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No deco time proximity.

Discussion in 'Advanced Scuba Discussions' started by CUMBIAMAN, Nov 20, 2018.


    CUMBIAMAN Angel Fish

    I am having a long time discussion with a buddy about the no deco time shown in a computer. How close can you get to the zero time while diving, without risking too much. Some opinions are to go up when you reach the 10 min mark, some consider the 5 minute mark.
  2. lermontov

    lermontov Manta Ray

    # of Dives: 500 - 999
    Location: christchurch
    theres no decisive line - you can do everything right and still get bent , and vice versa. the computer programmes are simply best practice using statistical proabablility that if you follow the plan you have a high chance of off gassing safley
    D_Fresh, oldschoolto and Bob DBF like this.
  3. arew+4

    arew+4 Manta Ray

    # of Dives: 200 - 499
    Location: Pa
    I use a fairly liberal computer, and generally try to keep the NDR above five minutes on a multi level dive. On the third, fourth, and occasional fifth dive of the day I get pretty religious about it.
  4. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: New York City
    That depends on your definition of "too much".

    The point at which you reach NDL=0 depends on (1) what gas you are breathing, (2) your depth and time (profile), (3) the algorithm that your computer uses, and (4) any adjustments that you have made to that algorithm to increase or decrease conservatism.

    For pretty much any modern algorithm, the risk of decompression sickness if you ascend at the calculated No Deco Limit (NDL=0) is acceptably small. Meaning that it's very unlikely, but not impossible to get bent. And there are many other factors that increase or decrease your risk of ascending at NDL=0 (e.g. fitness, hydration status, areas of previous injury, vascular anatomy, water temperature, activity level, etc...). So even if you got someone to make up a percentage chance of DCS for Algorithm X, profile Y, and conservatism settings Z, that number still wouldn't be accurate since no computer knows if you have a PFO or if you are hung over or if you are a 25 year old Navy Seal.

    If you have the option for multi level diving, you will see that NDL number grow again when you ascend. Certainly, ascending at NDL 10 would involve less decompression stress than ascending at NDL 0, all other things being equal. But all other things aren't equal, and that difference in decompression stress might not be clinically significant.

    Hedging your bets by padding things a bit if you are diving a lot is not a bad idea, as mentioned above. Just remember that no matter what the magic bracelet tells you, DCS is a risk of breathing compressed gas under water.
    Preach, Jack Hammer and Ghost Diver # like this.
  5. Lorenzoid

    Lorenzoid idling in neutral buoyancy ScubaBoard Supporter

    # of Dives: I just don't log dives
    Location: Atlanta, USA
    What does "too much" risk mean? It has been said that estimating where the line not to cross lies, whether that's using a computer or tables, is to "measure with a micrometer, mark with chalk, cut with an axe." All computers display a very precise number as NDL time remaining, but different computers display different numbers. Realizing that the amount risk increases or decreases with those differences in numbers can't be estimated as precisely--that's like cutting through that precisely marked number with an axe. Nobody can estimate with any precision how much difference on any single given dive it makes to your actual risk of DCS symptoms.

    Opinions are worth little, but my personal decision is to set my computer on medium conservatism setting and give myself a buffer of at least two or three minutes or so from whatever displayed NDL time "medium" may result in. If I have been diving a lot that week or don't feel particularly well, I might pad it considerably more than that.
  6. seeker242

    seeker242 DIR Practitioner

    # of Dives: 200 - 499
    Location: Pompano Beach, FL
    That depends on your personal definition and perception of "too much", which will be different for everyone.
  7. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
    In addition to defining "too much risk" you also need to define "go up". Are you ascending directly from (e.g. 90ft) to 15ft for a 3 min safety stop and getting out? Or going up (from 90ft) to 35ft and spending another 20mins at 30-40ft before ascending again to 25ft for 10mins and finally doing 5mins at 15ft.
    maj2, Zef and Steve_C like this.
  8. KenGordon

    KenGordon Rebreather Pilot

    Depth also matters. 5 minutes left at 40m is more margin than 5 minutes at 20m.

    NDL ivaries with depth and becomes very sensitive towards zeo time remaining so small changes in depth can have a large impact on the number.

    There are a bunch of factors other than time which matter and it is good to properly control those.

    The algorithm/settings matter to an extent too. I am personally happy to do deco and thus get out on the limit with some computers but not happy to do repeated NDL dives with others. For example I will not be getting out with a GF of 95 unless I really have to.
    meyerelie and Zef like this.
  9. Bob DBF

    Bob DBF Solo Diver

    # of Dives: I just don't log dives
    Location: NorCal
    You make your own limits, depending on your knowledge and the risk you are willing to take. 0 is the computers line in the sand, 5 min prior is good, 10 is better. In some cases it is better to pad the number incase you wind up with a longer stay at depth than planned as in finding an interesting critter or in case of an emergency. I also like to spend time shallow at the end of the dive to increase that number, regardless of what number I used to start up.

    racanichou, northernone and Steve_C like this.
  10. TerryC

    TerryC Master Instructor

    # of Dives: 2,500 - 4,999
    Location: St Kitts, West Indies
    Excellent advice here.
    What you need to remember is that your Dive Computer is just another "tool" in your diving Skill Set and it is there to assist you in minimising the risks of DCS.
    How you interpret the information it gives you is your decision. You may have a computer where the level of "conservatism" can be adjusted, this can easily affect your NDL by as much as 20%
    Diving is about managing risk, understanding the factors involved in DCS and understanding how computers work will help you in this.
    There have been comparisons done in controlled environments with different computers and it is very rare that 2 computers (from different manufacturers) give exactly the same NDL time. The reason for this is that different decompression models are used by different manufacturers.
    The more you read about this, the more you will realise that nothing is "set in stone".
    There is no right or wrong here, has to be a personal decision, just make sure it is an informed decision.

    Two articles for you:

    A introductory guide to managing DCS.
    Technical Specs for 47 different computers
    EireDiver606 likes this.

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