Vaisala HMP42 - Manual (Page 7)

HMI41 AND HMP42
M210317EN-A Operating Manual
3
2. TO BE NOTED IN THE MEASUREMENT OF 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 is 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
surroundings and the humidity is high. Although the humidity sensor reacts
rapidly to changes in the amount of water vapour in the air, the probe
temperature changes more slowly. To avoid errors caused by temperature
differences the probe must always be left to stabilize to ambient temperature
before starting measurements: the bigger the temperature difference, the
longer the stabilization time.
Indoors relative humidity should be measured in a place where the temperature
is as close to the average temperature of the room as possible. Measurements
taken close to heat sources will not give a true picture of the relative humidity
in the whole room.
Temperature (°C)
dR
H
(%
R
H
)
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 difference
between the ambient and the sensor temperature is 1
°C