almitywife
Vegemite Mod
in reference to this original thread; http://www.scubaboard.com/forums/rebreathers/210392-comparison-contrast-available-rebreathers.html
Gill Envy, ScubaBoard - View Profile: Gill Envy, has spent alot of hours in formatting this very infomative article on current rebreathers and options
many thanks
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Gill Envy, ScubaBoard - View Profile: Gill Envy, has spent alot of hours in formatting this very infomative article on current rebreathers and options
many thanks

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Part I
Introduction
As promised I have gone through and made some corrections and suggested additions. I have diverged from my original intent of having a relatively short synopsis in order to be more comprehensive.
Understanding what the various rebreathers have to offer is a formidable challenge and there are practically as many varying opinions out there as there are rebreather divers. I remember how mind boggling it was when I first started out trying to make sense of it all, and how frustrating it was to ask for opinions of which rig is the best, only to be given some nebulous one line answer like, “it depends on what you want to do” or being given what amounted to a sales pitch from a distributor or instructor who has a stake in promoting one type over another. Getting an objective answer is nearly impossible so it’s best to get as wide a variety of opinions as you can. While I respect the opinions of manufacturers, retailers and instructors, I thought it might be helpful to put something together that was from the perspective of a diver who has no commercial interest to protect or promote and is not beholden to any certifying agency or manufacturer. Some of this write up is fact and some is just my personal observation and opinion.
The bad news is that the answer to “which unit should I buy?” does ultimately depend on what your needs and preferences are, and ultimately you are the only one that can determine that. Now that I am a couple of years into the journey, I have attempted to go back and explain some of the basic differences as I now understand them in hopes that it might be helpful to others just getting started. The good news is that there are a lot of wonderful choices out there. It’s an exciting time to be getting into rebreathers as the industry seems to be hitting a critical mass where many solid options are available. I would recommend gathering information from as wide a variety of sources as is practical for you to determine for yourself what would best suite your needs before taking the final plunge. My hope is not to impart “the truth” but rather to leave you with helpful questions and perspectives that will help you on your quest to develop your own understanding of your priorities and needs. While I started out in pursuit of the one perfect unit, I have begun to respect them all for their own unique qualities.
To give you some idea of my background, to date I have 130 hours of CCR diving under my belt, my wife and I have owned and are mod 1 certified on an eCCR and a mCCR, the Evolution and now the Shearwater Copis Meg (about 230 combined CCR hours between us). We enjoy both tropical and cold water dry suit diving. I have trained to the dive master level and done over 700 dives since getting my original C-card 13 years ago. That said, I by no means wish to promote myself as an authority on the subject of rebreathers and, try as I might to be objective, I have my own evolving experiences, opinions and bias, which I am choosing to express openly. I encourage anyone to post variations in opinion or corrections. The feedback has been very helpful so far, thanks to all who have helped on this “little” project. I’ve done my best to incorporate new information and update my original, somewhat sloppy post.
Before diving in to the individual makes and models, I’d like to go into some general background on current rebreathers and trends that I am aware of.
There are two main differences between available CCR's. There are eCCR’s and mCCR’s. ECCR's rely on an automatic computerized set point controller with a solenoid (an electronically controlled piston valve) to help monitor and maintain o2 balance automatically by injecting o2 into the loop as needed. MCCR’s require manual o2 injection, assisted by a constant mass flow orifice. Near continuous vigilance is required for the safe operation of both types. In general, the design of mCCR’s and eCCR’s appear to be aiming at a similar goal, balancing safety and convenience, they just go about solving some of the main challenges in very different ways. Many deep dives are being done successfully on both mCCR and eCCR’s, so the choice between the two major styles is no longer thought of as a matter of depth; it ultimately comes down to a matter of personal preferences and priorities…there are merits to both.
Research and Design of each rebreather manufacturer is currently governed by a wide variety of individual manufacturing standards (often not available to the public) and even in some cases governmental standards such as CE certification. Unfortunately there is not much agreement on basic standards.
One of the challenges that CCR’s address in different ways is that while oxygen consumption ebbs and flows with changes in metabolism, the basic metabolic tolerances for o2 on a cellular/molecular level do not change with depth, while on the other hand the loop volume and gas density changes dramatically with depth. Keeping po2 relatively constant while gas volume and density in the loop change throughout the dive is key to enjoying the decompression benefits of a rebreather as well as staying within life sustaining o2 limits. Essentially, a rebreather is a nitrox blender on your back, optimizing the gas mix to match the ideal ratio of o2 and nitrogen for changing depths throughout a dive. The ideal threshold for o2 on a cellular level is measured in partial pressure of oxygen (po2). Too much o2 or too little can both be just as deadly.