MadSci Network: Engineering

Re: Formula for calculating wind velocity through a tube using a manometer?

Date: Mon Jan 29 17:01:10 2001
Posted By: Greg Culler, Staff, Mechanical Engineer, Industry
Area of science: Engineering
ID: 980105790.Eg

Hi Eric

This is an interesting experiment.  Manometers are very useful devices.

The formula you are looking for is:

 V=1096*(square root(VP/d))

Where V= Velocity
      VP= velocity Pressure
      d= density of the air 

The velocity pressure is the amount of pressure required to accelerate air 
from zero to some velocity and is proportional to the kinetic energy of 
the airstream.  The velocity pressure is only exerted in the direction of 
airflow and is always positive.  The other pressures, ie total pressure 
and static pressure can be positive or negative depending on which side of 
the fan the measurement is taken.

The actual density of air varies with temperature and humidity.  The 
standard density of air is said to be 0.075 lbm/ft^3.   If you assume 
standard air, the above equation will reduce to:


In order to measure the velocity pressure, you need to set up your 
manometer as shown in the sketch.  As you can see from the equation, the 
tube or duct diameter is not needed to determine the velocity of an air 
stream with a manometer.  The tube diameter becomes important if you want 
to move a certain quantity of air at a certain velocity:


Where Q= the volume of air
      V= the Velocity of the air
      A= the cross sectional area of the tube or duct.

The diameter of the tube or duct is also important if you want to control 
the pressure within a duct.

I hope this info is helpful.  If you want to know more, you may want to 
find a copy of the ACGIH Industrial Ventilation publications.

Good Luck

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