114 · BIOMEDICAL SENSORS Figure 3.2, which shows the transformation from a volume-time plot to a flow-volume plot. At the beginning of the forced expiration, flow changes in an almost steplike fashion from zero to the peak expiratory flow rate (which can be around 12 L s -1 ). Performance of an adequate forced expiratory maneuver requires a rapid flow acceleration to peak flow within 5% of FVC (which typically means within approximately 0.2 seconds; American Thoracic Society, 1994). If a flowmeter is to be used to measure this, then it is important that the dynamic response of the instrument be both adequate for following this abrupt change and appropriately damped so as not to introduce oscillation artifacts into the measured flow signal. 3.2. PNEUMOTACHOGRAPHY 3.2.1. FLEISCH PNEUMOTACHOGRAPH One of the most common forms of respiratory flowmeter is the Fleisch pneumotachograph, originally developed by Adolph Fleisch in 1925. In essence this is simply a device placed in the airstream that provides a very slight resistance to flow, generating a small pressure drop across it when flow occurs. The device exists in numerous forms with various designs for the resistive element, but in all cases the principle of operation is the same (Fig. 3.5). In the example in Figure 3.5, the resistive element consists of a series of tubes, each about 0.8 mm in diameter and about 32 mm in length, collected into a bundle that forms the flow path. The outer tubes of this bundle are perforated near each end and connect to annular pressure ducts around the cir-