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Ditching run time

Discussion in 'Hogarthian Diving' started by lefrogster, Feb 27, 2014.

  1. lefrogster

    lefrogster Angel Fish

    # of Dives: 100 - 199
    Location: Hong Kong
    30
    8
    0
    I did the standard PADI course, which I remember was 1 confined, 3 open water dives and a check out dive. I don't have any issues with buoyancy or meeting the run time, but I don't see sticking to the run time to be THE most important thing in deco: better to ensure the you're breathing the correct gas at the correct depth. I also have an issue with the NOTOX procedure, where you're basically doing the first steps of the switches on your own (perhaps while ascending to the next stop) and then checking your partner, only AFTER the switch (which might be too late for him if he switched to the wrong gas).

    When I moved over to IANTD, basically, you do the switches sequentially, not simultaneously, which allows your partner to check your gas and depth with you and watch you while you do the switch. It takes longer, but I feel is more secure and certain. Only after you both have successfully switched do you start the deco time, so its a time relative to when you complete the switch (not an absolute time).

    Most of the time this just follows the run time, but if there is an issue -- free flow, entanglement, down current, problem with bottle rotation, problem with DPV -- you spend time to fix that problem first before starting your deco count. Else you risk ascending to the next stop before the problem is fixed. If you overrun the run time, you then have to recalculate your remaining stops by adding x minutes to each of them, which can further cause error.

    ---------- Post added February 28th, 2014 at 02:26 PM ----------

    hey everyone, thanks for the valuable input. I just want to clarify that I'm not advocating diving without a plan. Its diving with a relative run time for deco rather than an absolute one.

    For instance, in my PADI instruction, if we spent 30 min bottom time, 1 min to reach first deco stop at 21 m and 2 min per stop for the 50%, our plan would be something like:

    - 21 m at 31 min switch to 50%
    - 18 m at 33 min
    - 15 m at 35 min
    - 12 m at 37 min
    ... etc.

    Let's say someone takes longer getting to 21 m for whatever reason and is 30s late. Then you have to pad the remaining times by 30s in your head. Then if at 6m switch, there is another delay, you have to pad that time and the 30s to your original run time.

    At IANTD, we would just start the count at 21 m only after everyone has switched regardless any delays. It would be:

    - 21 m at 0 min switch to 50%
    - 18 m at 2 min
    - 15 m at 4 min
    - 12 m at 6 min

    So you're just starting the count T0 at the moment of the deco switch and abandoning the absolute run time of the dive itself.
     
  2. theskull

    theskull Divemaster

    # of Dives: 2,500 - 4,999
    Location: St. Louis, MO
    1,478
    22
    38
    As all things tech, this is not a PADI/IANTD issue, but an instructor choice issue. I was taught both methods (run time vs. stop time) and I teach both methods, but I absolutely prefer diving by stop times. I calculate the expected run time and report it to the surface support, but like the flexibility of the stop times.
    Wreck dives -- pretty much on schedule all the time; down, then bottom, then up.
    Cave dives or Shore dives -- schedule is planned for our maximum allowable dive given gas supplies and personal comfort level, but our dives are nearly always a little shallower or a little shorter than originally planned for whatever reason. We then stick with our planned stop times even though we get to them ahead of the run time schedule, and usually shorten the last deco stop according to what our personal dive computers call for at that time.

    theskull
     
  3. Centrals

    Centrals Barangay Pasaway

    # of Dives:
    Location: Hong Kong
    6,383
    1,649
    113
    1. Run time was difficult in the beginning but after 14yrs of using it.....
    2. Plan the dive and dive the plan.
    3. Introduce couple of mins at 21m for the gas switch when planning the dive in the beginning.
    4. Adding extra time after delay or whatever is not that difficult!
     
  4. tomfcrist

    tomfcrist NAUI Instructor

    # of Dives: I'm a Fish!
    Location: Virginia, USA
    2,889
    1,789
    113
    I think pretty much everyone ditches the run time at some point. It's not a task loading issue as much as it's ultra conservative if you end up shallower or for less time on your bottom portion. Everything is flexible, but if I end up shallower but for the same bottom time I will follow run times....if I stay for less time I will switch to stop times. That's why we write down both right?
     
  5. waterpirate

    waterpirate Solo Diver

    # of Dives:
    Location: Delmarva peninsula
    1,393
    418
    83
    I think the choice of the instructor at the intro or tec 1 level is obvious.... You have to start somewhere, and rigid is safe, when you are new and builds confidence and skills. After you get some dives under you, everything tech is a team or personal choice.

