Operation; Operating Cycle; Figure 26 Typical Operating Cycle - Michell Instruments Optidew User Manual

Optidew User's Manual

4. OPERATION

4.1. Operating Cycle

Figure 26 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'.
32 97551 Issue 4.1, October 2019