Duct Traverse Point Layout Calculator

Concepts behind this calculation

Plan round or rectangular duct traverse measurement points with TSI-106 log-Tchebycheff tables, insertion depths, a cross-section diagram and coordinates; optionally convert clear-wall positions to a supplied outer-face marking reference.

A traverse layout defines where local velocity measurements will sample the duct opening. This planner scales its named method’s dimensionless positions by clear inside dimensions. Round layouts place depths along complete directed diameters; rectangular layouts form a grid from independently selected width and height positions.

The coordinate origin matters as much as the distances. Round depths begin at the negative inner-wall endpoint of each axis, while rectangular coordinates begin at the left and bottom clear walls. Outer-face marking adds only the entered offset. It does not infer wall construction, locate access holes, or prove that the probe can reach each point.

How to use this calculator

1. Choose Cross-section and enter Clear inside diameter or Clear inside width and Clear inside height.
2. For round layouts, select Points per diameter and Traverse axes compatible with the method and access.
3. Choose Marking reference; for an outer face, supply Clear-wall to marking-face offset.
4. Read the coordinate origin and numerical table before taking field measurements.

Calculation method and applicability

Insertion depth = inside diameter × published fraction; rectangular X = inside width × width fraction, Y = inside height × height fraction; outer-face mark distance = clear-wall position + supplied reference offset; total points = axes × depths or width positions × height positions.

Input definitions and units

Cross-section

Both shapes use the named TSI-106 method. They share this planner rather than separate calculator pages. Choices: Round — insertion depths; Rectangular — point grid.

Marking reference

Inner-wall mode reports the TSI clear-wall coordinates. Outer-face mode adds only the offset you supply; it does not locate a hole, account for probe hardware or verify field access. Choices: Clear inner wall; Outer face with entered offset.

Clear-wall to marking-face offset (in)

Enter the measured or documented distance from the clear inner wall to the face used for marking. This is a geometric offset only. Applies to Marking reference: Outer face with entered offset.

Clear inside diameter (in)

Measure the open air passage after liner thickness is removed; do not enter the outside duct diameter. Applies to Cross-section: Round — insertion depths.

Points per diameter

TSI describes six as common at 10 in or smaller, and eight or ten above 10 in. This planner requires eight or ten above 10 in; confirm your project specification. Choices: 6 points (up to 10 in); 8 points; 10 points. Applies to Cross-section: Round — insertion depths.

Traverse axes

Three axes at 60° are preferred in TSI-106; two perpendicular axes are its alternative. This choice records a layout, not proof that physical access or measurement conditions are acceptable. Choices: 3 axes — 60° spacing; 2 axes — 90° spacing. Applies to Cross-section: Round — insertion depths.

Clear inside width (in)

Horizontal open-air width in this cross-section view. Below 30 in: five positions; 30–36 in inclusive: six; above 36 in: seven. Applies to Cross-section: Rectangular — point grid.

Clear inside height (in)

Vertical open-air height after any liner is accounted for. The same 5/6/7 position rule applies independently to this dimension. Applies to Cross-section: Rectangular — point grid.

How to interpret the result

The output is a coordinate plan and point count, not measured airflow. Use the numerical positions for marking and a separate traverse reduction after collecting readings.

Worked examples

Assumptions and limits

• The implemented tables are TSI-106 Rev C, US version dated April 7, 2021, pages 2–3. They are one published instrument-method guide, not a certificate of compliance with ASHRAE 111, AMCA, AABC, NEBB, TABB or an adopted code.
• TSI gives round depth fractions of 0.032, 0.135, 0.321, 0.679, 0.865, 0.968 (6), 0.021, 0.117, 0.184, 0.345, 0.655, 0.816, 0.883, 0.979 (8), and 0.019, 0.077, 0.153, 0.217, 0.361, 0.639, 0.783, 0.847, 0.923, 0.981 (10). Circular axes are complete diameters; two axes contain fewer measurement points than three.
• Rectangular fractions are 0.074, 0.288, 0.500, 0.712, 0.926 (5), 0.061, 0.235, 0.437, 0.563, 0.765, 0.939 (6), and 0.053, 0.203, 0.366, 0.500, 0.634, 0.797, 0.947 (7). This produces 25–49 points. A 2×2 grid is not offered as this method.
• Dimensions and clear coordinates refer to the inside air passage. Outer-face marks add only the supplied offset; the planner does not calculate drill-hole positions, probe reach, wall/liner stack from nominal data, inaccessible points or obstructions.
• Point placement alone does not verify straight-run length, flow uniformity, swirl, negative velocity, probe wall clearance, instrument resolution or uncertainty. TSI’s location guidance and ASHRAE’s hydraulic-diameter guidance have different rectangular reference definitions; this tool does not combine them into an automatic pass/fail check.
• The rectangular 30 and 36 in thresholds come from the TSI US guide. Do not substitute SI dimension bands or rounded example coordinates from another publication. Flat oval, equal-area and log-linear methods are outside this implementation.
• The diagram shows the cross-section layout. Use the numerical coordinate table and the instrument/project specification for field marking; displayed numbers are rounded to three decimals. There is no airflow, pressure, balancing or code-approval result.

Sources

• TSI-106 Rev C: Traversing a DuctUS version, April 7, 2021; directly inspected pages 2–3 supply the implemented circular 6/8/10 fractions, 60°/90° patterns and rectangular 5/6/7 fractions and inch dimension bands. TSI states that this guide does not guarantee conformance to a particular specification.
• ASHRAE 2025 Fundamentals, Chapter 38: Measurement and InstrumentsContext for velocity traverses and measurement limitations. Its Figure 8 and SI dimension bands are not substituted for the named TSI US selection; its rounded example coordinates are not exact TSI multiplication fixtures.