Log-Tchebycheff Traverse Calculator
For TAB technicians balancing rectangular ductwork on site, who need ASHRAE 111 Log-Tchebycheff pitot positions fast — the asymmetrical multipliers (0.074, 0.288, 0.500 for a 5-point grid) push the outer readings inboard so wall-friction errors never enter the traverse average.
Enter a duct dimension to see the traverse points.
How to Use This Calculator
- Type the duct dimension in inches for the side you are traversing (e.g. 24).
- Choose points per side — 5, 6 or 7.
- Read the numbered insertion depths in inches from the list, each measured from the same reference wall.
- Change the dimension or the point count and the list updates immediately; copy the list if you want it with you at the duct.
FAQ
Q: How is each pitot insertion depth calculated?
A: Depth = duct dimension × the Log-Tchebycheff multiplier, so a 24 in duct with 5 points gives 24 × 0.074 = 1.776 in, then 6.912, 12.000, 17.088 and 22.224 in.
Q: Why are the Log-Tchebycheff traverse points not evenly spaced?
A: The ASHRAE 111 multipliers are deliberately asymmetrical — 0.074, 0.288, 0.500, 0.712, 0.926 for 5 points — so the outer readings are not taken too near the wall, which is what prevents wall-friction errors in the traverse.
Q: How many points per side should I use, and what multipliers does that give?
A: The tool offers 5, 6 or 7 per side: 5 uses 0.074/0.288/0.500/0.712/0.926, 6 uses 0.061/0.235/0.437/0.563/0.765/0.939, and 7 uses 0.053/0.203/0.366/0.500/0.634/0.797/0.947.
Q: Do I measure every point from the same wall?
A: Yes — each depth is measured from one reference wall, and because the multipliers pair to 1.000 (0.074 with 0.926, 0.288 with 0.712), the pattern mirrors itself if your probe enters from the opposite side.