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Weird Day of Diving with Petrels

Discussion in 'Shearwater Research' started by WetLens, Jun 3, 2015.

  1. BFRedrocks

    BFRedrocks ScubaBoard Sponsor ScubaBoard Sponsor

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    More interested in the second "issue" as using lithium energizers in my Petrel 2 has been fine (changed out the batteries that came with it the first day so I knew when I made a battery change). I'm going to assume that the Buhlman algorithm for NDL doesn't apply (or "zeros out") at a depth shallower than 40 feet, which is why the computer goes from "99" to whatever the NDL was below 40 feet once that depth is crossed.
     
  2. JohnnyC

    JohnnyC PADI Pro

    # of Dives: I'm a Fish!
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    The SAFT batteries don't work with the fuel gauge b/c of their discharge curve. It will show a full energy bar right up until the point the voltage bottoms out, then you get what happened with WetLens, where it shows low, then critical, all in the span of just a couple minutes. Using the energy bar as an indicator when using the SAFTs is worthless.

    Other batteries have a discharge curve that's more linear, so the energy bar will actually display the "fuel" level.
     
    Divin'Hoosier likes this.
  3. nakatomi

    nakatomi Solo Diver

    # of Dives: 1,000 - 2,499
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    I'm aware of that, that's why I try to swap them out before they die. The reason I mentioned that they still showed the full energy bar was to indicate that they were still in full working order when I pulled them.
    To me it was pretty impressive to get over 110 hours of diving from one battery.
     
  4. JohnnyC

    JohnnyC PADI Pro

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    You missed the point, "full working order" for two minutes is not "full working order." Hour 110 and 3 minutes and it might be dead. The energy bar is not an accurate metric to measure the usable life of the SAFT's at all. You cannot tell me that they were working with any degree of certainty other than at that specific point in time they had yet to trip the low voltage warning. That does not equate to "full working order"
     
  5. tcoen

    tcoen Shearwater Research

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    Hi,

    I have two things to respond to in this thread, so I'll start with the Saft battery warnings.

    We monitor the voltage level to determine the remaining capacity of the battery. This isn't perfect, but it works fairly well for most battery types. With the Safts (or any Li-SOCl2 chemistry battery), they have a very flat operating voltage plateau for most of their life, then die relatively quickly towards the end. By relatively quickly, I mean an operating life of maybe 100 hours, then dying over a few hours time. So it should be possible to give a warning a few hours in advance.

    However, with the Saft batteries, while the voltage plateau level is flat its actual level varies greatly with several factors. Temperature and current draw are two of the biggest factors affecting the voltage plateau. We need to detect a drop of about 0.15V to determine the battery is dying, but temperature and current draw can cause the voltage to fluctuate by over 0.3V! Fortunately, we can measure temperature and estimate current draw based on screen brightness and therefore compensate the warning voltages for these effects (this is why when someone asks at what voltage a Saft should be replaced we cannot give a simple number).

    The problem you are seeing where the Saft battery gauge indicator is dropping from full to the battery completely dying is probably because the current draw compensation is not tuned correctly. Improvements to efficiency have been made so current draw is now lower. However, the tuning has not been updated. Most of us at Shearwater are using 1.5V alkaline or 1.5V Energizer Advanced Lithium batteries since they offer better value, so we haven't noticed this (these battery types aren't nearly so sensitive to current draw).

    I do appreciate hearing about this. We are working on a firmware release cycle, so we will be sure to measure and adjust this tuning as needed.

    Best regards,
    Tyler Coen
    Shearwater Research

    ---------- Post added June 4th, 2015 at 07:27 PM ----------

    This response is regarding the NDL quickly dropping due to small depth variations during a slow ascent.

    We received the log files (thank you), and while we have not yet performed complete analysis, it does not look unreasonable to me. We will still perform detailed re-simulations and verify the NDL numbers are correct.

    The situation I believe is happening is this:
    - A dive is performed to around 90ft. You stay there for a while and NDL goes down because tissue nitrogen loading has gone up.
    - NDL gets down to a few minutes.
    - The ascent then starts.
    - As you get shallower, the NDL starts going up. However, it is important to remember for most of the tissue compartments, the nitrogen tissue loading is still pretty high. That is, you were almost in deco when at depth, so except for the fastest tissue compartments, most compartments still have significant nitrogen loading.
    - The ascent is slow, maybe looking at a reef, just taking some time. So some tissue compartments are still on-gassing, or just balanced around on-gassing and off-gassing.
    - You can get in situations where a bunch of factors are aligned, like the current tissue loading, on/off-gassing rates, ascent rate, M-values, etc. such that ascending or descending just a few feet can impact NDL significantly.
    - Note that this only happens when ascending towards the end of a dive and tissue loading is high. You don't see this near the start of a dive at these depths, because tissue loading is then low.

    Like I mentioned, we will of course perform a more detailed review to ensure there are no bugs or other problems here. This will take a few days. I just wanted to share my first analysis, which is that it seems reasonable and I have seen this before (during development simulations mostly).

    Please let me know if you have any questions.

    Best regards,
    Tyler Coen
    Shearwater Research
     
    Oldbear and undrwater like this.
  6. Oldbear

    Oldbear Teaching Neutral Diving

    # of Dives: 1,000 - 2,499
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    As I am reading Deco For Divers...your comments are making so much more sense to me. Thanks.
     
  7. nakatomi

    nakatomi Solo Diver

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    I completely understand that at some point of the cycle, the battery does become unpredictable and may die within a short period of time.
    And yes, I also understand that with the SAFT batteries the battery gauge indicator is not working properly due to the characteristics of the battery.
    My point was, once again, that the batteries had not died on me when I swapped them, nor had the gauge indicated they were going to die. I just swapped them to put in fresh batteries.
    Since my dives are in the same area as Aotus's, I figured I might share this information with him because I feel it could help him get a better prediction as to how long the SAFT batteries may last for him.
     
    Aotus likes this.
  8. clownfishsydney

    clownfishsydney Loggerhead Turtle

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    Re the second "problem", I have a OSTC 2N which runs identical Buhlmann algorithm. It sometimes does the same thing. My old Aladins also did this as well. Seems strange, but this is what happens on some dives where you are close to NDL but then go up a bit.
     
    Lorenzoid likes this.
  9. Stoo

    Stoo NAUI Instructor

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    And once again, Shearwater Research demonstrates it's amazing customer service...

    Can you even magine any other manufacturer engaging with a customer to discuss the specifics of some "oddities" in their device... Hell, other manufacturers require you go through 4 levels of distributors just to get through to them, where you will be met with silence...

    Nice!
     
  10. Jim Lapenta

    Jim Lapenta Dive Shop

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    This is why buying any other computer, unless you simply can't afford or don't really need it, than a shearwater is nuts. I've spoken with Tyler, Lynne, and Christina at shearwater and have always gotten amazing service. When I got my predator and downloaded the manual one of the first things that stood out was, and I'm paraphrasing, this computer will fail at some point so be ready. Honesty! Holy crap! They admit that stuff happens and to be ready for it. Look at the gibberish and evasive techniques used by other mfgs. There is a thread going on now about a "tech" computer that would have me taking my 12 gauge to it if I was getting the bs answers others are. Shearwater rocks! Their people are the best.
     
    bryanmc57, RickyF, Dr. Lecter and 4 others like this.

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