Newsletter 2

A.A.R.G. JOURNAL

ISSUE NO. 2

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 JUNE 1998

Editors Message

We received positive responses from many people to our first newsletter. A few even came up with some money to cover postage and printing but due to the poor response in this regard I had considered not bothering with this edition. However, this endeavor is solely for the purpose of spreading information about rebreathers with the hope of educating people in their safe use. The information is intended to get you thinking. Knowing how a system works and being aware of its fundamental operation and failure points is the one thing that may save you if something goes awry. On top of this I received $64.40 from some articles that were reprinted in Dive Log Australia so this edition has made it to print. If you are lucky enough to be reading this and you find the information useful please send us your A$5 so you can receive the next years worth of issues. Also, if you have any information or stories about rebreathers, no matter how trivial you may consider it, please send them to me for inclusion. I can't put this together by myself so your input will be much appreciated. I would like to thank those who have contributed so far, including Richard Pyle and Joe Dituri from Hawaii and Walter Stark, Rod Nairne, Suzie Dudas and Jason Rogers from Australia.

Good luck and Safe diving,

Scott Leimroth

Given the inherent risk of unconsciousness that is a part of rebreather diving, many users are tempted to use full face masks in order to provide airway protection. Adapting masks intended for open circuit diving to fully closed does however present some difficulties. Most OC masks are designed either for surface respiratory protection (firefighting) applications and adapted for underwater use, or intended for use with large amounts of surface supplied gas. Common examples of these are the Interspiro AGA, and the Kirby Morgan Bandmask.

In the case of the Interspiro type of mask, the water seal comes from a tight fit, combined with a positive internal pressure, maintained through the use of spring loaded exhaust valve, and spring loaded diaphragm. Very little water enters the mask but any which does is expelled through the exhaust valve, located at the bottom of the mask. (In the case of the AGA, the exhaust valve is combined with the demand valve, but in many other surface use masks, the exhaust valve is a separate unit) Cool dry air entering the mask is directed across the faceplate, and then into the oralnasal mask built into the mask. Use of flapper valves separates the path of the inhale and exhale gases, reducing dead space, and keeping the faceplate from fogging up.

The problem with this is that the rebreather only provides for ambient pressure within the mask, and so leaks may leak in, not out of the mask. Leakage into the mask will have to follow the gas path back into the loop. In fully closed sets, or semiclosed sets that have high mounted dump valves (e.g. the Atlantis/Dolphin) this water will accumulate within the loop. Additionally, the gas coming out of the loop and being directed across the faceplate is warm and moist. When it contacts the cool faceplate, the gas will dump moisture, and the faceplate will tend to fog up. All this makes this type of mask unsuitable for use with a rebreather.

The Bandmask style of FFM is even worse. Intended for surface supply diving, little or no attempt is made to provide a gastight face seal. Rather it is intended to leak like a sieve, and the water that comes into the mask is expelled via the exhaust valve located in the bottom of the demand valve housing. Additionally a separate hand operated valve can be opened which provides a constant flow of gas into the mask, to defog the faceplate and blow water out of the mask. Gas usage can be prodigious, as can be water ingress. It's more than enough to make this type of mask useless for rebreathers.

There is however another type of mask. Here in Australia, they go by the generic term of "CressiSub", however they are produced not only by CressiSub, but also by TechniSub and ScubaPro. They are similar to a normal scuba half mask but with an extension. The lower skirt has 'grown' to cover the lower half of the face, and the skirt goes across the forehead, down the cheeks and under the chin, sealing the entire face off from the water. There is a simple oval hole, corresponding to the user's mouth, and the mouthbit of the rebreather is simply jammed through the hole. This provides the user with a normal mouthbit to grasp with the teeth and a dry space covering the rest of the face. There is normally no flow of gas within the mask, much like the situation in a normal scuba half mask, and the defogging techniques used with a half mask work as well with this type of FFM. A 'spider' or head harness retains the mask, and so most of the loads from the hoses that would normally be taken by the user's mouth, are transmitted to the whole head, greatly increasing comfort, and allowing the use of larger bore or more rugged breathing hoses. Any leakage will accumulate in the lower part of the mask, and by exhaling via the nose the user can clear the mask, just as with a half mask. In the event of loss of consciousness, the bit will usually be retained in the mouth, but in any case, the nose and mouth both remain dry, thereby preventing drowning. Users switching from a mouthbit to a CressiSub will find that the integrity of the rebreather/user interface is markedly improved and the quantity of water accumulating in the loop is quite minimal. Users of sets that have stiff loop over pressure valves (or no valves at all as is the case with some homebuilts) will find that they can dump gas from the loop nasally, just as they can with a half mask. Also it is possible to remove the mouthbit, while the mask is in place, allowing the diver to speak. This is useful during the prebreathe phase of the dive, where the diver can exchange information with others, without wasting oxygen by breathing atmospheric air and so requiring a second O2 flush.

