h w = head water (m Water, ft Water)
γ w = ρ w g = specific weight water (9807 N/m 3 , 62.4 l b f /ft 3 )
ρ w = density of water (1000 kg/m 3 , 1.940 slugs/ft 3 )
g = acceleration of gravity ( 9.81 m/s 2 , 32.174 ft/s 2 )
If the flowing fluid is water - then ρ f = ρ w , and (2) can be modified to
h w = v 2 / 2 g (3)
Or simplified to
h w (m H2O) = 0.051 v 2 (3b)
h w (mm H2O) = 51 v 2 (3c)
v = velocity (m/s)
h w (ft H2O) = 0.0155 v fps 2 (3d)
h w (in H2O) = 0.186 v fps 2 (3e)
v fps = velocity (ft/s)
h w (ft H2O) = 0.0155 (v fpm / 60) 2 (3f)
h w (in H2O) = 0.186 (v fpm / 60) 2 (3g)
v fpm = velocity (ft/min)
This calculator can be used to calculate water velocity in a pipe or water stream from a measured head of water column. The dynamic pressure as head is typically measured with a pitot tube .
|Water Flow Velocity |
- v -
|Velocity Head |
- h w -
If the flowing fluid is air with ρ f = 1.205 kg/m 3 (20 o C) and the reference fluid is water with ρ w = 1000 kg/m 3 - (2) can be simplified to
h w (m H2O) = ((1.205 kg/m 3 ) v 2 ) / (2 ( 1000 kg/m 3 ) 9.81 m/s 2 )
= 6.1 10 -5 v 2 (4)
v = air velocity (m/s)
h w (mm H2O) = 6.1 10 -2 v 2 (4b)
Note! - these simplified equations must be relaculated for other air temperatures and densities .
h w (ft H2O) = (v / C) 2 / 12 (4c)
h w (in H2O) = (v / C) 2 (4c)
v = velocity (ft/min)
C = constant (4005 for temperature 70 o F)
The constant C and temperature:
| Air Temperature |
( o F)
This calculator can be used to calculate air velocity in a duct or air stream from a measured head of water column. The dynamic pressure as head is typically measured with a pitot tube .
Charts are based on air density 1.205 kg/m 3 and water density 1000 kg/m 3 .
Table based on ρ f = 1.205 kg/m 3 (C = 4005, temperature 70 o F) and ρ w = 1000 kg/m 3 ( 1.940 slugs/ft 3 ) .
|Air Flow Velocity |
- v -
|Velocity Head |
- hw -
The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
Dynamic pressure is the kinetic energy per unit volume of a fluid in movement.
Major and minor loss in pipes, tubes and duct systems.
Typical fluid flow velocities for common liquids, gases and vapors.
Pitot tubes can be used to measure fluid flow velocities by measuring the difference between static and dynamic pressure in the flow.
Static pressure vs. pressure head in fluids.
Flow rate or discharge in an open conduit, channel or river can be calculated with the velocity-area principle.
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