Contrary to popular belief are actually a much older technology than open circuit regulators

The Long (his)Story About focus Scrubbers & Sensors From Fleuss to Evolution There is more and more talk of Text: Michel Tagliati rebreathers, stated Additional reporting by Peter Symes as being the future of diving. However, not many people have tried them. So what is all the fuss about?

he greatest majority of the Treaders of this magazine will have learned to dive with a regulator originating from the Aqualung, which the renowned and Emile Gagnan invented in 1940. The regulator was the first , an English naval offic- commercial, er from , worked out the generally principles for a re- appa- available dive- ratus, and produced a prototype at the system, and end of the 1870’s. He stayed down in a this promoted to re-use the air that has been exhaled, water tank for nearly an hour, and later the later great circuit, because the exhaled air by using a closed system. went down to 5 meters in a lake using expansion of piece of passes straight out into the surrounding his system. Fleuss was the first “diver” in sports . It is reli- water, and is thereby lost. As only about Closed circuit systems history with a re-breathing apparatus. At everywhere. able, more or less foolproof, a quarter of the available has Closed circuit systems are a far from the beginning of the 1900’s the military And it must and its simple construction is been taken up by the body, and the rest new idea. It was Giovanni Borelli in the were quick to take up this idea. Oxygen- be said that very robust. These are good properties expelled, it is a rather ineffective utilisa- 1700’s who first thought of re-using the rebreathing equipment consisted of the regula- to have when one’s underwater breath- tion of a scanty resource. (See table next exhaled air. His idea was to recirculate an oxygen tank together with a bag of

WWW.AMBIENTPRESSUREDIVING.COM tor is a fine ing depends on them. However, as we page.) the air through a copper tube which was potassium hydroxide and a breathing will see, there are also a number of dis- In addition, as the amount of air used cooled by the sea water, and thereby loop, and was used to rescue submarine Ambient Diving’s Closed Circuit advantages. And it is these that make increases proportionally with depth, open “cleaned” the air before re-use. Luckily, crews and attack divers (there were no Evolution is the latest step in CCR re-breathers an interesting alternative systems become more and more ineffi- it was never made. The mining industry revealing bubbles on the surface). The ...erh... evolution. system. cient, and there are therefore major limi- and its problems with gas in the mine German manufacturer Dräger released When diving with a regulator, it is tations to how long a diver can remain shafts also stimulated the relevant tech- several models for military use in connec- said that one is diving with an open underwater. It seems obvious, then, to try nical developments during the 1700’s. tion with World Wars I and II.

