Prandtl probe calculator

flow velocity calculator

online since 2001

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Calculation setup
Fluid properties
Flow rates and velocity
Flow is subsonic
Pipe dimensions

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pt - total (stagnation) pressure
Measured pressure of the stream when flow velocity is brought to zero isentropically, in the center of the Prandtl probe
pd - dynamic pressure
Differential pressure calculated as the difference between the total (stagnation) and the static pressure.
p - static pressure
Measured pressure of the flow without the affect of the fluid movement (kinetic) energy
ρ - fluid density
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
T - temperature
Fluid temperature for gas density calculation based on the ideal gas state equation
q - volumetric flow rate
Fluid flow rate in terms of units of volume per unit of time
ṁ - mass flow rate
Fluid flow rate in terms of units of mass per unit of time
V - velocity
Flow velocity in terms of units of distance per unit of time
c - speed of sound
Sound speed for given gas and actual conditions in terms of pressure, density and temperature
M - Mach number
Flow velocity and actual speed of the sound ratio
D - internal diameter
Internal circular pipe diameter
A - area
Internal pipe cross section area
h - channel height
Height of the channel for rectangled closed pipe line
w - channel width
Width of the channel for rectangled closed pipe line

Calculation setup

Select value to input. You should enter selected one. The other one will be calculated.
total (stagnation) pressure
dynamic pressure
Select value to input. You should enter selected one. The other one will be calculated.
fluid density
Select pipe shape
round pipe
full cross section fluid flow
rectangle pipe
for rectangle channels and full cross section flow

Why Prandtl probe?

You can use a Prandtl probe as flow velocity metering instrument mainly for open flow around solid bodies. However, it is also suitable for velocity measurement of air flow in a rectangle and round channels.

What has to be understood to perform the calculation?

With a Prandtl probe, you can measure flow velocity indirectly, by measuring a difference between stagnation pressure and static pressure of flow in a single point of a flow streamline.

A Prandtl probe measures stagnation pressure in the central part of a Prandtl probe, where flow velocity equals zero. When flow velocity is zero at some point of streamline, measured pressure in that point equals total pressure.

On the outer part of the Prandtl probe where flow streamlines are tangent to the Prandtl probe, the flow velocity is unimpaired, and the pressure measured at that part of streamline is called static pressure.

The difference between stagnation and static pressure in one point of flow streamline is called dynamic pressure. Dynamic pressure represents kinetic energy, and flow velocity can be calculated using this calculator.

What about Pitot tube?

You can use the calculator for Pitot tube as well. A Pitot tube metering instrument is measuring stagnation pressure in one point of a streamline. After you measure static pressure by some other instrument like a barometer, you can compute dynamic pressure, and the flow velocity using this calculator.

When is this calculator suitable?

You can use this calculator for incompressible flow of liquids and compressible flow of ideal gases.

Available in download version

save/open multiple results
export to word and excel
print results
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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