The disadvantage of this type of FFM, inherent in all FFMs, is that in the event of a loop failure, the entire mask must be removed in order to change to bailout supplies. Apparently there is a new FFM on the market which provides some form of docking port, allowing you to swap supplies without disturbing the whole mask, but I've not had the opportunity to dive this type of mask and see if it will work in a CC application.

As with all FFMs, underwater vomiting will present major problems. I recently scrubbed a dive due to fear of seasickness causing me to vomit into my FFM. I don't have a shut off type mouthpiece, and my best guess for how to handle this would be to try to remove the mouthbit, and vomit into the mask, then replace the mouth bit, flood the mask with water, and then clear it. I have never tried this, and if anyone has met this situation with success perhaps you'd like to contribute to the next newsletter!

If using the spit 'n wash defogging technique, the user will also find that rinsing a mask that is attached to the set quite awkward. A small squeeze bottle of water will greatly facilitate this process!

Another slightly unexpected disadvantage I found when I made the switch to FFM on my rebreather was that I no longer liked to dive alone! The advantage of having a protected airway is that you won't drown, but this is of little use if you simply sink to the bottom to die. I now much prefer to dive in the company of others, who can swim over and push a few buttons on my set in the event of my passing out.

DYK

Did You Know..... that the liquid breathing medium used in the movie Abyss is actually real. Liquid breathing is currently used by the medical community after the development by Alliance Pharmaceuticals of perfluorooctyl bromide, a fluorochemical (oxygenated flourocarbon) that was given the generic name perflubron. Useful as a blood substitute and for liquid ventilation, perflubron (under Alliance Pharmaceutical's brand name LiquiVent) is instilled directly into the lungs of patients with acute respiratory failure (caused by infection, severe burns, inhalation of toxic substances and premature birth), whose air sacs have collapsed. Once inside the lungs, perflubron enables collapsed alveoli (air sacs) to open and permits a more efficient transport of oxygen and carbon dioxide. Current tests are on "preemies," but trials with adults are ongoing. One baby girl actually lived for 4 weeks on this liquid medium. It can also go by the name of RIMAR 101 and is available from 3M.

Liquid Breathing: a brief history.

information from Joe Dituri.

The use of liquid as an alternate breathing media dates back to the early 1900s wherein intratracheal administration was used as a type of treatment for the inhalation of poisonous gas. In addition to the clinical utility for pulmonary hygiene, the saline filled lung has been employed subsequently as an investigative tool to elucidate a number of respiratory structural-function relationships. The saline solution contains only 3% by volume that, as you know, is insufficient to sustain life. Stein and Sonnenschein proposed that if the saline was pressurized, the PPO2 could be raised high enough to sustain life. In 1962, Kylstra demonstrated how mammals submerged in hyperbarically oxygenated saline could breathe liquid and resume gas breathing. This approach proved to be problematic. CO2 retention and profound acidosis were associated with hyperbaric saline breathing.

In 1966, Clark and Gollan were the first to use a normobarically oxygenated fluorocarbon liquid to support the respiration of animals immersed in the breathing fluid. FX-80 was the first and most commonly used fluorocarbon liquid with suitable properties. Other fluorocarbons used include FC-80, Caroxin-D, Caroxin-F and Rimar-101, which was used on the only human subject. Several other liquids were tried for this application (silicone, vegetable and animal oils) but found toxic.

Initial liquid breathing investigations used either total body immersion or gravity assisted ventilation. Both methods proved to be inadequate for prolonged ventilation. CO2 retention and acidosis were still problems due to flow limitations associated with total body immersion. The advent of demand-regulated and mechanically assisted breathing systems allowed effective CO2 elimination during liquid ventilation. Shaffer says that oxygenation and ventilation can be effectively maintained in a liquid filled lung for up to 30 hours. That's not a lot of time, and the one test subject had several problems.

