Application Note

3 Fluke Corporation Fundamental IAQ/HVAC measurements with the Fluke 975 AirMeter™
How the 975 measures it
The Fluke 975 measures CO
2
automatically and also uses it
to calculate the percentage of
outside air. Select “% OUTSIDE
AIR” and you’ll be prompted for
“CO
2
” or “Temp” calculations.
Next enter the return air value,
the mixed air value, and then
the outside air value. The greater
the differential between indoor
and outdoor temperature or CO
2
values, the greater the accuracy
achieved. If the outdoor to indoor
temperature differential is less
than 20° F, then a calculation
based on CO
2
is likely to be more
accurate. ASHRAE ventilation
requirements for most spaces
at full occupancy will result
in a 700 ppm CO
2
differential
between outdoors and indoor
occupied spaces. (See ASHRAE
standard 62.1 for commercial
ventilation, or AHSRAE standard
62.2 for residential ventilation.)
Air Velocity Functions
Fundamentals
Control of air movement enables
us to condition it, clean it, heat
it, cool it, humidify it, dehumidify
it, exhaust it, ventilate it, dilute
it, mix it, deliver it, accelerate
it, position it, maintain occupant
comfort zones, maintain healthy
buildings, and the list goes on.
Proper air volumes within HVAC
ducts are essential in equipment
performance. When air volumes
within HVAC ducts are incorrect,
the air cannot be conditioned
as designed, operating costs are
elevated, and equipment life
expectancy is shortened.
Velocities from registers and dif-
fusers are integral in maintaining
adequate air patterns for condi-
tioning the space, while main-
taining acceptable noise criteria
and occupant comfort levels. Air
velocities from registers and dif-
fusers must create air patterns
that evenly distribute and mix
conditioned air to room air while
avoiding uncomfortable velocities
in the occupant zone. The occu-
pant zone is normally considered
to be below occupant height and
one foot or more from walls and
air velocities should generally be
less than 50 fpm (feet per min-
ute) in order to avoid complaints.
Air volumes from registers and
diffusers must be sufficient to
provide a change in room heat
content at least comparable
to the heat loss or gain of the
space. Air volumes into exhaust
grilles must be sufficient to
control odors, gases, or fumes
as required by the purpose of
the exhaust system. Many cir-
cumstances warrant the mea-
surement of air velocities or air
volumes and the 975’s velocity
probe allows for both quick spot
measurements of air velocities
or patterns and precise duct tra-
versals.
How the 975 measures it
The Fluke 975 velocity probe
contains a thermal anemometer
and a compensating temperature
sensor that will telescope for
readings 33 inches beyond the
handle, either in free air, at a
GRD (grille-register-diffuser) face,
or within a duct. Thermal ane-
mometers are accurate through
a wide range of velocities, and
are especially preferred for low
velocity readings.
Since air density is related to
temperature and absolute pres-
sure, the temperature sensor in
the probe tip and the absolute
pressure determined at meter
startup compensate the readings
to actual conditions.
Reading CO
2
levels in an office space.
Using the velocity probe for measurements at the air intake.