Lilly type pneumotachograph

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. Erratum Eur Respir J 1995; 8: 1629.