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Stack timer question (rebreather use)

Discussion in 'Shearwater Research' started by broncobowsher, Jan 20, 2019.

  1. JohnnyC

    JohnnyC Divemaster

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    I don’t have a problem with the current implementation of the stack timer. The timeline for breakthrough is not a hard line in the sand. There are so many variables that relying on a timer as anything other than a VERY general indication of time on the lime isn’t a good practice to get into.

    That being said, the amount of time on the loop outside of an actual dive is, for all intents and purposes, insignificant in terms of the overall life of the scrubber fill.
     
    rjack321 and doctormike like this.
  2. KenGordon

    KenGordon Rebreather Pilot

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    How about counting time when the solenoid is going off? Eg any minute when it activates was a minute equivalent to diving. Or n activations within p minutes. Does the handset know when the solenoid is activated?
     
  3. Kevin A.

    Kevin A. Rebreather Pilot

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    I run the stack timer 'count when on' on the stand alone only.. which is basically running just when I'm about to dive (breathing phases) and few irrelevant minutes to do any changes/updates etc.. well, not sure if it'll prove useful one day, but the little extra fiddle to have these 'warnings' there seems legit, for the time being..
     
  4. KenGordon

    KenGordon Rebreather Pilot

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    I don’t know, I’d call that comment dismissive and unhelpful so he was justified in biting back.
     
  5. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    Neilwood and tbone1004 like this.
  6. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

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    how is asking a question dismissive?
    Stack timer is useful, but as we all know, scrubber duration is based on quantity of CO2 absorbed. It can take 20 minutes, or 20 days, it may be 18 minutes or 18 days if it's cold. The computer doesn't know any of that so they just count down a timer that you arbitrarily set based on your knowledge of the scrubber. If you are doing extended prebreathes, then factor that into the timer.
    I pointed out an issue with his suggestion for starting the timer when .19 is turned off. I said that because you may prebreathe on .19, you may just not turn it off. If @Shearwater is going to implement a change to their computers, it has to be something that isn't going to cause them a liability issue. Having it set the way they have it now is fairly idiot proof. If the computer is on, we count. If the rebreather is diving, we count. The real important part is the dive, and unless you do a truly extensive prebreathe at the surface, you aren't going to have anything significant to worry about. The suggestion of turning off .19 means that you could go an entire dive without the stack timer ticking down and that brings on risk.
    If there is a way to implement it that doesn't remove the computers ability to auto-start for the important part, then it may be worthwhile. I.e. his suggestion could work provided there is an auto start as soon as the sensors detect depth, or even if there was an option to manually start the stack timer ahead of the "dive" mode, but I wouldn't implement a third option without some sort of override.
    The unintended consequences of a "mode" need to be taken into account during evaluation of suggestions and that is what I was pointing out.
     
    doctormike likes this.
  7. w3dge

    w3dge Nassau Grouper

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    I'd suggest that any accuracy gain in the Stack Timer implementation would be lost in the noise of inaccuracy around scrubber lifetime anyway, and wouldn't be worth the added complexity.
     
  8. JohnnyC

    JohnnyC Divemaster

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    To be a frank, I'm sort of surprised Shearwater even implemented a non-sensor based scrubber "monitor" at all. I can see someone going, "hey, it says I've got 4 hours left! woohoo!" then ignores all of the signs of a CO2 issue because, "I've still got time left!"

    That being said, if a person is unwilling to bail out irrespective of what the little tv on their wrist is saying, that's their own problem, but I'm kind of surprised they went with it anyway. Keep in mind people dive on 2 year old cells, don't check for linearity or limiting of their sensors, etc. I wouldn't be surprised if these are the type of people that look at a stack timer as an excuse, not a safety feature.

    I enjoy having the feature though.
     
    tbone1004 likes this.
  9. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    RIght, that was the problem I had with overthinking the "accuracy" of any implementation of a stack timer. It's a simple reminder, that's all. I was taught to just time my scrubber duration, and not try to be too clever about it, in the absence of any sort of CO2 monitoring system.

    So this does that. I don't bother nickel and diming the system to get another 15 minutes that the scrubber "owes" me.

    Sort of like how people use the term "undeserved hit", to refer to DCS that you got, even though your magic bracelet said you were fine.
     
    tbone1004 likes this.
  10. Kevin A.

    Kevin A. Rebreather Pilot

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    Location: Malta
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    Indeed, it is a simple reminder, but we have grown to trust our 'magic bracelets'. I don't think it's meant to replace common sense and reason when it comes to scrubber time. The good thing; a warning at -1hr and alarm -30min, even if it's not factual, will drive the point home better than just a mental reminder. "...I've set up this timer, am I going to ignore it now?"
     

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