Operate the air conditioning system normally and use the ductblster manometer to measure the static pressure in the supply plenum or trunk. You should measure after the coil but before any takeoffs. Record this pressure. Now, turn the system off and connect the ductblaster to the blower on the return side. You will need to block the return air trunk off so air only goes through the unit and into the supply ducts.
Now turn on the ductblaster. Once again, measure the supply air static at the same location where you measured it with the unit operating. Dial the ductblaster up until the measured supply air static equals the reading you took when operating the system. The amount of airflow the ductblaster is moving is the same as the airflow through the system when it was operating.
How does this work? You are matching the airflow required to create a supply static equal to the supply static created by the system blower. Note that this does not require any particular manufacturer’s data. This procedure allows you to make an accurate airflow measurement using a tool you may already have. It also allows you to get more from your investment in the ductblaster.