    I was taught to use every tool available on every dive. Run time, stop time, ratio and puter. boiling all those together makes for a well understood and executed deco. Run times as was mentioned is crucial to boat staff. It lets them know when to expect you on the line or see a bag.
    Eric
     
  6. Centrals

    Centrals Barangay Pasaway

    # of Dives:
    Location: Hong Kong
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    1,649
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    It also keep the team together instead of scattering over various depth.
     
  7. RJP

    RJP Scuba Media & Publications

    # of Dives: 1,000 - 2,499
    Location: New Jersey
    13,460
    6,035
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    [

    For simplicity sake, let's take deco gas, switching, etc out of the equation for a moment. (Assume a lost-gas scenario, if you will.)

     

    Suppose you plan a dive to 140ft, with 24%, and a planned bottom time of 25min.   What are your deco obligation (stops and time) and gas requirements for that plan? (Hint: 25min at 140ft = 37min of deco time for a run time of 62min and 133cf of gas.)

    Now, considering the same dive, imagine if you stay at 140ft for 5min longer than you planned - 30min instead of 25min. What are the changes to your deco obligation (stops and time) and gas requirements? (Hint: 30min at 140ft = 52min of deco time for a run time of 82min and 167cf of gas.)

    Still not seeing any benefits to being "forced to stick to a strict schedule"?  Asked another way, which would be more stressful: sticking to your planned runtime... or trying to complete a deco obligation requiring 167cf of gas on a set of double 80's that only contains 160cf of gas?

    My tech instructor (who has posted above) drilled us on "schedule, schedule, schedule" which was stressful during the class, but following a rigorous runtime plan outside of a course setting actually reduces stress because you understand where the limits are and plan accordingly.  In a non-course setting, there would only be stress if you are pushing the limits. 

    If you find that having a plan - and sticking to it - is stressful, perhaps tech diving is not for you.
     
  8. tomfcrist

    tomfcrist NAUI Instructor

    # of Dives: I'm a Fish!
    Location: Virginia, USA
    2,889
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    113
    Not sure what you are doing to plan this RJP, but I came to two conclusions reading this.

    1) You are doing very conservative deco.

    2) Your RMV is abnormally high for a tech diver.

    Running the same plans using Buhlmann ZHL16/C i come up with this

    @ 25min BT= 57 min run time/114.4cuft
    @ 30min BT= 74 min run time/143.1cuft

    Still not the best plan on AL80's, but I dont dive those things.

    Edit....I see you are using 35/80... Which explains the runtimes, but what RMV are you planning with?
     
    Last edited: Mar 10, 2014
  9. RJP

    RJP Scuba Media & Publications

    # of Dives: 1,000 - 2,499
    Location: New Jersey
    13,460
    6,035
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    I come to the conclusion that you've missed the point. Or rather that I obscured the point with irrelevant details that you've focused on instead...

    I started to write the above post with no specifics, and it was not clear. I should have have been clear that I simply downloaded some free deco app during a slow meeting at work and plugged numbers in without changing any of the base settings. (I use V-planner with my RMV, conservativism settings, etc to plan my dives... but I don't have V-Planner on my work iPad.) The point was NOT about the merits of that specific plan, but rather that for ANY PLAN the idea of "ditching the plan" because "following the plan" is stressful... is a bad idea.

    In the case of the hypothetical plan above - irrespective of its merits - the idea that an extra few minutes at 140ft could commit you to an extra 20min - and an extra 30cf of gas - at 15ft is the point. Which point is also illustrated with your numbers as well.
     
    nimoh and PfcAJ like this.
  10. tomfcrist

    tomfcrist NAUI Instructor

    # of Dives: I'm a Fish!
    Location: Virginia, USA
    2,889
    1,789
    113
    Oh no...I got your point...But was highly confused by the numbers. I, in no way, find following the plan as stressful or even remotely task loading. But I tend to over plan my dives, getting my deco plan based on the longest possible time I will be on the bottom....I rarely find a need to stay as long as I planned, therefore i can choose to either deco on the fly..or follow the original deco plan. If a person has the ability to accurately plan deco on the fly..there is nothing wrong with doing so. Many of the people that are held up on a pedistal for some reason by a buttload of divers on this board consistently deco'd on the fly....to include the DIR bubbas.
     
    RJP likes this.

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