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Perdix Ndls

Discussion in 'Shearwater Research' started by Bigeclipse, Apr 20, 2016.

  1. KenGordon

    KenGordon Rebreather Pilot

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    Good post.

    Don't be obsessed with NDL, it is tangential to the conservatism as it depends on the ascent rate, whether the programmer takes the ascent into account and which limit to saturation they choose.

    I suggest you set the perdix to any value, then dive the other one to NDL but do the stops the perdix gives you. Then you will be confident that your risk is the same as before but get used to doing stops. You may find that those stops are very short when you actually get to them.
     
    Last edited: Apr 22, 2016
  2. vixtor

    vixtor Solo Diver

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    This is what I'm talking about. Below, a recreational profile on EAN32, which for Suunto was a NDL dive. With the Petrel, at minute 17 I would have been already into deco, but nevertheless at the end of the dive I would have been able to get out of the water at the same time as I really did. So NDL is not important, total run time is.
    Screenshot.png

    Another example. This time it was a dive made with trimix, for which at that moment I did not had a computer, so it was performed with printed schedule. Still, the deco obligation of the Petrel 30/70 would have been comparable (in fact shorter, which is to be expected from a computer). Again, this setting proves to be alright for my expectations:
    Screenshot-1.png

    In the end, by doing this, I have discovered that every and every time, the 30/70 setting would get me out of the water almost at the same time as my old Suunto did in the end. Yes, the computer would show a deco obligation which the Suunto did not, but still, it would take me out of the water, deco obligation or not, at the same minute. And for my current expectations this is what I want to happen. I don't want more conservative, because I'm comparing with what is already considered a very conservative computer. I don't need more aggressive, because right now, with the size of tanks that I'm using, available gas is the really limiting factor. Also, even in a drysuit, I am getting cold towards the end of the dive, because most of the time surface temperature is very hot (like 40 degrees celsius) and water is very cold (6-7 degrees celsius), so no matter what I do, I am already sweating when I jump in the water, which really doesn't help with the cold at the bottom. So, as I don't want longer dives, I am really happy with the 30/70 setting.
     
  3. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

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    so the TLDR version of that is your Suunto is assuming you're actually going to do your safety stops and it calculates that into your NDL times, the Perdix assumes you are making a straight ascent at 30ft/min. The reason that our advice to change the GF's based on the variables helpful is because you set a top risk tolerance, for me that is GF85, and I won't go above that. Depending on the myriad of variables that are considered, that GF may go down depending on my risk aversion that day. IT is important because when you choose a GF, you have to understand why you chose it instead of just randomly picking one based on low/med/high conservatism
     
  4. vixtor

    vixtor Solo Diver

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    The biggest problem when you lower the GF is that all you can know is that lower might be safer (even if sometimes too low might create other issues). Nobody knows statistically how much lower is how much safer (for example, you just have no idea whether decreasing the GFhi from 85 to 70 will reduce your DCS risc from 0.001% to 0.0001% or to 0.00001%). There just isn't enough data to know this. Further more, there is not even a common scientific view about the real DCS cause after all. And even if some attempts were made to build databases of dives, in order to get such insight, my guess is that they aren't accurate enough (because not all dives get reported, to know for sure the real fail rate of some specific setting).

    So you can understand why you want to lower it, but there's no way you can say that you understand why you have chosen exactly that specific value over another nearby value.
     
  5. KenGordon

    KenGordon Rebreather Pilot

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    Can you provide some evidence to support those assertions?

    I think we can only guess about the implementations of either computer's NDL calculations.

    I'd try to compare my NDL calcs to those of my petrel but I am normally overcome with an urge to slit my wrists when I try planning on it - I have to change to OC mode, gas, disable a bunch of gases etc etc, all with risk of passing the menu item and having to go round again.

    If you want to provide a set of NDL times for a few depths (metric please) I'll see how they line up. A mix of low and high gf would be interesting too. Which do you believe they use to limit surfacing?
     
  6. uncfnp

    uncfnp Solo Diver

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    and personal factors. My Petrel is set so conservative I have to dive in tec mode even during recreational dives. My partner was accustomed to pure DSAT with never an issue. He dives on the low conservative rec mode.
     
  7. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

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    I can't, but I was responding to the guy who apparently ran the profiles using those algorithms.
     
  8. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

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    you can't say anything statistically, but if you are going to make that statement, what is the point of gradient factors in the first place? or NDL's for that matter, your Suunto is calculating completely different decompression profiles than your Shearwater and you say it doesn't matter, so why dive a computer at all? Why bother to choose which decompression algorithm makes the most sense to you, etc etc?
     
    Last edited: Apr 22, 2016
    KWS likes this.
  9. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    I see it every time I do a recreational dive with my Perdix or Petrel. I have it set on tech mode, with a 40/70 GF setting. It gives me a NDL countdown, just as a Suunto will. When I exceed it and go into deco, it tells me where I have to do a deco stop and for how long--10 feet for 1 minute. If I go further into deco, it will eventually require me to do what a Suunto would call a 3 minute safety stop.
     
  10. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    One more point--having a lower first number in your gradient factor is considered being more conservative. Why exactly is that? I personally changed from the default 30 to 40 thinking that 40 is safer than 30, and I did not make that decision out of the blue. I may even go higher than that for the same reason. I guess I am not sure what people mean by "conservative."
     

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