Underwater weighing is the most cumbersome method of body-fat testing, but it's also the most accurate method that's anywhere near affordable. You sit on a scale in a tank of warm water about the size of a Jacuzzi. Then comes the unnerving part: You blow all the air out of your lungs and bend forward until you're completely submerged. If there's air trapped in your lungs, you score fatter than you really are. Knowing this fact makes you try really, really hard to blow out your air, which makes you feel like you're about to explode. You stay submerged for about five seconds while your underwater weight registers on a digital scale. The result is then plugged into a mathematical equation.
This method of testing is based on the premise that muscle sinks and fat floats. The more fat you have, the more your body wants to float when dunked under water. The denser you are, the more you sink, and the more water your body displaces.
The margin of error for this test is 2 to 2.5 percent for young to middle-aged adults. The results are less accurate for children, older adults, and extremely lean people. This is because lean body tissue is made up of other things besides muscle.
Here are the results:
Jennifer: 37.3%, Caution range; Lean weight 161.84 lbs.; Fat weight, 96.16 lbs.; Recommended range, 22-26$
Heather: 36.2%; Caution range; Lean weight 139 lbs.; Fat weight 79 lbs.; Recommended range, 22-26$
Rodney: 33.1%; Caution range; lean weight 210.16 lbs.; Fat weight 99.46 lbs.; Recommended range 16-20%