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3.1.3 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
value output.
In high humidity, sensor warming prevents the sensor and filter from becoming wet. High
humidity may be present when the dew point 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 dew point measurement. A wet sensor and filter would otherwise result in a dew
point 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.
When measuring in temperatures below 0 °C, the supply voltage must be 24 ... 28
VDC.
More Information
‣ Power Supply Requirements (page 25)
3.2 DMT143 Startup Sequence
When the transmitter is powered up, it will perform the sequence (applicable from software
version 1.1.5 onward) shown in
will measure normally for 40 seconds, after which it will perform the sensor purge and
autocalibration functions. This can take up to 5 minutes, during which time the output is
frozen: it will display the last measured values before the sensor purge began.
The autocalibration function may be postponed if the measurement environment is not
suitable. For example, the environment may be changing rapidly or have too high humidity.
Normal operation and output is typically achieved no later than 6 minutes after startup. The
sensor purge repeats every 24 hours (12 h default interval for the DMT143L variant with
DRYCAPâ 180S sensor if used with purge), and autocalibration every hour. The output will
again be frozen for their duration.
Figure 5 (page
20). After a 2-second startup, the transmitter
Chapter 3 – Functional Description
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19
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