The BOD POD Body Composition System uses the relationship between pressure and volume to derive the body volume of a subject seated inside its fiberglass chamber. Derivation of body volume, together with measurement of body mass, permits calculation of body density and subsequent estimation of percent fat and fat-free mass. This method is called Air-displacement plethysmography.
The BOD POD is a single fiberglass unit composed of two chambers. The test chamber accommodates the subject during testing and the reference chamber contains instrumentation for measuring changes in pressure between the two chambers. The volume of the test chamber is determined by pressure changes precipitated between the test chamber and reference chamber by a moving diaphragm mounted on the common wall between the chambers.
The pressure ratio relationships between the chambers are inversely related and are characterized by Boyle's Law:
where V1 and P1 are the volume and pressure prior to subject entry into the test chamber and V2 and P2 are the volume and pressure while the subject is in the test chamber. Therefore, subject body volume will equal the volume of the test chamber before subject entry less the test chamber volume with the subject present.
Because of difficulty maintaining isothermal conditions in the enclosed environment of the test chamber, the BOD POD functions under adiabatic conditions (i.e. air temperature is gaining/loosing heat), thus Poisson's Law more accurately characterizes the pressure volume relationship in the testing chamber:
where V1 and P1 are the volume and pressure prior to subject entry into the test chamber, V2 and P2 are the volume and pressure while the subject is in the test chamber, and γ is the ratio of the specific heat of a gas at constant pressure to constant volume (1.4 for air).
Moreover, isothermal air present in the test chamber during a body volume measurement will result in an underestimation of body volume because isothermal air is more easily compressed (40%) than an equivalent volume of adiabatic air, resulting in a lower pressure output signal for a given body volume. There is one source of isothermal air (i.e. air in the lungs) and several sources that are "isothermal-like" (air trapped within the fabric of clothing and air trapped within hair on the head and body).
Body Metrics follows the manufacturer's instructions and procedures which correct and control for these sources of error, ensuring that your body composition test with us is as accurate as possible.