82 X-RAY MAG : 4 : 2005 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED

Respiration corresponding increase in A REBREATHER TIMELINE In a modern medical textbook on physi- pressure. Therefore, when 1500´s In and France, full diving ology, one of the several chapters on using a closed circuit there 125 l/min suits made of leather with metal helmets. The aspects of is entitled “Physical is the same consumption of diving helmets were already some sort of principles of gaseous exchange; air, whatever the depth. At 100 l/min rebreather, but needed surface supply and of oxygen and carbon dioxide through rest, 0.3 to 0.5 liters of oxy- were without scrubber 75 l/min the respiratory membrane”. And it is these gen are used per minute 1680 Giovanni Borelli designed a closed very important principles that are of great (l/min), and with maxmum 50 l/min breathing circuit. The idea was to recirculate interest to divers. How we take in and work up to 3.0 l/min are Open systems - i.e. regulator and tank 25 l/min air through a copper tube which was cooled utilise oxygen, and how we get rid of the used. Of course, there are Closed circuit rebreathers by sea water. The assumption was that al the 3 l/min carbon dioxide produced by . differences between indi- Gas consumption impurities would then condense out of the air viduals, depending on the 10m 20m 30m 40m inside the tube. Cellular metabolism is size of and his or Depth 1726 Stephen Hale designed the first independent of pressure her physical condition. scrubber: a flannel liner, soaked in salt and The amount of air which is needed to Comparison of gas consumption in open and tarter, used in a helmet for mine disasters. closed systems. In closed circuit rebreathers the take a normal breath at a depth of 20 The carbon dioxide 1772-4 Oxygen independently discovered gas consumption doesn’t vary with depth. meters is three times greater than at the problem by Swedish chemist, Carl Wilhelm Scheele, in surface, and therefore the consumption As we have seen, cellular 1772, and the English chemist Joseph Priestly, of air with open systems increases with metabolism depends on oxygen, which diver can tolerate a build up to a CO2 in 1774. Soon followed the first known ideas depth. However, more importantly, cel- is inhaled from the atmosphere. The fol- of about 10%, increasing to use Oxygen for diving and first functional lular metabolism does not depend on the lowing table gives the composition, in respiratory volume to compensate for ideas to build autonomous rebreathers. pressure. volume percent, of inhaled atmospheric the increased CO2. However, beyond 1876 Henry Fleuss began to develop an Oxygen is required to make adenosin air (on an average cool, clear day), and the 10% level the respiratory center in the oxygen rebreather. He used a rubber face trifostat, ATP, the fuel of the human cell. also the corresponding composition of brain stem begins to be depressed rather mask and a breathing bag connected to And a molecule of oxygen is a molecule exhaled air. (There is also a minor content than stimulated, and the diver’s respira- a copper oxygen tank. The carbon dioxide, of oxygen, wherever it is to be found, at of the non-reactive noble gases such tion then actually begins to fall rather scrubber was a rope yarn soaked in a solu- the beach, or 100 meters deep, with the as argon, helium, etc., but this has been than to compensate. As a result of this, tion of caustic potash. Enabling Fleuss to walk ignored here.) It will be seen that the varying degrees of lethargy, narcosis, along a river, this was the first SCUBA Dive. uptake of oxygen in the lungs has caused and, finally, anesthesia will occur. The 1879 Fleuss builds a Mining-Rescue the oxygen content of the inhaled air CO2 in the exhaled air must therefore be Rebreather for Siebe/Gorman to be reduced from its original 20.84% removed before the remaining oxygen 1881 A special rebreather scrubber using to 15.7% in the exhaled air. It will also be can be re-used. Luckily, this is quite easy a barium hydroxide is patented by Khotinsky seen that the concentration of carbon to do, using calcium hydroxide, Ca(OH)2, and Lake. dioxide, a biproduct of metabolism, has which reacts with, and therefore fixes, the 1904 Siebe and Gorman patent Oxylite, a gone up by about ninety times, to 3.6%. carbon dioxide. It is called CO2-scrub- potassium- and sodium-peroxide mixture that bing and the filtering material, in this case produces oxygen on contact with water. COMPONENT INHALED AIR % EXHALED AIR % teh calcium hydroxide, which comes as 1907 A Dräger rebreather is used as sub- N2 78.62 74.5 a granulate, is referred to as the scrub- O 20.84 15.7 marine rescue equipment 2 ber. CO 0.04 3.6 2 1912 Dräger helmet-diving-rebreathers are H O 0.50 6.2 In a rebreather, CO2-scrub- 2 available Total 100.0 100.0 bing therefore takes place by circulating the 1913 Dräger performs a successful simu- So, it is not so much the reduction in through a canister with the lated 40 minute dive to 80m in chamber. amount of oxygen that is the problem, for scrubber granulate in it. 1914 Dräger introduce a selfmixing - there is still some 16% available in each Rebreather for max. 40m. expelled breath, which should be avail- Scrubber, in this case 1926 Dräger presents the first early recrea- able for re-use. It is the increase in carbon Sofnolime, comes in tional rebreather, the ‘Bade-tauchretter’. dioxide, CO2, that is the real problem. A small granules, 1941 Dräger “Kleintauchgerät 138” 1953 Dräger Leutnant Lund II and No, rebreathers are not new. In this Expression: Breathing loop Barakuda Delphin I 1912-footage we see Drägers helmet The whole volume of circulating gas in a rebreather, mainly 1967 Russians experiments with predeces- diving rebreather in use. the hoses, one or two counterlungs and also the diver’s lungs. sor to AKA-60 Photo: Dräger’s archive.

