11
Procedure for Measuring Diffuser, Register and Grille
Velocities with Diffuser Probe
To calculate the air volume passing through a diffuser, register or grille
outlet, measure the velocity at the outlet, applying the air flow factor that
is provided by the outlet manufacturer, and use the following equation:
Q = K
x
V
Where
English Units
Metric Units
Q = Volume flow rate
ft
3
/min
m
3
/s
K = Manufacturer’s flow factor
—
—
V = Average measured velocity
ft/min
m/s
Instructions for positioning the Diffuser Probe at the outlet for measuring
the velocity, as well as an appropriate flow factor, are available from the
outlet manufacturer. Shown below is a typical example:
Typical Diffuser Air Flow Measuring Procedure
To determine the volume of air discharged, a Velometer with a Diffuser
Probe may be used as shown below.
Note that the bottom of the positioning fins on the probe are level with
(and touching) the bottom of the intermediate spinning of the diffuser. The
probe is held vertically. The flow factors shown in the accompanying table
are applied to an average of six velocity readings taken at equally spaced
positions around the periphery of the diffuser.
Diffuser
Air Flow
Size
Factor
10
0.42
12
0.66
14
0.9
17
1.3
20
1.8
24
2.8
32
4.9
40
7.7
