Flow rate through a venturi meter from pressure difference and geometry using Cd.
Fluid Mechanics IIDiameter units (applies to both D₁ and D₂)
Venturi flow equation:
Beta ratio and area ratio:
Typical β range: 0.3 – 0.75. Outside this range, Cd correlations become less accurate.
Throat and upstream velocities:
Where:
Typical Cd values:
| Venturi type | Cd |
|---|---|
| Classical venturi — machined convergent | 0.995 |
| Classical venturi — rough-cast convergent | 0.984 |
| Universal venturi tube | 0.977 |
| Short-form venturi tube | 0.96 |
| Venturi nozzle (ISO 5167) | 0.975 |
Venturi meters have significantly higher Cd than sharp-edged orifices (≈ 0.61) because the gradual converging section minimises energy losses.