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Turn pressures

Discussion in 'Advanced Scuba Discussions' started by KenGordon, Feb 17, 2019.

  1. KenGordon

    KenGordon Rebreather Pilot

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    I have had several people, in two contexts, suggest that a turn or turnaround pressure of a 100 bar is a thing.

    Does anyone have practical examples of when that would be the case in simple single tank diving?
     
  2. Jeremy Williams

    Jeremy Williams Barracuda

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    Just fill your tank to 150 bar before the dive?
     
  3. KenGordon

    KenGordon Rebreather Pilot

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    A long time ago I owned a 2250 ish psi cylinder, but this is a question for modern diving with normal, full cylinders.
     
  4. barth

    barth DIR Practitioner

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    Start a reefdive with 200 bar, go to 30 meter depth. Go slowly shallower and turn the dive at 100 bar at 18 meter (20-25 minutes divetime) or something like that. Go slowly shallower to 10 meter (40-50 minutes divetime). Do a safetystop. Surface with a tankpressure of 40-50 bar.
     
  5. KenGordon

    KenGordon Rebreather Pilot

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    So exactly what do you mean by ‘turn’? It doesn’t sound like you are returning the way you came but just using that to mean ‘start to finish’.
     
  6. decompression

    decompression Instructor...seriously...

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    Rock bottom calculation could easily have 100 bar as turn pressure for a dive profile.
     
  7. KenGordon

    KenGordon Rebreather Pilot

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    Can you give an example? I am wondering if this is a language thing. By ‘turn’ do you mean pressure at which to leave the bottom? Or actually turn back?
     
  8. Dominik_E

    Dominik_E Barracuda

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    I also think it may be a language thing. There are dive sites where profiles with a notably deeper begin and a shallow tail are well possible.
     
  9. decompression

    decompression Instructor...seriously...

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    Certainly,
    So, if we know a few things...
    Divers sac rate, tank size and fill pressure
    and profile.
    SACxATAxascent time x divers = RB

    1.0 x 2.5 x 8 x 2 = 40
    So 40 cuft =1500 psi
    So in metric it’s still 50% of total bar (200)
    This is still very conservative as it’s using a SAC of 1 but in a non-emergency you could easily use your actual SAC which is likely lower than 1.0. Also, this incorporates min deco ascent times so you can recalculate with faster rates (100’ = 6 min at 30’/min)

    So, at 100 bar you must leave the bottom and start your ascent. Now, if “turn pressure” is used to describe the point in the dive when we head back to start and then start the ascent then the BAR would be significantly higher. So, 10 bar descent, 20 Bar travel, 20 bar return and 100 bar ascent. You can influence this profile by bringing more gas, shortening the bottom time or changing the depth.
     
    wetb4igetinthewater likes this.
  10. AdivingBel

    AdivingBel Diver ScubaBoard Supporter

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    Sounds like a fairly typical warm water, rec wall dive. 3000 psi AL80. Dive to the wall. Drop to your max depth (70/80 ft.). Cruise along the wall until 1500 psi. Turn and ascend to 30/40 ft. or top of the wall. Work your way back to the boat. Paddle around under the boat. Surface with about 500 psi. 50/60 min. dive. Also works in Bonaire & Curacao with it's ubiquitous "swim to the drop," but usually with a longer dive time given overall shallower average depth and shallow swim out/in given it's a shore dive. Pretty standard in the Caribbean.
     
    yle and Sevenrider860 like this.

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