How Everything Works
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Question 602

I was wondering if a pitot tube is a very efficient way to measure airflow and, if so, what would be the conversion formula to cfm? - BN
A pitot tube determines airspeed by measuring the pressure rise that occurs when the airstream is slowed to a stop. Any time moving air encounters a closed chamber head-on, the air stops and it exchanges its kinetic energy—its energy of motion—for pressure potential energy—energy stored in the form of an elevated pressure. By measuring this elevated pressure, you can determine what the air's kinetic energy was while it was moving and thus how fast it was moving.

Pitot tubes are used to measure airspeed in airplanes. They're the cigar-shaped objects that project forward from the undersurfaces of airplanes near their noses. I suppose that you could use a pitot tube to measure the speed of air flowing through an air duct, but to determine the volume of air flowing through that duct, you'd need to know the dimensions of the duct. The relationship between pressure in the pitot tube and the airspeed is complicated and so is the relationship between airspeed in a real duct and the volume of air it's carrying. Overall, this doesn't look like an easy job.


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