It comes with a chart.With that chart, you can accurately determine the airflow of a fan:

• In all the operating positions within a pulley

• Size of replacement pulley if necessary

• Operating position in the replacement pulley

• Size of the belt at all operating position.The Pulley GPS is your resource to analyze fan performance and proffers solution in a minute so that you can get the work done flawlessly at minimum time.Changing Airflow is a Breeze – See chart in pictures.Desired air flow= 5000 CFMMeasurement= 4500 CFMPulley: 4.75Belt size: BYou want to increase air flow by 500 CFM, 10%Solution: Using Pulley GPS OP, is #6.From OP #6 on the chart you move left.You can increase flow by either 8% or 11%, its your call.For 8% close the pulley by 3 half turns to OP #3.For 11% close the pulley by 4 half turns to OP #2If the pulley is too small or large for desired airflow, the lower chart will guide you to the right size.Other features

• Used at both digital and analog modes• Used as a micrometer• Built in belt sizer