balancing a column of liquid of known weight
against air pressure. The units of measure used
are pounds per square inch, inches of mercury
using mercury as the fluid, and inches of water
using water as the fluid.
The simplest form of manometer is the basic
U-tube type. Several variations of the basic type
are presently used in air movement applications,
for example, the inclined type (draft gauge) and
the combination inclined and vertical type. An
inclined manometer with a pitot probe is shown in
figure 13-10. Many commercially installed central
manometers connected to duct interiors with static
Rotating Vane Anemometer
The rotating vane anemometer (fig. 13-11)
consists of a propeller or revolving vane connected
through a gear train to a set of recording dials that
indicate the number of linear feet of air passing in
a measured length of time. It requires correction
factors and frequent calibrations, and it is not as
accurate as the velometer.
The primary application for a rotating vane
anemometer is the measurement of grille velocities
on heating, cooling, and ventilating installations;
however, it may not be suitable for exhaust
measurements or for measurements on very small
Figure 13-10.Inclined manometer with pitot probe.
In addition to the air balancing instruments,
there are other miscellaneous devices required.
Thermometers are necessary for making tempera-
ture measurements at various duct and room
locations; a tachometer is needed to determine fan
speeds; and a multimeter is needed to check fan
motors for proper operation.
PREPARATION FOR BALANCING
The following preliminary procedure is
necessary before proper balancing can begin. These
steps are general in nature and should apply to
1. Review applicable mechanical drawings and
job specifications. This review will provide
necessary data on the ducts, air handlers, and
outlets. Information pertaining to design airflow
can also be taken from these drawings.
2. Prepare a simple working sketch of the
entire duct system showing dimensions, airflow
volumes and velocities, and the location of all
components such as dampers, fans, coils, and
filters. Duct outlets should be numbered on the
sketch starting at the farthest one from the fan
Figure 13-11.Rotating vane anemometer.