A.A.R.G. JOURNAL

ISSUE NO. 1

JOURNAL OF THE AUSTRALIAN AMATEUR REBREATHER GROUP

c/o Tyacona Old Hume Hwy BERRIMA 2577 TELEPHONE:(02) 97596501

Email: diver@chilli.net.au Internet: http://www.chilli.net.au/~diver MARCH 1998

Editors Message

Well here it is as promised, the very first A.A.R.G. newsletter/ journal. Hopefully the first of many more to come, this journal will be presented on a quarterly basis. I will try to keep the information presented varied and up to date however NO responsibility is taken for any errors or incorrect information. I will also try to keep to the aims of the A.A.R.G. by providing information that will hopefully make your rebreather diving safer along with a few dive reports and any other interesting tidbits I come across. If you have any information for inclusion please post or email it to me at the address on the cover page.

At the time of writing this our membership has grown to about 85 and includes a few people outside Australia. This first issue is free however to receive subsequent issues of the hard copy you should post AU$5 if you live in Australia or AU$10 if you live elsewhere to me to cover postage and printing. Alternately, as many members have Internet access, I can email the text or send the MSWord file to you as an attachment for free but if you want the hard copy I need some money to cover the cost.

Good luck and Safe diving,

Scott Leimroth

WELCOME

Welcome to the AARG! If you are a member you will already know this is a group set up to promote the safe use of rebreathers and disseminate commercially unbiased rebreather oriented information. Our aim is also to represent the end-user in any future government regulations regarding recreational use of rebreathers, although we hope legislation is never required.

Background history: I originally conceived the idea of a non-profit group with the goals listed above in early 1997. At that time to our knowledge I was the only recreational diver regularly diving a closed circuit system in Australia. Concurrently I was reading a lot of articles in the diving press concerning rebreathers and how wonderful they were. For example apparently fully closed and advanced semi closed units were going to let you dive longer with less deco, cost less to buy and run in the long term, and were safer.

On the other hand diving doctors in particular believe rebreathers are unsafe and unsuitable for use for recreational divers. Basically that rebreathers should remain the exclusive domain of the military.

The truth of course lies somewhere between these two extremes and that's the balanced view we hope to uphold through the AARG. We hope to represent the recreational rebreather diver in any future government. regulations, hence we must remain non-profit and unbiased but still co-operate where necessary with both commercial rebreather suppliers and technical training agencies.

Of course a good idea is one thing, carrying it out is another. That's where Scott Leimroth has excelled in producing arguably the best rebreather web site in the world and monthly promotional articles in Dive Log Australia.

The other founding member of the AARG is the silent achiever Jason Rogers. Jason provided the inspiration for my successful attempt at building a rebreather, which we both dived back in 1995, and also recently in his own successful semi-closed rebreather design. All 3 founding members units feature what we call "Jason Bags" counterlungs, which have proven to be an excellent material choice.

Rod Nairne

RECOMMENDED READING

Life Support Systems Design by

Nuckols/Tucker/Sarich from Simon and Schuster Custom Publishing ISBN 0-536-59616-6

Oxygen and the Diver by Kenneth Donald ISBN 1 85421 176 5 published by: The SPA Ltd Units 7/10, Hanley Workshops Hanley Road, Hanley Swan, Worcs. England.

What is a "Rebreather"?

Richard Pyle has kindly given us permission to reprint this article. It is included for those members who may be unfamiliar with rebreather technology as a beginners guide, however, even if you think you know all about how rebreathers work you may well find something new in this thorough article.

To understand what a rebreather is and how it works, it is useful to understand how conventional scuba works.

Nearly all diving apparatus presently available to the public falls into a class known as open-circuit scuba. This type of system was first introduced to recreational divers by Cousteau and employs a compressed gas supply and a demand regulator from which the diver breathes. The exhaust gas is discarded in the form of bubbles with each breath, hence the term "open-circuit". Open-circuit scuba is inherently inefficient: because only a small fraction of each inhaled breath is actually used by the diver for metabolism, there is a tremendous waste of useable oxygen (O2) with each breath. Furthermore, the quantity of O2 lost in this manner increases with increasing depth.

