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Perdix Ndls

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

  1. Stoo

    Stoo NAUI Instructor

    # of Dives: 5,000 - ∞
    Location: Freelton & Tobermory, Ontario, Canada
    I agree completely... But understanding what's happening that determines if a profile is conservative or otherwise isn't a bad thing.

    I suspect that for many people, none of these computers will let you bend yourself, IF you follow the profile they suggest accurately.
    uncfnp likes this.
  2. uncfnp

    uncfnp Solo Diver

    # of Dives: 500 - 999
    Location: North Carolina
    To the OP. You are way over thinking this. You have been happily diving a computer that I am assuming was set to DSAT without conservative factors. All of the options on your Perdix in recreational mode will be more conservative, including the low setting. So pick low and go diving.

    My understanding, the purpose of the Gradient Factors (and SF and CF) is to balance dive times and risk based on an individuals risk tolerance modified by diver and dive conditions. Its not meant to be adjusted just to prolong dive times between dives. If you want to get the longest bottom times with the safest profiles in the scenarios you have given, dive nitrox rather then air, don't push NDL's and consider setting your Perdix for a 5 min safety stop.
    Last edited: Apr 20, 2016
    KWS likes this.
  3. Bigeclipse

    Bigeclipse Barracuda

    # of Dives: 100 - 199
    Location: USA - New York
    Thanks Tbone. So reef diving in warm waters such as bonaire would warrant different settings than colder mid Atlantic or lake diving and depends on the work you are or are not exerting...water conditions, currents...etc. may not be a big deal to be on low conservatism during a warm deeper calm dive of 80ft but might be a big deal if cold...heavy currents and task loading at the same depth
    Last edited: Apr 21, 2016
  4. rhwestfall

    rhwestfall Solo Diver

    # of Dives: 200 - 499
    Location: "La Grande Ile"
    that, and your overall well being...
  5. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    yes, so here are factors that will cause me to drop my GF high

    Didn't sleep well
    Drank too much *of the decompression aids the nights before...*
    Didn't drink enough *water*
    Cold-both air and water. Cold water slows down N2 intake, but also slows release and since cold is cumulative, it slows progressively throughout the dive. If diving in water below about 80F I will usually dive a GF of 75-80, below 60, it is no more than 70
    Work-if I am drift diving or on a DPV, my body is in a very different state than if I am kicking in current
    How do I feel that day-I could just be feeling a little weird, tired, sore, etc etc and that will cause me to pad decompression.

    Being deco certified, I can't tell you the last time I paid attention to NDL's. If you are doing a 3-5 minute "safety stop" you're already performing decompression, you may just not have theoretically saturated your tissues yet. Removing the stress of worrying about your NDL's allows you to lower some of the time pressure associated with diving deep. You have three factors that will cut your bottom time. Time, Air, Temp. If you remove time, you have cut your stress factors considerably and you just plan your ascent based off of a predetermined pressure. Much less stressful than having to watch both, especially if you violate that NDL as many will start reeling in their own heads about having violated that limit and think they're going to get bent. You get to the surface with a sore knee and think you were bent despite forgetting you knocked it into the bulkhead that morning. Have a rash and think it's skin bends even though you might have had a slight reaction to the neoprene, etc etc. It's funny, but by controlling the conservatism on the computers and KNOWING what it does and why you chose to set it at that limit, you are making the decision.

    Quick example that are numbers taken out of thin air, so don't run this into pastodeco or something because I doubt they'll work.
    Which is safer?
    NDL limits at 100ft for 30 minute bottom time, GFhi set to 95, and you perform a 5 minute "safety stop" *keep in mind the Shearwaters do not force you into safety stops so they are not factored into your NDL's which may also be contributing to the extra conservatism. If the recreational computers have a 3 or 5 minute stop factored into their NDL's, that will extend bottom time*. You surface with a theoretical GF Hi of 80
    Same dive, but your computer is set for GFhi of 80, and you blow your safety stop, make a slow ascent to the surface and come up with a GF Hi of 80.

