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  1. #11
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    Paul Raymakers did some very interesting simulations here http://www.revo-rebreathers.com/uplo...en_sensors.pdf of the different strategies, and I think makes a convincing case for staggered swapping to minimise the risk of simultaneous cell failure. I have the n@90 cell checker and use this to determine which cell is the weakest, and to verify all cells are readin linearly and not current limited before each trip, and I err on the side of caution. It's always painful getting rid of perfectly good cells but, given the other thread re ox tox, am happy to spend money to reduce theoretical risks as far as I can.

    I read recently, and I imagine it was on here or the other place, of a 3 cell rebreather where 2 cells failed simultaneously, and the two that failed were closest to each other and hence out voted the third good cell.

  2. #12
    Old but keen Mark Chase's Avatar
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    Personaly I have in the past changed cells on a 6 month rotation swaping one and then two.

    So 6 months in I replace cell 2 and six months later replace cells 1 & 3 then repet replacing cell 2 6 months later again.

    The reasion behind this was I was keeping at least one tried and tested cell and not replacing all three with cells which could have a batch fault.

    However since the great cell disaster of 2011 I have changed stratagy to replaceing one cell at a time. And yes this meens a cel will get to 18months old. And i have done this because prior to the launch of the reliable Narked @ 90 cells I was using my old BUD cells from AP and waiting wating for new cells.

    In the end i discoverd two things.

    1: Cells last 18months + no problem

    2: When an old cell fails its no diferent to when a new APD rubbish cell fails.


    As a result I have begun to question the logic of just arbitrerialy changing cells at 12 months. I have for the last two years just swaped out cells which start to run hot or have failed.

    I have had more troubble putting in new cells than i have ever had with old cells failing.

    ATB

    Mark

  3. #13
    rEvo-lution iamyourgasman's Avatar
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    Changing 02 Cells

    I also use the staggered method as I have 5 cells in the rEvo, replacing them at once is not a viable option. I just changed a perfectly good one last week (it went in 07/2011! so had a good few hours out of it). As a result currently all my cells are <12months for a few more days until the oldest will celebrate its first birthday and 3 are <6 months. My next scheduled change will be in August/September. If any of the five will fail, I'd be quite happy just to run the unit on 4 cells, but I can still put the perfectly good old one back (and probably watch it dying on the first dive!) if I wish,until I got a replacement cell from Paul.

  4. #14
    Purine hurts my feet Major Clanger's Avatar
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    Changing 02 Cells

    Quote Originally Posted by Stonybonytony View Post
    Paul Raymakers did some very interesting simulations here http://www.revo-rebreathers.com/uplo...en_sensors.pdf of the different strategies, and I think makes a convincing case for staggered swapping to cell.
    My OP was made after reading that article elsewhere and my views about deep tox hits and maximising avoidance. To me, the staggered approach has merit so will be changing my replacement procedure to try to minimise the impact of insidious cell behaviour. Was interested in what others do.
    GaryL

  5. #15
    Old but keen Mark Chase's Avatar
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    Quote Originally Posted by Major Clanger View Post
    My OP was made after reading that article elsewhere and my views about deep tox hits and maximising avoidance. To me, the staggered approach has merit so will be changing my replacement procedure to try to minimise the impact of insidious cell behaviour. Was interested in what others do.



    To be honest the easiest thing to do is just buy Narked @ 90 cells and go diving

    And this isnt because i hate APD. I used APD cells exclusivly right up untill Alex Dias screwed it all up for us. I did that because the old APD cells were very reliable. The new ones are a disaster and i wont dive them till they sort it all out.

    Only good thing about the new APD cells is it teaching us all how to cope with cell failure and making us much more vigulent

  6. #16
    Purine hurts my feet Major Clanger's Avatar
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    Changing 02 Cells

    Quote Originally Posted by Mark Chase View Post
    To be honest the easiest thing to do is just buy Narked @ 90 cells and go diving
    I would do if I didn't have a meg and buy direct from ISC Good cells so far, barely dropped a mV in a year.
    GaryL

  7. #17
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    What struck me about Paul's paper, is that the risk of 3 of 5 cells failing in the simulation was 6 in a million. I can cope with 1 cell fail, no problem, and 2 is probably ok, though complicated in they were both on the shearwater and hence would affect automatic oxygen injection, but 3 and I wouldn't even have a trustable majority, and would effectively have no idea what the ppo2 was. At this point I would of course bail or at least go to scr mode depending where it was/ what I was doing. Though Sod's law it would happen right in the middle of some other crisis when brain is already busy dealing with something else.

    Anyhow, 6 in a million, 1 in 166,000, doesn't sound bad. But if I do 50 dives a year, then it's a 1 in 3,000 risk in any year. And if I dive for the next 25 years, then there's a 1 in 133 chance of experiencing a 3 cell failure at some point. Which to me starts too look much too much like "something that could happen".

    There is an apocryphal story about a Boeing executive who only flew on 4 engined planes. When asked why, he replied "because we don't make 6 engined planes"

  8. #18
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    ...though I'm thinking of cell failure as "giving an incorrect reading" rather than reading zero, which I think may be Paul's definition of failure. If 3 cells fail and read zero, at least it is obvious to ignore them, and rely on the two remaining cells.

  9. #19
    Old but keen Mark Chase's Avatar
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    Quote Originally Posted by Stonybonytony View Post
    ...though I'm thinking of cell failure as "giving an incorrect reading" rather than reading zero, which I think may be Paul's definition of failure. If 3 cells fail and read zero, at least it is obvious to ignore them, and rely on the two remaining cells.



    Thats more the point.

    I have had cells read zero but usualy pre dive. i have never had a cell fail to zero which i properly tested during the set up and during the dive its self.

    I have had cells drift out of acepted parity with other cells, I have had slow reacting cells and i have had cells which wouldent read higher than X PP02.


    None of thease events caused me to go off loop during a dive. I meerly did the necessary testing to find out which cell was relaible and ended the dive on that cel or cells. I have had cells that wouldent read above 1.5pp02 so i just did the dive on 1.3 and regularly checked i could get 1.4 if i needed too.

    WHen i first started diving CCR I was most feerfull of 02 levels and cell issues.

    Now they hardly bother me at all and I only have one feer of CCR, and that is C02.

    Cell issues can be checked difinitivly, even mid dive and the results can be worked arround, even if the work arround results in the decision to go OC and go home.


    ATB

    Mark

  10. #20
    Nicotine, valium, vicodin... notdeadyet's Avatar
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    Re: Changing 02 Cells

    I used to stagger the cells on my KISS. The Mk15 cells are made to order from AII (not worth keeping inventory of) and I can't decide if that makes them more or less likely to suffer from bad batch syndrome as the more common cells seem to. They have been very steady and reliable unlike the AII KISS cells I tried. That said, I still stagger them.

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    Caliph Hamish Aw-Michty Ay-Ya-Bastard, Spiritual leader of Scottish State in England


 
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