Vaisala HUMICAP HMP155 Guide de l'utilisateur télécharger pdf (Page 47)

Chapter 4 ________________________________________________________________ Operation

VAISALA _______________________________________________________________________ 45

Sensor Functions

The active output version of HMP155 includes optional functions such as

chemical purge and sensor heating. These functions are described in

more detail in this section.

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 11 below. The sensor polymer absorbs the

inte

rfering c

hemical; and this reduces the ability of the polymer to absorb

water molecules and consequently the sensor gain decreases. In chemical

purge, heating the humidity sensor to a temperature level of

approximately +180 °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 probe returns to

normal mode. The whole cycle takes about 6 minutes.

NOTE

Chemical purge function locks the output values for about 6 minutes.

NOTE

Avoid using the purge function in low (below freezing) temperatures.

Low temperatures decrease the effectivity of purge. Also, the lower the

temperature, the longer it takes for the sensor to return to normal

functionality.

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Vaisala HUMICAP HMP155 Guide de l'utilisateur Page 47