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Anywhere to Overfill Tanks Near the Keys?

Discussion in 'Wreck Diving' started by Addison Snyder, Dec 21, 2020.

  1. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
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    95' and 133's are not the same size. 95's and 117's are, and 108's and 133's are. The 117's are only 1.5lbs heavier than the 95's but both are 15L bottles, and the 133's are also about 1.5lbs heavier than the 108's with both being 17L bottles
    These bottles are all the same diameter, and the length difference is negligible for the 108/133 and less than half an inch for the 95/133's. I would hardly call that much larger and heavier.

    @Addison Snyder the bottles are not identical to the Euro tanks that are 250/300bar rated. They do have different specifications per Faber themselves. I have the spec sheets for them and they have different alloys. If they were in fact the same, then the manufacturers wouldn't have had to get special DOT permits for the HP bottles to keep their weight reasonable.
     
    rjack321, drrich2 and jborg like this.
  2. RyanT

    RyanT ScubaBoard Supporter ScubaBoard Supporter

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    @tbone1004, I was trying to provide a quick explanation of the advantage of overfilling LP tanks. I wasn't trying to make an apples to apples comparison in terms of tank water volumes. Sorry if that wasn't clear. A cave filled LP 95 gives close to 130 cf of gas (a little less for nonlinear compression). A service filled HP 133 is 133 cf. The empty weight of an HP 133 is 5.5 lbs heavier than a LP 95. In a set of doubles (I should have mentioned I was thinking that as well), 133s are 11 pounds heavier. In an already heavy set of lp 95 doubles, 11 pounds would be real weight to me for similar gas volumes.
     
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  3. Addison Snyder

    Addison Snyder Nassau Grouper

    # of Dives: 200 - 499
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    So it seems to me I'm out of luck as far as a place that fills tanks worth a darn. Anyone that consistently isn't total crap with fills (like not recognizing + ratings)?
     
  4. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
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    except apples to apples is important. A cave filled 95 does not give 130cf. It holds 0.53cf of water. 3442psi=237.3bar. 237.3*.53=125.7cf. Compressibility factor at 250bar and 300k=1.033. 125.7/1.033=121cf, a far cry from 130cf. Same math with 17L is 138cf.

    Your example proves exactly the issue when people start talking about relative tank volumes. The FX133's will hold 13% more gas than the 95's when filled to the same pressure. The FX177's will hold the exact same amount of gas and it's a whopping 3.5 lbs more for a set of doubles. You won't notice that.
     
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  5. RyanT

    RyanT ScubaBoard Supporter ScubaBoard Supporter

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    I'll go for 3.5 less pounds, but then I probably need to work out more :D!

    I was trying to illustrate to our EU counterparts why we overfill LP tanks, but your point is well taken.
     
  6. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
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    they have tanks over there with different working pressures and the same water volumes. It makes a lot more sense there because they would say I have a 15L 200 bar, or 15L 250 bar, or 15L 300bar and it makes sense. It's only over here where we measure tanks indirectly that it's an issue.

    For the Euro people the real reason we use LP bottles. We don't have any true 300bar tanks in the US, we only have 2 currently made pressure models, 2640/~200bar, and 3442/~250bar. The HP versions were not readily available in this country until relatively recently and they are significantly more expensive than their LP counterparts. They are also more negatively buoyant which is an issue in cave country where the history of overfilling began.
     
  7. RyanT

    RyanT ScubaBoard Supporter ScubaBoard Supporter

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    Or like when used to have to read Army Corps water gauge increments of 1/10 of a foot. I continue to be amazed that we still use imperial units.
     
  8. jborg

    jborg Lurker ScubaBoard Sponsor

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    Thanks, I'm kinda getting it and kinda not. :) Over here the default tech-ish setup is the twin 12x232, which looks like about the same tank as the HP100. OptimalBuyancyCalculator quotes the HP100 at -0.3 kg negative and the 12x232:s at -0.5 kg to -0.9 kg, so about 1 lb difference or roughly the same... That's considered a quite light and buoyant tank here, I certainly wouldn't want anything floatier. (The 300 bar tanks are indeed heavy, though.)
     
  9. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
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    I have straight edges at work that have tenths of inches on them as well...

    @jborg we don't have anything quite like your 12L's, but the 100's are as close as we have. Those are 13L
     
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  10. RyanT

    RyanT ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 500 - 999
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    @jborg, yes, the buoyancy characteristics are something else that I failed to mention. LP tanks are floatier than HP tanks. This seems especially important for cave divers who are carrying multiple stage bottles. @tbone1004 can speak to this as I'm not a cave diver. If you are planning to move spent stage bottles out of a cave, you don't want a bunch of negative bottles. AL tanks (commonly used for deco bottles in open water) just don't hold as much gas.
     
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