# Converting CFM on an Anemometer help? 4.5 inch orifice on vacuum

I have an anemometer that I bought that does not have the ability to put in the ID of the orifice I am trying to measure. In this case it is a 4.5 inch orifice-stays the same for me 95 percent of the time. Does anyone know how I convert the reading manually so that I can get an accurate reading on my anemometer?

Below is the link to the meter I bought on ebay-it is a cps avm 03.

Thank you!

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CWatters
Homework Helper
Gold Member
I think you are asking how to calculate the volume per min given the diameter and velocity?

= area * velocity.

If you want it in cubic feet per min then the area needs to be in square feet and the velocity in feet per min.

The area is..

= ∏r2/144
= 3.14 * (4.5/2)2/144
= 0.11 square feet

So if the velocity is say 4 feet per second then the cfm is

= 0.11 * 4 * 60

If the anemometer reads in feet per min then you don't need to multiply by 60.

(Edit: I don't know how accurate you need the result but if the annemometer reads velocity in feet per min then it might be sufficient to just divide that by 10 and call it cfm)

All this assumes the anemometer doesn't block and reduce the airflow significantly.