Vaisala HM70 - Manual (Page 38)

User's Guide ______________________________________________________________________
36 __________________________________________________________________ M210297EN-E
Sensor Preheat (Optional)
Sensor preheat is an optional feature of the HM70 series humidity
probes (B-version probes).
In some measurement applications, unwanted dew formation makes
humidity measurement difficult or even impossible. High humidity
combined with rapidly changing outdoor temperature can condense
the water vapour in the air onto the sensor. Until this dew evaporates
or dries, it is impossible to obtain a true reading. In sensor preheat the
sensor is heated to a temperature level of approximately +100 °C for
up to 4 minutes. This is to prevent possible condensate from the
sensor.
When setting the probe from outdoor conditions into warm and humid
conditions there can be large temperature difference between the
probe and external environment. In such a conditions turn on the
sensor preheat immediately before installing the probe to prevent
condensation forming on the probe.
Figure below shows how response time can be dramatically shortened
and water condensation on the sensor avoided when using sensor
preheat.
Response time to high RH/T enviroment
0
20
40
60
80
100
120
0 10 20 30 40 50 60 70 80
tim e (s)
%RH
No Preheat Preheat for 12s
Figure 6 Response time to high RH/T environment
This curve shows how
relative humidity
increases as a
consequence of
condensation on the
sensor. The correct
response is achieved
not until the sensor is
dried.
When using preheat,
response time is
shortened.