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Body fat Percentage.

Discussion in 'Advanced Scuba' started by Teljkon, Jan 17, 2014.

  1. Teljkon

    Teljkon Contributor

    71
    5
    This question while it may belong in dive into fitness. Poses a question that I think is advanced. Since I weight around 230 give or take for flux and I take 12 lbs to become neutral and we know that fat displaces .903 gm/l. Shouldn't I be able to figure out my Ww or Wet weight to plug into the formula for body density. So most of the math is straight forward as shown below I cant seem to figure out how to get a good number Ww from the known variables. I have never worked with specific density before or medical equations so I am not completely familiar with the concept or how things apply. even if I just take the 12lbs as the difference I get screwy results.

    Body density formula
    Wa / (((Wa - Ww) / Dw) - (RV + 100cc)), where Wa = body weight in air (kg), Ww = body weight in water (kg), Dw = density of water, RV = residual lung volume, and 100cc is the correction for air trapped in the gastrointestinal tract.

    Once I have body density I can use the Siri equation to get a body fat % for an Anglo.

    % Body Fat = (495 / Body Density) - 450

    Simply taking 12lb as the difference yields a result of.
    Wa = 230
    Ww = 218
    Dw = 1.025 (think its a little higher in hawaii but no matter)
    Rv = I don't know how to get my RV yet but for the sake of estimation 1.2L the male average = 1200cc

    230/ (((230 - 218) / 1.025) - (1200+100CC))=
    230/ ((12/1.025) - 1300CC)
    230/ (11.70- 1300CC)

    1cc air= .013grm
    1300CC to lbs = .0373lb

    230/ 11.6627 = A 19.7 body density

    -424.87% body fat?????

    Any dive Doctors know where I am screwing the pooch and any idea of how to get to the best Ww??
     
  2. Storker

    Storker ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    17,098
    13,496
    I'm not a dive doctor, but... You say you need 12lbs (5.5kg) to become neutral. Is that in the buff, or including your exposure protection? If it's without your exposure suit, that looks a bit... on the high side to me. And if it's including your exposure suit, you have to include the suit's buoyancy in your calculations.
     
  3. DevonDiver

    DevonDiver N/A

    # of Dives: 5,000 - ∞
    Location: Subic Bay, Philippines
    15,396
    8,192
    An XXXXL 3mm wetsuit is considerably more buoyant than an XXXXS wetsuit.... extrapolate.
     
    LeadTurn_SD and swimmer_spe like this.
  4. danvolker

    danvolker Dive Shop

    # of Dives: I'm a Fish!
    Location: Lake Worth, Florida, United States
    5,884
    3,009
    While the formula might get you "close" to an estimate......ultimately the formula will not be any where near as effective in getting you to the PERFECT weighting as would getting in the ocean with your chosen gear, and adding and subtracting weights until you have hit dead neutral with a near empty tank and have practically no air ...or no air...in your BC or wing. Your BHB playground is a perfect venue for this.

    If this is NOT about diving, but is a desire for getting the extremely accurate body fat percentage test.....where they drop you in a tank with water, have you expel all the air you can from your lungs, and then predict your body fat from a formula that includes the water you just displaced.....this IS available locally --I do know body builders that have had this done in the PAlm Beach or Lauderdale area.....

    And while not theoretically as accurate, there are plenty of Bodybuilder gyms that have trainers well skilled with the skin caliper test, and their skill level will often get their body fat measurements to be as good as a facility that is doing the water immersion tests.

    Even a gym like LA Fitness ( Boynton on Congress--not far from your Lantana address) could be expected to come quite close.....

    The scales you can buy to weigh yourself that also use electrical impedance to suggest body fat percentage, are heavily effected by your hydration level--or any significant inflamation from excercise--and this can through them off by several percents....though if you use these scales at the same time every day...say 7 am every morning, before eating, it is reasonable to expect that this will be a good indicator of body fat going up or down...and worth while for this reason. Not so much for their absolute suggestion as to what your percentage is..

    What are you guessing your body fat percentage is now?
     
  5. tracydr

    tracydr Divemaster

    # of Dives: 500 - 999
    Location: North Carolina, 3 miles from South Carolina
    2,730
    743
    i didn't go through the math but I do know, from my nutrition and exercise phys classes that body fat percent methods are notoriously inaccurate. I seem to remember that even underwater measurements can be 25-50% off.
    I don't know how accurate the newer scales are that you can get for $35-$50 but I seriously doubt that they are terribly accurate,either.
     
    AfterDark likes this.
  6. redacted

    redacted Guest

    It seems to me that Ww should be a fairly small value. It is what the scale would read if you were standing in water rather than air. If you can exhale and sink in SW, it would be a positive value. If you exhale and still float, it would be negative.
     
  7. jbark

    jbark Contributor

    # of Dives: 25 - 49
    Location: Michigan
    87
    31

    I am not a dive doctor, or any other kind of doctor for that matter, but the physicist in me sees that you have a problem with your units in that equation for body density. That is unless you actually do have a mass of 230 Kg ( which correlates to a weight of ~500 lbs).

    There is also a potential problem with the density of water. You use 1.025 g/cm^3, but it may be that you should be using 1025 Kg/m^3 to provide for consistency in units.

    A little dimensional analysis goes a long way at times. . .

    Cheers
    Jerry
     
    LindaSSF likes this.
  8. Foxfish

    Foxfish Solo Diver

    # of Dives: 200 - 499
    Location: Perth, Australia
    717
    118
    I didn't wade through all the formulae. Try this. Get a really big big tub. A smallish rainwater tank would be ideal. Get your dive gear on and jump in. Now fill the tank with water. Climb out. You may need a helicopter to assist. Measure the change in water level and determine your displaced volume. Next, jump on the scales with your gear on. That gives you your mass. Mass divided by volume is your diving density. If that is greater than the density of sea water you'll sink. If less then you'll float. Do this with a full tank and an empty tank. With a lung full of air and empty lungs.

    A simpler approach would be to jump in the sea with your gear on and BCD inflated. Let the air out of your BCD and take a deep breath. You should float with the water level just below your nose and sink when you exhale. Add or remove weight to suit. No calculator required.
     
    AfterDark likes this.
  9. Teljkon

    Teljkon Contributor

    71
    5
    I am so sorry I didn't make my self clear I am doing this to try to figure out my Body fat% not really as an exercise for diving simply related. I am very much trying to avoid going to the university or into a gym to do this. I have relocated from Palm Beach to Hawaii. So thank you for pointing that out Dan. I need to update my location. I have not yet settled in and do not have a good gym to go to for this kind of thing. Frankly to some extent I am being cheap I much rather spend my money on dives another thing that is more expensive in Hawaii.

    To be more clear about the specifics I do not dive with a wet suite at all just a swim suite and nothing else I also never inflate my bc when I dive. Most likely there is no extra buoyancy provided by my gear to interfere with he calculation chances are I should probably be adding a few units of weight for the equation due to gear. The last dive I did I had to give my bc a little shot of air so I am actually probably going to trim 2 lbs down to 10 on my belt.

    Also Jerry/Jbark I am 230lb not 230 kg. Do you think I should be using KG and not LB?? I grabbed the formula form an American web site it would surprise me if they used kg and not Lb but its possible. I also would like to thank you for the tip on the water density. I wont be redoing the math right now but will try those changes in the near future.
     
  10. Foxfish

    Foxfish Solo Diver

    # of Dives: 200 - 499
    Location: Perth, Australia
    717
    118
    It's complicated. 3D ultrasound scanning anyone?
     

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