[almitywife]

Part IV

The Redundancy Debate

One of the areas of great debate with respect to design of all rebreathers is the need for true electronic redundancy and what design actually amounts to true redundancy. Just about everyone knows from experience with cell phones, computers and other electronics that electronics are not 100% reliable, and the more complex they are, the more likely they are to have problems. Just about every rebreather diver agrees that carying OC bail out is a must but many will debate the value and definition of electronic reduncancy. Many folks believe that a fully integrated system that lacks true redundancy can not be trusted, as a problem in one function would leave doubt about the function of the rest. True redundancy is important so that if there is a failure in one system that you still have another, presumably unaffected system to compare on and rely on for bail out, as many folks like the idea of being able to stay on the loop if they can verify that it's safe to do so. Having a completely independent po2 reading through a truly separate device in order to compare readings to determine if it’s safe to stay on the loop is considered by many to be essential. This comparing process also helps to keep the mind engaged. The other major reason for true redundancy is that you will have more options available to continue diving on a trip in a remote area after the dive has been ended and the problem identified.

There seems to be two very divergent trends in rebreather design. Some eCCRs are going in the direction of more fully integrated systems, a kind of all in one approach. Fully integrated systems offer the advantage of global monitoring of the system. The disadvantage of integration is thought to be that it leaves one vulnerable to a malfunction leading to a lock out or one element causing problems in the entire system requiring factory service before additional dives can be made. Quite a few manufacturers of both eCCRs and mCCRs are going in a very different direction with the development of more fully modular systems, with elements that can be removed and replaced if there is specific malfunction, increasing the prospects for in-field repair by the diver. For instance, some eCCRs are made in such a way that a constant mass flow valve can be easily added for running manually, quite a few divers are finding that it’s surprisingly easy to add a constant mass flow valve to the save a dive kit, leaving them with more options in case of controller failure. One manufacturer of eCCR electronics even comes stock with an mCCR's cmf valve and a solenoid…a very wise step forward in design in terms of convenience, IMHO.

At this point all eCCRs, no matter how redundant they are only have one solenoid and this leaves even redundant set point controllers vulnerable to a common failure point. MCCRs, lacking a solenoid, lend themselves particularly well to modularity because they lack such a common failure point. The disadvantage of modularity with the intent of leaving some fixes up to the diver is that they may encourage one to venture into the realm of technician, beyond one's actual abilities, in order to swap out malfunctioning aspects of the system and many people believe this is too much to expect of the average user, even arguing that this could invite divers to put themselves at unnecessary risk, and continue diving it when it should instead be sent back to the manufacturer to be worked on by a certified technician. The flip side to that argument is that modularity often means replacing something is a matter of unplugging it, unscrewing the port in the head and adding a new element and replacing it without having to do anything more technical...leaving that to a certified technician when it's convenient to send the offending widget in for proper service. Each diver has to make a call on the level of involvement and risk they are comfortable with, ultimately deciding how much it’s worth to them to have options to continue diving in the event of a malfunction.

Debates are also made as to whether a HUD can adequately act as a redundant back up. It is my opinion that for a HUD to be considered a true back up it needs to be independent (separate battery and brain) and able to communicate actual po2.

In many ways it is argued that true redundancy is a matter of both safety and convenience. More and more mCCR’s are being designed with the idea that the user will add their favored back up system for such redundancy. It is my feeling that all rebreathers should come with provisions for running them manually with a mass flow assist option if for no other reason than to give a diver options for continuing to safely dive on an extended or remote trip, should some aspect of the automatic set point controller fail where certified repair is not possible.

There are several electronics packages that are being offered by third party manufacturers for just about every eCCR and mCCR rebreather. They are often used to create added redundancy (even a 4th cell on an eCCR) or to replace the stock electronics altogether. The Shearwater GF, Pursuite and the Shearwater HUD , Hammer Head electronics , The Universal Rebreather Monitor, and the VR3 are all third party electronics that are commonly used. Third party computers are often used “in line” to a 4th cell, for additional redundancy on eCCR’s. To date it is my understanding that the Shearwater HUD is the only truly independent real time po2 HUD monitor, offering continuous po2 display with it’s own separate battery and wiring, making it a convenient and simple addition for true redundancy in many systems.

Certainly the single most impacting factor with respect to safety and CCR diving is the awareness and attitude of the diver. Ultimately rebreathers don’t kill divers. All to often what kills divers are lapses in attention and making the kind of simple mistakes that anyone at any level of training and experience can make if they become too confident and fail to maintain adequate vigilance. The best attitude to take while diving rebreathers of any kind, no matter how much experience and training one gets, is that you are 100% responsible for what is happening and you should be prepared and practiced for a variety of failure modes. Getting certified to operate a CCR takes about a week of fairly solid training and is unit/brand specific. Much of the training involves drills on how to deal with various challenges and failure modes and calculating adequate OC bail out to return safely to the surface should the unit fail during a dive.

With all the choices and opinions out there, it can be very challenging to choose a rebreather. Getting enough experience to distinguish one’s actual needs and preferences from the variety of opinions takes extensive time, and ideally, first hand experience in a swimming pool on each of the available units; or even better, to really understand the differences between CCRs you’d need to put quite a few hours on each unit in the field. This would be completely impractical for most of us because it would require getting certified for each one! Maybe when I win the lottery I’ll write that report ;~).