ANEMOMETERS
Introduction
Airflow through outlets (sidewall grilles and registers or ceiling diffusers) or inlets (return air
grilles) can be measured using a rotating vane anemometer.
Diagrams 1 and 2: Anemometers
Anemometers usually have timing circuits and give digital read outs in metres/second (m/s).
Vane anemometers can not be used for low velocities (0.25 m/s or less) due to the friction of
the vane bearings.
When using anemometers, the manufacturer’s grille or correction factors (typically between
0.8 and 0.9) need to be taken into account for accurate velocity readings. Grille factors
account for the unrestricted area of the grille being measured, eg a grille with a 0.85 grille
factor would have an 85% open blow area and a 15% grille area.
The anemometer may also have a calibration or correction factor that must be taken into
consideration.
Actual Velocity (m/s) = Average velocity reading (m/s) x correction factor/s.
Example: A wall grille is found to have an average velocity of 10 m/s. The grille factor for the
wall grille is 0.9. Calculate the actual velocity of the air leaving the grille.
Solution: Actual velocity = Average velocity reading x correction factor/s. Actual velocity =
10 m/s x 0.9. Actual velocity = 9 m/s
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Note:
Area in the following formula is the free area which is the area that is free of obstruction in a
coil or grille or diffuser. Free area = Gross area x grille or coil factor
Volume is a quantity measurement
Unit of measurement is m3/s
Volume involves 2 factors: Area and speed (velocity)
The formula is: Volume = area x velocity or m3/s = m2 x m/s.
Diagram 3: Wall mounted grille
Anemometer readings are taken using either the patch or sweep methods depending on the
type of meter used.
Patch method
Starting from one corner, place the anemometer on the coil/grille (position 1) and record the
reading in m/s. Move the anemometer to position number 2 and again record the reading.
Continue this method until the entire coil/ grille has been covered. This method only applies
to anemometers that measure velocity without a timing factor.
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Diagram 4: Illustration of a patch method for using an anemometer
Average the readings using:
Velocity (m/s) = Total of readings
Number of readings
Sweep method
Again start from one corner, place the anemometer on the coil/grille (position 1) and start the
anemometer and the timing mechanism. Move the meter in a continuous motion over the
coil/grille and once you have covered the entire coil/grille record the meter reading and the
time taken. You will either have the final reading or you will be required to do the following
calculation to calculate the average velocity:
Velocity (m/s) =
Total of reading on meter
Time taken to attain the reading in seconds
If you have an old meter that does not have a timing mechanism built in you will need to use
a stopwatch.
Always check the manufacturer’s instructions for details of the meter you are working with.
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