Uses include assisting human survival in high and low pressures, and under great acceleration. In addition to decompression studies, other liquid breathing hyperbaric experiments have explored mammalian tolerance to high pressure. The first experiments demonstrated that convulsions occurred at 50-80 atm in animals breathing the

fluorocarbon. Other tests yield an 85 atm threshold for the onset of convulsions. These results for liquid breathing are in contrast to helium thresholds of 90-122 atm. There is definitely a limit to human tolerance, and we are almost there.

In my capacity as a U.S. Navy diving officer, I am unaware of any such applications within the military, but I am a small fish in this big pond.

Ed Note:

Obviously there are many hurdles to be overcome before it could be considered for use by divers but who knows, perhaps some day we will all be breathing liquid, just like a fish.

For further reading:

Masden, J., Henriques, U. 1969: Fluorocarbon Liquid Breathing. A Preliminary Report. Revue De Subaquatique Et Medecine Hyperbare. Tome 1. Janvier-Fevrier-Mars 1969. No 3 pp 171-174 (English)

Kylstra, J.A 1968: Liquid Breathing: An experimental approach to deep diving without inert gas narcosis and decompression hazards. R. Phys., Subaq., Med., Hyperb., T. 1, Mars-Avril, Mai. pp45-49

Pegg, J.H., Horner, T.L., Wahrenbrock, E.A. 1963: Breathing Of Pressure-Oxygenated Liquids. Proceedings of the Second Symposium on Underwater Physiology. National Academy of Sciences. National Research Council Publication 1181: p.166.

Clark, L.C., Gollin, F: Survival of Mammal Breathing Organic Liquids Equilibrated with Oxygen at Atmospheric Pressure. Science, 152:1755.

Rebreather News

Two mammoth cave dives were conducted using Halcyon rebreathers in May.

On Saturday, May 23, Dr. Reinhard Buchaly and Sandro Madeo dived the Ressel, an underwater cave located in southern France. Olivier Isler had penetrated the cave for 2 km in 1990, reaching an air filled room, but could not check for continuation because diving alone it was too risky to remove his gear (double semiclosed rebreather RI 2000) and leave the water. Buchaly (Halcyon) and Sandro (OC) reached the room and Sandro explored the dry cave for leads. The deepest point is 81m approx 1500m from the cave entrance. The dive took 7 hrs in 13 degree water and the Halcyon performed faultlessly.

This dive was on top of the new record cave dive conducted by the WKKP in Wakulla springs. George Irvine, Jarrod Jablonski and Brent Scarabin used Halcyon rebreathers to explore 12,730 feet into the cave. The dive was for a 3hr 40min bottom time with the following deco:

From 295 on 11% oxygen 60% helium, it went: 240 for 5 on 19/35, then by tens, 2, 4, 2, 3, 4, 6, 8,10,10,10,10, then from 120 on 35%, 20,13,13,13,20, then from 70 on 50/50 20, 30, ( all 20 on, 5 off to back gas with out changing the stops ), then 20 at 50 on 19/35 ( cleanup break), and 50 at 50 on 35%, then 50 at 35 on 50%, then 230 at 28 on oxygen with breaks to 19/35 every twenty for five, and then 15 at 28 on 19/35 and straight up to the surface for a 13.5 hour run time.

Gas consumption using the Halcyon was about 105 cubic feet of bottom gas ( 11/60), plus break and inflation gas from the back tanks ( as well as "reality" check gas"), 40 cubic feet of 19/35 trimix deep deco and break gas, 80 cubic feet of 35% nitrox, 40 cubes of 50/50, and 100 cubic feet of oxygen. 3:40 bottom time, 9:50 deco.

With these two dives, on top of all the others the unit has been used for, the Halcyon has proven it is the right tool for the job in deep long cave exploration. If it can perform faultlessly in such scenarios then it would certainly do so under any conditions.

Next issue we will do a full report on the ins and outs of the Halcyons' unique design.

Homebuilding News

I recently made a small oxygen breather using a wider version of the lay flat hose bag material and a smaller 100mm sewer pipe scrubber. It is chest mounted and worked ok on its test dive however the breathing hydrostatics were noticeably different with the chest mounted bag. It was easy to inhale but there was some WOB on the exhale, this was probably not helped by the old mouthpiece I used on the test dive.

