Flow (V') is being measured in a tube with a small, fixed resistance. In this type of pneumotachograph the resistance to flow comes from an array of capillaries arranged in parallel with the direction of flow (ref. 1). The pressure drop across the resistance relates linearly to flow at relatively low flows, when the flow pattern is laminar. Higher flows give rise to a turbulent flow pattern, when the pressure drop across the resistance changes more than proportionally with flow. Accurate measurements are best performed when the flow pattern is laminar and flow linearly related to pressure drop. Pneumotachometers according to Fleisch are available in various sizes, suitable for different flow ranges. Their resistance to flow may increase due to accumulation of secretions or other contaminants, and from condensation of water vapor, changing their flow characteristics. The flow characteristics also vary with the viscosity of the gas measured, which varies with the gas mixture, and with gas temperature (ref. 2).
Ref. 1 - Pneumotachometer according to Fleisch Fleisch A. Pflügers Arch 1925; 209: 713. Fleisch A. Pflügers Arch 1929; 223: 364. Fleisch A. Nouvelles méthodes d’étude des échanges gazeux et de la fonction pulmonaire. Benno Schwabe & Co, Editeurs, Bâle, 1954, pp. 79-85.
Ref. 2 - Measuring gas flow A synopsis of conditions which affect the measurement of gas flow with a pneumotachometer can be found in Appendix A, pp. 30-32 in: Quanjer PhH, Tammeling GJ, Cotes JE, Pedersen OF, Peslin R, Yernault JC. Lung volumes and forced ventilatory flows. Eur Respir J 1993; 6 suppl. 16: 5-40. Erratum Eur Respir J 1995; 8: 1629.