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
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.