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Thread: buoyancy

  1. #1
    Grouper
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    buoyancy

    ok after reading some of the posts from the thread input on buying a tank I'm beginning to question the way I figure my weighting.

    I've always tried to add just enough weight at the beginning of the dive to allow me to be able to barely get under the surface all air totally out of the wing.
    sometimes it's where I have to push it to get to about 7-10 ft then it evens out.

    i've always felt that the compression of the wetsuit at depth would compensate for any positive buoyancy from the tank being breathed down.

    I've not had problems hovering at 15 ft on the ascent doing this.

    it's been trial and error but I know that with my LP95 and full suit,hood and camera, pony tank that it's going to take 12 lbs in freshwater to get my under while with the AL80 it's more like 16-17 lbs.

    the question is in your opinion is this backwards should i be more concerned about the weighting at the end of the dive then the start?

  2. #2
    Barracuda
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    You should be properly weighted for the end of the dive to avoid being too light and floating up. You may be already properly weighted without realizing it. It's been my experience that when first getting in the water there is a bit of trapped air in and on gear that makes one more positively bouyant than one may think they are. I've found that best way for me to have a solid weight check is at the end of the dive, this tells me more reliable infomation than the amount of effort to sink when first starting out does.

  3. #3
    Barracuda
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    Jack Hammer is right - always weight yourself to be neutral, with a dead empty tank and at the surface with no air in your BC/drysuit. If you cannot do this with an empty tank, you can use tank specs to take a perfectly full tank and then add the specified lead. This lead is to compenate for the air used during the dive or the swing of the tanks buoyancy.

    The reason you do this is to ensure you are never positive during the dive and you can always achieve neutral buoyancy, no matter what happens during the dive. (think losing all of you air in your tank, on your buddy's octo trying to hold a safety stop before ascending).

  4. #4
    Grouper
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    thanks for the replies.

    I've never had a problem with being too buoyant during a dive sometimes I think just the opposite that I'm too heavy. ike Jack Hammer said I may have just lucked into proper weighting.

    but here's a very real scenario that i need help with.

    at home I'm normally diving as described in the first post maybe without the hood at times and i have that fairly dialed in (perhaps thru dumb luck) but when on vacation i'm not using my own tanks and not diving the same setup i.e less neoprene and different size tanks.

    most places are not going to let you drain a tank to 500 psi or less to do a weight check(or are they?) what's the procedure then?

  5. #5
    Barracuda
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    Quote Originally Posted by BUDMAN View Post
    most places are not going to let you drain a tank to 500 psi or less to do a weight check(or are they?) what's the procedure then?
    There are specs posted for tanks provided by the manufacturers. I always google Huron Scuba Tank Chart when I want to look it up.

    From these specs, they will give a buoyancy full and buoyancy empty. This represents the weight of the air in the tank or the swing of the tank. You will need to know this difference. For an AL 80 - its around 6lbs. For a HP80, its also around 6lbs. This value is determined by the cft of air in the tank, not the material the tank is made of or the pressure the tank is filled to. All 80cft tanks have the same swing wieght from full to empty.

    So, using a full tank, you would do the wieght check just like you would for an empty tank and get perfectly neutral. Once you have done that, add the 'swing' wieght from the tank to compensate for the air you will be using during the dive. For an 80cft tank - add around 6lbs. A 100cft tank is around 8lbs of swing so you add around 8lbs of lead. These are rough numbers which are good enough for weighting for most divers.

  6. #6
    Grouper
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    thanks

    knew i could rely on you all for good info

  7. #7
    Grouper
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    Quote Originally Posted by BUDMAN View Post
    most places are not going to let you drain a tank to 500 psi or less to do a weight check(or are they?) what's the procedure then?
    Cavediver's approace sounds great. Wish I had known. When I went to Bonaire this January, before taking my checkout dive I askd for and got an empty tank, did my weighting, came vback and got a full tank and did my checkout dive. I could have saved myself the trouble if I only had known. Well, now I do. Thanks Cave.

  8. #8
    Guppy
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    Quote Originally Posted by in_cavediver View Post
    ...So, using a full tank, you would do the wieght check just like you would for an empty tank and get perfectly neutral. Once you have done that, add the 'swing' wieght from the tank to compensate for the air you will be using during the dive. For an 80cft tank - add around 6lbs. A 100cft tank is around 8lbs of swing so you add around 8lbs of lead. These are rough numbers which are good enough for weighting for most divers.
    So all things being equal. If you knew what your lead requirements were with a AL80 you would basically have to add "approx" 2lbs of lead if going to a HP100?

  9. #9
    Barracuda Founding Member
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    Quote Originally Posted by Airborne! View Post
    So all things being equal. If you knew what your lead requirements were with a AL80 you would basically have to add "approx" 2lbs of lead if going to a HP100?
    No because the steel 100 (faber fx100) is -8.41 full and -0.59 empty and the AL80 (luxfer 80) is -1.4 full and +4.4 empty so regardless of the larger swing (due to more air) you will still be REMOVING lead, not adding it due to the boyancy of the empty tanks. I think I would remove 4lbs to start.

    I think the example you are responding to is the weight to add compared to the same tank full.
    Flatliner
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    On the road to Grand Master Spammer...

  10. #10
    Quote Originally Posted by Airborne! View Post
    So all things being equal. If you knew what your lead requirements were with a AL80 you would basically have to add "approx" 2lbs of lead if going to a HP100?
    Probably not... In the other thread about tanks it was said that going from an AL80 to a HP100 would let you drop 6lbs of lead since the steel tank is more negative in the water than the aluminum tank.

    EDIT: Or 4lbs like Flatliner said
    -Aaron

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