Fluke 975 AirMeter Application Note page 3

Taking measurements at the intake of a
rooftop unit.
Air measurements
at Grilles-Registers-
Diffusers (GRDs)
Supply air GRDs are selected and
positioned to deliver specified air
volume in velocities and patterns
that result in acceptable comfort
and ventilation within the occu-
pant zone. The occupant zone is
considered to be one foot from
walls and below head height.
Velocity from a supply GRD nor-
mally does not exceed 800 FPM,
and velocity into a return grille
should not exceed 400 FPM in
applications where noise would
be objectionable. Velocity must
be sufficient to mix the supply
air with the room air outside of
the occupant zone, while creat-
ing comfortable air patterns and
temperatures within the occu-
pant zone.
Throw is the distance the air
travels from a GRD before reach-
ing terminal velocity. Throw is
normally 75 % to 110 % of the
distance from the GRD to the
next intersecting surface (wall)
or terminal velocity point of
adjacent GRDs. Terminal veloc-
ity is simply the velocity at the
point within the throw that is
chosen to stop measuring throw
for engineering design reasons.
Terminal velocity is typically
50 FPM to 75 FPM in residen-
tial and office spaces, but may
be specified by the engineer to
be as high as 15 FPM to 150
FPM in commercial applications.
Generally, air velocities in the
occupant zone at 50 FPM are not
objectionable. Stagnant zones are
created when velocities fall to
15 FPM. To determine space air
patterns, use the velocity probe
to "follow" the throw of GRDs .
To determine air volume
delivered by a GRD, it's best to
perform a duct traverse with the
velocity probe in the duct run-out
3 Fluke Corporation Measuring air velocity with the Fluke 975 AirMeter: Using the velocity probe
leading to the GRD. Alternately,
use a traverse with the velocity
probe at the face of a GRD, along
with the GRD manufacturer's
engineering data, to determine
air volume.
Unlike a section of duct, the
area of a GRD cannot be mea-
sured in the field due to the fact
that the air changes direction
and accelerates through the vena
contracta (the vena contracta
is an effect that occurs when
air flowing through any open-
ing "sticks" to the edges of the
opening, effectively reducing the
size of the opening). Even careful
field measurements of the free
area of a GRD to determine air
volumes will result in gross mis-
calculations of air volume. The
GRD manufacturer will publish
an "effective area" (A = effec-
k
tive area in square feet) that can
only be determined by laboratory
tests that measure actual air vol-
ume and GRD face velocity (V
= average face velocity in feet
per minute). This effective area
can be used in the field for air
volume calculations.
For a given GRD, the manu-
facturer will normally publish the
effective area along with a range
of face velocities with the result-
ing volume flow in cubic feet per
minute (CFM) and pressure drop
for each face velocity. These val-
ues are determined with straight
duct connected to the GRD car-
rying non-turbulent air evenly
distributed across the duct.
Calculating air volume from a
GRD requires taking enough face
velocity readings to get an aver-
age velocity. Set up a grid of test
points across the face of the GRD
that will result in a good average
when finished. Grid spacing is
typically three to five inches, no
more than six inches, and a min-
imum of six stable velocity read-
ings per throw direction. Position
the velocity probe sensor flush
with a supply GRD, or one inch
(± .031 in) away from a return
grille, and center the probe in
the opening. Select the Fluke
975 AirMeter volume flow rate,
rectangular duct, and enter 1
inches by 1 inches dimension.
This will result in a CFM calcula-
tion that equals the average FPM
calculation. The calculated CFM
is then multiplied by the GRD
manufacturer's A factor for the
k
actual CFM.
CFM (cubic feet per minute) = A
A = Effective area in square feet
k
V = Average face velocity in feet per minute
avg
Miscellaneous velocity
readings
Ventilation air is often supplied
through the outdoor air hood of a
packaged rooftop unit. Within the
hood is a bank of bug screens
that can be traversed in a similar
manner as return grilles. Enter
the volume flow rate func-
tion of the Fluke 975 AirMeter,
select rectangular duct, enter
the dimensions of the bank of
bug screens, capture a velocity
reading approximately every six
inches, and let the AirMeter cal-
culate the CFM of ventilation air.
When the balance between
outdoor air intake and exhaust
avg
air is incorrect, a potential for
roof or building damage exists,
and occupants entering a build-
ing can be confronted with an
objectionable wind when the
doors are opened. Building pres-
surization should be limited to
0.0 in to 0.1 in water column
(w.c.) and best if kept below
0.05 inches w.c. The velocity
probe can be used at the build-
ing entrance to help evaluate
building pressure. A 1300 FPM
air velocity through an open door
equates to over 0.1 inches w.c.
building pressurization, and a 15
mph wind in the face.
Fluke. Keeping your world
Fluke Corporation
PO Box 9090, Everett, WA USA 9806
Fluke Europe B.V.
PO Box 1186, 560 BD
Eindhoven, The Netherlands
For more information call:
In the U.S.A. (800) 443-5853 or
Fax (45) 446-5116
In Europe/M-East/Africa +31 (0) 40 675 00 or
Fax +31 (0) 40 675 
In Canada (800)-36-FLUKE or
Fax (905) 890-6866
From other countries +1 (45) 446-5500 or
Fax +1 (45) 446-5116
Web access: http://www.fluke.com
©006 Fluke Corporation. All rights reserved.
Printed in U.S.A. 10/006 78647 A-EN-N Rev A
x V
k avg
up and running.
™