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# Closed Loop Air Circulation Wind Tunnel, Pressure Differential Measurement

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## Closed Loop Air Circulation Wind Tunnel, Pressure Differential Measurement

(OP)
Hello everyone,
I am new here. I am hopping to get some replies to clarify and help with my task at hand.
I have a 6" x 12" sheet metal duct, about 24 feet long, with a fan inside it to circulate the air. For my application, I need to measure the air speed at the center of the duct at about 8 feet away from the fan outlet, on its pressure side. I am using a pitot tube and manometer to measure the pressure differential, I can calculate speed of air from there (the ranges of differential pressure that I am expecting is from 0.1 to 7 Pascals (.03 inch w.c.). Well, both plastic tubes coming from the pitot tube and going to the manometer are reading negative pressure (vacuum). There is a difference between the two line vacuums. Air is blowing by the pitot tube, but I do not see pressure. Could I still use these vacuum measurement to calculate my air speed? I would appreciate any comments. thanks.

### RE: Closed Loop Air Circulation Wind Tunnel, Pressure Differential Measurement

The dynamic pressure in a fluid flow can be expressed as

pd = 1/2 ρf v^2
where
pd = dynamic pressure in fluid (Pa, psf (lb/ft^2))
ρf = density of fluid (kg/m3, lb/ft^3)
v = fluid velocity (m/s, ft/s)
The dynamic pressure is measured between the two ports on the pitot tube.

Measuring pressure differentials this small with a manometer with any repeatability is really hard. Getting an appropriate Digital Manometer will make the job much easier. Link to one of many sources

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