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Petrel users have you been able to closely match NDL times with other PDC''s?

Discussion in 'Computers, Gauges, Watches and Analyzers' started by Nasser, Apr 15, 2015.

  1. Nasser

    Nasser ScubaBoard Supporter ScubaBoard Supporter

    I'll be doing some NDL dives over a few days with a buddy who's on a Suunto Vyper and was wondering if anyone has actually been able to closely match NDL times on their Petrel with other dive comuters?

    I understand that computers using different algorithms will have different NDL times and levels of conservatism, but I'm trying to dial in GFs on my Petrel to come close to the NDL's of a Suunto Vyper/Zoop.

    I saw a thread here on SB where someone put up the NDLs of a Petrel on GF 45/95 against Suunto NDL's (with no conservative settings added) and to my surprise the Petrel was more conservative than the Suunto at some depths. I don't know how that will figure on repetitive dives as the Suunto's do get more conservative.

    For the record, I feel fine diving GF's of 45/95 (Petrel's least conservative setting in OC Rec mode) ... although I'm actually diving in OC Tec mode (and yes I do understand how GF's work and using the LO GF on deco dives to generate deep stop depths).

    Has anyone compared these two PDC's in real diving situations - not NDL planers or charts - over several repetitive NDL dives?
  2. victorzamora

    victorzamora Solo Diver

    Matching NDLs to Buhlmann-based computers is typically a piece of cake. I go to "plan" mode on my Rec computer, get a range of depths, and fiddle with my Petrel until I get the same results. Understanding how GFs work is the crucial component there.

    The problem lies with Suunto, though. Their algorithm is based on a bubble-model, and they have a BUNCH of arbitrary factors thrown in on top of them. So, on that particular dive the Petrel might have been more conservative.....but had you have done a reverse profile or an aggressive saw-tooth profile the Suunto would penalize you without you realizing it. It's hard to match a computer when you can never know what it's really doing.
  3. sheeper

    sheeper Public Safety Diver

    # of Dives: 5,000 - ∞
    Location: Vero Beach, Florida, United States
    I've not had much luck matching the Petrel to other people's computers on NDL dives. Suunto seems the hardest to match up for reasons mentioned above.
    The Suunto are more conservative especially to the Petrel's rec mode (depending on how the GF is set). What I do is agree to follow someone's lead. So i'll hang a bit longer with my Suunto friends on their safety stop even though I met my obligation. after all, a 4 minute stop is not "worse" than a 3 minute stop!
    Nasser likes this.
  4. hroark2112

    hroark2112 Tech Instructor

    # of Dives: 0 - 24
    Location: Raleigh, NC
    Why would you need to, unless you're just doing it for poops & giggles? If they're close to NDL, dive is done.
    kr2y5 likes this.
  5. Nasser

    Nasser ScubaBoard Supporter ScubaBoard Supporter

    Yes exactly... when I ran the NDL planner on the Petrel against that of the Suunto I found a very slight variance of a couple of minutes here and there.. something that is fairly negligible, but it is these arbitrary factors you've mentioned on the "Suunto RGBM" that make me wonder how this translates into real dive situations over a few dives between the two PDC's.
  6. victorzamora

    victorzamora Solo Diver

    So, there's no way of matching it up over multiple dives. There are too many unknowns.

    The best way to dive it safely is for everybody to dive the computer they're comfortable with, set to a conservatism they're comfortable with, and the person who calls the dive first calls it. Whether they've reached low air, NDLs, or their own extra-conservative buffers for either.
  7. RickyF

    RickyF Manta Ray

    # of Dives: 500 - 999
    Location: Oahu
    I have been able to closely match my Petrel to my Suunto D6, but only for the first dive. On the second dive the Petrel allows a lot more bottom time and the Suunto will throw me into Deco, but i just hang on the safety stop until it clears.
    Nasser likes this.
  8. Lorenzoid

    Lorenzoid idling in neutral buoyancy

    # of Dives: I just don't log dives
    Location: Atlanta, USA
    The Suunto algorithm seems to give a lot of weight to surface interval, and unless one takes a long surface interval it will compute less no-deco time for the subsequent dive than a Buhlmann model. I've been using my old D6 as a backup since I got a Petrel.
    Nasser likes this.
  9. Nasser

    Nasser ScubaBoard Supporter ScubaBoard Supporter

    Where you dialing in your own custom GF's in tech mode or using one of the fixed GF presets in rec mode?
  10. kr2y5

    kr2y5 Solo Diver

    # of Dives: 200 - 499
    Location: Seattle
    Like some others, I also do not understand why you would want to tinker with GFs so hard, until they "match" your buddy's, I would never do that. The computer should be tracking whatever you think is appropriate for your body, and the conservatism should reflect how you feel on that day, how fit you are, and how much sporty you want to get. To me, this takes precedence over matching GFs. If you are doing an NDL dive, then as said earlier, whoever gets close to the NDL first, or whoever's computer fails first, simply aborts the dive, period. This should ensure that when you begin ascent, even if neither computer works, nobody's in deco, and a basic 30fpm ascent strategy will suffice. If this is not an NDL dive, on the other hand, then you should have redundant computers (or tables, or a rule of thumb, or redundant whatever sources of deco information float your boat), and either a concrete plan you will closely follow, or at least agreement on the basic dive parameters, like max bottom time, max deco, etc.

    I myself have definitely been guilty of getting complacent. On a recent dive with a very experienced buddy, we got lazy, and we did not agree on the maximum amount of deco (and, to preempt a certain type of criticism, we did not set a fixed plan and fixed deco schedule, because this really was hardly more than a simple rec dive). Both of us had enough redundancy to do the dive solo, and both of us had plenty of gas. We just agreed we will stick around at the end of the dive, and do whatever deco was required in the shallows. At the end of the dive, I was happily doing my deco with a little under 10 minutes on my Suunto and 20 minutes on my Petrel, while my buddy was getting all excited, and kept poking me to try and get out. Turns out, we used wildly mismatching conservatism settings, we had slightly different gases, his obligation cleared up, and he was frantically trying to dell me that my computer is ****ed. We had more than enough gas to stay longer, but he was cold and in pain. After my Suunto cleared as well, we went up. On a subsequent dive together, I finally gave up and cranked down my excessive conservative settings from 30/70 to 20/80. But, more than anything, I think what went wrong is not so much the difference in conservatism, but the lack of prior communication, and setting boundaries for the dive... which I accept as my fault, because the parameters I was diving with were pretty off from one could consider standard, and it was my responsibility to recognize that, and to either change the parameters I was using, or establish sensible limits to preempt possible problems.

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