Figure 13-5.Pitot probe.
total and static pressure can be easily measured,
the velocity pressure can be found by subtracting
static from total pressure. In most problems
concerning duct systems, air pressure is expressed
in terms of inches of water (1 pound per square
inch = 27.74 inches of water.)
At the time of initial installation of a duct
system, the design data should be recorded. After
initial start-up, the system should be balanced so
that each air outlet is adjusted to the design rate of
flow. During the initial balancing procedure, the
actual design rate of flow is sometimes not
achieved, but the flow is within the range of
acceptable standards. When such conditions exist,
they should be noted on the design data sheet
where they may be considered by maintenance
personnel during repairs or the rebalancing of the
system. After the system is balanced and proper
operation is assured, static pressure measurements
are taken throughout the system. Also, the total
pressure difference across the fan (the difference
between the suction total pressure and the
discharge total pressure) is noted. Although these
initial measurements can be used for checking the
design of the system, their main function is to
serve as reference data for future tests. If the
system fails to function properly at any time,
another set of measurements should be taken and
compared to the original set.
AIR BALANCING INSTRUMENTS
Numerous instruments designed for air
balancing requirements are available from dif-
ferent manufacturers. Those that are most
commonly used are discussed in this section.
This instrument is particularly adaptable to
maintenance work because of its portability, wide
scale range, and instantaneous reading features.
Its accuracy is suitable for most air velocity and
static pressure readings. Since velometers are
made by several manufacturers, the instruction
sheets for any instrument should be thoroughly
understood before attempting to use it. A
functional velometer set consists of the basic meter
with hoses and accessories as shown in figure 13-4.
MEASURING DEVICES. There are four
measuring devices used with the basic meter for
determining air velocities and pressures. They are
the pitot probe, low flow probe, diffuser probe, and
static pressure probe.
The pitot probe (fig. 13-5) is a stainless steel
measuring probe with a standard length of 12
inches and a diameter of 1/2 inch. It is suitable for
measuring velocities at supply openings and at
return openings. Its primary purpose is to measure
velocities within ducts. It is not recommended for
velocity ranges below 300 fpm.
The low flow probe (fig. 13-6) is used for
measuring velocities in open spaces. It connects
directly to the meter and permits measurement of
air by placing the instrument directly in the air
currents. It is useful for measuring drafts in rooms
and air velocities at ventilation hoods and spray
Figure 13-6.Low flow probe.