Different manufacturers use different methods for measuring a spirograph.
Most methods measure the volume indirectly: the volume is calculated from the measured flow.
The most frequently used methods to measure a spirometry are:
Pneumotachographs measure the flow according to the Venturi effect. The Venturi effect is the phenomenon that occurs when a flowing fluid is forced through a narrow section, resulting in a pressure decrease and a velocity increase.
These spirometers measure the pressure drop when a patient blows in the device.
There are 2 types of pneumotachographs: Fleish and Lilly. The Lilly type measures the difference in pressure over before and after a membrane with known resistance. Fleisch types use a series of parallel capillaries. A laminar flow is very important in order to obtain correct results.
Fleish pneumotachographs are more reliable than the Lilly ones.
The biggest disadvantage of this method is that they are very sensitive to temperature, humidity and athmospheric pressure of surrounding air.
This means that these spirometers must be calibrated very often: at least daily and after each displacement.
Pneumotachographs without thermostat are not reliable, since they don't hold the change of temperature into account, which is a very important influence on results.
|pneumotachograph type Lilly||pneumotachograph type Fleisch|
Turbine spirometers use a tubine to measure expiratory flow. The harder the patient blows, the faster the turbine rotates. These rotations are measured (usually by infrared).
Results are reliable and reproducible and these spirometers need no calibration and no thermostat if the turbine is made of carbon or kevlar.
There is no influence of pressure or humidity on the spirometry results.
Spirometers that use ultrasound are the latest development in this list.
These spirometers measure flow by determining the Doppler effect in the expired and inspired air, using ultrasound. Not all studies show reliable results.
These spirometers require no calibration and do not need a thermostat.
These spirometers measure the electronic resistance through a hot wire. This resistance is dependant on the temperature of the wire. Temperature in the wire drops when the patient blows air in the spirometer.
These spirometers are not very reliable and do not know the direction of the flow (inspiration <--> expiration).
Results are not very precise and calibration is difficult and must be done very often (at least once daily).