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Thread: how to be Neutral Buoyancy?

 


  1. #1
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    how to be Neutral Buoyancy?

    recently during Jan 2013, my wife finally overcame the worry of diving. now she is pretty well in the open water except during 2 incidents, which had happened during her 1st and 2nd night dives with an instructor.

    before we went for the training of AOW of SSI, we have went to Tulamben for shore dives, which was 3 dives on the 1st day and 2 dives on the 2nd day. After that we break for one day before heading for night dive. the following day, we went for 2 deep dives, which was around 33m and followed by a night dive. next day, we went for 2 deep dives which was 38m.

    During her day dives, she use 10 pounds of weights and with the BCD deflated, she is negative buoyant.

    But during the night dives, she was unable to sink down during the 1st night dives, and was given additional 2 pounds of weight. during the dive, all of a sudden, she became positive buoyant and shot up like a balloon and i went up to catch her down to 10m.

    During her 2nd night dive, she used a total of 12 pounds, and after her safety stop, she became positive buoyant again...

    Both of the cases, she is having 80 bar of air left, after the dives.

    is it due to panic?

    After a week later, she got decompression sickness... which i think, it's caused by the positive buoyancy.
    It can't be caused by the deep dives, as we were within the no decompression limit, went we are left with 2 mins, we proceeded to ascend.

  2. #2
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    Unless she ate a REALLY big dinner before the night dive, nothing physically changed to effect her weighting. That means another factor must have been responsible for her needing more weight. Most likely, this was anxiety... causing a change to her breathing pattern (more air in lungs = more float) and/or incorrect application of descent procedures (not exhaling fully, not fully emptying her BCD and/or finning upwards whilst at the surface).

    Adding more weight didn't do her any favors. She had to add more air to her BCD to compensate for this superflous extra weight. This meant that, on ascent, there was more air in her BCD to expand. From 10m to the surface, that air expands to double it's compressed volume. 2kg of weight needs ~2litres of air to balance it for neutral buoyancy. That extra 2 litres of air becomes an extra 4 litres of air as you ascend from 10m to the surface. That's a lot more air expansion to deal with....a lot more air that needs dumping on ascent.

    Having to dump more air on ascent increases chances of a run-away ascent. The more air, the more severe the run-away will be. This accounts for most novice diver buoyancy issues.

    In short... get the weighting right. Once you know the right weighting, don't add more weight unless a weight check proves the need to do so. If there is a problem with descent, rule out any skill-based issues first.

    Adding extra weight might seem like a logical way to reduce the risk of fast/uncontrolled ascent. It's counter-intuitive though... it makes that risk far worse.

    Here's my article series on buoyancy... it goes into more detail: Scuba Buoyancy Masterclass

    Positive buoyancy doesn't cause DCS. Ascending faster than the recommended maximum ascent speed can do that... although that itself is determined by the amount of nitrogen absorbed (depth + time). That relationship includes repetitive dives also... so don't fixate on depth alone.

    If you and your wife were diving to within 2 minutes of a no-decompression limit, then you did have a lot of nitrogen absorption. Remember the advice given in training... dive conservatively. A no-decompression limit is not a 'magical barrier' to DCS. A fast ascent pretty much invalidates the no-decompression limit... such limits are calculated on strict parameters - ascent speed being a primary factor.

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    What a great respnse from Devon Diver. I would only add that another factor may be the amount of air consumed, especially on the second example. The more air depleted from the tank, the more positive you will be. Also, when people are excited or anxious, they tend to breath quicker and more shallow, essentially maintaining more air in their lungs on constant average, which will render them more buoyant. And Devon's comments on the impact of being overweighted and the impact of needing more air in bcd no doubt added to the problems. Practice makes perfect when it comes to buoyancy control. So does being calm and comfortable in the water, and being properly weighted.
    DivemasterDennis

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    Per DCS:

    Saying it "can't be caused by deep dives" and then saying you were only 2 minutes within the NDL after days of repetitive deep dives is... (insert politically correct adjective here), those charts are based on a generalization of the "average diver" or even worse, someone fit in pique condition (it's just easier to assume it's a hard to achieve goal to be safe). You're always supposed to use conservative measures and safety stops, if you fear you're going to get DCS because you either exceeded your NDL or were close to, do a 8-15 minute stop, you had the air to do so. Working on the buoyancy, will allow you to do so as stated above. Even at 10m however, that's a pretty decent depth to still remain safe for, it's not good to exceed the ascent rate but it still gives you a decent ATA to off gas at in a "worst case".
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    Both of "the DD's" covered it, but I saw this....

    Quote Originally Posted by cool79 View Post
    But during the night dives, she was unable to sink down during the 1st night dives, and was given additional 2 pounds of weight. during the dive, all of a sudden, she became positive buoyant and shot up like a balloon and i went up to catch her down to 10m.
    "First" Night Dives are a perfect example of an extreme task loading demonstration.

    If you can eliminate "anxiety induced over breathing", which I don't think we have, there are mechanical possibilities.

    The only other way this can really happen?

    She lost a weight~ did that hastily added 2# ... did it fall away?
    Or gained buoyancy.... her BC inflator hose valve started to leak air into her BC.

    I have seen extremely slow insidious leaks in inflator valves- all but imperceptible- when enough time has elapsed, when a certain shallower depth is reached- buoyancy reaches a tipping point in a flash.

    Pilot error: I have also seen many newer divers dutifully "purging" their BCs on ascent while unknowingly pressing the inflator button instead.
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    Being a newbie, I am working on my bouyancy too. One thing that caught me on a dive was that I didn't fully empty my BCD when swimming up to my SS and as the air in the BCD expanded it made me positively bouyant. It didn't happen all at once but I didn't realize it until I was cruising towards the surface. By the time I realized the error, I was bobbing in the waves.

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    What your wife experienced isn't uncommon. The answers above are pretty much spot on with my experiences working with new divers. New experiences ... particularly at night when your vision is limited ... produce anxiety. This, in turn, causes physical responses that you may not even notice ... breathing becomes more rapid and shallow, retaining air in the lungs. This increases your buoyancy ... think of lungs as another, internal BCD which you are retaining air in when you breathe like that. There's also the possibility that she was unconsciously kicking upward, or just not noticing changes in depth due to limited visibility ... and not responding quickly enough by dumping air when she should.

    The good news is that these are all things that improve rather quickly with practice. The liklihood is that there is nothing "wrong" ... there is simply a need for more experience.

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    Yes, Devon D says it best. DCS related to weight problems? Something I haven't read about since joining SB. As he said-- get the weighting right. I did a weight check 7 years ago and none since. Trim has changed somewhat due to equipment, but not a whole lot.
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    I did notice the comment that she experienced DCS a week later - from all I have read and learned, this is highly unlikely. DCS will typically manifest within 24 hours.
    "Boys need to get together and do boy things with other boys occasionally. I'm okay with that." My Wife.

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    the trim was ok during the day dives, but when during the night dives...

    I think it's more towards anxiety, which made her ascent up to rapidly.
    The dive computer was borrowed from the dive centre, and it was stated that she ascent faster than 6 sec from 5m to surface.

    question for the future night dives, should I hold her hands to prevent this from happening again?
    I was told during my AWO SSI with the course director not to touch her or hold her hands. Let her swim on her own, which she ascent up too quickly during her night dives...

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