Seymour Fisch
Contributor
So, I did wade through the formulas, and awap and jbark hit the nails on the head: you were using the wrong value for Ww and the wrong units.
So, assuming the 230lbs in the air and 12lbs of weight to get neutral in the water, this gives you a Ww of -12lbs. Converting to kgs to make the units work out, we get 104.55kg for Wa and -5.45kg for Ww. Plugging these into the first part of the formula (Wa-Ww)/Dw, we get (104.55 - (-5.45))/1.025 = 107.3 liters. That's an estimate of the volume of water you are displacing.
That volume includes the air in your lung and your intestines, so you subtract that off. Since the units are in liters, we subtract about (1.2+0.1) liters (your estimate of your Rv and 100cc correction). So that leaves 106.0 liters as the volume you (and your equipment) displace in the water.
Now, going back to the whole formula, we compute Wa/106, and that gives you your density, or 104.54/106 = 0.9862.
Plug that into the body fat formula (which is an empirically fitted approximation), we get 495/0.9862 - 450 = 52% body fat. :shocked2:
But you can see that with the right units, at least you're getting into the ballpark of a correct answer.
I'm guessing that BCD has some buoyancy (I don't know, but do typical BCDs float when completely empty?), you might have had some air in the BCD, and did you do the weight check with your tank empty? An Al tank will be positively buoyant when it's empty, and that will throw the figures off, too. The problem with trying to compute body fat % from your scuba weighting is that the scuba weight is based on the entire system of you, plus all your gear (and your exposure protection if you had been wearing any). I'm also guessing the 12lbs is actually quite a bit more than your true Ww, since the Ww would be what you need to be exactly neutral while maximally exhaling, whereas your scuba weight assumes a normal breath and errs on the heavy side (since you can compensate with the BCD when diving).
So, assuming the 230lbs in the air and 12lbs of weight to get neutral in the water, this gives you a Ww of -12lbs. Converting to kgs to make the units work out, we get 104.55kg for Wa and -5.45kg for Ww. Plugging these into the first part of the formula (Wa-Ww)/Dw, we get (104.55 - (-5.45))/1.025 = 107.3 liters. That's an estimate of the volume of water you are displacing.
That volume includes the air in your lung and your intestines, so you subtract that off. Since the units are in liters, we subtract about (1.2+0.1) liters (your estimate of your Rv and 100cc correction). So that leaves 106.0 liters as the volume you (and your equipment) displace in the water.
Now, going back to the whole formula, we compute Wa/106, and that gives you your density, or 104.54/106 = 0.9862.
Plug that into the body fat formula (which is an empirically fitted approximation), we get 495/0.9862 - 450 = 52% body fat. :shocked2:
But you can see that with the right units, at least you're getting into the ballpark of a correct answer.
I'm guessing that BCD has some buoyancy (I don't know, but do typical BCDs float when completely empty?), you might have had some air in the BCD, and did you do the weight check with your tank empty? An Al tank will be positively buoyant when it's empty, and that will throw the figures off, too. The problem with trying to compute body fat % from your scuba weighting is that the scuba weight is based on the entire system of you, plus all your gear (and your exposure protection if you had been wearing any). I'm also guessing the 12lbs is actually quite a bit more than your true Ww, since the Ww would be what you need to be exactly neutral while maximally exhaling, whereas your scuba weight assumes a normal breath and errs on the heavy side (since you can compensate with the BCD when diving).