Hot wire anemometry
In the hot wire anemometry an electrically heated wire (or screen or film) is placed in the gas pathway, which is cooled by the gas flow (Fig. 1). The degree of cooling depends on the gas flow rate, which can thus be calculated.
Fig. 1 (from http://www.frca.co.uk/article.aspx?articleid=100390)
The hot wire (usually Pt) is incorporated into a balanced Wheatstone bridge circuit. Cooling the wire changes its resistance and unbalances the bridge. Most designs work on the constant temperature principle, whereby a correcting current is applied through the hot wire to compensate for the cooling effect of the gas, maintaining a constant wire temperature and thus restoring the balance in the Wheatstone bridge. This current is measured and from it the gas flow rate is determined. To compensate for changes in the gas temperature, a second wire is usually incorporated, which is maintained at ambient temperature. Minor corrections are also made according to the gas composition, to accommodate the variation in specific heat capacity, but hot wire anemometry is generally extremely accurate.
The cooling effect occurs with flow in either direction, and so to measure exhaled tidal volume the hot wire anemometer is placed in the expiratory branch of the circuit. It can be modified to provide information about the direction of flow by using an additional heated wire placed just downstream from a small bar, as shown in Fig. 1b. This bar shelters the wire from the full cooling effects of flow in one direction but not the other, and thus inspiratory and expiratory flows can be calculated separately. For this purpose the sensor must be placed in the Y-mouth-piece before the bifurcation. This technique is particularly useful to control neonatal ventilation.