Vaisala HMP230 User Manual
Page 9
HMP230 SERIES
M210225en-B
User's Guide
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 ±5.4 %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
Figure 1
Measurement error at 100 %RH when the difference between
the ambient and sensor temperature is 1 °C