peak flow meter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A portable instrument that detects minute decreases in air flow, used by people with asthma to monitor small changes in breathing capacity.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun medicine An instrument that monitors the peak expiratory flow rate of air from a person's bronchi; used to manage asthma.