Vaisala HUMICAP HMM170 - Condensation Prevention Functions

3.3 Condensation prevention functions
Condensation prevention heating prevents water from condensing on the sensor and
interfering with humidity measurement. Use of condensation prevention is controlled by a
single setting that can be changed with Insight software or using Modbus protocol.
By default, condensation prevention is o. However, HMM170 can be ordered from Vaisala
with the feature enabled.
When condensation prevention is enabled, the HMM170 will automatically use its available
condensation prevention functions. When warming is active, values of output parameters that
depend on temperature measurement (for example, relative humidity) are unavailable unless
temperature is written to register 0334
hex
from an external source. Output parameters that can
be measured or calculated without this external temperature information, such as dew point
temperature, are available even without the temperature input.
Sensor warming
The sensor warming functionality in HMM170 warms the humidity sensor when necessary to
keep its temperature above the dew point of the measurement environment. When sensor
warming is activated, all temperature dependent output parameters become unavailable
unless external temperature information is provided to the module. The output parameters
become available again 4 minutes after warming is stopped.
Sensor warming is intended for improving the condensation tolerance of the
probe in measurement environments that may occasionally cause condensation
to form. It is not intended for environments where continuous warming is needed.
Probe heating
HMM170 supports probe heating. Probe heating warms up the entire probe head instead of
just the sensor. When probe temperature is heated above dew point temperature,
condensation on the probe can be avoided while measuring the dew point temperature of the
process. The functionality is intended for use in challenging measurement environments where
condensation on the probe would otherwise present a problem.
Probe heating is normally disabled. If you select to enable probe heating, output parameters
that are dependent on temperature measurement (such as relative humidity) are disabled,
unless the true temperature of the measured environment is updated to the temperature
compensation register of HMM170 from another measurement instrument. Output parameters
that can be measured or calculated without this external temperature information (such as
dew point temperature) are available even without the temperature input.
If the ambient dew point rises close to current sensor temperature, the heating power is
increased to avoid condensation. Heating power also increases as supply voltage increases.
HMM170 User Guide M212259EN-D
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