Pitot Traverse Points and Average Velocity Calculator
Plan a duct traverse with a pitot tube or anemometer: get the log Tchebycheff measuring positions for a round or rectangular duct, then enter the velocities to get the average velocity and flow.
- Measuring positions from the inner wall = fraction × 600 mm (see table)
- Average velocity = arithmetic mean of all point velocities (log Tchebycheff positions are equal area weighted)
- Q = vavg × A
Traverse positions along each diameter
| Point | Fraction | Distance from wall |
|---|---|---|
| 1 | 0.032 | 19.2 mm |
| 2 | 0.135 | 81 mm |
| 3 | 0.321 | 192.6 mm |
| 4 | 0.679 | 407.4 mm |
| 5 | 0.865 | 519 mm |
| 6 | 0.968 | 580.8 mm |
Traverse Method
vavg = Σvi ÷ n
Q = vavg × A
Log Tchebycheff positions place points so a simple average of the readings gives the area weighted mean velocity. Round ducts are traversed along two diameters at 90°; rectangular ducts use a grid of points in both directions. Average the velocities, not the pressures, when using a pitot tube.
Worked example
A 600 mm round duct traversed at 6 points on 2 diameters (positions 19, 81, 193, 407, 519 and 581 mm) with the readings shown averages 9.45 m/s, a flow of about 9,620 m³/h.
Frequently Asked Questions
What is a log Tchebycheff traverse?
A set of measuring positions chosen so that the simple average of point velocities equals the true mean velocity of a developed profile.
How many points are needed?
Typically 6 to 10 per diameter in round ducts and 5 to 7 per side in rectangular ducts, more in disturbed flow.
Why average velocities and not pressures?
Velocity is proportional to the square root of pressure, so averaging pressures and then converting overestimates the mean velocity.
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