Fluke 975 Application Note

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Measuring air velocity with
the Fluke 975 AirMeter:
Using the velocity probe
Application Note

Air velocity is a key parameter in evaluating airflow system performance. As part of basic testing, adjusting
and balancing of HVAC air distribution systems, most
HVAC technicians now use an anemometer to measure air velocity at grilles-registers-diffusers, within a
duct, or in open spaces.
Anemometers are typically very accurate tools,
especially at low velocities, but they must compensate
for air temperature, absolute pressure, and ambient
absolute pressure. The Fluke 975 AirMeter tool has an
accessory velocity probe that uses a thermal anemometer to measure air velocity. A temperature sensor in
the probe tip compensates for air temperature, a sensor in the meter reads absolute pressure, and ambient
absolute pressure is determined upon meter initialization. For users who prefer to calculate their own compensation factors, the meter will also display air velocity
or volume at standard conditions.
This application note describes how to take accurate
air volume measurements within a duct, air measurements at grilles-registers-diffusers, and other locations.

Air volumes within a duct
The ultimate goal of any duct
system is to move the required
air volume, while keeping all
other factors within acceptable
limits, and to deliver it in quantities and patterns that serve the
intended purpose: heating, cooling, ventilating, exhausting, mixing, humidifying, dehumidifying,
or otherwise conditioning the air
within a space. Velocity within
a duct is determined not only
by application, but also by how
the duct is designed. Key design
factors include: The level of
available static pressure that can
be overcome by the fan due to
friction losses and pressure drops
of devices within the air stream;
the cost of duct work; the space
available for duct work; and
acceptable noise levels.

To determine the air volume
delivered to all downstream terminal devices, technicians use a
duct traverse. Duct traverses can
determine air volume in any duct
by multiplying average velocity
readings by the inside area of
the duct. Traverses in main ducts
measure total system air volume,
which is critical to HVAC system
performance, efficiency, and even
life expectancy. The difference
in air volumes between the main
supply duct traverse and the
main return duct traverse results
in outdoor air volume. A traverse
in run-outs is the most accurate
way to determine the air volume
delivered by the terminal device
(grille-register-diffuser). A traverse in exhaust ducts reveals
exhaust air volume.

Measuring air velocity in a duct.

From the Fluke Digital Library @ www.fluke.com/library

on one side of rectangular ducts,
and two to three holes in round
ducts, in order for the telescoping anemometer probe to access
the traverse points. To ensure
the anemometer is used in the
direction of calibration, align the
mark on the velocity probe tip
with the impact direction. When
extending the probe, align the
wand section with the handle to
help maintain the correct direction inside the duct.
Before taking measurements,
slide the protective sheath
toward the wand handle in
order to expose the sensors in
the probe tip. For volume flow
rate calculations, the Fluke
975 AirMeter™* will prompt for
rectangular or round duct, then
prompt for rectangular side
dimensions or round diameter.
Take the required number of
velocity readings one at a time
by pressing the “capture” key. If
a velocity reading is taken prematurely, the Fluke 975 allows

you to re-take it. When all velocity readings are complete, the
AirMeter™ averages the readings
and multiplies by the duct cross
sectional area to get air volume,
both at standard conditions and
compensated for absolute pressure and temperature.
The velocity readings (FPM)
are averaged and multiplied by
the inside area of the duct (sq ft)
which provides the air volume
(CFM).
Q	 =	V * A
Q	 =	Air volume, CFM (cubic feet per minute)
		 or M3/s (cubic meters per second)
V	 =	Velocity, FPM (feet per minute) or m/sec
		 (meters per second)
A	 =	Area of duct, inside dimension of duct in
		 square feet or square meters
*For determining air velocity greater than 600 feet per minute
(FPM) within a duct, an HVAC technician may also use a Pitotstatic tube with an inclined manometer. Anemometers are the
preferred choice below 600 FPM and are quite acceptable at
higher velocities, too. The Fluke 975 AirMeter’s anemometer
measures over a range of 50 to 3000 fpm. In low pressure
duct systems where sound is a concern, such as residences
and health care facilities, velocity usually ranges from 400 to
900 FPM, while in high pressure duct systems, velocities can
approach 3,500 FPM.

0.074 D
0.074 D
0.288 D
0.288 D
0.500 D
0.500 D
0.712 D
0.712 D
0.926 D
0.926 D

A duct traverse consists of a
number of regularly spaced air
velocity measurements throughout a cross sectional area of
straight duct. Preferably, the
traverse should be located in a
straight section of duct with ten
straight duct diameters upstream
and three straight duct diameters downstream of the traverse
plane, although a minimum of
five duct diameters upstream and
one duct diameter downstream
can give adequate results.
The number of measurements
taken across the traverse plane
depends on the size and geometry of the duct. Most duct traverses result in at least 18 to 25
velocity readings, with the number of readings increasing with
duct size. The industry accepted
measurement points across the
traverse are determined by the
Log-Tchebycheff rule for rectangular duct, and by the Log-Linear
rule for round duct. Usually, technicians drill five to seven holes

