AIR BALANCING INSTRUMENTS

Numerous instruments designed for air balancing requirements are available from different 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.

booths. Only velocity ranges from 0 to 300 fpm are applicable to this device.

The diffuser probe (fig. 13-7) is used for measuring air output at duct supply diffusers. It can also be used with some meters on return air diffusers. The meter reading and the K factor established by the diffuser manufacturer can be used to determine air volume outputs.

Figure 13-7.-Diffuser probe.

The static pressure probe (fig. 13-8) is used for measuring drops across blowers or fans in duct systems. The probe is carefully placed over an opening in the wall of a duct so as to form a positive seal. The hole should not be less than onequarter inch in diameter.

RANGE SELECTORS.- These are devices (fig. 13-9) that permit a rapid change of measuring ranges without the need for shifting to separate jets for each range. They are provided with connections that accept the various probes. These probes can also be connected to the meter. With the exception of the low flow probe, measurements may require a range selector.

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