Vaisala HMP260 - Manual (Page 9)

HMP260 SERIES
HMP260-U017en-1.5 Operating Manual
1997-11-25 3
2. TO BE NOTED WHEN MEASURING HUMIDITY
In the measurement of humidity and especially in calibration, it is essential
that temperature equilibrium is reached. Even a small difference in
temperature between the measured object and the sensor causes an error. If the
temperature is +20 °C (+68 °F) and the relative humidity 50 %RH, a
difference of ±1 °C between the measured object and the sensor causes an
error of ±3 %RH. When the humidity is 90 %RH, the corresponding error is
±6 %RH.
The error is at its greatest when the sensor is colder or warmer than the sur-
roundings and the humidity is high. A temperature difference of a few degrees
can cause water to condense on the sensor surface. In an unventilated space
evaporation may take hours; good ventilation accelerates evaporation. The
HUMICAP sensor starts to function normally as soon as the water has evapo-
rated. If the condensed water is contaminated, the life span of the sensor may
shorten and calibration may change.
Temperature (°C)
dRH (%RH)
0
1
2
3
4
5
6
7
8
9
10
-40 -20 0 20 40 60 80 100
Fig. 2.1 Measurement error at 100 %RH when the difference between the
ambient and sensor temperature is 1 °C