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Rebreather or not?

Discussion in 'Rebreather Diving' started by Dave Bevan, May 30, 2017.

Should I move to CC or stay on OC.

  1. Go on OC tech, get more dives and experience and switch later.

    12 vote(s)
    44.4%
  2. Start to train CC now.

    11 vote(s)
    40.7%
  3. You are an idiot.

    4 vote(s)
    14.8%
  1. Gareth J

    Gareth J Nassau Grouper

    # of Dives: 1,000 - 2,499
    Location: UK
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    The AP units CAN have CO2 sensors, these are a plug and go accessory (they are a consumable, like lime and O2 sensors). I am aware that there where early issues with reliability - i.e false positives. Again, having been through the learning curve they are a lot more reliable.

    A lot of the CO2 issues are down to careless assembly, poor pre-checks, and not changing your lime.

    The BIG issue with CO2 hits on CCR are, as I understand it, breathing a high PO2 gas mask a lot of the 'normal' CO2 symptoms until they are at the serious end of the spectrum.

    All those I have talked to who have successfully survived a CO2 hits say exactly the same thing.
    When they where first aware of the fact that they had a CO2 problem, they where unable to get off the loop. Their breathing rate had reached a point where, despite there realisation that they had a CO2 issue, they where unable to hold their breath for a fraction of a second, let alone make the switch to their bailout. Rationally, they new they had to get off the loop, but the 'lizard' part of the brain kept saying take another breath.

    One of the 'best buys' is a 'Bailout' valve, one quick rotation of the valve and you are back on OC. No need to hold your breath. The biggest issue, is how fast you go through the DIL, on a shallow dive, the general theory is that this will give you enough gas to control your breathing (3l on board DIL cylinder). Then you can bailout.
    For deeper diving you really need your off board bailout plumbed into the bailout valve to ensure you have sufficient volume of gas to get your breathing under control. Ideally you don't really want to have to take the mouth piece out full stop. But simplicity rules, so the expectation is to bailout to the deco' gas once you are shallow enough.
    Also, the shallower you are the more likely you can get back on the loop, and a better deco schedule. The scrubber is more efficient shallower than deep - less gas passing through the scrubber. But once you have breakthrough you need to balance the risk / benefit. Frequent flushing the loop is still more efficient than being on OC.

    As I have said previously in the thread. I still think most CCR divers fear CO2 more than anything else.
    That said I STILL haven't invested in a bailout valve. But CO2 is still the thing I fear the most.

    Gareth

    PS - Also remember CO2 brings on paranoia, and significantly elevated breathing rates, neither of which are things to look forward to.
     
  2. EricTheDood

    EricTheDood DIR Practitioner

    # of Dives: 100 - 199
    Location: California
    279
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    Thanks for posting this. I've read a lot of articles and posts about bailouts and BOVs. Also watched the documentary, "Diving Into the Unknown". They had me scratching my head. Now it all makes sense.

    What are the mechanics involved in CO2 buildup? More specifically, if one is able to "overbreathe" an otherwise fresh scrubber under heavy workload, why isn't the solution as simple as having a larger scrubber?
     
  3. KevinNM

    KevinNM DIR Practitioner

    1,954
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    There are experts who have put together presentations on this, and I'm not an expert. But basically it's that they produce more CO2 than they can exhale, and this triggers a vicious cycle that becomes ventilatory failure and culminates in death.
     
  4. Gareth J

    Gareth J Nassau Grouper

    # of Dives: 1,000 - 2,499
    Location: UK
    189
    91
    28

    From your post, you don't understand the operation of a CCR unit. To explain it i a post is not ideal but I will try to give you a simplified overview.

