Optidew User’s Manual
Michell Instruments 29
OPERATION
4 OPERATION
4.1 Operating Cycle
Figure 24
Typical operating cycle
At initial switch-on, the instrument enters a DCC cycle for 2 minutes. During this time
the mirror is heated above the prevailing dew point to ensure that all condensate
is driven off the surface of the mirror. The degree of heating is determined by the
configuration of the ‘Type’ and ‘Setpoint’ parameters in the DCC menu (see section 3.2
for further information).
The mirror is maintained at this temperature for the DCC period (default 4 minutes) or
2 minutes on switch-on. During the DCC process, Data Hold fixes the analog outputs
at the same value(s) as before DCC commenced. Data Hold typically lasts 4 minutes
from the end of a DCC cycle, or until the instrument has reached the dew point. This
procedure is in place to prevent any system which is connected to the outputs from
receiving a 'false' reading.
After the DCC period has finished, the measurement period commences, during which
the control system decreases the mirror temperature until it reaches the dew point.
The sensor will take a short amount of time to form a film of condensate and control
on the dew point. The length of this stabilization time depends upon the dew-point
temperature. When the measurement is stable or tracking very slow changes in dew
point, the Sensor indicator in the Operational Status display will indicate ‘Control’. Note
that at dry dew points (below around -20°C) the sensor may display ‘Control’ when the
mirror temperature is still slowly oscillating, always use the trend graph on the display
as a secondary indication
The end of a DCC cycle re-sets the interval counter, meaning that another DCC will start
(by default) after 4 hours have elapsed. Once the measurement is stable, Data Hold will
release, and the analog outputs will resume their normal operation. At this point the
Status area of the Operational Status display will change to ‘Measure’.
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