Vaisala HMP140 - Manual (Page 6)

HMP140A SERIES
Operating Manual
U175en-1.5
2 1999-12-08
2. TO BE NOTED WHEN MEASURING HUMIDITY
In the measurement of humidity and especially in calibration, it is essential
that the temperature equilibrium is reached. Even a slight difference in the
temperature between the measured object and the sensor causes an error. For
example, at +20
°
C (+ 68
°
F) and 50 %RH, a temperature difference of +1
°
C
between the measured object and the sensor causes an error of +3 %RH. If
relative humidity is 90 %RH, the error is about +5.4 %RH.
The error is at its greatest when the temperature of the sensor differs from that
of the surroundings and the humidity is high. A few degrees’ difference in
temperature may cause water to condense on sensor surface. Efficient
ventilation accelerates evaporation whereas in an unventilated space, it may
take hours. The HUMICAP

180 sensor returns to its normal functioning as
soon as water has evaporated. Any contaminated water condensing on the
sensor may shorten its life span and change the calibration.
Temperature (°C)
dRH
(%RH
)
0
1
2
3
4
5
6
7
8
9
10
-40 -20 0 20 40 60 80 100
Figure 2.1 Measurement error at 100 %RH when the temperature
difference between the ambient and the sensor is 1
°
C