Peak Flow Meter
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As part of your daily asthma self-management plan, your doctor may recommend that you use a hand-held device called a peak flow meter to monitor how well your lungs are working. 
How to Use
You use the peak flow meter by taking in a deep breath and then blowing the air out hard into the peak flow meter. The peak flow meter then gives you a peak flow number that tells you how fast you moved the air out.
You will need to find out your personal best peak flow number. You do this by recording your peak flow number every day for a few weeks until your asthma is under control. The highest number you get during that time is your personal best peak flow. Then you can compare future peak flow measurements to your personal best peak flow, and that will show if your asthma is staying under control.
Your doctor will tell you how and when to use your peak flow meter and how to use your medicine based on the results. You may be advised to use your peak flow meter each morning to keep track of how well you are breathing.
Your peak flow meter can help warn you of a possible asthma attack even before you notice symptoms. If your peak flow meter shows that your breathing is getting worse, you should follow your emergency asthma action plan. Take your quick-relief or other medicines as your doctor directed. Then you can use the peak flow meter to see how your airways are responding to the medicine.
See Also
For a step-by-step Peak Flow Meter Tutorial.
Peak Flow Meter vs Spirometer/Spirometry
A peak flow meter is a device used to measure the rate at which a person can forcefully exhale air out of the lungs. This rate is measured in peak expiratory flow (PEF), and peak flow meters can be used by a person in a home setting.
Spirometry is a term used to describe a group of pulmonary function tests. This includes measuring PEF, a test that is performed by a doctor in a hospital setting. The information gathered during these tests is useful in diagnosing certain types of lung disease like asthma.
Daily PEF monitoring is often confused with spirometry measurements. It is not meant to replace spirometry testing, but rather to reinforce spirometry by providing a daily warning system to help people manage their asthma.
*copied from National Heart Lung and Blood Institute