We have been contacted in the last few months by a number of divers in Australia and New Zealand who are interested in, or are currently, building their own rebreathers. Locally, Bruce Stewart is nearing completion of his unit and has utilized the resources available at his workplace to machine up parts very professionally. Andrejs Kristovskis is also in the final stages of construction so it looks like there will be two more homebuilts in Sydney very soon. If any homebuilders have any photos or info about their units they would like published please send it to the address on the front cover.

Other News

Any members that need absorbent should contact Paul Davis at Drager Australia who tells me that they have a product called Divesorb Pro available. This is basically a high grade of the Dragersorb 400 and comes only in a 5l (4.5kg) container @ $40.00 ea. Freight is added ex-Melbourne. It is also available from their distributors at around the same price. Paul mentioned that this is the current Military preferred product and they go for the 5l only now as it keeps fresher and reduces danger to contamination or exposure. Paul Davis can be contacted at Drager Australia Pty Ltd 6/7-9 Kent Road Mascot NSW 2020 PH.(02) 96674172 Fax. (02) 96695294 Mobile. 0412147871 Email. paul.davis@drager.com.au

Paul also informed me that the purple color indicator used in some absorbents can be dangerous. Part of the chemical process for the colour indicator is that when wet it gives off an ammonia gas. Now ammonia is a dangerous gas as the nose becomes de-sensitized quickly if introduced slowly and you can get dangerous short term exposures without even know of this. Drager will not supply nor recommend this to anyone and colour indicators should be avoided wherever possible.

In the dive press recently I noted that Uwatec has released its Air XO2 air integrated SCR computer and the Oxy2 inline sensor. The cost is an astronomical AU$1500 (approx) for the sensor and computer. The AirXO2 can be used with gas mixtures up to and including 99% O2 while the Oxy2, which contains two oxygen sensors, is inserted into the air flow of the inhalation hose of a SCR. The sensors can be individually calibrated and transmit data to the Air XO2. The advertising for it states that "With reliable O2 measurement and appropriate warnings, it's possible to optimise SCR gasflow to improve the performance and subsequent bottom times." Being able to monitor O2 levels and decompression will make SCR's much safer however the diver should always be diligent to use the correct flow rate and gas mix for the dive and use the sensors as only a guide. They should at least give the diver a way of detecting reduced flow due to particle impingement and decreased O2 levels on ascent. Hopefully they will make SCR diving safer but they should not be used for CCR diving on their own, or to foolishly attempt to dive a SCR in a way contrary to it's design.

Another product, the Drager Oxygauge will become available this month. This is an in bag oxygen sensor that sends a hard wired message to the analog gauge giving constant readings of PPO2 in the loop. In the near future a computer will be available for the unit and when fitted to a console will take an infa-red signal into the dive calculations.

Rebreather Profile

The A.P. Valves 'Buddy Inspiration'

The Inspiration is a fully closed circuit rebreather manufactured by A.P.Valves in England. It uses a twin over the shoulder breathing bag design and comes with two 3ltr cylinders, one for diluent and one for oxygen. It has a computerized Oxygen Controller system consisting of two micro-processor control units with digital displays and separate power supplies, three oxygen cell sensors and a solenoid oxygen injection valve. The electronic controller is a voting logic system which automatically instructs the solenoid to inject oxygen into the mix to replenish the oxygen the diver has metabolized and maintain the ppO2 at the chosen Setpoint. The Inspiration has two default Setpoints for the partial pressure of oxygen in the breathing mix - 0.7 bar and 1.3 bar. As with many electronically controlled units the setpoint of .7 is used at the surface to avoid having the solenoid continually fire in an effort to keep a setpoint of 1 or higher. The Inspiration uses a setpoint of 1.3 for main part of the dive.

It also has a specially designed harness, available in small, medium and large, and 16kg wings style Buoyancy Control Device with an Auto Air - emergency demand valve/jacket breathing valve/inflator. There are manual diluent and oxygen feeds to the left and right counterlungs which come in two sizes, medium or large, depending on your lung volume. The cost of the unit is £3009 incl. VAT with the Auto Air and £2962 incl. VAT with the AP200 Standard Inflator.

Having only seen photos and read reports of the Inspiration it appears to be a well manufactured unit. It has certainly undergone testing for WOB and sensor accuracy. One fault I would find with it is that the size of the tanks which can be fitted is limited by the moulding of the housing. There is only room for the 3 ltr cylinders and any extra bailout gas needs to be carried externally by the diver.

