Vaisala SHM40 - Manual (Page 21)

Chapter 3 ____________________________________________ Humidity Measurement in Concrete
VAISALA _______________________________________________________________________ 19
Importance of Temperature Differences
CAUTION Temperature of the probe and the concrete must be equal before starting
the measurement!
In humidity measurement, and especially in calibration, it is essential that
temperature of the probe and measured environment is the same. Even a
small difference in temperature between the measured environment 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 the probe may
require adjustment.
Temperature (°C)
Δ
RH
(%RH)
0
1
2
3
4
5
6
7
8
9
10
-40 -20 0 20 40 60 80 100
1111-102
Figure 4 Measurement Error at 1 °C Temperature Difference
Figure 4 above shows the measurement error at 100 %RH at
temperatures ranging from -40 to 100 °C, when the difference between
the ambient and sensor temperature is 1 °C.