Hydrostatic (Underwater) Weighing
This technique determines body fat from body density (the ratio of body weight to body volume) or: D = Mass/Volume. It is based on the whole-body principle of Archimedes, which states an object’s loss of weight in water equals the weight of the volume of water it displaces. In this procedure the subject sits strapped in a chair that is submerged into a water tank and weighed. While submerged, the subject must expel all the air from their lungs and hold their breath for 10-15 seconds until the scale can be read. This process is repeated up to 10 times to ensure accuracy.
Average Test Time: 30-60 minutes
Advantages:
- An established reference method for measuring body density.
- Known to be very accurate when compliance issues can be followed properly.
Disadvantages:
- Test time is lengthy, arduous, and very difficult to perform correctly.
- Technician must be highly skilled in order to obtain accurate results.
- Some populations are impossible to test, including the extremely obese, disabled, elderly, infants and small children, and those with an aversion to water.
- Ongoing maintenance issues are high. Water temperature needs to remain constant and the tank should be drained after each measurement, since dirty water can skew results.
- A wide range of equipment, protocols, calibration, and methods for determining residual Residual Volume can contribute to measurement errors.
- Hygiene issues.