Lilly type pneumotachometer
 Flow
(V') is derived from the pressure difference over a small,
fixed resistance, offered by a fine metal mesh. 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. |
The
trumpet-like configuration of the pneumotachometer head
is designed to achieve laminar flow over a wide range
of flows. On that account Lilly type pneumotachometers
are mostly linear from 0 to 12 L/s.
The 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. 1).
Ref. 1 - 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.