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Rebreather Question

Discussion in 'Rebreather Diving' started by Sidediver77, Jan 7, 2018.

  1. JohnnyC

    JohnnyC Divemaster

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    I didn't take it to mean surface, calibrate, and descend. I took it as end the dive. The recalibration would be at a later time, say, the next time you build the unit. Recalibrating with a wet cell will even further exacerbate non-linearity. Most likely meaning you'd never get a viable reading in air anyway. I'm not sure how it would mask anything since you're literally throwing every other reading out of whack at the same time since you're not doing a 2-point calibration.

    Guess it depends on what he means. I assumed what I did because no one in their right mind would re-dive a unit with obvious cell problems. Maybe I assume too much. Then again I'm not the kind of guy that throws his unit in the back of a truck, throws it in the corner of a garage, and picks it up in a week to go dive it without doing anything else to the unit.....
     
    Last edited: Jan 13, 2018
  2. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

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    Fair enough but let's consider what a calibration actually does. We have a cell which has an expected output of between 8mv and 13mv when exposed to air. These numbers should accurately reflect the partial pressure of oxygen the cell is exposed to but does not reflect anything meaningful to a diver. When we calibrate a cell we tell the computer that 8mv = .21 PO2. That number is meaningful to the diver. When we calibrate that cell in O2 at one atmosphere we get 38.08mv from the cell but the display shows 1.0 PO2. If the cell is outputting 13mv in air or 61.9 mv in O2 those numbers don't mean much until we tell the computer what those numbers mean in PO2.
    That is a multiplier value that is applied to the reading so the diver can understand it. That number does not change from dive to dive. If your cells vary by any significant amount from dive to dive you have a cell problem not a calibration problem. The multiplier value does not change, the cells have. Constantly changing cells are a big concern and I personally would scrap cells that were't stable.
    As far as 2 point calibration goes I have had several discussions with Leon about it and my belief that it masks problems, not fix them. I am happy to report ISC no longer uses 2 point calibration (not on my account) but I'm sure a number of rebreather luminaries have shared my thoughts on the matter.
     
  3. JohnnyC

    JohnnyC Divemaster

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    Here's the issue I have with your math though, as it pertains to a wet cell. We're never calibrating anything in air. The computer doesn't know whether it's 8mV or 13mV or anything in between or maybe even higher until we give it a reference value. O2 cells come in a variety of mV outputs in air, some as high as 27mV!!!

    Without giving it an air reference value, its air mV level is meaningless to the computer, and only partially meaningful to the diver (we already know we're breathing air at a correctable atmospheric pressure). Essentially, we're doing a mental 2-point calibration, the computer is just not using the initial value as a reference. So, how do we make sure that what our cells see, is what our computer is telling us. Easy, give it 100% (or close to it) O2 at 1 atmosphere of pressure, and tell the computer that this is O2. Great, so now the computer knows what O2 is, and will extrapolate down to air based mV values and pressure. Think about a dive computer at altitude. It displays a lower PO2, even though the FO2 of air is the same, because the computer is reading the lower mV output of the cells due to decreased atmospheric pressure. However, it knows the "loop" FO2 of the gas because it knows it's at altitude, and using that 2-point reference of known O2 and pressure, can accurately display PO2. So now we get to the issue with the wet cell re-calibration.

    The computer never knows what mV value the cells are in air, they only know what mV value the cell is in O2 (remember we're only single-point calibrating). So if you recalibrate your computer, it now has a new reference value for O2, thus changing the multiplier value, thus changing the value that it would display for air as well. So, recalibrate with a wet cell, do a dil flush, and you won't get .21 on the surface, thereby confirming the failure state of your cells. In this situation, a 2-point calibration would mask a wet cell issue, however as long as the cell is not current limited, it will adjust linearity accordingly, and as long as the cell stays wet, it will be accurate. Of course as soon as the face is clean and dry, your entire calibration goes out the window. Whether or not 2-point calibration is a useful thing is purely a matter of opinion, although I recognize your reasons for disliking the procedure. Anyway.....

    If you recalibrate with cells that are in some form of error state on the surface (wet, electrolyte leak, etc.) and try to validate against air, it will show failure. If you try and validate cells with an O2 flush down to 6m depth (again, we're assuming some semblance of intelligence in the diver), it will show failure, whether it be current limited, calibration is out, whatever the failure state is. This is why having a known-good calibration is so important, and why a recalibration where cell health is in question can be beneficial for confirming discrepancies in cell health. BUT, it MUST be followed up with proper validation procedures.

    Thoughts?
     
  4. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

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    two points;
    1. the computer saves the calibration value when you turn it off so if there is a need to calibrate before each dive you have cell issues.

    2. a calibration, validation, whatever will not show current limiting in a 1ata environment. You need a pressure pot to test for the early (and most dangerous) stages of current limiting.
     
  5. joshk

    joshk Nassau Grouper

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    Why not validate at 20' on the descent?
     
  6. JohnnyC

    JohnnyC Divemaster

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    Yes, i’m aware of both. However as cells age their characteristics change, and can require recalibration. I have yet to see a cell that will hold calibration for a year. And according to the AI engineers I spoke to when I had a wonky cell out of the box, this is expected behavior.They continuously generat current, whether you’re using them or not. As the electrolyte is consumed, output voltages change. As long as a cell calibrates and can be spiked beyond max PO2 and is not current limited this is not an issue.

    And we can validate cell limiting by flushing the loop with O2 at a safe depth that will spike the PO2 beyond normal bottom PO2 limits, as well as spiking them beyond deco limits. 1.8 for A few seconds isn’t going to tox a diver at the beginning of the descent, and I don’t know anyone who doesn’t validate their cells to 1.6 at 6m on the initial descent, and many who go to 1.7 or 1.8 to make sure they are not current limited. Of course you can always have a cell failure during a dive. We hedge our bets against this by running multiple O2 cells and validating with dil and O2 spikes during the dive.

    What I’d like to hear is your thoughts on how a recalibration with wet cells, which will drastically alter the computers multiplier value, will not manifest itself during normal cell validation checks.
     
  7. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

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    It is a good method of validating a cell but does not work well with my style of diving. When starting a dive I like to get where I'm going fast so having a pure O2 loop at the start of the dive will severely limit me descent rate. I do however flush with O2 at 20ft on my ascent as I will not only validate my cells but it helps with my deco.
     
  8. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

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    I'm not really sure what you're asking here. Why would you recalibrate wet cells? I won't.
     
  9. Sidediver77

    Sidediver77 Angel Fish

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    These questions are just fun easy questions for rebreather divers. I know these answers. I took the Tec 40 ccr course.
     
    Last edited: Jan 14, 2018
  10. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

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    Dr Doug, I don't think you're the a##hole here.
    I'm done
     
    rjack321 likes this.

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