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Yet Another Proper Weighting Question

Discussion in 'New Divers and Those Considering Diving' started by Straegen, Mar 15, 2009.

  1. Straegen

    Straegen New Fish ScubaBoard Supporter

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    I feel kinda stupid since this should be easy, but I just want to make sure I understand all this properly.

    If I get in the pool with nothing but a swimsuit and float, I could add weight until I was neutral at the surface (my eyes at water level). Lets say that is 9lb. This weight no matter what depth will not change at least not enough to matter I assume. Sound right so far?

    Now I have my BPW which has a 12lb plate (custom... I know they don't normally come this big), 2lb regs and a 10lb tank full. So around 24lb of negative buoyancy. At the end of my dive my tank will weigh around 2lb. I need to compensate for this loss but I will get to that a bit later. Still sound right so far?

    Next is the exposure suit. Lets say my 3mm is buoyant by 11lb. Now as I go deeper air is squeezed out of it and it looses buoyancy. I assume we plan for 0 buoyancy from the wetsuit in the worst case scenario. I also have to make sure my BPW is heavier than my wetsuit so it can sink me. If it is lighter, I need to add the difference between the buoyancy of the exposure suit and the BPW to the weight system. Hopefully I am still on track.

    In the above scenario, I am on the surface 20lb positive (9lb for me and 11lb for the wetsuit). My BPW is 24lb in the beginning but will be around 16lb at the end of the dive due to air loss. I should add at least 4lb to a weight system to compensate (20lb of potential buoyancy - 16lb for the BPW at the end of the dive). Correct?

    Now my wing size should be at least 28lb (24lb for a fully aired BPW and 4lb for compensation weight) plus enough to comfortably lift my head out of the water at surface say around 7lb. Meaning a 35lb wing minimum with a couple pounds to spare would be appropriate. I don't think this is right, but not sure where I went off the logic rails.
     
  2. don Francisco

    don Francisco Manta Ray

    # of Dives: 500 - 999
    Location: Metro New York
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    Imperically, it all sounds somewhat reasonable, but I'd be a bit concerned about the lack of ditchable ballast.

    As described, you'd be pretty negatively bouyant w/o ballast at depth as your suit compressed. I'd probably opt for a lighter BPW and add lead so that in the event of a BC problem at depth I'd be able to ditch enough ballast to get close to neutral.

    With enough lift capacity for managing at depth, floating higher at the surface shouldn't be a problem, since the wetsuit will be uncompressed and contribute to your floatation.

    I'm more a fan of adjusting weight by experiment, while making sure that both the ditchable ballast and BC lift are adequate.
     
  3. Steve02

    Steve02 Solo Diver

    # of Dives: 25 - 49
    Location: Dublin, Ohio
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    The way I read that, he has 9 lbs of weight (presumably on a belt) to make himself neutral. This would/should be ditchable.
     
  4. don Francisco

    don Francisco Manta Ray

    # of Dives: 500 - 999
    Location: Metro New York
    993
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    I agree it would, but unless I mis-read the post the 9#s referenced was only to measure his body's bouyancy, In the paragraph describing his final configuration he describes his body as 9#s bouyant, which I read to mean he no longer had the 9#s on the belt, and was carrying only 4#s of ballast.
     
  5. Straegen

    Straegen New Fish ScubaBoard Supporter

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    This is just an example to see if I am understanding how the numbers all work. My buoyancy is very different than the numbers above, but your point is well taken about being neutral after you ditch weights in case of a wing failure at depth.
     
  6. rstofer

    rstofer Loggerhead Turtle

    # of Dives: 100 - 199
    Location: Northern California
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    Over in the Equipment -> BC's and Weight Systems, there is a handy spreadsheet for calculating all this stuff.

    The one thing it doesn't account for is that your wetsuit doesn't decompress as easily as it compresses. So, even though it might seem that you would be getting too positive to make a safety stop with an empty tank, it probably isn't true.

    Tobin from Deep Sea Supply (manufacturer of a terrific BP/W) has a different way to set initial buoyancy. It is substantially different. With an empty BC and a full tank, set your weighting so you are eyeball level. This is about 8# less positive than conventional wisdom, assuming a 100 cf tank. The idea is based on the fact that with a mostly empty tank, you can make the safety stop because your drysuit is still compressed. This is consistent with the idea that 2d dives are colder than first dives.

    According to the spreadsheet, it takes about 10# of lift to get your head out of the water. Most people can float about head high so if it took 9# to get down to eyeball level, I wouldn't be surprised. But, I haven't tried it.

    What I do know is that I could use an Al 80 (4# positive at the end of the dive) in salt water with only a 6# weight belt.

