Please explain (teach me)

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

  1. ozorowsky

    ozorowsky Angel Fish

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    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 Great White

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    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.

    flots.
     
  3. willembad

    willembad Nassau Grouper

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    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?

    Willem
     
  4. wedivebc

    wedivebc CCR Instructor ScubaBoard Supporter

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    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

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    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.

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    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 Administrator Staff Member

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    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 Surface Interval Member

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    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

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    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.

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    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.
     
  11. ianr33

    ianr33 Orca

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    What was your plan in the event you ran out of air during your deco stop?

    (And yes, I'm just playing Devils Advocate to point out there is more to diving than just blindly following a computer)
     
  12. NetDoc

    NetDoc Chairman of the Board

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    Your first dive was probably a multilevel one as well. PDCs (Personal Dive Computers) do a wonderful job of accurately tracking our depth and time in a way a human can not hope to keep up. As was previously pointed out, using a table for a multilevel dive is like using a hammer to set a wood screw. You can do it, but its the wrong tool for the job!

    That being said, with the longer dive times, please consider increasing your safety stop from 3 to five minutes as well as doing a two minute "half stop" when your depth is greater than 60 fsw.
     
  13. fnfalman

    fnfalman Single Diver

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    Others have already addressed the dive computer versus table issue.

    Let's talk about something else. YOU DID NOT do a "Deco" stop. Yuo did a "Safety" stop. Big difference.
     
  14. NetDoc

    NetDoc Chairman of the Board

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    If it was suggesting five minutes, then it probably WAS a obligatory deco stop. As for a safety stop NOT being a deco stop, there are a number of us who would disagree with you. All stops involve decompression of some sort. Some are optional and some are not, but they are still deco stops.
     
  15. ozorowsky

    ozorowsky Angel Fish

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    Point taken thank you for clarifying.

    and to ianr33: during my "safety stop" I still had 1800psi. Also there were 2 dive buddies with me, and 3 classes going on with about 5 students each.

    That was part of the reason I picked this dive spot "controlled environment" without an instructor to get my feet wet and get more and more comfortable with diving.

    Thank you all for sharing your experience, and not slamming me for potentially making a mistake.

    One thing I am trying to gain a better grasp on is the safety stop. I know per the dive computer when to make one and per the table approximately when to make one, but what if you mess up? Will you go to the surface feeling sick? Go into convulsions, die instantly, is there any physical forewarning?

    Thank you.
     
  16. NetDoc

    NetDoc Chairman of the Board

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    Probably none of those things.

    There are two types of stops: obligated and optional (Safety).

    Violating either of those will not necessarily give you DCS. However, doing them does not completely keep you safe from them either. There are many factors that affect DCS including ascent rate, breathing rate, temperature, hydration, sickness, elevation and so on. Its up to you to dive in a reasonable manner and mitigate those risk factors by decreasing your dive time (conservancy) or not diving until things are normal.

    If you miss an optional stop, there is nothing to be done. You can continue to dive normally.

    If you miss an obligatory stop, then you should stay out of the water for 24-48 hours and monitor yourself for symptoms of DCS. Call DAN and get to a hospital/hyperbaric chamber if you are suspicious.

    Signs and Symptoms of DCS
     
  17. rhwestfall

    rhwestfall Surface Interval Member

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    I applaud you for asking why, but am disturbed as to the apparent lack of education by a person with a c-card. What is in dive training/certification these days?:confused:
     
  18. boulderjohn

    boulderjohn Administrator Staff Member

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    I would say that in most cases the problem is where it always was: teaching tables with square profiles only to the OW student. Multi-level and computer diving is not usually part of OW training. This is changing.
     
  19. DevonDiver

    DevonDiver Tech Instructor ScubaBoard Supporter

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    5 minutes on a PADI table is an emergency deco stop. It is mandatory, in the event of exceeding an NDL. Full rules for emergency deco is written in the special rules on the table.

    Likewise, a 5 minute STOP on a computer is also highly likely to be deco. I don't know of any computer that gives more than 3 minutes as a safety stop. If it was deco on the computer, then it would also indicate a ceiling, that you could not ascend beyond for the given stop time.
     
  20. fnfalman

    fnfalman Single Diver

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    Safety Stops are optional. Deco Stops are not.

    You may or may not get a DCS hit by skipping a Safety Stop. I'd like to see somebody does a 45-minutes dive at 150-ft and skip a Deco Stop while not getting the bend.

    The OP's second dive profile was nowhere close to having to do a deco. Maybe the computer was generous in the 5-minutes safety stop.

    I suppose we need to clarify a bit about deco stops. If a rec diver were to push the No Decompression Limit for a little bit, then the computer will recommend a "deco" stop that either longer than the typical safety stop, or maybe a "deco" stop somewhere deeper and then a longer safety stop. This is a lot different than technical/commercial divers doing some serious deep bottom time and have to use the hyperbaric chamber or have to hang for an extended period of time while sucking on high-O2 mix while doing a true obligatory deco stop. Skip this stop and you'll be in a world of hurt for sure and not just maybe.

    Decompression diving (even if it were just a little bit over the NDL) should not be executed until the diver knows what he or she doing. Trust the computer to do a "deco" stop for you is a recipe looking for trouble.
     

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