A rebreather is a fundamentally different kind of diving apparatus. There are three basic types of rebreathers presently being used in government and industry: oxygen rebreather, semi-closed rebreather, and closed-circuit rebreather. Each has specific advantages and disadvantages, as will be discussed briefly below. All kinds of rebreathers, however, have certain basic components in common. All designs start with a breathing loop equipped with a mouthpiece, through which a diver breathes. If the entire breathing loop is of rigid construction, the diver would be unable to breathe because there would be nowhere for the exhaled gas to go into, nor the inhaled gas to come from (analogous to trying to breathe in

and out of a soda bottle). Thus, there must be some sort of collapsible bag attached to the breathing loop that inflates when a diver exhales, and deflates when a diver inhales. This bag is referred to as, appropriately enough, a counterlung. If a diver were to continue breathing in and out from this breathing loop, the carbon dioxide (CO2) exhaled by the diver would soon build up to dangerous levels. Therefore, the breathing loop must also include a CO2 absorbent canister containing some sort of chemical (e.g., HP Sodasorb, Sofnolime®, or lithium hydroxide) that absorbs CO2, removing it from the breathing gas. Of course, the CO2 absorbent canister alone will not permit the diver to continue breathing from the rebreather indefinitely; the oxygen in the breathing loop will eventually be consumed by diver via metabolism. Therefore, the rebreather must have some means to allow oxygen to be injected into the breathing loop in order to continue sustaining the diver.

Furthermore, to prevent the diver from simply inhaling the same gas that was just exhaled, the

rebreather must be designed to ensure that gas continues to circulate in one direction around the breathing loop. This is usually accomplished with an upstream check-valve, and a downstream check-valve, located on either side of the mouthpiece; these allow inhaled gas to come from only one direction in the breathing loop, and allow exhaled gas to go only in the opposite direction.

Another feature common to most rebreather designs is some sort of shut-off valve in the mouthpiece which can be shut if the mouthpiece is removed underwater, to prevent water from flooding the breathing loop.

The fundamental difference between the three kinds of rebreathers is the way in which they add gas to the breathing loop, and control the concentration of oxygen in the breathing gas.

Oxygen Rebreather

The oxygen rebreather is the simplest kind of rebreather system, and will form a starting point for discussion of more complex systems. An oxygen rebreather consists of the basic components described above, with a cylinder of pure oxygen as the supply gas to replace the oxygen consumed by the diver. Some types of oxygen rebreathers add oxygen into the breathing loop at a constant rate, which is chosen to closely match the rate at which the diver's metabolism consumes it. However, the diver's rate of metabolism may vary during the

course of the dive due to variations in the diver's workload. Hence, such an active-addition system is prone to adding too much oxygen during periods of rest (resulting of wasteful venting of gas from the breathing loop), and/or not enough oxygen during periods of heavy work (resulting in the need for the diver to add oxygen via a manual bypass valve). Many oxygen rebreathers incorporate some sort of passive-addition system, whereby oxygen is added to the breathing loop at a rate that matches the metabolic consumption rate of

the diver. A simple method for achieving this sort of gas addition system involves a mechanical valve which is triggered when the counterlung is completely collapsed. As the diver's body converts the oxygen to carbon dioxide via metabolism, and the carbon dioxide is removed by the CO2 absorbent, the total volume of gas in the breathing loop decreases. Eventually, a diver's full inhalation will cause the counterlung to "bottom-out" (completely collapse), thereby triggering the mechanical valve to add more oxygen. The hazard with this type of system on an oxygen rebreather is that it is vitally important to flush the breathing loop with pure oxygen prior to the commencement of the dive. If a large enough volume of other gasses are in the breathing loop, the diver may suffer from hypoxia (insufficient oxygen) before the counterlung collapses enough to trigger the mechanical oxygen-addition valve. From a design standpoint, oxygen rebreathers are very simple because they do not require a complex O2 control system. However, they are also extremely limited in function because the potential for CNS oxygen toxicity (too much oxygen) prevents safe operation of oxygen rebreathers at depths in excess of about 20 feet/6 meters. In order to safely descend to greater depths, the gas mixture in the breathing loop must contain some constituent other than pure oxygen (e.g., nitrogen or helium). Such mixed-gas rebreathers usually come in one of two forms: semi-closed rebreathers and closed-circuit rebreathers.

Semi-Closed Rebreather

Unlike oxygen rebreathers, semi-closed rebreathers are a form of mixed-gas rebreather, in that they incorporate gas mixtures other than pure oxygen. There are two fundamentally different categories of semi-closed rebreathers: active-addition, and passive-addition. By far, the most common are the active-addition systems. They are similar in design to the active-addition oxygen rebreathers, except that the supply gas contains a mixture other than pure oxygen. The supply gas is usually injected into the breathing loop at a constant-mass rate. In other words, regardless of the depth, a constant number of molecules of gas are injected into the loop in a given period of time. The rate of injection in such systems must be adjusted according to the fraction of oxygen in the supply gas, such that the rate of oxygen addition to the breathing loop meets or exceeds the rate at which the diver consumes oxygen in the breathing loop.

