Vaisala HMP155 - Manual (Page 17)

Chapter 2 __________________________________________________________ Product Overview
VAISALA _______________________________________________________________________ 15
Warmed Probe Option
In changing temperature conditions with high humidity, the probe
temperature can lag behind the temperature of the surrounding air and
this can cause a risk of condensation on the sensor. A wet probe cannot
observe the actual humidity in the ambient air. If the condensed water is
contaminated, the life span of the probe may shorten and the calibration
may change.
The HMP155 warmed probe is heated continuously so that its
temperature is always higher than that of the environment. This prevents
condensation on the probe. With the additional temperature probe it is
possible to calculate the true RH based on the non-heated (ambient)
temperature information. If the warming is selected but the additional
temperature probe is not present, HMP155 only produces dew point and
mixing ratio output.
Connection Cover Option
1206-039
Figure 3 HMP155 with Optional Connection Cover
An optional connection cover is available for enhanced protection in wet
environments such as coasts or rainforests.