How to Measure the CFM of Ductwork

You can measure the cubic feet per minute (CFM) that pass through a duct using a pilot tube assembly. This device uses two probes to measure the pressure differential across the duct. This pressure drop is directly proportional to the air flow speed through the duct, which is a linear measure. This value, in turn, relates directly to the volumetric flow rate.

Static pressure probe
Total pressure probe
Differential pressure gauge

Instructions

- 1
  Insert the static pressure probe into the "low" port of the differential pressure gauge.
- 2
  Insert the total pressure probe into the "high" port of the differential pressure gauge.
- 3
  Place the static pressure probe at a right angle to the flow of air from the filter.
- 4
  Place the total pressure probe parallel to the flow of air from the filter.
- 5
  Read the pressure drop, or velocity pressure, from the gauge. For this example, imagine a pressure drop of 0.8 inches of water.
- 6
  Find the square root of the velocity pressure; in this case, 0.8^0.5 = 0.8944.
- 7
  Multiply the result by 4,005, which is a constant conversion factor: 0.8944 x 4,005 = 3,582. This is the rate of flow through the duct, or flow velocity, measured in feet per minute.
- 8
  Multiply this rate by the duct's cross-sectional area. With an area of, say, 3 square feet, this would be 3,582 x 3 = 10,746 cubic feet per minute.

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