The advantage of this type of rebreather compared with an oxygen rebreather is that it allows divers to descend to greater depths without excessive risk of oxygen toxicity. The disadvantage, however, is the fact that the part of the supply gas that is not oxygen (usually nitrogen or helium, or both) is also added to the breathing loop at a constant rate. Because the diver's body does not consume this "other" gas, it continues to build up in the breathing-loop. To prevent the obvious consequence of over-expansion, this excess gas must be periodically

vented out of the breathing loop. In an ideal world, only the non-oxygen component of the breathing gas would be vented from the loop, saving the oxygen for consumption by the diver. However, because the gas in the breathing loop is more-or-less homogeneously

mixed, a certain fraction of the vented gas is wasted oxygen.

Another problem with active-addition semi-closed rebreathers is that the concentration of oxygen in the breathing loop is variable. First of all, the oxygen fraction in the breathing loop necessarily "lags" somewhat behind the oxygen fraction in the supply gas. The reason for this is that the diver's body is "pulling" oxygen out of the breathing gas much faster than it is "pulling" out the other constituents of the supply gas. Also, the oxygen is being added to the loop at a constant rate, but the rate at which

the diver's body consumes the oxygen varies according to the diver's workload. A given diver's metabolic oxygen consumption rate can vary by a factor of 6 or more in normal conditions, and as much as 10-fold in extreme conditions, depending on the level of exertion. These fluctuations affect the magnitude of the "lag" between the fraction of oxygen in the supply gas, and the fraction of oxygen in the breathing gas. To minimize the risk of hypoxia, the concentration of oxygen in the supply gas and the rate at which the supply gas is injected into the breathing loop must be high enough to accommodate the needs of a diver during heavy exertion. The higher the oxygen fraction in the supply gas, the more restrictive the depth limitation due to the risk of oxygen toxicity during periods of low workload. Furthermore, the greater the gas injection rate, the less time a given volume of supply gas will last (i.e., the less efficiently the supply gas is used). Thus, because of the (usually unpredictable) variability of oxygen needs by the diver during the course of a dive, and the inability of constant-mass flow semi-closed rebreathers to compensate for this variability, active-addition semi-closed rebreathers are inherently inefficient compared to other kinds of rebreathers.

An alternative approach to semi-closed rebreather design is some sort of passive-addition system.

Passive-addition designs attempt to adjust the rate at which the supply gas is added to the breathing loop to match more closely the metabolic needs of the diver. The simplest way to make this adjustment in real-time is to "key" the gas injection rate to the diver's breathing rate. In most circumstances, breathing rate, or respiratory minute volume (RMV), will be directly proportional to metabolic oxygen consumption rate. Thus, most passive-addition semi-closed rebreathers inject supply gas into the breathing loop at a rate determined by the diver's RMV: more gas is injected during periods of high RMV, and less gas is injected during periods of low RMV. While this approach reduces the problem of large fluctuations in the oxygen content of the breathing gas at different workloads, there is still the need to periodically vent excess gas, thereby reducing gas efficiency.

Closed-Circuit Rebreather

Although the term "closed-circuit rebreather" is often used to refer to any kind of rebreather device, in this context the term will be used specifically in reference to fully closed-circuit, mixed-gas rebreather systems. Like semi-closed rebreathers, closed-circuit rebreathers are a type of mixed-gas system, enabling descent to much greater depths than can be safely reached with oxygen rebreathers. However, there are several important and fundamental differences between semi-closed rebreathers and closed-circuit rebreathers.

The first difference has to do with the way oxygen is added to the breathing loop. Whereas semi-closed rebreathers inject oxygen along with other gases, closed-circuit rebreathers generally consist of at least two independent gas supplies. One of these contains pure oxygen, which is injected into the breathing loop to make up for the oxygen that is consumed by the diver. The other gas supply is called the diluent. The diluent usually consists of either compressed air or a special gas mixture such as nitrox (nitrogen-oxygen, usually with higher than normal oxygen concentration than for compressed air), heliox (helium-oxygen, usually with lower than normal oxygen concentration than for compressed air), neox (neon-oxygen) or trimix (usually helium-nitrogen-oxygen). The diluent gas mixture usually contains enough oxygen such that it can be breathed directly from the cylinder via an open-circuit system at the operating depth of the dive. This supply is used to maintain system volume during excursions to depths where the volume of gas in the breathing loop is compressed. In some rebreathers the diluent is also used for the emergency open-circuit bailout gas supply in the event of a total system failure of the rebreather apparatus.

