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