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Weighting Paradox

Discussion in 'Advanced Scuba Discussions' started by cainslie, Jan 4, 2017.

  1. cainslie

    cainslie Angel Fish

    # of Dives: 500 - 999
    Location: Ballito, South Africa
    20
    5
    3
    Hi all.

    Hopefully some of you can help out with a bit of a weighting paradox I have:

    I'm constantly on the lookout for tips and advice on being a better diver and I came across this article recently: How to Reduce Weight When You Dive • Scuba Diver Life All good and well, nothing really new or revelatory in there.

    Generally, I consider my weighting to be good (4kg). On most dives I only need to add a little air in my BC once I'm near the bottom and I'm happy as Larry.

    But, with my current setup, if I empty my BC at the safety stop, I sink. Conventional wisdom says that means I'm over weighted. However! When I start my dive I always have to work a little to get down the first few meters. I flap like an injured seagull on the surface when I'm trying to descend. Once I'm down to about 3m, I then sink just fine.

    My descent strategy is to empty BC, tuck my knees to my chest, exhale and roll forward. I end up with my fins out the water and require some ungraceful kicking and flapping to start my descent.

    So, am I underweighted, overweighted or just descending with bad technique? I realise this is one of those things that's impossible to diagnose in a forum, but any tips or comments?

    Thanks.
     
  2. chris kippax

    chris kippax Divemaster

    # of Dives: 500 - 999
    Location: Australia
    573
    342
    63
    All I do is breath out slowly until you I feel myself become negative then I just hold my breathe for a few seconds until a metre or so deep, take a small inhalation breath and start finning then I'm on my way.
     
  3. Diving Dubai

    Diving Dubai Instructor, Scuba

    # of Dives: 500 - 999
    Location: Dubai UAE
    3,110
    2,806
    113
    Often, BC's trap air. Even when "empty" you still have atmospheric pressure. Also exposure protection if dry can be more buoyant than when wet.
    At the end of a dive water pressure pushes out the air - hence not having additional buoyancy (being heavy)

    Also subconscious anxiety (or sudden shock of cold water) as you splash can lead to a greater lung volume, that when you relax goes away.

    It maybe as simple as squeezing your BC some more at the surface and chilling, making sure you make a complete exhale for the descent.

    I typically have a very slow descent for the 1st meter or so (but never have to duck dive) but then am a bit over at the end.

    I would not recommend adding any more weight - it may just need some more relaxing
     
    manicminer likes this.
  4. The Chairman

    The Chairman Chairman of the Board

    # of Dives: I just don't log dives
    Location: Cave Country!
    55,617
    22,819
    113
    Stop chasing your tail. If you're sinking SLOWLY at the end of the dive with no air in your BC, I wouldn't worry about it. I was at a spring recently when a fairly well known cave diver came up to me wanting a single pound. At first I could only find two pounders, but he was adamant that he wanted just one pound.

    Wow. Talk about cutting hairs! Your weight is to compensate for your wetsuit and sometimes your body fat. Your BC and your lungs are used to compensate for compression of the same as you descend. So, if you can float at the end of the dive with a bit of air in your BC, you're OK.

    BTW, the biggest problem most people have with their buoyancy is actually their trim. If you're not horizontal and your thrust is also horizontal. If you don't understand this, check this out: Master Neutral Buoyancy: The Importance of Horizontal Trim (Simple Vector Physics) - ScubaBoard
     
    mnjhuz76 likes this.
  5. mi000ke

    mi000ke Manta Ray

    # of Dives: 100 - 199
    Location: Massachusetts & Grand Cayman Island
    755
    778
    93
    I raised this same issue recently in this thread - do you weight to get below the surface easily, to control buoyancy at depth, or to hover at a safety stop?

    Weighting for descent vs bottom
     
  6. archer1960

    archer1960 Manta Ray

    # of Dives: 50 - 99
    Location: Southern New England, USA
    572
    135
    43
    For me, it's the safety stop. If I can't hold that, it can be somewhat risky, or at least aggravating. If I am neutral at the safety stop, I have no trouble getting down at the beginning of the dive when I have the extra weight of the air in my tank helping me.
     
  7. halocline

    halocline Solo Diver

    8,469
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    I personally weight for neutral at 10 ft with a near empty tank. If I'm a bit heavy on the first dive, I'll take a couple of pounds off. What Diving Dubai said about the BC is definitely true. Not to hijack your thread into a dead horse beating fest, but you might try a dive with a simple BP/W, webbing harness, no padding, steel plate, and see if that changes your ability to descend easily.

    What Netdoc said about not worrying if you sink slowly at the safety stop is also good advice. I get the desire to dive with as little weight as possible, but what's more important is that you don't cork to the surface from your safety stop, even if had very little air in your tank for some reason.
     
    Dish likes this.
  8. The Chairman

    The Chairman Chairman of the Board

    # of Dives: I just don't log dives
    Location: Cave Country!
    55,617
    22,819
    113
    Fair question. Here is how I weight my students.

    At the side of the pool, breathing on your reg with an empty BC, cross your legs and cross your arms (important to do ALL of this!) Have a buddy guesstimate how far the top of your head is above the water after a normal inhale. Then, add a pound for every inch the top of your head sticks out of the water plus one or two. With your arms and legs crossed on a normal inhale, your head should be just awash (water just barely over your head). At this point if you exhale, you should start to descend slowly. Assume the Scuba Position (Horizontal) and point your head a bit down. You'll continue to descend until you take a breath. No problem, take a breath and your descent will stop. Just simply exhale again in order to continue your descent. Once you're at the depth you want, simply breath normally. At this point you should be able to control your depth with just your breathing. Oh, you can add a bit or remove a bit from your BC, but once you get comfy near the bottom, stop playing with your BCD. Just use your breath to ascend and descend. The only caveat is to never, ever occlude your airway. Super breaths are made with your chest muscles (Intercostal levators) only. Don't use your glottis to hold your breath ever.

    BTW, don't forget to add 3 to 4 pounds when you get in salt water. If you're wearing a wet suit, just toss it down the neck. They'll do just fine under your wet suit.
     
    manicminer likes this.
  9. nolatom

    nolatom Captain

    # of Dives: 200 - 499
    Location: New Orleans
    1,151
    514
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    Even tropical BCs have about what, 25 pounds of lift fully inflated? So overweighting yourself a little on the first dive is much less a problem than being underweighted. then fine tune til you're good. Make a point of logging that fine-tuning-- what was a little heavy, what was light, and what size and material tank you're carrying, what neoprene you wore. Then look that up before the dive

    From a full inhale to full exhale is worth maybe what, about four pounds? Check your logbook, do what worked before, and you should be within half a breath of "okay".

    Also, if you descend head-first, become friends with your bottom dump valve on the BC. It helps.
     
  10. chrisch

    chrisch Solo Diver

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    How much gas do you have at the safety stop? The key thing is to remember that if you are at that stop with 20 bar in an Alu tank you will have different buoyancy to 60 bar in a steel.

    I agree it is a worthy goal to reduce lead as much as possible but it is also important to be able to hold a mandatory deco stop in an OOG (empty tank) situation, breathing on your buddy's gas, if you are decompression diving. You might find the difference between - for example - 60 bar and empty is enough that you can consider yourself "correctly" weighted.
     

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