Vaisala HMM211 - Manual (Page 29)

Chapter 6 ___________________________________________________________ Chemical Purge
CHAPTER 6
CHEMICAL PURGE
This chapter provides you with information on chemical purge.
In some applications, the sensor gain may decrease gradually due to an
interference caused by some chemical present in the ambient air (see
Figure 7 below). The sensor polymer absorbs the interfering chemical;
this reduces the polymer's ability to absorb water molecules and so
decreases the sensor gain. In chemical purge, the interfering chemical is
evaporated by heating the humidity sensor to a temperature level of
approximately +160 °C. Automatic chemical purge takes place at startup.
Chemical purge
Calibration
values
Measured values
after chemical
exposure
Humidity
O
utp
ut s
igna
l
Figure 7 Decrease of the Sensor Gain Due to an Interfering
Chemical and the Effect of the Chemical Purge Process
The sensor with chemical purge option is a composite sensor in which
the HUMICAP
®
and Pt 100 temperature sensors are attached to each
other. Chemical purge option requires that the sensor is protected with a
stainless steel sintered filter (part no. HM46670).
VAISALA _______________________________________________________________________ 29