Vaisala HM70 - Manual (Page 36)

User's Guide ______________________________________________________________________
34 __________________________________________________________________ M210297EN-E
RH and the lower limit so low that it never will be reached (for
example 0 % RH). Thus, the HM70 will alert when RH rises
above 50 % RH.
8. Select Alarm ON/OFF, press ON to activate the alarm and return
back to the basic display.
9. A note picture sign appears on the upper left corner.
10. When the alarm level is reached, you can stop alarming by
pressing OK. To reactivate the alarm function, answer YES .
To stop the alarm function completely, answer NO .
NOTE The alarm is not in function when the device is turned off. Remember
to turn off the automatic power off, see page 25.
Chemical Purge (Optional)
Chemical purge is an optional feature of the HM70 series humidity
probes (B-version probes). In some specific applications the sensor
gain may decrease gradually due to an interference caused by some
particular chemical present in the ambient. The sensor polymer
absorbs the interfering chemical; this reduces the polymer's ability to
absorb water molecules and so decreases the sensor sensitivity. In
chemical purge the interfering chemical is evaporated by heating the
humidity sensor to a temperature of +160 °C for 2 minutes.
For the most applications it will be sufficient to perform the chemical
purge once a week. For applications where HM70 is subject to high
concentrations of chemicals, chemical purge should be performed
more frequently, in extreme cases before each measurement.
Turning Chemical Purge ON
The following applies only with humidity probes HMP75B, HMP76B
and HMP77B: