A spirometer helps to measure how well your lungs are functioning by recording the amount of air you breathe in and out and the speed of your breath. The three main values you will see are: PEF which is the maximum amount you can exhale, FEV1 which is the amount of air you can force out of your lungs in one second, and FVC which is the greatest total amount of air you can forcefully breathe out after breathing in as deeply as possible.