Fluke 975 AirMeter Application Note page 2

A duct traverse consists of a
number of regularly spaced air
velocity measurements through-
out 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 diam-
eters 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 geom-
etry of the duct. Most duct tra-
verses result in at least 18 to 5
velocity readings, with the num-
ber of readings increasing with
duct size. The industry accepted
measurement points across the
traverse are determined by the
Log-Tchebycheff rule for rectan-
gular duct, and by the Log-Linear
rule for round duct. Usually, tech-
nicians drill five to seven holes
0.437 D 0.437 D
0.563 D 0.563 D
0.785 D 0.785 D
0.939 D 0.939 D
No. of points or traverse lines
5
6
7
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
on one side of rectangular ducts,
and two to three holes in round
ducts, in order for the telescop-
ing 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 direc-
tion 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 pre-
maturely, the Fluke 975 allows
0.061 D 0.061 D
0.235 D 0.235 D
Position relative to inner wall
0.074, 0.38, 0.500, 0.71, 0.96
0.061, 0.35, 0.437, 0.563, 0.765, 0.939
0.053, 0.03, 0.366, 0.500, 0.534, 0.797,
0.947
you to re-take it. When all veloc-
ity 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 pres-
sure 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 /s (cubic meters per second)
3
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 Pitot-
static 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.
0.321 D 0.321 D
0.579 D 0.579 D
0.865 D 0.865 D
0.968 D 0.968 D
D D
No. of measuring
Position relative to inner wall
points per diameter
6 0.03, 0.135, 0.31, 0.679, 0.865, 0.968
8 0.01, 0.117, 0.184, 0.345, 0.655, 0.816, 0.883, 0.981
10 0.019, 0.077, 0.153, 0.17, 0.361, 0.639, 0.783, 0.847, 0.93,
0.032 D 0.032 D
0.135 D 0.135 D
0.981