Pneumotachograph
Purpose
Allow continuous recording of airflow, and thus calculation of volume changes across time
Principle
Based on Hagen-Poiseuille equation
Measure flow by sensing the pressure drop across a fixed resistance in laminar flow
Setup
- Resistance
- Pressure sensors (transducers) on either side of the resistance
- Heating element to keep temperature constant, and to prevent condensation
1. Fleisch type
Resistance is by using corrugated foil wound into a spiral to form a series of parallel laminar resisters
2. Lilly type
Resistance is by using a layer of metal or plastic gauze
Factors affecting accuracy
Pressure drop depends on
- Characteristics of the resistance
- Viscosity of the gas
* thus temperature, and (somewhat) pressure
- Composition of the gas
- Water condensation
Other notes
- Pressure difference is kept low to minimise effects on gas flow
