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Cave Fills on LP tanks

Discussion in 'Technical Diving Specialties' started by ScubaFeenD, Oct 15, 2010.

  1. Fruitographer

    Fruitographer Angel Fish

    # of Dives: 200 - 499
    Location: Crystal River, FL.
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    I spoke with a hydro guy the other day and he was saying the issue with overfilling is the stretch of the metal. The tanks are designed to go through 1,000's of hydro's although I can't remember the exact number. My hydro buddy said the tanks will stretch during hydro but then they will relax back to their original state over a month or two time period. By continually over filling, the tanks never get a chance to settle and go back to the original state which they could do on a proper fill. The metal weakens and eventually the tank will be condemned.

    I think the condition of the tank should also be taken into consideration. We all know there are tanks out there with pits and rust that should definitely not be used but people do it anyway. At least if you're gonna overfill, do it on some good clean tanks and not something with a bunch of corrosion, rust or pitting.
     
  2. KevinNM

    KevinNM DIR Practitioner

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    Sure, if you are pressurizing them past the point where they have an infinite life. I'm not sure if that is something that can be accurately calculated or just requires testing a large number of tanks to produce a statistically valid sample. But even the standard 10,000 cycle life is a LOT of cave diving. Like 13 years of daily diving with two fills per day every day.
     
    JohnnyC and PfcAJ like this.
  3. rjack321

    rjack321 Captain

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
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    Except with 3AA steel cylinders this hasn't proven to be the case. I have seen and used 25+yo 104s that have been nearly continually overfilled and they pass hydro just fine. 1,000,000 fills at 2640psi or 200,000 fills at 3600psi is still longer than any of us will actually cave dive. Overfilled "LP" steel tanks are outliving divers not the other way around.
     
  4. Fruitographer

    Fruitographer Angel Fish

    # of Dives: 200 - 499
    Location: Crystal River, FL.
    50
    6
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    The majority of the tanks I've seen in Florida are all cave filled and don't seem to have a problem. I wouldn't be worried to overfill my LP85's, of course I would never do that because it's illegal. The main thing I was just mentioning in the last post was that the 10,000 hydro life cycle is if the cylinder has around 2 months to relax back to the original stretch of the metal. If the tank is continually overfilled, it causes the steel to swell ever so slightly and never gets a chance to relax back to it's original form. Even if it's only 2,000 life cycles that's still a lot of diving.
     
  5. CuzzA

    CuzzA Solo Diver

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    I'm not an expert in the hydrostatic testing world, but if the tank doesn't return to it's original size doesn't it get condemned? I thought that was the whole point. I've never heard of this two month to go back to normal suggestion.
     
    Coztick, Lorenzoid, PfcAJ and 3 others like this.
  6. gobuyastick

    gobuyastick DIR Practitioner

    # of Dives: 50 - 99
    Location: Florida
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    Im three years older than my lp104s, i bet theyll outlive me :)
     
    rjack321 likes this.
  7. Diver-Drex

    Diver-Drex Nassau Grouper

    # of Dives: 200 - 499
    Location: US east coast
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  8. Charles2

    Charles2 Barracuda

    # of Dives: I just don't log dives
    Location: Montgomery, Texas
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    For DOT specification tanks (includes 3AA & 3AL) -- "A cylinder must be condemned...when permanent expansion exceeds 10 percent of total expansion."

    The tank does not need to return to its original size in order to be requalified. Most tanks do have an almost imperceptible permanent expansion. I have never heard of the 'two month' relaxation theory either.
     
    Scuba J7 likes this.

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