whats involved in building a pitot style bench
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by Mouse » Wed Apr 06, 2005 2:04 pm
A Pitot tube is basicly a little air scoop that points upstream into the airstream and produces a pressure inside the tube caused by the air impacting the tube opening. Place a similar tube facing downstream and that tube produces a pressure caused by the vacuum created at the opening of the tube by the airstream. Now, the difference of these two pressures is the total pressure and you can calculate the velocity of the air using the total pressure.
If these Pitot tubes are located inside a bounded path, and you know the area of the path at the location of the Pitot tubes, using the velocity and area of the path, you can calculate the cfm.
Typical Pitot tubes require a long straight run before and after the Pitot tube location to stabilize the velocity profile of the airstream inside a bounded path. The problem I have found is that the longer the run from the location that the test pressure is being measured (settling chamber or fixture) the higher degree of unlinearity exists between the test pressure measuring point and the Pitot tube. So while long straight runs are required, they still should be as short as possible. The catch 22.
A Pitot tube setup generally has a much lower pressure drop than an orifice style setup, requiring much less air power to achieve desired test pressures, among other advatages, and disadvantages.
If designed properly, a Pitot tube has about the same accuracy as an orifice.
It is only my opinion, based on my personel thinking, that a well designed Pitot tube setup gives a better representaion of actual air flow conditions because it is so much more unobtrusive and less disruptive to the flow.
John
If these Pitot tubes are located inside a bounded path, and you know the area of the path at the location of the Pitot tubes, using the velocity and area of the path, you can calculate the cfm.
Typical Pitot tubes require a long straight run before and after the Pitot tube location to stabilize the velocity profile of the airstream inside a bounded path. The problem I have found is that the longer the run from the location that the test pressure is being measured (settling chamber or fixture) the higher degree of unlinearity exists between the test pressure measuring point and the Pitot tube. So while long straight runs are required, they still should be as short as possible. The catch 22.
A Pitot tube setup generally has a much lower pressure drop than an orifice style setup, requiring much less air power to achieve desired test pressures, among other advatages, and disadvantages.
If designed properly, a Pitot tube has about the same accuracy as an orifice.
It is only my opinion, based on my personel thinking, that a well designed Pitot tube setup gives a better representaion of actual air flow conditions because it is so much more unobtrusive and less disruptive to the flow.
John
- Mouse
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