Anemometers are used to measure the velocity of airflow. Measurement of air velocity must be performed when evaluating the total operation of an air distribution system. It must also be done when adjusting an air distribution system to balance the volume of air in cubic feet per minute (CFM).
When evaluating and/or balancing an air distribution system, the anemometer is used primarily to measure the airflow being delivered to the supply grilles and at the input to the return air grilles. Refer to the discussion on air velocity meters (velometers) in this section for other instruments that are commonly used to measure airflow velocity in air distribution systems.
Rotating-vane, swinging-vane, and hot-wire anemometers are three types in common use. Rotating-vane anemometers require that a stop watch be used to time the air measurement and that calculations and/or corrections be made in order to convert the measured rate of airflow into air velocity in feet per minute (FPM). Swinging-vane and hot-wire anemometers give direct readings of air velocity in FPM. For this reason, they are often called velometers.
The rotating-vane anemometer (Figure 1-29) has three calibrated velocity dials that are mechanically driven by a nearly frictionless propeller (vane) that rotates on impact with the airstream being measured. In addition to the calibrated dials, the rotating-vane anemometer has a zero reset lever to return all dial readings to zero prior to the start of each measurement. It also has a brake lever to turn the velocity dial mechanism on and off at the start and finish of each timed measurement. Accompanying the instrument should be a calibration chart used to correct the readings.
As shown, the outer scale of the rotating-vane anemometer reads directly from 0 to 100 fee t in 1-foot increments; a 100-foot scale on the left reads from 0 to 1,000 feet in 100-foot increments; and a 1,000-foot scale on the right reads from 0 to 10,000 feet in 1,000-foot increments. To measure air velocity in FPM, each measurement made with the anemometer is timed for one minute with a stop watch. The anemometer shown in Figure 1-29 is recording a velocity of 1,860 FPM, which is read from the dials as follows: the 1,000’s dial on the right reads more than 1,000 but less than 2,000 = 1,000 feet; the 100’s dial on the left reads more than 800 but less than 900 = 800 feet; and the direct dial reads 60 feet. After each measurement or series of measurements, calculations and/or corrections are usually required to convert the measured value of airflow into the real value for airflow velocity in FPM. It is best to make several measurements and then average the results to achieve greater accuracy. To maintain its accuracy, the rotating-vane anemometer must be calibrated periodically against known velocities.
The swinging-vane anemometer (Figure 1-30) gives direct readings of velocity. When positioned to make a measurement, the airstream being measured passes through the anemometer sensor probe, causing a precision-balanced vane to tilt at different angles based on the air velocity. The position of the vane determines the velocity reading o f the air displayed on the meter scale in FPM. Various attachments such as static pressure tips or pitot tubes are used with the anemometer when measuring supply and return airstreams. Swinging-vane anemometers are available in several velocity ranges from 0 to 2,500 FPM.
A hot-wire anemometer (Figure 1-31) provides direct readings of air velocity in FPM. This instrument uses a sensing probe containing a small resistance heater element. When the probe is held perpendicular to the airstream, the temperature of the heater element changes due to variations in the airflow. This causes its resistance to change, which alters the amount of current flow applied to the meter circuitry, where it automatically calculates the air velocity for display on the meter scale or a digital readout.