83 X-RAY MAG : 4 : 2005 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED 1968 First electronically controlled So, at the beach, oxygen in the air has a On the other hand, a prologued expo- The matter with oxygen rebreather, the Electrolung.is marketed And while we are at it, why not increase partial presure of ca 0.21 bar. And if the sure to high ppO2 leads to oxygen toxic- the oxygen level in the ínhaled air, even diver is using compressed air at a depth ity and the condition which 1970 BioMarine CCR-1000 perhaps right up to 100% pure oxygen? of 10 meters, then the corresponding oxy- manifests itself in sudden convulsions, 1972 BioMarine Mark 15 This way we could also avoid breathing gen is 0.42 bar (0.21 x 2 which under water inevitably will lead to 1975 Dräger LAR V in nitrogen, which is the cause behind bar), at 20 meters it is 0.63 bar, at 30m its . Therefore a ppO2 of 1.6bar depth narcosis and illness 0,84 and so forth. In the literature the par- is usually considered the upper limit in 1977 BioMarine NM-6 and give away with all concerns about tial pressure of oxygen is written as ppO2. , and only for a limited 1978 Interspiro ACSC decompression limits. If it only was so time. And 1.3 bar the level not to exceed 1982 Interspiro Oxydive easy! The apparently paradoxical answer The importance of pp02 in general. is that pure oxygen quickly becomes poi- When discussing respiration this is prob- For these reasons it should be clear that 1984 Dräger Tieftauchsystem CCBS for operating depths of max. 600m sonous under pressure. ably the most important parameter to maintaining the ppO2 of the breathing be considered. Not only must the excess gas within specific bounds is of utmost 1985 AKA-60 (Russian) Partial pressure of oxygen carbon dioxide be removed from the air, importance. It becomes, quite litterally, a 1991 Dräger Assume that we have a mixture of gases. but the partial pressure of the oxygen matter of life and death. Then the total pressure of the mixture is must also be maintained at a reasonably 1992 Interspiro DCSC PETER SYMES equal to the sum of the exerted constant level to ensure correct cellular Diving conventionally with compressed 1995 Dräger SCR Atlantis (1998 renamed separately by each of the individual metabolism. air on open systems, as most regular holi- in Dräger-Dolphin) Lt.Lund rebreather, 1953 components in the mixture. So, if the mix- Gaseous diffusion from the alveoli day makers do, controlling ppO2 is usual- 1998 Buddy Inspiration Scrubbers ture we are considering is atmospheric of the lungs to the pulmonary blood is ly not an issue as normal air only reaches 1999 Dräger-Ray Although easy to do in principle, getting air, we have: determined by the partial pressure of the a ppO2 of 1.3 bar at a depth of 54m - far a CO2-scrubber to function in a safe and oxygen. The partial pressure determines beyond where they should venture. 2001 Halcyon RB80, Cochran CCR and reliable way is not completely without its Patmos = PN + PO + PCO2 the exerted in diffusion through the But as soon as we start tinkering and SCR Azimuth PETER SYMES complications - which many divers have pulmonary membrane. raising the O2-content of our breathing 2004 Diving’s Evolution found out to their cost. As with walking Where In other and more plain words, it is the gas, these matters change, as we are or running, it takes work to breath under- Patmos is the ppO2 and not the O2 % that determines taught already at entry level Nitrox cours- water. If the resistance to breathing is too PN is the partial pressure of nitrogen whether we live or die. es —such as PADI’s Enriched Air Specialty, PCO is the partial pressure of carbon dioxide high, there will be physiological problems 2 Consequently, ppO2 should be maina- or the various Basic Nitrox courses offered which will limit the use of this system. PO is the partial pressure of oxygen tained within a certain range to sustain elsewhere. Also, hypercapnea, the condition life. where there is too much CO2 in the body Now, according to the ideal , as Normal ppO2 (at sealevel) is, obvi- can become an issue. The material used the components are all at the same tem- ously, 0.21 as air contains 21% oxygen. If to remove the carbon dioxide in the sys- perature, the partial pressures must be the partial pressure falls below 0.12 (12% tem, the scrubber, has a limited life, and proportional to the volumes of each gas oxygen at sealevel) most individuals will can only fix a certain maximum amount present in the mixture. In the table given lose their ability to function. Below 0.10 of carbon dioxide. above, oxygen has an volume % of approx- bar ppO2 they will lose conciousness and As these filters cost money, people imately 21 of atmospheric air. This means below that die from asphyxiation. are disinclined to change them often that its partial pressure is 0.21 of an atmos- enough. The maximum limit for how phere, or 0.21 bar. This is oxygen’s contribu- long these filters should be used is thus tion to the total atmospheric pressure. some times transgressed, which increases the risk of acci- dents.

