Fleisch type pneumotachometer
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
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
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.