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How much BCD lift do you really need?

Discussion in 'New Divers and Those Considering Diving' started by Fun2dive, May 11, 2004.

  1. Fun2dive

    Fun2dive Garibaldi

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    Hello Everyone!

    I am a newly certified open-water diver who just returned from diving in ST. John. I was wondering why divers purchase BCD's that have such high lift capacities? From what I understand, the BCD's purpose is to fine tune bouyancy at depth and float you comfortably at the surface (not to say overcome the amount of weight your carrying). I guess what I'm asking is, once I'm properly weighted (regardless of exposure suit or equipment) wouldn't 5LBS. of lift make me positively bouyant?

    Thanks!
     
  2. ScubaRon

    ScubaRon Manta Ray

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    I posted the weight calculation method in this thread and if you read it you will see what the requirements are for.

    The high lift capacities are primarily for technical diving with multiple tanks.
     
  3. ScubaFriend

    ScubaFriend Instructor, Scuba

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    Well i am very lucky i dive in warm clear seas and dont even need to wear a wetsuit at anytime of the year, i have just purchased a BCD that i am very happy with it has 18 ponds of lift....yes just 18 pounds and it is great it is enough to keep me high out of the water when i need it. Other details are i wear as i said no wet suit and 4lbs of weight ans i weigh about 200 pounds
     
  4. Don Burke

    Don Burke Loggerhead Turtle

    # of Dives: 200 - 499
    Location: southeastern Virginia
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    Let's look at the dive backwards.

    You end up at your shallowest stop with your tank at about five hundred and your BC flat. Before the ascent to the stop, you were at the bottom with a compressed wetsuit which makes you displace less water. When you first got to the bottom, you had a nearly full tank and a compressed wetsuit.

    My Bare 5/3 is about sixteen pounds positive on the surface, about twelve pounds positive at ten feet, and about four pounds positive at one hundred feet. If I'm weighted for ten feet, I'll be about eight pounds heavy at one hundred feet just due to the suit.

    An AL80 is about five pounds heavier at three thousand psig than it is at five hundred psig.

    That's thirteen pounds that the BC needs to carry at the beginning of the dive. Bigger tanks and thicker wetsuits will change more.

    Add a little for constriction of the BC against the tank or your body. Add a little more for a little higher ride at the surface.

    Generally, twenty-something pounds of lift will get the job done for me with a single tank. I happen to think those people using seventy to one hundred pounds of lift are misguided.
     
    starick likes this.
  5. Fun2dive

    Fun2dive Garibaldi

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    Thanks, for the great info. It seems then that wetsuit compression is a major factor in needing extra lift. However, this is only temporary as you return to the surface compression decreases and your bouyancy increases. In your example your Bare 5/3 loses 8 pounds between 10 and 100 ft. so it makes sense then that your BCD needs to be able to provide 8 pounds @ 100ft. But as you decrease your depth your BCD no longer has to compensate for wetsuit compression.
     
  6. jbd

    jbd Dive Shop

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    are you dealing with wetsuit compression but also there is compression of your abdomen as well. Those of you who dive witha weight belt will undoubtably notice that the weight belt gets real loose the deeper you go. Much of this is due to abdominal compression.
     
  7. Don Burke

    Don Burke Loggerhead Turtle

    # of Dives: 200 - 499
    Location: southeastern Virginia
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    True enough. That's one reason you vent air from a BC on ascent.

    If you don't have enough BC to carry that eight pounds (plus whatever gas weight there is) at one hundred feet, how do you stay neutral while you're down there? The temporary problem still needs to be solved.

    There's a thread with some math in it already on the board.

    http://www.scubaboard.com/t39683.html
     
  8. Fun2dive

    Fun2dive Garibaldi

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    I agree, but it's a little misleading when BCD lift requirements are described as if wetsuit compression, tank weight, body compression, surface flotation, equipment, and bouyancy control all are needed at the same time at the same depth. For example, you need your BCD to provide postive flotation at the surface, however at that time your BCD doesn't need to provide lift for a full tank at depth as well as wetsuit compression. Conversely, at depth, your BCD doesn't need to provide surface flotation; yet people group all these factors together and lump all the lift together and come up with BCD lift requirements. Seems a little strange.
     
  9. Don Burke

    Don Burke Loggerhead Turtle

    # of Dives: 200 - 499
    Location: southeastern Virginia
    2,155
    5
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    Actually, the BC does need to lift a full tank on the surface.

    The displacement change due to body squeeze gets lost in the other factors.

    Bouyancy control isn't an additional factor. It's what you do about all of the other factors.

    I see what you're saying. It's easier than it looks.
     
  10. scubacowboy

    scubacowboy Nassau Grouper

    # of Dives: 500 - 999
    Location: CLEARWATER, FLORIDA
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    That is surely true in a single tank straight forward recreational dive. However I would venture a guess and say that most people using a lift cap. that high are diving big doubles and carrying quite a bit of gear. Myself I find 60 pounds to be just right diving my doubles where a full set of tanks are right around 20 pounds negative before manifold, regs, ect.
     
    starick likes this.

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