Vaisala PTU300 Guia do Utilizador descarregar pdf (Página 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

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Vaisala PTU300 Guia do Utilizador Página 143