The second major difference between closed-circuit rebreathers and semi-closed rebreathers is how the two systems maintain the concentration of oxygen in the breathing loop. Whereas most semi-closed rebreathers maintain a (more or less) constant fraction of oxygen (FO2) throughout the course of the dive, closed-circuit rebreathers maintain a relatively constant partial pressure of oxygen (PO2) in the breathing loop. To accomplish this, virtually all closed-circuit rebreathers incorporate some sort of electronic oxygen sensors which monitor the concentration of oxygen in the breathing gas. In most cases, closed-circuit rebreathers also incorporate an electronic O2 control system, which automatically adds oxygen when the PO2 drops below a certain level (this level is called the PO2 set-point).

Closed-circuit rebreathers have advantages and disadvantages when compared to open-circuit scuba and semi-closed rebreathers. All of these diving technologies have important applications.

CARBON DIOXIDE AND THE REBREATHER DIVER: A SURVIVAL GUIDE . By Rod Nairne. ©copyright

Hypercapnia is the term applied to the effects of increased tension of carbon dioxide in the blood. Open circuit divers are at little risk of this unless deep diving on air or by deliberate hyperventilation to conserve gas supplies.

By comparison rebreather divers are more commonly effected due to re-breathing exhaled CO2.

Fact: If you dive a regularly dive a rebreather you will experience hypercapnia at some point. As we can't eliminate this problem the goal for

rebreather divers should be effective management by taking both preventative action and corrective action.

Some ways to help minimize the risk of hypercapnia:

The rebreather should have fresh absorbent. If you intend to use one absorbent fill for several dives, it is imperative you keep accurate records of dive time on each canister fill. Some divers even log the amount of oxygen metabolized to get a closer estimation of the amount of CO2 which has been scrubbed, but for most purposes dive time is sufficient.

Always be conservative when deciding if a change of absorbent is necessary: the stuff is relatively cheap.

Keep water out of your scrubber: if your absorbent becomes wet it just won't work as well. If you do wet your scrubber and it seems OK it's illusory as any increased output of C02 will just not be absorbed. In other words it will work until you really need it. The cause is water soaking the porous granules where much of the CO2 is absorbed, and nothing short of an oven will dry it out again.

Other things to watch: ensure you have the gas flow in the correct direction, some units are less tolerant of this mistake than others. The mouthpiece check valves are also a common failure point if they become "hung up" or just old. A good pre dive check is to crimp one side of the mouthpiece hose off and check the valve can hold a little pressure.

***This method is preferred to just listening for the "slap" of the valve as many valves will still slap when hung up.***

When it happens: what you can do.

Switch to open circuit is sound advice as a first step, just to get a clear head and evaluate the situation whilst breathing uncontaminated gas. Carbon dioxide can hinder cognitive processes. After this you may decide to go back on the loop if the cause of the problem was just poor breathing patterns, heavy exertion, or if necessity demands it even a failing scrubber.

Always restrain and minimize any exertion if you get in this situation, and STOP and catch your breath BEFORE it gets out of hand. There is a lag between exertion and the output of CO2 so don't get overconfident you have sorted a situation; in reality it's going to catch up to you in a big way real fast. If you do overstep the mark, be ready to switch to open circuit immediately. Symptoms rapidly disappear with the inspiration of CO2 free gas.

A good technique if your scrubber is not performing is switching to manual semiclosed mode, exhaling each 4th or 5th breath will allow you to continue even if the scrubber is almost completely inoperative. Clearly this should not be a common practice however it may get you out of a serious situation unscathed.

Lastly just be plain professional and maintain a high standard of equipment, training and awareness.

Carbon Dioxide Levels

1% PCO2 0.01 US navy cutoff limit for closed circuit system CO2 absorbers.

3% PCO2 0.03 Most divers sense stimulated breathing rate and depth.

12% PCO2 0.12 Breathing stimulus does not increase much above this level.

25% PCO2 0.25 Resting patient still conscious due to depression of the central nervous system.

30% PCO2 0.30 Becomes anesthetic.

On the Internet

For those of you connected to the Internet there is a wealth of information available on rebreathers. One mail list that is often interesting (but at other times quite stagnant depending on who's away diving) is the rebreather list. To subscribe send an email to rebreather-request@nwdesigns.com with the word subscribe in the body of the message. Through this list you can contact many of the manufacturers and most of the regular RB divers around the world for information and advice. The nwdesigns archives also have lots of good info. Oh and don't forget our very own A.A.R.G. website at http://www.chilli.net.au/~diver

Seven Deadly Sins or Some facts about Scrubbers

1.Removing carbon dioxide from the breathing gas is a "chemical reaction". The canister is not a "filter".

