From a sales brochure kindly provided by Emanuele Isnardi. The entire sales brochure can be downloaded here.
“Our PULMOSTAR SMB is a whole-body plethysmograph of the constant-volume type, i.e. it measures pressure changes due to thoracic volume-change differences. Measurements are made against the pressure in a compensating chamber. The compensating vessel is built into the plethysmograph and communicating with it through a high resistance (time constants of 50, 10 and 20 seconds may be selected). This offsets slow, e.g. thermally induced pressure changes while the pressure fluctuations occurring simultaneously with respiration are registered. This also eliminates or dampens interfering influences reaching the plethysmograph chamber and the compensating vessel cophasically. The zero position can be restored at any time by allowing both the plethysmograph chamber and the compensator to assume atmospheric pressure through a valve system.”