Apparatus
A Fleisch No. 3 pneumotachometer connected to a differential pressure transducer (Validyne model MF-45) and a carrier demodulator (Validyne model CD18) equipped with a 200-Hz low-pass filter was used to measure airflow (V) (corrected to BTPS). The outputs were digitized at 800 Hz by a 12-bit analog to digital converter (Labmaster, Scientific Solutions, Inc) driven by a microcomputer (NEC Fowermate 386/20) equipped with appropriate software (Labtech Notebook, Laboratory Technologies, Inc).
Each peak flowmeter tested was connected in series with the distal end of the pneumotachometer. The mouthpiece of the flowmeter was 5 cm from the site of flow measurement on the pneumotach (Fig 1).
Calibration
Two Collins rotameters, one calibrated between 0 and 90 L/min, and the other calibrated between 60 and 900 L/min, were used to calibrate the pneumotachometer. We found that the presence of a peak flowmeter in series with the pneumotachometer caused minor changes in the calibration of the instrument. For this reason, calibration was performed with the peak flowmeters in place to allow for errors due to back pressure and turbulence caused by the peak flowmeter.

Figure 1. Schematic of peak flow measuring apparatus.