From “New methods of studying gaseous exchange and pulmonary function” by Alfred Fleisch, published by Charles C. Thomas, 1960, page 51.
“The great interest currently enjoyed by pneumotachography has induced us to improved our old instrument which separated the air flow into a number of parallel tubes 130 mm long and 1.8 mm interior diameter. The length of the air passages is now reduced to 32 mm, thus greatly decreasing the dead space. To improve the proportionality the inner diameter of the air passages is reduced to 0.8 mm.
“The system of air passages is obtained by rolling together a thin corrugated nickel strip and a flat strip (Fig. 13). The outer corrugated layer 2 is used for the pressure tappings. The brass tube 3 has two rows of small holes 4 and 4a distributed over its whole circumference and communicating with the annular ducts 5 and 5a. The pressure is the transmitted through the tubes 6 and 6a to the differential manometer 8. If the air flow passes through the pneumotachograph from right to left the pressure will be greater in 5 than in 5a; the membrane 9 will therefore move to the left and the ray of light will be deflect upwards by the mirror 10. If on the other hand the current passes fro left to right, the pressure will be greater in 5a than in 5, the membrane will move to the right and the ray of light will be deflected downwards. We would like to stress the fact that the membrane will only respond to differences in pressure, not to the absolute pressure.
“When the speed of the air flow does no exceed a certain value the air particles move in the pneumotachograph under laminar conditions, thus ensuring strict proportionality between speed and the deflection of the ray of light. One this limit has been passed a state of turbulence ensues, easily recognized by the slight oscillation of the light spot.
“The metallic membrane manometer connected to the pneumotachograph has a natural frequency of about 20 cycles per second; since the physiological oscillations of the air flow are slower, a rapid reaction and a reliable recording are assured.”