Measuring flow using differential pressure

[Greenpower]

I recently calibrated a flow meter at a water BPS using some of the original specifications data left by the employees before me, after I finished the calibration and everything was said and done, I was questioned on whether it was reading accurately or not because the readings were different then what the water plat was use to seeing. I thought that is way I was told to calibrate it, because it seemed wrong before I did the work. Anyway being fairly new to measuring flow using a differential pressure sensor, I was needing help coming up with some calibration factors. I have found many formulas, but cannot decide with one to use, we do not compensate gat this timeh for temperature, we just measure the fluid assuming that it will not be affected enough by temp to get to far off.
My question is how can I figure up inches of h20 per GPM of water flowing though a certain size pipe. One of the formulas I am looking at Qv=Kãhw/P.
Qv = volumetric flow rate
hw = differential pressure
P = flowing density
K = flow factor
Although it gives a volumetric and I was wanting mass flow rate, I am sure you can convert it.
The reason I need to know is so I can check the pressure sensor,h because it was made in 1972, and may no longer be accurateh, and check the specs left behind for me to calibrate the flow meter by.

Any help or ideas would be greatly appreciated.

 

[saxon]

Green, What are you using as the primary flow sensor? An orifice plate, venturi, flow nozzle, pitot, annubar, elbow taps? All of these will give different dP's for agiven flow rate. Also, what type of dP cell are you using and is the sq. root extractor still operational? Are there e/p or i/p transducers in the instrument/control circuit anywhere that may be out of calibration?

If it's an orifice plate, use the following formula, other meter elements will be different:

q=Kb^2AF*(2g(dp/r))^1/2

Where:
q= volumetric flwo rate, ft^3/sec
K= orifice flow coefficient, dimless
b= dia. ratio (d/D)
A= inside area of pipe, ft^2
F= area factor for thermal expansion dimless
g= unit conversion factor=32.17 lbmass-ft/lbforce-sec^2
dp= measured diff. press, lb force/ft^2
r= fluid density, lb mass/ft^3

 

[Greenpower]

I am sorry it took me so long to get back to you, but I had to get the meter running, I got the unit clabrated and is running smoothly now. I am interested in learning more about the differant types of flow measurement. We have many differant types of flow sensing in our system that I need to learn more about. The flow senser Is a venturi type, and I am using the square root extractor in my equation. I am breaking the flow down into a percentage and using the square root of that perentage and multipling that by the max flow.