    Which is safer? Which is less stressful? Which has higher risk of DCS? You have to analyze yourself and determine which you deem safer. I will always choose the second dive profile, but others may not. I always have the option to increase the conservatism on that profile by adding a safety stop that will lower the theoretical tissue loading before surfacing. I also have a bit of a buffer at the bottom if I have to stay down for an extra minute or two due to an emergency without running into mandatory decompression.
  6. dberry

    dberry Hydrophilic ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Philadelphia
    At some point I'll have to ask this in new thread, but for now: At this point I seriously doubt I'll ever want to do true tech, and I may never move to any form of doubles. But I like the idea of carrying a redundant supply (probably a 40?) and being trained enough with deco to safely use that supply if I ever do need it for deco. OK, I might never exceed NDLs with a single AL80 on a reef, but I probably could diving with the HP119s I got last summer. I plan to do Rescue Diver first, but I'm confused by the options offered by the different agencies for the next step. Advanced Nitrox / Tek40 / ?? What's the best route to get what TBone1004 describes as "some sort of decompression training" without going whole hog into Tech?
  7. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    TDI Deco Procedures is probably the closest that I know of. It is typically done with advanced nitrox and/or advanced recreational trimix *for narcosis management* so you can do accelerated O2 decompression, but it isn't required.
    Jax likes this.
  8. Jax

    Jax Deplorable American ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: AZ TX
    Bwaaahahahahaha! That is the same excuse I had when I took the TDI Advanced Nitrox / Decompression Procedures!!!

    It's a slippery slope! :eyebrow:
    BenjaminF likes this.
  9. uncfnp

    uncfnp Solo Diver

    # of Dives: 500 - 999
    Location: North Carolina
    The best route is to go with the best instructor in your area, be it Tech 40, AN/DP, etc. Either will get you want you want.

    I am partial to AN/DP (Trimix not needed but certainly a valid option). Tech 40 is probably more in line with your stated goals and AD/DP overkill but when it comes to deco I think more is better.
    Last edited: Apr 21, 2016
  10. vixtor

    vixtor Solo Diver

    # of Dives: 200 - 499
    Location: Bucharest, Romania
    When I got my Petrel, I had similar doubts about what to use. Truth is that reading books and articles on deco theory just tell you how it works, and will give you more doubts as soon as you understand how this science has no hard/crystal clear facts to rely on (being more statistics than absolute truths). Reading forums, and specially those huge threads about deep stops where some people with lots of experience in the area had argued over, will give you more doubts. Trainings are good as long as you don't ask yourself too many questions, but sometimes the agency version will be outdated, or maybe your instructor will tell you facts that worked for him, which are not always the most accepted version, so you will still end up asking yourself what is the best for you.

    The people telling you that they adjust their algorithm based on how they feel are also not really helpful, because their choice (except for the most generic common sense that conservative is better when there is a supplementary risk factor involved) has no solid foundation when they get to select the GF values. In the end, the truth is that even Buhlmann without GF is almost safe enough statistically, so no matter the GF value, for normal diving (with no or light/medium deco involved) it will probably just work.

    Unfortunately this was not enough for me to make me happy. But I could decide for myself that my diving before Petrel was good enough for me (i was happy with the bottom times, I was happy with the deco stops duration), so as long as I would have similar outputs from Petrel (+/- 1 minute) I would probably be happy in the future. So what I did was to use Subsurface or Diving Log to run ZHL16-C on my previously recorded profiles, and check what GF setting would take me out of the water at almost the same time as my previous Suunto. What I have discovered was that 30/70 would be identical in virtually all cases. Please note: the Petrel ZHL16-C will tell me sooner than Suunto that I'm in deco, but with GF 30/70, the deco obligation would clear at the same time as with the Suunto during the ascent/safety or deco stop (so what I get is the same bottom time and total duration, but not necessarily the same deco steps).

    So for me the default 30/70 (which also has some backing in forum discussions and articles) proved to be good enough. I don't want to push deco and get out of the water faster, because this is currently just good enough for the depths that I'm diving, and until we find any wreck in the area at a much bigger depth than all current wrecks, I'm 100% happy.

    My suggestion for you is to do the same. Put all your current profiles in Subsurface (which is a very nice free software) and try to find what GF setting will give you the same time as you dived. Doesn't matter whether you get some deco obligation or not (so don't compare NDLs), just look whether in the end you can get out of the water at the same minute.

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