A drawing from patent #3,154,068, submitted 1961, approved 1964.
“Another object of the invention is to provide a high dynamics spirometer. The instrument is designed to have small travel of the parts in motion corresponding to large volume differentials or changes in volume. Therefore large volumetric acceleration with correspond to small linear acceleration. Since for a given mass in motion, the lower the accelerations, the higher will be the frequency response.
“Another object of the invention is to provide a spirometer which will offer all the known functions of previous spirometers, and at the same time provide an electrical readout of both volume change and flow changes. These two measurements are entirely independent. Previous to this invention, no patient-driven spirometer has been capable of independent readout of volume changes and flow changes.
“The low linear acceleration results in small forces required to actuate the moving elements. Since the patient’s lungs in mechanical spirometry are required to generate the forces to drive the spirometer, it can readily be seen that the lower linear accelerations result in lower forces in the lungs to drive the spirometer. Thus the patient’s breathing is more nearly normal during the process of measurement. The effect is magnified at the point of higher dynamics, such as rapid breathing and coughing.
“In an ordinary spirometer, a patient coughing is not only affected physiologically by the higher forces of the ordinary spirometer, but the actual cough is recorded subject to the errors of the instrument at high frequencies.
“A further object of this invention is to provide a spirometer in which the ambient pressures within the spirometer and the patient’s lungs remain as close to atmospheric pressure as possible. Thus the patient does not have the feeling that something is impeding his breath, which is the case in the ordinary spirometer.”