Differential-pressure meters trade a calibrated restriction for a square-root flow reading. Calibrate the meter factor C from one trusted point, hold beta between 0.2 and 0.75, provide 10-20 diameters of straight run upstream and 5 downstream, and compensate liquid density for temperature. Custody transfer needs the full ISO 5167 treatment with discharge coefficient and expansibility.
Formulas
dP to flow
Q = C x sqrt(dP / rho)
C from calibration; rho at flowing conditions.
Calibration
C = Q / sqrt(dP/rho)
One trusted point fixes C for the run.
Worked example
Water run at 25 kPa
Inputs: C 50; dP 25; rho 1000.
Calculation: Q = 50 x sqrt(0.025) = 7.91 m3/h.
Lesson:
Frequently asked questions
What is C?
Calibrated factor linking dP to flow for one meter run.
How much straight run?
Roughly 10-20D upstream and 5D downstream, plus conditioner if short.
Does density matter?
Yes for liquids; gases need compressibility and expansion corrections.
Sources and further verification
- Orifice plate Wikipedia
- Venturi tube Wikipedia
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