A pitot tube traverse is a method for measuring air velocity at multiple points across a duct cross-section to calculate total volumetric airflow. The technician positions a pitot tube at various locations in a grid pattern, typically 16 to 25 points, recording velocity pressure at each point. This systematic approach accounts for non-uniform velocity distribution caused by friction and turbulence.
Technical Procedure
The pitot tube measures dynamic pressure while a manometer displays results in inches of water column (in. w.c.). Proper technique requires holding the tube perpendicular to airflow and allowing 10-15 seconds for readings to stabilize. ASHRAE standards recommend traversing at least one-third of the duct depth from the entrance. Calculations convert velocity pressure to actual velocity using the formula: velocity (feet per minute) equals 4005 times the square root of velocity pressure.
Application in HVAC Systems
This method provides accurate baseline measurements for commissioning new systems and troubleshooting existing installations. It identifies airflow imbalances, duct leakage, and equipment performance issues. Traverse measurements typically take 20-45 minutes per duct location, making it more time-intensive than alternatives but significantly more accurate for critical applications.