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.
Pressure probes measure air flow through ducts.
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. .

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 0.75

0.75 ^ 0.5

= 0.866

Step 7

Multiply your answer by 4,005 to convert it to feet per minute.

0.866 * 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 2.5 feet respectively:

2 * 2.5

= 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