P

where point 1 is in the free stream just upstream of the probe and point 2 is just inside the open end of the probe (the stagnation point). Since the friction loss is negligible in the free stream from 1 to 2, and v2 = 0 because the fluid in the probe is stagnant, Eq. (10-3) can be solved for v1 to give v, = (M))"2 (10-4)

The measured pressure difference AP is the difference between the "stagnation" pressure in the velocity probe at the point where it connects to the DP cell and the "static" pressure at the corresponding point in the tube connected to the wall. Since there is no flow in the vertical direction, the difference in pressure between any two vertical elevations is strictly

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