The Bod Pod uses a method called air displacement plethysmography to measure body volume and body density. It's a good alternative for oeople who cannot or do not want to perform underwater weighing.
The Bod Pod is a fiberglass chamber with a window and takes 5-10 minutes to complete.
Instead of displacing water, as is done with underwater weighing, air can be displaced from the Bod Pod's closed chamber with known volume, and no one has to get wet.
The Bod Pod and Boyle's Law
The principles and assumptions on which the Bod Pod is based are founded in the relationship between pressure (P) and volume (V) as well as the influence of temperature on gas pressure within a closed chamber. When the chamber is empty, a measurement is taken of the pressure (P1) and volume (V1). After the student enters the chamber and the door is closed, another measurement of pressure (P2) and volume (V2) is taken. Taking into account the inverse relationship between pressure and volume (Boyle's Law) we can see that:
(P1/P2) = (V2/V1)
Bod Pod Standardizing
Clothing is isothermal (doesn't change temperature) and may result in an underestimation of body volume. Body hair is also isothermal. This is why it is important to standardize the clothing by having you wear a tight-fitting swimsuit and a swim cap when undergoing air displacement plethysmography. Two other conditions affecting isothermal conditions are body surface area (derived from height and weight) and thoracic gas volume (the air in lungs and thorax). You do not need to calculate these two factors; the Bod Pod computer software handles all this.
How The Bod Pod Determines Body Volume and Density
When you look into the Body Pod chamber, you see a fiberglass partition with a built-in bench to sit on. That partition and its bench separate the front part of the chamber from the back part of the chamber. There is a diaphragm in the back part of the chamber, and it moves back and forth to create small volume changes. These changes are equal in magnitude but opposite in sign between the front and back halves of the chamber.
For example, if the volume in the front chamber decreases by 1.5L, then the volume in the back chamber increases by 1.5 L. Poisson's law is then applied to compute the volume of the front chamber. Volume is computed twice - once before the student enters the Bod Pod and again after the student enters and sits down with the chamber door shut.
The student's raw body volume (BVraw) is then calculated by subtracting the volume of the front chamber when the student seated from the volume of the empty front chamber.
The body surface area and thoracic gas volume corrections are then applied so that body volume (BV) can be computed as follows:
BV = BVraw - surface area artifact + 40% thoracic gas volume
Just like in the underwater weighing lab, body density (Db) is calculated from BV. Then you can select the appropriate conversion formula and convert Db into percent body fat. Although you can change the formula used by the manufacturer's software, the Bod Pod software defaults to the Siri conversion formula to determine the body fat percentage.
Bod Pod vs. Underwater Weighing
Research comparing body density measurements from the Bod Pod to those from underwater weighing has shown good agreement between the two methods on average. As expected, there are individual differences; some of these individual differences are quite large while others are very small. The precision of air displacement plethysmography is equal to or better than that of underwater weighing; however, factors such as excessive body hair, ability to sit still, clothing, and wet hair or loose clothing can adversely affect Bod Pod results.