Large-Scale Airflow Testing
Pitot tube traverse methodology for measuring airflow in large commercial duct systems, fan airflow verification using manufacturer performance data, and airflow measurement at supply outlets and return grilles.
- Perform a pitot tube traverse in rectangular and round commercial ductwork to determine total airflow
- Calculate airflow using velocity pressure readings and duct cross-sectional area
- Verify fan airflow using manufacturer fan curves and measured static pressure and speed
- Measure and document airflow at individual supply registers and return grilles in commercial buildings
Lesson 1
Pitot Tube Traverse Fundamentals
Why Pitot Traverses Are Essential in Commercial Work
In residential systems, airflow can often be estimated using temperature rise calculations or measured at individual registers with a flow hood. In commercial systems, the duct sizes are larger, the airflows are higher, and the accuracy requirements are greater. The pitot tube traverse is the industry standard method for measuring airflow in commercial ductwork with the highest accuracy.
A pitot tube traverse measures the velocity pressure at multiple points across the cross-section of a duct. These velocity pressure readings are converted to velocity (feet per minute), averaged, and multiplied by the duct cross-sectional area to calculate total airflow in CFM.
How the Pitot Tube Works
A pitot tube is an L-shaped metal tube with two concentric tubes. The inner tube has an open tip that faces directly into the airflow and measures total pressure (the sum of static and velocity pressure). The outer tube has small holes around its circumference that measure static pressure only. When connected to a manometer, the difference between total pressure and static pressure gives the velocity pressure.
Velocity Pressure (VP) = Total Pressure - Static Pressure
The manometer displays this differential directly when the pitot tube's total pressure port and static pressure port are connected to the positive and negative sides of the manometer respectively.
Converting Velocity Pressure to Velocity
The relationship between velocity pressure and air velocity is:
Velocity (FPM) = 4,005 x square root of VP (inches w.c.)
For example, if the velocity pressure reading is 0.10 inches w.c.:
Velocity = 4,005 x square root of 0.10 = 4,005 x 0.316 = 1,266 FPM
This constant (4,005) assumes standard air density at sea level and 70 degrees F. For non-standard conditions (high altitude, high temperature), a correction factor is applied.
Traverse Grid for Rectangular Ducts
For rectangular ducts, the cross-section is divided into a grid of equal-area rectangles, and a velocity pressure reading is taken at the center of each rectangle. The number of measurement points depends on the duct size:
| Duct Dimension | Points Per Side | Total Points |
|---|---|---|
| Up to 18 inches | 3 | 3 x 3 = 9 minimum |
| 18 to 36 inches | 4 | 4 x 4 = 16 |
| 36 to 54 inches | 5 | 5 x 5 = 25 |
| Over 54 inches | 6+ | 6 x 6 = 36+ |
The minimum number of points for any traverse is 9 points (3 x 3 grid). More points provide greater accuracy, especially in turbulent conditions.
Traverse Points for Round Ducts
For round ducts, the traverse uses the log-linear or equal-area method. The duct cross-section is divided into concentric rings of equal area, and measurements are taken along two perpendicular diameters. For a 6-point traverse on each diameter (12 points total), the measurement positions from the duct wall as a percentage of the diameter are:
4.4%, 14.6%, 29.6%, 70.4%, 85.4%, 95.6%
Most digital manometers and airflow instruments have these positions pre-programmed and can guide the technician through the traverse.
Traverse Location Requirements
For accurate results, the traverse plane must be at least 7.5 duct diameters downstream and 3 duct diameters upstream of any disturbance (elbow, tee, damper, transition). In commercial systems where straight duct runs are limited, choose the best available location and note the non-ideal conditions in your report.
The pitot tube traverse is the standard method for measuring commercial duct airflow. Velocity is calculated from velocity pressure using V = 4,005 x square root of VP. For rectangular ducts, use a grid of at least 9 points (3 x 3). The traverse plane must be located at least 7.5 duct diameters from upstream disturbances for accurate results.