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Cell linearity checks

Discussion in 'Rebreather Diving' started by JonG1, Dec 4, 2019.

  1. JonG1

    JonG1 Nassau Grouper

    # of Dives: 2,500 - 4,999
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    I have recently been reviewing my knowledge of cells and have read several useful threads on here. This prompted me to resurrect a thread on a UK board that I started some time ago.

    I am now a bit concerned that my understanding of the concept is flawed and would appreciate a sense check.

    It's probably easiest to understand by reading the following thread if you have time:

    Current Limiting and Linearity - Page 8
     
  2. taimen

    taimen ScubaBoard Supporter ScubaBoard Supporter

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  3. rjack321

    rjack321 Captain

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    The SB threads you linked on TDF had a bunch of posts by me so I thought I would try to clear this up. E.g. like this one
    Calibration question

    I have lost ppO2 output on a cell due to corrupted flash memory. I discovered it on the surface. I had mV but no ppO2 at all. I recalibrated and it fixed it. Sent the head in and they reflashed the memory for me but otherwise there was nothing "wrong". Normally I don't look at mV output on a dive.

    Regarding calculating what your "expected mVs should be" this is something I was taught twice. In my original TDI MOD1 with my Meg and again in my PSIA Kiss crossover. There is no way to confirm linearity in the water. You can confirm cells aren't current limited but this is not the same thing.

    Short of putting your cells in a pot, there is no way to test for linearity but there can be hints of a failing cell at calibration. There should be a 1:4.78 relationship (1/0.209 = 4.78) between mV in air and mV in 100% O2. If the mVs don't rise proportionally, the cell is out of spec. 100% should be 4.78x the mV of air. Calibrating an out of spec cell is masking a problem.

    To illustrate crudely:
    Green line is a "perfect cell"

    Orange is a good cell

    Red is an out of spec cell that is linear but not performing as adequately. A deviation of more than 3% between ideal and expected for this kind cell in 100% O2 at calibration will mean that you will get 1.3 on your handset but the true O2 is at ppO2 of 1.4 (not fatal but more than 3% is a reject). You can even spike the red cell to 1.5. It is not current limited in an immediately dangerous way

    Purple is linear then becomes current limited. It will never reach 1.3, 1.4 etc. The inflection point moves lower and lower as the cell gets worse and worse. Red type cells eventually turn into purple cells.

    Checking that mVs are as expected in 100% is an effort to catch cells behaving like the red one. Spiking to 1.4+ briefly on a dive is trying to catch cells like the purple one. There is rarely a "usual" cell but I have had far more cells gradually go out of spec with the red line shifting lower and lower vs detecting any pronounced inflection point from current limiting. I change all my cells at 12 months which is probably why I have never gotten to the point of having a purple cell, I have seen lots of cells gradually perform poorer and poorer as the red line shifts down - still linear just going out of spec compared to the ideal green line.

    The easiest way to check that your cell isn't "too bad" yet on the spectrum of red cells (assuming they don't fail abruptly, all good cells eventually go red like before they get to the really bad purple stage) is to confirm that your mV in 100% O2 is close to what they "should be" based on where they started in air. Since using a pressure pot isn't part of any MOD1 courses that I know of, that's why verifying that your cells are linear in the air to 100% range is part of calibrating in both my MOD1 and my crossover with different agencies and instructors. 3% deviation from expected sounds small but ends up being a shift of 0.1 ppO2 units after calibrating when you're trying to dive in the 1.2-1.4ish range.

    (ps I know there exceptions, the figure and explanations were an effort to simplify)
     

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    taimen, JonG1 and sunnyboy like this.
  4. JonG1

    JonG1 Nassau Grouper

    # of Dives: 2,500 - 4,999
    Location: Glossop UK
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    Thanks very much rjack for taking the time. I thought I had it straight in my head but I think that point that confused me in the threads listed was what I took to be an inference that divers were carrying the extrapolated mv data from calibration/bench checks on slates/tape to reference linearity during dives.

    Were you taught this math in your entry level CCR class?

    Post #80 for instance seems to infer this?

    I am also still not clear on why a full flush at say 1.4m with o2 couldn't be used to check linearity if you did have extrapolated mv data for that ppo2 point, not withstanding the small differences exerted by vapour, completeness of flush etc. How does this differ from effecting the same thing in a pressure pot, albeit that the pot would be sterile, dry etc.

    Arguably the in water check would be more akin to actual operating conditions and loop performance?
     
