Fluke 975 AirMeter Application Note

Measuring air velocity with
the Fluke 975 AirMeter:
Using the velocity probe
Air velocity is a key parameter in evaluating airflow sys-
tem performance. As part of basic testing, adjusting
and balancing of HVAC air distribution systems, most
HVAC technicians now use an anemometer to mea-
sure 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 anemom-
eter to measure air velocity. A temperature sensor in
the probe tip compensates for air temperature, a sen-
sor in the meter reads absolute pressure, and ambient
absolute pressure is determined upon meter initializa-
tion. For users who prefer to calculate their own com-
pensation 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 measure-
ments 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 quanti-
ties and patterns that serve the
intended purpose: heating, cool-
ing, ventilating, exhausting, mix-
ing, 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.
F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y
To determine the air volume
delivered to all downstream ter-
minal 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 tra-
verse in exhaust ducts reveals
exhaust air volume.
Application Note
Measuring air velocity in a duct.