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.

Velometer

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.