  5. rjack321

    rjack321 Captain

    # of Dives: 1,000 - 2,499
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    I do put a piece of tape on my handset with what the mV should be at 1.2. If I lose calibration but not the cell outputs (again,like I did on the surface 3 yrs ago) I can actually run the machine manually off that mV. I had a friend who ran his meg off 2 cells and a 3rd that gave mV but no ppO2 data for a week. He didnt know it was a really just a sign of lost calibration.


    Are the cells <exactly> 1.4m? Not your handset the cells.
    I can't do a full flush of anything even half of an exhale screws it up.
    Can you hold your breath long enough to allow the cells to stabilize?
    Can you do at least 4 or 5 full exchanges of the loop? (that's a lot to gas) without exhaling into the loop and negating your known gas?
    You need all that to see a difference from expected.

    The pot is small, a perfectly known gas, fully flushed, with enough time (30-60secs) for the mV to actually stabilize. I dont want to hold my breath. Can't do that complete of a flush (8+L) and can't hold the cells at that precise of a stop depth.

    Looking for a <3% difference from expected at 1ata 100% is a pot-free way to look at if the slope of your calibration line is going to be too flat due to cell age.

    Spiking to 1.4+ doesnt confirm your calibration is good, only that you dont have really bad cells like the purple ones.
     
    taimen likes this.
  6. Pandora's Box

    Pandora's Box Angel Fish

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    Some good stuff here and I think this thread clears up a lot of the confusion when divers talk about cells. The graph from rjack321 explains it in an instant. A cell can be both linear (i.e. mv response to o2 is a straight line in the portion that matters), and not limited (it can read higher than required for target max o2), but still be a bad cell if the linear slope deviates enough from ideal.

    One part I didn't grasp was:

    Should this be 3mv instead of 3%? Or maybe 3% when calibrated in Air?
     
  7. JonG1

    JonG1 Nassau Grouper

    # of Dives: 2,500 - 4,999
    Location: Glossop UK
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    So the same caveats apply in terms of a flush for dil, 02 flush at 6m etc. and there is an obvious disconnect between reality and theory but at a level mv could be used during dive provided the caveats are understood.
     
  8. rjack321

    rjack321 Captain

    # of Dives: 1,000 - 2,499
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    3 mV would be big (5-7%) in a cell that spans ~10-50mV air to 100%

    Hard for me to draw this but imagine
    The green line is what you want, the slope is 1:1
    The red line has a flatter slope, it works to point. What's that point for a red-like cell to fail and get tossed though?

    Imagine a cell's mV is 3% less than ideal at a 1ata, ppO2 = 1.0 during calibration.

    Go up from your 100% calibration point to a ppO2 of 1.3 and that 3% is magnified. The distance between red and green is getting bigger the further away from the 100% calibration point you get. All CCRs assume linearity although I think the liberty has a 3-multipoint calibration. All Shearwaters have a 1 point calibration, Megs have a 2 point. Not sure about the rest but nobody calibrates above 1ata so the principle that you are extrapolating above your cal point remains.

    0.3 bigger ppO2 is 30% more O2. Because the red slope is too flat that is your ppO2 is further away from the green line. Your calibration says you have ppO2 of 1.3. But because the red slope is too flat, the reality its a ppO2 is higher (exactly how much is squishy because the cell is rarely exactly the same mV to ppO2 slope throughout the measurement range)

    This is why if you go into the Narked@90 pressure pot spreadsheet they only allow a 2% deviation. I was taught 3% as a limit. my 1 year old cells right before retirement are usually right at 3-3.5% low up at a ppO2 of 1.8-2.0. If they were that low at ppO2 of 1 that would be pretty bad, they'd be 10%+ low at the high end of their range (1.8+) At that point they are starting to act more like the purple cell.
     
  9. rjack321

    rjack321 Captain

    # of Dives: 1,000 - 2,499
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    Get a pressure pot and then you dont have to try this during a dive. They only bother plotting above the calibration point but the first tab of their spreadsheet is doing exactly what my green vs red lines is doing. 1) Is the slope too flat? and 2) Is there a high end drop in output (like purple cell in my pic)
     

    Attached Files:

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  10. JonG1

    JonG1 Nassau Grouper

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    Location: Glossop UK
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    Doesn't Bobby reference an absolute max of 10% linearity deviation in his draft paper , subject to certain considerations in terms of dive, set point etc.
     

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