Read flow from differential pressure with a calibrated factor Q = C sqrt(dP/rho), then verify beta, straight runs, and density compensation.
Calibrate C from one trusted point
Run a known flow, record dP and density, and solve C = Q / sqrt(dP/rho). That single factor absorbs the discharge coefficient, beta, and tap geometry for screening use on the same run.
Beta and straight-run discipline
Keep diameter ratio beta in 0.2-0.75 and respect upstream/downstream lengths, adding a conditioner when pipe is short. Swirl and asymmetry bias the root reading far more than most expect.
Density and custody limits
Liquids need temperature correction; gases need compressibility and expansion factors. Screening with constant density is fine for operations, but money measurement follows ISO 5167 in full.
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.
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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