How to Measure Airflow in Ducting

To maintain your building's ventilation system, you need to regularly monitor the rate of air flow in different air vents. A duct's air flow depends on two factors: the duct size and the air velocity. You can calculate the cross-sectional area by measuring the duct with a ruler or measuring tape. Air velocity, however, requires special equipment to measure. Use a pilot tube assembly, which uses two pressure probes and a differential pressure sensor.

Pressure probes measure air flow through ducts.

Step 1

Plug the static pressure probe into the "low" port of the differential pressure sensor. 

Step 2

Plug the total pressure probe into the "high" port of the differential pressure sensor. 

Step 3

Position the total pressure probe so it points along the duct's air flow. 

Step 4

Stand the static pressure probe perpendicular to the duct's air flow. 

Step 5

Note the air flow's velocity pressure from the differential pressure sensor's display. 

Step 6

Find the square root of this answer.  If the velocity pressure is 075 075 ^ 05 = 0866.

Step 7

Multiply your answer by 4,005 to convert it to feet per minute 0866 * 4,005 = 3,468. 

Step 8

Measure the length of the duct in feet. 

Step 9

Measure the width of the duct in feet. 

Step 10

Multiply the length by the width.  If the length and width are 2 and 25 feet respectively: 2 * 25 = 5 square feet.

Step 11

Multiply the answer by the answer to step 7 3,468 * 5 = 17,340 cubic feet per minute. 

Things You Will Need

  • Differential pressure sensor
  • Static pressure probe
  • Total pressure probe
  • Ruler or measuring tape

About the Author

Ryan Menezes is a professional writer and blogger. He has a Bachelor of Science in journalism from Boston University and has written for the American Civil Liberties Union, the marketing firm InSegment and the project management service Assembla. He is also a member of Mensa and the American Parliamentary Debate Association.

Photo Credits

  • air conditioner vent image by Tammy Mobley from Fotolia.com