2.Heat from the canister is a positive indication that the absorber is working. A cold canister is a bad indication.

3.Do not reuse materials as the chemical action depletes their effectiveness.

4.Excessive water will "melt" the absorbent materials. Remove all excess water from the canister.

5.Quality of the absorbent materials can not be determined in the field. Depleted materials may give an immediate reaction to carbon dioxide. But depleted materials will stop reacting within minutes of use. Insure a quality material by knowing your supplier and keeping the materials sealed air tight prior to use.

6.Do not use a canister that is below 20 deg, Fahrenheit temperature. For cold water operation the canister must be pre heated to above freezing to insure the chemical process will start.

7.Never use a rebreather with a used canister as: a)You do not know how long the unit will last. b)The previous user may be infectious.

Some facts about Rebreathers

Also from Rich Pyle

1. Know your PO2 at all times.

2. Open-circuit scuba experience is not as useful for rebreather diving as a good grasp of

diving physics and physiology.

3. Training should emphasize failure detection, manual control and bailout procedures.

4. Cover your ass.

This is probably the most important piece of advice that my rebreather instructor, Bill Stone, gave to me. This point doesn't need much elaboration, but is nevertheless vital to rebreather survival. It is fundamentally the same principle that all cave divers and mixed-gas divers should already understand: always have an safe alternate pathway back to the surface. For open-circuit divers, this usually means a second regulator and following "rule of thirds" for gas consumption.

On rebreather dives, especially those requiring extensive decompression, the logistics of providing for an alternate means to safely return to the surface, even in the event of catastrophic, unrecoverable breathing loop failure, can be difficult.

Rebreather Dive Checklist

I. Pre-Dive

In addition to general gas mixing, equipment testing, rig preparation, team briefing, and other obvious pre-dive activities, rebreather divers should perform several additional pre-dive routines.

A. Loop Leak Test

An essential pre-dive test for any rebreather is a loop leak (or "positive pressure") test. This step

involves adding gas to the rebreather loop until the over-pressure relief valve vents, and observing for a subsequent drop in remaining loop volume or pressure that might indicate a poorly sealed connection or leak somewhere in the breathing loop.

B. Oxygen Control System Test

Another test prior to commencing the dive is a verification of the oxygen control system function.

Minimally, this test involves flushing the loop with diluent, activating the oxygen control system, and verifying that the solenoid fires correctly. If the unit allows the user to easily adjust the PO2 set-point, the test could be conducted with a low set-point (such as 0.3 atm) to verify that the solenoid stops firing after set-point has been achieved. If this latter test is conducted, it is imperative that the PO2 set-point be returned to the correct value prior to the dive.

C. Final Checklist

Beyond the standard checklists frequently used by open-circuit mixed-gas decompression divers, a separate checklist should be developed specifically for the particular rebreather unit that is to be used. Minimally, this checklist should include verification of absorbent type and remaining canister life, accurate oxygen sensor calibration, correct PO2 set-point, oxygen and diluent cylinder pressures, diluent gas composition(s), and correct position (open or closed) of all valves in the system. Additional model-specific verifications may also be required for certain rebreathers.

BIOCHEMICAL DECOMPRESSION:

A Fundamentally New Approach.

Susan R. Kayar (Reprint from: Pressure, Undersea and Hyperbaric Medical Society April 1997,Volume 26, Number 2)

Our research team at the Naval medical Research institute in Bethesda, Maryland, is working on a radically different method for decompressing divers. Safe decompression currently depends on the loss by passive diffusion of breathing gases that went into solution in the diver's tissue's while at depth. Our new approach envisions an additional active removal of some of these gases by biochemical processes, utilizing bacterial metabolism as the source for the biochemical machinery. The bacteria are packaged, swallowed, and delivered to the large intestine of the diver before the start of the dive. During the dive, some of the gas that is carried in the blood diffuses into the intestine, down the partial pressure gradient created by the metabolism of that gas by the bacteria. End products of this metabolism have a safe route for elimination from the intestine. Judicious selection of the bacterial species prevents a pathological response to the bacteria, which are outcompeted by the native intestinal flora and eliminated over a period of one to a few days following the dive.

