Fluke 975 Service Guide

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

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.

Application Note

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.

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 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.

A duct traverse consists of a

0.061 D

0.235 D

0.437 D

0.563 D

0.785 D

0.939 D

0.074 D

0.288 D

0.500 D

0.712 D

0.926 D

0.135 D

0.321 D

0.032 D

0.579 D

0.865 D

0.968 D

D

0.135 D

0.321 D

0.032 D

0.579 D

0.865 D

0.968 D

D

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

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 M

3

/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.

No. of points or traverse lines Position relative to inner wall

Patterns of holes drilled in rectangular and round ducts when conducting a duct traversal. Taken from ANSI/ASHRAE Standard 111-1988.

2 Fluke Corporation Measuring air velocity with the Fluke 975 AirMeter: Using the velocity probe

5 0.074, 0.238, 0.500, 0.712, 0.926
6 0.061, 0.235, 0.437, 0.563, 0.765, 0.939

7

0.053, 0.203, 0.366, 0.500, 0.534, 0.797,

0.947

No. of measuring

points per diameter

6 0.032, 0.135, 0.321, 0.679, 0.865, 0.968
8 0.021, 0.117, 0.184, 0.345, 0.655, 0.816, 0.883, 0.981

10 0.019, 0.077, 0.153, 0.217, 0.361, 0.639, 0.783, 0.847, 0.923,

Position relative to inner wall

0.981