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Nitrox course. What's the point?

Discussion in 'New Divers and Those Considering Diving' started by Dody, Jan 9, 2021.

  1. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
    Just a little story about that. When I started tech diving, we did not use computers at all. We preplanned the dive and followed the plan using bottom timers as our guide. When two divers in our group of friends got bent, they had no idea why--they had followed the plan perfectly. Well, one of them was using a computer in gauge mode, so he was able to get a profile of the dive. It turned out they had not been nearly as perfect as they thought--not even close. Ascent rates, depths, and stop times were far from perfect. These were educated people, one with a Ph.D in afield requiring a high level of mathematical skill.

    I think of that whenever I see people say they don't use a computer and just follow a written plan instead. Since they really have no way of knowing, they really don't know how well they followed it.
    peocro, Esprise Me and DiveClimbRide like this.
  2. lermontov

    lermontov Manta Ray

    # of Dives: 500 - 999
    Location: christchurch
    yep hard to argue with real time data
    peocro likes this.
  3. Gareth J

    Gareth J Manta Ray

    # of Dives: 1,000 - 2,499
    Location: UK

    Rule 1 - Never forget to use the computer between your ears.

    Modern electronic devices are fantastic, but never forget rule 1

    I would suggest that if you do invest in a computer, compare it to your regular practice (table). Work to the most conservative, even if the computer is giving you time, make changes slowly.

    I was told when I first started using computer planning tools for runtime. To set the software so that it produced results similar to what I had already been doing. Once the software was producing runtimes comparable with my historic profiles, then to start adding accelerated stops etc.
    If you wanted to modify the settings to make it more aggressive, do it in small steps over a long period of time.
    i.e. you could make the software do anything, giving almost no decompression, the question was, would this be a sensible profile.​
    Graeme Fraser and mac64 like this.
  4. BRT

    BRT not a soft touch ScubaBoard Supporter

    Yes, barely into deco and it is cleared on ascent. As I said, the computer doesn't know how you are going to ascend and calculates it on the go_Obviously in a non rec computer knowing the total of your deco stops is an important detail. But you are missing the point. If the computer knows as you approach deco at 100' that it needs to add ascent time, why does the NDL increase if you go up to 90 feet? And if you run out your time at 90 and go to 80 the same thing happens. You think that the computer thinks you are that many minutes closer to the surface?
  5. MichaelMc

    MichaelMc Working toward Cenotes ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: Berkeley, CA
    Having the dive profile logged by a computer with near-perfect memory is great.

    Even better if it can tell you information derived from that profile during the dive, to help your decision making.

    Doing more than 'look at the fish' dives without logging info would leave me with no good record of whether I hit the dive profile plan or not.
    peocro likes this.
  6. mac64

    mac64 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 5,000 - ∞
    Location: Ireland
    I’ve always been meticulous with planning and carrying out a dive plan. But like you said there’s always room for error.
  7. mac64

    mac64 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 5,000 - ∞
    Location: Ireland
    My plan is to bring the computer as backup and slowly make the switch over a year. But I’ll always still plan the diving. I’ll start small until I’m happy with the profiles. I use Buhlmann ZHL16/C
  8. Belzelbub

    Belzelbub Manta Ray

    # of Dives: 200 - 499
    Location: Largo, Florida
    Interesting how times have changed, and maybe how things are different in other areas.

    When I first got certified, both dive computers and EAN were around. I recall that my instructor was not a fan of either for recreational divers. It was tables and air. To him at least, EAN was for tech divers only, and I seem to recall him saying the same about computers.

    For years, that’s how I dove. When some of my friends were diving with computers, I decided to see what it was all about. One of the first things I noticed is that I had been ascending too fast. Without a computer to alert me to it, I hadn’t noticed it before. It’s a heck of a lot easier looking at the ascent rate on the computer than it is watching the depth gauge and my watch at the same time.
    peocro and Esprise Me like this.
  9. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
    As you ascend, you are decompressing to some degree simply by ascending at a safe rate. That is why you can go directly to the surface on an NDL dive. Ascend too fast, and you can get bent. If you have only gone a minute or so into deco, that may be all you need to clear.

    You will see something similar if you watch divers on a team during a decompression ascent when they are using computers. Due to minor variations in their depths during the dive and especially to variations in the mix of gases they were breathing during the dive, their computers will not match perfectly during the ascent. Let's say Diver A's computer clears the diver at the depth they are at and says to ascend to the next level and stay there 5 minutes. Diver B's computer, however, still has a couple of minutes to go at the current depth, so Diver A waits until Diver B's computer clears for that depth so they can stay together. When Diver B is ready, they go to the next level, but when they get there, Diver A's computer now says only 4 minutes are required. That is because there was decompression going on at the deeper depth, just not as fast as it would have been at the shallower depth.

    What I wrote in the previous paragraph is a little simplistic, though, and not always accurate. What you are describing is in the gray area between true NDL diving and true decompression diving. What I have discovered in my research is there really is a difference, despite what the "all dives are decompression dives" people will tell you. If you begin your ascent on an NDL dive before going into deco, you can take your sweet time on your ascent. As long as you do not overstay your time in the deeper water and go into deco, you can take as long as you want to ascend, and in most cases, you will increase your NDL time as you ascend and not require a stop. (That is what makes multi-level diving possible.) Once you are really into decompression, dawdling on ascent can significantly increase your decompression time because while your faster tissues are off-gassing (as I was describing in the last paragraph), your medium and longer tissues are ongassing, and they are increasing your required decompression time.
    peocro, Esprise Me and BRT like this.
  10. yle

    yle Instructor, Scuba

    # of Dives: 500 - 999
    Location: Southern California
    Funny you should ask... dividing by 7 is an interesting situation.

    1 / 7 is .142857 to 6 dec places

    2 / 7 is double this, of course... which happens to be .285714 (notice the six digits)

    3 / 7 is triple the first number. What are the first two digits? Well, 14 x 3 is 42. And you might see where this is going

    3 / 7 is .428571 (again, notice the digits...)

    and it doesn't stop there!

    4 / 7 is .571428

    5 / 7 is .714285

    and 6 / 7 is .857142

    So you can easily divide any number by 7 with six decimal place accuracy if you just remember that

    1 / 7 is .14 28 57 (notice that the order of the digits in all of the above fractions is the same...)

    SO... what is 22 / 7 ? It's 3 + 1 / 7 which is


    Of course this is why it's a decent approximation for pi... 3.14159... the difference is only about 0.04%

    But the above fractions can be used to divide any number by 7:

    e.g. 63458 divided by 7 is

    63000 + 420 + 35 + 3 div by 7

    or 9065.428571

    Thanks for asking! (oh, and yes... I do this to my students also. Not my scuba students, my college students. Most of them just humor me, some ignore.)
    lowwall likes this.

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