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Thread: CO2 retention in OC vs CCR

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    RBW Member manni-yunk is an unknown quantity at this point manni-yunk's Avatar
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    CO2 retention in OC vs CCR

    I have been reading a lot about CO2 and I know Co2 retention has been discussed on here as I have done a ton of searches in my last 12 months of research. I am about to make the move to CCR.


    That being said - I am having difficulty with something. Stay with me as I explain.

    I completely understand the concept of "breakthrough". If the wrong scrubber material is used, or poor scrubber packing occurs resulting in channeling, if someone tries to extend the life of their scrubber to long, if a flapper valve fails, etc.......all of these and more could result in CO2 not being properly removed from the breathing gas. I get it. Im not talking about any of these situations in my question.


    I am talking about actual CO2 retention. Either from a physiological fastor or any other reason - a person who is retaining CO2 at a higher than normal rate.

    This seems to be a topic that comes up a lot in conversations about rebreathers.


    What I dont understand and I am hoping someone can explain to me - is - what is the difference between CO2 retention between a CCR and OC diver and why does it seem to be more of a perceived problem with CCR? In other words - if I am retaining CO2 for any reason other than a problem or issue with my gear - why is it more of a concern on CCR? ( I am not asking why CO2 in general is more of a concern on CCR as that is clear - I specifically mean co2 retention not due to the unit itself). If my scrubber is working as designed and none of the C02 in my system is due to the rebreather itself- wouldnt one have just as much of a pending problem regardless of OC or CCR?



    ( I do understand that the CO2 retention could in fact be due to a poor WOB in a unit - but I consider that analogous with a poorly tuned regulator. It seems most of the major units on the market have proven an acceptable WOB or they would not exist).


    I guess the actual question is that if someone is a C02 retainer on OC.......would they be any worse off on a properly functioning CCR? If so why?



    Sorry if this question seems ridiculous. Im very detail oriented in major decisions..and I ask a lot of questions until I believe I have a complete understanding..especially when it comes to a CCR since not that long ago I was a member of the "I would never dive CCR" crowd and now Im pretty sure Ill have one sooner rather than later.

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    Re: CO2 retention in OC vs CCR

    When diving OC I used to get head aches I always assumed to be from CO2 retention.
    When I switched to CCR they never re occurred,even though the WOB of my OC regs should have been less than my CCR (IDA71 modified originally).

    Co2 retention is an issue with both OC and CCR divers. I think CCR divers are simply more aware/concerned with it.

    While I am unsure of your last question about a CO2 retainer being worse off on CCR than OC, something to consider is a diver being less reactive to CO2. (eg freedivers or perhaps smokers). If they dive CCR they may not notice CO2 build up..... until its too late.

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    Re: CO2 retention in OC vs CCR

    The issue of CO2 retention being more prevalent with CCR divers than OC Divers primarily has to do with the management of loop volume and gas exchange. Your search for CO2 retention is well founded and is not universally understood by many CCR divers and/or instructors. Most CO2 retention occurs by the reasons listed below and more commonly while under heavy workload. When it does occur under heavy workload, the diver is often taught that this is due to "over breathing" the scrubber. In most cases, over breathing a scrubber is a myth when it comes to any CE compliant machine that is working properly. Over breathing a scrubber is VERY possible among rebreathers that are NOT CE compliant or a CE unit that has been modified with aftermarket BOV's, counterlungs, loop hoses, etc. Since prevention is always the key to any issue, start be making sure you purchase a CE compliant, non-modified unit and then learn from an EXPERIENCED rebreather instructor, not someone that teaches rebreathers part time.

    In the case of a properly functioning, CE rated, Non-Modified unit here are the most common ways you retain CO2:

    The goal of any CCR diver is to maintain Optimal Loop volume, which is slightly more than one full breathe. This means after the diver exhales into the rebreather fully, he then has enough gas to inhale without activating the ADV.

    However, too many divers have much more than Optimal. More than Optimal leads to having too much gas overall and prevents the diver from fully inhaling and exhaling. Imagine taking a paper bag and filling the bag with air. Then imagine inhaling while not on the paper bag and filling your lungs with air. Then imagine trying to then breathe off that bag. You would not be able to inhale or exhale because your bag and lungs are already full. When you have too much gas the effects are obvious and you cannot breathe. But when you are riding on the edge of just too much gas you are not able to exchange gas properly and therefor retain CO2.

    The next problem is attempting to maintain what is called "minimal" loop volume. By not having at least one full breathe in the loop you are required to short breathe, thus leading to poor gas exchange and then CO2 retention. You can get away with this when you are not working hard, but the effects are swift when you begin to work.

