Maxcool Function; Frost Assurance Technology (Fast) - Michell Instruments Optidew User Manual

Optidew User's Manual
DCC Plus
DCC Plus is a feature designed to further control the build-up of contaminants on the mirror
surface without physical intervention from the operator. The function operates immediately prior
to either a scheduled automatic DCC or a manual DCC, by cooling the mirror for a few seconds
before heating it.
This cooling causes additional condensation on the mirror, which dissolves water-soluble matter,
and dislodges non water-soluble matter. When the surface is then heated and the water
evaporated, the contamination will cluster together leaving areas of clean mirror between, which
has less overall impact on the optics.

4.4.2. MAXCOOL Function

The MAXCOOL function overrides the dew point control loop and applies maximum cooling
drive to the Peltier heat pump. It can be used to determine:
• What temperature the mirror can be driven down to with reference to the sensor body.
• Whether or not the instrument is controlling at the dew point and whether it is able to reach
it. This situation could, for instance, arise when attempting to measure very low dew points
where, possibly due to a high ambient temperature, the Peltier heat pump is unable to depress
the temperature far enough to reach the dew point.
• Whether the instrument is controlling by switching MAXCOOL on for a short period and then
switching back to MEASURE. This will depress the mirror temperature briefly and when it is
switched back to MEASURE the control loop should be able to stabilize the mirror temperature
at the dew point again.
The MAXCOOL function can be turned on by touching the MAXCOOL button on the Main Screen.

4.4.3. Frost Assurance Technology (FAST)

In carefully controlled laboratory conditions, super-cooled water can exist in temperatures as low
as –48°C. However, when using a chilled mirror instrument it only occurs on the mirror at
temperatures down to around –30°C.
A gas in equilibrium with ice is capable of supporting a greater quantity of water vapour at a
given temperature than a gas in equilibrium with liquid water. This means that a measurement
below 0°C taken over water will read approximately 10% lower than the same measurement
taken over ice.
There are two modes of FAST operation, enabling and disabling FAST acts on both modes:
Following DCC: the Optidew makes an initial dew point measurement. If the initial measurement
is between -3°C and -30°C, then the mirror is driven down to below -35°C to ensure the formation
of ice on the mirror surface. The instrument then continues operation as normal.
40 97551 Issue 4.1, October 2019