Plethysmograph with device for compensating fluctuations in the ambient air

摘要

Plethysmograph comprising a plethysmograph chamber (10) which can be sealed in an air-tight manner, a reference chamber (16) having an essentially rigid wall, so that its internal pressure is largely independent of fluctuations in the ambient pressure, a first differential pressure sensor (12) which supplies an electrical plethysmograph chamber pressure signal, is connected by one connection to the plethysmograph chamber and by its other connection to the reference chamber (16), and furthermore a second differential pressure sensor (18) which supplies an electrical ambient pressure signal, is connected by one connection to the reference chamber and by the other connection to the environment, a subtractor circuit (26) whose inputs are coupled to the differential pressure sensors, and a matching circuit (24) which is connected between the second differential pressure sensor (18) and the subtractor circuit (26), and which matches the dependence of the plethysmograph pressure signal on fluctuations in the ambient pressure to the dependence of the ambient pressure signal on the fluctuations in the ambient pressure. <IMAGE>