Quote:
Originally Posted by 3-Wheeler
I'm not trying to be picky here, because I understand the point your trying to make, and you are making a good point. 
However, the pitot tube, or at least the pitot tubes I have seen at work, work on the differential pressure between the static pressure (potential energy) and the dynamic pressure (kinetic energy).
Static pressure would indicate the generalized pressure rise inside the duct that feeds the flow meter pitot tube, and the air velocity component measures the air pressure *reduction* due to the high velocity air. Yes, I did say pressure *reduction* or vacuum if you wish.
I should point out as a matter of air flow discussion, that the higher the air velocity inside a closed conduit, the higher the *vacuum* generated. This is exactly how vacuum aspirators work. They take compressed air and convert it into a vacuum due to a tiny restriction flow path inside the device, that highly accelerates the air stream, which in turn creates a high vacuum. I have to demonstrate this phenomena to new staff in the lab from time to time as some of them have a hard time believing the effect.
And so it is with pitot tubes that measure the air stream velocity. I believe that the really accurate tubes measure the differential pressure, and not just the stagnation pressure as alluded to above.
Hope this helps, Jim. |
According to the diagrams here, the pressure inside the tube is higher than static pressure
Pitot Tubes
Look at the equations. They depend on stagnation pressure being higher than static, because you can't take the square root of a negative number
V = \sqrt{\frac{2 (p_t - p_s)}{\rho}}
http://en.wikipedia.org/wiki/Pitot_tube
Since the tube is pointing into the airflow, it has to be higher pressure inside than outside. Stick your head out the car window and you can feel the pressure increase in your mouth. Ram air intakes on cars wouldn't work otherwise.