Like all closed circuit rebreathers, care needs to be taken in checking the O2 levels and referencing them against the known variables of drive gas and depth. While my own personal preference is against having an automatic addition because of the tendency to become complacent and reliant on it, the Inspiration uses a number of audible alarms and other automatic cross checks to attempt to keep errors to a minimum. My own preference again would be to run a lower setpoint, around 1, as running 1. 3 like the Inspiration does leaves less room for error at the top end of the scale. A.P. Valves appear to be quite confident in the ability of their electronics to keep the loop PO2 stable.

At present the only Inspirations in Australia are owned by Barry Hallett in Sydney who was still waiting for them to clear through the docks when I spoke with him last.

Some comments on failure modes in electronic CCRs By Walter Stark

Hypothetical failures in hypothetical designs are somewhat hard to talk about. The possibilities are limitless. I will instead suggest some ways to avoid or minimize them as much as possible.

Electronics and sensors situated together and using short fixed Teflon insulated cables present minimal opportunity for cable failure. Electronics situated atop a back pack and operating at ambient pressure are least likely to leak and if so most likely to let gas out rather than water in. If transparent plastic is used for the absorbent canister, electronics housing and breathing bag simple visual inspection for absorbent condition, condensation and water leakage is possible. Cylindrical canisters and electronics housings can be strong, rugged, easy to make in a lathe and lend themselves to highly reliable radial o-ring seals.

The greatest danger of water entry comes via the mouthpiece. A breathing circuit whereby exhalation goes direct to the counterlung and inhalation is drawn in through the absorbent canister past the electronics provides double water/condensation removal before the sensors. Outlets from both counterlung and absorbent canister should be free standing and away from the bottom to reduce the likelihood of picking up any water which may have gotten in. Counterlungs should be provided with drains which can be used underwater.

Readouts should operate at high impedance so that any failure of them or their cables have minimal effect on the control circuit. Redundant sensor circuits should incorporate signal limiting or clipping to prevent failure in one from dragging the others past safe limits. Switchable redundant batteries should be used. Only batteries constructed with hard wired connection between cells should be used. An extra ppO2 readout which is totally independent of the control circuit is worth considering.

With a well designed unit by far the greatest source of failure is operator error and failure to pay attention.

Some common errors to watch out for:

1. Trying to squeeze more time out of absorbent, batteries and sensors.

2. Careless or rushed setup. Lack of thoroughness.

3. Black box syndrome. The calibration is way off so just crank up the trim pot until it reads what you want rather than finding out why it's so far out.

4. Not thinking about what you are doing. Leads to mistakes like valving in pure O2 at depth instead of inert gas.

5. The She'll be Right assumption. The alarm is sounding and the readouts are high but if I take a few more breaths it will probably drop back.

6. Leaky mouth washout. This is all too common. Experienced OC divers are pretty casual about letting some water in around the mouthpiece. Even when it's gurgling away with each breath in a CCR they may ignore it until the system is well on the way to flooded. When a mouthful of pH 13 seawater wakes them up they blame the unit, saying, "It flooded!".

7. Drunken diving. Related to 4. above. If you have N2 on the inert side whether you know it or not you are impaired at depth and more prone to not pay attention or to make simple mistakes. There is a lot of individual variation in this. Using a mix which keeps the ppN2 below 3 ata is a good idea.

Other than water leakage by far the most likely

equipment failure is manifested as solenoid not operating or stuck open and readouts too high, too low or one is drifting away from the others. For solenoid problems over ride with manual control. You should be able to hear if a solenoid is working or not. If all readouts are high cut off O2 and flush with inert. If this bring them back use manual control and abort dive. If it doesn't you have major electronic problems. Go to manual SCR mode and abort dive.

If all readouts are low use manual O2 control. If the readouts respond properly continue with manual control to abort dive. If they do not. Flush with inert premix and abort in manual SCR mode. If one sensor is drifting but the others are responding normally simply abort dive permitting the remaining two to effect control. If the errant one is erratic or extreme go to manual SCR mode.

This all presumes the inert gas supply is premixed with a breathable % O2. Manual SCR simply means valving in the inert premix, taking two to four breaths (depending on depth and %O2), exhaling overboard and refilling from the inert. If a decompression obligation is involved you can switch to O2 at 40 fsw. If your first stop is deeper than that then you should have a standby breathing system available at the first stop.