We have had recent success with the early stages of demonstrating the feasibility of biochemical decompression , using hydrogen as the diluent gas in the breathing mixture and hydrogen-metabolizing bacteria introduced into the intestines of laboratory animals. In our experimental model, bacteria that metabolize hydrogen and carbon dioxide to methane and water were placed in the large intestines of rats. When the rats were pressurized in a hyperbaric chamber, the rate at which they released methane increased with increasing pressure of hydrogen in the chamber, starting within minutes from the introduction of the hydrogen. Methane release rate decreased as hydrogen was later flushed out of the chamber. This demonstrated that hydrogen breathed by the rats was reaching the bacteria on a time scale of seconds to minutes and that the environmental conditions in the intestine were suitable for these bacteria to metabolize the hydrogen . By measuring the total volume of methane released during the dive, we estimated the minimum volume of hydrogen removed from the rats. We had a sample dive profile for the rats breathing hydrogen and oxygen in which we knew that the occurrence of decompression sickness for untreated animals was approximately 50%. We predicted that the volume of hydrogen removed by the bacteria was sufficient to reduce the risk of decompression sickness on this dive profile to 20%. Our prediction was found to be accurate for animals up to 24 hours following bacterial treatment.

Hydrogen diving was originally proposed by Arne Zetterstrom, an innovative young engineer with the Swedish Navy in the early 1940's. Zetterstrom recognized that hydrogen, due to its small molecular mass, would reduce the difficulties with ventilating lungs when breathing high-density gases, such as encountered by divers breathing helium-gas mixtures at great depths. Arne Zetterstrom met a tragic death during an experimental hydrogen dive in 1945m and research in hydrogen diving died with him for the next several decades. However, interest was revived in the 1970's. This past summer COMEX announced success with their twelfth human trial, using a trimix of Hydrogen, helium and oxygen to a depth of 210 meters in the open sea.

Biochemical decompression with the use of hydrogen-metabolizing bacteria has thus a very real application in human diving today. When we scale up from rats to humans, our results predict that hydrogen biochemical decompression could potentially shorten decompression time from deep saturation dives by several days. Hydrogen biochemical decompression also could reduce counterdiffusion effects potentially encountered during gas shifts, when divers switch from a gas mixture containing hydrogen to one containing helium or nitrogen. Even more exciting is the possibility that nitrogen metabolizing bacteria can be used to achieve biochemical decompression for air dives. While we are still years of animal and then human research away, the possibility exists that swallowing a few capsules the night before a dive might make sport divers safer from the risk of decompression sickness.

This work supported by NMRDC work unit no. 61153N MR04101.00D-1103 The opinions and assertions contained herein are those of the author and are not to be construed as official or reflecting the views of the US Navy or the naval service at large.

A.A.R.G. Member News

Barry Hallett from Southern Cross Divers is eagerly awaiting the arrival of his TWO Buddy Inspiration Rebreathers which he has had pre-ordered for the past 18 months. It will be interesting to see them up close and personal and even more interesting to go diving with Barry when they arrive.

On the home-building front Andreas Kristovskis is finally going from the planning stage to the construction stage and has all the parts and is beginning construction. He intends building a unit similar to the 'Top Gun' design but initially as a O2 rebreather and Semi-closed. He is still deciding if he should build a there and back style scrubber or a straight through type. He has managed to order one of the new Apollo drysuit inflators which apparently compensates the pressure automatically.

Bruce Stewart is in the final stages of construction of his unit and only needs to finish the scrubber and final bits on the mouthpiece. As Bruce has access to a machine shop (and he knows how to use it) his unit should at least look very professional. His mouthpiece certainly does being machined from delrin and stainless.

Other News

Pro-Dive at Coogee has a 2 compartment 6 man Decompression Chamber in the front of their shop. Dives and courses are available. This could be useful for anyone wanting to test their rebreather in a controlled, dry and supervised environment.

Rebreather Profile

Each issue we hope to profile one of the commercially available rebreathers on the market. This months profile is on the Drager Dolphin, formerly the Atlantis, which is definitely the most readily available and popular RB in this country.

I was at Camp Cove a few weeks ago and got talking to a diver who was there with a new Dolphin. As it turned out he was none other than Paul Davis, Drager Australia's market manager for defence and diving. He was accompanied by an engineer from Drager Germany and we had a brief but interesting discussion about rebreathers in general. Paul kindly sent me an information pack about various Drager products so if any members are interested in this or other information about Drager please contact me.

The Drager Dolphin

The Dolphin is a semiclosed rebreather for use with nitrox. It uses a constant mass flow dosing system which includes an additional demand bypass. Depending on which nitrox mixture you are using there are different orifices to set the rate of flow. The loop has a total volume of approximately 10.5 litres and depending on your individual lung volume and the adjustment of the pressure relief/dump valve the volume can be varied by 4.5 to 7.1 litres. The scrubber holds approx. 2.25 kg of absorbent. It comes with either a 4 or 5 litre 200bar steel cylinder and a separate 2 L, 200bar open circuit bailout cylinder. It is 520mm long, 370mm wide and 235mm deep. On land it weighs approx. 17kg and in water is about 1 kg negative.

