Minimum diameter of the velocity-type respiratory air flow transducer

Respiratory tubes with the length of 35 mm and the diameters of 10, 15, and 20 mm were specially made and both the static(Ps) and dynamic (Pd) pressures were simultaneously measured for steady flow rates ranging 1-12 l/sec. Least squares analysis resulted successful fitting of Ps and Pd data with quadratic equations with correlation coefficients higher than 0.99(P<0.0001). The spirometric measurement standards of the American Thoracic Society(ATS) were applied to Ps data, which demonstrated the smallest tube diameter of 15 mm to satisfy the ATS standards. The maximum Pd value of the velocity type transducer (the functional single use respiratory air flow tube) with the diameter of 15 mm was estimated to be 75 cmH2O, implying more than 7 times larger sensitivity than the widely used pneumotachometers. These results showed that the velocity type respiratory air flow transducer is a unique device accomplishing miniaturization with the sensitivity increased, thus would be of great advantage to develop portable medical devices.