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Calculating turn pressures (gas matching) for a pair of cave divers.

Discussion in 'Cave Diving' started by CaveSloth, Mar 13, 2020.

  1. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    Sometimes the Rules need to be revisited.
    Team foxturd: DIR Rules. Explained
     
  2. wKkaY

    wKkaY DIR Practitioner

    # of Dives: 500 - 999
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    I'm a new cave diver who trained in Mexico and in metric.

    When I visited Florida I found gas planning a lot more bothersome as there were different sizes and manufacturers of tanks, and it wasn't always clear what their internal volume is, and fill pressures differ from one shop to another or from one day to the next.

    Tank factor was something new to me when someone flipped to the page in his wet notes. I don't think he's ever had much practice with them, as he struggled and gave up after one try In the end we just did it the metric way with our best estimation of our tank internal volume (that I took other peoples word for).
     
  3. Superlyte27

    Superlyte27 Cave Instructor

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    I took my cave training in 1996. Gas matching was taught back then. Not sure how far back you have to go to find someone who wasn't taught gas matching by a good instructor.
     
  4. Manatee Diver

    Manatee Diver Manta Ray

    # of Dives: None - Not Certified
    Location: Tampa Bay, FL
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    There are four common tank sizes and tank factors
    AL80s 2.5/5.0
    LP85s 3.2/6.4
    LP95s 3.6/7.2
    LP100ish 4/8 - Covers all in the LP100 range as you have 104, 105, and 108 each with their own tank factor.
    HP100s 2.9/5.8

    Yes metric is easier as the tank size is stamped on the tank, but these turn it into metric like calculations.
     
    wKkaY likes this.
  5. wKkaY

    wKkaY DIR Practitioner

    # of Dives: 500 - 999
    Location: Malaysia
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    Just curious how did those numbers come about e.g. "3.2" for LP85? are they in gallons or some other kind of unit?
     
    grantctobin likes this.
  6. O-ring

    O-ring Solo Diver

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    cf3 per 100 psi

    In the LP85 example (2650 is working pressure in an LP85)

    2650/100 = 26.5
    85/26.5 = 3.2
     
    wKkaY likes this.
  7. Manatee Diver

    Manatee Diver Manta Ray

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    wKkaY likes this.
  8. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    wKkaY likes this.
  9. Manatee Diver

    Manatee Diver Manta Ray

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    Of air too, though the Z factors for nitrox are close enough.

    Although these caveats also apply to metric calculations too.
     
  10. CptTightPants21

    CptTightPants21 Solo Diver

    # of Dives: 200 - 499
    Location: NY/NC/FL
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    It depends on the environment--diving off a boat or a shore dive with a lot of surge, do it before hitting the water. Cave, quarry, or easy walk in shore dive-do it in the water.

    There are less distractions when you are all in water about to start the dive than on the surface--either people are still setting moving gear around, doing other checks, or you have everyone standing around overheating in their drysuit when this all could be done in the comfortable setting of in the water. In the water everyone is on the same page and can better control pace.

    To go back to your example, a team that skimps on the s drill or is haphazard isn't going to be any better in or out of the water.

    From a cave diving perspective, every dive has one or more stages. It's easier to the stage calculations when you have all your bottles ready to go then to try to remember back what you thought you said 20 mins ago in the parking lot.

    It is cuft of volume per 100psi. so (100 psi/working pressure psi) * tank size

    Working Pressure would be 2640 for LP tanks, 3000 for Al80s, 3442 for HP tanks.

    LP85s = (100 / 2640) * 85 = 3.2 cuft (x2 for doubles)

    HP 100s = (100 / 3442) *100 = 2.9 cuft

    It's difficult to explain calculating turn pressures and volumes in metric vs imperial in a clear and concise way. I would just remember the below key numbers and use them as needed in your own calculation process.

    For Metric =
    AL80s = 11 liter tanks
    hp100s = 12.2 liter tanks
    LP85s = 13 liter tanks
    LP108s = 17 liter tanks

    28.3 liters in each cu ft
    1 Bar = 14.7 psi
     

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