Unique failure modes of electronically controlled fully closed circuit rebreathers

In the last issue, Rodney Nairne dealt with some failure modes of constant mass flow semiclosed circuit rebreathers. Now I'd like to look at similar situations, in electronically controlled fully closed circuit rebreathers.

First (as always seems to be the case) some background information.

Electronically controlled 'breathers tend toward similar lines, and while there are of course many model specific differences, they usually have these major components:

A system of mechanical devices such as;

An ambient pressure loop

A system for providing medium pressure diluent and oxygen

A diluent add valve

A over pressure dump valve

As well as the electronic components that we will be dealing with here;

O2 sensors

These produce a tiny voltage in proportion to the partial pressure of oxygen present on the actual sensor.

An O2 add valve

This controls the flow of pure oxygen into the loop, and is usually driven electrically

A primary electronics package

This is the "smarts" of the rebreather, and it decides when to open the O2 add valve based on information it receives from the O2 sensors

A primary display

A simple display, intended to stay in the diver's field of vision at all times, it is normally limited to displaying that the O2 level is in one of three states, "low" "correct" or "high". Some displays are set up to show other things, but this comes as a trade off with readability.

A secondary display

Often erroneously called "backup", this more complex display, it usually there to provide a numeric display of the sensor outputs, and is useful during calibration of the setpoint, and so the diver can check the outputs from the multiple sensors for agreement.

A system of cabling

This connects the elements of the electronic systems together, and allows them to send information from one element to another, usually in the form of an analog variation in voltages. Most cables need to carry more than one type of information, and so they are multiple wire cables, with several information carrying lines, and one common ground line.

Now to examine the possible failure modes, we can play the "what if" game with each element that makes up the electronic system. The main failures for systems like this will be, flooding or breakage, in other words, short circuit and open circuit. The effects of these failures unique to each element, and I'll discuss them in turn.

O2 sensors

These are the beasties that evaluate the O2 levels in the loop. Known for being unreliable they're almost always used in sets of three, with the primary electronics programmed to make some type of evaluation of the information that each sensor is giving, and work out which one to believe

The most common type used has a semipermiable membrane covering an electric cell. This type of cell uses oxygen as part of it's workings. When there is no O2, the cell stops working and the output voltage falls to zero. As the partial pressure of O2 in the gas around the cell increases, more O2 diffuses into the cell, and the output voltage rises.

Because it is consumed by the reaction that generates the current, it can go 'flat' like any other battery. When it is going flat, it will become non-linear, in other words, the amount of voltage produced will no longer be in direct proportion to the O2 level of the gas in which it is immersed. What happens is that the cell is no longer able to create high currents, and high O2 levels will result in less voltage, or conversely, higher O2 levels will be required to create the same output. This is of particular significance to users who check the calibration of the set at an O2 level less than the setpoint. For instance, there is no guarantee that sensors that worked at say a PPO2 of 0.95 will remain linear all the way up to 1.4 Since it is impossible to calibrate O2 sensors at 1.4 on the surface, there is no way to tell what actual O2 level is going to be needed to make the sensors output a voltage corresponding to 1.4

The sensors are also subject to variations due to the collection of water droplets on the surface of the sensor. If the membrane becomes covered in water droplets (whether due to condensation or water ingress) the loop O2 level will be uncoupled from the sensor voltages. If the coverage happens when the loop O2 level is higher than the setpoint, the rest of the electronics will assume that all is well, and the O2 add valve will not be opened. Both the primary and secondary displays will show that the O2 level is within range, and the diver will deplete the loop O2 and pass out when it falls below life support range. If the O2 level is below the setpoint when the sensors are covered, the O2 add valve will remain open. Both the primary a secondary displays will show that the O2 level is within range, but the O2 level will increase unchecked, rising above the life support range.

Water ingress has obvious causes, and obvious solutions, but condensate on the sensors can be quite subtle. Mature designs will have had it determined that the sensors remain warmer than the dewpoint of the gas passing over them under all conditions. Experimental designs may not have determined this factor. Additionally, such things as reversing the direction of the loop may mean that there are local variations in the temperature and dewpoint within the loop. Where the sensors may have been protected from condensate, they may now be at a point where large amounts of water will form. This can arise from something as simple as installing the mouthpiece check valves incorrectly, or fitting the hoses back to front.

