Thermal behavior of the constant volume body plethysmograph.

The thermal behavior of the constant volume body plethysmograph under the thermal effect of the patient and the thermostatic bag is modeled by two relations: the first is between the temperature in the box and the heat flux generated by the patient and the bag, and the second is between the pressure in the box and the same flux. When the heat flux evolves as a step function, the temperature and the pressure are characterized by four time constants, the theoretical values of which are functions of the heat transfer coefficients by natural convection between the faces of the walls, and the air inside and outside the box. Only two of these four theoretical constants can be experimentally measured with reasonable precision. One of these is responsible for an important delay between the installation of the patient in the box and the disappearance of the disturbance thereby created in the pressure signal. Investigations with patients and with three different plethysmographs each built of a different material (wood, PVC, and Perspex) show the influence of the construction material on the waiting period before the disappearance of the disturbance in the pressure signal.