6.2 Traverse Velocity Conversion and Airflow Calculation

Traverse Velocity and Airflow Mathematics

Convert each point before averaging

For standard air, velocity at a traverse point is:

V = 4005 × square root of Pv

Velocity pressure is proportional to velocity squared, so averaging velocity pressures and then taking one square root is generally wrong. For equal-area points, convert each pressure to velocity, add the velocities, and divide by the number of points. For unequal represented areas, multiply each local velocity by its area and sum the local flows.

Example: four equal-area readings are 0.04, 0.16, 0.25, and 0.36 in. w.g. Their square roots are 0.20, 0.40, 0.50, and 0.60. The mean square-root term is 0.425, so mean velocity is 4005 × 0.425 = about 1,702 FPM. Averaging the pressures gives 0.2025 and about 1,802 FPM, which overstates this example.

Calculate area and flow

For rectangular duct with clear internal width W and height H in inches:

A = W × H / 144

For round duct with internal diameter D in feet:

A = pi × D squared / 4

Then Q = Vavg × A. A 24-inch by 18-inch duct has 3.00 sq ft net area. If mean velocity is 1,402 FPM, airflow is about 4,206 CFM.

Account for internal liner and obstructions according to the governing method. Equivalent diameter is used for location criteria in some procedures; it does not replace actual cross-sectional area in the airflow calculation.

Density

When actual density is required, use V = 1096 × square root of (Pv/density), with consistent units. Apply the same density basis to every point. Do not “correct” the final CFM again if density was already included in the point velocities. State whether the result is actual volumetric flow, standard flow, or another referenced quantity.

Point identity and signs

Retain each coordinate and raw signed reading. A negative or unstable value can indicate reverse local flow, probe misalignment, a reversed connection, or a poor plane. Do not discard it merely because it complicates the average. Check setup, repeat the point, and follow the selected method for directional flow.

Round-duct methods often use two perpendicular diameters to represent variation in two directions. Rectangular methods use a defined grid such as Log-T or equal-area points. The coordinate table determines what area each reading represents; there is no universal six-to-ten-point rule for every round duct.

Spreadsheet or calculator workflow

A defensible calculation table contains:

• point number and coordinate;
• raw velocity pressure and sign;
• density used, if applicable;
• square root or calculated velocity;
• represented area or weight;
• local airflow; and
• inclusion, repeat, or limitation note.

Keep full precision through intermediate steps and round the final flow to a level supported by the instruments and geometry. Protect formulas against blank cells, text, and accidental averaging of the pressure column.

Reconciliation

Compare the traverse with a suitable independent measurement collected in the same system mode. Terminal sums may differ because of duct leakage or omitted devices; fan inlet and outlet traverses may differ because of location quality; an airflow station may have an incorrect area or factor. A material discrepancy triggers checks of dimensions, point coordinates, zero, tubing, density, simultaneous conditions, and leakage.

Repeatability alone is not proof of accuracy. A probe can repeat a biased result at a disturbed plane. Record plane location and disturbances with the math so the CP can evaluate the result.

Exam method

For calculation questions, write the relationship, convert units, preserve the square-root order, and estimate the expected magnitude before selecting an answer. If a 0.25 in. w.g. point under standard air does not produce about 2,000 FPM, recheck the operation. If a 2 ft by 2 ft duct is treated as 4 square inches or 16 square feet, recheck the unit conversion.

Multi-method comparison

Suppose a main traverse reports 10,200 CFM and simultaneous terminal readings sum to 9,700 CFM. The 500-CFM difference is not automatically leakage. Check whether every terminal was included, whether the fan and VAV system were stable, whether branch leakage lies inside the chosen boundaries, and whether the uncertainties overlap. If the terminal readings were collected an hour later under a different load, direct subtraction is not a valid continuity check.

For a round duct, use actual circular area for Q even if an equivalent or hydraulic diameter was used to evaluate straight-run distance. Mixing those two uses of diameter is a common source of otherwise polished but incorrect answers.