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Dry suit diving: BCD vs suit for bouyancy

Discussion in 'Basic Scuba Discussions' started by gedmondson, Oct 10, 2008.

  1. gedmondson

    gedmondson Solo Diver

    # of Dives: 100 - 199
    Location: West Linn, OR
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    I took my drysuit diving class last evening. We were taught to use the suit for bouyancy control, and I found this method to be intuitive and gave me finer control than using the BCD. I have seen comments on this board advocating the BCD for bouyancy control while diving dry. I would like hear comments and rationale both ways from experienced dry divers. Thanks in advance!
     
  2. FireDiver443

    FireDiver443 Divemaster

    # of Dives: 200 - 499
    Location: Western NY
    145
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    To each their own. I've tried both ways, and found, depending on the dive, I had to manipulate my body/change my position in the water quite a bit to get the air bubble in the suit to the exhaust port on my left arm. I prefer to add just enough air to get rid of suit squeeze and use my BP/W to control buoyancy.

    From the mental perspective of diving, since I also dive without a drysuit, using a single method to control my buoyancy (i.e. BP/W) develops some muscle memory and makes my response to dump air automatic in a stress/task loading situation, instead of having to take an extra moment and think about HOW I'm going to go from positive to neutral/negative if I suddenly find myself in an unexpected ascent.

    Go with your training and what you're most comfortable with.
     
  3. JeffG

    JeffG Solo Diver

    # of Dives: 500 - 999
    Location: Edmonton, Alberta
    10,028
    75
    48
    A drysuit is made for keeping you dry and makes a mediocre BCD.

    Now if a diver is weighted right (starts the dive negative by the weight of the gas on his back)....
    and is using a small tank, the bubble of gas to manage in a drysuit isn't that big and is relatively easy to manage. (Think an Alum 80. ~6 lbs of gas)

    Now if you have a doubles diver with twin 130's of nitrox. (~20 lbs of gas) The gas bubble required to maintain buoyancy would be unwieldy in a drysuit. The BCD is a better choice.

    Inbetween, you could find all sort of configurations.
     
  4. Rick Inman

    Rick Inman Advisor ScubaBoard Supporter

    9,468
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    Well, you may not get a lot of feedback, because this issues has been beaten to the ground, and a search will give you hours of reading, with everything to say having been said.

    However, I will point out that dynamic instability is the main reason I use my BC as a BC and my DS as a DS. All that gas able to move around in the suit adds instability to trim. But one bubble that rides directly over my balance center and my natural lung sack stabilizes me and makes trim and buoyancy a snap.

    Do 10 dives each way and see what you think.
     
  5. texdiveguy

    texdiveguy Orca

    # of Dives: 500 - 999
    Location: DFW,Texas
    6,965
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    Yep and Yep on the 2 above posts.

    Experiment!
     
  6. NWGratefulDiver

    NWGratefulDiver Mental toss flycoon ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: Pugetropolis
    47,330
    28,175
    113
    Pros and cons both ways (like most things) ... I've tried it both ways, explain it both ways to my students, and let them decide which works best for them.

    Basically, the reasoning behind using your drysuit for buoyancy control boils down to simplifying management ... it's easier to inflate and vent one source than two. There is some merit to this, although if you leave the dump valve on your drysuit open (or mostly open on adjustable valves), then dumping air from a drysuit pretty much manages itself ... when you lift the BCD inflator hose to dump air, the left shoulder (or wrist, for those with wrist dumps) becomes the highest point on your drysuit, and the place where the air bubble in your suit wants to go. It will come out without any conscious effort on your part.

    The reasoning behind using your BCD for buoyancy control boils down to stability ... as you move, any trapped air is going to travel to the highest point it can get to, and a BCD bladder offers much less ability to move around than a drysuit. Keep in mind that as that trapped air travels, it affects your trim, and you have to compensate with body movements to maintain good trim.

    Now, a properly weighted diver is weighted for the buoyancy of their cylinder at the end of a dive ... which means at the beginning of the dive you will be several pounds overweighted. You have to compensate for that by adding air to your BCD or drysuit ... whichever you're using for buoyancy control. So it's at the beginning of the dive where which choice you make will matter the most. With a smaller cylinder, such as an AL80, it amounts to about 6 lbs of extra weight that you're compensating for, and that's a reasonably manageable bubble. With larger cylinders, the difference can be more than 10 lbs. With doubles, it can be considerably more than that. The larger the bubble, the less easy it is to manage it using your drysuit ... especially if you have to break trim and get vertical for any reason, at which point all that air comes up around your shoulders ... and possibly starts to bubble out your neck seal.

    For the recreational diver, it really boils down to choice ... use whichever method is more intuitive for you. For the more aggressive diver using larger cylinders ... or for the recreational diver who's grossly overweighted to begin with ... using the BCD is advantageous.

    ... Bob (Grateful Diver)
     
  7. micklock

    micklock Solo Diver

    # of Dives: 200 - 499
    Location: Michigan
    694
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    This is from my drysuit manual.

    The key to maintaining buoyancy control with your drysuit is to only
    add enough air to the suit so that it is comfortable. You may always feel
    a slight squeeze from the suit, but after time, you will not notice it,
    especially when in a horizontal position. One of the key elements in
    drysuit diving is to dive with the minimum volume of air needed inside
    the drysuit. Use your BC, just as youÂ’ve always done, to achieve proper
    buoyancy.
     
  8. NWGratefulDiver

    NWGratefulDiver Mental toss flycoon ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: Pugetropolis
    47,330
    28,175
    113
    Different manufacturers recommend different approaches. The manual I got with my first (Diving Concepts) suit recommended using the suit for buoyancy control.

    DUI recommends using the BCD. In fact, at the DOG Days event, John Boyer demonstrated having the user get vertical and raise their arm ... the proper amount of air would fill a bubble from the wrist seal to about midway between wrist and elbow. That's obviously a rule of thumb ... the proper amount will actually depend on the type of undergarment you're wearing, since the purpose is to provide adequate loft for warmth ... but it's a good starting point.

    ... Bob (Grateful Diver)
     
  9. NOVIZWHIZ

    NOVIZWHIZ Barracuda

    # of Dives: 500 - 999
    Location: Beaufort, SC
    354
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    try both - do what's comfortable and works for YOU - I don't even hook my power inflator to my DS. I tried using it for bouyancy control and because I dive heavy, the air I needed to ascend built pressure up around my neck seal - felt like I was choking to death.

    best advice I've seen is to use your BC and you always control your bouyancy the same way, not this way with one set-up that way with another.

    other thing that always bothered me is the possibilty of a feet-first ascent if you lose control.
     
  10. Blackwood

    Blackwood DIR Practitioner

    # of Dives: 500 - 999
    Location: Redondo Beach (SoCal, not Washington)
    5,537
    315
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    I use my suit. I've not taken a drysuit course. Rather, I learned from other drysuit divers who do the same.

    One of the things I learned early (by taking a few rocket rides topside) when I bought my suit is that it doesn't dump fast. I like the knowledge that I can go negative quite easily, which leads me to using the BCD.
     

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