Vaisala PTU300 - Manual (Page 143)

Chapter 4 _________________________________________________________________ Operation
VAISALA _______________________________________________________________________ 141
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 63 on page 141. 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 63 Decrease of Sensor Gain
Before starting the chemical purge, note the following:
- the sensor is protected with a PPS grid with stainless steel netting, a
stainless steel sintered filter or with membrane SST filter