With the GM8901 to obtain an accurate airflow measurement you would multiply the speed in feet per minute by the area of the hose opening in square feet. Hoses have different diameters and thus different surface areas at the opening, so there is no one size fits all factor. If you are measuring a 1 1/4 inch diameter hose you would multiply the observed airspeed in ft/min times the surface area 0.0088 square feet. If you were measuring a 35mm hose on something like a Miele you would multiply the observed airspeed in ft/min by 0.0104. Ect. Each hose diameter will have a different square foot opening to use in the calculation.
Likewise with the HP-846A, directly from the operating manual, you have to manually enter the surface area of the hose opening of the vacuum being tested. If you are not entering the correct value the resulting CFM calculation will be incorrect. As before for accuracy there is no one size fits all factor to multiply airspeed by.
A general shortcoming with rotating vane anemometers are bearing friction and the amount of obstruction to airflow inherent in their design. Hot wire anemometers, like you find in automobile Mass Airflow Sensors do not create as much of an obstruction to the airflow so there are less losses and greater accuracy. HVAC pros will use capture hoods with differential pressure sensors or a thermal ( hot wire ) sensor.
