From a Fenyves & Gut sales brochure kindly provided by Emanuele Isnardi.

“Our PULMOREX body plethysmograph incorporates the pressure-corrected flow principle, but also be equipped with the constant-volume mode of operateion in addition. Presently, this type of body plethysmograph is probably the most perfect.

In order to explain the function of this plethysmograph we will proceed from the simpler constant-pressure plethysmograph.  In 1960 Mead described his “volume displacement” plethysmograph which he used in conjunction with a Krogh spirometer to determine changes in chamber volume. We replaced the Krogh spirometer with a pneumotachograph and a linked integrator.  The resulting assembly is an “open-box” “volume-displacement”, i.e. constant-pressure plethysmograph.”