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Cave Fills on LP tanks

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

  1. Lobzilla

    Lobzilla Contributor

    # of Dives: 100 - 199
    Location: North Carolina, Maryland
    673
    167
    Bob is correct to be somewhat more conservative with Aluminum.

    And, both Aluminum and Steel alloys used for common engineering applications, will experience first elastic and then plastic deformation, when subjected to increasing mechanical load. We try to avoid brittle fracture whenever possible and certainly in the case of scuba tanks that get banged around.

    To understand the differences between these two materials, let's take a tension spring made out of Aluminum and one made from Steel and pull them. For each spring, there is a point at which the spring has been permanently stretched and will not 'spring' back to its original length. They yielded once we exceeded the yield strength of the material.

    When we cram gas in our scuba tanks the tank will expand and hopefully spring back to its original size when emptied. We do not want tanks that have 'cheeks' (however small) after a fill.

    The difference between Aluminum and Steel alloys used for scuba tanks is that the modulus of elasticity (spring rate) and yield strength is lower for Aluminum. That is the reason why an Aluminum tank has thicker walls, has more external volume, and is heavier for the same pressurized capacity. Aluminum is not a very efficient material for pressure cylinders (and springs) but it is cheaper to make tanks from it and the buoyancy characteristics may be more appealing, depending on our needs.

    Once we pressurise a cylinder beyond the yield strength of the material, the wall of the cylinder will get thinner permanently. It should be clear that at this point the end is near. Either the pressure will continue to rise (filling or heating in the trunk of a car) until the cylinder blows or the next couple of fill cycles (again reducing the wall thickness) will cause catastrophic failure.

    During hydrotesting, any plastic deformation beyond a 'grace' limit is considered reason for condemnation. However, hydrostatic testing is performed at 1.5 times the maximum operation pressure for most cylinders. This pressure is stamped into the cylinder after the letters TP.

    Now we could hastily assume: If the hydro facility did not detect worrisome plastic deformation at 1.5 times the operating pressure then we should be fine with a cave fill to the same level. There is one little catch here - Fatigue. And this is where Aluminum looses hands down against Steel.

    In the early days of pressurized airplanes, engineers where really surprised when fuselages failed catastrophically and blew the lid well below their elastic design limit. Back in the lab it was discovered that Aluminum will fatigue during repetitive load cycles at any(!) load, even if that load is well below yield stress. Another reason why we are not going to find many springs made from Aluminum.

    Steel will fatigue too if stressed close to yield strength but in the case of scuba cylinders most of us may not get old enough to experience failure, UNLESS
    a) the cylinders were of marginal design and had trouble passing even the first hydro
    b) we exceed the elastic deformation limit through intentional or accidental (heat in car) overpressurization
    c) corrosion reduced the wall thickness
    d) pitting caused stress risers that aggravate fatigue
    (The last two go typically hand in hand)

    I am not suggesting what we do or not do with our tanks, just trying to shed some light on the factors involved.

    Here is an interesting Navy study on scuba tanks. But please let's read the section about the destructive potential before getting any bad ideas while looking at the reported burst pressures.
     
    Last edited: Oct 17, 2010
    northernone likes this.
  2. Meister481

    Meister481 Tech Instructor

    # of Dives: 0 - 24
    Location: Greenwood, Indiana
    642
    4
    That's what I have found to work the best. I don't like using 32 at JB for big dives, Usually end up with 26 or so for a dive like this. Any longer deco and I'll go with a second deco gas. The only reason I posted was for using O2 as the only gas for a dive with this much back gas. It's the limit of such for me. Only fried my lungs once, and got them irritated once. That was enough for me.

    The WKPP dives are well beyond my abilities and knowledge level. Andrew's dives are well beyond me as well. Have a while to go before I get there. Rebreather is the next step in that direction.

    It was definitely BG related, there is no other explanation. Anything under 20 minutes, no breaks, 20-60 minutes 12 on 3 off, over that 12 on 5 off. As far as the slow ascent goes, I do 3 stops in the chimney at JB.

    I'm not going any further with my planning in this thread, I don't need some douche killing himself by using my gas/deco schedule instead of getting trained.

    32 is fine for the short dives, keeps me off the bottle and resting on shore.

    The breaks save you, that's for sure. I upped mine after my issue. No longer happens, ever. Been a long time since I did so.
     
  3. Cave Diver

    Cave Diver Divemaster

    17,866
    2,519
    No problem. I've copied the discussion to a more appropriate venue for anyone wishing to discuss it. :)
     
  4. Meister481

    Meister481 Tech Instructor

    # of Dives: 0 - 24
    Location: Greenwood, Indiana
    642
    4
    Thanks John.
     
  5. Lobzilla

    Lobzilla Contributor

    # of Dives: 100 - 199
    Location: North Carolina, Maryland
    673
    167
    Hi guys,

    If I promise not to pull a 'douche' stunt, could you please PM me the continued thread. I am curious how you practically connect the physiologically different beasts of pulmonary and CNS toxicity.
     
  6. PfcAJ

    PfcAJ Contributor

    # of Dives: 5,000 - ∞
    Location: St Petersburg, Fl
    7,922
    7,293
    There appears to be come confusion regarding CNS% and OTUs. They are not the same thing. OTUs relates to pulmonary damage, CNS refers to neurological issues.
     
    LiteHedded likes this.
  7. Meister481

    Meister481 Tech Instructor

    # of Dives: 0 - 24
    Location: Greenwood, Indiana
    642
    4
    You might want to try a few books on the subject. I like the IANTD technical diving encyclopedia. I don't buy all the karate mumbo jumbo, but the science behind the book is solid. It's the same beast, O2, just different symptoms.

    And there always will be....lol

    Back on topic!
     
  8. Lukas Ircha

    Lukas Ircha Registered

    # of Dives: 200 - 499
    Location: Vancouver BC
    9
    0
    Seriously those 40 year old tanks can handle that much pressure 3000 psi or 3100. Wow yes that is nice 88 cubic feet is lovely.
     
  9. Littlerayray

    Littlerayray Regular of the Pub

    # of Dives: 100 - 199
    Location: Cornwall, Ontario, Canada
    780
    295
    I have double l.p. 108s and my dive shop fills them an additional 10% my tanks are always at about 3200 3400 never had a problem
    I'm 5'8" 220lbs I can lug those suckers around np the rig weighs 130lbs no extra lead needed I honestly don't notice any drag
     
  10. LiteHedded

    LiteHedded Contributor

    4,075
    921
    I take it back. Have since got burning throat and lungs on bigger dives than I was doing back then. It’s a real thing
     

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