0.032
0.032
D D
0.061
0.061
D D

0.135
0.135
D D

0.235
0.235
D D

0.321
0.321
D D

0.437
0.437
D D

0.579
0.579
D D

0.563
0.563
D D

0.865
0.865
D D

0.785
0.785
D D

0.968
0.968
D D

0.939
0.939
D D
No. of points or traverse lines
5
6
7

D D
Position relative to inner wall
0.074, 0.238, 0.500, 0.712, 0.926
0.061, 0.235, 0.437, 0.563, 0.765, 0.939
0.053, 0.203, 0.366, 0.500, 0.534, 0.797,
0.947

No. of measuring
points per diameter
6
8
10

Position relative to inner wall
0.032, 0.135, 0.321, 0.679, 0.865, 0.968
0.021, 0.117, 0.184, 0.345, 0.655, 0.816, 0.883, 0.981
0.019, 0.077, 0.153, 0.217, 0.361, 0.639, 0.783, 0.847, 0.923,
0.981

Patterns of holes drilled in rectangular and round ducts when conducting a duct traversal. Taken from ANSI/ASHRAE Standard 111-1988.
 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 meaTaking measurements at the intake of a
rooftop unit.
sured in the field due to the fact
that the air changes direction
Air measurements
and accelerates through the vena
at Grilles-Registerscontracta (the vena contracta
Diffusers (GRDs)
is an effect that occurs when
Supply air GRDs are selected and air flowing through any openpositioned to deliver specified air ing “sticks” to the edges of the
volume in velocities and patterns opening, effectively reducing the
size of the opening). Even careful
that result in acceptable comfort
field measurements of the free
and ventilation within the occuarea of a GRD to determine air
pant zone. The occupant zone is
volumes will result in gross misconsidered to be one foot from
calculations of air volume. The
walls and below head height.
GRD manufacturer will publish
Velocity from a supply GRD normally does not exceed 800 FPM, an “effective area” (Ak = effective area in square feet) that can
and velocity into a return grille
only be determined by laboratory
should not exceed 400 FPM in
tests that measure actual air volapplications where noise would
ume and GRD face velocity (Vavg
be objectionable. Velocity must
= average face velocity in feet
be sufficient to mix the supply
per minute). This effective area
air with the room air outside of
can be used in the field for air
the occupant zone, while creatvolume calculations.
ing comfortable air patterns and
For a given GRD, the manutemperatures within the occufacturer will normally publish the
pant zone.
effective area along with a range
Throw is the distance the air
travels from a GRD before reach- of face velocities with the resulting volume flow in cubic feet per
ing terminal velocity. Throw is
minute (CFM) and pressure drop
normally 75 % to 110 % of the
for each face velocity. These valdistance from the GRD to the
ues are determined with straight
next intersecting surface (wall)
duct connected to the GRD caror terminal velocity point of
rying non-turbulent air evenly
adjacent GRDs. Terminal velocdistributed across the duct.
ity is simply the velocity at the
Calculating air volume from a
point within the throw that is
GRD requires taking enough face
chosen to stop measuring throw
velocity readings to get an averfor engineering design reasons.
age velocity. Set up a grid of test
Terminal velocity is typically
points across the face of the GRD
50 FPM to 75 FPM in residenthat will result in a good average
tial and office spaces, but may
when finished. Grid spacing is
be specified by the engineer to
typically three to five inches, no
be as high as 125 FPM to 150
more than six inches, and a minFPM in commercial applications.
imum of six stable velocity readGenerally, air velocities in the
occupant zone at 50 FPM are not ings per throw direction. Position
objectionable. Stagnant zones are the velocity probe sensor flush
with a supply GRD, or one inch
created when velocities fall to
(± .031 in) away from a return
15 FPM. To determine space air
grille, and center the probe in
patterns, use the velocity probe
the opening. Select the Fluke
to “follow” the throw of GRDs .
975 AirMeter volume flow rate,
To determine air volume
rectangular duct, and enter 12
delivered by a GRD, it’s best to
perform a duct traverse with the inches by 12 inches dimension.
velocity probe in the duct run-out This will result in a CFM calculation that equals the average FPM

calculation. The calculated CFM
is then multiplied by the GRD
manufacturer’s Ak factor for the
actual CFM.
CFM (cubic feet per minute) = Ak x Vavg
Ak = Effective area in square feet
Vavg = Average face velocity in feet per minute

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 function 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 calculate the CFM of ventilation air.
When the balance between
outdoor air intake and exhaust
air is incorrect, a potential for
roof or building damage exists,
and occupants entering a building can be confronted with an
objectionable wind when the
doors are opened. Building pressurization should be limited to
0.02 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 building 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 Corporation Measuring air velocity with the Fluke 975 AirMeter: Using the velocity probe

Fluke.	Keeping your world
	
up and running.™
Fluke Corporation
PO Box 9090, Everett, WA USA 98206
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
In the U.S.A. (800) 443-5853 or
Fax (425) 446-5116
In Europe/M-East/Africa +31 (0) 40 2675 200 or
Fax +31 (0) 40 2675 222
In Canada (800)-36-FLUKE or
Fax (905) 890-6866
From other countries +1 (425) 446-5500 or
Fax +1 (425) 446-5116
Web access: http://www.fluke.com
©2006 Fluke Corporation. All rights reserved.
Printed in U.S.A. 10/2006 2786472 A-EN-N Rev A



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