Pitot Traverse Airflow from Point Readings (Velocity-Averaged)
The correct way to average a Pitot traverse: convert EACH equal-area point's velocity pressure to a velocity (V = 4005 x sqrt(VP)) and average the velocities, because velocity is what integrates to flow -- not the velocity pressures. Averaging the VPs first and taking one square root always reads high, since sqrt is concave. Four readings 0.09/0.16/0.25/0.16 in wc in a 24x12 duct give 1,602 fpm and 3,204 CFM velocity-averaged, where the single-average-VP shortcut reads 3,254 CFM, 1.55 percent high. Space the points on an equal-area or log-Tchebycheff traverse per NEBB/AABC/ASHRAE. A field measurement, not a substitute for a calibrated flow station.
Formula and source
V_i = 4005 sqrt(VP_i) at each equal-area point; V_avg = mean(V_i); A = (w x h)/144; CFM = V_avg x A. The single-average-VP shortcut V = 4005 sqrt(mean VP) reads high because sqrt is concave.
Pitot-traverse airflow averaged per ASHRAE Fundamentals / AABC / NEBB field practice: average the point velocities, not the velocity pressures. By name.
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