and logistics.