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Bouyancy characteristics for hp steel tanks

Discussion in 'Tanks, Valves and Bands' started by formernuke, Nov 27, 2019.

  1. halocline

    halocline Solo Diver

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    You have to understand that the empty buoyancy is part of your weighting calculation. If a tank is neutral empty, it does not impact how much lead you need for a given wetsuit/BC configuration. If it's positive (like AL tanks) you need to add lead matching the amount the tank is positive empty. If it's a heavy steel tank that is negative empty, you remove lead by that amount.

    This is useful information IF you know what your lead requirement is with a different tank but the same suit/BC. If you are just trying to figure out which tank works best for you with a specific wetsuit/BC, it can start with a guess based on the thickness of the wetsuit and the specs of the tank, but you will always need to do an actual weight check.

    Almost all divers wearing a wetsuit/BC will need weight with an AL tank. There are some heavy steel tanks that will result in a diver being overweighted even without any additional lead; it depends mostly on the thickness of the wetsuit. You definitely want to avoid that scenario.

    The full-to-empty part of your question is entirely due to the capacity of the tank. More air, more weight swing. It's really that simple. It just means that if you are ideally weighted with say a steel 120, you will be about twice as negative at the beginning of the dive as you would be with an AL63, because twice as much air weighs twice as much and you always adjust your weighting for an empty tank.

    Im not sure if this is helpful info to you, but it's a starting point.
     
    Kupu, FreeFlyFreak and Doctor Rig like this.
  2. Doctor Rig

    Doctor Rig Nassau Grouper

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    @Bob For the less informed, why would a -7.25 empty be good for cold water? Because extra weight is required anyway for thicker rubber?
     
  3. drk5036

    drk5036 Barracuda

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    Right. If you have more neoprene, you’re going to need more lead to descent. It just means you need less lead on a belt or in pockets, as it’s taken care of by the tank.
     
    Doctor Rig likes this.
  4. Bob DBF

    Bob DBF Solo Diver

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    Yes, rubber or drysuit. When you are wearing 25 to 30 something # of ballast, the tank negative buoyancy can replace some of the belt weight.
    Of course, if you are in Fl and not using any ballast, you could get into trouble if you are negatively buoyant and have a BC problem.

    As I remember from an Al 80 to an LP 80 you can drop around 6# off the belt, and your overall weight will drop a couple of pounds, all due to changes in weight and buoyancy between tanks. And you will pick a couple of cuft of air while you do it.



    Bob
     
    Doctor Rig likes this.
  5. Diver below 83

    Diver below 83 Regular of the Pub

    # of Dives: I'm a Fish!
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    I’m looking to purchase my first tank currently and am in this same boat trying to figure out the ideal size. Currently I’ve always dove with rented al80’s with rash guards and jacket rented bc. I’ve been using 14 lbs but always felt over weight. Tried 12 and found that much better but still felt over. Haven’t tried 10 yet.

    now I’ll be using my bp/w. It is stainless but it’s the dive rite lite. So it’s only about 2# of usable weight. I also just ordered my deep 6 3mm wetsuit. I’ve never dove with a suit so I have zero clue how much that’s going to change my trim. I would like to buy a steel tank along with a pony. I do plan to solo dive. Currently I was considering everything from hp80(smaller and would save some weight since I want to do a pony too) lp85(gives me weight and a little extra air) up to hp100 again with a pony as well. Considering I’ll probably do either a 13 or 19 pony I feel like hp100 might be overkill. So I was leaving more towards 85 then. Any suggestions are highly welcome
     
  6. Bob DBF

    Bob DBF Solo Diver

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    I’d get my weighting down before I picked up a tank, other than an Al 80. You need to know how negative you might be during your dives, and whether you are good with no ditchable weight, if it turns out that way.

    Oh yeah, the pony is negative when full as well.


    Bob
     
    Diver below 83 likes this.
  7. formernuke

    formernuke Manta Ray

    # of Dives: 50 - 99
    Location: New England
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    I got a faber steel 100 for a good price so I went for it.

    Weight wise in a dry suit in fresh water with a stainless steel backplate, single tank adapter and steel tank I need 6 pounds of lead, I weigh 185 pounds.
     
    Bob DBF likes this.
  8. Doctor Rig

    Doctor Rig Nassau Grouper

    # of Dives: I just don't log dives
    Location: Michigan
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    Reference: “Buoyancy Calculator” post and program by @rsingler

    If you haven’t used RSingler’s program to predict lead on a proposed tank & rig, you might give it a try. He has an extensive list of tanks and their properties included with his program.

    if you have tried RSingler’s program, how accurate has it been for you?
     

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