The accuracy of measurements of changes in air volumes within a plethysmograph
is improved by addition of a manifold to provide a common source of exterior air
to the plethysmograph test and reference chamber. The preferred plethysmograph
includes test and reference chambers divided by a common separator wall, pneumotachs
in communication with the chambers, and a differential pressure transducer having
a first inlet in communication with the test chamber and a second inlet in communication
with the reference chamber. The manifold includes an exterior air inlet and a passageway
extending between the two pneumotachs and the air inlet. The passageway may be
in the form of separate tubes or conduits, or a common housing covering the pneumotachs.
Preferably, the distances from the exterior air inlets to the two pneumotachs are
approximately the same.
