Bod Pod Measures Fat with Thin Air

The company (Life Measurement, Inc. of Concord, California) explains the workings of its plethysmograph device:

The BOD POD technology is fundamentally the same as underwater (hydrostatic) weighing, but uses air instead of water. The BOD POD measures the volume of air a person's body displaces while sitting inside a comfortable chamber, rather than measuring how much water their body displaces when dunked in a tank. By using air instead of water, the BOD POD offers a fast, safe, and easy-to-use tool for measuring body composition, without sacrificing accuracy.

Since it is based on the same whole-body measurement principle as hydrostatic weighing, the BOD POD first measures the subject's mass and volume. From these measurements, whole-body density is determined. Using this data, body fat and lean mass can then be calculated.

The BOD POD itself is a dual-chambered, fiberglass plethysmograph that determines body volume by measuring changes in pressure within a closed chamber. The front, or test chamber, has a seat that forms a common wall separating it from the rear, or reference chamber. The door to the front chamber is secured by a series of magnets during the brief data collection period. The BOD POD also has a large acrylic window, providing the subject with a wide field of view, making the test process a comfortable and enjoyable one.

Bod Pod product page