Venturi tube flow calculator

flow rate measurement calculator

online since 2000

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Calculation setup
Flow rates and velocities
Venturi dimensions and coefficients
Fluid properties

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p1 - inlet pressure
Absolute pressure at the Venturi inlet
p2 - throat pressure
Absolute pressure at the Venturi throat
Δp - pressure drop
Measured pressure difference
D1 - inlet diameter
Venturi internal inlet diameter
D2 - throat diameter
Venturi throat diameter
e - expansion factor
Coefficient used for compressible flow calculation
C - coefficient of discharge
Coefficient used for Venturi tube based on ISO 5167
ReD - inlet Reynolds
Reynolds number calculated in front of the Venturi tube
Red - throat Reynolds
Reynolds number calculated at the Venturi tube throat
q1 - volumetric flow rate
Fluid flow rate in terms of units of volume per unit of time on the inlet conditions
q - standard flow
Flow rate at standard conditions. Used only if the selected fluid is gas.
ṁ - mass flow rate
Fluid flow rate in terms of units of mass per unit of time
V1 - upstream velocity
Flow velocity at the Venturi tube inlet where flow diameter is D1
T1 - upstream temperature
Fluid temperature for gas density calculation based on the ideal gas state equation
ρ1 - upstream density
Fluid density at the Venturi inlet in terms of mass per unit of volume
R - gas constant
Gas constant in terms of energy per unit of mass and temperature, for gas density calculation using ideal gas state equation
κ - isentropic coefficient
Specific heat ratio
ν - kinematic viscosity
Result of fluid particles colliding to each other and moving at different velocities in terms of area per square unit of time
μ - dynamic viscosity
Result of fluid particles colliding to each other and moving at different velocities in terms of mass per square unit of distance and time

Calculation setup

Select Venturi tube type based on its manufacturing technology
Venturi tube manufactured by welding
Venturi tube manufactured by casting
Venturi tube manufactured by machining
Select value to input. You should enter selected one. The other one will be calculated
pressure in the throat of the Venturi tube
measured pressure difference in front and in the throat of the Venturi tube
Select value to input. You should enter selected one. The other one will be calculated
kinematic viscosity
dynamic viscosity
Select value to input. Available only if gas is selected. You should enter selected one. The other one will be calculated
upstream temperature.
upstream density.
Select fluid type
for compressible flow where ideal gas state equation is applicable
for incompressible flow of liquids
Application of ISO 5167 constraints
ISO constraints
Deselect if you don't want to check if the Venturi tube and the flow conditions are within the limits of the ISO standard.

Why should you use this calculator?

Use this calculator to quickly calculate the actual flow rate through the Venturi tube flow meter with only a few inputs. This calculator performs the flow rate calculation from the measured pressure difference caused by the flow contraction in the throat of the Venturi tube.

Due to the contraction in the diameter from the inlet to the throat of the Venturi tube, the flow velocity is changing while flow rate remains constant. That change in flow velocity is creating the increase of the dynamic pressure and drop of the static pressure, as total pressure remains uniform, according to the conservation law.

The change of the static pressure can be easily measured. That measured value is the input value for the calculation of the flow rate in the Venturi tube. You should measure pressure in front of the Venturi and the Venturi's throat and use that value in the calculator.

This calculator also calculates total pressure drop created by the Venturi tube, although it is much lower than in other pressure differential metering devices like an orifice.

You can use the calculator for the Venturi effect (change of the static pressure due to the flow stream contraction) analysis, also. Many devices like Venturi vacuum pump, gas burners, wine aerator, aspirators, grills, and others use Venturi effect for their operation. These devices are creating the drop in static pressure, and that pressure change is inducing the flow of the other fluid.

When is this calculator suitable?

The calculator is relevant for a subsonic flow single-phase liquid or ideal gases. It is not applicable for pulsating flow, or non-steady flow when the flow rate is changing in time.

When is this calculator not relevant?

This calculator is not suitable for flow of multiphase fluids, like a stream of liquids that contain solid particles or stream of liquids that contain undissolved gases. Also, it is not suitable for gases that are not ideal, i.e., gases that don't relate to the ideal gas law.

Limiting factor for Venturi effect is when the flow reaches chocked flow and mean velocity is close to the local speed of sound.

What has to be understood to perform the calculation?

To calculate flow rate, you have to enter the Venturi tube inlet and throat diameter, together with fluid properties - density and viscosity.

For a gas as flowing fluid, instead of the density, you can enter gas constant, pressure and temperature at actual conditions. Density is then calculated using a perfect gas state equation.

You should enter density on real flow conditions - real pressure and real temperature.

How is the calculation executed?

The calculator is doing calculation according to ISO 5167-4.

You can expect to have accurate and reliable results solely if the Venturi tube satisfies the requirements from the standard.

What are the standard ISO 5137-4 conditions?

For the flow of a perfect gas, the pressure reduction produced by the Venturi tube (p2/p1) must be higher than 0.75.

Limits of use for Venturi tube with an "as cast" convergent section are:

100 mm < D < 800 mm
0.3 < β < 0.75
2x105 < ReD < 2x106

Limits of use for Venturi tube with a rough welded sheet iron convergent section are:

200 mm < D < 1200 mm
0.4 < β < 0.7
2x105 < ReD < 2x106

Limits of use for Venturi tube with machined convergent section are:

50 mm < D < 250 mm
0.4 < β < 0.75
2x105 < ReD < 1x106

where is:
β - diameter ratio d/D

What happens if a calculation is not within these limits?

If the Venturi tube characteristics or flow conditions are not according to the ISO 5167-4 calculator presents the message. If you still want to perform calculation regardless of the conditions in the standard, you can choose not to use ISO constraints in the calculation.

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custom fluid properties
K factor for fittings, resistance coefficient
pipe surface roughness selection
select between gauge and absolute pressure
compressible isothermal flow
dry air isothermal flow
gas offtake flow
natural gas flow


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