    Having just enough gas to breathe while not having too much is an intuitive sense that is developed by someone that is taught how to do it. It will never come without learning it. Make sure you get an experienced rebreather instructor that can teach you how to do this. They all talk about it, but few of them actually teach you how to do it and make it an important part of the skills taught while diving.

    In addition, poor management of true Optimal loop volume leads to overweighting, poor posture and poor buoyancy control. The number one reason many rebreather divers complain of never having the buoyancy control they had with OC is due to poor Loop volume, which leads to adding too much weight to counter the additional loop volume which narrows their available neutral buoyancy range. They usually dive head high and feet low. Its clearly obvious and could be easily fixed.

    Best of luck with your rebreather diving, it will change your life in a good way.

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    So many CCR So little etc Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase's Avatar
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    Re: CO2 retention in OC vs CCR

    The only C02 hit I ever had is on OC and I have been CCR since 2004

    I am baffelled by claims of C02 hits due to scrubber bypass or beakthrough because with the exception of a couple of tricky radial scrubbers, most are dead easy to fill propery and prety much all of them will easily do double the duration passed by CE mark


    The big issue for CCR divers imho is loop valve failure and increased work of breathing.

    Failure of the mushroom valves in the unit is very possable and often overlooked.

    WOB issues are common. Just having the unit too low on your back can mess with WOB and increased WOB can lead to retaining C02 and a full on hit.

    I consider CCR safer than OC in most areas where multy gas OC diveing is an option but I also consider the risks of C02 retention to be considerably higher on CCR than on OC

    Its my last great fear when diveing a CCR

    Many people get hung up on high or low PP02 but for me C02 is the big one to watch out for

    ATB

    Mark

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    Re: CO2 retention in OC vs CCR

    I have never had a CO2 hit on CCR either, but I have had on OC a few times.

    Now, except all the obvious reasons for getting a CO2 hit on a CCR, I think the reason I have never had one on a CCR is that I do not have to watch my rate of breathing.

    I breathe as much as I like, whenever I want, while on OC, you have to watch your gas-consumption, and you also have to breathe in a forced way to be in control of your buoyancy.
    (Yes, I know when you are experienced, you don´t have to think about it, but that is because your brain does it automatically)

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    RBW Member oya has a spectacular aura about oya has a spectacular aura about oya has a spectacular aura about oya has a spectacular aura about oya has a spectacular aura about oya's Avatar
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    Re: CO2 retention in OC vs CCR

    https://www.youtube.com/watch?v=GW1yVFF_FK4

    Starting at about minute 7 or so.

    Well, watch the whole thing because it's awesome. But starting at about minute 7 I think it might start discussing just what you're asking after.

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    RBW Member manni-yunk is an unknown quantity at this point manni-yunk's Avatar
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    Re: CO2 retention in OC vs CCR

    Quote Originally Posted by oya  View Original Post
    https://www.youtube.com/watch?v=GW1yVFF_FK4

    Starting at about minute 7 or so.

    Well, watch the whole thing because it's awesome. But starting at about minute 7 I think it might start discussing just what you're asking after.

    I watched the whole thing and it was great. Other than WOB and counterlung position and its impact on pressure differential - it seems everything he said is really not rebreather specific, but SCUBA specific - right?

    I do love the idea of an end tidal CO2 monitor.

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    Re: CO2 retention in OC vs CCR

    Quote Originally Posted by manni-yunk  View Original Post
    I guess the actual question is that if someone is a C02 retainer on OC.......would they be any worse off on a properly functioning CCR? If so why?
    I think the shortest answer (as I understand it anyway) is "no."

    There is a slightly bigger dead air space in even the smallest DSV than in a regulator. But you breath funny on a regulator to draw out your gas supply, whereas you can breath normally on a rebreather to get rid of all that nasty, nasty CO2.

    In the end, just how much of a CO2 retainer one is is probably second fiddle to how much of a CO2 producer one is. Close your eyes and imagine the standard rebreather diver's general body type. Does the person in your head look like they'd get winded running to catch a bus? Then they will probably get winded swimming against a stiff current, too.

    Frankly, though, I'm not that smart. And I'm very curious to hear what the bigger brains than mine have to say on the matter.

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    Re: CO2 retention in OC vs CCR

    I have had nasty little surprises from CO2 retention on both OC and CCR. I have come to the conclusion that with properly tuned and prepped gear, that the biggest concern (at least for myself) is poor breathing habits. Deep, full breaths along with complete full exhalation seems to solve my problems. Maximum lung ventilation to clear out the CO2 seems appropriate.
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    RBW Member Mr.Dave is an unknown quantity at this point Mr.Dave's Avatar
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    Re: CO2 retention in OC vs CCR

    Only CO2 issue I had was during shallow breathing due to minimum loop volume. I put it down to being under weighted and as I was task loaded (training) I was breathing fast and shallow

    Grim experience but good lesson learnt.

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