Glossary term
Venturi Meter
A flow measurement device that infers flow rate from pressure change through a converging-diverging section.
Definition
deviceA Venturi meter is a differential-pressure flowmeter that estimates flow rate from the pressure drop caused by accelerating fluid through a converging throat.
A Venturi meter uses a converging section, throat, and diffuser to create a repeatable relation between flow rate and pressure difference. Compared with an orifice plate, it usually has lower permanent pressure loss but is larger and more expensive. Its accuracy depends on geometry, discharge coefficient, Reynolds number, fluid properties, pressure taps, installation length, and calibration.
A Venturi meter narrows the flow area at a throat. Continuity requires velocity to increase at the throat, and Bernoulli’s equation links that velocity increase to a pressure decrease. The measured pressure difference between upstream section and throat is then converted to flow rate using geometry and a discharge coefficient.
A simplified incompressible-flow form is:
where C_d is discharge coefficient, A_t is throat area, \Delta p is differential pressure, \rho is density, and \beta is throat-to-pipe diameter ratio. Compressible flow, multiphase flow, low Reynolds number, and cavitation require additional treatment.
Engineering use
Venturi meters are used in water systems, process plants, laboratories, HVAC, combustion air measurement, and slurry or dirty-service applications where lower permanent pressure loss is valuable. They are more tolerant of some fouling than sharp-edged orifice plates, but they still require correct straight-run installation and pressure tap maintenance.
Installation and calibration
Accuracy depends on the machined throat geometry, pressure-tap location, upstream flow profile, downstream recovery, and the calibration basis for the discharge coefficient. Bends, valves, reducers, swirl, deposits, or partially filled pipes can alter the differential pressure for the same flow rate. High-accuracy installations usually specify straight-run lengths, flow conditioners, tap inspection, transmitter range, temperature and pressure compensation, and periodic calibration checks.
Common mistakes
A common mistake is applying the ideal Bernoulli equation without discharge coefficient, Reynolds-number limits, density correction, or installation effects. Another is ignoring throat pressure: if it falls near vapor pressure, cavitation can occur and invalidate the measurement. A strong Venturi-meter review states pipe diameter, throat diameter, pressure taps, fluid density and viscosity, Reynolds number, discharge coefficient, upstream disturbance, calibration basis, and uncertainty.