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Just starting in doubles

Discussion in 'Basic Scuba' started by Frackingawesome, Jul 3, 2019.

  1. Frackingawesome

    Frackingawesome Contributor

    # of Dives: 100 - 199
    Location: Lancaster, Pa
    91
    42
    I am just getting started in doubles. But I have a question. Can I use 2 takes from different companies? For instance, I have an HP100 from Faber and a Genesis HP100. can I band them together or do I need 2 identical tanks? I know I can not mix tank sizes.
     
  2. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    18,255
    10,535
    @Frackingawesome you can, but I really would not recommend it. The reason you don't want to mix sizes is because the lengths being different will make sitting quite annoying. Underwater it doesn't really matter, but sitting is going to suck. I'd try to find a mate to either the Faber or the Genesis. If the Genesis is an Asahi, then you'll be rolling to whatever side that bottle is on.
     
    Compressor likes this.
  3. Frackingawesome

    Frackingawesome Contributor

    # of Dives: 100 - 199
    Location: Lancaster, Pa
    91
    42
    that I what I though. thanks
     
  4. drainaps

    drainaps Registered

    # of Dives: 200 - 499
    Location: Shanghai / Singapore / Cebu
    55
    40
    Your trim will be difficult to keep unless you balance things out between the tanks with weights. I'd not do it, especially at the learning stages (I was particularly slow at getting comfortable with doubles after being quite proficient with a single tank, so any complications such as the one you have in mind seem like a bad idea to me).
     
    Compressor likes this.
  5. melanie.

    melanie. Contributor

    # of Dives: 200 - 499
    Location: Ontario, Canada
    298
    227
    You can do whatever you want.

    The tank length isnt the biggest issue - you can fix that with tank boots and spacers to make them level on the ground.

    It's your trim. Different tanks float differently... even the same brand but different years can. So you might feel rolly because of the way the tanks get lighter through the dive.

    That being said, if money is an issue, you can always try the tanks you have. Worst case you don't like it, and buy another tank so they match (which is what you were going to do anyways).
     
    Bowers likes this.
  6. Graeme Fraser

    Graeme Fraser Tech Instructor

    # of Dives: 1,000 - 2,499
    Location: Narnia
    927
    1,608
    Another thing to consider is whether the diameters are exactly the same, otherwise the cylinder bands might not tighten evenly. This could result in overstressing the manifold, particularly if there's a difference in length.
     
  7. TrimixToo

    TrimixToo Regular of the Pub

    # of Dives: 200 - 499
    Location: New York State
    1,026
    1,109
    I used to think that scuba tanks were built to precise dimensions. Then I chucked a few in the lathe to cut them, face the cuts, and try to bevel them. (Think "porch bells" and "umbrella stands.") I was amazed at how much runout they had. You don't even need a micrometer to measure it; you can use a ruler. They all had to be beveled with a file.

    IMO, the effective diameter of a tank under the bands depends on it's rotational position. That in turn is controlled by the essentially random thread starts for each tank and valve. There's enough compliance in the bands to let them be snugged up with some amount of mismatch. I'm sure that "some amount" varies by band manufacturer, at least. If they didn't snug up I'd flip them and try them the other way before giving up.

    I have one mismatched set of doubles. Both tanks are Fabers! The height difference is just enough to be annoying once in a while. I could trim one of the boots to even them up, but (a) I'm too lazy and (b) I would inevitably reinstall them on the wrong tanks, or even on a different set of doubles, making things worse.
     
    melanie. likes this.
  8. ScubaSam

    ScubaSam Sister of Shenanigans ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: Cave Country
    40,441
    7,881
    This is the exact reason given by my instructor when I moved from single tank to doubles.

    Good luck, and welcome to doubles!
     
  9. Compressor

    Compressor ScubaBoard Supporter Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: NYS
    3,374
    1,798
    Great explanations above. That's why many recommend 2 tanks from the same manufacturing batch which for us common divers means buying the tanks at the same time from one location.
     
    ScubaSam likes this.
  10. divad

    divad ScubaBoard Sponsor ScubaBoard Sponsor

    8,364
    2,339
    I have three sets of Fabers (how did that happen?) They are all "matched", but from different years. The older set is 1/2" taller at the neck than the other two sets so they get the lower profile Halcyon manifold. The other two (shorter) sets get the taller Thermo manifolds so they're all in about the same place.
     

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