    With a 7/8mm wetsuit, a 6# plate, 2# regulator, -10.5# tank and 20# of lead, I sink like a stone. I figure I am at least 5# overweighted. Next time out I'm going to remove 4# and see how it goes.

    You need to think about how much weight is ditchable. You don't want to get to the bottom with a highly negative tank and a very heavy backplate only to blow out your BC or wing. The spreadsheet will show you how negative you are at depth - make sure you can swim it up after losing your ditchable weight.

    Richard
     
  7. Puffer Fish

    Puffer Fish Captain Happy ScubaBoard Supporter

    # of Dives:
    Location: Knoxville, TN
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    In the above example, you are 4lbs heavy at the start of the dive, with no ditchable weight..the lack of ditchable weight is the first issue.

    The second is, at the bottom, you may loose most of that 11lbs from the wetsuit... which would mean you would need 15lbs of air just to say level at the bottom. On the surface, you will need more lift that just the weight of your head, because at some point the tank valve and regulator will be in the air...as well as part of the tank.. Most of the time, with a single tank, around 40 lbs of lift is normal.. assuming you are not really small.


    Remember, you may need lift at the start of the dive also.
     
  8. Straegen

    Straegen New Fish ScubaBoard Supporter

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    I don't think a wetsuit will normally loose much more than 2/3 buoyancy. A drysuit can loose all of it though. It would take 15lbs of air to offset in the worst possible scenario but the tank is probably going to be lighter due to some air loss and the wetsuit is going to have some buoyancy most likely. In this scenario, 15lbs is probably the right number for ditch weight.

    Don't think it works that way. Underwater your head is closer to neutral due to air spaces and such. Above water your head becomes completely negative so most in water weighting hasn't accounted for it being negative. A reg is always negative. It will effect the buoyancy of a person slightly thanks to gravity but not very much in this instance. At least that is my understanding.

    I don't think 40lbs is normal for a BPW design at least in warm tropical waters using a single tank (or cold water really). I think 30lbs is normal for most people with a SS backplate and for many 20lbs using an aluminum BP. Generally speaking your wing size is determined mostly by loss of buoyancy of your exposure suit. I was just putting out the idea of a heavy BPW setup (steel tank, very heavy BP) to stretch the numbers. I think your BP selection should probably offset your personal buoyancy (not including exposure suit) as much as possible.

    At the start of the dive at least in a wetsuit, you have max buoyancy working for you and I would assume your wing is inflated unless you are doing a drop and dive without surfacing. It is really a complete wing failure that I am trying to plan for or rather understand the weighting formula.
     
  9. Puffer Fish

    Puffer Fish Captain Happy ScubaBoard Supporter

    # of Dives:
    Location: Knoxville, TN
    8,072
    141
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    Straegen,

    The lighter travel wings are not designed for SS back plates...they might work for some people, but the margin for error gets pretty small. Take a SS back plate, add reg, all the hardware, and you already have 8 or 9 lbs...if one happens to be diving a steel tank, you now could be another 8 negative at the start...we are now up to 17 lbs... with only 13 left for lift...and nothing is ditchable. The wing would have to be pretty full, just to prevent it from sinking by itself.

    I think you will find just about every major mfg of wings makes their normal one around 40 (plus or minus a few pounds), and the light weight ones for tropical, light weight systems.

    Wet suit material vary in how heavy and how much it looses lift... I have 11 suits, made by 6 different mfg... the heaviest weighs in at over 20 lbs..

    Many of the newer high stretch ones can go neutral to negative on a dive.

    Some recover almost completely (the best nitrogen blown ones), some don't.

    Most of my suits seem to recover everything but about 2 -3 lbs or so, with a 7 mm and around 1 lb for a 3mm... which means being just a bit lighter at the end, when low on gas.

    It also means one is the most negative at the start.

    If you are concerned about a wing failure, then ditchable weight is really important, and particularly in tropical conditions where one does not have a suit to help as much. Your comment about offsetting personal buoyancy would seem to have merit, but there a shops in the keys that rent LP steel tanks....and others that rent al 80's...as an example... weighted out for a AL 80 then, kind of sucks.
     
  10. Slonda828

    Slonda828 Divemaster

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    I think it is important to note that your suit will not instantly return to its initial buoyancy upon your return to shallower depths. I have found that if I am neutral at the surface with a full tank, that when I am at my safety stop with 500 PSI or so that my wetsuit is crushed enough to not pull me up even though I am 5 pounds light on gas. Test this for yourself, but it has worked for me with both 3mm and 7mm wetsuits. Tobin from DSS has covered this topic numerous times on here.
     

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