occurs, a reading is still displayed
by
the Analyzer
but
alternates with a
DEWP
0 I HT
UNSTABLE
message. This alternating Display will remain
for
two
minutes until a new
HUMIDITY measurement has been obtained.
One possible cause
of
an unstable
dewpoint
may be the
proximity
of
human beings
(usually the operator). Therefore, care should be taken
by
the
operator
to
ensure that
the
Humidity
Transducer is
positioned
as
far
from humans as possible.
Additional
prob-
lems
may
be caused
by
the presence
of
pollutants in the atmosphere which
may
inter-
fere with the dew
formation
process.
4.5.3.
Accuracy
of
Humidity
Measurements
38
Air
temperature
Dewpoint
Relative
Vapour
Ia
oc
td
oc
Humidity%
Pressure
kPa
-20
-20
100
0,13
0
-20.
21
0,13
10
-15
13
0,17
20
-5
.
17
0,38
30
5
20
0,87
50
25
25
3,18
T00682GBO
*
Lower
limit
for
dew
point
is
-20
°C
Table. 4.1. Relationship between Dewpoint, Relative
Humidity
and
Wa-
ter
Vapour Pressure
for
different
air
temperatures
The range
of
humidity measurements
that
can be obtained using the
Indoor
Climate
Analyzer and Humidity Transducer is dependent upon the
air
temperature as defined by
the relationship:
where
ta
is the air temperature and
td
is the dewpoint.
As there is an
absolute
lower
limit
of
-
20°C
for
the
dewpoint
measurement, the
accu-
racy
of
the calculation
of
relative
humidity
and
water
vapour
pressure is dependent on
the dewpoint.
For an
air
temperature
of
20°C, the accuracies stated in Table
~.2
can be obtained with
an
accuracy
between ±
o,soc
and ± 1
,0°C
for
the
dewpoint
measurement.
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