    BASIC EXPLANATION

    A rebreather is a recirculating breathing system.
    Expired gas (air) passes through a one way valve into the exhale counterlung.
    From the counterlung the gas passes into the bottom of the scrubber.
    The gas passes through the scrubber - which should remove any CO2
    At the top of the scrubber, the PO2 (partial pressure of Oxygen) is measured.
    - based on the PO2, the set point, and the 'control system' [1], Oxygen is added to maintain the set point.
    The gas passes into the inhale counterlung
    The gas passes from the inhale countering through a one way valve and back to the mouthpiece.


    CO2 is removed by the scrubber, the one way valve ensure that the exhaled gas is forced through the scrubber and inhaled gas is from the exit point of the scrubber.

    SCRUBBER FAILURES

    I am not sure of the ratio of the following failures (CO2) only, but I have put them in the order I think is the most likely.

    1. Poor unit assembly.
    This includes leaving out O-rings, dirty O-rings, damaged components etc.
    2. Failure to change the Scrubber material
    The scrubber material is a consumable, and changes its chemical composition as the CO2 passes through it, combining with the CO2. All rebreathers have an estimated operational time for a specified grade of sofnalime [2] (or alternate material), at a specified depth, at an expected work load.
    Poor storage of Sofnalime can result in it degrading and becoming less efficient.
    3. Breakthrough
    - Breakthrough is the result of CO2 remaining in the gas that has passed through the scrubber.
    Breakthrough can be the result of poor assembly - leaving out the upper scrubber O-ring allowing gas to bypass the scrubber (poor assembly)
    - Channeling, poorly packed scrubber allowing gaps in the scrubber that bypass the sofnalime.
    -Bad Scrubber material - it doesn't clean out the CO2.
    - Overused scrubber material - each x kg of sofnalime will remove x litres of CO2, once you have pushed that volume of CO2 through the sofnalime it stops working. NOTE - its efficiency degrades progessively
    - Overbreathing the scrubber. The manufacturer specifies the scrubber efficiency on an assumed workload. It you exceed the workload, more CO2 is passing through the scrubber than expected so its operational time is reduced.
    - Cold Scrubber, the chemical reaction results in a thermal reaction, the thermal reaction improves efficiency. You 'should' prebreath the scrubber. This gets the reaction going AND MAY give you an indication of a problem BEFORE you get in the water.
    - Depth (gas volume/CO2 volume). One of the fundamentals of CCR's is the fact that we produce a definable amount of CO2 for a given workload. THIS DOES NOT CHANGE WITH DEPTH. However, as you get deeper, the volume of gas in the breathing loop needs to increase (boyles law). So the amount of CO2 is increasingly diluted in the gas in the breathing loop, which makes it harder for the scrubber to select the CO2 molecule from the other gas molecules. i.e a CO2 molecule will pass further through the scrubber before it reacts with the sofnalime. It may well pass through the complete scrubber and exit before it reacts with the sofnalime - BREAKTHROUGH. As the scrubber is used the lower section becomes dead, so there is less distance to 'catch' the CO2 molecule in the scrubber.


    HOW THE SCRUBBER WORKS
    The piece below is not mine, I think its Gorden Hendersons, but it explains things very well.

    Greetings from planet Zorg...

    Here on Zorg, we abducted some humans to test your resistance to CO2 and the efficiency of our patented CO2 grabbing demon chamber.

    We took a human and connected a hose to them. The hose supplies gas and has one-way valves. The exit of the hose goes into a box. Inside this box are 1000s of little demons. These demons adore CO2. They will grab a passing molecule of CO2 and hang onto it for the rest of their lives. They can only hold one each. After the CO2 demon box there is another box with different demons inside - these count the number of O2 molecules you have used and replaces them.

    We observed that humans when in a steady state consume the same amount of O2 per breath, regardless of the pressure we subjected them to. When given 100 molecules of our gas, they would use 4 molecules of our oxygen and turn this into 3 molecules of CO2 and 1 molecule of water vapour.

