Venturi Flowmeter Calculator

A fluid passing through smoothly varying constrictions experience changes in velocity and pressure. These changes can be used to measure the flowrate of the fluid.

To calculate the flowrate of a fluid passing through a venturi, enter the parameters below. (The default calculation involves air passing through a medium-sized venturi, with answers rounded to 3 significant figures.)

Inputs

Pipe diameter upstream of venturi, D_a:
Diameter of venturi neck, D_b:
Pressure difference measured by venturi, Dp:
Fluid density, r:
Discharge Coefficient, C:

Answers

Velocity at A, V:	2.21 m/s
Volume Flowrate, Q:	17.0 l/s
Mass Flowrate:	0.0220 kg/s

Select desired output units for next calculation.

Equations used in the Calculation

As long as the fluid speed is sufficiently subsonic (V < mach 0.3), the incompressible Bernoulli's equation describes the flow. Applying this equation to a streamline traveling down the axis of the horizontal tube gives,

From continuity, the throat velocity V_b can be substituted out of the above equation to give,

Solving for the upstream velocity V_a and multiplying by the cross-sectional area A_a gives the volumetric flowrate Q,

Ideal, inviscid fluids would obey the above equation. The small amounts of energy converted into heat within viscous boundary layers tend to lower the actual velocity of real fluids somewhat. A discharge coefficient C is typically introduced to account for the viscosity of fluids,

C is found to depend on the Reynolds Number of the flow, and usually lies between 0.90 and 0.98 for smoothly tapering venturis.

The mass flowrate can be found by multiplying Q with the fluid density,