There have been a few small changes in the Dolphin compared to the Atlantis. The first thing you might notice is the different BCD and technical style harness that comes with it. Also, the loop components are now color coded so that red fittings are for exhaled gas flow and blue fittings for inhaled gas flow. The breathing bags are made of a sturdier looking black coated fabric material compared to the BCD bladder type material that was in the Atlantis. The bags also have small water drains at the bottom so they can be drained after the dive and for cleaning/drying.

They have retained the 'muffler' looking scrubber. I questioned the engineer as to the reason behind this design and he replied it was so it would fit in the backplate. I asked about the effects of this shape on gas flow and breakthrough/channeling and he claimed there was little difference to a cylindrical scrubber design. All scrubbers are prone to some channeling along a flat surface. The scrubber has inserts that stop gas flowing directly along the walls to reduce channeling. These result in an L shaped fitting with the bottom of the L protruding into the absorbent bed. Gas flow is diverted by the protrusion from flowing directly along the walls however for every benefit in rebreather design there is always a trade off. In this case it is that the protrusion into the scrubber bed reduces the area of gas flow through the scrubber at these points. Considering this unit is solely for 'recreational' diving depths the effect on work of breathing should not be too noticeable.

The Dolphin is probably the most popular rebreather on the market today and certainly the most accessible. In Australia and the Pacific region it is one of the few RBs, besides a few homebuilt units, that is being dived on a regular basis for recreational or non-professional dives. As with all rebreathers thorough training on the particular unit is required for safety and while these units are for recreational diving a recreational divers usual lax attitude to safety is dangerous when using any rebreather. ALWAYS go through ALL recommended pre-dive checks and seek proper training and advice from the manufacturer. Drager Australia can be contacted on 1800 67 77 87 if you require further information about purchasing or training on the Dolphin.

Unique failure modes of semiclosed circuit constant mass flow (CMF) rebreathers.

by Rod NairneÓ

CMF semi closed circuit rebreathers are simple and reliable devices, and may be second only to oxygen rebreathers in terms of common usage. The whole system is dependent on the correct and accurate functioning of the CMF orifice, which meters the flow of the mixed gas into the breathing loop.

As the whole purpose of a rebreather is greater gas efficiency, the CMF is usually set with a small margin of safety, most commonly to give a fraction of oxygen in the loop equal to 19-21% (to avoid hypoxia on the surface) assuming a maximum oxygen uptake (VO2) of 3 liters per minute. The F02 in the loop can be estimated by the formula:

F02= ((flow rate *F02 in supply gas) - Max Vo2)/(flow rate -V02)

For example a nitrox mix with 60% oxygen assuming a flow rate of 6 liters per minute and a diver with a V02 of 3 liters per minute will give us a F02 of 20%.

The upper VO2 limit represents a reasonable upper limit of oxygen metabolism that the average diver will not be able to exceed for prolonged periods. Actual VO2 uptakes have been recorded twice this level however, which would result in hypoxia if repeated for a reasonable period underwater. Instantaneous hypoxia does not occur due to the loop acting as a oxygen reservoir and the increased pressures whilst diving giving an increased PO2 for a low fraction of oxygen.

Whilst this formula can be manipulated to calculate the flow rate required for any gas mix, in reality the flow rate will be limited to one of 3 as determined by the CMF orifices available in the rebreather. Each of the 3 orifice sizes flows a different mass per unit time of gas, and is designed to be used with a supply gas with a specific fraction of oxygen and a specific inert gas diluent, in all but rare cases this is nitrogen. The most common mixes are nitrox 60, 40 and 32 for diving to 16, 30 and 40 msw respectively. This depth limit is calculated by the maximum safe PO2 of 1.6ata of oxygen. (note: military divers may use a max safe depth calculated on a P02 of 2.0 ata, or 23, 40 and 52msw respectively. )

The CMF orifice will operate according to the following formula, provided the supply pressure is 2 times the ambient pressure, and the supply gas is composed of only nitrogen and oxygen.

flow rate in cubic feet per minute = 11(supply pressure)(diameter of orifice in inches)squared.

e.g. for a supply pressure of 450psi and a required flow fate of 6 liters per minute as in the example above, the approximate orifice size is =0.0065 inches or 0.185mm.

