The PUMPZ POD is an Air Displacement Plethysmograph (ADP) that uses whole-body densitometry to determine body composition (fat vs. lean). Similar in principle to underwater weighing, the POD measures body mass (weight) using a very precise scale, (accurate within a gram) and volume by sitting inside the POD for two fifty second increments of air measurement. Body density can then be calculated.

By comparison it uses the same densitometric principles as hydrostatic weighing to accurately determine body composition.  While hydrostatic weighing uses water displacement in the tank to measure body volume and an underwater scale to measure weight, the POD uses air displacement in the POD chamber to measure body volume and a separate floor scale to measure weight.  The POD therefore is much easier to operate and the protocols are much simpler to adhere to.  The following is a list of important facts that prove it is very difficult to obtain accurate and reliable results with mobile hydrostatic (or underwater) weighing tanks and why the POD is a better choice.

The mobile hydrostatic weighing tanks use a predicted residual lung volume versus most stationary tanks that have the ability to measure the residual lung volume. POD results should only be compared to hydrostatic weighing results with residual lung volume measured.  

Measurement of density by hydrostatic weighing should include determination of residual lung volume to correct for air remaining in the lungs after maximal exhalation.  Any air remaining in the body (e.g. lungs or gastrointestinal tract) will have the effect of increasing buoyancy. 

When residual lung volume predicted values are used, residual lung volume is usually over or underestimated and can result in errors of up to 4 % body fat in normal, healthy adults.

When residual lung volume is measured research has shown an error of only +/- 100 ml which equates to an error of 0.8% body fat by hydrostatic weighing.

With regards to the hydrostatic weighing measurement site; the tank should have a water heater and filtering system for subject comfort and protocol compliance.  Water temperature should be close to skin temperature 89.6 – 95 degrees F (32-35 degrees C).  Water quality must be maintained through filtering and regular chemical treatments to maintain chlorine levels at the manufacturer’s recommended levels for spas.  Water pH must be maintained between 7.4 and 7.6. 

The POD also measures air in the lungs but the amount is different than residual lung volume and is called thoracic gas volume.  This is a much larger amount of air.  The POD can measure thoracic gas volume but also has predicted thoracic gas volume equations (available for healthy lung functioning persons) based on their height, weight and gender. 

Additionally, the POD results can be obtained in as little as 5 minutes, while hydrostatic weighing takes much longer (depending on subject behavior).  For accuracy, reliability, speed and comfort the POD is the choice of many respected researches and health & fitness experts! 


April Wigand, Partner & Operator