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Air Speed Meters
The type of measurement in this section refers to portable instruments that are used to measure the speed of air and
wind. The measuring devices are referred to as anemometers. There are 2 primary sensing systems used.
The vane type employs a rotating fan blade arrangement while the wand type has a sensing system built into the tip of the wand.
The wand type is generally used for determining air speed inside ducts or pipes, while the vane
type is primarily for use in free air space.
Most of the units have hand held digital displays. Standard unit costs are in the $100 to $400 range. The
cost is influenced by the accuracies and features.
At the upper end of the scale, ranges are selectable from 0.2 to 20 meters/second (resolution 0.1 m/s) to 1
to 31 knots (0.1 knots) with accuracies of +/- 3%. Scales include meters/second, kilometers/hour, feet/minute, miles/hour, knots. Temperature sensor is
included and NIST traceability is available. Computer interface is possible with a built in RS-232 port.
At the lower end of the cost scale, ranges are selectable from 0.5 to 28
meters/second (resolution 0.1 m/s) to 1.0 to 54 knots (resolution 0.3 knots) with unstated accuracy. Scales include meters/second, kilometers/hour,
feet/minute, miles/hour, knots, Beaufort Force. Temperature sensor is included. Neither computer interface or NIST traceability are available.
A common use for the meters is to provide air velocity inside pipes so that the
volume of air flowing through the pipe can be calculated (some meters come with calculation feature built in). Once the velocity of air inside the pipe is
known (feet/minute for example) and the area cross section of the pipe is known (in square feet for example), the volume per unit of time (Cubic
Feet/Minute or CFM) can be easily calculated. For high precision, it is very important to sample across the cross section of the pipe. If possible,
sampling should not be done close to size transitions or elbows, because of the turbulent air patterns. The air speed is not the same at the center of the
pipe as it is close to the side walls - particularly for large diameters and low air velocities.
Another issue in the volume calculation involves the temperature of the air.
Conversion tables are available that can correct for temperature so that the more accuate designation for volume of flow can be calculated - Standard Cubic Feet/Minute or SCFM.
Capture velocity is another important parameter that can be easily be
measured with the air speed meter (anemometer). Dust and fume collection systems are generally designed for specific air velocities to capture the dust
or fume. The velocity of air needed to move the dust/fume into the collection system varies depending on the application. A much higher velocity is
required to capture moving dust from a grinder than fumes from a tank. The idea of measuring the actual velocity at the extreme limits of the area being
controlled is to confirm that there is sufficient air speed at all points to capture it and move it into the collection system. Designers often underestimate the
effect of ambient air movement and design systems as though they were operating in a still air room. So, simply confirming the design air velocity
inside ductwork and piping is not sufficient to assure that the particular dust/fume is contained.
 Wind speed is also measured with anemometers. The small hand held units
can be used but generally a permanent mounting is done so that wind speed can be determined at any time from a remote location. They are availble separately
but often are part of a weather kit to allow determination of conditions including temperature, rainfall and barometric pressure.
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