Vaisala HMT330 - Manual (Page 135)

Chapter 4 _________________________________________________________________ Operation
VAISALA _______________________________________________________________________ 133
Sensor Functions
Chemical Purge (Optional)
In some specific applications the sensor gain may decrease gradually due
to an interference caused by a particular chemical present in the
measured gas, for example. The decrease of sensor gain due to an
interfering chemical and the effect of the chemical purge process are
illustrated below, see Figure 66 below. The sensor polymer absorbs the
interfering chemical, and this reduces the ability of the polymer to absorb
water molecules. Consequently the sensor gain decreases. In chemical
purge, heating the humidity sensor to a temperature level of
approximately +160 °C for several minutes evaporates the interfering
chemical.
The purge function starts with heating stage, continues with settling and
when the temperature of the sensor is decreased the transmitter returns to
normal mode. The whole cycle takes about 6 minutes.
NOTE Chemical purge function locks the output values for about 6 minutes.
0508-035
Figure 66 Decrease of Sensor Gain