Hydrostatic weighing, also known as Hydrodensitometry or underwater weighing, is a classic measure of body composition.
- purpose: the aim of underwater weighing is to measure the density of the body, and from that figure calculate percentage body fat
- equipment required: Hydrostatic stainless steel weighing tank, including underwater mounted chair and scale, weighted belt and nose clip. A more simple set up may include a chair and scale suspended from a diving board over a pool or hot tub.
- procedure: The dry weight of the subject is first determined. The subject, in minimal clothing, then sits on a specialized seat, expels all the air from their lungs, and is lowered into the tank until all body parts are emerged. The person must remain motionless underwater while the underwater weight is recorded. This procedure is repeated several times to get a dependable underwater weight measure. See videos about Hydrostatic Weighing.
- scoring: Body density = 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. The body density (D) can be converted to percent bodyfat (%BF) using the Siri equation. For more accuracy residual lung volume (RV) should be physically measured, though there are calculations for RV estimation. One estimation of residual volume is one third of forced vital capacity (FVC). See lung function tests.
- advantages: Underwater weighing is the most widely used test of body density and in the past was the criterion measure for other indirect measures.
- disadvantages: The equipment required to do underwater weighing is expensive. The tanks are mostly located at university or other research institutions, and there is generally not easy access for the general population.
- validity: This method may underestimate body fat percentage of athletes as they tend to have denser bones and muscles than non-athletes, and may overestimate body fat percentage of elderly patients suffering from osteoporosis.
- comments: The water temperature is often elevated to provide a comfortable experience for the subject, however the density of water is dependent on the temperature and should be factored into the equation.