Vaisala DMT143 - Manual (Page 18)

USER'S GUIDE ____________________________________________________________________
16 ___________________________________________________________________ M211435EN-E
Sensor Purge
Sensor purge is also an automatic procedure that minimizes the drift at
the wet end readings of the dewpoint measurement. Sensor purge is
performed once a day or when the power is switched on. The sensor is
heated for several minutes which will then evaporate all excess
molecules out of the sensor polymer. This, together with the auto-
calibration results in a very small drift of the sensor due to the very linear
behaviour of the polymer technology. During the sensor purge the
transmitter outputs the T
d
value prior to the procedure.
Sensor Warming in High Humidities and
Freezing Conditions
DMT143 has a sensor warming feature that improves the sensor’s
performance in high humidities and freezing conditions. Sensor warming
does not interrupt the transmitter’s T
d
value output.
In high humidity, sensor warming prevents the sensor and filter from
becoming wet. High humidity may be present when the dewpoint
temperature rises close to the gas temperature.
Sensor warming is switched on automatically when the humidity in the
measured gas increases to a level where dew can start to form. The
advantage of sensor warming is the rapid response of dewpoint
measurement. A wet sensor and filter would otherwise result in a
dewpoint equal to ambient temperature (that is, RH = 100 %).
If in spite of sensor warming the sensor gets soaked, it will recover fully
back to normal operation after it dries out.
In freezing conditions, DMT143 uses sensor warming to maintain the
sensor temperature above 0 °C.
NOTE When measuring in temperatures below 0 °C, the supply voltage must be
24 ... 28 VDC. For more information on DMT143 supply voltage
requirements, see section Power Supply Requirements on page 23.