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bottom time definition?

Discussion in 'New Divers and Those Considering Diving' started by Pilaar39, May 12, 2004.

  1. Pilaar39

    Pilaar39 Guest

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    I am a new diver, and all of my dives to date have been off a dive boat in the Caribbean/Florida. For the most part, they have been with certified PADI instructors/divemasters.

    However, this last dive trip I went with a group that was SDI/TDI certified. We explained that we were newbies, with only our PADI OW certification that allowed us to a max dept of about 60 feet.. however, it was my understanding we could go deeper as long as we were accompanied by a divemaster.

    There was what I believed was an appropriately certified crew person who said she would guide us on a dive to a wreck at 100 feet. We stayed down for about 30 minutes, but did extra stops on the way up.. 1 minute at 40, 1 minute at 25, and 4 minutes at 15. This instruction was given to all divers on the dive.

    When I later looked at my dive tables and log, I did not know how to fill them in! The tables did not have a pressure group for 100 feet at 30 minutes.

    Later we talked to the group about what actual bottom time was.. some said it was from when we entered the water to when we got out.. others said it was until we started to ascend, while still others said it was everything minus the decompression times.

    Can someone advise me exactly how bottom time is calculated? And what should I have used when filling out my log book using the PADI tables?

    It is my understanding that PADI tables are for no-decompression diving.. but it appears that this SDI/TDI group took us on a decompression dive.
     
  2. Don Burke

    Don Burke Loggerhead Turtle

    # of Dives: 200 - 499
    Location: southeastern Virginia
    2,155
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    Bottom time is from the start of descent to the start of ascent. It doesn't include stops.

    Run time is from the start of descent to the return to the surface.

    Thirty minutes at one hundred feet is beyond any no-stop limit I have ever seen for air.

    My old Navy tables call for three minutes at ten feet for that dive. I'd do quite a bit more than that.

    Were the other folks on EAN32? Maybe they thought you were as well.
     
  3. Drew Sailbum

    Drew Sailbum Instructor, Scuba

    # of Dives:
    Location: Grand Cayman
    2,188
    103
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    Welcome to multi-level diving.

    The strict PADI definition of bottom time starts from the beginning of descent until the beginning of final ascent to the surface. It does not include the time spent during the final ascent and any safety stop.

    The rules for using the PADI dive tables have you check the bottom time against the maximum depth reached during the dive. If the exact maximum depth or bootom time is not on the table, use the next greater depth or time. This treats all dives as if they were at the deepest depth (or possibly deeper than your max depth) for the entire bottom time. Ths is not very realistic.

    Taking a multi-level approach to diving takes a lot of this rounding error out of the equation. Thus a multi-level dive is planned by taking A minutes at X depth, followed by B minutes at Y depth, followed by C minutes at Z depth. So long as you start at the deepest depth (X>Y>Z) and proceed shallower, you gain progressively more time as you reach shallower depths. This may provide a total bottom time (A+B+C) which may be much greater than the NDL at depth X.
     
  4. MechDiver

    MechDiver Divemaster

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    The first problem was those divers taking you on a dive that you were not qualified for, the second problem was you agreeing to go on a dive that you were unqualified for.

    And I don't mean unqualified as far as depth, I mean you were not able to determine if the dive was dangerous, or what the dive parameters were.
    That is a very good way to hurt yourself.

    You also cannot plot a dive like this using PADI NDL tables. Take the entire time as the dive time, the max depth as the mas depth, call it a "deco" dive, and put in comments. "I shall not do this again until I know why I did what I did".

    Good luck,
    MD
     
    DannyD and Dive-N-Bike like this.
  5. Sideband

    Sideband Loggerhead Turtle

    # of Dives: 200 - 499
    Location: Carol Stream, IL
    1,514
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    I understand how all of that works for actual Nitrogen loading, but how do you log the dive? From what I see, the log and tables go hand in hand. When you break from the tables and start using a computer that allows for multilevel diving the log gets misleading.. As I see it anyway.. My computer doesn't give a letter group for example.
     
  6. crispos

    crispos Instructor, Scuba

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    Pilaar39, you sound like the prom queen that wondered what that fuzzy warm feeling was between her legs last night.

    Yeah, that was a decompression dive. Way to go. Next time ask about the dive profile and check your tables, BEFORE the dive.

    The PADI definition for bottom time is breaking the surface on descent, to the first (and only) safety stop, at 15 feet.

    P.S. You can't log it as a PADI dive. Just log it as a dive dive.
     
  7. Don Burke

    Don Burke Loggerhead Turtle

    # of Dives: 200 - 499
    Location: southeastern Virginia
    2,155
    6
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    You can come up with a pretty good approximation of letter group by matching the no-stop bottom times the computer gives you against the adjusted bottom times on table three. It certainly isn't perfect, but it does give you an idea of where you are if the computer craps out before or during the next dive.
     
  8. String

    String Master Instructor

    # of Dives: I just don't log dives
    Location: Grand Cayman
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    "dive time" here is from starting the dive to stopping at your first stop depth (6 or 9 meters if using our deco tables).

    "Bottom time" as mentioned from start dive to start of ascent.

    Then you have "Total time" or "Surface to surface" time which is the time from starting dive to ending dive.
     
  9. rickyd

    rickyd Solo Diver

    # of Dives: 500 - 999
    Location: Southern California
    389
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    Out of curiosity, do you have a computer with a nitrogen loading bar?, and did you look at it!!!!!!! I just went into my computer, in plan mode, and I show a max bottom time of 16 minutes at 100 fsw, before I go deco. This is on an Aeris 500ai. The SSI tables put 100 FSW at 17 minutes max. So, if you actually stayed down at 100 for 30 mins, then you are waaaay beyond NDL - even if we assume a few minutes to descend.

    Glad you're OK.
     
  10. Drew Sailbum

    Drew Sailbum Instructor, Scuba

    # of Dives:
    Location: Grand Cayman
    2,188
    103
    63
    Simple.
    Max depth = (whatever your computer says)

    Bottom time = (Your choice: total run time or run time minus safety stop and ascent time)

    A computer assisted multilevel dive in which I reach a maximum depth of 102' and spend a total run time of 34min gets logged as 102' for :34. I don't worry about the pressure group.
     

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