4.4. AIR VELOCITY MEASUREMENTS
The movement of air (i.e. draught), particularly when localised, is the most common
reason for discomfort in a given environment. There may, however, be
several reasons
for these air movements. Because
of
rapid fluctuations and variations in direction, air
velocity is normally difficult to measure accurately, and thus mean values are often used.
The
relationships between mean air velocity and % dissatisfied at different temperatures
is shown in Fig. 4.4.
ISO 7730 recommends that the mean air velocity (over a
3-minute
period) should not
exceed 0,15
m/s
(29,5
ft/min)
in the winter
or
heating period. This limit may be increased
to 0,25
m/s
(49ft/min)
in the summer
or
cooling period. ISO 7726 also recommends that
the standard deviation about the mean
value should be measured, but no limits
for
this
have been set.
%
80
60
40
20
-o
.!!!
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<0
10
(/)
(/)
0
8
6
4
2
0
0,1
0,2
0,3
0,4 0,5
m/s
Mean Air Velocity 841207
Fig. 4.4. Relationship between Mean
Air
Velocity
and
percentage
dissa tis tied.
4.4.1. Use of the Air
Velocity Transducer
34
Five different air velocity measurements can be obtained from the Indoor Climate Ana-
lyzer using the
Air
Velocity Transducer MM 0038. When the Analyzer is in the "Display"
mode, activation
of
the "Mean Value" pushkey produces two values on the Display
of
the Analyzer. The left-hand value is calculated using a five-second time constant, which
damps the
fluctuations sufficiently to enable the displayed value to be read by the hu-
man eye. A 3-minute
linear mean air velocity value, which is updated every
10
seconds,
is
displayed on the right-hand side
of
the Display. Activation
of
the "St. Dev." pushkey
also produces a double display
of
the calculated standard deviations (the 5-second
value on the left-hand side and the 3-minute value on the right-hand side
of
the
Display).
In
addition to the two displayed air velocity measurements, an instantaneous air velocity
measurement
on
a linear scale can be obtained, by connecting the X-output on the Rear
Panel of the Analyzer to a suitable recording device such as a Level Recorder Type 2317
or
an
X-Y
Recorder Type 2308 (see Chapter
5).
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