# 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 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