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Perdix Ndls

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

  1. Bigeclipse

    Bigeclipse Barracuda

    # of Dives: 100 - 199
    Location: USA - New York
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    All...I'm used to diving a GEO 2.0 and following PADI dive tables. I know the Perdix follows it's own type of model along with GF to determine it's NDLS but I have also seen that as far as NDL goes it seems to be quite conservative... at least for the first dive. From what I read the Perdix will not penalize you for your repetitive dives that other computer like suunto might. What question is what suggestions do you have for me as far as setting this computer up until my tec course next year where I will be likely using only OC REC mode? Which mode will get me closest to PADI dive tables (maybe conservatism setting low in OC REC mode?) I know that for recreational diving limits I have always felt fine after dives if I stick to the tables...even if I approached NDLs. Let me ask two scenerios...

    First scenerio:
    Lets say you had a deeper dive of 110 ft on a cool wreck so you want to maximize your NDL to be as close to the PADI table as possible and you don't mind a long surface interval before second dive...maybe you set the conservatism to low?

    Second scenerio:
    You plan on doing 3-5 repetitive dives through out the day at mixed recreational depths so for thus scenerio maybe you set conservatism to "medium"?

    I hope what I'm asking makes sense.
     
  2. rongoodman

    rongoodman ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Albany, NY
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    The Perdix, like all Shearwater computers, comes with the Buhlmann GF ZHL-16C deco algorithm. You can modify its behavior by adjusting two parameters know as gradient factors. As an alternative, you can, for a extra fee, unlock the built-in VPM algorithm, which has its own set of conservatism factors. You can and should learn more about both models, especially if you're planning future technical training, since they are the most widely used in that world. Deco for Divers is the obvious place to begin.

    I haven't used PADI tables for many years, so I can't comment on them. Until you understand more about gradient factors, and what changes to them imply, I'd just set it to Medium(40/85) and leave it alone.
     
    Bratface likes this.
  3. uncfnp

    uncfnp Solo Diver

    # of Dives: 500 - 999
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    First, the Geo has multialgorithm options as well. What settings did you dive on it?

    The SW in rec mode gives you three options, all in Buhlman GF ZHL-16C, High, Medium and Low. If you want the most liberal option available, put your DC setting on Low. This will give you the longest NDL's and most closely match the Geo without any GF set. Then obviously a little more conservative would be Medium and High would give you the most conservative diving you can do in rec mode.

    SB links that might help

    New diver: Suggested conservatism settings for a Petrel?

    Links to posts by Shearwater

    # 133. Shearwater coming out with new DC?
     
    Last edited: Apr 20, 2016
  4. rongoodman

    rongoodman ScubaBoard Supporter ScubaBoard Supporter

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    Be sure an keep an eye on the tissue loading graph as you use the Perdix. It provides a lot of information as to what's going on with deco the model you're using in a very easy to understand format. I'll be on a liveaboard in Belize next month with multiple dives on multiple days, and will be watching the slow compartments.
     
  5. markmantei

    markmantei Professional Photographer

    # of Dives: 200 - 499
    Location: Calgary, Alberta, Canada
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    If you do plan on moving to tech in the future I would echo the suggestion to learn more about GF and how changing the GF's change the decompression curve. The values you choose are very much a personal decision based on many factors, but the default GF's on the Shearwater computers were chosen for a reason (I heard a story about how they changed their defaults due to doppler tests on RB divers).

    Are the defaults more conservative than the Geo on the most aggressive setting? Yes. The question is whether the Geo is too liberal or the Shearwater is too conservative for you, and both can be adjusted to compensate. Personally I dive both a Shearwater (on the default GF's) and an Oceanic (on the most aggressive setting). My oceanic is my get out of dodge quick and everything should be fine backup if things go pear shaped, while the Shearwater is what I use as the controlling computer, anecdotally I feel better diving the more conservative profile, I also feel safer doing technical dives with the extra padding on the decompression schedule.
     
