User's Guide ______________________________________________________________________
22 ___________________________________________________________________M210983EN-A
0910-033
Figure 4
HMP155 Probe with Additional T-probe
Chemical Purge (Option)
Chemicals in the measured gas may interfere with the humidity sensor
of the HMP155. The sensor polymer absorbs the interfering chemical,
gradually reducing its ability to absorb water molecules. Consequently
the sensor gain decreases.
The chemical purge function heats humidity sensor to a temperature of
approxi180 °C for several minutes, which evaporates the
interfering chemical and any moisture on the sensor. The purge function
starts with heating stage, continues with settling and when the
temperature of the sensor has decreased, the probe returns to normal
mode. The entire cycle takes about eight minutes. For the duration of the
purge, the output from the HMP155 is locked to the last measured
values.
If your HMP155 is equipped with the chemical purge function, perform
it always before RH calibration (see section
) or when there is a reason to believe that a
sensor has become exposed to an interfering chemical. The purge can
be activated from the
Functions
menu of the MI70 indicator. Make sure
that the temperature of the sensor has come down to normal temperature
before starting a calibration.
If the chemical purge function is enabled, and the HMP155 is left
continuously powered, it will perform the chemical purge at a very long
interval of approximately 22.7 days (automatic purge). This interval can
be changed using serial line; see the HMP155 User’s Guide.
