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Please explain (teach me)

Discussion in 'Basic Scuba Discussions' started by ozorowsky, Apr 24, 2011.

  1. ozorowsky

    ozorowsky Angel Fish

    Hi everyone,

    I am curious here. I went diving yesterday and think I got super lucky I followed my dive computer.

    First dive I was down 47 ft for 59 minutes. Simple right leaves me in pressure group C.

    Then, my fiance and I got out of the water, went to lunch for a bit, had a surface interval of 3 hours 20 minutes.

    Then we went out to a different dive site. This time visibility was absolutely terrible and I went a little deeper than expected. We went down 78 ft (at that depth for about 12 mins) and when I noticed how deep we were we went up to 35 ft for a bit hung out there, did a 10 minute deco stop at 15 ft then ended our dive. According to my dive computer total bottom time on this dive was 68 minutes.

    I am doing my Dive log and am very confused. According to this we should be dead. Here is what I am factoring please tell me where my calculations are wrong.

    1) 47 ft Down for 59 minutes puts us at pressure group S
    2) Surface interval of 3 hours 20 minutes puts me at pressure group A.
    3) Assuming our residual nitrogen time at 80 ft in pressure group A is 4 minutes, and we then went down to 80 ft for tbt of (68 + 4) 72 minutes, I don't understand.

    The RDP says 80 feet for 30 mins is pressure group R with mandatory 5 minute decompression stop. I more than doubled that according to above.

    Can someone explain to me why we are fine and what is wrong with my calculation? I watched my computer nitrogen calculator cautiosly when we were hanging out at 15/20 feet.
  2. flots am

    flots am Solo Diver

    # of Dives: I just don't log dives
    Location: Wherever you go in life, that's where you are.
    A comparison between tables and a computer is invalid for multilevel diving. Table data is based on a square profile, which you didn't dive.

    Computers measure actual depth and time and will allocate an appropriate amount of nitrogen for time spent at various depths.

    A dive that's perfectly fine on a computer could easily be way off the end on the dive tables.

  3. willembad

    willembad Solo Diver

    # of Dives: 500 - 999
    Location: As far away from salt water as you can get in Flor
    You wrote that your computer said total bottom time for dive 2 was 68 min. What it really told you was TRT (total run time) of the dive was 68 min. You yourself said you left 78 ft after 12 minutes. No computer will ever match up with tables since tables assume square profiles and computers use real time info to calculate NDL. That's one of the reasons divers choose to use computers.

    You mention a deco stop. Was that a called for obligation by your computer or you chose to stop yourself?

  4. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

    Your mistake was adding the bottom time from the first dive. The TBT = the Actual bottom time of the second dive + the residual nitrogen time (RNT) you obtained from the table after the surface interval.
    Calculating dives from tables is almost becoming a lost art. I encourage you to continue thinking along these lines, even if you eventually become a computer diver as it will make you a more aware diver.
    Sinbad Scuba likes this.
  5. Bubble_Boy

    Bubble_Boy Nassau Grouper

    # of Dives: 25 - 49
    Location: Montreal, Canada
    The OP says the computer gave him that bottom time for the second dive only....
    Did you hang out for 30 min at the surface before going down? Next time just keep an eye on your computer.... it shouldn't start counting before 5 feet or so but maybe something is wrong with the depth meter.... unlikely, but maybe....
  6. TSandM

    TSandM Missed and loved by many.

    # of Dives:
    Location: Woodinville, WA
    You have just been introduced to the basic issue with dive tables. They are designed for square profile dives, where you go down to a wreck or reef and stay at the same depth for the entire bottom time. When you don't do that, the tables don't really model the nitrogen absorption you had, and the more you multi-level a dive, the worse that discrepancy gets.

    When I started diving, I was doing a lot of shore dives with max depths in the 60 to 70 foot range, and the dives were often an hour long. I couldn't figure out how to log them at all, because they all came out serious mandatory deco dives . . . but the profiles were big "U" or "V" shapes, where we swam downhill for a while, and then uphill for a while, and those dives are just about the worst to try to plug into tables.

    You can use a multi-level table, like the PADI Wheel or e-RDPML, to try to get a better sense for what you are actually doing, or you can use your computer, which is doing what it is designed to do.
    beckbailey and wedivebc like this.
  7. boulderjohn

    boulderjohn Technical Instructor Staff Member

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
    Your second dive was a multilevel dive. It is as if you did a 78 foot dive followed by a 35 foot dive followed by a 15 foot dive, each with no surface interval. Think of it that way and you will see why you are OK.

    As mentioned earlier, tables assume a square profile. In contrast, a computer constantly recalculates your dive based on actual depth. The next time you are diving a similar profile to your second dive, look at what your computer is telling you in terms of how much no decompression time you have left when you are at your deepest depth. As you ascend, watch what happens to those minutes. Look especially at what happens when you get in the 35 foot range.
    WetBoy101 likes this.
  8. Hoomi

    Hoomi Manta Ray

    # of Dives: 100 - 199
    Location: Tucson
    Additionally, both tables and computers default to a conservative calculation. Basically, they work on the assumption of what is safe for most people, and then factor in a bit extra for safety.

    I.E., if most people's nitrogen absorption is fine at Y depth for X minutes, then the computer or table will reflect that data minus a percentage to accommodate those who aren't quite so tolerant. At the other end of the scale, though, are those who are more tolerant, who would be just as fine at Y+10 depth at X+10 minutes, for example. The safety margin also accommodates variations that will occur in the same person under different conditions.

    Both tables and computers are conservative guidelines that, if followed, should keep a diver safe. However, neither is so definitive and cut-close that, if you accidentally go beyond the limits stated, you're definitely going to die. It's much like the stopping distance ratings for automobiles, and the guidelines for distance between your car and the car in front of you. Your car might be able to stop in much less distance than the average, but for a general guideline, the average is good to keep in mind. Even if you're following too close, you may still be able to stop in time, but the advice of safety experts, just as with the tables/computers, is, "why chance it?"
  9. ozorowsky

    ozorowsky Angel Fish

    Thank you all for your prompt replies. I think BoulderJohn explained it perfectly.

    So as long as I follow my dive computer I should be safe right?

    and to willembad: Yes the computer recommended a 5 minute deco stop. I did 10 to be safe.

    Thank you all again for your reassurance.
  10. TSandM

    TSandM Missed and loved by many.

    # of Dives:
    Location: Woodinville, WA
    Your dive computer will most likely keep you safe, although no decompression program works flawlessly for all people. It is best not to push no-deco limits, and "riding the computer" (ascending just fast enough to keep the gauge from clicking down to zero) is a good way to see if it doesn't work for YOU. It is also a very good thing to have some idea of what the computer OUGHT to be telling you, because no electronic gadget is without the possibility of malfunction -- if your computer is feeding you nonsense, you ought to be able to recognize that.

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