A peak flow meter is a small tool that is used by asthma sufferers to check on the strength of their exhalations and gauge how well their lungs are functioning. While helpful and useful, peak flow meters do have some downsides associated with them that users should know about.
A peak flow meter can only measure the air flow through the major airways of your lungs. These major airways are where nearly all of your exhalation strength comes from. However, minor airways in your lungs can be affected by asthma just as major airways can and a peak flow meter does not give you the ability to check the strength of those airways. This can lead to people taken by surprise when their minor airways swell and begin to cause asthma symptoms, while the major airways are still doing fine.
A peak flow meter, while it can measure the strength of your lungs' exhalation, cannot in fact measure your lung function. For instance, you could have diminished lung function, but still be able to exhale strongly. This becomes problematic only if you associated the exhale strength with how well your lungs are working. If you just use the peak flow meter as an estimation, as it is meant to be used, then you should be able to detect most early warning signs of impending asthma attacks.
A peak flow meter's name comes from the fact that you are supposed to measure your peak flow with it. Your peak flow is the strongest exhalation you can have in two or three week's time. You judge all your other exhalations by that peak exhalation. However, it can sometimes be hard to figure out what your average exhalation strength is, especially if you have severe asthma or diminished lung capacity. So the peak flow meter is only useful if you can identify your average exhalation strength in the first place to judge your current exhalations by.