A peak flow meter for asthma is like a thermometer for a fever. It’s a tool that helps you monitor what’s going on inside your body. In some cases when you are not feeling well, you may feel "hot" or "feverish," but when you take your temperature with a thermometer, it is normal. With asthma, sometimes you may feel your breathing is fine, but when you measure it with a peak flow meter, your lung function is slightly decreased. A peak flow meter can help you determine airway changes and thus, better manage your asthma.