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(P_S) and dynamic (P^D) pressures were simultaneously measured for steady flow rates ranging 1-12 l/sec. Least squares analysis resulted successful fitting of P_S and P_D 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 P_S data, which demonstrated the smallest tube diameter of 15 mm to satisfy the ATS standards. The maximum P_D 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