A weigh tank uses Archimedes’ principle to determine body density and calculate body fat percentage. The subject is submerged in water and their underwater weight is recorded. The Siri Equation is used to determine body fat percentage. The method is mainly available in research institutions and colleges.
A weigh tank, or hydrostatic weigh tank, works by using Archimedes’ principle to determine the density of a person’s body and then calculate the percentage of fat in the body. After being normally weighed out of the hydrostatic weigh tank, the subject is seated on a specialized seat and lowered into the tank filled with water until fully submerged. The person’s underwater weight is then recorded. It is important that the person exhales as much air as possible before diving so that the air in their lungs does not create additional buoyancy. The process is typically repeated several times to obtain an accurate measurement of the subject’s underwater weight, which is one of the pieces of information used to calculate their body fat percentage.
Calculating the percentage of fat in the body has a lot to do with the density of each part of the body in relation to the density of water. Fat is less dense and has more volume than the skeleton and muscles. It is also less dense than water. Therefore, fat placed in a tank of water must float, because fat is less dense than water.
The skeleton and muscles are denser, with less volume than water and fat. Muscles and skeleton placed in a tank of water must sink because it is denser than water. In other words, when a body is submerged in a weigh tank, it will sink more or float more depending on the amount of lean tissue or fat. This is based on Archimedes’ principle, which says that when an object is placed in water, the weight of the displaced water will be proportional to the weight of the object.
Along with the subject’s weight in and out of the water, several other numbers are needed to calculate a person’s body density and percentage of body fat. These include residual lung volume, water density, and an estimate of the amount of air trapped in the gastrointestinal tract. After calculating the density of the body, the absolute maximum is 1.10, which means no fat, and the minimum is 0.90, which means the whole body is fat: an equation called the Siri Equation is used to determine the percentage of body fat of the person. This equation says that to calculate body fat percentage, one should divide 495 by body density, then subtract 450. For example, a body density of 1.07 would yield a body fat percentage of 12.6 percent, because 495 divided by 1.07 is 462.6 , and 450 less than that number is 12.6.
Using a weighing tank to calculate your body fat percentage is a complicated process that requires a great deal of space. Therefore, it is mainly available only in research institutions or colleges and universities. The cost to use it can range from $10 USD to $75 USD, depending on where it is done. As long as it is done correctly and the calculations are accurate, this is the most effective method of measuring body composition.
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