The 1/8 D Velocity Profile

To calculate the flow rate of water in a full pipe it is necessary to determine the average velocity.  When upstream and downstream conditions create a relatively laminar flow profile (see illustration) determining average velocity is simple.  

Using a Marsh-McBirney  insertion electromagnetic flowmeter the sensor can be placed at a position in the flow that is equal to one-eighth the inside diameter (1/8 D) of the pipe.  Velocity readings taken at this location will be very close to the average velocity.

For example:  In a 24 inch pipe the sensor should be inserted to three inches into the pipe.