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

Optidew User's Manual
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.
Dynamic: whenever the measured dew point drops below the value set as 'FAST SetP',
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.
Note that Data Hold is active whenever FAST is active.
For further information, see section 3.2
4.4.4

STANDBY Mode

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OPERATION
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