  6. Bigeclipse

    Bigeclipse Barracuda

    # of Dives: 100 - 199
    Location: USA - New York
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    I understand what everyone is saying. I'm not just trying to have the most NDL possible or anything just trying to figure out settings to start with and it sounds like medium is a good place until I really figure out my own body and start my tec courses. I have always followed the PADI tables and my GEO on aggressive settings and I have fortunately never been hurt or sick so I believe the Perdix on REC low conservatism would probably be fine. I'm just concerned about my deeper dives and NDLs on a first dive as I'm used to getting a bit more time than he Perdix states for medium conservatism. It will seem my friends and their Suuntos will have longer NDL times for the first dive. No big deal.
     
  7. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

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    Hi Bigeclipse,

    I think you will find low conservatism (45/95) will run reasonably close to DSAT/PADI RDP. Why don't you run both computers and check for yourself? You can always end your dive whenever you want, based on either computer. Let us know what you find out

    Good diving, Craig
     
    uncfnp likes this.
  8. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

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    I kind of skimmed through the responses above, but basically the beauty of the Shearwater computers is your ability to really decide what level and how that level of conservatism is implemented. It is less noticeable on recreational dives, because you don't hit the gf-lo numbers.

    The super watered down basics are the Perdix let's you out at the same theoretical tissue loading regardless of whether it is dive 1, or dive 6 of the day, and doesn't make any adjustments to that theoretical loading based on rapid ascents, short SIT's, sawtooth profiles etc etc. It simply tracks the 16 compartments and let's you up when they have reached whatever predetermined level you told it to. For NDL's it just counts down until that theoretical limit is reached. The algorithms in the other computers generally do not follow those limits and are often based on completely different algorithms to predict decompression. They are also generally modified in some way to add conservatism based on certain parameters *sawtooth profiles, rapid ascents, short SIT's etc etc*.

    Innovation ยป Computers
    Your Geo 2.0 has the option of choosing two different algorithms. The DSAT is based on the same models that were used to compute the PADI RDP. The PADI RDP is optimized for multiple dives per day with relatively short intervals, at the sacrifice of ultimate bottom time for the first dive. Perfect for what it was designed for which is repetitive reef diving.
    They also offer a Z+ mode which is based on Buhlmann ZHL-16C, which is conveniently what your Shearwater runs off of. The difference here is that the Shearwater allows you to modify the conservatism within that algorithm as opposed to letting Oceanic choose the theoretical numbers for you. The conservatism mode basically makes the computer think you are diving at altitude which makes the algorithm think you are a few feet deeper than you really are.

    Now, what does all of this mean. If you dive PADI tables, or the DSAT mode on your Geo 2.0, your Shearwater needs to be on conservatism low, which is 45/95 gradient factor. This maximized bottom time, but if you hit your NDL and make a somewhat rapid ascent from the last 15 feet, you have the potential to supersaturate some of your tissues because they are at a theoretical loading of 95%. In reality, even at conservatism low, it is likely that you will have longer NDL's using the PADI tables or DSAT algorithm on the Geo than on the Shearwater. When decompression comes into play, you have to determine the balance of bottom time and decompression time against your risk aversion of the bends. If you are diving cold water, working hard on the bottom, and didn't get a lot of sleep *i.e. most mid-Atlantic wreck diving*, and dive that same conservatism setting, it is significantly more likely for you to get bent than if you are doing a drift dive in the Caribbean. You have to tailor those conservatism settings to what you are comfortable doing.

    In Tec mode the Shearwaters default to 30/70 I believe which is incredibly conservative compared to any dive tables or recreational computers. Not great if you don't have any decompression training, but significantly lowers your risk of DCS. It is up to you, but before you go monkeying around with settings and choosing which algorithm to use on the geo, you should understand the hows and whys

    Flexible Control of Decompression Stress - Shearwater Research
    Gradient Factors | Dive Rite
     
    tridacna, Bratface, Jax and 1 other person like this.
  9. uncfnp

    uncfnp Solo Diver

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    If you have been diving agressive/low conservative profiles on the Geo/ tables, and the low setting on the Perdix gives a very similar dive, why change just because its a different computer?
     
  10. KenGordon

    KenGordon Rebreather Pilot

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    It is a mistake to compare tables and computers. The tables assume a square profile at maximum depth. The computer gets the real profile. By diving tables you are likely to be adding significant conservativism even if the NDL times for it are longer for the tables than the computer.

    Since the two gf numbers are interpolated between first stop and surfacing it would be interesting to know what the gf would be when the NDL expires.
     

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