Measurement Principles for Spirometers
There are different measurement principles that spirometers use to measure the flow-volume loop. The most important ones are pneumotachometers, turbine spirometers and ultrasonic spirometers.
Pneumotachometers measure using the Venturi effect. This is the name given to the physical phenomena where dynamic fluids and gases accelerate when going through a narrow opening. At the same time, pressure is reduced.
A pneumotachometer measures the pressure difference before and after a membrane (Lilly type pneumotachometer) or capillaries (Fleisch pneumotachometer) with known resistance.
Turbine spirometers measure the rotations of a turbine: the higher flow is, the faster the turbine rotates. An infrared detector detects the rate at which the light from the infrared source is interrupted by the passing of the turbine. With this information flow can be calculated.
Flow can also be measured using ultrasonic waves and the doppler effect.