Air flow measurement

[fri52tz]

For a boiler combustion air application I have a venturi type of flow measurement device in the ductwork. I want to verify the dp across the device. The ductwork is 18' x 31", the venturi throat is 12 5/8". Can anyone recommend the appropriate formula to calculate dp for air flow at 120 F, 25 in w.c, and 892,000 pph?

 

[JEB66]

you should traverse the uct with a fechiemer probe and determine the calibration curve of the venturie.