Oxygen rebreathers In water pure oxygen rebreathers are re- stricted to use in shallow water of less than 6 meters - to keep ppO2 below the1.6 bar limit.

WWW.DIVESAFE.NET For this reason the pure oxygen rebreathers are of limited and mainly military use. Oxygen rebreathers however also have a use on land with , rescue crew and miners.

84 X-RAY MAG : 4 : 2005 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED Whether we dive on open systems or general limits, this optimum Oxygen percentage Open, semi-closed and fully closed circuits semi-closed rebreathers (see explana- oxygen content corresponds to a at various depths tion below) we chose, prior to the dive, constant oxygen partial pressure of 100 % By Open Circuit one usually refers to Another important distinction between our breathing gas with a predetermined 1.3 bar oxygen regardless of depth. the omni-present regulator - a second the popular Dräger SCRs and a CCR 75 % oxygen-content. This may be air or we and a first stage connected to on a such as the Inspiration, is that the SCRs Closed circuit may op for another specific gas suitable Controlling the pp02 50 % tank of compressed air. The expres- have a present oxygen-% and a vari- Rebreathers for the depths we are aiming at. In most To ensure that the correct oxygen sion is partially misleading because able pp02, whereas in the Inspiration it

cases this will be Nitrox–which is now be- level is obtained, i.e. its partial pres- Oxygen % 25 % the air doesn´t go in a circuit. It travels is the pp02 that is fixed and the oxy- coming quite a household word among sure, it is necessary to measure its Diving on air from the tank and via the regula- gen-% which varies with depth. divers. concentration. For this purpose a tor, through the diver before getting 10m 20m 30m 40m 50m Nitrox with 32 or 36% oxygen is now special transducer is needed, an exhaled into the water in a one-way quite routinely being offered at divecen- oxygen sensor. (see box below) Depth process. tres, resorts and worldwide as these two “standard-blends” will cover Modern closed A semi-closed circuit is a type of most needs in the typical recreational breathing systems rebreather in which (most commonly) range with near optimum benefits. With the advent of oxygen sensors Comparison of the breathing gas is being continously However, chosing the right oxygen-% and microprocessors, it is now pos- no-stop times injected from the tank into the breath- will always be a compromise between sible to reduce the amount of gas 150 min Diving ing loop where it circulates a number on air longer no-decompression times versus taken down by a diver to a mere Closed circuit of times. On average the gas is re-cir- maximum depth. For example, diving on bagfull of atmospheric air and a 120 min Rebreathers culated 4-5 times before being vented to the outside through an over-pressure Nitrox with 36% oxygen gives a generous few liters of compressed oxygen. 90 min no-deco time of 50 mins at 26m - rather These closed circuit rebreathers, 40m 50m . It is because gas is continously than the usual 20 min when diving on air. CCR, use oxygen sensors to control 60 min being vented that these circuits are That is nice, but Nitrox 36 can´t take you the partial pressure of oxygen in the referred to as being semi-closed (or Dräger Dolphin SCR No stop time any deeper, as pp02 is already 1.3 bar inhaled gas. This means that the 30 min rather semi-open). The breathing gases around that depth. diver can continually control and used in these semi-closed (circuit) And if you dive any shallower you maintain oxygen partial pressure at 10m 20m 30m 40m 50m rebreathers or ‘SCR’ are various pre- don´t get the full benefits of Nitrox at that certain levels during a dive. And this set Nitrox-blends. The popular Dräger particular oxygen content. So, for each is where the fully closed rebreathers Depth rebreathers (Dolphin, Ray...) are SCRs. depth there seem to be an optimum excel. oxygen % in the breathing gas. With the In a fully closed (circuit) rebreather Say, for example, that a fullly closed loop. The rebreather then will then compensate in a or ‘CCR’ all the gas is continuously breathing system has been set to compensates to keep the controlled way by injecting being recirculated and no gas is vent- maintain a given oxygen partial oxygen partial pressure some more oxygen into the ed. The CCR is generally being consid- pressure at depth, typically 1.3 constant. Likewise, if the loop. ered the thoroughbred of rebreathers. bar. If the diver then descends to diver ascends, the pres- As the graphs above In CCR’s the only gas being con- another depth, the ambient pres- sure decreases, and the clearly demonstrate, the sumed is the oxygen metabolised by sure increases, and with it the oxy- oxygen partial presure will potential extension of no- the diver (not considering gas used for gen partial pressure in the breathing decrease. The apparatus deco times are huge. inflating suits, wings etc and gas lost during ascent and decent). APD Inspiration CCR

