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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: V=4005*(squareroot(VP)) 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: Q=VA 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 Greg

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