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Air Integration & NDL Accuracy?

Discussion in 'Shearwater Research' started by CaveSloth, Nov 12, 2019.

  1. ls23

    ls23 DIR Practitioner

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    @Jan_oi Independently from your computer you should know the minimum gas for the depth you are diving to. This should account for some problem solving time at depth and bringing an out of gas buddy up with you. Going over NDL a little bit while solving a problem won't be critical and the deco will likely clear on the way up. If you plan to go into deco you should account for that in your gas planning.
     
  2. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    I believe so but it uses a different algorythum. there a like 5 models and buhlmen has a dozen versions for different purposes. uwatec has kind of its own form what i have read.
     
  3. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    ;to the op some computers with AI monitor gass usage in PSI and compute how much time you have left based on what you set up for on the boat pressure

    one of my puters has dive time remaining time and it displays teh shorter of NDL O2 level, consumption rate. sopa tp100 ft it may say i have 5 in left based on NDL and then got to 40 ft and it says i have 20 minutes based on tank psi.
     
  4. Caveeagle

    Caveeagle Rebreather Pilot

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    The old uwatec Aladdin AirX (mid 90s) claimed to track your gas consumption and adjust your NDL based on an assumed workload.

    This is a very interesting idea, but I think you run a serious risk of layering way too many assumptions that could end up making a computer less safe.

    I am far more interested in technology that can track live biometrics and use some sort of voting logic to give me alerts. Some of the new Doppler tools for post dive analysis are pretty cool, but I’m not sure we have figured out how the typical diver can benefit from this data yet.

    If I had a magic wand... I would stick with by existing algorithm based computer model, but have some sort of “onboard” wearable sensor that could give me an independent alert if I was Fizzing too much for a safe ascent.
     
  5. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    I'm sorry, maybe I don't understand the question.

    You are diving single tank. You reach NDL at 40m. The amount of time that I would recommend that you reasonably stay at 40m would be zero, since you have no redundant gas supply to safely do staged decompression.

    Am I missing something?
     
  6. Steve_C

    Steve_C Divemaster

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    Ok, true case, instabuddy looses mouth piece and cannot find her octo since in a new location. Put her onto mine for a few minutes while we sort things out. Put her back on her octo and we continue the dive. If our computed NDLs had anything to do with air consumption they would both be wrong.
     
  7. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

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    From Deco for Divers, Mark Powell, 2014, pp 42-43:
    Unlike the first two models, which used 16 compartments, the newer model ZH-L8 ADT uses only 8 compartments. It also includes the effects of temperature and work of breathing rate during the dive as well as taking into account microbubble formation. Unfortunately, the ZH-L8 ADT model is described only very superficially in the latest edition of Buhlmann's book and doesn't include many details of the parameters of the model. The model was adopted by Uwatec and is the basis for their range of dive computers, as such, it has been extensively dived by recreational divers around the world.

    Here is a description of Human Factor Diving from Scubapro: https://ww2.scubapro.com/media/382612/human_factor_diving.pdf

    Here is a section of the G2 owners manual discussing the workload calculation and that it can be turned off: I can't find any information regarding specifically how heart rate, breathing rate, or skin temperature are used to adjust the baseline decompression algorithm.
    upload_2019-12-9_11-11-40.png

    From my own examination of several ScubaLab computer tests of 4 simulated dives in a hyperbaric chamber, the baseline performance of the Scubapro ZH-L8 ADT MB or ZH-L16 ADT MB decompression algorithm appears to be middle of the road to conservative. In 2017, ZH-L16 ADT MB was consistently in the conservative group along with Cressi and Mares.

     
    Andrei Astra likes this.
  8. Belzelbub

    Belzelbub Barracuda

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    I understand what you are asking, but as has been said, the time is zero. Unless you planned to do a decompression dive, and are equipped and trained to do so, you shouldn't be doing that. As you know, the Perdix turns off GTR when you have a deco obligation. I think that's Shearwater's subtle way to tell you that you've overstayed your welcome at this depth and need to complete your deco obligation and get shallower. That's the correct answer.

    Now, you can switch to OC tech mode during the dive, but you should have some familiarity with this before hand. Also, you can use the Dive Planner+ in Dive setup to plan a deco dive. Again, it would be advised to get training in deco diving before planning one. Plus, the time to plan a deco dive is not when you've exceeded the NDL. The time to plan it is well in advance.

    I gather that your question was more around gas supply since you referenced GTR off. Gas supply is no longer the limiting factor on this dive, so is irrelevant provided you have enough gas to get you through your stops and all the way to the surface. GTR measures the amount of time you can stay at your current depth without getting into your reserve. So, as long as you had time here before going into deco, you should be good. But again, your time remaining to wait here is zero. You haven't planned as a deco dive, so shouldn't linger any longer.

    In your second scenario, you qualified it with an issue with your buddy that you had to deal with. Again, same answer. Your time at that depth is expired, and need to make a proper ascent now.
     
    FreeFlyFreak likes this.
  9. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    I do not understand why people who plan to go to 40m and possibly exceed NDL also use REC mode.
     
  10. KenGordon

    KenGordon Rebreather Pilot

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    Well, 32% is too rich for 40m. 25 or air. I expect a no stop time of about 8 minutes, but you are using a computer designed to introduce deep stops. I am also uncomfortable with a single at that depth, with diving that much nitrogen and with having a buddy on a single that cannot look after themselves.

    I did a dive to 34m on air recently and had to think hard about wither to use a twinset (simpler for my buddy) or a rebreather (more protective for me). I decided to take the rebreather, left the trimix at home and regretted it as it took me ages to sort out a simple issue with a dropped SMB. On the up side I had a pleasant warm feeling from the buzz... when I lost my buddy all I had to worry about was excuses for the coroner, I knew I was ok as I had two sources of gas.

    There is an impression given that 40m is not terribly deep, people will claim, with a staring face, that OW divers are qualified to 40m. Really 40m is a serious depth and should be regarded with care and respect.

    No dive to any depth for 3 or 6 minutes before having to ascend is sensible. If there is something worth seeing then do the training it takes to see it, use the kit required and don’t push your luck. Something like 99.999% of stupid dives don’t kill anyone, but that doesn’t mean they are clever, just the numbers did not come up that day, next week might not be the same.

    If you want to see schooling hammerheads, learn to use a twinset and go to the southern Red Sea. Get plenty of gas, no worries and loads of time. Or go shallower.
     
    Andrei Astra and FreeFlyFreak like this.

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