Based on the energy conservation low, Venturi tube is one of the easiest to use, not expensive and very accurate instrument for flow rate measuring in pipe systems. Determine pressure drop from the inlet to the throat and calculate flow using calculator.

Theory

Calculation of flow rate using orifice plate calculator is for incompressible flow, based on the Bernoulli principle:

where is:
p - pressure
rho - density
V - velocity
g - gravitational constant (9.81 m/s2)
z - geodetic height

Assumption that pressure lost is negligible (pressure drop is obvious and included with coefficient of discharge which is introduced bellow):

and:

and if velocities substituted with flow rate:

where is: Q - volumetric flow rate
D - diametar

Pressure drop through the orifice because of velocity increase can be calculated as follows:

or:

Expressing flow rate from the previous equation leads to:

Substituting:

flow rate can be determined as:

where C is coefficient of discharge. The above equation is main one used for flow calculation in calculator. Other values are calculated using following equations:

mass flow:

velocities:

If calculator is used for gas flow, then gas is considered as incompressible and ideal. Equation for ideal gas:

can be used for calculation of temperature T:

as well as density rho:

where R is gas constant (R=287 J/kgK for air).

Coefficient of discharge C

As fluid exits a reservoir through a small hole and enters another one, or flows out to the open air, stream lines tend to contract itself, mostly because of inertia. Coefficient of discharge C is used to include this effect.
For the Venturi tubes with diameters in range of D = (200 - 1200 mm), D2/D1 = (0.4 - 0.7) and
ReD = (2 *10⁵ - 2 *10⁶) the coefficient of discharge is C = 0.985.
In this calculator for coefficient off discharge C following equation has been used:

where a, b, and c depend on the type of Venturi tube.
For welded tube, these coefficients are:
a=0.70304970
b=0.00490015
c=-0.00024547

For casted tube are:
a=0.60892370
b=0.00659844
c=-0.00033123

And for machined are:
a=0.49670179
b=0.00873339
c=-0.00044367

Reynolds number on inlet ReD is calculated using well known equation:

Appliance

Flow through Venturi tube calculator can be used for both liquids and gases. Fluid is considered as incompressible, so density (rho) and temperature (T) are constant through tube. Also, gas is considered as ideal.

Units of measure are both in SI and English system.

Flow through Venturi tube calculator can be used for calculation of:

volumetric flow - Q
mass flow - G
velocity on inlet, bigger diameter - V1
velocity on smaller diameter - V2
For calculation of those values, necessary values for input are:

inlet, bigger diameter - D1
smaller diameter - D2
inlet pressure - p1
either pressure on smaller diameter - p2, or pressure drop p1-p2
kinematic viscosity - ni, or dynamic viscosity mi
for gases only:
either temperature - T, or density - rho
gas constant - R
for liquids:
density - rho

Beside four values (Q, G, V1, V2), which calculation is main purpose of calculator, values that are not defined by user are determined in process of calculation (for example: if pressure p1 and temperature T are specified for gas flow - value for rho is calculated together with four main values (Q, G, V1, V2))