    So in the test, with 100 molecules of gas in the loop. The human push/pulled this through the box with the CO2 demons in it. Every breath, 3 lucky demons grab a CO2 molecule each and are happy for the rest of their lives. We repeated this for many of your earth hours, pushing 100 molecules of gas through the CO2 box at a nice steady rate - the happy demon front line progressed linearly through the CO2 demon box until eventually they are all happy. At that point, the loop gas has some CO2 in it and we observed that the humans started to show signs of unease, panic and general ill-feeling. They eventually died a rather uncomfortable death.

    To continue our experiments, we abducted more humans and carried on, this time we subjected them to a pressure of 2 bar. This is the same as being under 10 metres of your water. There is now 200 molecules of gas in the loop, but the human still only uses 4 molecules of O2 and turns these into 3 molecules of CO2 and 1 water vapour. Each breathe pushes 200 molecules through the CO2 demon chamber, so the demons have to work faster to grab the CO2 molecules and die happy. Sometimes a front-line demon misses, but the 2nd line catches it OK. This carries on and eventually all the demons are happy, then as above, the human dies painfully and horribly from CO2 poisoning.

    We needed to do more experiments, so we continued with our abduction programme. Now we're testing to 90m. There are now 1000 molecules of gas in the loop, but as observed before, then humans still only take 4 molecules of O2 out and metabolises these into 3 of CO2 and one of water with each breath, However, the poor CO2 demons now have 1000 molecules of gas going through their chamber like a hurricane, and in those 1000 molecules there are still only 3 molecules of CO2! It's now very hard for the demons to catch a CO2 molecule and hang on to it! The front-line demons have a real hard time catching the CO2 molecules and a lot more pass further down the line to be caught by the latter ones. Eventually, the front-line demons are full, but still the latter ones need to work to catch the CO2 and there will come a stage where there aren't enough latter ones who can catch the CO2 fast enough, so some will get through.

    Eventually so many will get through that the human starts to notice it and dies horribly as before - even when there are still some unhappy and empty CO2 demons left. Continuing our experiments with more abducted humans, we test again at 90m, but then we decide to ascend the human to some depth where the number of molecules in the loop is much less, so each breath the CO2 demons have more of a chance to catch the CO2 molecules left.

    Eventually, after 100's of trials, killing a great many humans every time,

    (And you should have seen our abduction budget! Off the scale!) we have come up with some rules for keeping humans alive and maximising the happiness of the CO2 demons. Our rules are many, long and complex but to simplify them for you humans we have reduced them to 3 simple rules..

    Rule 1: You have 3 hours maximum.

    Rule 2: For subsequent dives deeper than 20m: You must leave the bottom when the _total_time_ breathed through the system reaches 140 minutes.

    Rule 3: For subsequent dives deeper than 50m: You must leave the bottom when the _total_time_ breathed from the system reaches 100 minutes.

    I'm glad the Inspiration was machine tested at DERA. Glad I wasn't the human being killed every time. I wonder what other rebreathers would show given the same tests? I wonder why others don't bother with these tests, and instead resort to cycling in the garage with a unit on their backs.

    It's fairly obvious from reading above that things happen differently at depth. The deeper you go, the wider the reaction front becomes and eventually you'll run out of scrubber before the reaction front reaches the end.


    The above explanations are not a substitute for a training course, they are a very basic outline of how a rebreather works.
    Hopefully this helps a few people.

    Gareth


    [1] some units use computers, some units are manual
    [2] sofnalime is the most commonly specified and used scrubber material
     
  5. KevinNM

    KevinNM DIR Practitioner

    1,954
    698
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    There are all sorts of interesting other issues that can get you. For example see the linked set of papers in the thread:

    Gas density guidelines

    There is also a video of a gas density talk that I saw linked to somewhere on scubaboard that was somewhat explanatory about how you can get hypercapnia at depth while your scrubber is still working.

    Edit: it think it's this one
    vey interesting article by simon mitchell
     
    Last edited: Jun 15, 2017
    EricTheDood likes this.

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