An improved plethysmograph with servo control of hydrostatic pressure

One limitation of the conventional fluid displacement plethysmograph is that the hydrostatic pressure varies as recordings of volume change are obtained. Thus, use of this instrument can lead to inaccuracies that are proportional to the compliance of the biological system under study. This paper describes an improved design which utilizes a servo-control system and an open, mobile reservoir to ensure that the hydrostatic pressure is maintained constant as volume changes are recorded. The result is a device that succeeds in combining stability, sensitivity and accuracy in a fluid displacement plethysmograph.