# Calculate flow rates and velocity in pipes

Greetings all. This is a real world issue that is happening at work and there is much discussion as to the correct answer. The situation is this:

We need to test a Flowmeter that measures flow and velocity. Flow is in Gal/Min and Velocity is in Ft/sec.

There is an Omega Flowmeter installed in the pipe as our standard reference. Our product utilizes two (2) Transducers mounted on opposite sides of the pipe to measure the flow. They should read approximately the same amounts. The water is in a closed loop system driven by a centrifugal pump and controlled by a VFD Motor Controller. Both the reference meter and the transducers are mounted on an 8-foot horizontal pipe. The reference uses internal magnetic sensors to measure the flow.

The problem is that our product reads approximately 20 percent lower than the reference meter. One camp claims it is because there is a slight difference in the internal diameters of the pipe and the other believes it is noise generated by the Motor controller. Here are the particulars. Assume these numbers are EXACT (which they are).

Internal diameter of Omega Reference Meter is 1.87 inches.
Internal diameter of piping is 2.028

Unfortunately, the reference meter does NOT display Velocity, only flow, so the velocity can only be calculated.

I have done numerous calculations based on info found online, but  am still stuck.

What say you all?

Thanks
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Commented:
>>  There is an Omega Flowmeter installed in the pipe as our standard reference.
The reference uses internal magnetic sensors to measure the flow.

What is the model number?  What are its range and accuracy specifications?
Is it really a magnetic flow meter?
https://en.wikipedia.org/wiki/Flow_measurement#Magnetic_flow_meters
Or is it a propeller/turbine with a magnetic pickup to count revolutions?
What are its range and accuracy specifications?

>>  Our product utilizes two (2) Transducers
What kind of transducers?  Pressure, optical, acoustic?

How are you calibrating your product?  Analytically or with a hose, bucket, and stop watch?
What range and accuracy do you hope to achieve?

>>  Both the reference meter and the transducers are mounted on an 8-foot horizontal pipe.
Are both sensors well away from the corners and each other?

Flow meters work best with laminar flow.  The small restriction at the reference meter will cause some turbulence.  This may affect the reading on the reference meter.  It could also affect your transducers if they are located within a few pipe diameters on either side.
Commented:
If you haven't done so already, I think you need to perform a messy absolute calibration for your reference sensor and your transducers.  If you care about precision, you may need to worry about temperature effects.

Do your transducers calculate an average velocity or a velocity profile?
http://patents.justia.com/assignee/ocean-research-equipment-inc
Commented:
>>  I have done numerous calculations based on info found online, but  am still stuck.

Commented:
For every foot, your 1.87" pipe has 131.83 cubic feet of water. The 2.028" pipe has 155.05, which is 17.6% more than the smaller pipe. The smaller pipe then has 82.4% less volume as the larger. 82.4% of 20/gpm is approximately 16.48/gpm. I'm not sure if this is the correct way to figure this out, but it looks good on paper!

Flyster

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