This formula leads us to a failure mode unique to CMF rebreathers: particle impingement (PI). PI of an orifice as small as 0.188mm is possible with a wide variety of substances commonly found in diving equipment: corrosion particles can either block the orifice directly of indirectly, as can sand particles, and even (and most commonly) salt water that can bypass the

usual block for particles (brass sintered filters) in soluble form to form sodium chloride crystals either between the filter and orifice, in the orifice or after the orifice upon evaporation of the solvent.

To demonstrate the effect of particle impingement, assuming a moderate blockage of 10% of the orifice diameter and using the above formula we have a reduction of flow from 6 to 4.85 liters per minute. Substituting this into our original formula gives a negative value for F02, this means the gas mix will be insufficient to sustain life at a V02 of 3 liters per minute.

Unfortunately should blockage by salt crystals result in a fatality, it would be difficult to establish this as a cause. Firstly, if salt crystals were responsible, they would almost certainly redissolve in the time intervening the accident and the recovery of unit as it would have flooded. Secondly, if salt crystals are found, it is most likely they formed once the unit was recovered and sufficient time had elapsed for the water solution to have evaporated leaving the crystal solute.

The insidious nature of such a flow reduction, is that it is not reliably detectable by the diver once in the water (there is no reliable warning). Unfortunately pre dive checks will not prevent either subsequent formation of crystals or other particle impingement's which may occur during the dive. This is in stark contrast to open circuit equipment where a failure of the life support apparatus is instantly apparent by inability to take a breath. The rebreather diver may be able to continue to breath normally, unaware of any problem, until oxygen reserves in the loop are depleted and hypoxia occurs. This outcome is more likely in the deeper range of diving depths due to the oxygen fraction making up less of the loop volume.

Particle impingement is only one example of hypoxic outcomes possible in semiclosed circuit rebreathers. Should the supply cylinder run out un-noticed during a dive (an unfortunately common event in scuba diving) again the diver may not be aware of the flow stopping, as the only indication would be a cessation of a small amount of bubbling from the unit which reduces with depth. This could easily go un-noticed, especially if the diver is pre occupied with another task, is wearing a hood in cold water, or diving with comparatively noisy open circuit divers. Again the comparison with open circuit: if the cylinder runs out the diver can not breath so is instantly alerted.

Pre-dive flow rate checks may be compromised if a supply mix with a fraction of oxygen less than that required for the orifice size is used for

a dive. The flow rate check in this case will pass the rebreather for use whereas in fact the rebreather will give a mix containing insufficient oxygen. A possible mistake might be if the 32 mix is connected to the 60 orifice, of if the nitrox mix is substantially oxygen deficient, for example the 60 mix contains only 50%. (note should this occur, and hypoxia result it may be difficult to test for the nitrox mix in the tank after it has been totally discharged).

The responsible and properly trained semiclosed rebreather diver will be aware of these risks, an be vigilant in pre dive checks. The diver will also be aware, (despite pre dive checks) of the risk of reduced flow during the dive, and always flush the loop with fresh gas before ascents, where hypoxia is most likely due to the reduction of pressure and the resultant drop in PO2 to hypoxic levels should a flow impingement have occurred.

Dive Report

by Scott Leimroth

I dived my homebuilt CCR at night for the first time recently. The dive site was ShipRock in Sydney which is a wall dive covered in growth that has a maximum depth of 17m. The site is a marine reserve so fish life is prolific. At night the many decorator crabs, which look like moving sponges, come alive and are everywhere. My displays do not glow in the dark so if my torch failed I would not have been able to read them. Later I thought of a solution to this, I could simply attach a cylume stick to the displays so that I could read them if my torch failed. I spent most of the dive experimenting with diving semi closed and also by only adding O2 to the loop as the counter lung volume depleted. This ended up being reasonably effective when I checked the PO2 on the displays, however it would not be reliable over a long period of time.

Calling all Rebreather Divers

Our members are regularly conducting both shore and boat dives. If you are a rebreather diver or just interested in checking out a rebreather at work then please contact us to join us on one of our dives. We would especially like to get enough divers to book a whole charter so that we can fully extend ourselves and do some long bottom times with our rebreathers.

Please Note

The A.A.R.G. is dedicated to the dissemination of information about rebreathers and rebreather diving, particularly in Australia and the South Pacific region. Our aim is to increase the general diving publics' awareness about rebreathers and related issues and to provide you with the stimulus to make your rebreather diving safer. However NO responsibility is taken for the accuracy of any information presented in this newsletter. You should always seek out verification of any information you acquire about rebreathers and use your own judgment as to it's validity to your context. Be safe, be conservative and dive safe. Remember, this is not diving equipment, this is life support equipment and your life may depend on it.