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Anywhere to Overfill Tanks Near the Keys?

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

  1. jborg

    jborg Lurker ScubaBoard Sponsor

    # of Dives: 50 - 99
    Location: Sweden
    Gotcha. If you're carrying a bunch of tanks I can see the equation being entirely different. I'm limited to just regular backmount experience.
  2. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    primary "backgas" bottles, whether sidemount or twinsets need to be part of the "balanced rig" because they can't be ditched. The specific buoyancy characteristics of those bottles weigh heavily into that calculation and 3-4lbs of buoyancy can be huge depending on exposure protection and whatever else is going on with your rig. In general you want them to be pretty heavy so you don't need to carry any lead, but at the same time you don't want them so heavy that you are no longer diving a balanced rig.
    Stage and deco bottles need to be as neutral as possible so they don't disrupt the balanced rig, but also so they don't cause you to roll over when you clip one on, hence AL80's and AL40's being used when AL30's and LP72's are not because they are too heavy.
    jborg likes this.
  3. Capt Jim Wyatt

    Capt Jim Wyatt Hanging at the 10 Foot Stop Staff Member

    # of Dives: 5,000 - ∞
    Location: High Springs - Cave Country
    Rainbow Reef will not overfill your tanks. If you want tec trips go to Horizon Divers, hands down.
    The Chairman likes this.
  4. drrich2

    drrich2 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Southwestern Kentucky
    Interesting! On a brief tangent...what's stopping someone from bringing European-style tanks to the U.S., if they're cramming more gas into a (nearly) same-size tank? A lot of us like more gas. Is the DOT somehow blocking this via regulation?

    For us Imperial unit folks, how much gas in cubic feet can one back into one of the big, higher pressure European-style tanks? Wonder what PSI they fill at?
  5. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    The tanks must have DOT stamps to be allowed hydro. They have 300bar rated tanks, so 4350psi. The Z-factor tables only give the equivalent of 80F, so can't get down to 70f, but it's close enough. 300bar at 300k has a compressibility factor of 1.109. So your LP95's that would sound like they were 95*4350/2640=156cf, only hold 140cf. Not that much more than they do at a cave filled 250bar/3600psi at 121cf and most people deem it not worth the drastically increased wear on the first stages and compressors .
    Zef and drrich2 like this.
  6. Addison Snyder

    Addison Snyder Contributor

    # of Dives: 200 - 499
    Location: Gainesville, FL
    Mind sharing a good read on how that all works?
  7. Manatee Diver

    Manatee Diver Stop throwing lettuce at me! ScubaBoard Supporter

    # of Dives: None - Not Certified
    Location: Tampa Bay, FL
    Cliff notes version: As you pump more gas into a tank and the pressure increases the attractive and repulsive forces between atoms causes the gas to deviate from the ideal gas law, which for scuba gases means you get less and less gas as the pressure increases. We compensate that with applying a Z factor using a chart for the gas, temperature, and pressure.
    rjack321 and drrich2 like this.
  8. Wookie

    Wookie Curmudgeon Apprentice ScubaBoard Business Sponsor ScubaBoard Supporter

    European tanks don’t come with DOT stamps, and therefore can’t be filled in the US.
    rjack321, Johnoly, Zef and 1 other person like this.
  9. The Chairman

    The Chairman Chairman of the Board Staff Member

    # of Dives: I just don't log dives
    Location: Cave Country!
    Just bring a rebreather and be done with it. :D :D :D
    Johnoly likes this.
  10. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    1*2 does not always equal 2, sometimes it equals 1.9...

    Basically the molecules can only be packed in there so tightly. Up to about 2500psi at room temperature we assume that everything is linear, i.e. 1*2=2. After that, it gets exponentially harder to compress, so even though 1*4 should equal 4, it only gets to about 3.9.
    In "normal" open circuit diving it really doesn't matter because everything is gauged off of pressure.

    Contrary to the point that @The Chairman made though about rebreathers, is that rebreathers are one of two pieces of dive gear that caused us to pay attention to it in the cave diving community. On normal open circuit diving, we dive to thirds. The Z-factor actually adds conservatism to the dive because the "1st third" of 3600psi down to 2400psi is actually smaller than the "2nd third" of 2400psi to 1200psi, which is slightly smaller than the "3rd third" or safety/reserve third of 1200psi to 0. Happy days.
    When scooters are used though, there aren't any magic ratios that you can use because your "planned speed" is different than your "emergency speed" so everything now has to be done on gas volume. Rebreathers are the same. Now instead of the math being super easy and going "well I kicked in for 1200psi, so it will take me 1200psi to get out", it now becomes "well I kick 50fpm, and I breathe about .65cfm while I am kicking, and I am planning to dive at 100ft or 4ata's, so 4*.65=2.6cfm DAC, and at 50fpm it takes me 2 minutes to go 100ft, so I need 5.2cf/100ft. I'm planning a dive to 3000ft, so I need 30*5.2=156cf of gas to get out, plus a 1.5x reserve=234cf total. PHEW! So now the compressibility factor gets REALLY important. Someone will then say well I have a pair of LP85's and if I pump them up to 3650psi I'll have 235cf and enough gas to get out. The issue is the Z-factor at 300K/250bar is 1.067. 235/1.067 is only 220cf of gas and while they think they have enough, they really don't.

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