Unexpected water formation may also lead to a partial short circuit of the sensor leads, resulting in a suppression of the signal from the sensors. This will mean that an increase in loop PPO2 will be required to maintain the setpoint voltages.

O2 Add valve

Some type of electromechanical valve. As with all valves it can stick open, closed or partially open. Perhaps the least dangerous of all failures in an electronic rebreather, as any one of these three failures should be readily apparent to the user through the displays. In the situation of a stuck open valve, the user should have 1-2 minutes in which to spot the problem and react if they are using a sane setpoint, somewhere close to the middle of the life support range. This is because a sensible design will only flow a couple of litres per minute more than the greatest metabolic requirements, and so it will take some time to push the O2 level to the point where it will cause acute oxygen toxicity. Of course when using a setpoint up around 1.4 or 1.6, near the top of the life support range, the delay before reaching lethal levels is shorter, and due to the diver's exposure to hyperoxic mixtures, the delay before the onset of gross symptoms will be reduced.

Leakage of water into the electrical parts of the valve may overload the primary electronics, disturbing it's other outputs.

Primary electronics package

Normally mounted in some type of pressure proof housing, this is where the decisions are made in the rebreather. The housing will normally also contain the main battery, that is used to drive the primary electronics, the primary display and the O2 add valve.

Since it is inside a strong housing, it is normally immune to breakage, however since it needs to be regularly dismantled for battery changes and calibration, it is quite prone to flooding. Major leakages are not normally a problem, as the entire electronics package will fail suddenly and totally, making the diver instantly aware that something bad has happened and that they need to abort to the preplanned bailout. Of course a sensible diver will realize that the O2 sensors can no longer be trusted and that a manual bailout on the secondary display readings is foolhardy. The sensors can only produce tiny voltages, and after a primary electronics flood out, they'll be connected to a lot of wires, big batteries and salt water. Not the ideal state for a sensor that can only output milliwatts.

Minor floods are more serious, as they are difficult to identify, and their effects are difficult to predict. Small amounts of water somewhere in an analog system are just plain bad. They may effect the readings, but not the results, they may effect the results, but not the readings or they may effect both. It can damage the operation of the software in the primary, leading it to alter the way it handles (and displays) the results of disagreement amongst the sensor voltages. It may suppress the sensor voltages themselves, causing the set to increase the O2 levels or increase the sensor voltages, causing the set to allow the O2 level to drop.

Essentially, if the set begins to do anything weird, such as adding a lot of O2, fluctuations in either display, sudden variation in primary battery voltage or in fact anything unusual, then you should suspect primary electronics flood.

Primary display

Usually a very simple device using flashing lights, or shrinking/growing bar graphs. Beware that if it floods the short circuit in the display can effect the operation of the primary electronics. Analog circuits are naturally sensitive to variations in voltages, and the unexpected load of a flooded display will probably cause unexpected voltages to appear in the wrong places. If this effects the sensor voltages, then the secondary display may start giving you incorrect information.

Secondary display

Usually with it's own internal power source, in most designs it displays the sensor voltages (converted to a PPO2 reading) and some indication of the state of the primary battery. A floodout in this display can connect the sensors to the primary battery voltage, or to the secondary battery voltages, or both. It can also suppress the sensor voltages by providing a short circuit. Of course if this happens, then the primary electronics can no longer control, or display the O2 level correctly. The O2 level may go up a little, or a lot, or down a little or a lot. No way to tell.

Cable system

All the information that is carried within the rebreather's electronics goes via cable. If any of the cables leak, then the information is reduced to garbage, and the components that it's connected to (at both ends of the cable) will not behave correctly. For example: a cable carrying only O2 sensor voltages. If it leaks, then it may tie all the voltages together. All three sensors will read the same value, even if one or two of them fail. Clearly the O2 level would have to be much higher to maintain the same average voltage if one of the sensors dies. While this is happening, the apparent O2 level will remain constant, the electronics will perform as normal, and both displays will read normal levels, with agreement between the sensors.

As I think you will see, the electronics of a rebreather depend on the correct function of every part. No element can be considered to be working independently of the others. Often you will hear statements such as; "If the primary electronics fail, you can fly the unit manually just off the independent backup display, with some practice it's easy!". Now you will have the tools to judge for yourself the value of that person's knowledge of the subject.

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.