A peak flow meter is an inexpensive, pocket-sized device that measures peak expiratory flow (PEF), or how fast a person can exhale. To measure PEF, the person takes a deep breath and then blows (huffs) into a tube on the peak flow meter as hard and as fast as possible.
To get the most accurate PEF, the peak flow meter should be used three times and the best results recorded. PEF is lowest in the early morning and highest in the afternoon.
Results obtained by using a peak flow meter are not as accurate as those obtained by using spirometry, another test used to measure lung function. But the peak flow meter can be used at home, while spirometry has to be done in a doctor's office.
WebMD Medical Reference from Healthwise