Oxygen sensors solve in order to reach the sensing So, as the two processes take place, tion. The current produced can then - An oxygen sensor can be electrode. The flux of oxygen to the O2 + 2H2O + 4e- = 4OH a current is generated (represented be used, via a computer, to control considered as a small fuel- working anode is dependent on the here by the four electrons 4e- ). This an oxygen inlet valve. cell in which the chemical partial pressure gradient of oxygen These hydroxyl ions migr ate through current can be measured externally As the electrochemical reaction energy of oxygen is transformed across the barrier. This means that the a conductive electrolyte (typically by passing it through a known resist- results in the oxidation of the lead an- into electrical energy. output signal from the cell is propor- potassium hydroxide) to the metallic ance and measuring the potential ode, these sensors have have a lim- tional to the partial pressure of the lead anode, where they are involved drop across it. Since the current ited life. Once all the available lead A very thin, plastic membrane, oxygen in the gas mixture. in the oxidation of the lead to its produced is proportional to the rate has been oxidised they no longer placed over the top of the sensor, When oxygen reaches the working oxide. at which these reactions occur, its work. Typically, oxygen sensors have operates as a solid barrier in which electrode, it is immediately catalyti- measurement allows accurate deter- 1 – 2 year life times. - the oxygen molecules must dis- cally reduced to hydroxyl ions. 2 Pb + 4 OH- = 2 PbO + 2 H2O + 4 e mination of the oxygen concentra-

85 X-RAY MAG : 4 : 2005 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED Taking it further cally, the ‘base substance” of what number of tanks a diver who need US/Canada Distributor of Inpiration Nitrogen is the usual diluent for you breathe and what the oxygen to explore some extreme environ- and Evolution Rebreathers oxygen in our normal atmosphere. gets injected into, it is what inflates ments like deep wrecks and caves However, as all divers know, nitro- the breathing loop and what is used need to carry and for this reason gen dissolves in the blood under for in the wing or drysuit. the rebreather has become a pre- high pressure, and can cause nar- So with an octopus attached this ferred tool among many underwa- cosis or . To tank also can double as a normal ter explorers and scientists alike. reduce or eliminate this problem open circuit and as backup system divers would like to use as little nitro- should there be a problem with Next issue: gen as possible in the inhaled air. the rebreather. (Switching to open Diving the thing - what is it like and Remove the biologically inactive system by closing the rebreather’s what the difference in skills? nitrogen from the inhaled air and mouthpiece and breathing from the risk of being hit by rapture of the octopus instead is referred to as the deep disappears. And rebreath- bail-out). ers can solve this problem, too, with The diluent gas is normally not the non-narcotic gas helium replac- consumed except for what is used ing nitrogen in the inhaled air being to inflate the breathing loop and used at greater depths. maybe the suit and wing (some In CCRs like the Inspiration there carry separate tanks for this, i.e. are two tanks (see picture on previ- when argon is used for suit inflation). ous page). One contains the oxy- For divers this is very good gen which is being injected into to news as Helium is a very expen- breathing loop to maintain the cor- sive gas so using a rebreather rect pp02. also come a significant economic The other tank contains the dilu- incentive. Also it means a signifi- ent gas, which usually is just air, but cant reduction in and the it can be anything breathable such as trimix. This gas is, basi-

“Now, where Disclaimer: did that con- Diving with rebreathers requires tact lens go?” special training.

