
Chapter 4 ________________________________________________________________ Operation
VAISALA ______________________________________________________________________ 111
OPEN [aa]
where
aa = address of the transmitter (0 ... 99)
CLOSE
The CLOSE command switches the transmitter back to the POLL
mode.
Example:
>OPEN 2 (opens the line to transmitter 2, other
transmitters stay in POLL mode)
>CRH (for example, calibration performed)
...
>CLOSE (line closed)
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 44 on page 112. The sensor polymer
absorbs the interfering chemical; 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 +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.
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