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One cell slow to wake up. Your thoughts?

Discussion in 'Rebreather Diving' started by doctormike, Nov 5, 2019.

  1. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    OK, question for the group. I don’t have the head with me, so I don’t have the raw data download to post, but here is the general issue.

    JJ CCR, three cells from the same batch, dated March, 2019

    Typically when I build the unit and turn on the controller with the head exposed to air, I get something like this for mV output of cells 1, 2 and 3:

    10.6, 10.6, 9.6

    I calibrate with 100% O2, and while cell number 3 seems to read a bit higher during the dive (“waking up”?), it stays within acceptable range, so no issues.

    Last week, I got 10.6, 10.6, 9.0. on the build. Still within acceptable range. I calibrated with 100% O2, no problem. mV on calibration was 47.4, 47.5 and 40.7 (all a bit below linear, but not too far off).

    During the dive, cell 3 started reading fairly high PO2 (something like 1.3, 1.3, 1.47). I didn’t get a great dil flush, so I thumbed the dive. I figured that the third cell had woken up and started delivering voltage more in line with the other two, so the calibration earlier was giving me a high reading.

    On the boat I pulled the head and let it dry a bit. Then I reassembled it - now all three cells were reading around the same in air (10.6). Calibrated with 100% O2, all good, and went for a dive - no issues, all three cells held set point well and very close to each other.

    So my question is, should I replace cell #3, or change my build procedure to try to get it to wake up before doing the pre-dive calibration? I was thinking about exposing it to 100% O2 for a bit with the head open, and then waiting to see if the mV in air rises to the same general range as #1 and #2, and then doing the calibration after that.

    I have a mini-check from Narked at 90, so when I get back to where the rebreather is I could also check linearity at higher PO2, wasn’t able to do that after last weekend…
     
  2. _Ralph

    _Ralph Barracuda

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    With those mV results, I would have recalibrated as cells 1&2 were more than 2mV below the expected mV value ... per my training on the JJ.. cell 3 was pretty darn close too..

    HOWEVER, that wouldn’t do me or you any good with cell 3. It checked out to a reasonable mV with O2, but of course, it changed its output since calibration.

    How long between your initial calibration and your prebreath, and did you notice the difference there?

    _R
     
  3. Dusty123

    Dusty123 Angel Fish

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    maybe try rotating the cells around just to make sure the problem follows the cell and isn't a cable connector issue
     
  4. MegDiver792

    MegDiver792 Nassau Grouper

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    40mv at 1.0 PPO is pretty low. I get a cell below 44 I start watching it carefully.. Remember a new cell at 1.0 should be close to 50mv or slightly above.
     
  5. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    It's not an absolute number, it's a percentage. Specs for the cells from the JJ manual are a little tighter than elsewhere, but they are 9-13 mV in air, and I was taught about 5% off linearity at PO2 of 1.0. I have been calibrating cells on this unit for two years, Vandagraph and AI, and I almost always get about 5-6% off, throughout their service cycle. I have the calibrations in a spreadsheet, and they always come out about the same. Not sure what else to do about that, but seems reasonable.

    So for the cell showing 9.0 mV in air, linearity would be 43.0, and 40.7 is about 5.26% off (actually calibrates at 1.01).

    But this is a bit off topic. The question is what to do about the fact that two cells are stable and one "wakes up" at some point...
     
  6. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    Good question and I can't remember exactly, but 3 was a bit higher. Not as much of a difference at 0.7 as during the dive at 1.3. I did the calibration at around 11 PM, and prebreathe was probably around 9 AM the next day.

    As far as cell 1 and 2, no matter how often I calibrate, I get about 5% off linearity at 1.0 (same for #3).
     
  7. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    Good point, but not sure why the cable would wake up....
     
  8. MegDiver792

    MegDiver792 Nassau Grouper

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    and coincidentally, which cell is acting up? Oh, cell 3. Imagine that.

    Its right at the bottom range on air, 20% below new at 1.0. I'd have already been contacting someone for warranty replacement.
     
  9. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

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    I don't understand why cells that are exposed to air need to "wake up". Unless you are storing them in an anoxic environment the cells will already be "awake"
     
    sunnyboy likes this.
  10. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    What's "new"? Cell output is a range. JJ manual specifies 9-13, but many cells are 8-13. It's 5.26% off linearity, not 20%. And after O2 exposure, it's reading 10.6 in air, that's the point of my question.

    But I guess you are voting for replacement? I'm not saying that's a bad idea, I'm just trying to think this through. Will check it again next weekend on air to see if it goes back to the 9.6 that it has been at for the past 3 months.... obviously if it drifts down out of spec, it's going in the trash. Or maybe I'll see about a warrantee replacement!
     

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