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Cave Filling LP versus HP tanks

Discussion in 'Tanks, Valves & Bands' started by Compressor, Jun 23, 2018.

  1. 2airishuman

    2airishuman Solo Diver

    # of Dives: 100 - 199
    Location: Greater Minnesota
    Modern HP cylinders (that is, anything except the very thick walled 3AA3130 and 3AA3000 cylinders that have been out of production for decades) are made with different steel that is stronger than the steel used in 3AA cylinders.

    They are also heat treated, and are subject to destructive test requirements for each batch of cylinders.

    Though there are technical differences, the PST, Worthington, Asahi, and Faber special permits are largely the same and use broadly similar steels with similar test requirements. Asahi made a spun cylinder that was heavier due to a thicker base, the Faber permit uses metric units while the PST one uses imperial, the later permits allow ultrasound testing in place of hydrostatic testing, and there are very slight differences in the steel composition.

    But none of these are made from the same steel as 3AA cylinders.

    All gas cylinders are designed around their test pressure. For 3AA LP steels, that's 4000 PSI. For special permit HP cylinders, it's 5250 PSI. Metallurgically, theoretically, either should withstand repeated stresses to 90% of test pressure. There is some -- really not a lot in the greater scheme of things -- field experience with 3AAs that says that is safe. Whether that field experience is applicable to HP cylinders is, at this point, an open question. I don't want to be part of that experiment, and you'll have to decide for yourself whether you want to be a part of it.

    Field experience is vital because cylinders in SCUBA service are subject to things that they don't see in a lab. They are dropped, banged around, they suffer corrosion, they have threads that aren't perfect, some have thicker walls due to inherent uncertainties in the manufacturing process, etc.

    As pointed out upthread, there are diminishing returns from increasing the pressure much above 3500 PSI. The ideal gas laws -- that state that the mass of a gas is proportional to volume and absolute pressure and inversely proportional to absolute temperature -- don't hold at that point, because the diameter of a gas molecule becomes large relative to the space between molecules.

    Stay safe...
  2. DanaHunt

    DanaHunt DIR Practitioner

    # of Dives: I just don't log dives
    Location: Watertown, NY

    They are 104 when in plus service. They are roughly 95's at 2400.
  3. DanaHunt

    DanaHunt DIR Practitioner

    # of Dives: I just don't log dives
    Location: Watertown, NY
    Here's how I fill my lp108. As you can see the end result of the cubic feet is substantially different than a 3442 tank called a 130 but is considered its physical size equivalent

  4. runsongas

    runsongas Contributor

    # of Dives: 500 - 999
    Location: California - Bay Area
    3aa spec is low carbon steel that is lower tensile strength but better elasticity. the higher elasticity allows it to tolerate the additional stress of overfills better so that it doesn't fatigue prematurely and fail their next hydro.

    most 232 bar / 300 bar / 3442 psi tanks are made from high carbon steel which makes them stronger/tougher and thus have thinner walls/lower weight for a given capacity/service pressure. but their lower elasticity means more risk of permanent deformation at higher pressures and more likely to lead to premature failure at their next hydro.
    Scared Silly and MichaelMc like this.
  5. Lobzilla

    Lobzilla Contributor

    # of Dives: 100 - 199
    Location: North Carolina, Maryland
    There are a couple of different specs floating around on the internet.

    I will change the example to a different tank and research this further. The 104s have to go to hydro anyway and I can measure the internal volume and do the math from there
    Last edited: Jun 23, 2018
    DanaHunt likes this.
  6. taimen

    taimen Rebreather Pilot

    # of Dives: 200 - 499
    Location: Europe
    Absolutely true. What I tried to say was that "real" 300bar cylinders are so heavy that I doubt "identical" cyliders are sold in the US with lower rating.
  7. Storker

    Storker ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    As always, it depends. And not if you compare on a capacity basis rather than on a water volume basis.

    A 10L300bar weighs pretty much the same as a 12L232bar or a 15L200bar, and they all carry around 3000 bar-liters (the 10L a little less due to non-ideal gas behavior above ~250 bar). On the other hand, I once had a 15L300bar, and it was a beast. Hardly needed any weight on my belt. I sold it, since it was too heavy and had way more capacity than I need for a single dive, but not enough for two.
  8. taimen

    taimen Rebreather Pilot

    # of Dives: 200 - 499
    Location: Europe
    If you follow the earlier discussion, I was referencing to the claim that identical cylinders are sold in Europe stamped as 300 bar cylinder (this claim to support overfilling cylinders). That was not really a capacity based comparison. I have hard time believing that european 300bar cylinders weighting as much as they do would be sold in the US with much lower pressure rating (3442psi=237 bar) due to liability issues.
    Last edited: Jun 24, 2018
  9. northernone

    northernone Contributor Rest in Peace ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Currently: Cozumel, from Canada
    Does sound logical.
  10. Bob DBF

    Bob DBF Solo Diver

    # of Dives: I just don't log dives
    Location: NorCal
    My perspective is that it has always been a rumor, usually put forth as justification for overfilling cylinders. If the tanks were the same, it would be easy to verify as scuba cylinders are not state secrets.

    In addition if a manufacturer was making such a tank, why not add the info to the tank like they do with DOT and TC and simplify the marketing.

    Last edited: Jun 24, 2018
    taimen likes this.

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