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This is an example for determining the airflow in
a 6" duct. The Pitot tube reading was 0.025
inches of water. From the chart, this is 640 feet
per minute.
The 6" duct has a cross sectional area of:
=
[
3.14
x
(6"÷12)
2
]
÷4
= 0.2 square feet
The airflow is then:
640 ft./min.
x
0.2 square feet = 128 cfm
For your convenience, the cross sectional area
of some common round duct is listed below:
DUCT DIAM. (inches) CROSS SECTION AREA (sq. ft.)
5.0 .14
6.0 .20
7.0 .27
The accuracy of the airflow reading will be
affected by how close to any elbows or bends
the readings are taken. Accuracy can be
increased by taking an average of multiple
readings as outlined in the literature supplied
with the Pitot tube.
BUILT-IN
BALANCING
DAMPERS
BACK DRAFT
DAMPER
OUTSIDE
HOOD
INSULATED PIPE
WITH VAPOUR
BARRIER
26
MAGNEHELIC
DUCT
AIR
FLOW
Pitot tube
Magnehelic gauge
Note: For best results keep Pitot tube
well away from dampers.
Pitot Tube
Figure A
Figure B
Figure C
Pitot Tube and Gauge
Placement of the Pitot Tube
Push and turn with slotted
screwdriver. Damper
automatically locks when
pressure is released.
When connecting ductwork
to the collar, take note where
screws are located. Screws
should be located no further
than 1/2” from outside edge
of collar, so as not to impede
operation of the damper.
1/2”
Insulated flexible
ducting
Balancing Collar Instructions
Figure D