Chapter 3, To be noted when measuring humidity – Vaisala HM44 User Manual
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User's Guide _______________________________________________________________________
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CHAPTER 3
TO BE NOTED WHEN MEASURING
HUMIDITY
Temperature of the probe and the concrete shall be the same!
In humidity measurement and especially in calibration it is essential that temperature of the
probe and measured object is the same. Even a small difference in temperature between the
measured object and the probe causes an error. As the curve below shows, if the temperature
is +20 °C and the relative humidity 100 %RH, a difference of ±1 °C between the measured
object and the probe causes an error of ±6 %RH. When the humidity is 90 %RH, the
corresponding error is ±5.4 %RH.
A temperature difference of a few degrees can also cause water to condense on the sensor
surface. HUMICAP
sensor starts to function normally as soon as the water has evaporated. If
the condensed water is contaminated, the life span of the probe may shorten and calibration
may change.
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 3.1
Measurement error at 100 %RH when the difference
between the ambient and sensor temperature is 1 °C