pitot tube flow measurement

[wfeldmann]

I am using a pitot tube connected to a differential pressure gauge and I get the following readings. The static pressure (perpendicular) was connected to the low side and the total pressure (parallel) was connected to the high side of the gague.

Dp static Total
30Hz - .31inWC .13inWC .44inWC

These readings are the pressures that I got when I inserted a pitot tube into a 30 inch circular duct. I inserted the tube exactly 15 inches to be in the center and the tip was directly parallel with the flow. The flow transmitter says we were flowing 3,400 scfm, but my calculation I did comes up with 10,921 scfm. Can anyone help me determine what is the correct flow with these process parameters.

Process media - air
Temperature - 80F
Diameter of duct - 30 inches

 

[quark]

Your calculatin is correct for the given dynamic pressure. Check whether correct diameter of the duct is fed into the indicator calculation.

 

[sailoday28]

Something is not clear to me. The Pitot tube should be measuring total pressure, .44inWC.
Are you stating that your device is equivalent to a Pitot-Static or Prandtl Tube? And all three pressures are measured?

 

[DRWeig]

Verified your answer along with quark.

It's odd that you're off by such an odd factor in your flow transmitter -- can you give us info on the flow device and the parameters that are programmed into it?

Good on ya,

Goober Dave

 

[wfeldmann]

We did find the problem and it was in the parameters of the DP transmitter. It had to deal with the square root and linear settings and values assigned to these parameters. By the way it was a prandtl tube I was using and I had it hooked to a yokogawa pressure calibrator, so I could get static, total, and dynamic pressures easily by just removing and switching out my tubes going to the calibrator. Thank you to everyone that provided feedback and assisted.