When experience comes in the way

Diving with a fully closed ing of safety. This technical divers use fully of 3-liter tanks containing 200 system is, in many ways, can lead to a fail- closed systems at depths bars of compressed oxygen. significantly different ure to carry out the of more than 100 meters, However, the technique is so from diving with an open vital control checks in spite of the fact that nei- advanced that the neces- system. Several things of the equipment ther the equipment nor the sary knowledge, training and have to be re-learned, before and after sensors are approved for maintenance of this equip- and re-learning is much each dive. these depths because of ment is not yet fully integrat- more difficult if one has Oxygen sensors the doubts regarding the ed into the training courses has long been used to diving breathing system. degrade (oxidise) and must reduced reliability of their offered today. with an open system. Many For example, they may not be changed every year, or performance. (see, for To dive safely with fully experienced divers think that realise that the oxygen par- more often, according to example, the story about closed systems requires a they are quite competent at tial pressure can vary con- how much the system has David Shaws fatal dive to thorough theoretical and diving, and often find it dif- siderably. Also, there is a risk been used. The reliability of 271m in X-RAY #3) practical knowledge plus fre- ficult to accept that they are that a very experienced diver the sensors and their out- But, today, it is technically quent use in order to main- beginners again when div- will make serious elementary put is also a matter of some possible to dive to a depth tain the routines and reflexes ing with a completely closed mistakes, due to a false feel- controversy. It happens that of 100 meters with a couple learned during training.

86 X-RAY MAG : 4 : 2005 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED Water is unique in that it is the only natural substance that is found in all three states – liquid, solid (ice) and vapour – at the nor- mally found on Earth

density, ensures that all of a body of The Uniqueness of Water water (not just its surface) must be close to 0°C before any freezing can Water, obviously, is essential For a start, water is unique in that occur. The freezing of rivers, lakes to our very existence, with- it is the only natural substance that is and oceans is therefore from the top found in all three states – liquid, solid down, so insulating the water from out it we rapidly die. In spite (ice) and vapour – at the tempera- further freezing. The oceans do not, of this, everybody perceives tures normally found on Earth. And therefore, freeze from the bottom water to be a rather ordinary the fact that ice and liquid water can upwards until they result in just a thin sort of stuff, as it is transparent, coexist at the not abnormal tempera- layer of liquid water on solid ice. If ture of 0°C, is extremly important for all they did then life as we know it today odourless, and tasteless. life on Earth. could not have been possible. Many divers feel almost just at home Water has an unusually high den- The unusually high density of water in water as they do in air, and rarely, sity, and this density is further anoma- is mainly due to the cohesive nature if ever, think about its more unusual lous in that, unlike other liquids, it of the hydrogen-bonded network. This physical properties. Water appears to increases on cooling down to 3.984°C reduces the free volume and ensures be a very simple molecule, consisting and then decreases until the freezing a relatively high-density, compensat- of just two hydrogen atoms attached point of 0°C is reached. ing for the partial open nature of the to an oxygen atom. There are, in fact, The density of water is thus a maxi- hydrogen-bonded network. Why the very few molecules that are smaller mum at about 4°C, meaning that it density of water actually has a maxi- or lighter. In spite of this, it is a most expands both on heating and cool- mum at 4°C requires a much more remarkable substance, with many ing from this temperature. This density detailed thermodynamic explanation anomalous properties. maximum, together with the low ice than can be given here. ■

87 X-RAY MAG : 4 : 2005 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED